Hoplophoneus
Mounted specimen on display at the Zurich Natural History Museum.
When: Late-Eocene to Early Oligocene (~38 to 30 million years ago)
Where: North America
What: Hoplophoneus is a saber-toothed carnivore which is well known from the several deposits in the Western states and provinces of the USA and Canada. This animal is not a felid, but a member of the group Nimravidae. One day I will probably be a true saber-toothed cat on here, but today is not that day! Hoplophoneus is a lot closer to true saber-toothed tigers than Thylacosmilus is, today's fossil is a placental mammal, and has been closely linked to the order Carnivora, which is the order that includes cats. Exactly how closely linked has been a matter of debate for some time, pretty much since the first nimravid fossils were found in the late 1800s. At first nimravids, including Hoplophoneus, were thought to be felids. They were never thought to be sister taxa to the much later occurring saber-toothed forms however, so they have always represented another event of convergent acquisition of extremely enlarged canines.
Hoplophoneus and kin were kept as felids for several decades, but their placement was then called into question when relationships within Carnivora were re-examined using cladistic methodology. Since the 1970s they have been, at various times, true felids, non felids in feliformia, in caniformia, or outside of Carnivora all together! The most recent studies, the first to look at all parts of the skeleton, have placed Hoplophoneus and the rest of the nimravids outside of Carnivora, as the sister taxon to the rest of the group. They are neither caniforms (dog like carnivorans) or feliforms (cat like carnivorans).
This placement means that the leopard sized Hoplophoneus is an even more stunning example of convergent evolution than was first though. Not only are its enlarged canines convergent with some felids, but so are many other features of its anatomy. However, there are many differences as well, and these features are what place nimravids outside of Carnivora. Most of the differences are really specific anatomical features, most of which found in the ear region and the postcranial skeleton, but one easy one to see is the size of the brain. If you look at the reconstruction above the body looks very cat-like, but the head is quite different. According to one friend 'Its not a cat, its a weasel-cat! sabertooth... thing.'