sauropod dreaming
African bush elephants are today’s largest land animals. The heavier males weigh about 8 or 9 short tons on average. Ridiculously, heavy stuff can nowadays be measured in short tons (US), long tons (British) and metric tons, or tonnes. Come on scientists, instil some discipline here.
Many sauropod (meaning ‘lizard-footed’) dinosaurs were much heavier, though there’s much disagreement about actual weight, regardless of measurement methods. The semi-legendary Bruhathkayosaurus has been described as weighing some 120 tons (of some kind). If this is anywhere near correct, how did such beasts, notable for their long necks and tails, manage to carry themselves around with any sort of grace or agility over millions of years?
Brian Ford, in his recent book Too big to walk, argues that they must have been aquatic or semi-aquatic, a minority view that dates back to the 19th century.
Sauropods were herbivorous quadrupeds, the earliest known of which, Antetonitrus, dates from the Norian Age of the Upper Triassic, somewhere around 220 mya, and the last of which, the real titanosaurs, lived for some 70 million years (not individually) before the Cretaceous–Paleogene extinction event which brought the dinosaur age to an end. Clearly, these were very successful beasties in their day, but how did they manage to feed themselves to sustain their enormous weight, and to provide the enormous energy required to shift that weight around? These and many other sauropody questions have been the subject of lively debate in recent decades, raising questions of habitat and habits as well as anatomy, physiology, metabolism, circulation, respiration, nutrition, organic function and so on. For example, it’s estimated that a sauropod weighing ten times as much as an elephant (which spends about 80% of its waking life foraging) would require about 100,000 calories a day, or about 450 kilos of foliage. They probably swallowed this stuff whole, and relied on micro-organisms in their massive guts to break it all down. Of course, much of what we think we know of these creatures is speculative.
So what do we think we know about their environment, and how they negotiated it? Presumably, that environment wasn’t stable over 140 million years, and over the whole range of sauropod fossil discoveries. Early representations often placed them in aquatic environments, but in the late 20th century the consensus view that their bodies were ‘permeated with air-sacs’ required a shift of perspective, which I can’t easily follow. It seems the air-sacs would have, in effect, caused them to be totally buoyant and unstable in deep water, due to their maladaptive body proportions. However, sauropod fossils have often been found mingled with those of marine organisms, and their footprints have often been associated with floodplains and lagoons.
There’s a lot still to learn about how they moved and sustained themselves. Could they rear up on hind legs, supported, kangaroo-style, by their massive tails (a ‘tripodal stance’)? Often they’re depicted in museums and paintings in unusual stances and poses, more for visual effect than to capture a known reality. There’s some evidence that the very elongated Diplodocids, which had a lower centre of mass, were well adapted for the tripodal stance, unlike other titanosaurs.
Clearly there’s far more that we don’t know about dinos large and small, than what we know, and we’re living at a time of abundant fossil discoveries. It all makes for entertaining reading.
References
https://blog.nationalgeographic.org/2011/05/10/big-the-life-of-sauropod-dinosaurs/
Too big to walk: the new science of dinosaurs, by Brian Ford, 2018
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