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| Characteristics of | CELL |
From Wikipedia, the free encyclopedia.
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life.
Organisms can be classified as unicellular (consisting of a single cell; including most bacteria) or multicellular (including plants and animals). Humans contain about 10 trillion (1013) cells. Most plant and animal cells are between 1 and 100 µm and therefore are visible only under the microscope.
The cell was discovered by Robert Hooke in 1665. In 1835, before the final cell theory was developed, Jan Evangelista Purkyn? observed small "granules" while looking at the plant tissue through a microscope. The cell theory, first developed in 1839 by Matthias Jakob Schleiden and Theodor Schwann, states that all organisms are composed of one or more cells, that all cells come from preexisting cells, that vital functions of an organism occur within cells, and that all cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells.
The word cell comes from the Latin cella, meaning "small room". The descriptive term for the smallest living biological structure was coined by Robert Hooke in a book he published in 1665 when he compared the cork cells he saw through his microscope to the small rooms monks lived in.
| Characteristics of | Eukaryotes |
The eukaryotes are distinguished from prokaryotes by the structural complexity of the cells characterized by having many functionall semi-autonomous elements, usually referred to as organelles.
Many of these are membrane bounded and include the nucleus, the rough endoplasmic reticulum, dictyosomes (= golgi apparatus) lysosomes, and in protists contractile vacuoles and extrusomes. Other membrane bound organelles that are not always present include chloroplasts and mitochondria. Non-membrane-bound organelles include cytoskeletal elements (mostly tubulin = microtubule or actin = microfilament - based), contractile (actin-myosin assemblages, centrin) or other motility devices (mitotic spindle, myonemes, cilia, flagella).
The synapomorphic features of the eukaryotes is the nucleus with associated mitotic division system. The first eukaryotic cells probably also had a small array of cytoskeletal elements but these have yet to be identified.
| Characteristics of | Mitochondrial Eukaryotes |
Mitochondrial eukaryotes include all cells with nuclear genomes that at one time in their evolutionary history contained mitochondria. This broad class includes all plants, fungi, animals and most protists.
| Characteristics of | Crown Eukaryotes |
Crown eukaryotes are an artificial group of eukaryotic organisms found at the top of molecular phylogenetic trees including both eukaryotes and prokaryotes. They were originally thought to represent a late step of eukaryotic evolution (somewhat similar to a crown group) because they include multicellular and macroscopic lifeforms that represent the majority of the biomass of the planet while accounting for less than 1% of the genetic diversity. However, they are in fact the result of an artificial clustering of eukaryotic organisms with slowly evolving gene sequences. They are thus not a crown that excludes simpler eukaryotes, but correspond roughly to the initial radiation of eukaryotes. All eukaryotic lineages branching below the "crown" in phylogenetic trees are misplaced because of the long branch attraction phenomenon.
| Characteristics of | Unikonta |
The group includes eukaryotic cells that, for the most part, have a single emergent flagellum, or are amoebae with no flagella. The unikonts include opisthokonts (animals, fungi, and related forms) and Amoebozoa.
| Characteristics of | Opisthokonta |
Cladogram
| Opisthokonta |
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| Characteristics of | Holozoa |
Holozoa is a group of organisms that includes animals and their closest single-celled relatives, but excludes fungi.
Holozoa is also an old name for the tunicate genus Distaplia.
Because Holozoa is a clade including all organisms more closely related to animals than to fungi, some authors prefer it to recognizing paraphyletic groups such as Choanozoa, which mostly consists of Holozoa minus animals.
Perhaps the best-known holozoans, apart from animals, are the choanoflagellates, which strongly resemble the collar cells of sponges, and so were theorized to be related to sponges even in the 19th century. Proterospongia is an example of a colonial choanoflagellate that may shed light on the origin of sponges.
The affinities of the other single-celled holozoans only began to be recognized in the 1990s. A group of mostly parasitic species called Icthyosporea or Mesomycetozoea is sometimes grouped with other species in Mesomycetozoa (note the difference in the ending). The amoeboid genera Ministeria and Capsaspora may be united in a group called Filasterea by the structure of their thread-like pseudopods. Along with choanoflagellates, filastereans may be closely related to animals, and one analysis grouped them together as the clade Filozoa.
| Characteristics of | Filozoa |
The Filozoa are a monophyletic grouping within the Opisthokonta. They include animals along with their nearest unicellular relatives (those organisms which are more closely related to animals than to Mesomycetozoa. The Mesomycetozoa (or DRIP clade, or Ichthyosporea) are a small group of protists, mostly parasites of fish and other animals.
| Characteristics of | Metazoa |
All animals are members of the Kingdom Animalia, also called Metazoa. This Kingdom does not contain the prokaryotes (Kingdom Mondera, includes bacteria, blue-green algae) or the protists (Kingdom Protista, includes unicellular eukaryotic organisms). All members of the Animalia are multicellular, and all are heterotrophs (that is, they rely directly or indirectly on other organisms for their nourishment). Most ingest food and digest it in an internal cavity.
Animal cells lack the rigid cell walls that characterize plant cells. The bodies of most animals (all except sponges) are made up of cells organized into tissues, each tissue specialized to some degree to perform specific functions. In most, tissues are organized into even more specialized organs. Most animals are capable of complex and relatively rapid movement compared to plants and other organisms. Most reproduce sexually, by means of differentiated eggs and sperm. Most animals are diploid, meaning that the cells of adults contain two copies of the genetic material. The development of most animals is characterized by distinctive stages, including a zygote, formed by the product of the first few division of cells following fertilization; a blastula, which is a hollow ball of cells formed by the developing zygote; and a gastrula, which is formed when the blastula folds in on itself to form a double-walled structure with an opening to the outside, the blastopore.
Somewhere around 9 or 10 million species of animals inhabit the earth; the exact number is not known and even our estimates are very rough. Animals range in size from no more than a few cells to organisms weighing many tons, such as blue whales and giant squid. Most animals inhabit the seas, with fewer in fresh water and even fewer on land.
| Characteristics of | Eumetazoa |
A section of Metazoa that includes the phyla above the Porifera (Sponges).
Phylogenetic analysis suggests that the Porifera and Ctenophora diverged before a clade that gave rise to the Bilateria. The sponges (Porifera) were long thought to have diverged from other animals early. They lack the complex organization found in most other phyla. Their cells are differentiated, but in most cases not organized into distinct tissues. Sponges typically feed by drawing in water through pores.
Metazoans are divided into two basic groups: Radiata (organisms with radial symmetry, including jellyfish and their relatives) and Bilateria (organisms with twofold symmetry that gives them definite front and rear, and left and right, body surfaces). Bilaterians, the first of which likely evolved in the Precambrian, are a step up in multicellular complexity from Radiata. Whereas Radiata develop from two embryonic tissue layers (an inner endoderm and outer ectoderm), the Bilateria have a third tissue layer, the mesoderm, between the endo- and ectoderm. This layer forms the muscles and most organs located between the digestive tract and the outer covering of the animal. The vertebrate circulatory and skeletal systems also stem from the mesoderm. Most bilaterians also show cephalization: the evolutionary trend toward the concentration of sensory structures (such as the mouth, nerves, etc.) at the anterior end of the body -- the end of a moving animal that is usually first to encounter food, danger, or other stimuli.
| Characteristics of | Bilateria |
The bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back. The bilateralia are a subregnum (a major group) of animals, including the majority of phyla; the most notable exceptions are the sponges, belonging to Parazoa, and cnidarians belonging to Radiata. For the most part, Bilateria have bodies that develop from three different germ layers, called the endoderm, mesoderm, and ectoderm. From this they are called triploblastic. Nearly all are bilaterally symmetrical, or approximately so. The most notable exception is the echinoderms, which achieve near-radial symmetry as adults, but are bilaterally symmetrical as larvae.
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| Characteristics of | Deuterostomia |
There are four extant phyla of deuterostomes:
It seems very likely that 555 million years old Kimberella was a member of the protostomes. If so, this means that the protostome and deuterostome lineages must have split some time before Kimberella appeared at least 550 millions years ago, and hence well before the start of the Cambrian 542 million years ago. The Ediacaran fossil Ernietta, from about 549 to 543 million years ago, may represent a deuterostome animal.
