Mounting evidence suggests dinosaurs preyed upon our mammalian ancestors (Source: brotherxii/Flickr)
Fossilised mammal burrows that appear to have been clawed out by a predator suggests dinosaurs dug into mammal dens to get furry morsels.
Making the connection even stronger is that claw marks in the burrows are a pretty good match to the claws of dinosaur fossils found in rocks nearby, though slightly later in time.
"It's pretty tight," says palaeontologist Professor Edward Simpson of Kutztown University of Pennsylvania. "We can't say whether it's a troodont or a velociraptor," because the claw bones of those found nearby have lost their nails, or cuticles. But otherwise the match is a good one, he says.
Simpson and his students have published their 'trace fossil' discovery - that is, fossilised evidence of animal behaviours rather than the animals themselves - in the August issue of the journal Geology.
"To me there is almost no doubt," says trace fossil expert Professor Anthony Martin of Emory University. "It's very good circumstantial evidence."
No actual mammal bones or teeth have been found, though the burrows match the complexity of those of other mouse- or shrew-like mammals of that time and their present-day counterparts.
The criteria for calling something a fossilised mammal burrow were laid out a few years ago in a separate paper by Martin.
Other evidenceWith that criteria in mind, the team feels confident that they did find a mammal's subterranean abode. The researchers could go even further to make their case.
"One of the things we could do is to take the burrows and cut them up," says Simpson. There might be mammal teeth in them, which make an even stronger case.
But, he's hesitant to do that since it's a destructive process. There is also only a very small chance they will find any mammal bones or teeth.
So far Simpson and his students haven't even extracted the fossil burrow from the cliff in southern Utah in which it was found.
As for the dinosaur involved, it probably wasn't very big, judging by the claw marks in the burrows, says Martin.
Other evidence that has been reported to support the dinosaurs-bites-mammal story include the specialised jaw, teeth and skulls of some dinosaurs; mammal bones with bite marks; fossilised gut content; and faeces and trackways.
This case is different in that it points directly at how the dinosaurs hunted rather than just the fact that they ate mammals.
"This is an excellent example of how trace fossils can reveal animal behaviour," says Martin.