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The Trithelodontids, also and better known (especuially in earlier literature) as the Ictidosaurs, were tiny latest Triassic and early Jurassic cynodonts that neatly bridge the gap between advanced theriodonts and the primitive mammals.

Although previously separated from other theriodonts as a distinct infraorder, the Ictidosauria, study of the type and only specimen of the genus the Trithelodon has determined its close affinities with the typical Ictidosaurs Pachygenelus and "Diarthrognathus". For this reason, Hopson and Kitching suggested using the family name Trithelodontidae Broom 1912 for all of the forms called "ictidosaurs", and included all these groups under the Cynodontia.

The Trithelodonts (Ictidosaurs) are thus very advanced, probably insectivorous, cynodonts of small size in which some incisors and in some species the upper and probably postcanines have a transversely-oriented cutting edge, in others the uppers have an oblique and the lowers a longitudinal cutting edge. These are features connected, as in the tritylodonts, with the more efficient chewing of food. As in the Chiniquodontidae the secondary palate is long (so the animal could eat and breathe at the same time, a mammalian feature) and the postorbital bar (the bar of bone behind the eyes) is absent, another feature found also in Tritylodonts and primitive mammals.

But what makes these animals unique is the new mammalian joint between the squamosal and dentary had come into functional being. Thus, the ictidosaur Pachygenelius (formerly known as Diarthrognathus) possesses both the reptilian quadrate-articular jaw joint and a newly developed mammalian squamosal-dentary jaw joint. Because of this Simpson (1959) suggested that it might better be classified as a mammal, but Hopson and Crompton retain it in the Therapsida. Although certain primitive features in the skull suggest a derivation from scaloposaurid bauriamorphs, indicating the posisbility that the mammalian condition may have evolved twoice (once from advanced therocephalians via the Ictidosaurs and once from cyndonts via Tritylodonts) But both the structure of the dentary and the braincase structure have independently confirm a cynodont ancestry, and the polyphyly of mammals is no longer considered a valid hypothesis.

Pachygenelius stands truly at the dividing line between reptile and mammal in so far as this important diagnostic feature of jaw articulation is concerned. This is the only reason why the ictidosaurs are classified as reptiles rather than mammals. In the mammals, the quadrate and articular bones have migrated from the articular region of the jaw to the middle ear where they have been transformed into two of the bones concerned with the transmission of vibrations from the eardrum to the inner ear. Because, in the ictidosaurs, the tta formation of the quadrate and articular bones had not taken place, these animals can be placed arbitrarily within the reptiles.

Family Trithelodontidae Broom 1912b.

List of genera

Trithelodon riconoi Broom 1912b

Pachygenelius monus Watson 1913

Pachygenelius broomi Crompton 1958

Chaliminia musteloides Bonaparte

Comments: The only known Triassic Trithelodont, and part of a highly endemic fauna, which makes it difficult to correlate. However although the Los Colorados Formation is usually considered latest Triassic (Late Norian /Rhaetian), Jose Bonparte says somewhere (I can't find the exact passage...) that the fauna may not be isochronous. So it is possible that the upper part of the Los Colorados may be Hettangian (earliest Jurassic). That would fit in Chaliminia with the other Trithelodonts.

Theioherpeton

Comments: no further info at the moment

some Links and References

Trithelodontidae - Palaeos

JURASSIC CYNODONTS; Tritylodontidae and Trithelodontidae, an internet directory - by Trevor Dykes - the most complete listing (along with the present page) of genera and species

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 and 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

James A. Hopson and James W. Kitching, 1972, "A Revised Classification of the Cynodonts (Reptilia; Therapsida), Paleontologica Africa, 14. 17-85

Trithelodont jaw - a fine photo of a partial jaw (Nova Scotia Museum). Earliest Jurassic (early Hettangian) age.

The Parrsboro Fossil Site - more information, including a reconstruction of the live animal (scroll down)

Trithelodont jaw fragment with tooth - also from Nova Scotia, identified as Pachygenelus ?monus

Eucynodontia - cladogram and genus list - Mikko K. Haaramo

'La importancia turistica de Ischigualasto', preparado por Dr William Sill. a fossil inventory in Spanish - at the bottom it lists Chaliminia musteloides (under "cynodont"

Palaeos Page (incorporates some of this material, plus a lot of additional material)

Kingdoms of Life > Animalia > Vertebrata > Tetrapoda > Sauropsida > Therapsida

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