Repost: It’s a kangaroo… It’s a llama… No, it’s Palorchestes!

[This essay was originally posted on October 6, 2010.]

In his 1931 account of fieldwork in Patagonia, Attending Marvels, the 20th century paleontologist George Gaylord Simpson considered the appropriateness of the phrase “fossil hunting” to his profession:

Fossil hunting is far the most fascinating of all sports. I speak for myself, although I do not see how any true sportsman could fail to agree with me if he had tried bone digging. … It has uncertainty and excitement and all the thrills of gambling with none of its vicious features. The hunter never knows what his bag may be, perhaps nothing, perhaps a creature never before seen by human eyes. Over the next hill may lie a great discovery! It requires knowledge, skill, and some degree of hardihood. And its results are so much more important, more worth while, and more enduring than those of any other sport! The fossil hunter does not kill; he resurrects. And the result of his sport is to add to the sum of human pleasure and to the treasures of human knowledge.

The fate of fossils successfully hunted fits comfortably within this metaphor. The elusive quarry of the fossil hunters often winds up on display in the fossil halls of museums; vast trophy rooms representing the most impressive organisms to be rescued from the earth’s strata. One after another, cleaned and polished skeletons stand arrayed in evolutionary ranks, some frozen in action and others standing as if posing for Charles R. Knight, Erwin Christman, or one of the other early greats of prehistoric illustration. They are things of bone, glue, metal, and fiberglass; the gleaming end-products of long days in the field and countless hours in the prep lab.

As beautiful as the fully-articulated skeleton of an Apatosaurus or a saber-toothed cat can be, however, museum displays often mask the scientific process of restoring ancient life. A visitor to a museum’s fossil hall could be forgiven for thinking that most of the skeletons were found mostly-intact in the correct anatomical positions with little assembly required. Many cannot perceive which bones are real, which have been fabricated, and which skeletons have been cobbled together from multiple specimens.

Nor is it immediately apparent that many skeletons represent the latest iteration of ongoing attempts to reconstruct what extinct animals were like. The dinosaurs of today’s museums are very different from the ones I grew up with, for example, and both the pre- and post-“Dinosaur Renaissance” versions are wildly disparate from Victorian restorations. To a greater or lesser degree, any reconstruction or restoration of a fossil organism is subject to change.

Among the many creatures which have been restored, revised, and revised again is a little-known marsupial from Australia called Palorchestes. Over the past century and a half it has taken on a variety of forms, and University of New South Wales paleontologist Brian Mackness recently tracked the changing shape of Palorchestes in a 2008 paper published in the Proceedings of the Linnean Society of New South Wales.

The trail starts in 1873, when the British anatomist Richard Owen first described Palorchestes. A dedicated anatomist rather than a field paleontologist, Owen had fossils sent to him from outposts all over the British Empire, including Australia. Frustratingly, there was very little of Palorchestes to work with – only part of the anterior section of the skull was found – but Owen believed he could perceive enough tell-tale anatomical landmarks to designate the animal as “the largest form of kangaroo hitherto found.”

Owen’s interpretation of Palorchestes became the standard one. Even though other naturalists described scraps attributable to the same kind of animal, there was no complete skeleton to work with, and there did not seem to be any reason to doubt that Palorchestes was a really big kangaroo. In 1912 the Australian Museum event went as far as creating a sculpture of the animal, which was a hit among visitors for over three decades.

Uncertainty about the relationships of Palorchestes began to circulate by the middle of the 20th century – was it truly a kangaroo, or did it belong in its own unique group? No one knew, but the kangaroo imagery stayed in place. When the Australian Museum wanted an update of their Palorchestes restoration in the mid-1940’s, for example, they commissioned a sculpture which was a little smaller than the original but still unequivocally a kangaroo. Then, in 1958, J.T. Woods provided compelling evidence that Palorchestes was more closely related to wombats than to kangaroos, and the museum literally dumped their famous restoration. (Rumor has it that the sculpture may be buried somewhere under Sydney’s Centennial Park.)

The revised idea of Palorchestes as a large wombat was supported by the discovery of bits and pieces of several species made during the 1970’s, although the creature clearly wasn’t like any wombat known before. Its forefeet bore long, compressed claws, and more complete skull material showed that it had a recessed nasal cavity which probably would have supported a short trunk. With a tapir-like head and claws reminiscent of those of a giant ground sloth, Palorchestes was clearly more unusual than paleontologists had realized, but what the whole animal looked like was another matter entirely. Some restorations cast it as an okapi-like creature, while others clearly drew on living tapirs for a model. By the 1980’s the only thing everyone could agree upon was that Palorchestes was an animal marked by a “high coefficient of weirdity.”