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| Characteristics of | Chordata |
The nerve cord of chordates develops dorsally in the body as a hollow tube above the notochord. In most species it differentiates in embryogeny into the brain anteriorly and spinal cord that runs through the trunk and tail. Together the brain and spinal cord are the central nervous system to which peripheral sensory and motor nerves connect.
The visceral (also called pharyngeal or gill) clefts and arches are located in the pharyngeal part of the digestive tract behind the oral cavity and anterior to the esophagus. The visceral clefts appear as several pairs of pouches that push outward from the lateral walls of the pharynx eventually to reach the surface to form the clefts. Thus the clefts are continuous, slit-like passages connecting the pharynx to the exterior. The soft and skeletal tissues between adjacent clefts are the visceral arches. The embryonic fate of the clefts and slits varies greatly depending on the taxonomic subgroup. In many of the non-vertebrate chordates, such as tunicates and cephalochordates, the clefts and arches are elaborated as straining devices concerned with capture of small food particles from water. In typical fish-like vertebrates and juvenile amphibians the walls of the pharyngeal clefts develop into gills that are organs of gas exchange between the water and blood. In adult amphibians and the amniote tetrapods (= reptiles, birds and mammals) the anterior most cleft transforms into the auditory (Eustachian) tube and middle ear chamber, whereas the other clefts disappear after making some important contributions to glands and lymphatic tissues in the throat region. The skeleton and muscles of the visceral arches are the source of a great diversity of adult structures in the vertebrates. For example, in humans (and other mammals) visceral arch derivatives include the jaw and facial muscles, the embryonic cartilaginous skeleton of the lower jaw, the alisphenoid bone in the side wall of the brain case, the three middle ear ossicles (malleus, incus and stapes), the skeleton and some musculature of the tongue, the skeleton and muscles of the larynx, and the cartilaginous tracheal rings.
| Characteristics of | Craniata |
The Craniata are characterized by a skull (initially cartilaginous and fibrous), which includes three types of sensory organs derived in ontogeny from ectodermal placodes; that is, thickened patches of the embryonic skin that sink inward toward the brain where they develop into sensory chambers. Anteriormost of these is the olfactory organ, which is initially unpaired, and becomes paired in the Vertebrata. Behind it are the paired eyes, the photo receptors that develop as lateral outgrowths of the brain. The skin and connective tissues adjacent to the neural (photoreceptive) part of the eye add secondary structures in the Vertebrata (lens, intrinsic muscles, and eye lids). Posteriormost of these sensory organs in the head are the paired acoustic organs or inner ears. The inner ears are mechanoreceptors concerned with hearing, balance, and perception of position of movement. The sensory cells of the inner ear are enclosed in a cavity filled with a liquid, the endolymph, and which develops from one to three semicircular canals. The acoustic organs also comprise a special component, the lateral sensory system, which is lost in most terrestrial craniates (Amniota). It consists of lateralis nerve fibres derived from the acoustic nerve and superficial mechanoreceptors, the neuromasts, which are housed in grooves or canals on the surface of the head. These extend onto the body in the Vertebrata. True neuromasts, however, seem to be unique to the Vertebrata, and have never been observed in hagfishes.
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The craniates are characterized by a skull; that is, a complex ensemble of skeletal elements which surrounds the brain and sensory capsules. The skull of hagfishes (top) consists of cartilaginous bars, but the brain is mostly surrounded by a fibrous sheath underlain by the notochord. The skull of lampreys has a more elaborate braincase and comprises a large "branchial basket" surrounding the gills. In the gnathostomes, the braincase is generally closed.
The skull also encloses the brain, always comprising five parts referred to as the rhombencephalon, metencephalon, mesencephalon, diencephalon, and telencephalon. The metencephalon is developed into a cerebellum in the Gnathostomata and some fossil jawless vertebrates. The nerve fibres are primitively non-myelinated and become myelinated only in the gnathostomes. The brain is continued posteriorly by the spinal cord, which is ribbon-shaped but becomes thicker in the gnathostomes. As in cephalochordates, the dorsal (sensory) and ventral (motor) spinal nerves are initially separate, but unite in the gnathostomes. In all craniates, the olfactory (I), optic (II), trigeminal (V), facial (VII), acoustic (VIII), glossopharyngeal (IX) and vagus (X) cranial nerves are present. Additional cranial nerves, the oculomotor (III), trochlear (IV) and abducent (VI) nerves occur only in the Vertebrata. Some consider that the latter have been secondarily lost in hagfishes.
The olfactory organ opens into a median duct, the nasopharyngeal duct, which also serves the intake of the respiratory water. In most vertebrates, however, this duct becomes a blind tube and the intake of respiratory water is made through the mouth or the gill slits. The nasopharyngeal duct lies ventrally against the diencephalon and there, in ontogeny, induces the formation of an important gland, the hypophysis, or pituitary organ, which comprises neural (neurohypophysis) and glandular (adenohypophysis) parts. The adenohypophysis is particularly complex in the Vertebrata, but very simple in hagfishes.
Craniates possess a unique embryonic tissue, the neural crest, that appears dorsal and lateral to the neural tube and which contributes to a great variety of adult tissues and structures including: sensory neurons (nerve cells), some skeletal and connective tissues in the skull, and some pigment containing cells and other integumentary tissues. In the skull, the neural crest cells give rise to the gill arches, jaws and parts of the braincase floor. In the gnathostomes and a number of fossil jawless vertebrates, the neural crest cells are also involved in the formation of the dermal skeleton (scales, teeth, and dermal bones).
The gills of craniates comprise gill filaments, made up by primary and secondary gill lamellae which insure gas exchanges. In hagfishes, the gills have no skeletal support, and are enclosed in pouches connected to the pharynx. Among vertebrates, a similar structure occurs in adult lampreys only, but here skeletal supports (gill arches) are present. The gills are derived from tissues of the embryonic gut (endoderm), but cells from the embryonic skin (ectoderm) are involved in their formation in the gnathostomes. The respiratory water flow is ensured by a special pumping and anti-reflux organ, the velum, situated at the limit between the mouth and the pharynx. There is a theory that the jaws of the gnathostomes are derived from the velum.
As chordates, all craniates develop a notochord, which is primitively large (hagfishes, lampreys), but becomes transitory in most vertebrates and is replaced by elements of the vertebral column, the centra and arcualia.
All craniates (except most tetrapods) possess a caudal fin strengthened by a number of cartilaginous radials. In vertebrates appear dorsal and anal fins, as well as radial muscles which ensure undulatory movements of the fin web. In the gnathostomes and some fossil jawless vertebrates, there are paired pectoral fins. Only the gnathostomes possess both pectoral and pelvic fins, which are modified into locomotory limbs in tetrapods.
All craniates possess an endoskeleton, which is primitively cartilaginous but becomes mineralized in various ways (bone, calcified cartilage) in the vertebrates. Only the gnathostomes and a number of fossil jawless vertebrates possess a mineralized exoskeleton which develops in the skin tissues. The exoskeleton is made up by a variety of tissues (bone, dentine, enamel).
Craniates have a circulatory system of arteries, capillaries and veins, and a chambered, muscular main heart located ventrally and anteriorly in the trunk. In the Vertebrata, the circulatory system is entirely closed. The two heart chambers, the atrium and ventricle are well apart. There are additional accessory venous hearts in the head and tail, which help in venous blood circulation, but these are lost in the Vertebrata. In gill-breathing craniates, the heart pumps venous blood anteriorly into arteries and capillaries in the gills for gas (oxygen and carbon dioxide) exchange with water. Oxygenated blood then collects dorsal to the gills and flows anteriorly to the head and posteriorly to the organs and muscles, and back to the heart. In some Vertebrata (Osteichthyes) diverticles of the digestive tract (lungs or air bladder) supplements or replaces gills as the repiratory organ.
