Topic: Sabre-toothed cats and marsupials
Marsupials with giant fangs? Yes, not all of the extinct sabre-toothed cats were actually cats…
Probably everyone has heard of the sabre-toothed cats, now extinct carnivores that roamed the earth for more than 40 million years. They are notorious for their dagger-like canines, which in the large species Smilodon populator could reach almost 30 cm in length. However, sabre-toothed cats also represent one of the most spectacular examples of convergence, as only some of them are actually true cats (Felidae). The sabre-tooth morphology has evolved repeatedly in several groups, even in marsupials! It has furthermore been suggested that an analogy is found in the allosaurids, theropod dinosaurs from the Jurassic, although these reptiles lacked giant canines.
Convergent evolution of the sabre-tooth
Amongst the placental mammals, the sabre-tooth evolved at least three times – in the felid subfamily Machairodontinae (including Smilodon and Homotherium) as well as in two families that are not true cats but belong to the suborder Feliformia (“cat-like” carnivores), the Nimravidae (including Hoplophoneus) and the Barbourofelidae (including Barbourofelis). Most widely distributed were the machairodontines, which inhabited North and South America, Europe, Asia and Africa from about 23 million years ago until about 11,000 years ago. The nimravids lived in North America, Europe and Asia for almost 35 million years from the Eocene to the late Miocene, while the barbourofelids were found in Eurasia, Africa and North America during the Miocene period. Due to the considerable similarities between these groups, the phylogenetic picture has been quite confused. For example, barbourofelids were long included in the Nimravidae as a subfamily but have recently been re-ranked as a family.
Intriguingly, the sabre-tooth morphology also arose in the South American Thylacosmilidae, which survived until the late Pliocene. This family belongs to a major group of extinct marsupials known as the Borhyaenoidea, which most biologists judge to be relatively close to opossums (Didelphidae). It should be noted, however, that some taxonomists consider the borhyaenoids to be non-marsupial metatherians rather than true marsupials. Borhyaenoids were broadly dog-like, but the thylacosmilids represented an extraordinary morphological excursion that ended up with a strikingly similar arrangement to the sabre-toothed placentals.
The convergences between these two groups are, of course, not precise and include important differences. It has been suggested that thylacosmilid sabre-teeth might have been superior to those of their placental counterparts in that they possessed a protective flange, were more deeply inserted into the skull and equipped with a kind of self-sharpening mechanism. Furthermore, the canines of thylacosmilids grew continuously, which meant that should one be broken then ultimately it would regrow – certainly an advantage for a predator. Despite these disparities, the general evolutionary pathways that resulted in the substantial reconfiguration of skull morphology necessary for a sabre-tooth carnivore were evidently very similar.
Dirk-tooth and scimitar-tooth ecomorphs
In sabre-toothed cats, two distinct ecomorphs are recognised. Whilst the “classic” dirk-tooth morph (e.g. Smilodon) possessed elongate sabres, the canines of the scimitar-tooth morph (e.g. Homotherium) were somewhat shorter. Both evolved repeatedly and are evidently representative of different hunting strategies. It seems that dirk-toothed forms were ambush predators operating in thicker vegetation and employing not only the jaws but also their powerful forelimbs to subdue prey. The thylacosmilids converged on this ecomorph, and although they could not retract their claws (as cats do), they were evidently highly effective predators. In some ways, their attack probably resembled that by a bear (bears have non-retractile claws as well). Scimitar-toothed forms, on the other hand, were able to run fast and hunt down their prey by repeatedly inflicting slashing wounds. In addition to these two main ecomorphs, the placental Xenosmilus from the Pleistocene of Florida represents an interesting compromise, combining the dirk-toothed arrangement seen in Smilodon with a killing bite more similar to that of the scimitar-toothed Homotherium. It is widely assumed that sabre-toothed cats were mainly solitary hunters, but recent evidence has suggested that Smilodon fatalis might have hunted in groups.
Besides hypothesising about the degree of sociality exhibited by sabre-toothed predators, there has also been some speculation on the senses they employed to find their prey. Whilst vision was probably important, the thylacosmilids, for example, had only limited binocular vision. A particularly interesting instance of convergence is seen in the middle ear of Thylacosmilus and Smilodon. The thylacosmilid middle ear is very different from that of other marsupials, and the bones employed were uniquely recruited for this function. There is speculation as to why the middle ear region has such a distinct anatomy, and one possibility is that it was adapted to enhance the reception of low-frequency sounds, which might have helped in the localisation of prey.
In a flight of fancy, Simonn Conway Morris has even hypothesised that the large canines could have served as some kind of sensory antennae when inserted into the soil. While this might seem unlikely at first, some extant subterranean mammals such as mole rats do indeed employ their enlarged incisors for exactly this purpose.
An evolutionary dead-end?
Today, all these sabre-toothed cats are extinct, so do they, with their highly specialised morphology, represent an evolutionary dead-end? Maybe not quite. There is evidence that an extant felid is well on its way to re-evolving the sabre-tooth mode – the clouded leopard. Traditionally considered a single species, it has recently been split into two species on the basis of molecular evidence. The clouded leopard (Neofelis nebulosa) is found in mainland Southeast Asia, whereas the Sunda clouded leopard (N. diardi) is limited to the islands of Sumatra and Borneo. Not only does the clouded leopard have the relatively longest canines of any living felid (about 5 cm), but its general skull morphology is also very different from that of other extant felids, approaching that of primitive sabre-toothed cats such as Paramachairodus. Closer examination of the Sunda clouded leopard revealed that its craniodental features are even more similar to the sabre-tooth morphology. Unfortunately, the likelihood of these two big cats fully re-evolving the sabre-tooth mode seems remote, considering the continuing environmental destruction of their habitats. Both species are currently classified as vulnerable, with decreasing population sizes.
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Map of Life - "Sabre-toothed cats and marsupials"
February 23, 2019