The earliest mammals are often portrayed as minor elements of Mesozoic ecosystems, often literally in the shadows of dinosaurs as they scurried and scampered around their feet. During this time though, the earliest mammal precursors were already laying the foundations to become an incredibly successful evolutionary radiation.
Modern marsupials are totally outshined in diversity and geographic extent by placental mammals, which together with their ancestors make up the group Eutheria. But this was very different back in the Cretaceous, with early metatherians being more diverse both in numbers and ecologies than their early eutherian cousins.
Some of the earliest ancestors of marsupial mammals (those which carry their young in a pouch), known as ‘stem metatherians’, are known from the Cretaceous fossil record. Sadly, they are mostly only known from isolated teeth or jaw elements, so we really don’t know much about them and their early evolutionary history.
Now, new specimens of a metatherian have been discovered from the Late Cretaceous of North America, which despite being from a species already known to science, challenge our traditional views of mammals from this time. The specimens are from the Hell Creek Formation, making them around 66 to 69 million years old, and from right towards the end of the reign of the non-avian dinosaurs. Known from a near-complete skull, researchers showed that the specimens belong to a species actually described back in 1889 by renowned American palaeontologist, Othniel Charles Marsh, Didelphodon vorax.
“What I love about Didelphodon vorax is that it crushes the classic mould of Mesozoic mammals,” Dr. Gregory Wilson, Burke Museum Adjunct Curator of Vertebrate Paleontology and University of Washington Associate Professor of Biology, said. “Instead of a shrew-like mammal meekly scurrying into the shadows of dinosaurs, this badger-sized mammal would’ve been a fearsome predator on the Late Cretaceous landscape – even for some dinosaurs.” (source)
By incorporating new information from these specimens, Wilson and colleagues were able to show that marsupials and their closest Cretaceous relatives originated and radiated in North America, before making their journey down to South America.
Even though Didelphodon was quite small-bodied, it was still the largest of any therian mammal from the whole of the Mesozoic (Theria is the major group comprising Metatheria along with Eutheria). Body mass estimates based on the length of the skull show that it would have reached a maximum of 6.2kg.
They were also able to show that for its size, Didelphodon, packed quite a spectacular crunch. The animal had a bite much worse than its bark, and would have had a durophagous, or ‘bone-crushing’ feeding style, making it the oldest of its kind to adopt this strategy. Wilson and colleagues were even able to show that mandible was even strong enough to resist the sorts of movements that, for example, a struggling prey item would inflict upon being bitten. Neat. The maximum estimated bite force at the animal’s canines was 218N, around the same strength as a modern caiman.
Based on the size of the biggest Didelphodon specimen, the largest maximum prey size would have been around 5kg – it could have even chomped on a small dinosaur! This means that even before the end-Cretaceous mass extinction, mammals had diversified to such an extent as to occupy the predatory-scavenger ecological role.
“Our study highlights how, despite decades of palaeontology research, new fossil discoveries and new ways of analysing those fossils can still fundamentally impact how we view something as central to us as the evolution of our own clade, mammals,” Dr. Wilson said. (source)
Wilson, G.P., Ekdale, E.G., Hoganson, J.W., Calede, J.J. and Vander Linden, A., 2016. A large carnivorous mammal from the Late Cretaceous and the North American origin of marsupials. Nature Communications, 7, p.13734. doi:10.1038/ncomms13734 (link)