This post is updated. See August 11, 2016 for new data on Repenomamus.
As in traditional reptiles
I’m seeking basal relationships of basal taxa in mammals. For those you have to go to the Middle and Early Cretaceous, not the Paleocene.
Asioryctes (mid-Cretaceous, 85 mya, Kielen-Jaworowska 1975, 1984, Fig. 1) was a basal eutherian mammal, not far from Eomaia (Ji et al. 2002), the basalmost eutherian. In the large reptile tree (LRT, subset Fig. 5) Asioryctes nested at the base of Tupaia + Glires (including Multituberculates). It retained most of the original arcade of teeth, but with smaller canines. The overbite is a trait of this clade.
The second Cretaceous mammal
is the predator, Repenomamus (Li et al. 2000, Hu et al. 2005; Early Cretaceous, Figs. 3, 4). One specimen of R. robustus was the size of the opossum, Didelphis, and included digested baby dinosaur bones, R. giganticus was half again as large, one of the largest known mammals of the dinosaur era. Despite its predatory nature, note the reduction in the canine teeth.
is traditionally considered a non-therian gobiconodontid mammal. Here it nests as a stem mammal along with Pachygenelus.
You have to go with
phylogeny first, and then identify your clade traits, rather than the other way around. And be prepared for change, which has always been the nature of Science.
Chronology and Phylogeny
The nesting of Repenomamus indicates that certain mammal clades had already diverged prior to its appearance.
Yixian Formation, Early Cretaceous, 125 mya yields Eomaia and Repenomamus from this list. Other Early Cretaceous mammals include:
- Astroconodon – 2mm triconodont molars, likely a predator
- Jugulator – 5mm triconodont molars, likely a predator
- Pinheirodontidae – clade of multituberculates
- Spinolestes – 24 cm long triconodont teeth, skeleton and fur along with xenarthrous vertebrae.
- Vincelestes – a 4 cm skull with large canines described as a stem-therian, I’ll test it next to see how it nests.
The presence of
Early Cretaceous multituberculates implies the presence of more primitive coeval stem rodents and rabbits and perhaps stem tenrecs.
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. 2000. A new family of primitive mammal from the Mesozoic of western Liaoning, China [in Chinese]. Chin. Sci. Bull. 45, 2545–2549).
Ji et al 2002. The earliest known eutherian mammal, Nature 416:816-822.
Kielan-Jaworowska Z 1975. Preliminary description of two new eutherian genera from the Late Cretaceous of Mongolia. Palaeontologia Polonica 33:5-15.
Kielan-Jaworowska, Z 1984. Evolution of the therian mammals in the Late Cretaceous of Asia. Part VII. Synopsis. Palaeontologia Polonica 4: 173-183. online pdf