What Was on the Early Mammal Menu?

Dinosaurs are great. Don’t get me wrong. But just as their bulk literally cast shade on many of their prehistoric neighbors, they continue to overshadow their Mesozoic contemporaries in our imaginations. Our ancestors and cousins are a perfect example. The popular image of a mammal in the time of the dinosaurs is of a shrew-like furball scurrying through the night in search of insects, waiting for the giant reptilian monsters to disappear so that the Age of Mammals can begin.

But the real story of Mesozoic mammals doesn’t fit the dominant dinosaur trope. Yes, they were small, but they were not all itty bitty insectivores. Starting in the Late Triassic, when mammals first evolved, the beasts proliferated into a variety of body types that created new niches. There were beaver-like mammals that swam in ponds, gliding forms similar to flying squirrels, digging critters that tore open insect nests, and a host of others. We can look at them as evolutionary underdogs now, but the fact is that our ancestors and cousins were far more successful in the time of the dinosaurs than we give them credit for.

Part of this emerging narrative is diet. Traditionally, the earliest mammals have been reconstructed as generalist insectivores, nabbing whatever crunchy or squishy arthropods they could. In a Nature paper published earlier this year, however, University of Bristol paleontologist Pamela Gill and colleagues found that some of the earliest mammals had already evolved teeth suited to more specialized menus.

The jaws and skulls of Morganucodon (a, c) and Kuehneotherium (b,d), showing muscle attachments and bite points. From Gill et al. 2014.
The jaws and skulls of Morganucodon (a, c) and Kuehneotherium (b,d), showing muscle attachments and bite points. From Gill et al. 2014.

The beasts at the center of the study were Morganucodon watsoni and Kuehneotherium praecursoris. Both lived around 200 million years ago, and, superficially, fit the shrew stereotype for Mesozoic mammals. By using a combination of X-ray images, biomechanical studies of the jaw, and microscopic wear patterns on the prehistoric teeth, though, Gill and colleagues found that these mammals likely preferred different sorts of food.

Morganucodon was a hard biter. Gill and colleagues found that the little mammal had a bite about 50% stronger than that of Kuehneotherium, although Kuehneotherium made up for that relative weakness with a faster bite. And the jaws of the two mammals were different when it came to stress, too. The jaws of Morganucodon were most resistant to bending stresses – created by, say, struggling prey – at the back of the toothrow, while the jaws of Kuehneotherium were most resilient near the front molars, where the teeth meet to make a natural shear.

So Morganucodon had a stronger, crushing bite, while Kuehneotherium had a faster, shearing bite. And while paleontologists haven’t found tell-tale gut contents for these mammals yet, damage to their teeth gives away the sorts of food they preferred.

Gill and coauthors found that the teeth of Morganucodon and Kuehneotherium sported microscopic wear patterns comparable to what’s seen in modern bats. The scratches on the Morganucodon teeth most closely resembled damage on the teeth of bats that feast on hard-shelled insects, such as beetles, and the damage of the teeth of Kuehneotherium was in accord with that seen among bats that regularly dine on moths.

While Morganucodon crunched beetles and other hard-shelled prey, Kuehneotherium likely had a softer diet of scorpion flies and early members of the moth lineage. The two mammals were not the generalists they were so often cast as. Instead, as Gill and colleagues point out, they mark the early days of dietary specialization for the greater lineage of which we are a part. So don’t write off Mesozoic mammals as meek little things that could only flourish in the absence of the dinosaurs. From the very beginning, mammals have been marked by adaptability and resilience.

Reference:

Gill, P. Purnell, M., Crumpton, N., Brown, K., Gostling, N., Stampanoni, M., Rayfield, E. 2014. Dietary specializations and diversity in feeding ecology of the earliest stem mammals. Nature. 512: 303-307. doi: 10.1038/nature13622

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