The Opossum’s Tale

In the continental United States, there’s only one native marsupial – the Virginia opossum. The title’s a bit of a misnomer. Opossums aren’t just restricted to the “Mother of Presidents.” They raid garbage cans and scurry across streets up and down the entirety of the east coast and the west, really only becoming scarce in deserts and badlands. This resourcefulness truly makes them the American opossum, and they’re an echo of a prehistoric time when their kin used to be quite common around here.

North America used to be a hotspot for archaic marsupials and their pouched relatives. Over 66 million years ago, when the likes of Tyrannosaurus and Triceratops were still stomping around, ancestral marsupials and their closest relatives – called metatherians – were both diverse and abundant. Our own ancestors and close cousins – the eutherians – were marginal mammals by comparison. The mass extinction that wiped out the non-avian dinosaurs is what decimated the metatherians and changed the course of mammalian history.

Thomas Williamson, Steve Brusatte, and Gregory Wilson track the upset in a new ZooKeys paper. The first part is tracking the rise of the protomarsupials. While the first metatherians evolved late in the Jurassic, by around 160 million years ago, they didn’t really get going until the later part of the Cretaceous.

From teeth, bits of jaw, isolated postranial bones, and extremely rare partial skeletons, paleontologists have counted 68 metatherian species from Europe, Asia, and North America during the entirety of the Cretaceous. Of all these, 29 species were present around 75 million years ago – the height of Cretaceous metatherian diversity. And even though their global diversity dipped to 25 recognized species during the last part of the Cretaceous, they were still common mammals that came in a variety of shapes and sizes.

Figuring out how these metatherians lived is hindered by the fact that almost everything we know about them comes from teeth and scraps of jaw. Still, Williamson and coauthors write, Cretaceous metatherians ranged from shrew-sized insectivores to carnivores and omnivores the size of a small Virginia opossum, some of them with specialized crushing teeth to bust up insect carapaces or tough seeds. Along with a totally extinct lineage of mammals called multituberculates, the metatherians were doing quite well during the Late Cretaceous.

But the protomarsupial heyday couldn’t last forever. The fossils found in the 68-66 million year old rocks of western North America document the metatherian downfall.

In the famous Hell Creek Formation, Williamson and colleagues note, 12 of the 31 mammal species present were metatherians, and, depending on the locality, they make up 35-60% of the mammal species found. (I was even lucky to stumble across one a few years ago – part of the upper jaw and two teeth from a metatherian called Pediomys.) In the rock just above the Hell Creek, though, there’s just a single metatherian – Thylacodon montanensis. Whether this was a true survivor or a species that migrated to the area isn’t clear, but, despite the fact that the mammal was abundant during life’s recovery from the great ecological shock of extinction, North America’s metatherians were not able to reclaim their former diversity or numbers. The scales had tipped in favor of our own eutherian relatives.

The metatherians didn’t fade away into nothing, of course. During the latest part of the Cretaceous, some metatherians started to invade South America. This emigration continued after the mass extinction, with almost twice as many metatherian species in South America as in the whole of North America. This left metatherians – including early marsupials – with an entire isolated continent to undergo a new radiation upon and start spreading elsewhere around the planet.

And even after the great extinction, North America remained an important place for metatherian evolution. It may have even been the place where the very first opossums started to snuffle through the lush forests that once covered the American west. One of the earliest opossums, named Mimoperadectes houdei by Inés Horovitz and colleagues in 2009, was found in the 55 million year old rock of Wyoming. Along with other finds, Mimoperadectes hints that opossums, at least, originated in North America before spreading south. The American opossum continues the legacy of Mimoperadectes today, both an echo from ancient time and a testament to the metatherian ability to survive.

References:

Horovitz, I., Martin, T., Bloch, J., Ladevèze, S., Kurz, C., Sánchez-Villagra, M. 2009. Cranial anatomy of the earliest marsupials and the origin of opossums. PLOS One. 4 (12): e8278. doi: 10.1371/journal.pone.0008278

Williamson, T., Brusatte, S., Wilson, G. 2014. The origin and early evolution of metatherian mammals: the Cretaceous record. ZooKeys. 465: 1-76. doi: 10.3897/zookeys.465.8178