|INTRODUCTION: Three-toed footprints, like
those of tyrannosaurs and cranes, are often referred to as "tridactyl" footprints.
Tridactyl dinosaur footprints are among the most common dinosaur footprints found in the
rock record. This is because all carnivorous dinosaurs, whether large like Tyrannosaurus
, or small, had bird-like, three-toed functional feet (the small, backward pointing
fourth toe, did not reach the ground, and left no indication of its presence in footprints,
as you have already discovered). All such dinosaurs, therefore, made tridactyl footprints.
As determined in Lab #4, recognizing the footprints of carnivorous dinosaurs, and those of tyrannosaurs in particular, is based on assessment of footprint size, geologic age, and details of footprint shape. Thus, as we have now seen, footprints of tyrannosaurs are recognized as large tridactyl footprints from rocks of late Cretaceous age (about 68 million years ago), particularly if found in the same beds in which tyrannosaur remains have also been discovered.
TYRANNOSAUR STAND-INS: Undoubted tyrannosaur footprints
are very rare. This is an impediment to our progress because we do not have ready access to
the objects - footprints - which we need to evaluate. But, rather than lament this
situation, and in an effort to advance our analysis, we will modify our approach. What we
can do is examine footprints of a carnivorous dinosaur other than Tyrannosaurus - one
which produced large numbers of footprints. We can use this second type of dinosaur as a
stand-in, or analogue, for Tyrannosaurus. Study of the analogue may help us in
working toward the ultimate goal of estimating tyrannosaur running speed. At the very
least, we should be able to say whether the stand-in could have caught the Jurassic Park
DILOPHOSAURUS: There is, in fact, a type of carnivorous dinosaur that has left us countless footprints. This dinosaur is Dilophosaurus, the same animal whose
||double-crested skull you may have viewed during the AMNH
field trip, especially if you did the supplemental work describe on page 6 of the field trip
guide. Dilophosaurs are of late
Triassic to early Jurassic age (about 185 million years ago), and thus
lived long before tyrannosaurs appeared on the scene. They are smaller than
Tyrannosaurus, have relatively longer arms, and in some ways are more primitive than
tyrannosaurs, but are still good analogues for their more famous relatives - and they have
left us thousands of footprints (a few of which are shown at left).
For movie buffs it might be interesting to note that a highly stylized dilophosaur appeared in Jurassic Park as the frilled, poison spitting dinosaur that attacked Nedry, the devious computer hack, at the bottom of the waterfall. The movie version is rather misleading, however. Real dilophosaurs were larger than depicted in the movie, and show no sign of having possessed a colorful, expandable fold of skin along the neck as does the cinematic dilophosaur. Also, there is no evidence that they could spit poison. They did , however, possess the unusual pair of bony crests along the top of their heads that makes them appear so unusual in the movie. What function these crests served is unclear. Some authorities have suggested that they may have acted as a signalling device (especially if covered by colorful skin) for purposes of species or sexual recognition, or they may have been a heat exchanger to help regulate body temperature.
DILOPHOSAUR TRACKWAYS: A trackway is a series of footprints
made by an animal as it walks or runs. The diagram to the left is a trackway, but the
footprints shown in the picture above are not. They are simply random footprints made by
several different animals moving in different directions (although each footprint may be one
part of a trackway extending off the rock slab). A good trackway is what we need to examine
if we are to obtain a picture of dilophosaur speed.
The dilophosaur trackway we will use as a basis for studying the running speed of dinosaurs is painted on the floor of the corridor outside 4152N, the lab in which we met to view the film clip of Jurassic Park. You may have already noticed the trackway. The corridor trackway was made by placing cardboard sheets on top of real dilophosaur tracks preserved in sandstones originally comprising the soft, wet shoreline sands of a now long-gone, late Triassic lake. The outline of a footprint was traced onto the cardboard, and the cardboard was then cut into the shape of the footprint. The cut-outs were used as templates to paint the trackway on the floor. The trackway reproduced in the hallway faithfully duplicates not only the shape and size of the footprints, but also their relative positions in the trackway. Thus, it records the actual steps made by an individual dinosaur as it moved across a mudflat at the edge of this ancient lake.