Giant Mesozoic badger turned mammalian dogma on its head
October 5, 2010 Leave a comment
Check your biology book. If it says anything about mammals during the age of dinosaurs, it probably depicts the mammals as small, shrew-like animals scuttering around at night, barely scratching out a living as they scurry away from the thudding feet of a Tyrannosaurus rex.
Mammals ate dinosaurs–tasted like chicken
Banish the thought and rewrite the book. Yes, many of the mammals that lived in the Mesozoic—from about 248 to 65 million years ago—were shrew- or rat-like critters that probably stayed out of the way of most dinosaurs. But recent fossil finds demonstrate that some of the mammals in the age of the dinosaur not only got in the path of dinosaurs, they ate them.
In China, there is a famous fossil bed best known for the fossils of feathered dinosaurs it has yielded. But paleontologists have also turned up some other intriguing remnants, among them the mineralized bones of species from the Repenomamus genus. These animals were long, squat-bodied creatures with strong jaws and very sharp, pointy teeth. Researchers at the site had already reported finding R. robustus, a carnivorous mammal weighing in at about 15 pounds.
But two other finds reported in Nature flip common Mesozoic mammal dogma upside down. The first discovery was that of a fossil species now dubbed Repenomamus giganticus, a cousin of R. robustus, but with some distinctive features: this specimen probably weighed about 30 pounds and grew to be up to a meter long. Think about a mid-sized dog, say a large basset hound, with a badger-like face and rodent-like sharp teeth, and you’ve got your R. giganticus. Not something you’d want to go hand-to-tooth with when it’s in a bad mood.
Died with dinner inside (& more dog breed comparisons)
That, at least, is what researchers concluded after their second find: a fossil of R. robustus, the smaller species, with a juvenile dinosaur skeleton where the R. robustus stomach would have been. Not only did these hardy Repenomamus species look scary, for juvenile, leaf-eating dinosaurs, they were deadly. Experts estimate, based on mammalian habits of today, that mammals can kill and consume prey that is up to half of their body weight. If R. robustus could snack on a 5-inch dinosaur baby, then presumably R. giganticus could have put back a dinosaur the size of a dachshund.
The scientists who identified and named R. giganticus had a couple of hurdles to overcome. First, they had to determine that this was a genuine average version of R. giganticus, not simply R. robustus with a pituitary problem. The error that would result would be akin to finding the skeleton of the world’s tallest man and assuming that it represented our entire species.
Badger or human, your teeth show your age
But they looked at the teeth that accompanied the skull and jaw fossils, and the molars held the clues to the animal’s age at death. The last molar of the lower jaw appeared to have just erupted when the animal died, and it had little wear. Based on this clue, the researchers concluded that fossilized remains were from a juvenile representative of the new R. giganticus species.
Making the case that a Mesozoic mammal had actually consumed a dinosaur also required some consideration and discarding of various possibilities. The little dinosaur skeleton, from a Psittacosaurus, was a small patch of bones within the ribcage of some R. robustus fossil remains. The bones were located right where the stomach is on today’s mammals, and appeared to have been broken, torn apart, and displaced from one another. The fossil bones of the accompanying R. robustus skeleton were not in this condition. The Psittacosaurus specimen also had teeth, most of which were worn, implying that this animal was not scavenged from an egg as an embryo. Based on these clues, the researchers concluded that this R. robustus had caught and eaten the hapless Psittacosaurus—dismembering it and swallowing it in chunks—shortly before meeting its own death.