A three-dimensional record of dinosaur feet and movement comes from 200-million-year-old footprints made in wet mud. Comparisons of these prints with the tracks made by living birds clear up some of the mysteries about dinosaur toes and the tracks that they left.
It was no accident that Sherlock Holmes had the habit of rousing his faithful companion off to another adventure with the exhortation, “Come, Watson! The game is afoot”. Their creator, Arthur Conan Doyle, was fond of footprints — even fossil ones — and he knew how they could be useful forensically1. Alas for the many palaeontologists who, through the years, have looked down their noses at such tracks and traces. For footprints have revealed many secrets of locomotion, ecology and behaviour to those who have been patient and sharp enough to study them2,3,4. And on page 141 of this issue, Stephen M. Gatesy and his colleagues5 bring the science of fossil trackways into the twenty-first century, in ways that Conan Doyle and his creations could never have dreamed.
The story begins as the researchers explored the tree-barren fields of East Greenland. Lured to the Triassic (over 200-million-year-old) exposures near the Fleming Fjord Formation by the prospect of discovering early mammals and their relatives, Gatesy et al. found a surprisingly diverse fossil fauna, the components of which are still being described6. As well as the bones and teeth of various vertebrates, the authors uncovered strange trackways with features that would have been ignored by most workers because they are so indistinct. Instead, Gatesy and colleagues turned the find into a model for future work.
Baird's ‘First Law’ of ichnology — the science of footprints — states that a trackway is not a simple record of anatomy. Instead, it is a record of how a foot behaves under a particular locomotory pattern as it makes contact with a particular substrate7. The varying conditions of the substrate can have a substantial effect on the features of the trackway, as anyone who has walked along a beach, both close to and above the strand line, can tell — and aching calf muscles, after a good walk along the shore, attest to the influence of substrate on locomotion.
The tracks studied by Gatesy and colleagues ranged from clear imprints to virtually indistinct traces, depending on the condition of the substrate. They were made by a theropod (carnivorous) dinosaur in mud that was often so sloppy that there was little chance of preserving precise records of individual joints or skin impressions. But this sloppiness preserved the entry and exit ‘wounds’ made by the foot, which led to an interesting discovery — the deeper you sink, the more of the movement that normally occurs above ground level can take place below it instead.
In theropod dinosaurs, the fifth toe is completely reduced and lost. The first toe (hallux) is short, and it is suspended from the second metatarsal (sole bone), a bit less than halfway up the sole. In those theropods that are closer to birds, the first toe has descended in the course of evolution, eventually hanging from near the end of the sole. For instance, in Archaeopteryx, which is the first known bird, the first toe is fully opposable (that is, it faces the other digits on the same foot), and in later birds the claws enlarge for perching, suggesting the start of true arboreality8. So, the distinction between bird and other dinosaur tracks has sometimes been assessed on the basis of the imprint of the hallux — if it extends backwards and towards the midline, the maker of the track is often regarded as avian9.
In the Greenland tracks, the hallux seems to make such an impression going in — in apparently avian fashion — but on the way out it disappears. To find out why, Gatesy et al.5 sectioned the fossilized footprints and traced the disappearance to the fact that the toes were brought together as the animal lifted its foot. They then ran guineafowl and turkey through successively more sloppy mud to demonstrate that this kinematic pattern is simply inherited by today's birds from their theropod ancestors10. Moreover, the footprints elongate as the mud becomes sloppier. In Mesozoic Era trackways, this feature has sometimes promoted the inference that some dinosaurs were plantigrade (that is, they walked on their soles)2,3. But the sloppiness of the sediment now reveals that, in these dinosaurs, the heel was just carried a bit lower than in birds today (Fig. 1). This finding indicates that the stride was more strongly powered by the femur in basal theropods than in birds, where the femur is more stable and the lower leg and foot provide more of the power thrust.
The results of Gatesy and colleagues' investigation are dramatically shown by computer graphi (Fig. 3 of their paper on page 143), which graft the anatomy of a typical basal theropod foot onto the footfall pattern of living birds, allowing for differences in proportions and kinematics. The conclusions are clear — early Mesozoic theropods walked much, but not exactly, like their living avian descendants. And, more importantly, locomotion and limb function have evolved like any other features10.
Most of the fossil footprint literature documents new tracksites, describes the form and proportions of tracks, and tries to assign such tracks to trackmakers, usually with little in the way of direct anatomical reference2. At a landmark conference11 in 1985, there was consensus that two frontiers should receive renewed attention: kinematic patterns and ‘competency’ of the sediment. Unfortunately, few studies have since done so. But Gatesy et al.5 set the standard for future work, and show just how much we have to gain from such analyses.
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Intra-Trackway Morphological Variations Due to Substrate Consistency: The El Frontal Dinosaur Tracksite (Lower Cretaceous, Spain)
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