The answer to this question is still as unclear as it has been for many years. The Eucynodontia are an assortment of four groups: the Cynognathia, Probainognathia, Tritheledontia, Tritylodontidae. Even this division is not universally accepted. The traditional classification of the eucynodonts was into "traversodonts" and "gomphodonts," based on the structure of the teeth. Most workers now reject this division, since the tooth structure seems to correlate more with diet than with phylogeny. However, the branching sequence is still not well understood. Here, we treat the tritylodonts as part of the Cynognathia, with the probainognaths and tritheledonts on a separate branch. However, almost every permutation of these four taxa has some adherents.
In addition, it is uncertain whether the tritheledonts or tritylodonts led to the Mammaliformes. We treat the "you" in Eucynodontia as stemming from the tritheledont branch. This may now be, by a slight margin, the minority view; but, at this point, we have no real consensus to fall back on. ATW020531.
A large and powerful predator, and dominant animal for many millions of years, Cynognathus was one of the largest cynodonts. The head was proportionally very large (up to 30 to 45 cm in length) and somewhat dog-like in appearance, hence the name Cynognathus ("dog jaw"). The skull is rather narrow, with a short temporal region, which served for the attachment of very powerful muscles for closing the jaws. Practically the whole of the lower jaw on each side was made up of a single bone, the dentary, into which were set the teeth: the cutting incisors, stabbing canines and shearing cheek teeth. Behind the small, peg-like incisors was a diastema, a gap, followed by the large canine. A great bony flange (the coronoid process) at the back of the dentary articulated with the skull, and enabled the jaws to be opened wide. This flange also provided a large area to which extra jaw muscles could attach, giving the jaws tremendous bite-power.
The body also was strongly built, with its hindlimbs placed directly beneath its body. The knee pointed forward and the elbow backward, giving an erect mammalian posture. Clearly this was an efficient predator, perfectly adapted to its environment, which persisted with little or no change for some five or ten million years. Review of the cynodonts gives only a single species, but it is possible that there were several very similar species, possibly in different parts of Pangaea, and perhaps succeeding each other as the Triassic period progressed.
Although a very successful animal, Cynognathus died out during the middle Triassic period, to be replaced by the smaller carnivorous Chiniquodontids and the small to large herbivorous "traversodontids." It is likely that the very arid conditions of the Triassic worked against the mammal-like Cynognathus and in favor of the reptilian archosaurs which had become the unchallenged masters of the land by the later Middle Triassic. MAK000906.
from the Early Triassic.
$ supplementary dentary-squamosal jaw articulation [RS01]; $ Very large dentary; post-dentary bones reduced and rod-like [RS01]; $ dentaries fused at mandibular symphysis [RS01]; lower jaw pulled back into occlusion (?); carnivorous forms with blade-like post-canines; $ scapula with acromion process on leading edge [RS01]; $ phalangeal formula 23333 (mammalian) [RS01].
Links: A new carnivorous cynodont ... with comments on eucynodont phylogeny.
References: Rubidge & Sidor (2001) [RS01]. ATW020221.
Cynognathia: Stem group ~ Cynognathus > Sinocodon.
Range: Early Triassic to Early Cretaceous of Antarctica, South Africa, South America.
Phylogeny: Eucynodontia : Probainognathia + * : Cynognathidae + Tritylodontidae.
Characters: $ Zygomatic arch, posteroventral part expanded laterally at or behind level of quadrate, so that skull has triangular outline in dorsal or ventral view; $ dorsoventrally wide zygomatic arch, extending dorsally above mid-orbit [RS01]; $ jugal with ventrally directed suborbital process for masseter attachment [RS01]; $ deep groove on lateral surface of squamosal (analogous to external auditory meatus for aural connection to postdentary bones) [RS01].
Links: Biology 356
References: Rubidge & Sidor (2001) [RS01]. ATW030324.
Cynognathidae: Paraphyletic basal cynognaths. Cynognathus, Diademodon, Massetognathus, Trirachodon.
Phylogeny: Cynognathia : Tritylodontidae + *.
Large (2m?) carnivores and "gomphodont" herbivores; heavy, thick skull (~40cm); long snout; adductor chamber enlarged longitudinally or posterodorsally; narrow or absent post-orbital bar; jaw articulation primitive (articular & quadrate); dentary with large coronoid process; coronoid process with large masseteric fossa; articular, prearticular, angular, and surangular reduced to bar projecting posteriorly from behind the massive coronoid; large canines; cheek teeth laterally flattened, recurved and serrated; derived, "traversadont" herbivores with cheek teeth having two transversely aligned cusps anterior to basin; postcranial skeleton similar to more massive version of Thrinaxodon.
Links: Yale Peabody Museum: The Mural, Triassic; Evol.Page to Print: Clado Pic.Cynognathus (Mammal-Like Reptile); Mesozoic; Untitled Document; augtwoe.html; Cynognathus Printout - Enchanted Learning Software; PANGEA Italian); Untitled Document.
Notes:  NOT a clade. This is a temporary place-holder for a series of basal cynognathians. The characters given are general cynognathian features. This includes the groups traditionally known as gomphodonts and traversadonts.  Generic and specific distinctions within the Cynognathidae have been based on characters which vary with age tooth number and morphology, skull proportions) and are influenced by postmortem deformation. Karoomys, Cistecynodon, and Nythosaurus browni are based on tiny juveniles of Cynognathus. In their review of the Cynodontia, Hopson & Kitching (1972) recognize only a single species, pending a thorough revision of the family. MAK000906. ATW020222.