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Limbs in synapsids

Limbs in synapsids


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Within synapsids, there was a change from a lizard-like sprawling posture (like in pelycosaurs) to a more erect stance. Non-eucynodont eutherapsids seem to have had a facultatively erect hindlimb and sprawling or semierect forelimbs. Eucynodonts seem to have had an obligatorily erect hindlimb and semierect forelimbs. Yet so many mammals (especially monotremes) and one tritylodontid seem to have had non-erect hindlimbs. What could have driven them to a non-erect hindlimb? Were there mammals with facultatively erect hindlimbs?


I think you are confused about what erect limbs means, monotremes have erect hindlimbs. Erect is not the same as pillar erect (which is exceedingly rare) Just because a limb is bent or a joint can rotate does not mean it is not an erect limb. More importantly terms like semi-erect don't actually mean much, it is a weak gradation not discrete categories. Like most things in biology nature does not respect our attempts to pigeon hole. Many animals can in fact switch between locomotive style.

To answer your second question, limb posture is often modified for specialized environments like aquatic living or burrowing. Which is why seal and mole limbs also have odd arrangements.


Synapsida

This family of very primitive pelycosaurs includes two species, known only from several skulls and limb bones from the Early Permian of North America. Eothyris, from the Early Permian of Texas (Belle Plains formation, Artinskian age), is known from a single skull (right), 62mm in length. The teeth are simple conical structures, and there are distinct caniniform teeth. Oedaleops, from the Early Permian of New Mexico (Abo Cutler Formation, Sakmarian , is known from three partial skulls and some limb elements.

In terms of cranial morphology the eothyridids are the most primitive of any pelycosaur. For this reason they are, in modern cladistic classifications, considered a sort of ancestral group (or rather, the sister group to all other synapsids). However they appear quite late in the fossil record (they are one of the last groups of pelycosaurs to appear). This means either that they existed for a long time but in upland areas or isolated geographic regions away from any chance of fossilisation, or, alternatively, that they are only secondarily primitive. Either hypothesis is valid, and the answer, if it is ever known, must await further fossil discoveries.

The eothyririds share a few unique characteristics (synapomorphies) with the caseids, indicating they may be ancestral to the latter. The two groups are united in the clade (suborder?) Caseasauria. (MAK 000720).


Characteristics of Amniotes

All amniotes have three membranes surrounding the fetus of one offspring. These membranes are the amnion, or protective layer, the top chorion layer, and the waste-absorbing allantois. These layers can be seen in the image of a chicken egg, below.

While amniotes share a number of other characteristics in general (being vertebrates, tetrapods, etc.), they all developed from a common ancestor which developed the amnion character. The amnion is seen within egg-laying species, such as birds and reptiles, as well as in mammals. While human eggs have lost the shell, in many ways they are identical to chicken eggs as they develop within the uterus.


Evolution of Mammals

The evolution of mammals passed through many stages since the first appearance of their synapsid ancestors in the late Carboniferous period. Mammals are synapsids: they have a single opening in the skull. They are the only living synapsids as earlier forms became extinct by the Jurassic period. The early, non-mammalian synapsids can be divided into two groups: the pelycosaurs and the therapsids. Within the therapsids, a group called the cynodonts are thought to be the ancestors of mammals. By the mid-Triassic, there were many synapsid species that looked like mammals. The lineage leading to today&rsquos mammals split in the Jurassic. Synapsids from this period include Dryolestes (more closely related to extant placentals and marsupials than to monotremes) as well as Ambondro (more closely related to monotremes). Later, the eutherian and metatherian lineages separated. Metatherians are the animals more closely related to the marsupials, while eutherians are those more closely related to the placentals. Eutherians are distinguished from noneutherians by various features of the feet, ankles, jaws, and teeth. One of the major differences between placental and nonplacental eutherians is that placentals lack epipubic bones, which are present in all other fossil and living mammals (marsupials and monotremes).

Figure (PageIndex<1>): Cynodonts: Cynodonts, which first appeared in the Late Permian period 260 million years ago, are thought to be the ancestors of modern mammals.

Since Juramaia, the earliest-known eutherian, lived 160 million years ago in the Jurassic, this divergence must have occurred in the same period. After the Cretaceous&ndashPaleogene extinction event wiped out the non-avian dinosaurs (birds are generally regarded as the surviving dinosaurs) and several other mammalian groups, placental and marsupial mammals diversified into many new forms and ecological niches throughout the Paleogene and Neogene, by the end of which all modern orders had appeared.

