Sunday, 13 March 2011

Synapsids

Hello class! We are Stephanie, José and Cristian.

Our group's topic was the evolution from reptiles to mammals, so we are going to talk mainly about the synapsids, also known as mammal-like reptiles, proto-mammals or stem-mammals. This reptiles are the ones that are more closely related to mammals (although some other reptiles had some of the characteristics of the mammals before them, such as flesh without scales in the pterosaurus or the ictiosaurus, which were viviparous) and are characterized for having an opening low in the skull roof behind their eyes, leaving a bony arch beneath each, accounting for their name. They lived from the late Carboniferous to the middle of the Cretaceous.

There are two orders inside the group or the synapsids: the pelycosaurs and the therapsids.

The Pelycosaurs


Pelycosaurs had lived from the late Carboniferous to the end of the Permian. They are the predecessors of the Therapsids and all of them were able to control their own temperature. This is the case of the Dimetrodon, who had a tall sail, consisting of elongated vertebral spines, which supposedly had the capacity of regulating its corporal temperature. The same happens with Edaposaurus. Also, some of them used their teeth to make a previous mechanical digestion similar that the one that humans do nowadays, characteristic that was not common in reptiles. Some of them, like the Varanosaurus, had developed something new: they had canines.

Dimetrodon


They dominated the land during 40 millions of years since the beginning of the Permian, but most of them disappeared before late Permian.






The Therapsids




Therapsids are the successors of the pelycosaurs, having evolved from them 275 millions of years ago. The first fossil of a therapsid that have been found is dated on the lower Permian, and they have extinguished in the early Cretaceous. Some of their common characteristics are their differentiated teeth (with distinction between incisors, canines and molars, their complex jaws and the position of their legs, more vertical that the ones of other reptiles.

Biarmosuchia is an assemblage of primitive Permian therapsids. They were moderately sized, lightly built carnivores, intermediate in form between thesphenacodont pelycosaurs and more advanced therapsids. Biarmosuchia is found in Russia an in South Africa and they lived in the Late Permian.
The biarmosuchian skull is very similar to the sphenacodont skull, differing only in the larger temporal fenestra and single large canine teeth in both upper and lower jaws. The vertebrae are also sphenacodontid-like but the shoulder and pelvic girdles and the limbs indicate a much more advanced posture.


Proburnetia

The Anomodontia are a major group of therapsids. They were mostly toothless herbivores. During the Middle Permian they were very diverse, including groups like the Venyukovioidea, the Dromasauria the Dicynodontia, and early very primitive forms like Anomocephalus and Patranomodon.
Of these only the Dicynodonts survived to the Late Permian, and became the most successful and abundant of all Permian herbivores, filling different ecosystems. Only two dicynodont families survived the Permian–Triassic extinction, one of them gave rise to the Kannemeyeridae. These were large, stocky, beaked animals that remained the dominant terrestrial herbivore right up until the Late Triassic, when changing conditions caused them to die out.


Eodicynodon

Dinocephalia are a clade of large early therapsids that flourished during the Middle Permian, but became extinct leaving no descendants.
Apart from the Biarmosuchia, the Dinocephalia are the lest advanced among the therapsids, they retain a number of primitive characteristics shared with their pelycosaur ancestors, although they are also more advanced in possessing therapsid adaptations like the expansion of the ilium and more erect limbs. They include carnivorous, herbivorous, and omnivorous, some semi-aquatic and some fully terrestrial, and were also among the largest animals of the Permian period.
All dinocephalians are distinguished by having interlocking incisors allowing a shearing contact between upper and lower teeth. Most dinocephalians also developed pachyostosis of the bones in the skull, which seems to have been an adaptation for intra-specific behaviour, perhaps for territory or a mate.


Titanophoneus

Gorgonopsia is a suborder of therapsid synapsids. Gorgonopsians evolved in the Middle Permian, from a reptile-like therapsid that also lived in that period. The extinction of dinocephalians led the gorgonopsians to be the dominant predators of the Late Permian. The Gorgonopsia became extinct at the end of the Permian period, being the only theriodont line to be terminated by this mass extinction.



Therocephalians are an extinct suborder of carnivorous eutheriodont therapsids that lived from the middle and late Permian into the Triassic. The therocephalians are named after their large skulls, which, along with the structure of their teeth, suggest that they were successful carnivores. Therocephalia is the group most closely related to the cynodonts, which gave rise to themammals. This relationship takes evidence in a variety of anatomical features, possibly including whiskers and hair.
The fossils of therocephalians are numerous in South Africa, but have also been found in Russia, China, and Antarctica. Early therocephalian fossils discovered in Middle Permian deposits of South Africa support a Gondwanan origin for the group, which seems to have spread quickly throughout the world. Almost every therocephalian extinguished during the great Permian–Triassic extinction.
The therocephalians evolved from an early line of pre-mammalian therapsids called 'theriodonts', and are a sister group to the cynodonts which include mammals and their ancestors. Therocephalians are at least as ancient as a third large branch of therapsids, the gorgonopsids which they resemble in many primitive features. The therocephalians, however, outlasted the gorgonopsians, persisting into the early-Middle Triassic period.




As a curiosity, moving a little bit apart from the synapsids, we can talk about one of the most curious genus of mammal that remains existing in the present time. They are known as the pangolins, and in this class are included eight different species which live in Asia and Africa. The curious part about this placental mammals is that they have scales all over their body. This characteristic is probably a remaining of the reptiles that proves mammals to have evolved from them.


Pangolin

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