Eventually the analysis of new and previously-discovered fossil material – particularly the vertebrae of the neck – showed that Palorchestes could not have been the slim, okapi-like animal of some popular restorations. As might be expected given its relationship to wombats, Palorchestes was a squat and relatively rotund quadruped, but there was still plenty about it to nitpick in terms of soft tissues. The animal almost certainly had a trunk, and its deep lower jaws looked like the perfect anchor for a long, prehensile tongue, but just how long were these structures?

An Aboriginal rock painting found in Deaf Adder Gorge, Arnhem Land in 1976 was offered up as a possible answer to some of these questions. Painted thousands of years before the present, the artwork was similar to some of the modern restorations of Palorchestes, although the ancient art appeared to show that the animal had a sort of mane near its midsection. This connection was extremely tentative – no confirmation could be made that the painting truly was of Palorchestes – but the proposed association nevertheless caused a few modern restorations of the animal to sport a shaggy coat and other features which could be picked out in the rock art. In general, though, Palorchestes was often cast as a “marsupial tapir” – just as the predator Thylacoleo was called the “marsupial lion” – in order to draw an evolutionary correspondence between Australia and the rest of the world. Nevermind that the mammals of prehistoric Australia were very distinct from their namesakes; they were popularly cast as examples of evolutionary convergence between marsupial placental mammals, even if the actual correspondence was in name only.

At present, the best restorations of Palorchestes show it as a squat, trunked herbivore with stiff forelimbs tipped in heavy claws. This understanding did not emerge simply by collecting more and more fossil evidence. Since we cannot observe or dissect extinct animals every restoration requires some degree of informed speculation, and this is what allowed Palorchestes to be restored in so many different ways. To Owen, the anterior part of the skull – the only part he had – was very kangaroo-like, so it made sense to restore it as a kangaroo. Later, when more of the skull and jaws became known, the anatomy of those parts hinted at soft tissue structures which are seen among tapirs and okapis today, thus leading to slim, long-necked versions of Palorchestes. The examination of previously-discovered fossil evidence again revised these images, but even then the anatomy of the trunk, tongue, and hair of the animal remained unknown. Without a complete animal, paleontologists relied on comparisons with other animals and other sources of information (such as rock paintings) to fill out Palorchestes, and Mackness points out that the desire of Australian paleontologists to interest the public probably played a role in generating so many different perspectives of the same animal. Most of the restorations of the animal have appeared in books, magazines, and pamphlets during a time period when museums and scientists used the weirdest of Australia’s extinct mammals to engage the public. This gave scientists and artists a reason to keep going back to Palorchestes to keep trying to refine its appearance.

How the current motif of Palorchestes as a weird, trunked, sloth-like marsupial stands up over time will depend on a mix of influences similar to those which produced it, from comparison with more fossil material to the use of better-known animals to create an anatomical theme. (As Mackness notes, despite being known for decades there is still a lot of undescribed Palorchestes material rattling around in drawers. Palorchestes could certainly use more in-depth study.) We will probably never again see the disparity in reconstructions which proliferated during the 1970’s and 1980’s, but tweaks will continue to be made as the process of scientific discovery continues.

When you look at a skeleton in a museum, you are not just seeing the vestiges of ancient life. The arrangement of those old bones are the product of hours of preparation undertaken within a scientific framework in an attempt to determine what that animal was thought to have looked like and how it may have acted. A reconstructed skeleton is not a static thing – a three-dimensional piece of truth which can be safely filed away – but something which is constantly being reassessed and rearticulated as we learn more about prehistory. Any restoration is a combination of fact, theory, hypothesis, and imagination, and given this vibrant mass of interacting ideas it is only natural that even old bones should occasionally reposition themselves into different shapes.

Image: Four views of Palorchestes: as a kangaroo, a pseudo-okapi, a “marsupial tapir”, and a “marsupial ground sloth.” Drawn from original sources by Greg Luker and included in Mackness, 2008.

References:

Mackness, B.S. (2008). Reconstructing Palorchestes (Marsupialia: Palorchestidae) –
from Giant Kangaroo to Marsupial ‘Tapir’
Proceedings of the Linnean Society of New South Wales, 130, 21-36

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