Saturday, 1 October 2022

Miocene (Pt 35): Crash Bandicoot and the Giant Platypus

When we think of marsupials, the animal that's probably most likely to come to mind first is the kangaroo, likely followed by the koala. Both of these animals, along with wombats and possums, belong to the largest order of marsupials, technically referred to as the diprotodonts - a term that literally means "two front teeth" on account of the enlarged, vaguely rodent-like incisors that characterise the family. These are used to clip at vegetation, since the group is overwhelmingly herbivorous.

This was less true in the Miocene, and that's because of the existence of marsupial lions. Or, more accurately, of thylacoleonids, members of the "marsupial lion" family, since the truly lion-sized animal best known by that name didn't live until much later. The group originated towards the end of the previous epoch, when some herbivorous, probably wombat-like, animal switched to a more meaty diet, its front teeth becoming blade-like in order to cut into flesh. They seem to have prospered during the Miocene, with three different genera currently recognised.

The most numerous of these was Wakaleo, which survived through essentially the whole of the epoch, and is known, not just from the large Queensland fossil deposits at Riversleigh, but also from both South Australia and the Northern Territory. Those living at the dawn of the epoch have been estimated to be roughly cat-sized, and had flexible wrists that suggest they often climbed trees, perhaps to pounce on prey from above, or perhaps to hunt other arboreal animals. Even in the early Miocene these were joined by larger and more terrestrial relatives, thought to have weighed around 22 kg (50 lbs), similar to a mid-sized dog such as a springer spaniel or a border collie. 

As the epoch wore on, however, they became larger still, their legs increasingly suited to walking on the ground. By the time the epoch was drawing to a close, the largest species, W. alcootensis, which is known from a fossil site near Alice Springs, is thought to have been about 50kg in weight, close to a great dane, although earlier estimates put it closer to a lioness. By this point, as one would imagine, it certainly wasn't jumping out of trees, but it does seem to have been an ambush predator, grappling its prey to the ground much as cats do.

At the opposite end of the scale, the other two genera of "marsupial lion", which both lived early on in the epoch over 20 million years ago, were smaller than even the smallest examples of Wakaleo; both are thought to have lived in trees. The smaller of the two is Microleo, estimated to have weighed just 600g (21 oz.) and to have hunted for insects and nesting birds among the branches.

The earliest known bandicoot comes from the Middle Miocene and is known (I kid you not) by the formal scientific name Crash bandicoot. This seems to have already been similar to the living animal, although as we only have one of its upper jaws, it's difficult to say that for sure. However, while it's the oldest fossil that we can place in what we'd now call the bandicoot family, it is part of an older lineage that extends back to at least the beginning of the Miocene and (despite a lack of fossil evidence) probably much earlier.

Examples that date back further than Crash, to the Early Miocene, include Yarala, a primitive form that still retained features common to non-bandicoot marsupials that have since been lost in the modern group; it may represent a very early branch within the proto-bandicoot line. Galadi is another early example, and notable for its shout snout. This would have made it look distinctly different from modern bandicoots and is thought to have implied a carnivorous diet, feeding on larger prey than the insects  most often consumed by the living species. Madju may be more closely related to living bandicoots, although still outside the "true" bandicoot family; we have enough fossils of this to determine that they did not yet have the record-breaking short pregnancy of the living species, and spent longer rearing their young in the pouch.

The largest carnivorous marsupial alive today is, of course, the Tasmanian devil. The family that this belongs to consists primarily of much smaller animals that can best be described as marsupial shrews and its fossil history is somewhat obscure, possibly because they were less numerous in the past than they are today. The oldest known examples include Barinya, a rat-sized animal known from both Queensland and South Australia and Ganbulanyi, whose teeth look to be adapted for cracking bone, in the manner of a miniature hyena. Even these appear to be clearly distinct from the most primitive marsupials, implying that their ancestry must stretch back a good deal further, even if suitably old fossils have yet to be discovered. 

Living rather later, in the Mid to Late Miocene, Whollydooleya (yes, really) may have been more impressive than its predecessors. We only know of it from a single tooth, but that's shaped in a way that implies a heavily carnivorous diet, and it is surprisingly large. Assuming it was in the proportion we expect to the rest of the animal (which is hard to prove), it would have to have been twice the size of modern Tasmanian devil.

