Fins to Feet


Evolution of the Felidae, part 1
July 29, 2010, 12:11 am
Filed under: Uncategorized

NOTE: Some geological periods that might need some defining: Pleistocene = 2.5 million to 12,000 years ago, Pliocene = 5.3 to 2.5 million years ago, Miocene = 23 to 5.3 million years ago.

Evolution of the Cat Family

An ancient forest. Enter Proailurus, the Dawn Cat. Lithe and long-backed, she flits ghost-like through the leaf-shrouded tree canopy. Boughs sway with each muffled footfall. Her silhouette blots out branch-framed dapples of forest light like a cold wind snuffing out candle flames.
She is on the prowl.
Suddenly, frightened squawks and thumping wingbeats are heard as a group of birds scatter noisily from a high-flung knot of leaves and bark. Something savage has transpired in the treetops. Proailurus has made a kill.
She descends to a lower, sturdier branch, an inert mass of feathers and flesh securely fastened in the snare of her jaws.

Proailurus is a cat*. And recognisably so. She stalked the treed valleys and flatlands of Eurasia 25 million years before the first Leopards ever did. But despite this yawning temporal chasm, they share a number of important characteristics: binocular vision, a pair of highly specialized blade-like “carnassial” teeth designed for shearing meat, retractable claws, a short face (compared to Dogs and Bears) and an almost exclusively carnivorous diet (known as “hypercarnivory”). These features are common to all cats.

The Leopard is, however, substantially larger than the caracal-sized Proailurus and there are differences in the morphology of the teeth, limbs and vertebral column between the two species (and, more broadly, between the Ur-cats and modern cats) that we will discuss in some detail as we move along. For example, Proailurus had more teeth than modern cats do.

The body proportions of Proailurus are strikingly similar to those of the Fossa, a cat-like predator that is found exclusively in Madagascar. Like the modern Fossa, Proailurus was (probably) an accomplished tree-climber. It had short, grasping forearms, supple ankles capable of wide rotation and long, propulsive hind limbs. Check out this video of a Fossa to get an idea of how Proailurus might have moved about in the canopy.

* The taxonomic position of Proailurus has recently been the subject of some debate. Some studies argue that it belongs to the Felidae (the cat family) and was ancestral to all later cats, whilst others contend that it belongs to a group of animals that was ancestral to mongooses, civets and hyenas in addition to the true cats. I have decided – for the sake of simplicity- to stick to the orthodox view and call Proailurus the earliest known felid.

The next link in the chain of cat history is Pseudaelurus. The fossil genus was given that name in 1850 by Paul Gervais, an illustrious French zoologist who seems to have dabbled in everything from the study of Dinosaur eggs to fish stocking. The classification was based on a single mandible that had been described over a decade earlier by another Frenchman and scientific heavy-weight, Edouard Lartet. Lartet likened the mandible to that of a Hyena, noting that it sported more teeth than the standard-issue felid jaw bone.

I'm not even going to pretend like that's to scale, but it does illustrate one major trend in Cat Evolution: the loss of teeth. This is associated with "hypercarnivory".

Since then, Paleontologists have identified a dozen species of Pseudaelurus. So here’s a description to chew on:

Pseudaelurus was a genus of agile wildcat-to-cougar sized felines that roamed Eurasia, Africa and North America 20-8 million years ago. Their skeletons are very similar to those of modern big cats.

Pseudaelurus bore certain “primitive” features, including short metapodes (these bones are equivalent to the “hand” and “foot” bones), hind limbs longer than forelimbs and a long, flexible back. It was more successful than Proailurus in terms of geographical distribution; Pseudaelurus fossils have been reported from places as far apart as Nebraska, USA and  Ash-Sharqīyah, Saudi Arabia.

Green spots = Pseudaelurus finds, Red spots = Proailurus finds. It's not entirely accurate, but it's good enough.

It was, ostensibly, the first cat to immigrate into the New World. It did so by crossing a land bridge that connected Eastern Siberia to Alaska during the early Miocene (the bridge itself has the wonderfully Tolkienesque name of “Berengia”).

