Updated September 22, 2018
with a re-nesting of Liaoconodon with Repenomamus.
Yesterday we noted that Repenomamus was not a mammal, but nested with the stem- (pre-) mammal trithelodontids, like Pachygenelus. Likewise, today Liaoconodon hui (Meng, Wang and Li 2011; IVPP V 16051; early Cretaceous, Aptian, 120 mya; Figs. 1, 3), nests between Gobiconodon and Repenomamus in the large reptile tree (subset in Fig. 2).
Liaconodon has a dentary-squamosal jaw joint
and the scapula has a ventral glenoid, which are traditional mammal traits. It also has a mammal-like ilium without a posterior process, like a mammal… or a pre-mammal tritylodontid, like Oligokyphus and Kayentatherium. The narrow braincase of Liaconodon, like that of Repenomamus, tells us this is a pre-mammal. More importantly, phylogenetic analysis nests Liaoconodon outside the last common ancestor of all living mammals: Megazostrodon (Fig. 2). Thus, the dentary-squamosal joint appeared by convergence in Liaoconodon and mammals.
Liaconodon lacks large canines
and the lower incisors are enlarged. The postorbital bar is not complete.
Jin Meng of the AMNH
made a video posted to YouTube describing how ground-breaking it was to find post dentary bones in Liaconodon, which they considered a mammal. Those post-dentary bones are indeed clear and articulated, but are typical for pre-mammal cynodonts.
The manus and pes are well preserved
which is something we rarely see around this node. The caudals are nearly identical to those of Castorocauda and Repenomamus. The femora were likewise rather short.
It was a good week for finding errors.
As before, we all boggled this one. To those who are toying with the challenge I presented earlier about finding badly nested taxa in the LRT, sorry, you missed this one.
Meng J, Wang Y-Q and Li C-K 2011. Transitional mammalian middle ear from a new Cretaceous Jehol eutriconodont. Nature 472 (7342): 181–185.