The digestive tract of craniates is longitudinally differentiated into mouth and oral cavity, pharynx, esophagus, intestine, rectum and anus. A stomach is developed in the Gnathostomata and some fossil jawless Vertebrates. All craniates have a pancreas that produces digestive enzymes and hormones (insulin and glucagon) that regulate blood sugar level. The pancreas was ancestrally disseminated along the anterior part of the gut, but becomes condensed into a well-defined organ in the Vertebrata.
All craniates and the related cephalochordates have a liver or hepatic organ that serves many functions including food storage and production of fat emulsifiers (bile).
The kidneys are the chief excretory organs of vertebrates and these organs play an important role in water and salt balance. Although kidneys vary greatly in size, shape and position among species, all contain nephrons as the basic functional units. Each nephron is a nearly microscopic tubule that receives a filtrate of blood (lacking blood cells and very large molecules). The filtrate is processed by selective secretion and reabsorption of materials to produce an excretory product (generally called urine) that contains nitrogenous waste and other materials. Long and complex kidney tubules occur only in the vertebrates.
Cladogram
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| Characteristics of | Vertebrata |
The Vertebrata have all the characteristics of the Craniata but share, in addition, a number of unique characteristics which do not occur in hagfishes (Hyperotreti). These characteristics are:
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| Characteristics of | Gnathostomata |
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The Meckelian Cartilage, also known as "Meckel's Cartilage", is a piece of cartilage from which the mandibles (lower jaws) of vertebrates evolved. Originally it was the lower of two cartilages which supported the first gill arch (nearest the front) in early fish. Then it grew longer and stronger, and acquired muscles capable of closing the developing jaw.[1] In early fish and in chondrichthyans (cartilaginous fish such as sharks, which are not primitive in any sense of the word), the Meckelian Cartilage continued to be the main component of the lower jaw. But in the adult forms of osteichthyans (bony fish) and their descendants (amphibians, reptiles, birds, mammals), the cartilage was covered in bone - although in their embryos the jaw initially develops as the Meckelian Cartilage. In all tetrapods the cartilage partially ossifies (changes to bone) at the rear end of the jaw and becomes the articular bone, which forms part of the jaw joint in all tetrapods except mammals
| Characteristics of | Teleostomi |
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Teleostomi is a clade of jawed vertebrates that includes the tetrapods, bony fish, and the wholly extinct acanthodian fish. Key characters of this group include an operculum and a single pair of respiratory openings, features which were lost or modified in some later representatives. The teleostomes include all jawed vertebrates except the chondrichthyans and the placodermi.
The clade Teleostomi should not be confused with the similar-sounding fish clade Teleostei.
Teleostomes have two major adaptations that relate to aquatic respiration. First, the early teleostomes probably had some type of operculum, however, it was not the one-piece affair of living fish. The development of a single respiratory opening seems to have been an important step. The second adaptation, the teleostomes also developed a primitive lung with the ability to use some atmospheric oxygen. This developed, in later species, into the lung and (later) the swim bladder, used to keep the fish at neutral buoyancy.
| Characteristics of | Euteleostomi (Osteichthyes) |
| Euteleostomi is a successful clade that includes more than 90% of the living species of vertebrates. Euteleostomes are also known as "bony vertebrates". Both major subgroups are successful today: Actinopterygii includes the majority of extant fish species, and Sarcopterygii includes the tetrapods. This clade is sometimes called "Osteichthyes", but since that name literally means "bony fish" and traditionally is a paraphyletic group that excludes tetrapods, the name Euteleostomi was coined as a substitute.
Euteleostomes originally all had endochondral bone, fins with lepidotrichs, and jaws lined by maxillary, premaxillary, and dentary bones. Many of these characters have since been lost by descendant groups, however, such as lepidotrichs lost in tetrapods, and bone lost among the chondrostean fishes. The image shows on the left a sarcopterigyan fin and on the right an actinopterygian fin. |
| Characteristics of | Sarcopterygii |
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| Characteristics of | Choanata |
The only animals with choana are the tetrapoda, and they could as well be called Choanata (they are also the only ones with a vomeronasal organ, which has an embryonic origin from the olfactory structure).
These internal nasal passages evolved while the vertebrates still lived in water. At this point they already needed to gulp air to get enough oxygen, and rather than open their jaws each time to do this, those mutants who acquired small openings to breathe through were more successful at living in the new environment.
Fish
Fish do not have choanae, instead they have two pairs of external nostrils: each with two tubes whose frontal openings lie close to the upper jaw, and the posterior openings further behind near the eyes. Whether choanae of tetrapods are homologous to the posterior nostrils or not has been debated. Reasons for dispute have been that the posterior nostril in its evolution into choanae would have to switch position relative to other anatomical features, i.e. a nerve. Recent paleontologiclal findings support homology: a 400-million-year-old fossil lobe-finned fish called Kenichthys campbelli has something between a choana and the external nostrils seen on other fish, which makes it look like it has a cleft palate or cleft lip.
The reason seems to be that the posterior opening of the external nostrils has migrated into the mouth for some reason.
A similar evolution has taken place in lungfish. Here the inner nostrils have generally been accepted as homologous to the posterior nostrils, but the homology to true choanae as internal nostrils has been a matter of controversy. The fossil lungfish Diabolepis shows an intermediate stage between posterior and interior nostril and supports the independent origin of internal nostrils in the lungfish.
Tetrapods
Similar migration is still seen in the tetrapod embryo, and can cause a baby to be born with a cleft palate. Why it should migrate is a mystery, since the nostrils would be useless as a breathing device before their final position inside the mouth. They could also already breathe air through their spiracles.
Tetrapods are also equipped with a lacrimal duct, or tear duct. How it evolved is not known, but it has an internal connection with the choana. It is possible that the choana started as a natural crack between maxilla and premaxilla because of an incomplete fusion in air-breathing animals. If this gap got wider and deeper with time, the frontal part of it would have to fuse together to avoid weakening the upper jaw, creating a small opening on the upper lip. Some more migrating, and this gap would meet the anterior pair of the external nasal openings. The posterior pair of the openings was then free to form the lacrimal duct if a migration caused them to come in contact with the eyes.
Choanae analogues in other animals and fossils
This would not have been the first time the jaws evolved some sort of opening. For instance, snakes have evolved a cleft in the lower jaw, allowing them to stick out their tongues without having to open the jaw. For an animal living in water, the formation of a paired cleft on the upper jaw would be quite logical. Terrestrial vertebrates would in any case need a way to breathe without needing to open their jaws each time.
Some fossil species are said to have both conventional external nostrils and a choana, but only more fossils will give a real answer to how the choanas evolved.
Lungfish and hagfishes
In addition to tetrapods, the lungfish has internal nostrils too. These seem to have a different origin than those of the tetrapods, and lungfish have no tear duct either.
Hagfishes have a single internal nostril that opens inside the mouth cavity, while Chimaerae have open canals that leads water from their external nostrils into their mouth and through their gills.
| Characteristics of | Tetrapodomorpha |
Tetrapodomorpha is a clade of vertebrates, consisting of tetrapods (four-limbed vertebrates) and their closest sarcopterygian relatives. Advanced transitional fossils between fish and the early labyrinthodonts, like Tiktaalik, are called 'fishapods' by their discoverers. They are half-fish half-tetrapods, in appearance and limb morphology. Tetrapodomorpha contains:
| Characteristics of | Osteoleptiformes |
Osteolepiform fish are thought to the ancestors of the tetrapods because of the structure of their paired fins and also because they may have had choanae (an intrabuccal opening possible posterior nostril (excurrent) possibly shared with the tetrapods. Despite there being numerous species, structurally they where quite homogeneous. There are two recognised clades, the Tristichopteridae and Megalichthyids, although the early cosmine covered "osteolepid" fish (inc Osteolepis, Gyroptychius and Thursius) are unable to fit into a clade and are probably paraphyletic. They are first seen in the Emsian or Eifelian as Cosmine covered osteolepids, reached their maximus diversity in the Mid/Late Devonian and by the Late Carboniferous only the large megalichthyids remained.
| Characteristics of | Tristichopteridae |
TheTtristichopterids were the most diverse and successful of the tetrapodomorph fishes throughout the Late Devonian, but were extinct by the end of the Famennian.
