74 Scientific American December 1997

Tracking a Dinosaur AttackThe efforts of a sculptor and a paleontologist

reveal details of a 100-million-year-old skirmish

by David A. Thomas and James O. Farlow

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DINOSAUR PREDATOR,

in this case an Acrocanthosaurus,follows in the tracks of its four-

legged herbivorous prey, a huge

Pleurocoelus that probably strayed

from a herd. These giant herbi-

vores roamed the shore using the

same gait as modern four-legged

mammals, allowing the two legs

on one side to move in such quick

succession that both front and

hind feet can be off the ground at

the same moment.

American paleontologists seeking di-

nosaur fossils made some notable

finds in the years just before

World War II. One of the excavations un-

dertaken during this heyday produced no

bones at all, but it nonetheless proved to be

rich in information about dinosaurs. That

dig, along the banks of the Paluxy River in

central Texas, unearthed a dinosaur “track-

way,” a collection of footprints left on an

ancient mudflat. Those fossilized impres-

sions owe their preservation to the sedi-

ments that encased them, hardening and

then eroding 100 million years later to re-

veal the wanderings of dinosaurs in now

solid rock.

Dinosaur trackways of this kind are in-

valuable to paleontologists. Although there

has been a great deal of speculation about

dinosaur behavior, fossilized footprints

provide the only direct evidence of how di-

nosaurs actually moved. By studying exca-

vated trackways, paleontologists have been

able to glean information about dinosaur

gaits and postures. They have used such

fossil footprints to determine how quickly

different species walked and to deduce that

many kinds of dinosaurs traveled in herds.

One particularly intriguing observation from the Paluxy River track-way was made by Roland T. Bird, who discovered this curious set ofdinosaur prints in 1938 and partially excavated them in 1940. Beforecutting huge slabs of this footprint-studded rock to ship back to hisemployer, Barnum Brown of the American Museum of Natural Histo-ry in New York City, Bird mapped and photographed the paralleltracks most thoroughly. He saw immediately that one set of footprints,from a two-footed carnivorous dinosaur (probably an Acrocanthosau-rus, weighing perhaps two or three tons), ran parallel to the trail left byan even larger, four-footed herbivore (most likely a giant Pleurocoelus),which was apparently traveling in a herd. And he later noticed that thecarnivorous dinosaur seemed at one point to have taken a strange skip-ping stride, leaving two consecutive right footprints in the mud.

Bird believed these two sets of footprints with a peculiar hop in themiddle represented the moment the smaller carnivorous dinosaurstruck at its larger herbivorous cousin. Most paleontologists with aninterest in dinosaur tracks initially scoffed at that rather dramatic in-terpretation. But some now think Bird may have been correct. That re-vision in thinking came about because of an unlikely string of eventsthat prompted us to reevaluate this decades-old find.

Digging Up Papers

Six years after Bird’s death, in 1984, Texas Christian University un-dertook to publish Bird’s autobiography, Bones for Barnum Brown.

They contacted one of us (Farlow) to read the manuscript and to act asits scientific editor. And it came as a surprise that Bird’s writing referredto various charts and a film of the excavation that paleontologists hadnot known existed.

Interviews with Bird’s wife and sister revealed that he had stashedaway quite a bit of unpublished information about the Paluxy Rivertrackway. Bird’s nephew soon discovered a canister with the lost filmof the excavation; it was neatly stored in a basement refrigerator. Abox in Bird’s attic provided countless notes, along with some largecharts of the footprints in question. These diagrams were key finds, be-cause the trackway had deteriorated considerably since 1940: not onlywere large pieces no longer in place (Bird sent slabs both to New Yorkand to the Texas Memorial Museum in Austin), but seasonal floodingof the Paluxy River had eroded the rock surface and covered it with amantle of sediment.

Fortunately, Bird’s charts and photographs showed the placement ofeach of the prints in fine detail, enabling Farlow to study the site anew.That work resulted in an extensive report, published in 1987, whichstated that the two parallel tracks represented one dinosaur followingthe other, just as Bird had originally surmised. But the interpretation ofthe strange hop remained open to debate within the community ofprofessional dinosaur paleontologists—prompting a working artist toget involved.

Sculpting Science

In 1983 the city of Albuquerque commissioned one of us (Thomas)to produce a life-size dinosaur statue for the New Mexico Museum

of Natural History and Science, a job that led to many further oppor-tunities to cast dinosaurs in bronze and fiberglass for other museumsaround the world. To give those creations realistic stances, it was nec-essary to investigate how animals moved in general and how dinosaurswalked in particular.

