Dinosaur ichnology

Some aspects of dinosaur research have an almost sleuth-like
quality to them, perhaps none more so than ichnology – the study
of footprints.

There is no branch of detective science which is so important and so
much neglected as the art of tracing footsteps.
(Conan Doyle, The Study in Scarlet, 1891)
The study of dinosaur footprints has a surprisingly long history.
Some of the first to be collected and exhibited were found in
1802 in Massachusetts by the young Pliny Moody while
ploughing a field. These and other large three-toed prints were
eventually illustrated and described by Edward Hitchcock in 1836
as the tracks left by gigantic birds; some can still be seen in the
Pratt Museum of Amherst College. From the mid-19th century
onwards, tracks were discovered at fairly regular intervals in
various parts of the world. With the development of an
understanding of the anatomy of dinosaurs, and most particularly
the shape of their feet, it was realized that the large ‘bird-like’
three-toed prints that were found in Mesozoic rocks belonged to
dinosaurs rather than giant birds. Such tracks, though of local
interest, were rarely regarded as of great scientific value.
However, in recent years, largely prompted by the work of Martin
Lockley of the University of Colorado at Denver, it has begun to be
appreciated more widely that tracks may provide a great deal of
information.

First, and most obviously, preserved tracks record the activities of
living dinosaurs. Individual prints also record the overall shape of
the foot and the number of toes, which can often help to narrow
down the likely trackmaker, especially if dinosaur skeletons have
been discovered in similarly aged rocks nearby. While individual
prints may be intrinsically interesting, a series of tracks provides a
record of how the creature was actually moving. They reveal the
orientation of the feet as they contact the ground, the length of the
stride, the width of the track (how closely the right and left feet
were spaced); from this evidence, it is possible to reconstruct how
the legs moved in a mechanical sense. Furthermore, taking
observations using data from a wide range of living animals it has
also proved possible to calculate the speeds at which animals
leaving tracks were moving. These estimates are arrived at by
simply measuring the size of the prints and length of each stride and
making an estimate of the length of the leg. Although the latter
might seem at first sight difficult to estimate with great accuracy,
the actual size of the footprints has proved to be a remarkably good
guide (judging by living animals), and in some instances foot and
leg bones or skeletons of dinosaurs that lived at the time the tracks
were made are known.

The study of tracks can also reveal information about dinosaur
behaviour. On rare occasions, multiple tracks of dinosaurs have
been discovered. One famous example, recorded in the Paluxy River
at Glen Rose in Texas, was revealed by a famous dinosaur footprint
explorer named Roland T. Bird. Two parallel tracks were found at
this site, one made by a huge brontosaur and the other by a large
carnivorous dinosaur. The tracks seemed to show the big carnivore
tracks converging on the brontosaur. At the intersection of the
tracks, one print is missing, and Bird suspected that this indicated
the point of attack. However, Lockley was able to show from maps
of the track site that the brontosaurs (there were several) continued
walking beyond the supposed point of attack; and, even though the
large theropod was following the brontosaur (some of its prints
overlap those of the brontosaur), there is no sign of a ‘scuffle’. Very
probably this predator was simply tracking potential prey animals
by following at a safe distance. More convincing were some tracks
observed by Bird at Davenport Ranch, also in Texas. Here he was
able to log the tracks of 23 brontosaur-like sauropods walking in the
same direction at the same time (Figure 35). This suggested very
strongly that some dinosaurs moved around in herds. Herding or
gregarious behaviour is impossible to deduce from skeletons, but
tracks provide direct evidence.

Increased interest in dinosaur tracks in recent years has brought to
light a number of potentially interesting avenues of research.
Dinosaur tracks have sometimes been found in areas that have not
yielded skeletal remains of dinosaurs, so tracks can help to fill in
particular gaps in the known fossil record of dinosaurs. Interesting
geological concepts have also emerged from a consideration of
dinosaur track properties. Some of the large sauropodomorph
dinosaurs (the brontosaurs referred to above) may have weighed as
much as 20–40 tonnes in life. These animals would have exerted
enormous forces on the ground when they walked. On soft
substrate, the pressure from the feet of such dinosaurs would have
distorted the earth at a depth of a metre or more beneath the
surface – creating a series of ‘underprints’ formed as echoes of the
original footprint on the surface. The spectre of ‘underprints’ means
that some dinosaur tracks might be considerably over-represented
in the fossil record if a single print can be replicated through
numerous ‘underprints’.

If herds of such enormous creatures trampled over areas, as they
certainly did at Davenport Ranch, then they also had the capacity to
greatly disturb the earth beneath – pounding it up and destroying
its normal sedimentary structure. This relatively recently
recognized phenomenon has been named ‘dinoturbation’.
‘Dinoturbation’ might be a geological phenomenon, but it hints at
another distinctly biological effect linked to dinosaur activities that
may or may not be measurable over time. That is the potential
evolutionary and ecological impact of dinosaurs on terrestrial
communities at large. Great herds of multitonne dinosaurs moving
across a landscape had the potential to utterly devastate the local
ecology. We are aware that elephants today are capable of causing
considerable damage to the African savannah because of the way
that they can tear up and knock down mature trees. What might a
herd of 40-tonne brontosaurs have done? And did this type of
destructive activity have an effect upon the other animals and plants
living at the time; can we identify or measure such impacts in the
long term, and were they important in the evolutionary history of
the Mesozoic?