What is exceedingly odd about the disappearance of tristichopterids is that they are very similar to elpistostegalians, the immediate ancestors of Tetrapods. Almost all of the characters of Tristichopterids are shared by Elpistostegalians, whether by inheritance or convergence. One distinction: the tendency to reinforce the cranial arch anteriorly, rather than lighten the rostrum as in Panderichthys. This may be the only significant qualitative distinction.
The Tristichopteridae include the much celebrated Eusthenopteron foordi from Miguasha in Canada made famous by Jarvik, who spent almost quarter of a century studying it. Eusthenopteron has lost the cosmine of the osteolepids, has a somewhat diphyceral tail compared to epiceral tail of Osteolepis has more angular posteriorly placed dorsal fins and an elongated snout (although juveniles have a shorter snout).
The most notable features of Eusthenopteron are the powerfully built pectoral and pelvic fins. This led to early speculation that Eusthenopteron could use these fins to crawl out of the water and onto land. This in turn led to Eusthenopteron being classed by some as a link to the early tetrapods.
Today however, Eusthenopteron is more widely accepted to have stayed in the water, but developed the parts that would allow for the evolution of legs. Reinforcement for this view comes from the study of transitional fossils such as Tiktaalik, which seems to suggest that primitive legs would have evolved in creatures that were still primarily aquatic. These continuing adaptations including primitive fingers and leg joints for navigating dense weeds and shallow waters, also proved useful for terrestrial locomotion as well.
Eusthenopteron did still have some of the features that would become present in later terrestrial amphibians. The teeth displayed folded enamel like the labyrinthodonts, and it also had internal nostrils. The bones of the pectoral fins also display clear upper and lower portions with bones that are analogous to a humerus, ulna and radius. The pelvic fins also have a similar arrangement but the bones here would be the equivalent of a femur, tibia and fibula.
| Characteristics of | Elpistostegalia |
Elpistostegalia or Panderichthyida is an order of prehistoric lobe-finned fishes which lived during the Late Devonian period (about 385 to 374 million years ago). They represent the advanced tetrapodomorph stock, the fishes more closely related to tetrapods than the osteolepiform fishes. The elpistostegalians, combining fishlike and tetrapod-like characters, are sometimes called fishapods, a phrase coined for the advanced elpistostegalian Tiktaalik.
A rise in global oxygen content allowed for the evolution of large, predatory fish that were able to exploit the shallow tidal areas and swamplands as top predators. Several groups evolved to fill these niches, the most successful were the elpistiostegalians. In such environments, they would have been challenged by periodic oxygen deficiency. In comparable modern aquatic environments like shallow eutrophic lakes and swampland, modern lungfish and some genera of catfish also rely on the more stable, atmospheric source of oxygen.
Being shallow-water fishes, the elpistostegalians evolved many of the basic adaptions that later allowed the tetrapods to become terrestrial animals. The most important ones were the shift of main propulsion apparatus from the tailfin to the pectoral and pelvic fins, and a shift to reliance on lungs rather than gills as the main means of obtaining oxygen. Both of these appear to be a direct result of moving to an inland freshwater mode of living.
Professor Per Ahlberg has identified the following traits as synapomorphic for Elpistostegalia (and thus Tetrapoda):
Tiktaalik is a monospecific genus of extinct sarcopterygian (lobe-finned "fish") from the late Devonian period, with many features akin to those of tetrapods (four-legged animals).
It is an example from several lines of ancient sarcopterygian "fish" developing adaptations to the oxygen-poor shallow-water habitats of its time, which led to the evolution of tetrapods. Well-preserved fossils were found in 2004 on Ellesmere Island in Nunavut, Canada.
Tiktaalik lived approximately 375 million years ago. Paleontologists suggest that it is representative of the transition between non-tetrapod vertebrates ("fish") such as Panderichthys, known from fossils 380 million years old, and early tetrapods such as Acanthostega and Ichthyostega, known from fossils about 365 million years old. Its mixture of primitive "fish" and derived tetrapod characteristics led one of its discoverers, Neil Shubin, to characterize Tiktaalik as a "fishapod".
Tiktaalik roseae is the only species classified under the genus. The name Tiktaalik is an Inuktitut word meaning "burbot", a freshwater fish related to true cod. The "fishapod" genus received this name after a suggestion by Inuit elders of Canada's Nunavut Territory, where the fossil was discovered.
Tiktaalik provide insights on the features of the extinct closest relatives of the tetrapods. Unlike many previous, more fishlike transitional fossils, the "fins" of Tiktaalik have basic wrist bones and simple rays reminiscent of fingers. The homology of distal elements is uncertain, but the proximal series can be directly compared to the ulnare and intermedium of tetrapods. The fin was clearly weight bearing, being attached to a massive shoulder with expanded scapular and coracoid elements and attached to the body armor, large muscular scars on the ventral surface of the humerus, and highly mobile distal joints. The bones of the fore fins show large muscle facets, suggesting that the fin was both muscular and had the ability to flex like a wrist joint. These wrist-like features would have helped anchor the creature to the bottom in fast moving current.
Also notable are the spiracles on the top of the head, which suggest the creature had primitive lungs as well as gills. This would have been useful in shallow water, where higher water temperature would lower oxygen content. This development may have led to the evolution of a more robust ribcage, a key evolutionary trait of land living creatures. The more robust ribcage of Tiktaalik would have helped support the animalÕs body any time it ventured outside a fully aquatic habitat. Tiktaalik also lacked a characteristic that most fishes haveÑbony plates in the gill area that restrict lateral head movement. This makes Tiktaalik the earliest known fish to have a neck, with the pectoral girdle separate from the skull. This would give the creature more freedom in hunting prey either on land or in the shallows.
Tiktaalik is a transitional fossil; it is to tetrapods what Anchiornis is to birds, troodonts and dromaeosaurids. While it may be that neither is ancestor to any living animal, they serve as evidence that intermediates between very different types of vertebrates did once exist.
The mixture of both "fish" and tetrapod characteristics found in Tiktaalik include these traits:
| Characteristics of | Tetrapoda |
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Tetrapoda is a crown group including modern amphibians and amniotes, their most recent common ancestor and all of its descendants. The animals have obvious, separate digits (fingers and toes) and well-defined joints in their limbs. Such limbs are called chiridii (chiridium, singular) (Laurin, 1998). Non-tetrapod tetrapodomorphs also possess chiridii, but true tetrapods always possess five or fewer functional digits per limb.
Interestingly, the name Tetrapoda means "four feet," yet snakes, whales, and limbless amphibians are tetrapods. These creatures have simply lost some or all of their limbs. Even when lost, the limbs are not lost uniformly.
Cephalization is the tendency for the head and anatomical units close to the head to form early in ontogeny (life development) and to be well-developed. Cephalization is characteristic of vertebrates and would, therefore, create expectations for patterns in limb development of animals. In most animal embryos, the head and front appendages form first and the lower extremities appear later. This might cause one to assume that if a limb were going to be lost, it would be the hind limbs to be lost because the front limbs appear earlier in ontogeny. Indeed, this is the case in whales where the forelimbs are preserved and the hindlimbs represented by only a vestigal splint for the femur and (in some species) a splint for the femur. This tendency does not hold true, however, in snakes where the hindlimb is represented by a vestigal ilium in primitive forms, but the forelimb is completely absent.
Tetrapods fall into two broad groups, Amphibia and Amniotamorpha. The former is known today from caecillians (amphisbaenians), which are limbless; urodeles (salamanders), which look like amphibian "lizards"; and anurans (frogs and toads). These three groups compose the clade Lissamphibia.
It is difficult to ascertain exactly when and in which group the amniote egg first appeared, but it is believed that diadectomorphs were approaching that condition. It seems likely that these creatures were spending a large amount of their time on land.
Pederpes ('Peter's Foot') is an extinct genus of early Carboniferous tetrapod, dating from the Tournaisian age (lower Mississippian, 359 - 345 Ma). Pederpes contains one species, P. finneyae.
