The therapsids, also called mammal-like reptiles and paramammals, were advanced synapsid reptiles, and the direct ancestors of the mammals. The traditional Linnean classification groups the therapsids into several suborders - usually Phthinosuchia/Biarmosuchia, Dinocephalia, Anomodontia, and Theriodontia, this last often subdivided. A suggested cladistic tree is as follows:
<==Therapsida |--Tetraceratops `--+--Biarmosuchia |--Biarmosuchidae |?--"Phthinosuchidae" `-+-Eotitanosuchidae | `--Eutherapsida |--Dinocephalia | |--Anteosauria | | |--Stenocybidae | | `--Brithopodidae / Anteosauridae | |?--Estemmenosuchidae | `--Tapinocephalia | |--Styracocephalidae | `--+--Titanosuchidae | `--Tapinocephalidae `--Neotherapsida |-+-Anomodontia | `-+-Galechiridae | |--Otsheriidae | `-+-Venjukoviidae | `-+-Galeopsidae | `-+-Dicynodontia | `-+-Eodicynodontidae | `-+-+-Endothiodonidae | | `-+-Dicynodontoidea | | `---Pristerodontoidea | |-+-Emydopoidea | | `--Robertoidea | `--Kingoriidae `--Theriodontia |--Gorgonopsia `--Eutheriodontia |--Therocephalia `--Cynodontia `-+-Procyonsuchidae `-+-Galesauridae `--Eucynodontia |--Cynognathidae `--Probainognathia |--Tritylodontoidea | |--Diademodontidae | |--Trirachodontidae | |--Traversodontidae | `--Tritylodontidae `--Chiniquodontoidea |-+-Chiniquodontidae | `--Probainognathidae |--Tritheledontidae `--+?-Abelobasilus `--+?-Sinoconodon |?-Gobiconodontidae `-- Mammalia
The above cladogram is taken from Jack Conrad's Vertebrate Paleontology site, along with modifications by Gillian King, The dicynodonts, A Study in palaeobiology, (Chapman & Hall, 199)), and Christian Kammerer (basal Anomodontia). Note that this phylogeny is not necessarily the only, or even the best, possible one.
A suggested phylogeny of the Therapsid groups is as follows: Evolving from mid-Permian ancestors like Tetraceratops (a small reptile completely unrelated to the well-known dinosaur Triceratops), these reptiles evolved progressively more mammalian features, first in the (to us because of lack of knowledge) disorderly branching of poorly known basal forms like the Biarmosuchia, Phthinosuchia, and Eotitanosuchia, from which developed a threefold branching. The earliest to develop were the somewhat more advanced but still ungainly carnivores, omnivores and herbivores of the Dinocephalian lineages, then following them the two very distinct lines of adaptive evolution, the diverse and succesful dicynodonts, and the the very mammal-like theriodonts. The latter group were to become the ancestors of mammals, through the various intermediate stages suggested here.
These three principal subgroups are usually ranked as separate suborders - Dinocephalia, Anomodontia, and Theriodontia, although their diversity is such that I for one would not be upset as a bit of taxonomic inflation that would raise them to ordinal rank. As with so much in nature, it seems to all be about food, at least getting enough food, and processing it better. Quite simply, the three principal radiations within the Therapsida appear to be based on three solutions to the problem of bringing the food-processing surfaces of the upper and lower jaws into closer contact in order to achieve a more efficient system than existed in the sphenacodontids for subdividing both plant and animal foodstuffs.
Unlike the pelycosaurs, which are known mainly from the early Permian of North America (with a smattering of European forms), but this is due to accidents of preservation in the fossil record, and possibly also restruction to the equatorial belt during the bitter Permo-Carboniferous ice ages), our knowledge regarding the therapsids is more extensive. Members of all the basic Therapsid lineages apart from the Theriodonta occur in the earliest therapsid fauna of Russia, the early Middle Permian Ocher fauna (Wordian Epoch). It use dto be thought that even more primitive fragmentary remains from the slightly earlier San Angelo formation of Texas also were of this category (Olson, 1962), but it is now known that these were falsely identified - they were actually Caseid peycosaurs, not therapsids at all.
The fossil remains of these animals are also represented in the middle and upper Permian deposits of Africa, especially the spectacular Karroo formation of South Africa, and from other formations in Russia, and the uppermost Permian sediments of China, and the Triassic of those three countries (especially South Africa) plus also South America, with rarer specimens from Europe, North America, Australia, and Antarctica. Only the Tritylodontids survived into the Jurassic, and are known from Europe and China.
|some Links and References|
Synapsids - short overview of mammal-like reptiles, with emphasis on Russian forms. Includes an evolution diagram
FAQs - Mammal-like reptiles - Answers to school children's questions about the world and lives of a range of ancient reptiles that gave rise to the mammals.
Barren Karoo's fossil treasure house - very readable on-line article
The real prehistoric stars - the Therapsida of the Karoo basin and elsewhere
Therapsida - by Jack Conrad - includes an up to date Cladogram, essay, and technical diagnosis of the group
Therapsida Vertebrate notes - dense bu excellent technical definition
Therapsida - cladogram and short bibliography - Mikko K. Haaramo
Synapsida - Mammals and their kin - T. Mike Casey - cladogram covers both Pelycosaurs and Therapsids
Carroll, R. L. Vertebrate paleontology and evolution. -W. H. Freeman and company, New York, 1988
Edwin H. Colbert, Evolution of the Vertebrates, 2nd edition, 1969, John Wiley & Sons
James A. Hopson, "The Origin and Adaptive Radiation of Mammal-Like Reptiles and Non-Therian Mammals, Annals of the New York Academy of Sciences 167:199-216, 1969
James A. Hopson and Herbert R. Barghusen, "An Analysis of Therapsid Relationships", in The Ecology and Biology of Mammal-Like Reptiles ed. by Nocholas Hotton III, Paul D. MacLean, Jan J. Roth and E. Carol Roth, Smithsonian Institute Press, Washington and London, 1986, pp.83-106
Palaeos Page (incorporates some of this material, plus a lot of additional material)