In Defence Of Early Mammals – Part One

Guest Author – Kim Chandler, MSc
2020 Palaeobiology Graduate

When discussing Palaeontology the topics of most interest rarely include mammals earlier than the charismatic megafauna like the woolly mammoth and the saber-tooths. Mammals first appeared in the Late Triassic, evolving from a group of animals called synapsids (such as Dimetrodon). The mammals that were first on the scene in the Mesozoic, are usually dismissed as tiny rodent-like things of little interest, especially when compared to the enigmatic dinosaurs they shared their habitat with. However, these guys are interesting in their own way, either due to the transitionary period of evolution at which they sat, or their ecology.

A surprisingly large range of morphologies and diversities existed even at the early stages of their evolution. They survived through the extinction event that wiped out non-avian dinosaurs at the end-Cretaceous and went on to diversify into the wealth of species we know today, along with a huge number of extinct forms. It is a shame that early mammals are rarely represented. At best, they are depicted as the mid-afternoon snack of some carnivorous dinosaur. In my opinion, they can be as interesting as those larger beasts immortalised in pop culture. Here is my attempt to raise the profile of some early mammals which,  nevertheless, are our ancestors.

1. Repenomamus robustus

Li et al. 2001 – Lower Cretaceous, China

Repenomamus makes the list due to two surprising characteristics – its size and its diet. This Early Cretaceous mammal was comparable in size to a large badger, and at a robust 12-14kg, far outweighed the average mammal of the time. Discovered in Liaoning, China, this fossil occupied the same beds that contain feathered dinosaurs. The morphology of Repenomamus differs from the common thinking that Mesozoic mammals were small and quick, by having a wide sprawling gait and short, stocky legs. In fact, it was bigger than some of the dinosaur species that were around at the same time (such as the dromaeosaurid, Graciliraptor). Almost as if to prove that this mammal could fend for itself against dinosaurs, a Repenomamus specimen has been shown with vertebrate bones preserved in its stomach, including the bones of a juvenile ceratopsian dinosaur, Psittacosaurus. So not only was it larger than some dinosaurs, but it predated upon them too.

Postcranial skeleton of R. robustus with Psittacosaurus bones shown in the preserved stomach contents. Taken from Hu et al. (2005)

Repenomamus is one of a group of triconodonts (a group of mammals identified by having cheek teeth with triangle shaped crowns), Gobiconodonts. Gobiconodonts are named for the specimen Gobiconodon found in the Early Cretaceous rock of the Gobi Desert. This group tend to be large by Mesozoic standards.

2. Necrolestes patagonensis

Ameghino, 1891 – Cretaceous – Early Miocene, South America

Necrolestes was described in 1891 by the famed Argentinian palaeontologist, Florentino Ameghino, after the discovery of a relatively complete fossil skeleton in Miocene beds of Patagonia over 100 years ago. Since then it has been a cause of much debate to which mammalian group it belongs. It is an enigmatic little animal, with a distinctive upturned snout that has led to many reconstructions having a fleshy proboscis like that of the extant star-nosed mole. However, its teeth are rather primitive, causing difficulty in assigning its place within the lineage. Its mole-like affinities caused it to initially be considered a part of the wastebasket group ‘insectivora’ and the only known extinct placental from South America. However, studies in 2012 suggest it is part of a group called the meridiolestids, a few branches away from the Therians (marsupials and placentals). This would mean that Necrolestes was a very late survivor, persevering well past the Cretaceous extinctions where most of this group vanished and 40 million years after its closest relative went extinct.

Reconstruction of the skull of Necrolestes patagoniensis, showing the distinctive upturned snout which inspires mole-like reconstructions with a proboscis. From Rougier et al. (2012)

3. Ptilodus

Cope 1881 – Palaeocene, North America

My next choice for the list is the genus Ptilodus, not because it is particularly interesting, in itself, but because it is a representative of a group of now extinct mammals that has the longest duration on earth of any mammal group – the multituberculates. Multituberculates are named for their peculiar cheek teeth, which are long and narrow with multiple rows of cusps, called tubercles. The last premolar in the jaw is a large chisel-like blade, thought to be used for cracking open nuts or seeds or slicing into insects. Multituberculates first originated in the Jurassic but then survived the end-Cretaceous extinction and flourished into many forms, mostly squirrel-like and arboreal but some living on the ground, the size of a modern beaver like the Palaeocene Taeniolabis, and some with prehensile tails. It is thought that the flourishing of multituberculates after the Cretaceous was linked with the extinction of small dinosaurs and marsupials that would have occupied similar seed-eating/tree-climbing niches. The multituberculates finally went extinct after 165 million years at the end-Eocene, with unclear reasons why, although some think this may be due to competition with rodents which was rapidly diversifying.

Ptilodus skeleton and foot, showing the ability to turn its foot backwards, allowing it to climb down trees head-first. From Jenkins and Krause, 1983

Ptilodus inhabited jungles and had special ankle joints which allowed it to descend trees head first. Along with its grasping tail, it was well adapted to arboreal life. The genus was first described by the eminent palaeontologist Edward Drinker Cope, famous for his feud with fellow palaeontologist, Othniel Charles Marsh, resulting in the infamous “Bone Wars”.

These are just some mammals I have included. I think they are all animals people today would happily peer at in a zoo or desire to keep as pets. To further your knowledge of these charismatic creatures, check out Part 2 where I offer up some more interesting early mammals.

References:

  1. Hu, Y., Meng, J., Wang, Y. et al. (2005). Large Mesozoic mammals fed on young dinosaurs. Nature 433, 149–152
  2. Ameghino F. (1891) Nuevos restos de mamíferos fósiles descubiertos por Carlos Ameghino en el Eoceno inferior de la Patagonia austral. Especies nuevas, adiciones y correciones [New remains of fossil mammals discovered by Carlos Ameghino in the lower Eocene of southern Patagonia. New species, additions and corrections] Rev Arg Hist Nat 1, 289–328.
  3. Rougier GW, Wible JR, Beck RM, Apesteguía S. (2005). The Miocene mammal Necrolestes demonstrates the survival of a Mesozoic nontherian lineage into the late Cenozoic of South America. Proc Natl Acad Sci U S A. 109(49):20053–20058
  4. Jenkins, F. A. and Krause, D. W. (1983). Adaptations for Climbing in North American Multituberculates (Mammalia). Science, 220(4598), 712–715.

Further Reading:

Donald Prothero (2017). The Princeton Field Guide to Prehistoric Mammals. Princeton University Press

Edited by Rhys Charles

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