A quick study shows that a four-legged mammal, such as an elephant,routinely steps on its own tracks as it walks, with a rear foot often land-ing in the spot that the corresponding front foot has just vacated. It cando so because both feet on one side of the animal can be off the groundat once. But a modern reptile never places a rear foot exactly where its

Tracking a Dinosaur Attack76 Scientific American December 1997

MODERN MAMMALS, such as these lions andCape buffalo, show how predators often matchstride with their prey before attacking. Soon afterjoining the hunt, the lion in the foreground easilypicks up the cadence of the galloping buffalo: firstthe hind legs of the two animals move forward insynchrony (a), next the front legs (b) and then thehind legs again (c), just before the lion strikes at therear of the buffalo (d).

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front foot has trod. It walks with diagonal legs moving together, sothe rear foot on one side lands before the front foot on that side leavesthe ground.

Interestingly, the tracks of four-legged dinosaurs indicate that thesecreatures must have moved with gaits similar to those of living mam-mals and distinct from those used by most contemporary reptiles. Thisassociation is evident from the record unearthed from the banks ofthe Paluxy River: the four-footed herbivorous dinosaurs left imprintsfrom their rear feet that commonly overlap their front footprints.

Might modern mammals have more to teach about the extinct rep-tiles that once roamed this ancient strand? In an effort to assess Bird’shypothesis, Thomas studied the way mammalian predators hunt. At-tackers typically match the speed and direction of their game beforethey can strike. But often they do more. A carnivore on the attack willusually adjust its stride until it comes into exact rhythm with the run-ning animal that it hopes to bring down.

For example, a lion, cheetah or other swift cat will accelerate untilit has caught up with its target. The predator then changes the lengthof its stride to match that of its prey. Only by keeping pace in this man-ner can the attacker reduce its motion relative to its quarry, which fre-quently is a much bulkier animal galloping furiously in an effort to es-cape. Eliminating relative motion is critical. Otherwise the predatorwould have difficulty striking safely and effectively when, finally, itlunges.

Scientific American December 1997 77

FOSSIL IMPRINTS of the Paluxy River trackway wereunearthed by Roland T. Bird and his co-workers in1940 (left). A large slab taken from the excavation wassent to the American Museum of Natural History inNew York City for exhibit (top), where it can still beseen by visitors. Bird also provided the Texas MemorialMuseum in Austin with a segment of the trackway, onethat contains a print showing a conspicuous drag mark(bottom) made by the four-legged herbivore, perhapsjust after being struck.

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In an informal study of recorded attacks of various African predators (lions, leop-ards, cheetahs and hyenas), there proved to be only a few instances in which these an-imals clearly did not come into rhythm with their prey. Some of these exceptional oc-currences involved an attacker executing a sudden ambush. In such cases, the needfor surprise outweighs the desire to strike more carefully. Other examples in whichthe attacker failed to come into matching cadence were generally restricted to pair-ings of small prey and larger attackers, a combination for which it is neither practicalnor necessary for the predator to harmonize its stride.

Some caution is clearly warranted in using these observations to help understandthe fossilized Paluxy River tracks. After all, modern four-legged mammals are quitedifferent from two- and four-legged dinosaurs. For instance, all the modern predatorsexamined were in a gallop during the hunt, whereas the two-legged carnivorous dino-

Tracking a Dinosaur Attack78 Scientific American December 1997

SLIM

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CARNIVOREFOOTPRINTS

HERBIVOREFOOTPRINTS

RIGHT

FRONT LEFTHIND LEFT

HIND RIGHTFRONT RIGHT

LEFT

PaluxyRiver

Trackway

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BIRD’S CHART shows how the tracks of the two-legged carnivore (red prints) closelyparallel one of several sets left by four-legged herbivores through a broad turn to the left.Both dinosaurs must also have veered right where the excavated tracks end, because theirimprints do not cross the other two sets of tracks. The footprints of the predator and preyalso show a remarkable symmetry for about a dozen paces (yellow lines), where it ap-pears that they moved in synchrony—except for one point where the carnivore apparentlyhopped, leaving no left footprint (red arrow).

CLOSER POSITIONING probablyrequired the carnivore to shift

stride so that it moved its rightleg forward just as the herbi-

vore advanced its left legs.

RECONSTRUCTED ATTACK SEQUENCEsuggests that the carnivore ap-proached in step with his prey.

saur in question was probably in a fast walk or slow run, andthe four-legged herbivorous dinosaur was in a mammallikewalk. Still, it would seem that in the early Cretaceous period,just as today, predators must have learned the advantages ofmatching rhythm with their prey.

100-Million-Year-Old Attack

One might imagine that the two dinosaurs under consid-eration had taken similar routes along what is now the

Paluxy River simply because both were following the ancientshoreline, with their passages separated by many hours oreven days. But detailed analysis of Bird’s chart of the track-way shows that the proximity of the two sets of tracks couldnot have resulted from such happenstance.

