Relationships of the Liassic Mammals Sinoconodon, Morganucodon oehleri, and Dinnetherium

New skulls of Sinoconodon and Morganucodon oehleri are studied from the Liassic of Yunnan, China, and the Kayenta Formation of Arizona to address the lack of morphological information about the relationships of advanced cynodonts and early mammals.

Abstract

A lack of morphological information makes it difficult to interpret the relationships of advanced cynodonts and early mammals. To address this problem, we studied new skulls of Sinoconodon and Morganucodon oehleri from the Liassic of Yunnan, China, and Dinnetherium from the Kayenta Formation of Arizona. Sinoconodon possesses a mosaic of primitive and derived cranial features. Many of Sinoconodon's cranial characters are plesiomorphies compared with the successively more distant outgroup cynodonts, Pachygenelus, Probainognathus, Tritylodon, and Thrinaxodon. Sinoconodon retains a large septomaxilla with a transverse shelf and intermediate pterygoid crests on the palate; the prootic vein passes through the cavum epiptericum; the incisors were replaced alternately more than once, and the canines at least four times. The postcanine row is restricted to five multicusped longitudinally ovate teeth that possess only vestigial cingula and do not occlude with one another. The erupting postcanines were successively added to the posterior end of the postcanine row. There is no evidence of replacement of the first four postcanines, but the ultimate postcanine may have been replaced. At least two anterior postcanines were lost in the older specimens. Sinoconodon shares several derived characters with other mammals. The most notable are the expansion of the brain vault in the parietal region, complete ossification of the medial wall of the orbit, a dentary condyle, and a concave glenoid fossa in the squamosal. These characters suggest that Sinoconodon and other mammals form a monophyletic group. Sinoconodon lacks a number of diagnostic apomorphies shared by Morganucodon, Dinnetherium, and other mammals. In addition, Sinoconodon developed some autapomorphic characters: an extraordinarily large canine; a massive dentary condyle; a large occipital condyle; and, relative to skull length, postdentary bones more reduced in size than in the other known Liassic mammals. These characters suggest that Sinoconodon is the sistergroup to a taxon that includes all other mammals.