People talk of the first mammals scuttling around at the feet of giant dinosaurs but we seldom get as literal an example as a mammalian skull being found beneath the foot bones of a giant dinosaur. No doubt the enormous skeleton will expand our understanding of dinosaur evolution, but it is the small skull beneath that could rewrite our theories on both the spread of mammals and movement of continents.
The bones came from a 124-139-million-year-old stratum in Utah, exactly the sort of place you'd expect to find dinosaur remains. So Dr Adam Huttenlocker of the University of Southern California, lead author of a paper in Nature on the find, was much more surprised to conclude the small skull accompanying the dinosaur bones belonged to a haramiyidan.
Some palaeontologists think these rabbit-sized creatures count as early mammals, while others consider them transitional between reptiles and mammals. The skull is from a hahnodontidid and is so exceptionally preserved it confirms the theory, described in the paper as previously “tenuous”, that the hahnodontidids were a subgroup of harmiyidans.
Early harmiyidan fossils have been found in what is now Europe and Asia. A North American find indicates migration between the two continents was still possible around 130 million years ago. It had been thought the break-up of the supercontinent Pangea had made such a journey impossible.
A single find might not be so convincing – it could be a legacy of Hahnodontidae evolving earlier than we thought and a migration while Pangea was still together. "But it's not just this group of haramiyidans," Huttenlocker said in a statement. "The connection we discovered mirrors others recognized as recently as this year based on similar Cretaceous dinosaur fossils found in Africa and Europe."
Huttenlocker and co-authors named the fossil Cifelliodon wahkarmoosuch, the genus name after palaeontologist Richard Cifelli and the species meaning "yellow cat" in the Ute language of the region's first people.
Although we have many teeth and even jaws from Cretaceous mammals in this region of western North America, an intact skull is a more exciting find, even aside from the surprising family from which it appears to come. Cifelliodon was small by today's standards, let alone those of the dinosaurs it evaded, but large compared to the other known mammals of the area, which were mostly closer in size to mice and shrews.
“Even before the rise of modern mammals, ancient relatives of mammals were exploring specialty niches: insectivores, herbivores, carnivores, swimmers, gliders. Basically, they were occupying a variety of niches that we see them occupy today," Huttenlocker said.
Cifelliodon had small eyes and enormous olfactory bulbs, consistent with being nocturnal and surviving largely on its sense of smell. Its teeth resembled those of modern fruit-eating bats.