Repenomamus is not a mammal, but close…

Everyone boggled this one
including yours truly. Repenomamus (Li et al. 2001, Hu et al. 2005; Yixian Formation, Early Cretaceous; IVPP V 12549) never really found any ‘soul mates’ within the Mammalia within the large reptile tree (LRT). For one thing, mammals tend to have larger brains reflected in larger crania. Carnivorous mammals tend to not shrink the canines and they tend to lose the septomaxilla, but Repenomamus retains it. More below.

Figure 1. Three specimens of Repenomamus to scale with bones colorized.

Figure 1. Three specimens of Repenomamus to scale with bones colorized.

Now, in the LRT
Repenomamus nests with Pachgenelus and the Tritylodontids, Oligokyphus and Kayentatherium and this is a good match. Repenomamus represents the carnivorous half of this clade. O and K were herbivores. We can only assume (prior to analysis) that other gobiconodontids like Gobiconodon, are going to also nest with them. Pachygenelus had a reduced canine. These stem-mammals have always been tricky to work with.

Figure 1. Basal mammals and therapsid outgroups nest around Repenomamus in the proximal clade outside the Mammalia.

Figure 1. Basal mammals and therapsid outgroups nest around Repenomamus in the proximal clade outside the Mammalia.

 

Originally, Hu et al.
recognized the primitive and derived traits in Repenomamus. That’s why they named it Repen “reptile” + mam “mammal” as they note, “referring to the the animal’s both reptilian and mammalian features, and its large size.” 

He et al noted several derived (mammalian) characteristics:

  1. well-developed dentary/squamosal articulation
  2. reduced number of teeth
  3. differentiation of premolars and molars
  4. presence of dorsal process of the premaxilla not in contact with nasal
  5. closed medial wall of the orbit
  6. fingerlike promontorium on the petrosal
  7. structure of the cheek teeth is simple
  8. differentiation of cusps on upper molars is indistinct

Hu et al also noted
Repenomamus retained some primitive (non-mammalian) features, but did not list them in the abstract, but did list them in the text:

  1. large size, relative to other Early Cretaceous mammals
  2. upper dental formula 3.1.2.4
  3. upper canine subequal to third upper incisor
  4. several dental characters
  5. large septomaxilla
  6. sagittal crest short and low
  7. lambdoid (posterior) crests well-developed
  8. sloping occipital plate in dorsal view
  9. post dentary bar retained
  10. dentary lacks angular process
  11. most of these traits are common to tritylodontids, not mammals of the time.

I note 

  1. the scapula has a short spine along the anterior rim.
  2. The arc-like coracoid is retained
  3. the cranium is very narrow
  4. the lateral temporal arch is robust, as in tritylodontids
Figure 3. Repenomamus giganticus in situ. Here the overlooked coracoids are identified.

Figure 3. Repenomamus giganticus in situ. Here the overlooked coracoids are identified.

Tritylodontids
were proximal outgroups to the mammals in the Triassic. And they appear to have developed in parallel to mammals in several traits as they continued into the Jurassic and Early Cretaceous. This parallel development appears to be the source of the present confusion.

Figure 5. Repenomamus giganticus reconstructed.

Figure 5. Repenomamus giganticus reconstructed.

And once again,
a phylogenetic analysis that was not made for mammals and has too many taxa per character was able to lump and split taxa more accurately than larger studies — all without ever seeing the specimen first hand. Sometimes it just takes a long list of candidate taxa to provide adequate opportunities to make that happen.

I’ll update the pages in ReptileEvolution.com to reflect those changes shortly.

References
Hu Y, Meng J, Wang Y-Q and Li C-K 2005. Large Mesozoic mammals fed on young dinosaurs. Nature 433:149-152.
Li J-L, Wang Y, Wang Y-Q and Li C-K 2001. A new family of primitive mammal from the Mesozoic of western Liaoning, China. Chinese Science Bulletin 46(9):782-785.

wiki/Repenomamus

 

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