… and Liaoconodon is not a mammal…

Yesterday we noted that Repenomamus was not a mammal, but nested with the stem- (pre-) mammal tritylodontids, like Pachygenelus. Today Liaoconodon hui (Meng, Wang and Li 2011; IVPP V 16051; early Cretaceous, Aptian, 120 mya), is also not a eutriconodont mammal, but nests between Probainognathus and Pachygnenelus a little deeper into the phylogeny of the Cynodontia in the large reptile tree.

Figure 1. Liaocondon skull traced and reconstructed. In the LRT it most closely resembles that of Probainognathus.

Figure 1. Liaocondon skull traced and reconstructed. In the LRT it most closely resembles that of Probainognathus. Note the enormous size of the temporal fenestrae, the downturned squamosals and postdentary bones, all shared with Probainognathus. 

Liaconodon lacks a coronoid
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. Once again the narrow braincase of Liaconodon, like that of Repenomamus, tells us this is a pre-mammal.

Figure 2. Probainognathus skull(s) in several views along with a pectoral and pelvic girdle.

Figure 2. Probainognathus skull(s) in several views along with a pectoral and pelvic girdle. The lack of a femoral head neck is a trait shared with Liaoconodon. 

Liaconodon lacks large canines
and the lower incisors are quite enlarged. The postorbital bar does not appear to be complete, but the prefrontal, postfrontal and postorbital are still visible and unfused to other bones, as in Probaingnathus.

Figure 3. Liaoconodon in situ.

Figure 3. Liaoconodon in situ. The causals are similar in shape to those of Castrocauda. 

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 par for the clade in 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 Castrocauda. 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.

References
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.

wiki/Liaoconodon

 

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