Read Caption
The nearly-complete skeleton of Alcidedorbignya inopinata. From de Muizon et al., 2015.

Paleo Profile: Alcide d’Orbigny’s Dawn Beast

Vertebrate paleontology has a sample size problem. Only a fraction of all the creatures that ever lived became preserved in the fossil record, and an even tinier sliver of that array has been discovered, cleaned, and studied. Even the most famous animals, like the fearsome Tyrannosaurus, are known from a (figurative) handful of individuals scattered through swaths of rock spanning a million years or more. Finding enough fossils to even start to pick at the biology of an extinct species is a tall order.

But every now and then paleontologists strike just the right spot. One such locality, near Tiupampa, Bolivia, is simply known as “The Quarry”, but don’t be fooled by the lackluster name. This is one of the best places in the world to find the skeletons of the mammals that thrived just a million years after a wayward asteroid closed the Age of Reptiles for good, and, as paleontologist Christian de Muizon and colleagues report in a massive monograph, this place has supplied an exquisite record of one of the earliest placental mammals to skitter around in the end-Cretaceous aftermath.

de Muizon and Larrry Marshall named the mammal Alcidedorbignya inopinata in 1992. Back then, it was mostly known from teeth and pieces of jaw. But in the years since the initial finds paleontologists have uncovered a nearly-complete skeleton, several partial skulls, and hundreds of other scattered elements. With all this new material, de Muizon and colleagues set about piecing together this dawn beast in greater detail than possible before.

There’s not an exact modern equivalent for what Alcidedorbignya was. It was a tiny member of a totally-extinct group of mammals called pantodonts that thrived in the earliest days of the Paleocene. And unlike its larger relatives, Alcidedorbignya was a nimble little beast that was probably adept at running through the trees as well as scampering around on the ground, able to stand up on its hind legs to have a look around or grab a morsel when necessary. The restoration of the mammal by Justine Jacquot-Haméon makes me think “cat squirrel” isn’t too far off, although the beast wasn’t closely related to either.

What’s still unclear is what led so many Alcidedorbignya to become buried in the same place. Especially strange is that The Quarry has yielded 33 jaws of juvenile animals and 35 jaws from adults, raising the prospect that these were gregarious mammals or that the youngsters hung around with their parents. Were these mammals social, or did the a local flooding event hit at just the wrong time of the year, taking out the next generation as well as their parents? The case remains open. With so many bones of Alcidedorbignya to compare and scrutinize, though, this little mammal offers one of the best chances we have to envision the world just after it was freed from the claws of the “terrible lizards.”

View Images
Alcidedorbignya in its Paleocene environment. Art by Justine Jacquot-Haméon.

Fossil Facts

Name: Alcidedorbignya inopinata

Meaning: The genus name honors French naturalist Alcide d’Orbigny.

Age: About 65 million years old.

Where in the world?: Tiupampa, Bolivia.

What sort of critter?: A pantodont, one of the mammals that proliferated after the demise of the non-avian dinosaurs.

Size: About the size of a large squirrel.

How much of the creature’s body is known?: Multiple fossils including a nearly-complete skeleton, at least three juvenile skulls, several hundred dental specimens, and more.


Previous Paleo Profiles: