Simocyon, a Unique Extinct Carnivore

THERE ARE certain niches that seem to be filled by one species or another at any time period. In my recent post on mammalian evolution, for instance, I mentioned Castorocauda, a Jurassic mammaliform that seems to have fit into the niche now occupied by beavers or otters. But occasionally we run across an animal that seems to be adapted for a unique role in its ecosystem. One of these animals is Simocyon. This is a puma-sized caniform that lived about 14 million years ago, and died out by four million years ago. Simocyon has a variety of unusual adaptations.

The first unusual feature noted was a false thumb. This character is similar to the thumb of the panda and of the red panda. Since they shared this unusual feature and an odd diet of bamboo it used to be thought that red pandas and pandas are closely related, but it was later discovered that pandas are bears, and red pandas are more closely related to raccoons. The red panda (genus Ailurus) is placed in the family Ailuridae, and Simocyon likewise falls into this family. The false thumbs of the giant panda (1) and Simocyon (2) are shown in the figure to the right.¹ The false thumb is produced by an enlarged radial sesamoid bone and serves as a relatively immobile surface that the other digits can be brought into opposition with, allowing a pinching or grasping motion.

In the panda and red panda, the false thumb is a convergently evolved trait and is used to grasp bamboo while eating. Both the panda and red panda have teeth that are adapted for this diet, but Simocyon has more generalized caniform dentition as seen below.²

So why the false thumb? It appears that this evolved originally for tree climbing and was preadapted for bamboo eating. Red pandas are primarily arboreal, and Simocyon has skeletal adaptations for climbing. The common ancestor of the two was probably a primarily arboreal carnivore that evolved the false thumb to aid in gripping branches. Salesa and coworkers for the first time have analyzed the postcranial skeleton of Simocyon in detail to determine what its lifestyle was like.²

The odd thing about Simocyon is that it retains many tree-climbing adaptations while being much larger than thought practical for an arboreal carnivore. Indeed, other aspects of its anatomy are more suited to a ground-hunting, cursorial generalized carnivore. The authors suggest that this was Simocyon‘s usual mode of behavior, but that it retained its tree-climbing ability because of its unfriendly neighbors. Simocyon lived alongside amphicyonids (colloquially known as bear-dogs) and saber-toothed cats. While Simocyon at the size of a puma was not small (about 54 kg, or 120 lbs), these other carnivores could weigh twice that or more. Simocyon may have scavenged their kills, and its likely these carnivores would have come into contact with Simocyon not infrequently. Large carnivores tend to be aggressive to smaller carnivores, especially those big enough to provide some competition for kills. Lions and hyenas have a mutually antagonistic relationship, wolves kill coyotes, and Great Horned Owls kill smaller owls. So it would not be surprising if most of these confrontations ended up in Simocyon fleeing up a tree. As the authors say, “In this scenario, the strong muscles of the shoulder, forearm and lumbar region of this ailurid would produce the necessary force to propel its body in the vertical faster enough to escape from these encounters.” And I’m sure Simocyon would have been thinking “FASTER!”, so far as it was able to think.

These conclusions were drawn from features of the forelimbs and lumbar spine. The forelimbs possess paws with false thumbs, an increased ability to turn the wrist and hand inward (useful for hugging tree trunks), increased brachiation, and strong muscles in the shoulders. The lumbar spine has unusual adaptations similar to those allowing bounding locomotion in mustelids (think of the way ferrets move), but this type of locomotion is inefficient in an animal this size and this adaptation is probably related to stabilizing the trunk for vertical climbing. At this time the features of the pelvis and hindlimbs are unknown.

It’s interesting to think that the saber-toothed cats may have contributed to the formation of an ecological niche that is absent in modern ecosystems, as well as seemingly occupying one themselves.

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1. Salesa, M. J.; Antón, M.; Peigné, S.; Morales, J. (2006). Evidence of a false thumb in a fossil carnivore clarifies the evolution of pandas. Proceedings of the National Academy of Sciences, 103, (2), 379-382. DOI: 10.1073/pnas.0504899102
2. Salesa, M.J., Antón, M., Peigné, S., Morales, J. (2008). Functional anatomy and biomechanics of the postcranial skeleton of Simocyon batalleri (Viret, 1929) (Carnivora, Ailuridae) from the Late Miocene of Spain. Zoological Journal of the Linnean Society, 152(3), 593-621. DOI: 10.1111/j.1096-3642.2007.00370.x