Monday, 25 November 2019

Whatever happened to kangaroo-kicking dinosaurs?

While I am waiting for a certain piece of literature to arrive in my mail so I can write part 3 of my APW trope series, I decided to bridge the time by writing something completely different and hopefully shorter. A few months ago I read Predatory Dinosaurs of the World for the first time. Apart maybe from The Princeton Field Guide to Dinosaurs, this 1988 book is probably the most famous work by paleoartist and freelance researcher Gregory S. Paul. There's a lot that can be said about it, like how the art-style has influenced mainstream paleoart, Paul's controversial theories and how much of an influence he may or may not have had on Jurassic Park-author Michael Crichton. But we will discuss none of that for now and instead focus on an image that has immediately captured my curiosity:
  Fig. 1: A Ceratosaurus attacks a pair of Allosaurus by rearing up on its tail and preparing to kick. The allosaurs with their flimsy tails look mightily impressed. I would be too if I saw a half-ton predator doing this.
This was apparently Paul's first officially published dinosaur illustration, first appearing, if I read the bibliography correctly, in a 1978 article by a certain J. L. Marx in an issue of the journal Science. What it depicts is a hypothesis by Robert T. Bakker, for whom Paul worked as an assistant and illustrator. Bakker's observation was that more primitive theropods, such as Ceratosaurus, had larger foot-claws and stouter tails than later ones such as Allosaurus, so he suggested the idea that they may have occasionally balanced on their tails and kicked with their feet during fights, much like kangaroos do today. What I find so fascinating about this is that this was the first time I ever heard of such an idea as I have never seen it addressed in any other work. I did try to do some research on it, but all I found is that this became a minor paleomeme, as some artists copied (or, if you will, plagiarized) this image in their works. This of course gives no indication about the scientific accuracy of the scene, as even today artists in a hurry do not tend to do their research and just copy whatever fits the page of a children's dino book. This leaves me incredibly curious. Has this ever been debunked or confirmed by a later study? My gut-feeling tells me that no dinosaur-tail should be able to bend this way, but I am of course not an expert on these biomechanics and the tails of ceratosaurs may have significantly differed from other theropods. Ceratosaurus' tail was apparently unusual enough that Bakker also proposed that it may have used it to swim like a crocodile, but unlike the kangaroo-kick, this idea has at least met criticism. If you know anything more about these matters, please tell me. Is this just another piece of 70s/80s Dino-Renaissance weirdness, like viviparous, trunked sauropods and frill-humped ceratopsians, or is there more to it?

