Sunday 12 May 2019

The Big African Not-a-Lion

Spot the real lion
When it comes to fossil animals, there can be little doubt that the ones most popular with the general public are the large ones. There's a reason that dinosaurs seem to hold the most enduring fascination, and that even popular science books about the whole range of prehistoric life are almost bound to have them as part of their title. (And, while some dinosaurs were small, most of the ones we know about weren't, perhaps partly because it's easier to preserve large solid bones than small fragile ones).

It's not really any different with prehistoric mammals, with mammoths, mastodons, and sabretooth cats being the ones that most people would instantly recognise and be familiar with. Even dire wolves, which weren't really all that big, are surely better known than, say, extinct foxes or weasels of the same age. Which brings up a second point: even if an animal isn't as large as an elephant or Diplodocus, being unusually large for a carnivore will still help its public profile enormously.

It's both huge and dangerous. What's not to love?

Nonetheless, perhaps just due to the vagaries of what was discovered when and how science communication has progressed, there are some large fossil carnivores that remain relatively obscure compared with the great sabretooths. Because, while sabretooths were large and undeniably deadly, they weren't the largest land-dwelling mammalian carnivores ever.

That honour probably goes to an extinct bear of some kind. But there are some others that came fairly close and, among them are those remarkably well-known animals, with a name that just trips off the tongue: the hyainailourines.

Hyainailouros, after which these animals are collectively named, was first discovered in 1863, but it wasn't until the 1930s that the group was recognised as something distinct. They first appeared, probably in Africa, a little over 50 million years ago, and were initially unremarkable, perhaps around the size of a cocker spaniel. From there, they spread across the Northern Hemisphere where they competed with a number of other carnivorous mammals of the day, something they didn't face back in their home continent.

That lack of competition proved beneficial, and while the smaller, Northern Hemisphere, species had all died out by around 35 million years ago, the African ones survived. And those are the ones that, around 10 million years later, began to increase in size, becoming the top African predators of their day. ('Africa', for these purposes, includes Arabia, which at the time was attached to it, and not to Asia). In fact, these new, larger, hyainailourines were successful enough that some of them later crossed back into Asia, eventually outliving their African cousins and finally going extinct around 14 million years ago.

But what exactly were they? That's harder to answer, because they have no close relatives alive today. One thing we can tell is that they weren't carnivorans - relatives of animals such as cats, dogs, and bears. There are a number of ways to tell this, but one of the key ones is that they possessed no less than three sets of flesh-slicing carnassial teeth, where true carnivorans only possess the one. Exactly what we think they were related to has changed over the years as better information has come in, but we currently consider them to be a type of hyaenodont.

Hyaenodonts, despite their name, are not related to hyenas (which are, of course, true carnivorans). They are an ancient group of animals, one that had their heyday mostly before the carnivorans we now know got started. In fact, it was probably competition from animals such as early cats and so on that led to their eventual demise.

There were several different kinds of hyaenodont, with Hyaenodon itself being the best known - probably because one of its species lived in North America. However, by the Miocene, those better-known forms were all long extinct, with only the hyainailourines, and a related group that also happened to live in Africa, still being around. Both groups were, judging from their teeth, highly carnivorous, probably feeding exclusively on meat.

Trying to piece together how this particular group of animals evolved and how the different species of hyainailourine related to one another hasn't proved easy. It's likely fair to say that there has been less palaeontological work conducted in Africa than in North America or Europe and many of the fossil hyainailourines we do have are rather fragmentary. However, while it's far from a complete skeleton itself, a new find has managed to shed some further light on the group, and (being a big scary carnivore) did at least manage to garner some press coverage... even if the word 'hyainailourine' was unsurprisingly absent from much of it.

The fossil in question has been named Simbakubwa kutoaafrika, and was recovered from 25 million-year-old deposits in western Kenya, a site previously known for, among other things, early fossil apes. The name translates as "big African lion", although, of course, it's not really a lion, or even particularly lion-like, beyond being a large mammalian carnivore.

Just how big is it? While this is apparently the most complete fossil hyainailourine known from sub-Saharan Africa, it's still only a set of jaws and a few other bones that might not even be from the exact same individual. Which makes it very hard to know just how large the animal might have been. Perhaps, for instance, it had particularly large jaws.

So, instead of measuring the skeleton, all we can do is make some educated guesses, using mathematical formulae that compare the size of certain teeth to the overall size of living carnivores. It turns out that even the most conservative formula suggests a weight for Simbakubwa of around 280 kg (620 lbs), which would be unusually large for a tiger, never mind a lion. Other calculations, however, suggest that it would have been much larger, weighing somewhere around 1,400 kg (1.5 tons)... about the size of a black rhino.

If that's true, it suggests a drive towards gigantic size surprisingly early in hyainilourine evolution. There had been some earlier suggestions that the increase in size of members of the group was due to increasing competition from smaller, but possibly more efficient carnivores, in the form of animals more closely related to those alive today. However, there would have been relatively few such animals in Africa this early, which may indicate that other theories more closely connected with the evolution of the local herbivorous prey, could be closer to the truth.

The idea here is that the African landscape was, at the time, becoming more open and less heavily forested, in turn driven by changes in the climate as the world began to warm. The more open habitat promoted the evolution of larger herbivores, much as elephants and rhinos live in relatively open habitat today... and if there was larger prey, at least carnivores might have evolved to become larger in order to catch it.

The teeth of later hyainailourines show some adaptation to bone-crushing, but Simbakubwa doesn't seem to have gone quite this far, with teeth more suited to slicing flesh from a carcass. If it was really as big as some of the estimates show (and, as I say, there is some uncertainty in this) then the prey in question would likely have included elephant relatives such as deinotheres, and the hippo-like anthracotheres... both of which would have been far too large as adults for any other known predator to tackle.

While Simbakubwa itself was followed by many later hyainailourines, the group as a whole may simply have become over-specialised. Any change in the diversity of local large herbivores could have made things difficult for them over time, when there was little else they could feed on. Add to that competition from 'modern' carnivorans, with their larger brains, and their ultimate fate may well have been assured.

But they must have been pretty impressive while they lasted.

[Photo by Ćukasz Ciesielski, from Wikimedia Commons.]

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