The metapodes of Psuedaelurus are elongated compared to those of Proailurus, but shorter than those of modern cats. This suggests that while it was better adapted for locomotion on the ground than its fossa-like predecessor, Pseudaelurus was still (as also indicated by the structure of its heel bone and the nature of the padding between its toes) quite tied to life in the canopy. As the felids descended from the trees and adopted more “cursorial” lifestyles, the metacarpals and radius elongated. This elongation of the lower forelimb (radius+metacarpus) is greatest in the Cheetah – the most cursorial of all the big African cats.

The Cheetah and Pseudaelurus forelimb. Notice how the forelimb of the former is elongated. This is an adaptation for cursoriality. The humerus-to-radius ratio in Pseudaelurus is indicative of an arboreal lifestyle.

Unlike the Cheetah (and like Pseudaelurus), the Clouded Leopard and the Jaguar have short radii and metacarpals. This increases the grasping strength of their arms – an adaptation for scaling trees and sinuous branches.

Two magnificent feline dynasties trace their origins to a common Pseudaelurus ancestor. One line bought the farm just 11,000 years ago, whilst the other continues to persist, if somewhat tenuously, in the present era.

A cat evolution cartoon

Here we hit an evolutionary fork in the road, with the Sabre-toothed cats on one side and the “normal” conical-toothed cats on the other.

 The Saber toothed cats (Machairodonts)

A poster from the Roland Emmerich movie, "10,000 BC". The Saber-toothed cat depicted here is hugely exaggerated in size.

Saber toothed cats are, beyond a shadow of an inkling of a doubt, the most popular prehistoric mammalian carnivores in current times. Try to initiate a conversation with the words “Hey, I saw a Homotherium skeleton on display at the Museum!” and you’re likely to get blank stares. But pad a similar sentence with the wonderfully picturesque adjective-noun combination of “Saber-toothed cat” and ears perk, eyebrows arc – you might even elicit a disinterested “Oh?”

And hell, IMDB tells me that they’ve made about 6+ film appearances. Granted, nearly all of the motion pictures they’ve featured in are decidedly craptacular, but that’s 6 more movies than the Creodonts or the Bear dogs will ever get.

Here’s a quote from a (deservedly obscure) 2002 television film titled “The Saber-tooth”:

Trent Parks: We got to get out of here.
Casey Ballenger: Where’s Lola? Trent, where is she? You left her behind?
Trent Parks: It had her. There was nothing I could do. It’s one of those dinosaurs.
Casey Ballenger: What?
Trent Parks: Those tigers with the teeth.
Casey Ballenger: A sabretooth?
Bob Thatcher: What?
Leon Tingel: You’re talking crazy, man. They’re extinct.

Ah yes, “Those tigers with the teeth”. Painfully amateurish screenwriting aside, let’s spring the question: Is the Saber-toothed tiger ancestral to the modern Tiger? And furthermore, was it manifestly bigger than what passes for ‘Tiger’ these days?

The answer to the first question is no. The Tiger and Saber-toothed “Tiger” lines diverged in the Miocene with Pseudaelurus and the two groups aren’t particularly closely related. Nor is there any reason to think that Saber-toothed “Tigers” had striped coats.

As for the second question: Well, it depends. We will take the term Saber-toothed “Tiger” (a vague and unfortunate misnomer that we will hereafter dispense with) to mean the genus Smilodon. Smilodon comes in three flavours. Smilodon gracilis, Smilodon fatalis and Smilodon populator. The largest extant felid, the Siberian Tiger, was certainly heavier that S. gracilis. Predicted body mass values for S. fatalis range between 160-280 kg, making it comparable to the Siberian Tiger (usually 220-260 kg, although a few exceed 300 kg) in terms of size. And the South American Smilodon populator was enormous even by Tiger standards, with the largest specimens weighing in at over 400 kg. A quote from Christiansen and Harris (2005):

Smilodon populator is clearly one of the largest felids ever discovered, rivaled only by the giant Pleistocene North American lion (see Merriam and Stock, 1932; Anyonge, 1993; Turner and Anton, 1997), and even normal-sized individuals are predicted to have rivaled world record Siberian tiger males (see Wood, 1976; Nowak, 1991; Sunquist and Sunquist, 2002).

A sketch of Smilodon populator I did. Coat colour is unknown. It is similar to Homotherium in one major way: It had longer forelimbs than hindlimbs. Some general features are also marked.