Fig. 2: Robert Bakker reconstructed Stegosaurus (both times shown without the iconic back-plates for better clarity) as being capable of occasionally assuming the classic tripodal stance and interpreted it as an active high-browser. As evidence he shows that it had an anatomy better suited to this than, for example, an elephant, which is also capable of rearing up. Brontosaurus gets the same treatment.
What also makes this interesting, from an art-historic viewpoint, is that it seems to be an inversion of a classic trope in dinosaur reconstruction. Kangaroos are the only large animals around today that are bipedal and have long, muscular tails. Before it was realized that non-avian dinosaurs are close relatives of birds, they were therefore often used as reference when reconstructing the posture of bipedal dinosaurs. Until the 1970s it was the norm to reconstruct theropods and other dinosaurs like kangaroos... if they were fat, lazy lizards. The main trait that was often carried over was that kangaroos use their tail as a sort of third leg when standing upright. However, dinosaurs were not envisioned as hoppers and instead portrayed as awkwardly waddling around in this tripodal stance. They were therefore left with all the awkwardness of a kangaroo and with none of the dynamicity or gracility. What Paul and Bakker did here seems to be an inversion of this trend, giving Ceratosaurus one of the most dynamic and characteristic movements of a kangaroo and none of the awkwardness. This type of "rebellion" is sort of typical of the Dinosaur Renaissance, as we often see old, orthodox ideas of dinosaurs being subverted or inverted. In 1986's The Dinosaur Heresies, Robert T. Bakker did perhaps something similar with another dinosaur. Contrary to the view up to that time, which saw Stegosaurus as a lardy, slow and tortoise-like low-browser, Bakker instead chose to reconstruct it as a high-browser that could occasionally rear up onto its hind-legs and use its tail as a third leg to reach higher tree-branches. Here the ability to stand in a kangaroo-like tripodal posture is utilized to paint a dinosaur, otherwise portrayed as lethargic, as an active, dynamic animal. Bakker's reconstruction made sense in the 80s, as Stegosaurus was reconstructed with significantly longer hind-legs than fore-legs. Thanks to new fossil finds we now know that its hind-legs were somewhat shorter than thought, but also that its neck was longer, making it more similar to other stegosaur species. I therefore wonder how much Bakker's reconstruction still holds up, although I suspect it is still largely salvageable. 
Fig. 3: The jerboa-like pterosaur-relative Scleromochlus, as illustrated by Mark Witton.
There is of course more that could be said about the history of dinosaur-kangaroo comparisons. The idea that some dinosaurs may have actually hopped like kangaroos has come up on occasion. Edward Drinker Cope originally thought Trachodon and Laelaps moved in such a way, though this was later abandoned in favour of a more awkward reptilian shuffle. An infamous example comes from 1984, when Bernier et al. identified a trace-fossil, which they named Saurosaltopus (literally "lizard-jump-foot"), as footprints left behind by a small dinosaur that hopped similar to a wallaby. It later turned out that Saurosaltopus was not made by a dinosaur at all, but was actually produced by the flippers of a sea turtle dragging itself along a beach. To my knowledge there is currently no evidence of any member of the clade Dinosauria having moved primarily in a hopping motion. The feet of kangaroos, which enable them to locomote in this fashion, are plantigrade, meaning they primarily stand on the soles (or metatarsals if you want to be technical) of their feet, just like us. The feet of most bipedal non-avian dinosaurs were digitigrade like those of avian dinosaurs (birds), meaning they walked on their elongated toes (I say most, because some noasaurids seem to have walked on just a single toe, somewhat similar to horses). While small birds do hop on occasion, this is of course not comparable to the movement of a kangaroo. Things become more interesting if we move outside Dinosauria and look at other Avemetarsalia (the clade that includes the last common ancestor of pterosaurs and dinosaurs and all its descendants). It has frequently been proposed that some dinosauromorphs and dinosauriforms, like Lagerpeton and Lagosuchus, and other ornithodirans like Scleromochlus, had plantigarde feet and moved similar to small, saltatorial mammals, like jerboas, wallabies or rabbits (Lagosuchus even means rabbit-crocodile). The strongest case for a saltatorial locomotion can be made for Scleromochlus, followed by Lagerpeton. It is interesting to think that dinosaurs may have evolved from a hopping archosaur similar to these, but current cladistic models show them being the closest relatives of silesaurids, which were quadrupeds. Scleromochlus however is often theorized to be a close relative to the ancestor of pterosaurs and its hopping gait may have been a pre-adaptation for their flight-capability. In 2015 Mark Witton illustrated Scleromochlus interestingly as fuzzy and hopping through a desert, very similar to a jerboa or desert-kangaroo. The fuzz is speculative, but reasonable given how we know pterosaurs had such integument, called pycnofibers. More recent studies also suggest that pycnofibers and the proto-feathers of dinosaurs are one and the same and go back to the common ancestor of both groups. Like the works of Paul and Bakker, the illustration uses kangaroo-like characteristics to portray the animal as fast and active, not slow and ungainly like would have been done in older art.
Fig. 4: As a reward for reading to the end: Bennett's wallabies (Macropus rufogriseus rufogriseus) being cute and doing wallaby-things at the Zoo Zürich (image taken by me). Personally I have never seen them in a fighting or aggressive mood. However, the emus that they share their walkthrough-exhibit with do seem to have fun harrassing little children. 
Literary sources:
  • Bakker, Robert Thomas: The Dinosaur Heresies. New Theories Unlocking The Mystery of the Dinosaurs and Their Extinction, New York 1986.
  • Barrett, Paul/ Naish, Darren: Dinosaurs. How they lived and evolved, London 2016 (Second Edition). 
  • Barret, Paul: Stegosaurus. An extraordinary specimen and the secrets it reveals, London 2017.
  • Desmond, Adrian: The Hot-Blooded Dinosaurs. A Revolution in Paleontology, London 1975.
  • Paul, Gregory Scott: Predatory Dinosaurs of the World. A Complete Illustrated Guide, New York 1988.
  • Witton, Mark: Pterosaurs, New Jersey 2013.
  • Witton, Mark: Recreating an Age of Reptiles, Marlborough 2017.
  • Witton, Mark: The Paleoartist's Handbook. Recreating prehistoric animals in art, Marlborough 2018. 
Online sources:
Image sources:
  • Fig. 1: Paul 1988, p. 35.
  • Fig. 2: Bakker 1986, p. 189-191.
  • Fig. 3: Witton 2017, p. 77.

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