When the dinosaurs dominated the earth, most of our mammalian ancestors were barely larger than a rat. On the other hand, they showed an even greater variety in their way of life and menu. A fossil discovery in China now testifies to this: paleontologists have discovered the bite marks of a small prehistoric mammal on the bone of a 20-meter-long dinosaur. Apparently the animal nibbled on the carcass of the giant lizard around 160 million years ago. His tooth prints are the oldest evidence of our distant ancestors being scavenged – and the oldest bite marks left by a mammal on a dinosaur bone.
As successful and diverse as mammals are today, their beginnings were rather modest. “The early mammals lived in the shadow of the dinosaurs for more than 160 million years,” explains first author Felix Augustin from the University of Tübingen. “During this time, they remained small and inconspicuous and only weighed around a hundred grams on average.” But fossil finds show that these early mammals had already developed an astonishing biodiversity and occupied many very different ecological niches. In addition to the numerous soil dwellers, there were also semi-aquatic, tree-dwelling, digging and even sliding ways of life. The diet of these first mammals was probably similarly varied. Paleontologists believe that they ranged from herbivores to omnivores and insectivores to small predators and scavengers. So far, however, there is hardly any fossil evidence for this.
Tiny bite marks on the fossil bone
A discovery that Augustin and his colleagues made in northwest China is all the more exciting. As part of a Sino-German expedition in 2000, they found and recovered numerous vertebrate fossils from the Jurassic era. In addition to the bones of large dinosaurs, turtles and crocodiles, these included numerous relics of small mammals from 160 million years ago. When the scientists examined some of these fossil bones more closely, they noticed tiny gnaw marks on one of the bone fragments. These tracks consisted of two roughly parallel rows of flat, elongated pits in the bone. To find out who might have left these marks, they compared their shape and size to those of various smaller animals, including both mammals and reptiles.
Based on these comparisons, the paleontologists come to the conclusion that the bite marks must have come from a small mammal: “The gnaw marks were very similar to those of today’s insect-eating mammals, such as shrews,” says Augustin. According to this, a small prehistoric mammal must have nibbled on this bone 160 million years ago. This makes this find a real rarity: “Direct evidence such as bite marks on bones or stomach contents are very rare,” says Augustin. “In addition, all previously known evidence came from the Cretaceous period at the earliest; they are no more than about 100 million years old. This is why our discovery from around 160 million years ago is so special. “
Oldest evidence of scavenging
The piece of bone that the small prehistoric mammal nibbled on at the time comes from the neck rib of a sauropod – a herbivorous dinosaur that is probably around 20 meters long and weighs several tons. In view of the enormous differences in size between the two animals, it seems clear that the mammal probably did not chew on this giant lizard during its lifetime. Instead, the paleontologists assume that the bite marks appeared only after the death of the dinosaur. “The bite marks clearly represent scavenging behavior,” said the researchers. They assume that the mammal fed on the scraps of meat that larger predators left on the carcass. The bite marks on the bones arose rather by chance – but now, 160 million years later, they have provided valuable insights into the way of life of our distant ancestors.
“The mammal bite marks we have described represent the oldest direct evidence of a carnivorous diet in early mammals,” say Augustin and his colleagues. “They also represent the oldest evidence of scavenging in this group of animals.”
Source: Felix Augustin (University of Tübingen) et al., The Science of Nature, doi: 10.1007 / s00114-020-01688-9