But Smilodon was not the only Saber-toothed cat. It belongs to a large group of felines with elongated upper canines called the Machairodontinae. This group includes two major tribes, the Smilodontini (Dirk-toothed cats, including Smilodon) and the Machairodontini (Scimitar-toothed cats). I will mark Smilodontini species in blue and Machairodontini species in green.

Some of the earliest known Saber-toothed felids were quite similar to Psuedaelurus – Paramachairodus ogygia, for example, was a primitive leopard-like saber-toothed cat with robust fore limbs and a supple body. It appears in Europe (Spain) during the late Miocene alongside the much larger Machairodus aphanistus. The latter had elongated limbs compared to the Psuedaelurine form, indicating a transition to a more terrestrial lifestyle. It was a top predator in its day and, as far as we are aware, the first Machairodont to attain Lion-size. Its skull morphology reflects a mix of adaptations to the biting model of modern cats and that of later Saber-toothed cats. Interestingly, canine size in this species appears to differ greatly between the sexes. Canines play an important role as weapons in male combat and confrontation (principally over females and territory). Among modern cats, Leopards and Lions display the greatest sexual difference in canine size and, concurrently, male members of both species exhibit low levels of tolerance for other males of the same species. By inference, we may argue that male competition was intense in Machairodus aphanistus too.

So how did P. ogygia and M. aphanistus interact? This is an interesting question. Modern small-sized felids actively avoid encounters with larger felids (as is the case with Leopards and Lions) in areas where they coexist. This is often accomplished by refuging in the tree cover or hunting at times of day when larger competitors are inactive. The Cheetah, for example, hunts during the daytime, when Lions are least active. The Leopard preferentially occupies wooded portions of the environment where Lions are unlikely to venture. A similar dynamic may have existed between the arboreal P. ogygia and the more terrestrial M. aphanistus.

Now let’s leap forward in geological time to the Pliocene era, 3 million years ago, when the Lion-sized Homotherium, a possible descendant of Machairodus, was present alongside the smaller Jaguar-like Megantereon in many parts of the world. These later Saber-toothed taxa share some important features:

1)       A long, powerful neck, presumably for reaching the underbelly or throat of struggling prey.
2)       A shortened lumbar spine.
3)       A shortened tail.

There were differences between Homotherium and Megantereon too. For one, the former had high shoulder blades and longer forelimbs than hind limbs, giving it a stance not unlike that of a Hyena. These long forelimbs were not quite as muscular as those of the modern Lion and seem to have been better geared for long-distance running rather than swatting down large prey. Megantereon, on the other hand, had relatively short and stocky forequarters.

NOTE: What follows is overwhelmingly based off of a single paper: Anton et al. (2009). There are, of course, considerable differences of opinion regarding the narrative I’ve constructed below, so take what I say with a grain of salt.

One species of Homotherium, H. latidens, occupied a particularly broad temporal and spatial range in Eurasia and Africa, coexisting in various places and times with some of our own Hominid relatives. It first broke into Europe after a cooling event 3.2 million years ago, alongside Megantereon cutridens. The two species exploited different habitats and prey niches – with the scimitar toothed H. latidens tracking and preying (possibly in packs) on large migratory animals on the open plains and the dirk-toothed M. cutridens stalking riverine forests and woodland patches for moderate-sized prey. This state of affairs persisted for 2 million years.

Europe found itself on the receiving end of another cold and dry event 1 million years ago. Although climate change had initially facilitated Megantereon’s entry into the continent, it was now mopping up forests and replacing them with open, high-visibility grasslands, thereby depriving the cat of its usual prey sources and hiding places. Competition from migrant Hominid populations and other predators eventually drove Megantereon to extinction.

Homotherium latidens weathered the changes a little better, persisting into the late Pleistocene. However, it now shared the European “preyscape” with two formidable competitors: The Lion and Homo heidelbergensis, a possible ancestor of the Neanderthal. Giant, aggressive Hyenas of the genus Pachycrocutus also appeared on the scene.

The European lion (left) and the slightly smaller Scimitar-toothed cat, Homotherium (right), coexisted in Europe during the Pleistocene.

H. latiden’s lacklustre performance in the face of competition from the European Lion can be attributed to one major factor: It was a specialist.

