Wednesday, January 27, 2010

Marsupial Carnivore Evolution Revisited




The evolution of marsupial predators in Australia and South America have been the center of controversy regarding their alleged “inferiority” to eutherian, or placental mammals. This latter group includes all other mammal species, including humans. Marsupial mammals are considered to be the more primitive group. They became isolated in the Southern reaches of Gondwana at the end of the Cretaceous. Gondwana included South America, Antarctica, and Australia, and subsequently split up into those three landmasses supposedly before placental predators had penetrated this far south. While little is known about the fauna of Antarctica for obvious regions, both predators and prey in Australia were represented by marsupials, while in South America the herbivores were represented by primitive placental groups, while their predators were the marsupial borhyeanids, descended from the opossums. Both these island realms developed unique faunas found nowhere else. According to orthodox paleontology (at least for many years) it was believed that the extinction of the South American borhyenaids, and many of the placental herbivore as well, died out as result of competition from the more advanced placental canines and felines who invaded the south after the establishment of the Panama land bridge. It is also widely held that the extinction the thylacine or Tasmanian wolf on mainland Australia occurred as a result of competition with the introduced dingo. The same factor is also attributed to the extinction of the smaller Tasmanian devil from the mainland.

Some of the notions of marsupial inferiority have been recently rethought. According to Robert Paddle, author of The Last Tasmanian Tiger: the History and Extinction of the Thylacine, much of the supposed inferiority attributed to marsupials can be attributed to what he aptly terms “placental chauvinism,” the instinctive human tendency to regard one’s own group as the superior one. While far less obvious than the human bias responsible for racism (the early twentieth century belief held by white eugenicists that the white race was most highly evolved), the bias to regard one’s own group as superior even applies to the animal kingdom where humans are even marginally a factor; we tend to regard humanity as the ripe flower on the tree of evolutionary progress. This same bias can be observed in the twentieth century depictions of dinosaurs as sluggish, swamp-bound behemoths destined for extinction and replacement with the smaller, wilier mammalia. Such images dominated orthodox dinosaur paleontology for many years and are undoubtedly the source of the word “dinosaur” as something old and outdated, or someone whose views are ultra-conservative and destined for extinction. Our tendency to favor our own is also the reason why King Kong is the “hero” while battling the saurian inhabitants of his native island. However, while the newer, sleeker public image of the dinosaur has gained much more respect recently, popularized by the recurrent Jurassic Park image of wily, pack-hunting raptors, it occasionally bleeds over into excess. Michael Crichton’s raptors, capable of chimp-link problem solving abilities such as opening doors for example, are, as every paleontologist will admit, purely creatures of fiction.
The same observations apply to placental chauvinism as well as to mammal chauvinism. Among the observations Paddle makes in his book are that the thylacine showed signs of intelligence which were often disregarded or overlooked. Thylacines were frequently raised as pets when they were very young. These animals, according to Paddle, proved to be affectionate and loyal pets. Their owners were attached to them as much as any dog owner would be. Obviously, no one today has ever then opportunity of a thylacine for a companion. Paddle also notes that the thylacine displayed signs of group hunting and cooperation, according to observers in the wild. While often regarded as a solitary hunter, such observations indicate that animals preferred to hunt in small family groups. The thylacine has often been described as dull-witted, but this seems to have been derived from observations of captive specimens, who were confined to poor and unethical environments; very many captive animals will act “dull-witted’ in such circumstances. Paddle gives a heart-breaking account of the last thylacine in captivity (who became known as “Benjamin” as a result of apparent misinformation, even though records indicate she was actually female) who was locked out of den when the temperature was below freezing. The same unethical treatment was also responsible for other animals in the Hobart zoo that winter, including a magnificent black leopard.
There are two additional facts in the paleontological record that have caused major dents in assumptions of the inferiority of marsupial predators. One if the recent discovery of a 55 million year old tooth in Australia which appears to have belonged to a small placental condylarth. It was previously believed that the placentals never penetrated this far south, and this is what allowed the marsupials such a great opportunity for diversity. Condylarths are an ancient group of predators unrelated to the modern Carnivora, and much closer allied with deer, sheep and goats. Though carnivores, they bore hooved toes, and are sometimes referred to as “wolf-sheep”. They appear to be the direct ancestors of whales and dolphins, and included the gigantic land carnivore andrewsarchus. While the Australian tooth came from a small beastie, most of the marsupials at this time were small as well. The other major fact involves the great faunal interchange following the Panamanian landbridge to South America, previously thought to have spelled doom for the South American borhyeanids. It is now believed that the ancient marsupial predators were already extinct by the time the true cats and wolves appeared, and thus the placental order could not possibly have ousted them. At the time the smilodon, the jaguar, and puma arrived on the scene from the north, the dominant South American predators were the phorusrhacids, giant predatory birds. The marsupials were absent by this time and had been absent for more than a few million years.
There is, of course, one major flaw when it comes to using this argument against placental chauvinism. The marsupials were apparently wiped out by competition with the terror-birds, who in term met their end in competition with smilodon fatalis and his ilk. This is hardly a scenario that speaks favorably of the marsupials. To be fair, the borhyenanids thrived as major predators for millions of years alongside the terror birds. The competition between the two orders was a long and gradual one, though speed and ferocity of the phorusrhacids finally won out. The competition between the terror birds and the cats however, was fast and furious. The monstrous birds undoubtedly gave the sabor-tooths stiff competition, and even made forays into the north, with one species, Titanis wallerii, holding out in Florida, dying out only 1.8 millions years ago. But the fight was ended in short order. In contrast, the marsupial predators thrived successfully alongside the terror birds for millions of years. Representatives included the great marsupial saber-tooth, Thylacosmilus atrox, which undoubtedly preyed on toxodons and giant capybara by driving its great stiletto-like teeth into their thick hides. It also incduded borhyena, which included species resmebling a wolf, or Tasmanian thylacine. Early representatives included prothylacinus, which , as its name implied, resembled a missing link between an opossum like predator and a modern thylacine, and proborhyena, a cattle sized, slow-loving predator/scavenger, Andrewsarchus-like in its proportions. All these predators thrived side-by side with their ancestral cousins, the American opossums, which still exist in a variety of small-carnivore niches today. These include the mountain dwelling Andean opossum, and the yapok, or water-possum, and otter-like animal common to South American waterways.

