Chaoyangodens: a transitional monotreme with big canines

Hou and Meng 2014
described a new Jehol eutriconodont mammal, Chaoyangodens lii, (Fig. 1) from the Yixian formation, Early Cretaceous. “The new species has a tooth formula I5-C1-P1-M3/i4-c1-p1-m4, unique among eutriconodonts in having only one premolar in lower and upper jaws, respectively, and a distinctive diastema between the canine and the premolar. Its simple incisors and reduced premolars show a mosaic combination of primitive and derived features.” In other words, this is a transitional taxon, as most are.

Later, Meng and Hou 2016
described ‘the earliest known mammalian stapes’ from the same specimen. “The stapes of Chaoyangodens is reduced in size compared to those of non-mammalian cynodonts and is within the size range of extant mammals.”

Figure 1. Chaoyangodens lii in situ and restored skull in lateral view. This taxon is a monotreme basal to both the echidna and platypus.

Figure 1. Chaoyangodens lii in situ and restored skull in lateral view. At a screen resolution of 72 dpi, this image is twice life size. This mouse-sized taxon is a monotreme basal to both the echidna and platypus.

Figure 2. Subset of the LRT focusing on monotremes and Chaoyangodens.

Figure 2. Subset of the LRT focusing on monotremes and Chaoyangodens.

Here in the large reptile tree (LRT, 1137 taxa, Fig. 2) Chaoyangodens nests between Kuehneotherium and Akidolestes, basal  to the living monotremes, Ornithorhynchus and Tachyglossus.

The top of the Chaoyangodens skull is buried in the matrix. The shape of the skull in lateral view, or at least parts of it, like the position of the orbit (Fig. 1), can be surmised by phylogenetic bracketing.

Based on the nesting of Chaoyangodens
and relatives, like Brasilitherium and Kuehneotherium (Late Triassic), these taxa are all crown mammals, not stem mammals (contra traditional thinking).

Luo, Kielan-Jaworowska and Cifelli (2002)
also nested eutriconodonts within crown mammals and this was confirmed by many later workers. The LRT nests many traditional triconodonts and eutriconodonts elsewhere, both more primitive and more derived.

Recently
we looked at the echidna sister/ancestor, Cifelliodon, here. It also had fewer and bigger teeth in the jaws, though none of those erupted beyond the gum line.

References
Hou S-L and Meng J 2014. A new eutriconodont mammal from the Early Cretaceous Jehol Biota of Liaoning, China. Chinese Science Bulletin 59, 546–553.
Luo Z-X, Kielan-Jaworowska  z and Cifelli RL 2002. In quest for a phylogeny of Mesozoic mammals. Acta Palaeontologica Polonica. 47 (1): 1–78.
Meng J and Hou S-L 2016. Earliest known mammalian stapes from an early Cretaceous eutriconodontan mammal and implications for evolution of mammalian middle ear. Palaeontologica Polonica 67:181–196.

wiki/Eutriconodonta

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