The synapsid lineage became distinct from the sauropsid lineage in the late Carboniferous period, between 320 and 315 million years ago. The sauropsids are today&rsquos reptiles and birds, along with all the extinct animals more closely related to them than to mammals. This does not include the mammal-like reptiles, a group more closely related to the mammals. Throughout the Permian period, the synapsids included the dominant carnivores and several important herbivores. In the subsequent Triassic period, however, a previously-obscure group of sauropsids, the archosaurs, became the dominant vertebrates. The mammaliaforms appeared during this period their superior sense of smell, backed up by a large brain, facilitated entry into nocturnal niches with less exposure to archosaur predation. The nocturnal lifestyle may have contributed greatly to the development of mammalian traits such as endothermy and hair. Later in the Mesozoic, after theropod dinosaurs replaced rauisuchians as the dominant carnivores, mammals spread into other ecological niches. For example, some became aquatic, some were gliders, and some even fed on juvenile dinosaurs. Most of the evidence consists of fossils. For many years, fossils of Mesozoic mammals and their immediate ancestors were very rare and fragmentary however, since the mid-1990s, there have been many important new finds, especially in China. The relatively new techniques of molecular phylogenetics have also shed light on some aspects of mammalian evolution by estimating the timing of important divergence points for modern species. When used carefully, these techniques often, but not always, agree with the fossil record. Although mammary glands are a signature feature of modern mammals, little is known about the evolution of lactation. This is because these soft tissues are not often preserved in the fossil record. Most study of the evolution of mammals centers, rather, around the shapes of the teeth, the hardest parts of the tetrapod body. Other much-studied aspects include the evolution of the middle ear bones, erect limb posture, a bony secondary palate, fur and hair, and warm-bloodedness.

A key characteristic of synapsids is endothermy, rather than the ectothermy seen in most other vertebrates. The increased metabolic rate required to internally-modify body temperature went hand-in-hand with changes to certain skeletal structures. The later synapsids, which had more-evolved characteristics unique to mammals, possess cheeks for holding food and heterodont teeth (specialized for chewing by mechanically breaking down food to speed digestion and releasing the energy needed to produce heat). Chewing also requires the ability to chew and breathe at the same time, which is facilitated by the presence of a secondary palate. It separates the area of the mouth where chewing occurs from the area above where respiration occurs, allowing breathing to proceed uninterrupted during chewing. A secondary palate is not found in pelycosaurs, but is present in cynodonts and mammals. The jawbone also shows changes from early synapsids to later ones. The zygomatic arch, or cheekbone, is present in mammals and advanced therapsids such as cynodonts, but is not present in pelycosaurs. The presence of the zygomatic arch suggests the presence of the masseter muscle, which closes the jaw and functions in chewing.


Order Therapsida

Therapsids are the more advanced of the order Synapsid, the direct ancestors of mammals. The Therapsids grew to much importance after surviving the Permian extinction and became much more widespread than their ancestors, the Pelycosauria. Even though they split off into several suborders, the only one to survive into the Jurassic Period was the cynodont suborder. Many of the therapsids have been found in places like Europe, Africa (mainly South Africa), Antarctica, Asia (China and India), and South America (mostly in Argentina). This may seem like a great feat but it helps to remember that during their reign all land masses had come together to form one massive landmass called, Gondwanaland. Since these animals were so widespread, they had much variability in their size and morphology. Some a long tails, some had short tails, some were big and thick and some were skinny and set much closer to the ground. For example, one species called the Moschops was around 16ft tall, had a massive skull, and a very thick body, it would have towered over a male human. [2] Some animals were carnivorous while others were herbivores, and there were even a few insectivores the skulls and dentition of which would have changed depending on their diets. For example, the carnivores were outfitted with sharper and larger incisors and canines as well as the teeth in the back which would have been used for shredding the meat (much like modern day mammals).

Suborder Dicynodontia

The dicynodonts were the largest suborder of all the therapsids, and aside from the cynodonts, were the most advanced. The most important evolution for the dicynodonts was the development of their stronger jaws. The synapsid opening in their skuls was much bigger than their previous ancestors, allowing for their muscles to be strengthened. These animals were so widespread that they were able to adapt to many different environments and use many different niches. For example, the Cistecephalus were animals that were adapted for underground life, like our present day moles. Others like the Robertia were probably adapted for life in the forest, as seen by their tooth morphology which would have been adapted for a more leafy diet. [2]

Suborder Cynodontia

The suborder Cynodonts were the only species to survive the great Permian Extinction and into the Jurassic Period. These animals are the direct and closest ancestors to mammals. The word "cynodont" means dog teeth. [2]


History of the Collection

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Samuel W. Williston in New Mexico, 1911. From Shor (1971).