A compelling body of evidence suggests that it was heavily dependent on large, grazing mammals as a food source and worked in packs to bring them down. The replacement/extinction of various ungulate species in Europe as a consequence of climate change may have dealt a blow to Homotherium populations. New herbivore populations would display new body-size ranges (prey size is a major determinant of hunting success) and employ new predator-avoidance strategies.

As Europe’s climate oscillated between cold and warm conditions throughout the middle and late Pleistocene, versatility became key to survival. Long-term success hinged upon being able to operate in both closed forest (associated with warm, moist periods) and open plain environments. So how might  Homotherium have performed in these alternating episodes of steppe and forest vegetation, particularly in times of scarcity?

Arid spells on the plains would, naturally, be marked by an increase in inter-species competition for food resources. Cover on the open plains would also be reduced. Kills made by packs of H. latidens on the open plains would quickly draw attention from other major predators. It is not difficult to imagine a Lion pride driving a group of Homotheres away from a kill (Hyenas regularly lose kills to Lions on the Serengetti). On an individual versus individual basis, the European Lion outstripped Homotherium in terms of sheer muscle power and size. This sort of kill-stealing or kleptoparasitism (perpetrated by Lions and perhaps Hominids and Hyenas) may have effectively put H. latidens out of business.

Warm periods would have been characterized by more heavily forested environments. The typical grazing mammals that constituted the bulk of the Sabre-toothed cat’s diet would have been reduced in number. Ambush hunting would have been the name of the game in the woodlands. So how does Homotherium measure up as a solitary Ambush hunter preying on a large proportion of relatively small game?

Lions readily operate as ambush hunters when forced to live in forested areas. They thrived in all phases of the Middle and Late Pleistocene thanks to this facility.

H. latidens was incapable of the accelerated bursts of speed that modern cats, including the Lion, use to ambush and kill prey. It was an endurance runner rather than a sprinter. It is therefore unlikely that Homotherium would have made much of an ambush predator. And the specialized H. latidens may have had trouble tackling smaller prey, given the potential damage its long, latterally-flattened canines could incur in the process. Extant felids, however, can apply a bite to the nape or skull of a smaller animal without risk of injury to their rounded-section canines.

The European Lion was the dominant felid predator in Europe for hundreds of thousands of years, surviving until historical times. The youngest recorded Homotherium fossil – from the North Sea – dates to about 28,000 years ago, some 17 centuries shy of the invention of agriculture.

In North America, however, Homotherium (H. serum) and the American Lion shared their environment with a third felid, Smilodon fatalis. We have discussed the three species of Smilodon above. Of these, Smilodon fatalis is the best known. Enormous numbers of S. fatalis bones have been recovered from the La Brea tar pits in Los Angeles. Many of them reveal some kind of pathological condition – dental disease, genetic abnormalities etc. There is also evidence of injuries sustained during combat with other cats.

We will deal with Smilodon, the morphology of the tooth and skull among cats and the hunting strategies of the Saber-toothed cats in the next post.

References:

“Phylogenetic Systematics of North American Pseudaelurus (Carnivora: Felidae)” by Tom Rothwell, American Museum Novitates, 2003
“The Big Cats and Their Fossil Relatives” by Alan Turner and Mauricio Anton.
“Body size of Smilodon” P Christiansen, JM Harris – Journal of Morphology, 2005
“Co-existence of scimitar-toothed cats, lions and hominins in the European Pleistocene. Implications of the post-cranial anatomy of Homotherium latidens (Owen) for comparative palaeoecology”  – M Antón, A Galobart, A Turner – Quaternary Science Reviews, 2005
“Late Pleistocene survival of the saber-toothed cat Homotherium in Northwestern Europe” – JWF Reumer, L Rook, K Van der Borg, K Post, Dick Mos, John De Mos – Journal of Vertebrate Paleontology, 2003.
“Inferred behaviour and ecology of the primitive sabre-toothed cat Paramachairodus ogygia (Felidae, Machairodontinae) from the Late Miocene of Spain.” – MJ Salesa, M Antón, A Turner, J Morales. Journal of zoology, 2006.

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3 Comments so far
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Great article on cat evolution. Would like to see more, including Nimravides and marsupial cat evolution.

Comment by George Brenner

[...] Evolution of the Felidae, part 1 « Fins to Feet [...]

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I think this website has some really superb information for everyone. “He is able who thinks he is able.” by Buddha

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