If you, like us, were under the impression that two-legged dinosaurs like those in the Velociraptor genus were pacy beasts that could zip across the ground at around 40 mph, a team of researchers has some bad news. A new study suggests they were much, much slower than previously thought.
Researchers at Liverpool John Moores University in the UK have rained on our speedy parade with new evidence suggesting that preserved footprints misrepresent the pace of theropods in particular – one of the three major groups of dinosaurs along with Ornithischia and Sauropodomorpha – by not accounting for the resistance of such muddied, soft surfaces that enabled their tracks to be fossilized all those millions of years ago.
"Fossil trackways serve as a valuable tool in understanding the behaviour and locomotion of extinct animals," noted the scientists. "Calculating speeds from trackways has become a standard approach, particularly for dinosaurs. However, the original equation was derived from predominantly mammalian data. There have been few validation studies using modern birds, the descendants of theropod dinosaurs."
The team used high-speed video recordings of helmeted guineafowl (Numida meleagris) traveling across varying consistencies of mud. They were chosen to test the theory because they walk and run on two legs, just like non-avian theropods (Velociraptor, Compsognathus). Their limbs – the proportions, joint angles and foot structure – resemble those small dinosaurs, too. They're also decent trotters, traveling at different speeds across different substrates – which is key to this study.
The guineafowl's trackways were then digitized and analyzed to calculate speed, and these numbers were measured against R. McNeill Alexander's groundbreaking formula that, since its creation in 1976, has been used to work out how fast a dinosaur was traveling across the land based on its fossilized prints. The late British zoologist based his formula on stride length, hip height and a scaling factor that was derived from living animals. You can read more about it here.
It was revolutionary, because it offered scientists the ability to not just investigate morphology but movement of these long-gone species, just by looking at their trackways set in fossilized earth. And it helped us imagine living, roaming dinosaurs, rather than seeing them as static skeletons reconstructed in museums. Dinosaur biology became dynamic, rather than a boneyard.
However, it's now being questioned – and with valid reasoning. The original formula was calculated as if the animal would have been traversing hard, dry ground, or something like a treadmill. However, the trackways studied were made in varying degrees of soft mud, which would have fundamentally impacted locomotion and stretched and distorted footprints, leading to exaggerated stride lengths. As such, the Liverpool researchers argue that Alexander's famous formula overestimates speed, particularly when it comes to the preserved trackways that scientists have since applied the equation to.
So, when applied to existing theropod trackways, accepted pace calculated with the formula may overestimate speed by as much as 4.7x. Ultimately, this means that some "running" dinosaur trackways may just show walking, and conversely scientists may have missed running behaviors because they didn't leave clean tracks for us to later study. Speed-based behavioral reconstructions – such as predator-prey chases and migrations – need reassessment, the researchers noted.
And yes, this means that even a T. rex could have been more inclined to saunter around and maybe jog, rather than tearing across the land at top speeds of 27-29 km/h (17-18 mph).
So a guineafowl moving at a slow jog of 1 km/h (0.6 mph) – roughly the pace of a wandering toddler – through Alexander's formula would be estimated as 4.7 km/h (2.9 mph). Scaling this up to a large ornithopod walking at 4 km/h (2.5 mph), trackway calculations would report its locomotion as running at nearly 19 km/h (11.8 mph). But it doesn't just concern speed, but how we view behaviors. A theropod that was casually walking through mud could have instead, based on trackway analysis, been interpreted as running after prey.
"None of the equations accurately predicted speed from trackways made by guineafowl in mud," the team said. "Despite ichnologists being aware of the limitations in the methods, they are still frequently used to provide very specific speed estimates from fossil trackways. Our results demonstrate that while there is some value to the equations for general trends, specific values may be substantially wrong or at least carry major uncertainty, when an animal is moving freely over compliant substrates. The existence of data points where the same stride length was made by birds moving at different speeds exemplify this. Errors in the equation are particularly high at lower speeds, where stride lengths do not become progressively shorter as speed nears zero."
For the dinosaurs that never left footprints (that we've found yet, anyway), scientists have instead used limb proportions and mass estimates to simulate likely top speeds. But many of those simulations have been checked using trackway data – which may be fundamentally incorrect.
And while this study certainly doesn't say dinosaurs never ran – there's a general consensus that some did – the researchers hope their analysis will encourage debate and perhaps a revision of previous assumptions that may be way off, both in speed and behavior.
"While trackways may offer important insights into locomotor behaviour in extinct dinosaurs, using them for anything but broad comparisons of relative speeds currently carries too much uncertainty to be worthwhile," they noted. "We strongly advocate that calculations of speed from fossil trackways are presented in broad terms, rather than as specific values."
But, like with any dinosaur revision of a history we've had to interpret, many millions of years after the last one walked (or ran) across the Earth, the scientists also acknowledge there are limitations to their guineafowl model, too.
"It is possible that substrate cohesivity would have less of an effect on a much larger dinosaurian trackmaker, especially one that leaves shallow tracks and is moving at a faster pace," they concluded. "It is also possible that trackways formed in coarser sediments, such as sand, fit Alexander’s formula more closely as the ‘pull effect’ would be less pronounced. It is therefore apparent that far more extensive studies need to be carried out across a range of body sizes, grain sizes and foot morphologies to enable more confident reconstructions of locomotion in extinct animals from fossil trackways. We refrain from adding a ‘correction factor’ or trying to derive a new equation to fit our data, because such a calculation may not be generally applicable."
"Without additional studies of extant taxa, particularly birds, moving on compliant substrates, presenting speed calculated from trackways is likely to be wildly inaccurate," they added.
The new research was published in the journal Biology Letters.
Source: Liverpool John Moores University via Scimex