The history of The Field Museum's non-mammalian synapsid collection begins near the start of the 20th century, with the fieldwork and research of S. W. Williston and P. C. Miller at the University of Chicago, and E. C. Case, a student of Williston who went on to become a professor at the University of Michigan.

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Paul C. Miller (left) in New Mexico, 1911. Ermine C. Case (right) in 1891. From Shor (1971).

They made important collections of synapsid fossils from Early Permian-age rocks in Texas, Oklahoma, and New Mexico, and published a seminal series of works describing the specimens and the rocks in which they were found (e.g.,Case, 1907, 1915, 1919 Williston, 1911 Case et al., 1913). The fossils collected by Williston, Case, and Miller became part of the Walker Museum's collections at the University of Chicago. After Williston's death in 1918, A. S. Romer joined the University of Chicago faculty in 1923, and began his extensive fieldwork in the Permian of North America. Between 1923 and 1934, when Romer left for Harvard University, he and P. C. Miller added to the synapsid holdings of the Walker Museum, including an important series of fossils they collected in 1928-1929 in Middle Permian- to Middle Triassic-age rocks in the Karoo Basin of South Africa. Romer was succeeded in 1935 by his student, E. C. Olson. Olson continued the tradition of Permian research at the University of Chicago, and made large collections of synapsids and other vertebrates from Texas and Oklahoma that were deposited in the Walker Museum. Beginning in 1947, the Walker Museum collections were transferred from the University of Chicago to The Field Museum, and Olson continued to deposit specimens at The Field Museum until he left the University of Chicago in 1969.

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Left: Alfred S. Romer. Right: Everett C. Olson (far right) instructing University of Chicago students, including future Field Museum Curator John Bolt (far left), in 1963. © The Field Museum GEO82841.

Permian research in Chicago was continued by two University of Chicago students, J. A. Hopson, who joined the University of Chicago faculty in 1967 and whose research focuses on non-mammalian synapsids, and J. R. Bolt, a fossil amphibian specialist who became a curator at The Field Museum in 1972. Former Fossil Mammal Curator J. J. Flynn conducted a series of six expeditions to Madagascar between 1996 and 2003, and added Malagasy Triassic dicynodonts and cynodonts to the non-mammalian synapsid collection. Most recently, K. D. Angielczyk, whose research focuses on anomodont synapsids, joined the curatorial faculty of The Field Museum in 2007. He has collected synapsid specimens from the Permian and Triassic of Tanzania and Zambia, and although the specimens will be returned to their respective countries of origin, casts of important material will remain at The Field Museum.

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Left: John J. Flynn with a skull of the cynodont Menadon besairiei from Madagascar. © The Field Museum GEO86240_2c, photographer John Weinstein. Right: Kenneth D. Angielczyk excavates a skull of the anomodont Dicynodontoides nowacki in Tanzania. Photo by R. Smith.
Anchor: #nonmammalian-synapsid-collection-highlights


Characteristics of Reptiles

Reptiles are tetrapods. Limbless reptiles—snakes and legless lizards—are classified as tetrapods because they are descended from four-limbed ancestors. Reptiles lay calcareous or leathery eggs enclosed in shells on land. Even aquatic reptiles return to the land to lay eggs. They usually reproduce sexually with internal fertilization. Some species display ovoviviparity, with the eggs remaining in the mother’s body until they are ready to hatch. In ovoviviparous reptiles, most nutrients are supplied by the egg yolk, while the chorioallantois assists with respiration. Other species are viviparous, with the offspring born alive, with their development supported by a yolk sac-placenta, a chorioallantoic-placenta, or both.

One of the key adaptations that permitted reptiles to live on land was the development of their scaly skin, containing the protein keratin and waxy lipids, which reduced water loss from the skin. A number of keratinous epidermal structures have emerged in the descendants of various reptilian lineages and some have become defining characters for these lineages: scales, claws, nails, horns, feathers, and hair. Their occlusive skin means that reptiles cannot use their skin for respiration, like amphibians, and thus all amniotes breathe with lungs. All reptiles grow throughout their lives and regularly shed their skin, both to accommodate their growth and to rid themselves of ectoparasites. Snakes tend to shed the entire skin at one time, but other reptiles shed their skins in patches.

