Dinosaurs Rediscovered
Page 20
McNeill Alexander suggested his formula could be applied to dinosaurs. Here was a simple way to estimate running speed from a trackway – and palaeontologists had thousands of tracks to look at. Typical speeds were in the range of 1–3.6 metres per second (equivalent to about 2 to 8 miles per hour), and the debate became heated as palaeontologists like Bob Bakker used his estimates of faster speeds, such as 20 metres per second (45 mph) for Tyrannosaurus, to demonstrate that certain dinosaurs ran fast and so must have been warm-blooded, and argued, as did others, that we would never be able to use tracks to indicate maximum speeds, because nobody can run at their fastest on wet mud or sand. Fast running requires a hard surface, and this is just the kind of surface that does not preserve tracks.
Walking and running speeds of dinosaurs, compared to some modern animals.
Nevertheless, most calculated dinosaur speeds are modest. Much attention has focused on T. rex, but it has been hard to marry tracks directly to the adult animal. Juvenile tracks found in 2016 suggest a speed of between 2.8 and 5 miles per hour (1.3 and 2.2 metres per second), a brisk walk for a human, but pretty slow for a larger animal. So, life in the Mesozoic may have proceeded in slow motion, not at the speed of race horses and cheetahs. This is a maximum possible speed estimate for T. rex, and tracks provide matching speed estimates.
How do these speed estimates fit more detailed modelling of locomotion? Digital modelling must obey the laws of physics, of course, but also fit in with the observational data from fossil trackways. John Hutchinson, like all other biomechanics experts, had to work from first principles. He recalls, as he worked on his doctoral thesis:
the dissecting and museum visits were a ton of work. Once I finished, I then had to dive into the physics of locomotion, which was intimidating especially considering how much was unknown about T. rex. I had to devise a way to take these unknowns into account while still testing how T. rex could have moved. At times, I gave up hope for finding anything!
Digitizing dinosaurs: how did their legs work?
Hutchinson did not give up. He knew that engineers use a great deal of common sense in their work, but they are also thorough. When he and collaborator Stephen Gatesy thought about the true posture of dinosaurs, they decided they should try out every possibility, and chose Tyrannosaurus rex as the model. So, they generated an amazing array of postures – thousands of them (see selection overleaf). Some looked quite sensible, with the legs pacing along at mid-height; but they also tried the Russian Cossack walk, in which the animal was squatting down close to the ground and somehow propelling itself forwards with its knees round its ears, then the fairy twinkle-toes look, with the body as high as it could go, and the legs tripping along almost en pointe.
These extremes could be ruled out immediately, because they involved huge extra forces to keep the legs ramrod straight or in a crouching posture. The so-called ‘ground reaction force’ is the key – this is the force acting vertically upwards as the equal and opposite force to the body weight. If it passes vertically upwards in front of the knee, such as when the limb is held too straight, the animal falls over backwards; if it is too far behind the knee, the rotational force about the knee is too high and could not be resisted by any reasonable size of leg muscles. In this case, two postures (‘a’, ‘b’) can be ruled out, whereas ‘c’ might be feasible.
After numerous runs of the experiment, Hutchinson and Gatesy settled on a core set of postures and strides, and these were the ones most people would have agreed looked more or less plausible. The experiment, however, was helpful as it gave logical reasons, in terms of energy wastage, for why the others should be rejected. The question then was whether such a pure-biomechanics approach could be used to determine maximum running speeds from first principles. We will never be able to follow a running T. rex and time its speed, but if the speed calculations from footprints on the one hand, and the fundamentals of skeletal function on the other, give the same answer, then we might suggest this is correct. It’s not a perfect way of doing science, but it satisfies the demands of common sense – and can be said to constitute proof, in a legal sense at least.
Tyrannosaurus rex body outline, and various possible leg postures.