This most basal Carboniferous tetrapod had a large, somewhat triangular head, similar to that of later American sister-genus Whatcheeria, from which it is distinguished by various skeletal features, such as a spike-like latissimus dorsi (an arm muscle) attachment on the humerus and several minor skull features. The feet had characteristics that distinguished it from the paddle-like feet of the Devonian Ichthyostegalia and resembled the feet of later, more terrestrially adapted Carboniferous forms. Pederpes is the earliest-known tetrapod to show the beginnings of terrestrial locomotion and despite the probable presence of a sixth digit on the forelimbs it was at least functionally pentadactyl.
Pederpes was discovered in 1971 in central Scotland and classified as a lobe-finned fish and It was not until 2002 that Jennifer Clack named and reclassified the fossil as a primitive tetrapod.
Pederpes is an important fossil because it comes from the period of time known as Romer's Gap and provides biologists with rare information about the development of tetrapods in a time where terrestrial life was rare.
Pederpes was 1 m long, making it average-sized for an early tetrapod.
The shape of the skull and the fact that the feet face forward rather than outward indicate that Pederpes was well adapted to land life. It is currently the earliest known fully terrestrial animal, although the structure of the ear shows that its hearing was still much more functional underwater than on land, and may have spent much of its time in the water and could have hunted there.
The narrow skull suggests that Pederpes breathed by inhaling with a muscular action like most modern tetrapods, rather than by pumping air into the lungs with a throat pouch the way many modern amphibians do.
| Characteristics of | Reptiliomorpha |
Reptiliomorpha refers to an order or subclass of reptile-like amphibians, which gave rise to the amniotes in the Carboniferous. Under phylogenetic nomenclature, the Reptiliomorpha includes their amniote descendants though, even in phylogenetic nomenclature, the name is mostly used when referring to the non-amniote reptile-like labyrinthodont grade. An alternative name, Anthracosauria is commonly used for the group, but is confusingly also used for the "lower" grade of reptiliomorphs.
Early reptiliomorphs During the Carboniferous and Permian periods, tetrapods evolved along a number of parallel lines towards a reptilian condition. Some of these tetrapods (e.g. Archeria, Eogyrinus) were elongate, eel-like aquatic forms with diminutive limbs, while others (e.g. Seymouria, Solenodonsaurus, Diadectes, Limnoscelis) were so reptile-like that until quite recently they actually had been considered true reptiles, and it is likely that to a modern observer they would have appeared as large to medium-sized, heavy-set lizards. Several groups however remained aquatic or semiaquatic. The two most terrestrially adapted groups were the medium sized insectivorous or carnivorous Seymouriamorpha and the mainly herbivorous Diadectomorpha, with many large forms. The latter group is in most analysis the closest relatives of the Amniotes.
From aquatic to terrestrial eggs
Their terrestrial life style combined with the need to return to the water to lay eggs hatching to larvae (tadpoles) lead to a drive to abandon the larval stage and aquatic eggs. A possible reason may have been competition for breeding ponds, to exploit drier environments with less access to open water, or to avoid predation on tadpoles by fish, a problem still plaguing modern amphibians. Whatever the reason, the drive led to internal fertilization and direct development (completing the tadpole stage within the egg). A striking parallel can be seen in the frog family Leptodactylidae, which has a very diverse reproductive system, including foam nests, non-feeding terrestrial tadpoles and direct development. The Diadectomorphans generally being large animals would have had correspondingly large eggs, unable to survive on land.
Fully terrestrial life was achieved with the development of the amniote egg, where a number of membranous sacks protect the embryo and facilitating gas exchange between the egg and the atmosphere. The first to evolve was probably the allantois, a sack that develops from the gut/yolk-sack. This sack contains the embryo's nitrogenous waste (urea) during development, stopping it from poisoning the embryo. A very small allantois is found in modern amphibians. Later came the amnion surrounding the fetus proper, and the chorion, encompassing the amnion, allantois, and yolk-sack.
Origin of amniotes
Exactly where the border between reptile-like amphibians (non-amniote reptiliomorphs) and amniotes lies will probably never be known, as the reproductive structures involved fossilize poorly, but various small, advanced reptiliomorphs have been suggested as the first true amniotes, including Solenodonsaurus, Casineria and Westlothiana. Such small animals lay small eggs, 1 cm in diameter or less. Such eggs will have a small enough volume to surface ratio to be able to develop on land without the amnion and chorin actively effecting gas exchange, setting the stage for the evolution of true amniotic eggs.
Although the first amniote probably appeared as early as the latest Mississippian period (Middle Carboniferous), non-amniote (or amphibian) reptiliomorphs continued to flourish alongside their amniote descendants for many millions of years. By the middle Permian the non-amniote terrestrial forms had died out, but several aquatic non-amniote groups continued to the end of the Permain, and in the case of the Chroniosuchids survived the end Permian mass extinction, only to die out at the end of the Early Triassic. Meanwhile, the single most successful daughter-clade of the reptiliomorphs, the amniotes, continued to flourish and to inherit the Earth.
| Characteristics of | Diadectomorpha |
Diadectomorpha are a clade of large reptile-like amphibians that lived in Euramerica during the Carboniferous and Early Permian periods, and are very close to the ancestry of the Amniota. They include both large (up to 2 meters long) carnivorous and even larger (to 3 meters) herbivorous forms, some semi-aquatic and others fully terrestrial.
Diadectomorphs possess both amphibian and reptilian characteristics. Originally these animals were included under the order Cotylosauria, and were considered the most primitive and ancestral lineage of reptiles. More recently they have been reclassified as amphibian-grade tetrapods, closely related to the first true amniotes. Contrary to other Reptiliomorph amphibians, the teeth of the Diadectomorpha lacked the infolding of the dentine and enamel that account for the name Labyrinthodontia for the non-amniote tetrapodes.
The reproduction of the Diadectomorphs has been the matter of some debate. If their group lay within the Amniota as has at times been assumed, they would have laid an early version of the amniote egg. Current thinking favours the amniote egg being evolved in very small animals, like Westlothiana or Casineria, leaving the bulky Diadectomorphs just on the amphibian side of the divide.
This would indicate the large and bulky Diadectomorphs had non-terrestrial anamniote eggs. However, no clearly diadectomorph tadpole is known. Whether this is due to an actual lack of tadpole stage or taphonomy (many diadectomorphs were upland creatures where tadpoles would have a poor probability of being fossilized) is uncertain. Alfred Romer indicated that the anamniote/amniote divide may not has been very sharp, leaving the question of the actual mode of reproduction of these large animals unanswered. Possible reproductive modes include full amphibian spawning with aquatic tadpoles, internal fertilization with or without ovoviviparity, aquatic eggs with direct development or some combination of these. The reproductive mode may also have varied within the group.
| Characteristics of | Amniota |
The amniotes are a group of tetrapods (four-limbed animals with backbones or spinal columns) that have a terrestrially adapted egg. They include synapsids (mammals along with their extinct kin) and sauropsids (reptiles and birds), as well as their fossil ancestors.
Many amniote synapomorphies are widely interpreted as adaptations to the rigors of life on land. Indeed Amniota owes its name to what may be its most distinctive attribute, alarge and hard-shelled "amniotic" egg which possesses of a unique set of membranes: amnion, chorion, and allantois. The amnion surrounds the embryo and creates a fluid-filled cavity in which the embryo develops. The chorion forms a protective membrane around the egg. The allantois is closely applied against the chorion, where it performs gas exchange and stores metabolic wastes (and becomes the urinary bladder in the adult).
As in other vertebrates, nutrients for the developing embryo are stored in theyolk sac, which is much larger in amniotes than in vertebrates generally. Hatchling amniotes also possess an egg-tooth and horny caruncle on the snout tip to facilitate exit from their hard-shelled eggs. The amniotic egg, together with a penis for internal fertilization, loss of a free-living larval stage in the life cycle, and the ability to bury their eggs, enabled amniotes to escape the bonds that confined their ancestors' reproductive activities to aquatic environments.
Some components of the amniotic egg have been variously modified within Amniota. Placental mammals, for example, have suppressed the egg shell and yolk sac, and elaborated the amniotic membranes to enable nutrients and wastes to pass directly between mother and embryo.
