Tetraceratops insignis Matthew 1908
Originally considered a pelycosaur of uncertain relationships, this strange little reptile has only recently been shown (in an important paper by Michel Lauren and Robert Reisz) to be the connecting link between the Pelycosaurs and the Therapsids.
Although collected and originally described in 1908, its evolutionary significance was not recognized because little was known about its osteology. The only known specimen was embeeded in a surrounding rock matrix was very dimcult to remove.
At least three pairs of horns are present on this animal, one on each premaxilla, prefrontal, and angular bones of the skull. The generic name - Tetraceratops means "four-horned face" - was chosen because only four horn-bearing processes were exposed when W.D. Matthew erected this taxon. These give the animal a superficial similarity to the primitive therapsid Burnetia mirabilis, but this does not necessarily indicate close relationships, because the horns are not located on the same bones. It is interesting to coinsder that similiar hornlets are fond on a number of different (and only distantly related) theropod dinosaurs - Dilophosaurus, Cryolophosaurus, Allosaurus, Carnotaurus, Tyrannosaurus, etc). It is not unlikely that in all these instances the horns may have been brightly coloured and served as instruments of intra-specific display.
The above cladogram of non-mammalian synapsids shows the Intermediate position of Tetraceratops. This cladogram is from Lauren and Reisz's paper. The position of the Gorgonopsia and Dicynodontia (upper right) is controversial - usually the Gorgonopsia are placed close to (i.e. considered the sister-group of) the Therocephalia. In any case Tetraceratops is shown to be more closely related to the therapsids than any pelycosaur, and that the sphenacodontines (Dimetrodon, Sphenacodon, Ctenospondylus, etc) are more closely related to therapsids than the haptodines.
Tetraceratops and the therapsids form a monophyletic group because they share the following seven derived features (synapomorphies) which appear in the primitive condition in sphenacodontines (see diagram at left):
The presence of a number of diagnostic therapsid features cannot be tested in Tetraceratops because of the fragmentary nature of the specimen. However, all other therapsids would seem to form a monophyletic group excluding Tetraceratops because they share the following derived characters that appear in the primitive state in Tetraceratops and pelycosaurs: the septomaxilla is large and has a facial process that excludes the maxilla from the narial margin; the maxilla contacts the prefrontal; the maxilla contacts the nasal and excludes the lacrimal from the naris; a distinct incisiform region is present; all the incisors are equal in size; the basipterygoid articulation is absent.
This combination of primitive and advanced features shows that Tetraceratops is intermediate between pelycosaurs and all previously known therapsids, an can serve as pretty good ancestral type (or "sister taxon" in cladistic jargon) to the later Therapsida.
It is quite likely that proto-therapsids such as Tetraceratops (and many others taht never became fossilised) evolved in an upland environment where they were not easily fossilised, away from the swamps and deltas frequented by the Pelycosaurs (the only upland Pelycosaur lineages were the Caeseids and, most certainly, their Eothyrid ancestors). This early Permian upland evolution proceeded paraelle to the Permocarboniferous lowland fauna, and probably originally derived from reptiles that adapted to dry habitats during the long Kazimovian arid period. Irt was from this upland evolution, via intermediate forms like Tetraceratops, that the great Therapsid evolutionary radiation emerged.
|some Links and References|
Biology 356 - Major Features of Vertebrate Evolution - Permo-Carboniferous Synapsids - by Dr. Robert Reisz, University of Toronto
The osteology and relationships of Tetraceratops insignis, the oldest known therapsid - M. Laurin, and R.R. Reisz - abstract of paper
Michel Lauren & Robert R. Reisz, "Tetraceratops is the oldest known therapsid", Nature Vol 345-17 May 1990, pp.249-50
Robert R. Reisz, Pelycosauria, Encylopedia of Paleoherpetology, Part 17A, 1986, Gustav Fischer Verlag, Stuttgart and New York, pp.84-5
Palaeos Page (incorporates some of this material, plus a lot of additional material)