What's on the menu today?

IN DAYS gone by, many palaeontologists thought the reason the dinosaurs became extinct was that the big, lumbering reptiles were outcompeted by small, nippy mammals who ate their eggs and generally ran rings around them. This quasi-anthropocentric view, of the inevitable rise of humanity's ancestors, took a knock when closer examination showed that dinosaurs, too, were often nimble and warm-blooded. Then it was found that the extermination was an accident, caused when an asteroid hit the Earth. Until that moment, the dinosaurs had reigned supreme and mammals were just an afterthought.

Just how supreme is suggested by work carried out by Edward Simpson of Kutztown University in Pennsylvania and his colleagues. Dr Simpson's analysis indicates that the relationship between dinosaurs and mammals was actually that of a diner to his lunch.

His study, just published in Geology, is of two burrows fossilised in sandstone near Escalante, Utah. The rock in question is 75m-80m years old: 10m-15m years older than the cataclysm that did for the dinosaurs. One burrow is 22cm (9 inches) long, 5cm wide and includes a 15cm-wide chamber. The other has a funnel-shaped opening that tapers down to a width of 5cm and runs towards what must once have been another opening. If these burrows had been made recently, they would instantly be recognisable as the work of small mammals. And that is exactly what Dr Simpson thinks they are.

Which is interesting, but not extraordinary. Fossil teeth, and the occasional skeleton, show that small mammals were common at this time. That they should have lived in burrows is no surprise. But animals dig burrows for protection. The question is, from what was this protection sought?

The clue which Dr Simpson thinks answers this question is signs of digging near the burrows. When he and his colleagues analysed these scrapings they realised that their shape suggested they had been made by the claws of predatory, feathered dinosaurs related to a well-known species called Velociraptor. The claws of Velociraptor and its kin had hitherto been regarded as weapons to be deployed against beasts of the aggressor's own size—either other Velociraptors, in fights over territory or mates, or large items of prey. Dr Simpson's work suggests the predators were not above going after small prey, too, just as coyotes will dig for prairie dogs.

Dr Simpson and his colleagues reject alternative explanations—that the diggings might have been scrapes created by dinosaurs seeking to mark their territories, or perhaps to give themselves dust baths to help clean their feathers, as their relatives the birds do to this day. The digging, they think, is too deep for either of those explanations to make sense. It is possible that the dinosaurs' intentions were to build nests for their eggs, but the researchers argue that this is unlikely because known nests are of a consistent size that does not match that of these diggings. What is left, then, is the likelihood that the dinosaurs were digging up snacks—hard as that thought may be for those snacks' human descendants to digest.