In the tracks of dinosaurs

Trackways, the most tangible record of locomotor behavior, provide evidence for one aspect of an animal's walking and running capabilities, and the only independent test of anatomical reconstructions. When footprints are arranged into alternating left-right-left-right patterns, they demonstrate that all dinosaurs walked with a fully erect posture. But how can trackways also give us an indication of locomotor speed?

We begin with stride length; that is, the distance from the planting of a foot on the ground to its being planted again. When animals walk slowly, they take short strides, and when animals are walking quickly or running, they take considerably longer strides. This much is intuitive for anyone trying to catch a bus about to pull away from the curb. Now, consider the situation when you are being chased by something smaller than you. The creature chasing you must take long strides for its size, and more of them too, just to keep up. So there is clearly a size effect during walking and running, and these will likely be different for different kinds of animals under consideration.

How, then, to relate stride, body size, and locomotor speed? British biomechanist R. M. Alexander provided an elegant solution to this problem by considering dynamic similarity. Dynamic similarity is a kind of conversion factor: it "pretends" that all animals are the same size and that they are moving their limbs at the same rate. With these adjustments for size and footfall, it doesn't matter if you're a small or large human, a dog, or a dinosaur. All will be traveling with "dynamic similarity"; only speed will vary. That variable Alexander terms "dimensionless speed."1 It is dimensionless speed that has a direct relationship with relative stride length. Stride length, of course, can be measured from trackways, which in turns allows us, for the first time, to calculate locomotor speed in dinosaurs.

To see how all this works, let's use Alexander's example of the trackway of a large theropod from the Late Cretaceous of Queensland, Australia. The tracks are 64 cm long, which Alexander, from other equally sized theropods, estimated must have come from a theropod with a leg length of about 2.56 m. The stride length of these tracks is 3.31 m, so the relative stride length (stride length : leg length) is 1.3. The dimensionless speed for a relative stride length of 1.3 is 0.4. And from all these measures, this Australian theropod must have been traveling reasonably quickly, at about 2.0 m/s, or 7.2 km/h.

As complicated as this approach appears, it represents the best method for estimating the actual speeds implied by trackways. But what about the fastest speeds a dinosaur might have been capable of? In 1982, R. A. Thulborn ofthe University of Queensland developed a method by which absolute locomo tor abilities could be calculated. Thulborn's work relied heavily upon Alexander's slightly earlier studies on speed estimates from footprints and the relationship between body size, stride length, and locomotor speed among living animals. For both approaches, Thulborn determined that relative stride length has a direct relationship with locomotor speeds at different kinds of gaits (for example, walking, running, trotting, galloping). Explicitly (for the quantitatively oriented among you):

Locomotor velocity = o.25(gravitational acceleration)05 x (estimated stride length)167 x (hindlimb height)-117

Thulborn used this equation to estimate a variety of running speeds for more than 60 dinosaur species. The first group of estimates were for the walk/run transition, where stride length is approximately two to three times the length of the hindlimb. A potentially more important estimate - especially for dinosaurs fleeing certain death or pursuing that all-important meal - is maximum speed, which Thulborn calculated using maximum relative stride lengths (which range from 3.0 to 4.0) and the rate of striding, called limb cadence (estimated at 3.0 x hindlimb length-0 63).

Although we report some ofthese speeds elsewhere in this book, it is of some value to summarize the overall disposition ofThulborn's estimates. For small bipedal dinosaurs - which would include certain theropods and ornithopods - all appear capable of running at speeds of up to 40 km/h. Ornithomimids, the fastest ofthe fast, may have sprinted up to 60 km/h. The large ornithopods and theropods were most commonly walkers or slow trotters, probably averaging no more than 20 km/h. Thus the galloping, sprinting Tyrannosaurus, however attractive the image, did not impress Thulborn (or us) as likely.

Then there were the quadrupeds. Stegosaurs and ankylo-saurs walked at no more than a pokey 6 to 8 km/h. Sauropods likely moved at 12 to 17 km/h. And ceratopsians - galloping along full throttle like enraged rhinos? Thulborn estimated that they were capable of trotting at up to 25 km/h.

Are these estimates accepted uncritically by all? P. Dodson has argued that these calculations would suggest that humans can run as quickly as 23 km/h, which in life they cannot. So it is possible that these calculations overestimate the speeds at which dinosaurs could run. On the other hand, anatomically, humans are not dinosaurs and these calculations could be valid for dinosaurs but be unapplicable to humans. At a minimum, they give some indication of the relative speeds of dinosaurs; for example, how quickly T. rex might have run in comparison with Triceratops.

1. Dimensionless speed may appear oxymoronic, but is in fact equivalent to real speed divided by the square root of the product of leg length and gravitational acceleration.

This idea was strongly reinforced by the discovery in 2000 of what was controversially inferred to be a four-chambered heart with an aorta. The "heart" was preserved as an ironstone mass within the thoracic cavity of the basal ornithopod Thescelosaurus, and identified using computed tomography (a CT scan). Doubters doubted; advocates advocated; and the issue remains unresolved.

Minds. In the late 1970s, attempts were made to assess the intelligence of dinosaurs using the encephalization quotient or EQ (Box 12.4). The idea was that living endotherms (birds and mammals) have significantly higher EQs than do living ectotherms (reptiles and amphibians), presumably because their more refined levels of neuromuscular control require an endother-mic metabolism.

EQ was reconstructed in dinosaurs using brain endocasts, internal casts of the brain-cases of dinosaurs (Figure 12.3). Based upon EQ, coelurosaurs were likely as active as many birds and mammals, while large theropods and ornithopods were somewhat less active than birds and mammals, but more active than typical living reptiles. Using EQ as representative of activity levels, other dinosaurs appear to have been in the range of living reptiles.

The nose knows. Endothermy requires the lungs to replenish their air (ventilate) at a high rate. And high rates of ventilation lead to water loss, unless something is done to prevent it. What modern mammals and birds do is to grow convoluted sheets of delicate, tissue-covered bone, called respiratory turbinates, in the nasal cavaties. The mucus-covered surfaces of the turbinates pull moisture out of the air before it leaves the nose, thus conserving water (Figure 12.4).

What about dinosaurs? Although a number appear to have had olfactory turbinates (indicative of a well-developed sense of smell), apparently none - as far as we currently know - had respiratory turbinates to allow them to recoup respired moisture. Considered exclusively on this basis, dinosaurs could not have been endothermic in the way that most mammals and birds are today.

Histology

Fossil bone may preserve fine anatomical details that are visible under a microscope. To see the details, a thin slice can be mounted on a glass slide, and ground down so thin that light can be transmitted through it (Figure 12.5).

Haversian bone. Bones grow by remodeling, which involves the resorption (or dissolution) of bone first laid down - primary bone - and redeposition of a kind of bone called secondary

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  • alessandro
    What was the fastest dinosaur?
    9 years ago

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