But if such animals were comparatively rare, the opposite is true of the thylacinids, relatives of the now extinct "Tasmanian tiger". Several members of this family are known from the Miocene, a diversity that one would not guess based solely on the one modern example. The earliest examples were smaller than a cat, and were similar in many respects to the omnivorous Barinya and its kin, with the largest Early Miocene species reaching 8 kg (19 kg), similar to a border terrier. 

Around 15 million years ago, as the climate began to shift towards drier conditions, they underwent a burst of evolutionary change, becoming much larger and more thoroughly predatory. Maximucinus, for example, living not much later than this change, had already reached a respectably dog-sized 18 kg (40 lbs) and by the end of the epoch, some particularly close relatives of the recently extinct species reached three times this weight - close to some of the very largest breeds of domestic dog.

The best known of these early thylacinids, however, is probably Nimbacinus, a fox-sized Early Miocene animal that happens to be known from a relatively complete skeleton. Although primitive in a number of ways, analysis of its jaws and teeth suggests a comparatively powerful bite, and that the animal would have been able to take down prey of similar size to itself, perhaps including early wallabies and wombats and medium-sized birds.

A few early marsupials are harder to place. Naraboryctes is a likely ancestor of today's marsupial moles, not yet as specialised as those peculiar animals are, and still adapted to digging through rich forest soil rather than the dry earth of the outback. Malleodectes was just plain peculiar, with hammer-shaped teeth unlike anything seen in any other mammal. About the size of a ferret, it's probably related in some way to the dasyurids and thylacinids, but is distinct from both of those groups. Our best bet is that it used its odd-shaped teeth to crack open snail shells, suggesting an oddly specialised diet that may have resulted in it eventually being out-competed by skinks.

Yalkaparidon is stranger still, to the point that, while we're reasonably confident it is related to the other marsupials of the continent (as opposed to South American opposums) it probably represents a lineage within them that's otherwise unknown. Their teeth have a peculiar shape, ironically said to resemble a boomerang, and with odd microstructure to their enamel as well. The rear teeth look as if they may have eaten comparatively soft prey, such as worms or caterpillars, but the front ones are adapted for gnawing something tough; one suggestion is that they may have extracted grubs from trees as the living aye-aye does.

Marsupials were not, however, the only mammals in Miocene Australia. I've already mentioned the bats, which had flown across from the north, but we know of a few monotremes as well. Best known is probably the "giant platypus" Obdurodon, thought to have measured something like 108 cm (43 inches) in length. In addition to its large size, it was unusual in that kept its teeth throughout life, whereas the modern species loses them as an adult. The teeth in question appear suited to crushing hard prey - something that the toothless modern platypus obviously doesn't do - so it likely had a different diet, with a higher proportion of crayfish and shrimp. The enlarged, spoon-shaped bill and flattened skull have been compared to the skulls of some crocodiles, suggesting that it may have caught its prey swimming in open water, rather than foraging on the muddy river bottom as the living species does. Like the living platypus, along with other semi-aquatic animals such as otters, it probably had a poor sense of smell.

The Miocene also saw the earliest known echidnas, belonging to the genus Megalibgwilia. About the size of the largest modern echidnas, it likely ate caterpillars and large beetle larvae, rather than worms or ants as its surviving relatives do.

It isn't clear what mammals, if any, may have lived even further south, on Antarctica, during the Miocene. The warm worldwide climate, at least in the earlier part of the epoch and during the heat of the Middle Miocene climatic optimum, meant that the northern coasts would still have been relatively green, albeit perhaps more tundra than lush forest. But animals can survive in tundra, and the continent has not always been devoid of mammals. Unfortunately, there are no known fossil deposits of the right age that we've been able to find (Antarctic palaeontology is hardly easy), so if anything did struggle on into the epoch, we don't know about it. In any event, when the ice sheets advanced to the coast around 13 million years ago, the continent would have been left to the penguins.

But, while that concludes my survey of the world's continents during the Miocene, I am still not done with the epoch yet. Before I wrap up this unusually long-running series, I will also have to look at what was happening in the oceans...

[Image by Douglass S. Rovinsky​, Alistair R. Evans, Justin W. Adams from a PeerJ article licensed under CC-BY-4.0].

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