Development of extra embryonic membranes in an amniote egg (chick). In this early developmental stage, the yolk sac is expanding over the yolk. The amnion and chorion are expanding over the embryo and will eventually form the amniotic chamber. The allantois is expanding toward the chorion, with which it will form a respiratory membrane, in addition to storing metabolic wastes of the embryo.
The comparative aridity of the terrestrial environment affects all aspects of amniote biology, and not just their reproductive systems. Thus, amniotes have highly keratinized skins that are relatively impervious and reduce water loss. They also possess horny nails that, among other things, enable them to use their forelimbs to dig burrows into which they can retreat during the heat of the day.
The imperative to reduce water loss is equally evident in the density of renal tubules in the metanephric kidney of amniotes, in the larger size of their water-resorbing large intestines, and in the full differentiation of the Harderian and lacrimal glands in the eye socket whose antibacterial secretions help to moisten and, along with a third eyelid (the nictitans), to further protect the eye from desiccation.
The commitment of amniotes to a life on land is also revealed by anextensive system of muscle stretch receptors that enables finer coordination and greater agility during locomotion, their enlarged lungs (which are the only remaining organs of gas exchange owing to the loss of gills), and the complete loss of the lateral line system other vertebrates use to detect motion in water.
Many of these features are rarely preserved in fossils, but there are some novelties in the skeleton that are no less diagnostic of amniotes. For example, amniotes have at least two pairs of sacral ribs, instead of just one pair. They also have an astragalus bone in the ankle, instead of separate tibiale, intermedium, and proximal centrale bones. Finally, they have paired spinal accessory (11th) and hypoglossal (12th) cranial nerves incorporated into the skull, in addition to the ten pairs of cranial nerves present in amphibians.
| Characteristics of | Synapsida |
Synapsida is one of two great branches on the amniote family tree. This is the branch that includes us.
Synapsids (Greek, 'fused arch'), synonymous with theropsids (Greek, 'beast-face'), are a group of animals that includes mammals and every animal more closely related to mammals than to other living amniotes. They are easily separated from other amniotes by having a temporal fenestra, an opening low in the skull roof behind each eye, leaving a bony arch beneath each; this accounts for their name. Primitive synapsids are usually called pelycosaurs; more advanced mammal-like ones, therapsids. The non-mammalian members are described as mammal-like reptiles in classical systematics, but are referred to as "stem mammals" (or sometimes "protomammals") under cladistic terminology. Synapsids evolved from basal amniotes and are one of the two major groups of the later amniotes; the other is the sauropsids, a group that includes modern reptiles and birds. Their distinctive temporal fenestra developed in the ancestral synapsid about 324 million years ago (mya) during the late Carboniferous period.
Synapsids were the largest terrestrial vertebrates in the Permian period, 299 to 251 million years ago. As with almost all groups then extant, their numbers and variety were severely reduced by the Permian-Triassic extinction. Though some species survived into the Triassic period, archosaurs became the largest and most numerous land vertebrates in the course of this period. Few of the nonmammalian synapsids outlasted the Triassic, although survivors persisted into the Cretaceous. However, as a phylogenetic unit, they included the mammals as descendants, and in this sense synapsids are still very much a living group of vertebrates. After the CretaceousÐPaleogene extinction event, the synapsids (in the form of mammals) again became the largest land animals.
Characteristics
Temporal openings
| Synapsids evolved a temporal fenestra behind each eye orbit on the lateral surface of the skull. It may have evolved to provide new attachment sites for jaw muscles. A similar development took place in the Diapsids, which evolved two rather than one opening behind each eye. Originally, the opening in the skull left the inner cranium only covered by the jaw muscles, but in higher therapsids and mammals, the sphenoid bone has expanded to close the opening. This has left the lower margin of the opening as an arch extending from the lower edges of the braincase. | 
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| Characteristics of | Therapsida |
Therapsida is a group of the most advanced synapsids, and include the ancestors of mammals. Many of the traits today seen as unique to mammals had their origin within early therapsids, including hair, lactation, and an erect posture. The earliest fossil attributed to Therapsida is believed to be Tetraceratops insignis (Lower Permian). Therapsids evolved from 'pelycosaurs' (specifically sphenacodonts) 275 million years ago. They replaced the pelycosaurs as the dominant large land animals in the Middle Permian. They remained the dominant fauna until replaced by archosaurs and rhynchosaurs in the Middle Triassic although some therapsids, the kannemeyeriiforms for example, remained diverse in the Late Triassic. The therapsids included the cynodonts, the group that gave rise to mammals in the Late Triassic around 225 million years ago. Of the non-mammalian therapsids, only cynodonts and dicynodonts survived the TriassicÐJurassic extinction event. The last of the non-mammalian therapsids, the cynodont tritylodontids, became extinct in the Early Cretaceous, approximately 100 million years ago. Similar in some respects to modern large ungulates were tapinocephalids. Basal theriodonts were similar to similar our terrestiral carnivorans, and many of the smaller forms filled niches that might correspond to our rodents. It was actually the small forms that were able to survive the extinction event that hit many therapsid groups. Ironically it was these small, insectivoran-style therapsids that would eventually give rise to mammals while the hippo- and carnivoran- mimics went extinct.
Basal therapsids may or may not have had "improved" metabolic conditioning. It seem likely that the relatively advanced forms like basal cynodonts might have been advanced well above the common basal amniote metabolism. If this is true, then it is not unreasonable to restore them with some form of insulatory structure; in this case, hair. Tenrecs and basal mammals (i.e., the duckbilled platypus) show the type of metabolism to which I refer. More basal forms may or may not have had more advanced metabolic rates than modern reptiles.
While the early therapsids had skulls very similar to those of their pelycosaurian ancestors, they differed in the post-cranial skeleton.
Legs and feet
Their legs are positioned more vertically beneath their bodies than are the sprawling legs of reptiles and pelycosaurs. The feet were more symmetrical, with the first and last toes short and the middle toes longer, indication the foots axis was placed parallel to that of the animal, not sprawling out sideways. This would have given a more mammal-like gait than the lizard-like gait of the pelycosaurs.
Jaw and teeth
Therapsids' temporal fenestrae are greater than those of the pelycosaurs. The jaws of therapsids are more complex and powerful, and the teeth are differentiated into frontal incisors for nipping, great lateral canines for puncturing and tearing, and molars for shearing and chopping food.
| Characteristics of | Theriodontia |
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Theriodonts ("Beast Tooth", referring to more mammal-like teeth), are a major group of therapsids. They can be defined in traditional, Linnaean terms, in which case they are a suborder of mammal-like reptiles that lived from the Middle Permian to the Middle Cretaceous, or in Cladistic terms, in which case they include not only the traditional theriodonts but also their descendants the mammals as well (in the same way that, cladistically speaking, the theropod dinosaurs include the birds as a sub-clade).
