Hoplophoneus, the False Sabretooth
Willem is a wildlife artist based in South Africa. He says "My aim is simply to express the beauty and wonder that is in Nature, and to heighten people's appreciation of plants, animals and the wilderness. Not everything I paint is African! Though I've never been there, I'm also fascinated by Asia and I've done paintings of Asian rhinos and birds as well. I may in future do some of European, Australian and American species too. I'm fascinated by wild things from all over the world! I mainly paint in watercolours. . . but actually many media including 'digital' paintings with the computer!"
Prehistory time yet again! This is actually a picture I drew a couple of years ago to go along with a museum exhibit in Florida. It portrays a mammal that must have been a top predator in its time – Hoplophoneus primaevus. This looks very much like a cat, doesn't it? Yet, at the time when it flourished, there were no cats – it was very long before their time. Hoplophoneus lived around the late Eocene to the early Oligocene epochs, about 35 to 29 million years ago. It is known from well-preserved fossils found in the mid-west of the USA.
The Cats that Weren't Cats
Old Hoplophoneus and its kin belonged to a family called the Nimravidae. This family doesn't exist any more, and thus has no common name, but it was big in its own day, and included a great variety of genera and species. They lived long before any humans were around to name or appreciate them. The nimravids evolved from a small, forest-dwelling predator similar to the modern genets, around 50 million years ago, the early Eocene. They were among the very first of the modern carnivorans to become apex predators. Nimravids were astonishingly cat-like. They do share a common ancestor with cats that also lived during the Eocene; but the true cat-ancestors took a long time before they became the cats we're familiar with. The nimravids were actually the first and original cat-types.
The feature by which nimravids can most easily be distinguished from cats, lies in the structure of their middle ears. Cats have an inflated bony compartment or 'house' for the middle ear called an auditory bulla, which is itself divided into two chambers by a thin bony wall or septum. Nimravids either do not have a bony bulla (it's presumed the 'house' for their middle ear was mainly cartilage) or a bulla that's not divided into two by a septum. Nimravids were fairly large predators, especially for their time. The smallest were similar in size to a Bobcat, and the largest were comparable to a lion or tiger. They lived in America, Asia and Europe. The Atlantic Ocean was starting to form, and high sea levels meant that Europe much of the time was an archipelago of islands rather than a continent, but at times there were land connections allowing the nimravids to spread, and consequently there weren't significant differences between the ones living on different continents. They were typically long-bodied and short-limbed. They were muscular and strong, but likely not very fast. Their feet were short and they trod flat on their soles, or with most of the sole contacting the ground. They could retract their claws. Their heads were very cat-like, with short muzzles and a reduced number of teeth. While some had fairly typically cat-like canine teeth, several had lengthened canines, like those of the later sabretooth cats. Those with the largest fangs also had bony flanges on their lower jaws, projecting downwards, to support the canine teeth when their mouths were closed. Those with the largest sabre teeth were some species of Hoplophoneus that were previously classified in the genus Eusmilus, and also a group called the Barbourofelids. These were extremely similar to the largest of the sabretooth cats, and at some point were removed from the nimravids and considered a family of their own, but now seem to be best returned to the nimravids. Barbourofelids had sturdy, muscular, bear-like bodies. Many nimravids, again like modern cats such as the lynxes, and also all of the true sabretooth cats, had shortened tails.
The first clear nimravids are known from about 37 million years ago, the late Eocene, and the group reached its peak during the early Oligocene, about 28 million years ago. They appear to have thrived in forests or lush woodlands, and disappeared with the spread of grasslands and savannahs, and with the establishment on the scene of the true cats. The last nimravids survived in Europe until the late Miocene, about 7 million years ago. They died out without leaving any descendants. Modern cats only started coming into their own during the ensuing Pliocene. They recouped the typical cat-form established by the nimravids, and astonishingly also came to evolve into several sabre-toothed types. This yet again illustrates the phenomenon of convergent evolution, where similar features are evolved independently by different groups. The amazing sabre-toothed sparassodonts of the genus Thylacosmilus are another example. Sabretooth cats would probably be with us still, if it weren't for the early human hunters who either hunted them, or competed with them for the same prey.
Thus, the old cat-types passed away, only for a new cat-type to come to the fore. Sabretooths returned, and then passed away again … but the type seems to be so successful, that it might not be far-fetched to imagine a day when it evolves again...from cats, maybe, if cats survive still, or maybe from a whole new other group. It may even be that people are still around at that time, to properly appreciate them for a change.