The Galesaurids, represented by a number of genera, the best known of which is Thrinaxodon, were similiar to but even more mammal-like than their ancestors the Permian Procynosuchids. These were small, solidly built carnivores, capable of running quite fast to judge by the erect posture of its strong hind legs. An animal like Thrinaxodon, in its cranial anatomy and dental morphology is an ideal ancestor for the earliest mammals. Certain of its cheeck teeth hear a striking similarity to those of the latest Triassic mammals of the family Eozostrodontidae.
Another mammalian trend is seen in the lower jaw of Thrinaxodon. The teeth on either side were set into a single bone, the dentary, which had become larger at the expense of the smaller bones at the back of the jaw. The effect of this trend among cynodonts, toward a single lower jaw bone, was to make the jaws stronger. The zygomatic arch of bone at the rear of the skull, behind the eyes, was also was heavier; the masseteric fossa and coronoid process were essentially mammalian in size and shape, indicating the presence of well-differentiated masseter and temporal muscles. As with the procynosuchids, there is a secondary or bony palate, which completely separated the breathing passage from the mouth. This development allowed the animal to breathe at the same time as retaining food in its mouth for longer periods, chewing it up into small pieces for quicker digestion.
The body was long and - for the first time among vertebrates - divided into a distinct chest (thoracic) and lower back (lumbar) region. The division was marked by the extent of the ribs; only the thoracic vertebrae bore ribs, and these formed a distinct cage, which housed the vital organs such as the heart and lungs. As in living mammals, there was probably a sheet of muscular tissue, the diaphragm, that closed off the rib cage in Thrinaxodon. As the animal breathed, the movement of the diaphragm would have filled and emptied its lungs efficiently an essential development in the evolution of body temperature control.
Many other structural changes had occurred showed a number of progressive changes over their Permian procynosuchid ancestors. For example, one of the foot bones had developed a heel which, with the help of strong tendons, would have acted as a lever to lift the foot clear of the ground with each step. The toes were all of equal length, allowing the body weight to be evenly distributed over them.
Ecological niche/Guild: small terrestrial and semi-aquatic carnivores, insectivores, and piscivores
Modern equivalents: badger, weasel,
Horizon: Late Permian to early Triassic period: Beaufort Series (Upper Daptocephalus Zone to Cynognathus Zone, most abundant in Lystrosaurus Zone) of South Africa; Fremouw Formation of Antarctica.
Distribution: it is likely these animals had a worldwide ( Pangea) distribution
preferred food: small dicynodonts, small diapsids, procolophonids, fish, invertebrates
length: about 50 cm long
weight: less than a few kg
Metabolism: partially or completely endothermic
Potential Predators: Permian: small Gorgonopsia, Therocephalians, Triassic: Proterosuchid Thecodonts, Cynognathids
Replaced by: Chiniquodontidae
Taxonomic status - valid (albeit paraphyletic) Family
Cynosaurus suppostus (Owen 1876).Synonyms: Cynosuchus suppostus Owen 1876; Cynosuchus whaitsi Haughton 1918; Cynosaurus suppostus, Schmidt 1927; Cynosuchoides whaissi Broom 1931; Alygalesuchus peggyae Broom 1942; Baurocynodon gracilis Brink 1951.
Remarks: The last two synonyms are based on small juvenile specimens.
Nanictosaurus kitchingi Broom 1936Synonyms: Nanictosaurus robustus Broom 1940b; Nanictosaurus rubidgei Brink & Kitching 1953
Synonyms: Glochinodon van Hoepen 1916; Glochinodontoides Haughton 1924b.
Galesaurus planiceps Owen 1859.Synonyms: Glochinodon detinens van Hoepen 1916; Glochinodon to ides gracilis Ilaughton 1924b; Notictosaurus gracilis Broom and Robinson 1948a; Notictosaurus trigonocephalus Brink and Kitching 1951b.
Genus Thrinaxodon Seeley 1894a.
Thrinaxodon liorhinus Seeley 1894a.Synonyms: ? Nythosaurus larvatus Owen 1876, Ictidopsis elegans Broom 1912a; Ictidopsis formosa van Lloepen 1916; Thrinaxodon putterilli Broom 1932; Notictosaurus luckhoffl Broom 1936; Micrictodon marionac Broom 1937.
Remarks: The best known Galesaurid, and along with Cynognathus the best known cynodont. In modern cladistic arrangements it is usually placed in a seperate, mono-specific, family, but it can certainly be retained in the Galesauridae. Nythosaurus larvatus Owen is based on the natural mould of a skull bearing impressions of the postcanine teeth. It is probably synonymous with either Thrinaxodon liorhinus or Platycraniellus elegans.
Platycraniellus elegans (van Hoepen 1916).Synonyms: Platycranion elegans van Hoepen 1916; Platy craniellus elegans van Hoepen 1917.
Remarks: This species is known with certainty from the type only which comes from Liarrismith, Orange Free State. A second specimen referred to this species by Brink (1954) is a Galesaurus.
Tribolodon frerensis Seeley 1894a.
Remarks: This is the youngest species referable with certainty to the Galesauridae. Though frequently classified as a cynognathid, it is a typical galesaurid in its known features.
|some Links and References|
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
Barry Cox, R.J.G. Savage, Brian Gardiner; Dougal Dixon, 1988 Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals
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 Africana, 14. 17-85
Jacques van Heerden and Bruce Rubidge, 1990, "The Affinities of the early Cynodonts Reptile, Nanictosaurus," Paleontologica Africana, 27. 41-44
Thrinaxodontidae - Palaeos Vertebrate Notes - Toby White's excellent technical summary, heaps of links. Incorporates material from these pages.
CRANIAL ANATOMY OF THE CYNODONT REPTILE THRINAXODON LIORHINUS - Richard Estes - digital version of an article originally published in: Bulletin, Museum of Comparative Zoology, Harvard University. Pretty technical
Palaeos Page (incorporates some of this material, plus a lot of additional material)