Photo credit: Rpoort, Hamilton, ON

Have you ever wondered where opossums like me came from? Who are my ancestors? How long ago did I become, well, me?

No one knows much about my oldest relatives. Scientists think that the handsome critter, pictured below was my earliest ancestor. If they are correct, a version of me lived 225 million years ago. Evidence suggests I am a common ancestor for all modern mammals, or at least a very close relative to their common ancestor.

I think the artist’s reconstruction is interesting; lots of whiskers on the snout, five fingers like modern opossums, and the tail looks familiar! But back then I was tiny, no bigger than a house mouse and weighed perhaps 20 to 30 grams. These shy and nimble ancestors of mine probably slept in hiding all day only coming out at night to eat insects. Scientists call him (or her!) Adelobasileus Cromptoni. 

Here I Am Long Ago

I’d like to think my true story begins here. This drawing shows another ancestor of mine from between 80 and 100 million years ago.

Yes! Meaning opossums were here when T-Rex walked the earth (note we're still here and T-Rex isn't!).

Back then we were still small but getting bigger. This one weighed around 40 - 80 grams, about the size of a modern chipmunk who’s on the skinny side. Like today, back then we were good climbers, omnivorous, came out at night, moved quietly, and we were very good at hiding. We had to be. Modern scientists named us Herpetotherium.

Not Quite So Long Ago (But Still Pretty Far Back)

What happened next?

This illustration shows another ancestor of mine from about 40 million years ago. By this time dinosaurs were gone, but now we had other worries. We were a bit bigger at around 100 grams, but other mammals were also big - really big, and they were hungry.

With a big thanks for this wonderful restorative artwork to: Jorge González – La Plata, Argentina

Being eaten or stepped on was still a worry for us and as a result our stealth skills continued improving. Mimoperadectes is what they decided to call us. Scientists seem to enjoy giving us complicated fancy names. We had to wait a long time before we got a name we like.

Here I am Today!

With a big thanks for this photo to: Chelsea Carroll, Benson, Vermont

As you can see, I've changed a bit in the last 100 million years or so, but not much (except maybe I'm cuter than my ancestors). I’m also a lot larger now (up to 5 kg) and I’ve honed my survival skills even further.

Science is just now discovering we may have another sense we use, but more on that in another article. Today, my biggest concern is humans and how to live in their world. My scientific name is Didelphis virginiana and dates back to 1792 when I was named by Johann Friedrich Kerr.

But it was thanks to the Native Americans I finally got a name I like: opossum, meaning "white animal.”

Sources: National Library of Medicine, National Center for Biotechnology Information, Bethesda, Maryland. Florida Museum of Natural History. The Opossum: Its Amazing Story William J. Krause University of Missouri (2005). Opossums: An Adaptive Radiation of New World Marsupials by Robert S. Voss and Sharon A. Jansa (2021). Prehistoric-Wildlife, Adelobasileus, Benjamin Guiterez (2012). Lucas, S. G., & Luo, Z. (1993). Adelobasileusfrom the Upper Triassic of West Texas: the oldest mammal. Journal of Vertebrate Paleontology 13-3, 309–334.  https://dinopedia.fandom.com/wiki/Adelobasileus. Adelobasileus cromptoni, Lucas & Hunt, (1990).