Not only do the two trails run parallel, detailed examina-tion reveals that the paths of the two animals wavered slight-ly and in the same fashion. So the subtle movements of oneanimal must have influenced the other. And something that isnot seen at all provides additional evidence that the two setsof tracks were made at the same time. Near the end of the ex-cavated lane, the tracks of the carnivore and herbivore bothhead toward the left. Had either animal continued in that di-rection, its footprints would have crossed into the adjacentsets of tracks. But they do not. Both animals must have turnedright after leaving the area that Bird ultimately excavated. So,in all, the two trajectories make several jags and two broadturns together. These animals were undoubtedly interacting.

It indeed seems most likely that the carnivorous dinosaurwas following the herbivore. The predator probably ap-proached its prey from directly behind, lingering, at first,

some steps back as it measured its quarry. The carnivorecould then have come into rhythm by adjusting the length ofits stride, just as mammals do today. Evidence for this behav-ior comes from a stretch of the trackway where the foot-prints for the two dinosaurs form an amazingly symmetricalarray. For about a dozen steps, the carnivore placed its rightfoot near or into the print made by the left hind foot of theherbivore. This pattern is just what one would expect if thecarnivore was trailing the herbivore as closely as possiblewithout colliding—just a few steps back and slightly off tothe left.

The rest of the scenario suggested by Bird—that the carni-vore actually struck at its prey midway along the trackway—

is more speculative, but there is good reason to believe it tobe true. Hypothesizing an attack of this kind not only ex-plains the uncanny similarity in the spacing of footprints be-tween the two sets of tracks, it also accounts for the missingleft footprint as a hop made by the carnivore, and it eluci-dates one additional piece of the puzzle unearthed more thanhalf a century ago.

Bird’s original observations show that the herbivore’s rightrear foot dragged at one point. This drag mark can be clearlyseen in the slab housed at the Texas Memorial Museum. Itoccurs a few steps ahead of the spot where two consecutiveright carnivore tracks occur. The mark suggests that the car-nivore indeed hopped as it set upon the larger beast walkingslightly ahead, because it makes sense that the animal strug-gling to escape would have faltered just as it was hit.

The drag mark and consecutive right footprints supportthe notion that the carnivore and herbivore moved over thispatch of ground together, separated by only a few steps whenthe carnivore struck. And the location of these features pointsto a possibility that the herbivore attempted to carry out adefensive maneuver. It might have tried to throw its weightinto the attacker just before being hit, exaggerating the de-fensive motion by turning left.

We do not know with certainty that such an attack or de-fense actually happened, how many carnivores joined thechase or why this particular herbivore was chosen to beculled from the herd. Too much of the record of this intrigu-ing episode remains buried underground. But it now appearsperfectly clear that about 100 million years ago, on a limeymudflat in what is now Texas, at least one swift carnivoresingled out and possibly attacked a huge, lumbering herbi-vore. It seems that Bird was not only lucky enough to find re-markable evidence of this incident of natural history but thathe was also wise enough to recognize, document and exca-vate part of the record of this ancient hunt left on a soddenplain, now turned to stone.

Tracking a Dinosaur Attack

The Authors

DAVID A. THOMAS and JAMES O. FARLOW came to examine these dinosaur tracksafter following very different career paths. Thomas, a sculptor, cast the world’s first life-sizebronze statue of a dinosaur in 1985 for the New Mexico Museum of Natural History andScience in Albuquerque. Some of Thomas’s other dinosaur creations are displayed at theAnniston Museum of Natural History in Alabama, the College of Eastern Utah Museumand the National Museum of Natural Science of Taiwan. His desire to make these statuestechnically accurate led him into the study of animal gaits and tracks. Farlow, a paleontolo-gist, received a doctorate from Yale University in 1980. He is now a professor of geology atIndiana-Purdue University at Fort Wayne, where he does research on dinosaur footprintsand biology and on Ice Age mammals of the midwestern U.S.

Further Reading

Lower Cretaceous Dinosaur Tracks,Paluxy River Valley, Texas. James O.Farlow. South Central G.S.A., Baylor Uni-versity, 1987.

The Dinosaurs of Dinosaur ValleyState Park. James O. Farlow. Texas Parksand Wildlife Press, 1993.

The Complete Dinosaur. Edited by JamesO. Farlow and M. K. Brett-Surman. Indi-ana University Press, 1997.

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APPARENT HOP (two consecutive right footprints) maymark the point on the trackway where the carnivorefirst struck at its prey.

Scientific American December 1997 79