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Recent Research: Meat-eating Mammals

A recent article in the Proceedings of the Royal Society B caught my eye because of its title: “Unique pattern of dietary adaptation in the dentition of Carnivora: its advantage and developmental origin.” After all, who doesn’t like big, pointy teeth and the animals that bear them? My hope was that the article might provide some new insights into why mammals such as cats, dogs, weasels, and mongooses have been more successful evolutionarily speaking than other groups of meat-eating mammals, living and extinct.

Figure 1 from Asahara et al., showing the relative sizes of molars and parts of molars in different groups of meat-eating mammals.

In short, I was not disappointed; the authors (Asahara et al.) combined information about tooth size and structure with genetic and developmental data to argue that the teeth of these mammals - technically known as carnivorans - can evolve more quickly and easily than those of other meat-eating mammals. This could give carnivorans a competitive edge over geological time.

But before I continue... note that the word “carnivoran” is not the same as the word “carnivore.” A carnivoran is a member of the group Carnivora, just like a rodent is a member of the group Rodentia. By contrast, a carnivore is any animal that eats meat. (Check out my November 2015 post for more on this.) Carnivoran is a taxonomic term that depends on an animal’s evolutionary relationships, whereas carnivore is an ecological term related to an animal’s lifestyle. (See also Tony Friscia’s Jan. 2014 blog.) You can have non-carnivorous carnivorans, such as the panda (Ailuropoda melanoleuca) as well as carnivorous non-carnivorans (some bats). Which brings us back to the story...

An eastern quoll (Dasyurus viverrinus) with a meaty meal. Modified from “Quoll Feeding” by GregTheBusker CC BY 2.0.

Asahara and colleagues came to their conclusion by comparing the teeth of carnivorans to those of two other groups of meat-eating mammals (i.e., carnivorous non-carnivorans): creodonts, an extinct group of placental mammals that once lived throughout Africa and the Northern Hemisphere, and dasyuromorphians, a group of marsupial meat-eaters of the Australasian region that includes the modern Tasmanian devil (Sarcophilus harrisii), quolls (Dasyurus spp.), and many smaller species.

But what prompted me to write about the article isn’t what it included, but rather what it didn’t include: sparassodonts.

Some of the many sparassodont fossils housed in the collections of the Museo Argentino de Ciencias Naturales “Bernardino Rivadavia” in Buenos Aires. Photo by D. Croft. Reuse permitted under CC BY-NC-SA 2.0.

Sparassodonts (formerly known as borhyaenoids) were the main group of carnivorous mammals in South America for most of the past 65 million years. They were marsupials, or at least more closely related to them than to placental mammals, and fossils of them have been found throughout the continent. Nearly 60 valid species have been named, and more are described each year. Some sparassodonts were as small as a mink (Neovison vison), and others were as large as a black bear (Ursus americanus). The group’s poster child is Thylacosmilus atrox, a saber-toothed species that inhabited South America until only a few million years ago and is one of the most remarkable examples of convergent evolution between marsupials and placentals.

Partially reconstructed skeletons of early Miocene sparassodonts from Santa Cruz, Argentina including Cladosictis patagonica (top), Prothylacynus patagonicus (middle), and Borhyaena tuberata (bottom). Image modified from Sinclair (1906:pl. 61).

How could such a spectacular radiation of carnivorous mammals be off the radar of most mammalogists and paleontologists? I don’t really know, but that’s what happens to many groups of Southern Hemisphere mammals with no living descendants. Just as most of the Earth’s land is located in the Northern Hemisphere, so are most of the Earth’s paleontologists. And it seems that most people in the Northern Hemisphere have greater awareness of things to the east and west of them as opposed to south. At least most of the time.

With the Olympics in Rio de Janeiro next month, all eyes will be focused on South America. While you are cheering for your team, take a moment to think about the spectacular sparassodonts and some of the other late, great clades that once called that continent home.

-Dr. Darin Croft is an Associate Professor at Case Western Reserve University in Cleveland, Ohio

Further Reading:
Posted: 7/29/2016 8:22:48 AM by croftdarinadmin | with 0 comments
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