Theriodonts appeared almost the same time as the anomodonts, about 265 million years ago, in the Middle Permian. Even these early theriodonts were more mammal-like than their Anomodont and Dinocephalian contemporaries. Eutheriodonts refer to all theriodonts except the gorgonopsians (the most primitive group). They included the therocephalians, cynodonts and their descendants: the mammals . The name means "true beast tooth". The eutheriodonts have larger skulls, accommodating larger brains and improved jaw muscles. The theriodonts (eutheriodonts) are one of the two synapsid survivors of the great Permian Triassic extinction event, the other being the dicynodonts. Therocephalians included both carnivorous and herbivorous forms; both died out after the Early Triassic. The remaining theriodonts, the cynodonts, also included carrnivores such as Cynognathus, as well as newly evolved herbivorous (Traversodonts). While Traversodonts for the most part remained medium-sized to reasonably large (length of largest species up to 2 meters), the carnivorous forms became progressively smaller as the Triassic progressed. By the Late Triassic the small cynodonts included the rodent-like tritylodonts (possibly related to or descended from travsersodonts), and the tiny, shrew-like, trithelodonts, which evolved into the first mammals. The trithelodonts died out during the Jurassic, and the tritylodonts survived in the Cretaceous, but the mammals continued to evolve. Many mammal groups managed to survive the Cretaceous Paleogene extinction event, which wiped out the non-avian dinosaurs, allowing the mammals to diversify and dominate the Earth. |
| Characteristics of | Cynodontia |
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Cynodontia or cynodonts ("dog teeth") are a taxon of therapsids which first appeared in the Late Permian (approximately 260 Ma) and were eventually distributed throughout all seven continents by the Early Triassic (256 Ma). This clade includes modern mammals and their extinct close relatives. They were one of the most diverse groups of therapsids. They are named after their dog-like teeth. Cynodonts have nearly all the characteristics of mammals. Their teeth were fully differentiated, the braincase bulged at the back of the head, and many of them walked in an upright manner. Cynodonts still laid eggs, as all Mesozoic proto-mammals probably did. Their temporal fenestrae were much larger than in its ancestors, and the widening of the zygomatic arch allowed for more robust jaw musculature supporting the evidence of a more mammal-like skull. They also have the secondary palate that other primitive therapsids lacked, except the therocephalians, who were the closest relatives of cynodonts. Their dentary was the largest bone in their lower jaw, as other smaller bones moved into the ears. They were probably warm-blooded, and covered in hair. |  |
| Characteristics of | Eucynodontia |
Eucynodontia ("true dog teeth") is a grouping of animals that includes both mammals and mammal-like non-mammalian therapsids ("mammal-like reptiles") such as cynodonts ("dog teeth"). Its membership was and is made up of both carnivores and herbivores. The chronological range extends from at least the Lower Triassic, possibly the Upper Permian, until the present day.
| Characteristics of | Probainognathia |
 | The Probainognathians are one of the two major clades of the infraorder Eucynodontia, the other being Cynognathians. They were mostly carnivorous, though some species may have evolved omnivorous traits. The Probainognathia form into four groups: Probainognathidae, Chiniquodontidae, Tritheledontidae, and Mammaliaformes. The earliest and most basal Probainognathian is Lumkuia, from South Africa. Non-mammalian probainognathians lived from Triassic to Jurassic, making this clade one of the longest lived therapsid family. |  |
| Characteristics of | Mammaliaformes |
 | Mammaliaformes ("mammal-shaped") is a clade that contains the crown group mammals and their closest extinct relatives. It is defined as the clade originating from the most recent common ancestor of Morganucodonta and the crown group mammals; the latter is the clade originating with the most recent common ancestor of extant Monotremata, Marsupialia, and Placentalia. Early mammaliaforms were generally shrew-like in appearance and size, and most of their distinguishing characteristics were internal. In particular, the structure of the mammaliform (and mammal) jaw and arrangement of teeth is nearly unique. Instead of having many teeth that are frequently replaced, mammals have one set of baby teeth and later one set of adult teeth which fit together precisely. This is thought to aid in the grinding of food to make it quicker to digest. Warm-blooded animals require more calories than those that are cold-blooded, so quickening the pace of digestion is a necessity. The drawback to the fixed dentition is that worn teeth cannot be replaced, as was possible for the reptilian ancestors of mammaliforms. However, as small mammals are generally very short-lived compared to reptiles of the same size, this was not much of a problem during the early phase of their evolution, in which the trait was set. Lactation, along with other characteristically mammalian features, is also thought to characterize the Mammaliaformes, but these traits are difficult to study in the fossil record. While the early mammaliforms likely had some form of lactation, their mammary glands probably were not associated with distinct mammae with nipples but rather were distributed in patches on the belly side with the young licking milk from the fur. Prior to hatching, the same glands would provide moisture to the leathery eggs, a situation still found in monotremes. Some early mammaliaforms did have fur. An insulative covering is necessary to keep a homeothermic animal warm if it is very small, less than 5 cm (1.97 in) long. |
| Characteristics of | Mammalia |
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All mammals share three characteristics not found in other animals: 3 middle ear bones, hair, and the production of milk by modified sweat glands called mammary glands.
Mammals hear sounds after they are transmitted from the outside world to their inner ears by a chain of three bones, the malleus, incus, and stapes. Two of these, the malleus and incus, are derived from bones involved in jaw articulation in most other vertebrates.
Mammals have hair. Adults of somespecies lose most of their hair, but hair is present at least during some phase of the ontogeny of all species. Mammalian hair, made of a protein called keratin, serves at least four functions. First, it slows the exchange of heat with the environment (insulation). Second, specialized hairs (whiskers or "vibrissae") have a sensory function, letting the owner know when it is in contact with an object in its external environment. These hairs are often richly innervated andwell-supplied with muscles that control their position. Third, through their color and pattern, hairs affect the appearance of amammal. They may serve to camouflage, to announce the presence of especially good defense systems (for example, the conspicuous color pattern of a skunk is a warning to predators), or to communicate social information (for example, threats, such as the erect hair onthe back of a wolf; sex, such as the different colors of male and female capuchin monkeys; presence of danger, such as the white underside of the tail of a white tailed deer). Fourth, hair provides some protection, either simply by providing an additional protective layer (against abrasion or sunburn, for example) or by taking on the form of dangerous spines that deter predators (porcupines, spinyrats, others).
Mammals feed their newborn young with milk, a substance rich in fats and protein that is produced by modified sweat glands called mammary glands. These glands, which take a variety of shapes, are usually located on the ventral surface of females along paths that run from the chest region to the groin. They vary in number from two (one right, one left, as in humans) to a dozen or more.
Other characteristics found in most mammals include highly differentiated teeth; teeth are replaced just once during an individual's life (this condition is called diphyodonty, and thefirst set is called "milk teeth); a lower jaw made up of asingle bone, the dentary; four-chambered hearts, a secondary palate separating air and food passages in the mouth; a muscular diaphragm separating thoracic and abdominal cavities; highly developed brain; endothermy and homeothermy; separate sexes with the sex of an embryobeing determined by the presence of a Y or 2 X chromosomes; and internal fertilization.
The Class Mammalia includes around 5000 species placed in 26 orders (systematists do not yet agree on the exact number or on how some orders are related to others). Mammals can be found in all continents and seas. In part because of their high metabolic rates (associated with homeothermy and endothermy), they often play an ecological role that seems disproportionately large compared to their numerical abundance.
| Characteristics of | Theria |
Theria is a subclass of mammals that give birth to live young without using a shelled egg, consisting of the Eutherians (including the placental mammals) and the Metatherians (including the marsupials). The only omitted extant mammal group is the egg-laying monotremes (Prototheria: the other subclass of Mammals): Platypus and Echidna
| Characteristics of | Eutheria |
Eutherians are a group of mammals consisting of placental mammals plus all extinct mammals that are more closely related to living placentals (such as humans) than to living marsupials (such as kangaroos).
There are no living nonplacental eutherians, and so knowledge of their synapomorphies ("defining features") is entirely based on a few fossils, which means the reproductive features that distinguish modern placentals from other mammals cannot be used in defining Eutheria.
The features of Eutheria that distinguish them from metatherians, a group that includes modern marsupials, are:
| Characteristics of | Euarchontoglires |
Euarchontoglires (synonymous with Supraprimates) is a clade of mammals, the living members of which are rodents, lagomorphs, treeshrews, colugos and primates (including humans).
The Euarchontoglires clade is based on DNA sequence analyses and retrotransposon presence/absence data, combining the Glires clade, which consists of Rodentia and Lagomorpha, with that of Euarchonta, a clade consisting of Scandentia, Primates (which includes humans) and Dermoptera.
Euarchontoglires is now recognized as one of four major groups within Eutheria (containing placental mammals). These four clades are usually discussed without a Linnaean rank, but has been assigned the rank of cohort or magnorder, and superorder.
Relations within the four cohorts, Euarchontoglires, Xenarthra, Laurasiatheria, and Afrotheria, and the identity of the placental root, remain somewhat controversial.
Euarchontoglires probably split from the Laurasiatheria sister group about 85 to 95 million years ago during the Cretaceous, developing in the Laurasian island group which would later become Europe. This hypothesis is supported by fossil as well as molecular evidence. The clade of Euarchontoglires and Laurasiatheria is recognized as Boreoeutheria.
