Before the planet lost T. rex and other carnivorous giants, mammal-like reptiles began to grow whiskers. The findings, published in Scientific Reports last month, suggest that many mammal-defining features were already present in our distant ancestors 240 million years ago. And these may have contributed to the subsequent evolutionary success of mammals.
Therapsid reptiles are the distant ancestors of mammals, and they walked the Earth long before dinosaurs dominated. One group of dog-like therapsids called probainognathians eventually evolved into mammals as we know them today. But since soft tissue don’t readily fossilize, evidence of hair in the fossil record has been limited to just mammals.
Rather than look for fossil traces of hair with therapsid remains, a University of the Witwatersrand team led by Julien Benoit searched instead for the neural structures that innervated the hairs. By using X-ray micro-CT scanning to image 29 therapsid skulls unearthed in Karoo, South Africa, the researchers reveal that therapsids had likely evolved hair and whiskers. "Whiskers are an amazing sensory tool to have when you are nocturnal and the evolution of whiskers possibly assisted in the survival of the therapsids – and more specifically the probainognathians," Benoit said in a statement.
The team found that the maxillary canal – a bony tube in the snout – is shorter in therapsids than in reptiles. Housed within the maxillary canal is a nerve that provides sensitivity to the animal’s snout, called the trigeminal nerve. A shortened maxillary canal allows for nerve movement as it branches into lip and nose soft tissue. This is how whiskers are innervated. "This leaves the trigeminal nerve free to follow the movements of a flexible snout," Benoit explained. "In reptiles, this canal is long and the nerve is enclosed in the maxilla all along its length, which prevents any movement of the nose and lips."
Previous mouse studies revealed that a gene called MSX2 is involved in the development of mammalian traits including a large cerebellum, mammary glands, and body hairs. Based on this new study, the expression of the MSX2 gene underwent a massive change some 246 to 240 million years ago. This triggered the evolution of many features that define mammals – including a mobile snout. And it coincided with the appearance of prozonstrodontians – a member of the probainognathians and the direct ancestors of mammals we have today. That means traits we associate with mammals were already present in advanced therapsids, before mammals even showed up.
Evolution of the maxillary canal for the trigeminal nerve (green) in Therapsida. Wits University