World rulers who left big footprints

The most remarkable and fascinating image of ancient tracks I have encountered appeared on an old front cover of Nature. It showed the orderly tread of a pair of bipedal humans discovered at Laetoli in Tanzania.

Sadly, readers of this new guidebook to the plethora of fossil vertebrate tracks of the western United States will look in vain for a detailed description of those famous and intriguing Laetoli traces, although brief comparisons are made with other sites worldwide.

Dinosaurs, of course, have always had a good press: witness the recent discovery of a fossil raptor sitting on a nest of eggs, suggesting a link with birds. Each new finding adds to our sense of wonder at the sheer power and ferocity of these massive creatures - perhaps some reflection of our own darker sides. Footprints and traces seem to highlight poignantly their vulnerability as well as that of other animals past and present.

By far the most fascinating section is the introductory chapter, in which the authors describe how tracks are preserved and conserved. As Martin Lockley says in his preface, "the pace of discovery for trackers", particularly in the past decade has "picked up so rapidly that we are now overwhelmed by new data". He calls this amazing renaissance a "near-overload of opportunity". Lockley and Adrian Hunt claim that the western US - from Texas to California, from Arizona to Washington - is the best area in the world to study fossil prints. And all this thanks to the extensive exposures of the Carboniferous-Recent strata in such classic sites as the Grand Canyon and the Rocky Mountains.

Lockley says that this "phenomenal resurgence" should lead to a better understanding of dinosaur social behaviour, speed estimates, an animal's range, habitat and ecology. Apparently, there are tracks from the "age of amphibians" dating from more than 100 million years before dinosaurs, who lived 350 - 300 million years ago. After the dinosaurs' reign, birds and mammals contributed their own records. It is fascinating to learn how much information about body structure, weight, and species abundance can be gleaned from something that is not really there, but is "permanent".

The authors have made their text accessible to nonspecialists with a free-flowing text unencumbered with offending jargon. And occasionally they seduce more general readers by including some enticingly brief essays in separate boxes, with delightful, eye-catching titles like, "Mama, papa and baby dinosaurs" and "Which came first, the dinosaur or the track". With all the wealth of technical detail and the many figures, however, I would have appreciated even more concessions to the general reader.

Early on, the authors ask one of the most fascinating questions: "How are tracks preserved?" Unlike bones, prints can easily be destroyed by rain or obliterated by a new herd trundling across the plain. The fact is that "track production rates far exceed the production of skeletal remains". Also, "predators do not attack tracks in the way that they consume carcasses".

In the most popular and most potentially valid explanation - the cover-up hypothesis - "tracks are presumed to be laid on a wet surface, such as a mud flat, and then baked by the sun until they become rock hard. The tracks are now resistant to erosion by the next tide or flood, which may also cover them with a new layer of sediment as the water subsides". However, one other explanation that the authors favour involves the phenomenon of "underprints", which are impressions "transmitted to an underlayer beneath the actual sediment surface". As underprints are buried when they are made, this would explain how they can be covered without being vulnerable to erosion.

The bulk of the book, however, is concerned with a detailed review of the major eras involved, which will appeal more to the experts. Also, as these five sections cover other vertebrates like amphibians, reptiles, camels and bear-dogs, they may well appeal more to those whose perspective on dinosaurs has become rather jaded. In all, this is an admirable achievement.

Robbie Vickers is former science books editor, The THES.