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Therioherpeton

Therioherpeton cargnini (Bonaparte and Barberena 1975; Late Triassic) was originally considered close to the ancestry of mammals, at it is, but here it nests with Brasiolodon and Sinoconodon. The skull and ribs document a low, wide morphology. The femur lacks a proximal head and neck.

 
Brasilodon

Brasilodon quadrangularis (Bonaparte et al. 2003; PV 0611 T, PV 0716 T; Early Norian, Late Triassic, 208 mya) nests with Snoconodon in the large reptile tree. The canine was larger and the jugal extended further to the back of the skull. The anterior dentary tipped town. The small cheek teeth were inset from the jaw margin, quadrangular in palatal view, a symmetrical distrubution of cusps and little variation among them.

 
Haramiyava

Haramiyavia clemmenseni (Jenkins et al. 1997, Luo et al. 2015; Late Norian, Late Triassic, 208 mya) nests with Brasilodon in the large reptile tree. It is considered a basal member of the Haramiyida, but it may be the only member. Only a few parts are known, but enough to restore most of the skull.

 

Sinoconodon

Sinoconodon rigneyi (Patterson & Olson, 1961. Early Jurassic) almost had a mammal jaw joint, and continued to replace teeth throughout its lifetime.

Derived from a sister to the pre-mammal, Therioherpeton. Sinoconodon was already derived in losing the premolars to produced a diastema (space) posterior to the canines. This pattern of tooth reduction was repeated several times within the Mammalia by convergence.

The Sinoconodon clade is currently basal to the Megazostrodon clade and all subsequent mammals, including long-snouted Akidolestes and Ornithorhynchus among the monotremes. The genus probably originated in the Triassic and became widespread by the Early Jurassic.

No doubt Sinoconodon laid eggs. Live birth occurred later. probably in smallet taxa with tiny embryos crawling to the teet, as in marsupials and basal placentals.

 

At left the skull shown is life size if your screen resolution is 72 dpi.

early mammal skulls
 
Amphitherium and Docodon

Amphitherium prevosti (von Meyer 1832; Middle Jurassic, 170 mya). Amphitherium was traditional considered the earliest representative of the pantotheres, mammals that were basal to modern eutherians (placentals) and metatherians (marsupials). Here it nests with basalmost mammals in the clade Monotremata. The large number of molars is a common trait in this clade. Such mammals do not have ear flaps ore vibrissae (facial whiskers). The lower legs and tail were probably the only parts not covered in dense hair.

Docodon victor (Marsh 1881; Late Jurassic, 2 cm skull length) traditionally considered a docodont, Docodon nests here between Sinoconodon and higher montremes, like Akidolestes and Ornithorhynchus. Note the seven molars.

 
Triconodon and Trioracodon

Trioracodon bisulcus (Simpson 1928; Late Jurassic; ~5 cm skull length; YPM VP 010340) has three molars each with three equal cusps, a straight jaw tip and convex ventral dentary lacking an angular process (lower retro process).

Triconodon mordax (Owen 1859; Early Cretaceous, 145 mya; BMNH 47764) represents one of the three-cusped molar taxa. Triconodon has five molars and only three premolars.

 

Bonaparte JF and Barberena MC 1975. A possible mammalian ancestor from the Middle Triassic of Brazil (Therapsida–Cynodontia). Journal of Paleontology 49:931–936.

Bonaparte JF and Barberena MC 2001. On two advanced carnivorous cynodonts from the Late Triassic of Southern Brazil. Bulletin Museum of Comparative Zoology 156(1):59–80.

Bonaparte JF, Martinelli AG and Schultz CL 2005. New information on Brasilodon and Brasilitherium (Cynodontia, Probainognathia) from the Late Triassic of Souther Brazil. Revista Brasieira de Paleontologia 8(1):25-46.

Bonaparte JF, Martinelli AG, Schultz CL and Rubert R 2003. The sister group of mammals: small cynodonts from the Late Triassic of Southern Brazil. Revista Brasileira de Paleontologia 5:5-27.

Butler PM and Clemens WA 2003. Dental morphology of the Jurassic Holotherian mammal Amphitherium, with a discussion of the evolution of mammalian post-canine dental formulae. Palaeontology 44: 1-20. http://onlinelibrary.wiley.com/doi/10.1111/1475-4983.00166/

Jenkins FA, Jr, Gatesy SM, Shubin NH and Amaral WW 1997. Haramiyids and Triassic mammalian evolution. Nature 385(6618):715–718.

Luo Z-X, Gatesy SM, Jenkins FA, Jr, Amaralc WW and Shubin NH 2015. Mandibular and dental characteristics of Late Triassic mammaliaform Haramiyavia and their ramifications for basal mammal evolution. PNAS 112 (51) E7101–E7109.

Meyer H von 1832. Palaeologica, zur Geschichte der Erde und ihrer Geschöpfe. Schmerber, Frankfurt a/M, xi, 560 pp.

Marsh OC 1881. Notice of new Jurassic mammals: American Journal of Science, ser. 3, 21: p. 511-513.

Marsh OC 1887. American Jurassic mammals. The American Journal of Science, series 3 33(196):327-348

Patterson B and Olson EC 1961. A triconodont mammal from the Triassic of Yunnan. In Vandebroek G (ed.), International Colloquium on the Evolution of Lower and Non Specialized Mammals. Koninklijke Vlaamse Academir voor Wetenschappen, Letteren en Schone Kunsten can Belgie 129-191.

Simpson GG 1928. A Catalogue of the Mesozoic Mammalia in the Geological Department of the British Museum 1-215.

Simpson, GG 1929. American Mesozoic Mammalia. Memoirs of the Peabody Museum of Natural History. iii (i): 1-235.

wiki/Sinoconodon
wiki/Docodon
wiki/Triconodon
wiki/Therioherpeton

 
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