Mammals have been around for a very long time. The first beasts scurried through the Mesozoic world around 220 million years ago, about 23 million years after dinosaurs got their start. And, the story goes, mammals only really got a chance to flourish after an asteroid cleared the non-avian dinosaurs out of the way around 66 million years ago. In the long lead-up to the catastrophic cosmic smack, mammals were supposedly at the mercy of the dinosaurs.
But discoveries over the past two decades have shredded this trope. Mammals in the Age of Dinosaurs were generally small, yes, but they were not all meek little insectivores that shivered at the sound of thunderous footsteps. Among their ranks were the ancient equivalents of beavers, flying squirrels, badgers, and more. In fact, paleontologists just added two more species to the list of furry creatures that thrived during the Mesozoic. One, Agilodocodon scansorius, was a lithe little critter that skittered through the trees, and the other preferred an opposite lifestyle.
From a specimen found in China, University of Chicago paleontologist Zhe-Xi Luo and colleagues have named the 160 million year old creature Docofossor brachydactylus. It wasn’t quite a mammal, but it was close. A member of a larger group called mammaliaforms – from which mammals themselves emerged – Docofossor was part of the great proliferation of mammals and their close relatives during the days of dinosaur dominance, and the creature pushes back the date for when mammaliaforms started burrowing below ground.
Docofossor was the Jurassic equivalent of a mole. The three-and-a-half-inch-long beast had thickly-built, sprawling limbs and short hands tipped with flattened claws, all of which are hallmarks of a mammal that dug around in the dirt This doesn’t mean that Docofossor was a relative or ancestor of modern day moles, but rather than evolution has spun off mole-like forms multiple times since the Jurassic.
And Docofossor wasn’t the Jurassic’s only burrowing beast. A decade ago, Luo and John Wible named Fruitafossor windscheffeli from the 150 million year old rock of Colorado. This creature was actually a mammal – just falling within the boundaries of the family tree – but had also independently evolved a squat posture and digging claws, as well as tubular teeth much like those armadillos use to crunch insects.
This common lifestyle is why Docofossor and Fruitafossor share a suffix. Zoologists and paleontologists refer to burrowing animals as fossorial, and the mechanical requirements of digging often causes distantly-related animals to evolve robust limbs, flattened claws, and other adaptations for delving below ground. And given how popular digging has been as a mammalian occupation since the Jurassic, fuzzy burrowers will undoubtedly keep shifting soil for millions of years to come.
Luo, Z., Meng, Q., Ji, Q., Liu, D., Zhang, Y., Neander, A. 2015. Evolutionary development in basal mammaliaforms as revealed by a docodontan. Science. 347 (6223): 760-764. doi: 10.1126/science.1260880
Luo, Z., Wible, J. 2005. A Late Jurassic digging mammal and early mammalian diversification. Science. 308: 103-107. doi: 10.1126/science.1108875