There is also one additional fact overlooked by both opponents of placental chauvinism. It has not been proved entirely that the marsupial predators had disappeared entirely from their old niches by the time the placental competitors arrived. Almost all the fossils discussed have come from the ancient pampas. While phororhacas is a predator supremely adapted for running down prey on the open grasslands, it would be ill-adpated to a forest or jungle setting. It appears relatively little is known about the rainforests which undoubtedly covered much of the South American continent in prehistoric times, just as they do today. Predators such as thylacosmilus or borhyena (or their relatives) could have held on in the jungles as the top predator species, and remained so until the Panama invasion. In fact, there is little reason to believe that this could not have been the case.
Also, it must be acknowledged that the thylacine has a much smaller brain than a similar-sized dog. And while brain size is not the only indicator of intelligence, it is certainly a major one, especially when dealing with such gaping brain to body size ratios. Paddle brings into question whether competition with the dingo truly spelled the demise of the mainland thylacine, indicating that the blame may well belong to the aboriginal people instead. This is a strong possibility. But the dingo is hardly off the hook. Paddle also cites the fact that the thylacine was an easy match for a dog of similar size, if the confrontation took place one-on-one. This is especially true because of the thylacine’s enormous jaw gap. But the highly efficient pack-hunting strategy of true canines would undoubtedly have easily one-uped the marsupial predators. It would have been easy for a pack of canines to have driven a single thylacine away from his kill. Paddle states that thylacines are believed to have hunting in groups more than commonly thought, but that possessed anything like the coordinated group-hunting techniques of canids is simply false.
The condylarth tooth in Australia may present a different scenario, however. Paddle, and other opponents of placental chauvinism may in fact be right, though not in quite the same manner as they supposed. It must be remembered that condylarths were a far cry from the order Carnivora when it came intelligence and hunting strategy. Members of this early predator group had small brains, and often enormous bodies, relying on size and ferocity to make a kill. Early marsupial predators, in contrast, may have smarter and more agile in capturing prey. The marsupials were the ones who had a huge evolutionary headstart on them, after all. Also, it was undoubtedly isolation itself which played a role in the demise of South America’s marsupial predators. The majority of herbivores (all of whom were placentals) died out as well when competitors arrived from the north, and these included all the ruminant-like beasts. Also, one family of true Carnivora was in fact already present in South America at this time: then procyonids or raccoon family. They are believed to have arrived by virtue of psossibly a single pregnant female on a raft of vegetation, or some other similar mishap. While smaller members of this family literally thrive today on both continents, early embers of family, such as chapamalania, evolved into bear-like forms alongside marsupial saber-tooths and phororhacas. But these beasts died out when true bears (of the short-faced Artocid family)arrived from the north.
In final analysis, it appears that many factors govern a species susceptibility to extinction as a result of competition, as well as what determines which group qualifies as more primitive. The reproductive system of the marsupials is merely a single factor. It appears evident that the marsupial predators of today appear to be more “advanced” by good measure than the early placental predators of yesterday.

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