The knee moment arm is the rotational force around the knee, and posture (c) shows the correct position of the knee to minimize the force. In ‘a’ the leg is too straight and in ‘b’ it is too crouched.
Hutchinson’s idea, in joint work published in 2002 with Mariano Garcia, was to use their knowledge of skeletons and muscles to estimate speeds. There is a standard relationship between muscle volume and speed, based on the assumption that the force of a muscle is proportional to its cross section. We see this when we compare the legs of a sprinter and a non-athletic person – the sprinter’s extensor muscles, which provide the main power in running, may be twice the diameter of those of a normal, healthy, but not obese person. Likewise, with other animals – greyhounds and whippets are all muscle when compared to other dogs.
The muscle volume versus force or speed relationship scales with body mass. Smaller animals need relatively less muscle to achieve fast speeds (which themselves are proportional to body size). Hutchinson took the chicken as his example of a fast-running small animal. This might not seem a good choice, but chickens are built for speed on the ground, as anyone who has tried to catch one can attest. A chicken weighs about 1 kilogram (2 pounds), and Hutchinson calculated its main leg muscles comprised 10 per cent of that body mass. He then calculated the volume of leg musculature required by a 6-tonne chicken (the size of T. rex), and the answer was that it would have needed about 10-tonne leg muscles, that is 200 per cent of the animal’s body mass (100 per cent × 2 legs = 200 per cent), to power it at an equivalent speed to a charging chicken. This is not possible – no animal can devote twice its total body weight to leg muscles. Not even 50 per cent would be possible, because all the other organs are required to make a functioning animal.
Calculating likely speeds of movement and the muscle mass needed to achieve that speed, at increasing body mass.
In a famous image produced to highlight their work, Hutchinson and Garcia showed the 6-tonne chicken keeping pace with a T. rex. Their calculation was that T. rex could have devoted no more than 30 per cent of its body mass of 6 tonnes to the leg muscles, and this meant it scales as a gentle stroller, capable of speeds of, at most, 10–22 miles per hour. These are the absolute maxima, and speeds of half these values are more likely. Recall that the footprint data gave a speed of 2.8–5 miles per hour. This represents good independent corroboration based on two independent scientific observations – fossilized trackways on the one hand, and on the other a well-established rule of biomechanics concerning body size scaling and the relative size of the leg muscles.
Recent work by John Hutchinson and his group shows how much there is yet to learn. In a 2018 study of modern birds, they confirmed a formerly debated point, that in fact most modern birds can move from one gait to another – say, walking to running – in a smooth manner, simply by moving faster, whereas in humans and many other mammals, there are distinct switches from walk to run. Applied to T. rex, the new bird locomotion model predicts that it would have bounced along at a steady lope, making strides 4 metres (13 feet) long, but always with at least one foot on the ground (it did not have a so-called airborne phase, as seen in fast runners such as ostriches and racehorses).
These recent studies illustrate a new level of confidence in a field that McNeill Alexander pioneered forty years ago. According to John Hutchinson:
We can now do much better in animating dinosaur locomotion than just using intuition. Evidence from footprints, skeletons, biomechanics, and comparisons with modern forms allows us to test likely poses and likely gaits. We can even identify cases – as with many dinosaurs – where they were moving in ways no modern animal does.
How did T. rex use its arms and legs?
The legs of tyrannosaurs were clearly used primarily to support the huge bo
dy weight and for locomotion. Perhaps they were also used to hold down the prey. Today, vultures and other scavenging birds use their feet to hold a carcass steady while they rip at the flesh. The African secretary bird, so called because it has a long quill-like feather tucked behind each ear, chases lizards and snakes, traps them by slapping its foot down hard on top of its prey, and then killing and tearing with its beak. Owls and eagles do the same thing, snatching their prey in powerful foot claws and carrying it off, and then holding the prey animal down with the foot to stop it squirming as they tear off flesh.