Reptiles ventilate their lungs using various muscular mechanisms to produce negative pressure (low pressure) within the lungs that allows them to expand and draw in air. In snakes and lizards, the muscles of the spine and ribs are used to expand or contract the rib cage. Since walking or running interferes with this activity, the squamates cannot breathe effectively while running. Some squamates can supplement rib movement with buccal pumping through the nose, with the mouth closed. In crocodilians, the lung chamber is expanded and contracted by moving the liver, which is attached to the pelvis. Turtles have a special problem with breathing, because their rib cage cannot expand. However, they can change the pressure around the lungs by pulling their limbs in and out of the shell, and by moving their internal organs. Some turtles also have a posterior respiratory sac that opens off the hindgut that aids in the diffusion of gases.

Most reptiles are ectotherms, animals whose main source of body heat comes from the environment however, some crocodilians maintain elevated thoracic temperatures and thus appear to be at least regional endotherms. This is in contrast to true endotherms, which use heat produced by metabolism and muscle contraction to regulate body temperature over a very narrow temperature range, and thus are properly referred to as homeotherms. Reptiles have behavioral adaptations to help regulate body temperature, such as basking in sunny places to warm up through the absorption of solar radiation, or finding shady spots or going underground to minimize the absorption of solar radiation, which allows them to cool down and prevent overheating. The advantage of ectothermy is that metabolic energy from food is not required to heat the body therefore, reptiles can survive on about 10 percent of the calories required by a similarly sized endotherm. In cold weather, some reptiles such as the garter snake brumate. Brumation is similar to hibernation in that the animal becomes less active and can go for long periods without eating, but differs from hibernation in that brumating reptiles are not asleep or living off fat reserves. Rather, their metabolism is slowed in response to cold temperatures, and the animal is very sluggish.


Evolution of Reptiles

Reptiles originated approximately 300 million years ago during the Carboniferous period. One of the oldest known amniotes is Casineria, which had both amphibian and reptilian characteristics. One of the earliest undisputed reptile fossils was Hylonomus, a lizardlike animal about 20 cm long. Soon after the first amniotes appeared, they diverged into three groups—synapsids, anapsids, and diapsids—during the Permian period. The Permian period also saw a second major divergence of diapsid reptiles into stem archosaurs (predecessors of thecodonts, crocodilians, dinosaurs, and birds) and lepidosaurs (predecessors of snakes and lizards). These groups remained inconspicuous until the Triassic period, when the archosaurs became the dominant terrestrial group possibly due to the extinction of large-bodied anapsids and synapsids during the Permian-Triassic extinction. About 250 million years ago, archosaurs radiated into the pterosaurs and both saurischian “lizard hip” and ornithischian “bird-hip” dinosaurs (see below).

Although they are sometimes mistakenly called dinosaurs, the pterosaurs were distinct from true dinosaurs ((Figure)). Pterosaurs had a number of adaptations that allowed for flight, including hollow bones (birds also exhibit hollow bones, a case of convergent evolution). Their wings were formed by membranes of skin that attached to the long, fourth finger of each arm and extended along the body to the legs.



Evolutionary history

Archaeothyris and Clepsydrops were the earliest known synapsids. They belonged to a group called pelycosaurs and they lived in Pennsylvanian time of the Carboniferous Period. The pelycosaurs were the first successful group of amniotes, spreading and diversifying until they became the dominant large terrestrial animals, in the latest Carboniferous and Early Permian Periods. They are currently divided into two clades, the Caseasauria and the Eupelycosauria. They were sprawling, bulky, cold-blooded and had small brains. They were the largest land animals of their time, ranging up to 3 m (10 ft) in length. Many, like Dimetrodon, had large sails that may have helped raise their body temperature. A few relict groups lasted into the later Permian.

The therapsids, a more advanced group of synapsids, appeared during the first half of the Permian and went on to become the dominant large terrestrial animals during the latter half. They were by far the most diverse and abundant animals of the Middle and Late Permian, including a diverse range of herbivores and carnivores, ranging from small animals the size of a rat (e.g: Robertia), to large bulky herbivores a tonne or more in weight (e.g: Moschops). After flourishing for many millions of years, these successful animals were all but wiped out by the Permian-Triassic mass extinction about 250 Mya, the largest extinction in Earth's history, which may have been related to the Siberian Traps volcanic event.

Only a few therapsids and no pelycosaurs, survived the Permian extinction and went on to be successful in the new early Triassic landscape they include Lystrosaurus and (later in the early Triassic) Cynognathus. Now, however, they were accompanied by the early archosaurs (formerly known as thecodonts, although this term is not used in modern classifications). Some of these (like Euparkeria) were small and lightly built, while others (like Erythrosuchus) were as big as or bigger than the largest therapsids.

