The Killer Rat-Kangaroo's Tooth
By Stephen Wroe
Nature Australia Volume 27, no 1:28 - 31
Natural selection didn't come up with the best design; it just made the best of what was available.
The 'hand' of the Giant Panda (Ailuropoda melanoleuca) has six 'digits'. In processing its staple diet of bamboo, the Giant Panda drags the stalks between its sixth 'digit' and its paw to strip off the leaves. This sixth 'digit' or 'thumb' is a curious device. It is not, as one might expect, simply an additional finger of the type sometimes produced through congenital defect. In fact, the Panda's 'thumb' is not a real digit at all, but a greatly enlarged and specialised wrist bone called the radial sesamoid that lacks much of the flexibility of true digits. Why, one might wonder, did the Panda evolve a 'thumb' out of a wrist bone when it already had a 'real' thumb?
Evolution can be a fickle and opportunistic process. Often the end result can appear surprisingly imperfect, even 'sloppy'. The bottom line is that, at any juncture in its evolution, a species is constrained by accidents of history. When 'fashioning' new adaptations, natural selection can only work with what it's got. Because the real thumb (the first digit) of the Giant Panda was already modified and in use for another task (for walking on), evolution could only work with what was available, in this case, a radial sesamoid bone. It is this very fact of imperfection that underpins the reality of natural selection.
As pointed out by Stephen J. Gould (Harvard University) in The panda's thumb, text books paradoxically cite examples of 'optimal design' in misplaced efforts to illustrate the evolutionary process. But, it is these same examples that are often cited by creationists in defence of the notion of a Divine Creator (how can the complex 'perfection' of, say, an eagle's eye not be the product of anything other than conscious design?). A graphic way to expose the soft underbelly of this argument is to flag examples of 'imperfect design'. The Giant Panda's thumb is one. Another, more home-grown example, and the subject of this essay, is the second premolar of the Powerful-toothed Giant Rat-kangaroo (Ekaltadeta ima).
Giant rat-kangaroos (subfamily Propleopinae) are an extinct group of marsupials found in fossil deposits ranging from around 25 million to 50 thousand years or so in age. Ekaltadeta ima is the oldest of six known species, and it is also the best represented. Fossilised remains of this animal include the only two near-complete skulls known for the subfamily, as well as numerous upper and lower jaws.
Propleopine biology has been the focus of considerable debate among palaeontologists, ever since 1888 when Charles W. De Vis, once Director of the Queensland Museum, described the first known species of giant rat-kangaroo, Propleopus oscillans. In particular, scientists have argued about the food preferences of these fascinating beasts. In recent decades the weight of opinion appears to favour carnivory, or at least shows a leaning in that direction. This conclusion is based mainly on the size and shape of a large, buzz-saw-shaped cheek-tooth, the adult third premolar (P3), which is common to all propleopines. Whether or not this tooth was actually used in the butchery of carcasses is difficult to determine, but there can be little doubt that, when used in conjunction with powerful jaw muscles, the P3 of Ekaltadeta had the potential to bite through just about anything. However, it is not what this P3 was used for, but how it got there, and how it was held in place, that is of interest to us here.
Like other mammals, Ekaltadeta ima replaces its 'baby teeth' with adult teeth. Normally, tooth replacement involves the shedding (eviction) of a pre-existing deciduous (or milk-) tooth by an erupting adult tooth in the same position. These deciduous teeth are the ones kids tuck under their pillows in the hope of obtaining hard currency from tooth fairies. Marsupials are unusual in that only one premolar is generally replaced, the deciduous third premolar (dP3). Among marsupials, kangaroos are still more unusual in that the erupting P3 usually displaces not only dP3 but also the second premolar (P2). Ekaltadeta ima, however, is an exception here, not only among kangaroos, but among mammals in general. From the fossil record we know that in juveniles, the P2 performs the role of primary cutting blade. However, when P3 erupts, P2 is not ultimately discarded as in other roos. Instead, this P2 is withdrawn and reoriented to fulfil a completely different purpose - as a buttress for the whopping great P3.
When you think about it, this is pretty strange. A single organ has been used for two completely different roles in the course of the individual's life. This, to the best of my knowledge, makes Ekaltadeta unique among mammals. It is also a glaring example of evolutionary opportunism that undermines the notion of 'optimal design'. Using a normally discarded tooth to act as a structural support for a succeeding tooth is not the best way to do the job. A far 'cleaner' and more aesthetically pleasing solution to the problem would have been to completely eject the P2 and buttress the P3 with bone. In the case of the killer rat-kangaroo's tooth, as with the Giant Panda's 'thumb', natural selection didn't come up with the best design; it just made the best of what was available. And in that opportunistic imperfection, there is no clear vision of the hand of God.
Further reading
- Gould, S.J., 1980. The panda's thumb. Penguin Books Ltd: Harmondsworth, UK.
- Wroe, S. & Archer, M., 1995. Extraordinary diphyodonty-related change in dental function for a tooth of the extinct marsupial Ekaltadeta ima (Propleopinae, Hypsiprymnodontidae). Arch. Oral Biol. 40: 597-603.
- Wroe, S., 1999. Killer kangaroos and other murderous marsupials. Sci. Amer. 280: 68-74.
- Dr Stephen Wroe is a mammalogist at the Institute for Wildlife Research (University of Sydney) and at the Centre for Research into the Evolution of Australia's Total Ecosystems (CREATE) at the Australian Museum. He obtained his PhD on marsupial carnivore evolution from the University of New South Wales.
Article provided courtesy of Nature Australia Magazine
http://www.natureaustraliamagazine.com.au
Copyright © Australian Museum, 2003