Birds have to use their feet for holding and subduing prey because they cannot grasp things with their wings. Dinosaurs, however, had freed their arms by being bipedal, and surely the early theropods would have grasped their prey with their hands while they bit at it. Likewise, early bipedal herbivores such as the basal sauropodomorphs and ornithopods were able to grab at and clutch leaves in their hands. Most later plant-eaters became quadrupedal, so the arms became pillar-like limbs, and the fingers shortened and were equipped with small hooves. Their arms were no longer capable of grasping. These sauropods, ceratopsians, stegosaurs, ankylosaurs, and hadrosaurs must then have manipulated their plant food almost exclusively using their mouths, having given up their ability to use their arms for anything other than walking.
A modern secretary bird shows no fear in quelling a cobra.
Genus:
Tyrannosaurus
Species:
rex
Named by:
Henry Osborn, 1905
Age:
Late Cretaceous, 68–66 million years ago
Fossil location:
United States, Canada
Classification:
Dinosauria: Saurischia: Tyrannosauridae
Length:
12.3 m (40¼ ft)
Weight:
7.7 tonnes (16,978 lbs)
Little-known fact:
One T. rex specimen, nicknamed Sue after its discoverer, Sue Hendrickson, was sold to the Field Museum, Chicago for $8.36 million in 1997, the highest-priced dinosaur ever.
The huge size of Tyrannosaurus rex can be appreciated with a human for scale.
Most theropods reduced the number of fingers from five to four, three, or even two, in some species of Tyrannosaurus. At the same time, the overall size of the arms became reduced until, in Tyrannosaurus, the arms were 20 per cent of the length of the legs, compared to 50 per cent in an early theropod such as Coelophysis, or 70 per cent in humans. What on Earth were these arms used for? As noted in Chapter 6, this trend for reduction in the arms was reversed in the maniraptorans, who used their elongate arms in flight. But T. rex was stuck with short arms, and (like the legs) they even became relatively smaller as the animal grew from juvenile to adult, as John Hutchinson and colleagues showed in a 2011 study.
The tiny, but powerful, arm of T. rex.
The dinky little arms of T. rex have been the subject of much speculation. If T. rex held down a dead carcass, or killed its prey, with its broad, spreading feet, were the arms used in any way in hunting? The common agreement is that they were not, because they could not reach the mouth – so even if T. rex grabbed a tasty morsel in its hand or hands, it could not even shove it into its mouth. Other suggestions are that the arms were used to push the animal up off the ground after it had been asleep, to hold down prey while the death bite was delivered, or even to tickle members of the opposite sex to encourage them to mate. None of these ideas is testable, but studies of the lever mechanics of the arms show that they were strong, even if ridiculously small. Their function remains a mystery, one of those puzzles in dinosaur science that will keep future researchers happily engaged.
It might have been a different matter for another huge theropod with short arms – the unrelated Carnotaurus from the Late Cretaceous of Argentina. This dinosaur went even further, and had arms only about 12 per cent the length of the hindlimbs, and the wrist bones were much reduced. In fact, this forelimb has been described as ‘vestigial’, meaning it’s barely there – so reduced in size as to be almost without function, like the wings of the flightless emus and kiwis today. Maybe Carnotaurus had dispensed with using its arms and hands for grasping, but used them instead to twirl a tuft of feathers on each side – like a Cretaceous exotic dancer, seeking to attract members of the opposite sex.
Genus:
Carnotaurus
Species:
sastrei
Named by:
José Bonaparte, 1985
Age:
Late Cretaceous, 72–69 million years ago
Fossil location:
Argentina
Classification:
Dinosauria: Saurischia: Abelisauridae
Length:
9 m (30 ft)
Weight:
1.6 tonnes (3,528 lbs)
Little-known fact:
Carnotaurus had a pair of horns located on top of its head, and these might have been used by males for head-butting contests.
Would the arms even have eventually disappeared if the tyrannosaurs and carnotaurines had not become extinct at the end of the Cretaceous?
