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The hyaenodont Tritemnodon envisioned as a civet-like beast. From A History of Land Mammals in the Western Hemisphere by W.B. Scott.

Carnivorous Fossil Beast Lived Like a Civet

The entire Cenozoic history of the world is often laid out something like this: dinosaurs were in charge until 66 million years ago, when a wayward asteroid decided otherwise, and then mammals breathed a sigh of relief as they took over. About 60 million years after that the first humans swung down from the trees, and no one has been able to argue with absolute certainty whether this has been a good or bad thing.

Of course there’s far more to the story. The “Age of Mammals” doesn’t get the respect it deserves these days.

Even though the entirely of the Cenozoic is short compared to other eras – 66 million years can fit comfortably between dinosaur superstars Tyrannosaurus and Stegosaurus with over 20 million years to spare – it’s still a significant chunk of time where different groups of beasts have waxed and waned as evolution and extinction have shuffled them around the planet. It’s the making of the world as we know it, and even if we feel that fossil mammals of the Cenozoic are familiar the fact of the matter is that we barely know them at all. A slinky carnivore named Galecyon embodies this point.

Galecyon was a hyaenodont. These were some of the most abundant and diverse meat-eating mammals of the Eocene – the archaic equivalents to cats, dogs, civets, and hyenas that filled niches similar to those that would later be rediscovered by the carnivoran mammals – and paleontologists have uncovered a wealth of hyeaenodonts from the Eocene strata of the American west. In one five million year strip between 55 and 50 million years ago, called the Wasatchian, paleontologists have found about 20 different species of hyaenodont in eight different genera.

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The humerus (top) and ulna (bottom) of Galecyon. From Zack and Rose, 2015.

Given that mammals have such distinctive dentitions, and teeth fossilize more readily than bone, untangling hyaenodont relationships and lifestyle has primarily involved looking them in the mouth. These efforts have suggested that there were different morphs that occupied various niches – that there were various ways to be a hyaenodont. Getting postcranial proof of this notion has been difficult, though, partly because of a lack of study and partly because the scarcity of good skeletons. Now, drawing from a skeleton of Galecyon found in the Wasatchian age rock of Wyoming, paleontologists Shawn Zack and Kenneth Rose have tried to partially fill this gap in our understanding.

The skeleton, found by paleontologist Jonathan Bloch, includes parts of the jaws, vertebrae, pectoral girdle, arms, hips, and legs. Based on these parts, Zack and Rose write, Galecyon would have been between 11 and 17.5 pounds in weight, or about the size of a large skunk. More importantly, though, the various articulations and muscle attachment points on the bones suggest that Galecyon was a hunter that spent most of its time on the ground but could climb into trees when necessary. The mammal’s joints were primarily suited to the limbs moving in a forward-back motion, like a dog, but parts of the arm and ankle hint at a greater amount of flexibility and grasping power than other animals at the time that spent all their time terrestrially.

So Galecyon was an animal caught between two lifestyles. It wasn’t stuck on the ground, like the carnivorous “wolves with hooves” called mesonychids, but it wasn’t as adept in the trees as some of its relatives like Prolimnocyon. Instead, Zack and Rose write, Galecyon probably lived like a modern civet or weasel – content on the ground but able to run up a tree if needed. And this fills out just a little bit more of the Eocene world. This was an odd time in life’s history, when the great forests that spread across North America boasted life simultaneously familiar and strange. It’s not enough to simply plop each species in this leafy habitat within our mind’s eye. We want to envision how all the pieces fit together. Now we can start to do that for Galecyon, imagining the little carnivore padding along the forest floor before scampering up the nearest tree to glare back at us.