Therapsids (Therapsida) are the synapsid animals with more erect limbs than the earlier synapsid "pelycosaurs" like Dimetrodon or Edaphosaurus, that had sprawling gait similar to lizards. Therapsids also have more complex teeth than the earlier synapsids.
Therapsids, reptiles, and mammals
First therapsids and earlier synapsids were somewhat reptile-like but later therapsids were more mammal-like. Therapsids other than mammals and the "pelycosaurs" were once called "mammal-like reptiles", but the name is somewhat misleading because they are not reptiles in the true sense.
All therapsids are closer to mammals than any reptiles and it is long known that mammals are the only surviving descendants of the therapsid lineage called cynodonts. That is the reason why modern taxonomy includes mammals in Therapsida.
Modern mammals only have one bone in their lower-jaw called dentary when other land vertebrates like crocodiles and lizards have several bones (dentary, articular, etc.). In the evolution of therapsids, the jaw-joint bones quadrate and articular became middle ear bones of mammals, "anvil" (incus), and "hammer" (malleus).
The synapsid type holes behind the eye hole for jaw muscles in the skull of therapsids became the zygomatic arch seen in the skulls of modern mammals.
Most therapsids went extinct at the end of the Permian period that wasn't long before first dinosaurs appeared in Mid epoc of the Triassic. This end Permian extinction was the greatest known dying event in the Earth's history and almost all life died at the time. Because of this the Permian-Triassic extinction is also called as the "Great Dying".
During the earliest Triassic the surviving therapsids were still doing quite well. Lystrosaurus, for example, was the most common vertebrate land animal of its time. But soon therapsids lost their power when crocodile related reptiles became larger and more ferocious.
Large therapsids like Placerias became less common during these times. Also in the Late Triassic dinosaurs took over the world from their croc-like relatives and became the rulers of the world. In the following dinosaur times mammals and other surviving therapsids were usually very small.
Last therapsids that were not mammals, the cynodont group is known as tritylodontids disappeared in Early Cretaceous about 100 million years ago.
The rise of dinosaurs and their crocodile-like relatives is likely due to the fact that all the large therapsids died in the Permian-Triassic extinction. And dinosaurs were adaptable animals able to exploit the empty niches left from the extinct therapsids.
Therapsid groups and characteristics
There were many different types of therapsids. Some of them like the dicynodonts were plant-eaters. Others like gorgonopsids were fearsome predators.
Some characteristics of therapsids:
- Skull bone features
- Enlarged temporal fenestra (hole for jaw muscles behind the hole for the eye)
- maxilla bone (upper jaw bone with teeth) enlarged, separating nasal & lacrimal bones
- quadrate (bone near the upper jaw joint) reduced, dentary (outer lower jaw bone)anteriorly expanded
- Other features
- more than 3 sacral vertebrae
- Raranimus (a very primitive therapsid)
- Burnetiamorpha (several horns and protuberances at head)
- Dinocephalia (therapsids with a thickened skull, meat, and plant eaters) (NOTE: They have nothing to do with dinosaurs.)
- Anteosauria (meat-eaters)
- Tapinocephalia (mostly plant-eaters)
- Estemmenosuchidae (horned dinocephalians)
- Tapinocephalidae (Moschops etc.)
- Anomodontia (mostly plant-eaters)
- Dicynodontia (beaked two-tusked plant-eaters)
A synapsid called Tetraceratops is classified as a therapsid by some but others think that it does not have enough in common with the proper therapsids. Tetraceratops is known only from one skull so all of it's characteristics are not known. Anyways, it was closely related to therapsids if it was not one of them.