Could dinosaurs swim?
All animals can swim, even cats. They may not like it, but they do it when they have to. Therefore, there is no reason that dinosaurs could not have been swimmers, especially when they were trekking over long distances. It’s not known whether all dinosaurs migrated, but it seems likely from our knowledge of modern large mammals. Today, caribou and elephants, for example, are famous for their long migrations as they seek sufficient food supplies in the face of seasonally varying availability.
In the Cretaceous, North America was divided into two land masses, one to the east and one to the west of the Western Interior Seaway, which ran up through Mexico and Texas to Alberta and Northwest Territories. Martin Lockley, famed dinosaur track enthusiast, born in England but a long-time resident of Colorado, identified a number of what he called dinosaur megatracksites, locations with thousands of footprints, mostly in the form of trackways, on the western coastline of this inland sea. The megatracksites documented how herds of dinosaurs trekked north and south, perhaps covering 2,000–3,000 kilometres (around 1,250–1,850 miles) in a season, in search of lush vegetation. Presumably, during these migrations, the herds had to swim across rivers, just as caribou and wildebeest do today. We can only imagine the immensity of the herd as it passed, the adults, weighing up to 50 tonnes each, clapping their great feet, each as broad as a tree trunk, thunderously to the ground and stirring up great clouds of dust. The juveniles, some as tiny as a sheepdog, would stay in the middle of the moving herd for safety, but could shoot in and out between the legs of their parents.
Mass sets of dinosaur tracks at Dinosaur Ridge, Colorado – mostly heading in the same direction.
Map showing the Western Interior Seaway. Dinosaurs trekked north and south along the east coast of Laramidia.
The most likely explanation of the hand-only sauropod trackways.
Some rare tracks appear to support the swimming idea. One of the most peculiar was discovered by buccaneering dinosaur collector Roland T. Bird, and reported in 1944. He had found a series of large, hand-only sauropod prints in the Cretaceous of Texas, and speculated that the animal was moving through deep water, its hind quarters and tail floating, and using its hands to paddle or prod its way through the water, maybe to change direction, as the hindlimbs did a doggy paddle behind. The only other explanation could be that the sauropod was balancing on its hands, and doing something extremely acrobatic. Such a suggestion, amusing as it might be, would be impossible because of the huge weight of the back half of the animal’s body. Further, even if the dinosaur were an amazing gymnast, with all its weight expressed through the arms, these would surely have made deep prints in the sediment, not the light prods that are seen in the trackway. Such hand-only sauropod prints have been reported also from South Korea and China, and may represent a regular behaviour.
; A determined theropod swims in deep water, leaving the merest of scratch marks on the bed of the river.
Roland Bird worked as a collector for the American Museum of Natural History (AMNH), and he discovered sauropod footprints on the banks of the Paluxy River, near Glen Rose, in Texas. He heard about the site from local informants who said the farmers there were busy excavating human footprints from these ancient rocks and selling them to gullible visitors. Here was proof that humans and dinosaurs lived side by side! Bird devoted great efforts to photographing, mapping, and excavating Paluxy River tracks for the AMNH, and did his best to explain to the farmers what they were. The supposed ‘human’ footprints were just chance bits and pieces of footprints, often a single toe of a three-toed dinosaur print, and yet these continued to be cited as evidence for ‘creation science’ until quite recently.
Swimming tracks have also been reported for theropods, including scrape marks made by the feet as a Megalosaurus-like animal floated along in the Early Jurassic of Connecticut, dabbing occasionally at the bottom of a river with its extended toes to keep moving in the right direction. Debra Mickelson reported a series of theropod tracks from the Late Jurassic of Wyoming that first showed normal tracks as the ostrich-sized animal walked in shallow water, then lighter foot impressions as it moved to ever-deeper water and its body began to float, taking the weight off its feet, and then mere toe-tip scrapes when it finally launched into swimming.