The hypothesized relationship among the Euarchontoglires is as follows:
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| Characteristics of | Primates |
The Primates are an ancient and diverse eutherian group, with around 233 living species placed in 13 families. Most dwell in tropical forests. The smallest living primate is the pygmy mouse lemur, which weighs around 30 g. The largest is the gorilla, weighing up to around 175 kg.
Primates radiated in arboreal habitats, and many of the characteristics by which we recognize them today (shortened rostrum and forwardly directed orbits, associated with stereoscopic vision; relatively large braincase; opposable hallux and pollex; unfused and highly mobile radius and ulna in the forelimb and tibia and fibula in the hind) probably arose as adaptations for life in the trees or areprimitive traits that were retained for the same reason. Severalspecies, including our own, have left the trees for life on theground; nevertheless, we retain many of these features.
Primates are usually recognized based on a suite of primitive characteristics of the skull, teeth, and limbs. Some of these are listed above, including the separate and well-developed radius and ulna in the forearm and tibia and fibula in the hindleg. Others include pentadactyl feet and presence of a clavicle. Additional characteristics (not necessarily unique to primates) include first toe with a nail, while other digits bear either nails or claws, and stomach simple in most forms (sacculated in some leaf-eating cercopithecids). Within primates, there is a tendency towards reduction of the olfactory region of the brain and expansion of the cerebrum (especially the cerebral cortex), correlated with an increasing reliance on sight and increasingly complex social behavior.
The teeth of primates vary considerably. The dental formula for the order is 0-2/1-2, 0-1/0-1, 2-4/2-4, 2-3/2-3 = 18-36. The incisors are especially variable. In some forms, most incisors have been lost, although all retain at least 1 lower incisor. In others, the incisors are intermediate in size and appear to function as pincers or nippers, as they commonly do in other groups of mammals. In some, including most strepsirhines (see next paragraph), the lower incisors form a toothcomb used in grooming and perhaps foraging. In the aye-aye (Daubentoniidae), the incisors are reduced to 1 in each jaw and are rodent-like in form and function. Canines are usually (but not always) present; they vary in size, including within species between males and females. Premolars are usually bicuspid (bilophodont), but sometimes canine-like or molar-like. Molars have 3-5 cusps, commonly 4. A hypocone was added early in primate history, and the paraconid was lost, leaving both upper and lower teeth with a basically quadrate pattern. Primitively, primate molars were brachydont and tuberculosectorial, but they have become bunodont and quadrate in a number of modern forms.
Living primates are divided into two great groups, the
Strepsirhini and the Haplorhini. Strepsirhines have naked noses,
lower incisors forming a toothcomb,
and no plate separating orbit from temporal fossa. The second digit
on the hind foot of many strepsirhines is modified to form a
"toilet
claw" used in grooming. Strepsirhines include mostly arboreal
species with many primitive characteristics, but at the same time,
some extreme specializations for particular modes of life.
Most primate species live in the tropics or subtropics, although a few, most notably humans, also inhabit temperate regions. Except for a few terrestrial species, primates are arboreal. Some species eat leaves or fruit; others are insectivorous or carnivorous.
Here, we follow Anderson and Jones (1984) in formally dividing living primates into two suborders, the Strepsirhini and the Haplorhini. We differ, however, in that we place humans and their close relatives, the chimpanzee, gorilla, and orang in the family Hominidae.
| Characteristics of | Haplorhines |
The Haplorhines, the "dry-nosed" primates (the Greek name means "simple-nosed"), are members of the Haplorhini clade: the prosimian tarsiers and the anthropoids. The anthropoids are the catarrhines (Old World monkeys and apes, including humans) and the platyrrhines (New World monkeys). The omomyids are an extinct group of prosimians, believed to be more closely related to the tarsiers than to any strepsirrhines, and are considered the most primitive haplorhines. Haplorhines share a number of derived features that distinguish them from the strepsirrhine "wet-nosed" primates (whose Greek name means "curved nose"), the other suborder of primates from which they parted in evolution some 63 million years ago. The haplorhines, including tarsiers, have all lost the function of the terminal enzyme which manufactures vitamin C, while the strepsirrhine prosimians, like most other orders of mammals, have retained this enzyme and the ability to manufacture vitamin C. The haplorhine upper lip, which has replaced the ancestral rhinarium found in strepsirrhines, is not directly connected to their nose or gum, allowing a large range of facial expressions. Their brain to body ratio is significantly greater than the strepsirrhines, and their primary sense is vision. Haplorhines have a postorbital plate, unlike the postorbital bar found in strepsirhines. Most species are diurnal (the exceptions being the tarsiers and the night monkeys). All anthropoids have a single-chambered uterus; tarsiers have a bicornate uterus like the strepsirrhines. Most species typically have single births, although twins and triplets are common for marmosets and tamarins. Despite similar gestation periods, haplorhine newborns are relatively much larger than strepsirrhine newborns, but have a longer dependence period on their mother. This difference in size and dependence is credited to the increased complexity of their behavior and natural history.
| Characteristics of | Simiiformes |
The Old and New World Primates but Tarsius.
| Characteristics of | Catarrhines |
Platyrrhines have flat noses, outwardly directed nasal
openings, 3 premolars in upper and lower jaws, anterior upper molars
with 3 or 4 major cusps, and are found only in the New World
(families Cebidae and Callitrichidae).
Catarrhines have paired downwardly directed nasal openings, which are
close together; usually 2 premolars in each jaw, anterior upper
molars with 4 cusps, and are found only in the Old World (Cercopithecidae,
Hylobatidae, Hominidae).
| Characteristics of | Hominoidea |
Apes are Old World anthropoid mammals, more specifically a clade of tailless catarrhine primates, belonging to the biological superfamily Hominoidea. The apes are native to Africa and South-east Asia. Apes are the world's largest primates; the orangutan, an ape, is the world's largest living arboreal animal. Hominoids are traditionally forest dwellers, although chimpanzees may range into savanna, and the extinct australopithecines are famous for being savanna inhabitants, inferred from their morphology. Humans inhabit almost every terrestrial habitat. Hominoidea contains two families of living (extant) species:
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| Characteristics of | Hominidae |
Until recently, most classifications included only humans in this family; other apes were put in the family Pongidae (from which the gibbons were sometimes separated as the Hylobatidae). The evidence linking humans to gorillas and chimps has grown dramatically in the past two decades, especially with increased use of molecular techniques. It now appears that chimps, gorillas, and humans form a clade of closely related species; orangutans are slightly less close phylogenetically, and gibbons are a more distant branch. Here we follow a classification reflecting those relationships. Chimps, gorillas, humans, and orangutans make up the family Hominidae; gibbons are separated as the closely related Hylobatidae.
Thus constituted, the Hominidae includes 4 genera and 5 species. Its nonhuman members are restricted to equatorial Africa, Sumatra and Borneo. Hominid fossils date to the Miocene and are known from Africa and Asia.
Hominids range in weight from 48 kg to 270 kg. Males are larger than females. Hominids are the largest primates, with robust bodies and well-developed forearms. Their pollex and hallux are opposable except in humans, who have lost opposability of the big toe. All digits have flattened nails. No hominid has a tail, and none has ischial callosities. Numerous skeletal differences between hominids and other primates are related to their upright or semi-upright stance.
All members of this family have large braincase. Most have a prominent face and prognathous jaw; again, humans are exceptional. All are catarrhine, with nostrils close together and facing forward and downward. The dental formula is the same for all members of the group: 2/2, 1/1, 2/2, 3/3 = 32. Hominids have broad incisors and their canines are never developed into tusks. The upper molars are quadrate and bunodont; the lowers are bunodont and possess a hypoconulid. The uppers lack lophs connecting labial and lingual cusps and thus, in contrast to cercopithecids, are not bilophodont.
Hominids are omnivorous, primarily frugivorous or folivorous. All but humans are good climbers, but only the orangutan is really arboreal.
Members of this family are well-known for the complexity of their social behavior. Facial expression and complex vocalizations play an important role in the behavior of hominids. All make and use nests. Hominids generally give birth to a single young, and the period of parental care is extended.
Species included in database (Wikipedia):
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See also:
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