L is for Leaellynasaura

The dinosaurs I first met were terrors that thrived in a warm, swampy world. Books and films presented them as little more than gaudy lizards and crocodiles that had ballooned to immense sizes thanks to glandular imbalances – actually proposed as a reason for dinosaur evolution and extinction – who frittered away their days wallowing in Mesozoic muck. The fact that the books in my local libraries were out of date kept this imagery alive far past its sell-by date, well into the late 80s, but every restoration or movie I had ever seen was clear that dinosaurs inhabited a warmer world where the only two seasons were wet and dry. Even dinosaurs that ventured close to the cooler poles were supposed to be itinerants that made the most of vegetation’s summer abundance before tromping their way back to warmer latitudes. The Mesozoic was an endless summer dominated by monsters.

But Fantasia-esque scenes of dinosaurs restricted to algae-choked lakes and steaming rainforests too tightly constrain the wonderful variability of the animals themselves. Non-avian dinosaurs dominated the planet for over 160 million years (their avian descendants carry on their legacy to this day), and undoubtedly occupied a variety of ecosystems – from dense forests to deserts, from floodplains to chilly polar habitats where dinosaurs may have strode through the snow. Contrary to the humid prehistoric visions I saw as a kid, discoveries near both poles have shown that some dinosaurs lived full-time in cool habitats which were dark for much of the year. One of these dinosaurs, hailing from the southern reaches of the Cretaceous world, was Leaellynasaura amicagraphica.

Compared to the various and sundry other dinosaurs paleontologists have dug up, Leaellynasaura was not particularly impressive or imposing. The dinosaur was a svelte, bipedal herbivore that reached about six feet in length and belonged to the ornithopod tribe of ornithischian dinosaurs. (Exactly what kind of ornithopod Leaellynasaura was remains a matter of dispute. For the moment, the dinosaur is considered to be an archaic cousin of the iguanodontians, but the relationships of many such small “hypsilophodontids” need revision.) What makes Leaellynasaura special is its ecological context.

Described by Tom Rich and Patricia Vickers-Rich in 1989 from fossils found in Australia’s Dinosaur Cove, Leaellynasaura roamed the southern continent around 118 to 110 million years ago. At this time, Dinosaur Cove fell within the Antarctic Circle. Based on the Early Cretaceous position of the site and geological clues, Leaellynasaura lived in a place with prolonged periods of light and dark, with temperatures between 21°F and 50°F.

How did Leaellynasaura cope with these conditions? Paleontologists continue to debate the intricacies of dinosaur physiology, and it’s hard to say what allowed polar dinosaurs to live in habitats so different from similar forms found elsewhere. Still, the microscopic structure of bone contains records of dinosaurian growth, and might provide some indication of how Leaellynasaura coped with cold conditions. In a PLoS One study published last year, Holly Woodward and colleagues reported that Early Cretaceous “hypsilophodontid” bones from Victoria, Australia showed signs of a rapid, three-year growth spurt followed by a slower approach to skeletal maturity. The assessments of age were made on the basis of Lines of Arrested Growth, a brief cessation in growth often tied to harsh environmental conditions.

Strangely, the researchers found that the growth patterns in southern “hypsilophodontid” bones were very similar to those of similar dinosaurs found in the interior of Cretaceous North America. The polar dinosaurs did not show any special changes in growth related to their habitat. As Woodward and coauthors noted, maybe this is because the general dinosaur growth profile allowed these animals to successfully exploit a variety of habitats. Like other dinosaurs, Leaellynasaura grew up fast to escape problems with regulating body temperature and environmental stress, and, once big enough, was able to suspend growth during tough times. The secret to dinosaur success might have been a flexible growth profile that was just as well-suited to warm habitats as cold ones.

This doesn’t mean that Leaellynasaura entirely lacked adaptations to its polar environment, though. Histology can only take us so far, Woodward and collaborators pointed out, and a 2009 study by Tony Martin suggested that the little ornithopods might have burrowed to avoid extreme temperatures. Martin reported three burrows in the Early Cretaceous rock of Victoria, Australia that were probably made by dinosaurs similar to Leaellynasaura. In fact, the identification of these burrows was at least partly based on the prior discovery of burrows definitely created by the small ornithopod Oryctodromeus cubicularis from the Late Cretaceous of Montana.

Frustratingly, unlike some of the dens in Montana, the bones of the burrowing dinosaurs have not been found in the southern trace fossils. Perhaps a lucky paleontologist will someday stumble across such a find. And, as Martin noted, we don’t know whether these dinosaurs added plant material to insulate their burrows, had accessory body coverings to keep them warm, or huddled together for warmth. We can only test these ideas against what has yet been found in the rock, and what future generations of paleontologists will dig up.

Still, Martin’s hypotheses remind me of John Conway’s squee-inducing restoration of LeaellynasauraJohn Conway’s squee-inducing restoration of Leaellynasaura. Covered in coats of simple fuzz, these dinosaurs may have foraged together in the snowy dark of the southern Cretaceous. Such a view is speculative, sure, but why should it be impossible? Dinosaurs were stranger than we could have ever supposed. Conway’s work is a testament to that realization. Indeed, Leaellynasaura was not the biggest dinosaur, the scariest, or worthy of any of the other vaunted Mesozoic superlatives, but this meek creature has helped us imagine something  that would have been deemed too fantastic to be true a few decades ago – snowbound dinosaurs.

Previous entries in the Dinosaur Alphabet series:

K is for Kileskus

J is for Juravenator

A-I at Dinosaur Tracking.

References:

Martin, A. 2009. Dinosaur burrows in the Otway Group (Albian) of Victoria, Australia, and their relation to Cretaceous polar environments. Cretaceous Research. 30: 1223–1237

Woodward, H., Rich, T., Chinsamy, A., Vickers-Rich, P. 2011. Growth Dynamics of Australia’s Polar Dinosaurs. PLoS ONE 6, 8: e23339. doi:10.1371/journal.pone.0023339