Triassic therapsids included three groups, the specialised, beaked herbivores known as dicynodonts (such as Lystrosaurus and its descendants, the Kannemeyeriidae), some of which reached large size (up to a tonne or more) the increasingly mammal-like carnivorous, herbivorous, and insectivorous cynodonts (including, from the Olenekian age, the eucynodonts, an early representative of which was Cynognathus) and the therocephalians, which only lasted into the early part of the Triassic.

Unlike the dicynodonts, which remained large, the cynodonts became progressively smaller and more mammal-like, as the Triassic progressed. From the most advanced and tiny cynodonts (only the size of a shrew), came the first mammal precursors, during the Carnian age of the Late Triassic, about 220 million years ago (mya).

During the evolutionary succession from early therapsid to cynodont to eucynodont to mammal, the main lower jaw bone, the dentary, replaced the adjacent bones, so that the lower jaw gradually became just one large bone, with several of the smaller jaw bones migrating into the inner ear and allowing sophisticated hearing.

Whether through climate or vegetation change, ecological competition or a combination of factors, most of the remaining large cynodonts (belonging to the Traversodontidae) and dicynodonts (of the family Kannemeyeriidae) had disappeared by the Norian age, even before the Triassic-Jurassic extinction event that killed off all of the large non-dinosaurian archosaurs. Their places were taken by the diapsid archosaurs known as dinosaurs, which dominated the terrestrial ecosystem for the rest of the Mesozoic Era. The remaining Mesozoic synapsids were small, ranging from the size of a shrew, to the badger-like Repenomamus.

During the Jurassic and Cretaceous, the remaining non-mammalian cynodonts were small, such as Tritylodon. No cynodont grew larger than a cat. Most Jurassic and Cretaceous cynodonts were herbivorous, and some were carnivorous. The family Trithelodontidae, first appeared near the end of the Triassic. They were carnivorous and persisted well into the Middle Jurassic. The other, Tritylodontidae, first appeared at the same time as the Trithelodonts, but they were herbivorous. This group became extinct at the end of the Early Cretaceous epoch. Dicynodonts are thought to have become extinct near the end of the Triassic period, but there is evidence that this group survived. New fossil finds have been found in the Cretaceous rocks of Gondwana. This is an example of Lazarus taxon.

Today, the 4,500 species of living synapsids are currently the dominant land animals and include both aquatic (whales) and flying (bats) species, including the largest animal ever known to have existed (the blue whale).

The synapsids have dominated the world twice, once in the Permian and once in the Cenozoic (today).

Synapsids evolution into mammals is believed to be triggered by moving to a nocturnal (night) niche, one of the few niches that the increasing dinosaurs didn't dominate. In order to survive at night, proto-mammals had to increase their metabolic rate to keep their body warm. This meant consuming food (generally thought to be insects) more rapidly. To facilitate rapid digestion, proto-mammals evolved mastication (chewing) and specialized teeth that aided chewing.

Limbs also evolved to move under the body instead of to the side. This allowed the proto-mammals to be able to change direction quicker in order to catch small prey at a faster rate. Rather than out-running predators, instead proto-mammals adapted the strategy of outmaneuvering predators using this same ability, it is believed. [1]


Conclusions

Eocasea martini represents the earliest and most basal known caseid synapsid, extending the fossil record of Caseasauria into the Pennsylvanian. Its discovery fills a significant gap in the fossil record, indicating that this important clade of early synapsids is much more ancient than previously documented in the fossil record. Eocasea provides clear evidence that large caseid herbivores, the largest known terrestrial vertebrates of their time, evolved from small non-herbivorous members of that clade. This pattern is mirrored by several other Permo-Carboniferous clades that include early herbivores (Diadectidae, Edaphosauridae, and Captorhinidae). Among amniotes, it is the synapsids, on the mammalian rather than the reptilian side of higher vertebrate evolution, that were able to acquire herbivory early in their evolutionary history. The available evidence indicates that this innovation in feeding behavior led by the end of the Early Permian to the establishment of a modern type of trophic structure in terrestrial vertebrate ecosystems, one in which numerous herbivores support relatively few top predators [2], [49]. Significantly, it is synapsids that evolved first both herbivores and large terrestrial predators in the evolutionary history of terrestrial vertebrates, a pattern that was maintained until the end of the Paleozoic.


Watch the video: the evolution of mammals from synapsid reptiles (July 2022).


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