New Evidence For Hair And Whiskers In Therapsids

Hairs perform a variety of sensory and protective roles in mammals

While not a defining feature of the group, hair plays a vital role in the lives of most mammals alive today. It functions as a means of insulation and camouflage, but in many cases, it has been adapted for additional purposes.

Cats and dogs will raise their hackles in a show of aggression while in hedgehogs, porcupines, echidnas and tenrecs they have been heavily keratinised into defensive spines. Similarly, whiskers are highly derived hairs adapted to sensory function by the placement of nerve endings at their base.

Well preserved fossils of early mammals show the presence of hair. It seemed likely that the reptilian ancestors of the first mammals, the therapsids, also had hair, yet their fossils did not preserve any such structures, leaving its temporal origin unclear. A recent discovery, however, has shown that some therapsids did indeed have hair. In mammals with whiskers, the trigeminal nerve is responsible for sensory transmission. It is housed within a bony tube called the maxillary canal, which is shortened in mammals to allow the end of the trigeminal nerve to innervate the soft tissue of the face.

'This leaves the trigeminal nerve free to follow the movements of a flexible snout,' said Dr Julien Benoit from the University of the Witwatersrand. '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.'

By using a technique based on x-ray micro CT scanning, the researchers found that the maxillary canal in members of the prozostrodont group of therapsids was shorter compared to those of reptiles. Pits on the snout also pointed strongly to the presence of whiskers in the group and therefore hairs from which the whiskers must have evolved. Genetic research by the team showed that the development of body hair is controlled by the gene MSX2. It is also involved, however, in the development of mammary glands. It is effectively the gene that makes us mammals.

CT scans of ancestral therapsids, showing
the shortening of the maxillary canal (green)

The prozostrodonts are the direct ancestors of mammals and are a derived subgroup of the probainognathian clade of cynodont therapsids. In reptiles the parietal foramen in the roof of the skull houses an organ known as the third eye which is photosensitive and involved in Circadian rhythms and thermoreception.

In the probainognathians, the parietal foramen was lost by the ossification of the roof of the skull. This loss also trended with the enlargement of the cerebellum; both are mammalian traits.

Based on the anatomical changes within the prozostrodont probainognathians, the researchers suggest that changes in expression of the MSX2 gene 240 to 246 million years ago triggered the evolution of hair and whiskers, the enlargement of the cerebellum, complete ossification of the skull roof, and the development of the mammary glands - all classic mammalian features.

'Our research has shown that these features of mammals were already present in advanced therapsids, prior to the appearance of mammals,' said Benoit. 'It also has implications for understanding how mammals survived the domination of dinosaurs during the Mesozoic period and the subsequent evolutionary success of mammals.'