History Plants and Animals

Therapsids Developed Hair in Order to Survive Millions of Years Ago

The late Permian therapsid Lycaenops (Image credit: Dmitry Bogdanov)
The late Permian therapsid Lycaenops (Image credit: Dmitry Bogdanov)

It is hard to deny that T. Rex was the most ferocious creature in the jungle, but something as seemingly insignificant as being able to grow hair may have helped mammal-like reptiles to outlive such large beasts.

Dr. Julien Benoit and his colleagues from the University of the Witwatersrand, Professors Paul Manger and Bruce Rubidge scanned the fossil remains of mammal-like reptiles from the Karoo of South Africa. They discovered that these particular reptiles, known as therapsids, may have evolved hair as well as the use of whiskers as a sensory tool in order to make their way around at night long before the Mesozoic age when dinosaurs became the dominant terrestrial animals.

Benoit says whiskers are an amazing sensory tool to have when you are nocturnal and the evolution of whiskers is probably what helped the therapsids to survive. Probainognathians probably lived on thanks to the same evolution, which eventually evolved into the mammals we see around us today.

Thrinaxodon
The maxillary canal for the trigeminal nerve (in green, right picture) and the correlated pits on the snout (left) in a Therapsida Thrinaxodon. (Image credit: Wits University)

The Karoo rocks were deposited over a long period from 300 to 180 million years ago. These rocks are internationally renowned for their wealth of fossils, especially of therapsid reptiles which are the distant ancestors of mammals. The therapsids roamed planet Earth and dominated terrestrial ecosystems way before dinosaurs and are known from all continents in the world, but South Africa has by far the most diverse and long-ranging record of this very important group of animals. Thousands of therapsid fossils are curated in South African palaeontological research institutions. While their fossil record is quite extensive, to date there has not been any therapsid fossils found that contained evidence of hair, leaving the fossil record of hair limited to mammals. Instead of looking for fossilized hair in therapsids, Benoit and his team used scanning technology in order to search for the neural structures that would be present in creatures who indeed had hair.

Techniques based on computerized X-ray micro-tomography (CT scan) and digital three-dimensional modeling were used to scan ancient remains. After in-depth study, Benoit uncovered that the maxillary canal of therapsids (which is a bony tube within the snout of the animal that holds the trigeminal nerve) was shorter in therapsids than in reptiles. The trigeminal nerve is a nerve the offers sensitivity to the snout of the animal, and since the maxillary canal was smaller, it offered more movement of the nerve as it brushed into the soft tissue of the lip and nose where it would connect to whiskers.

According to Benoit, this leaves the trigeminal nerve free to follow the movements of the flexible snout. In reptiles this canal is long and the nerve is enclosing in the maxilla all along its length, which prevents any movement of the nose and lips. The research suggests that the mammal-like snout evolved around 240 to 246 million years ago with the appearance of a group of therapsids called Prozostrodontia, which are directly related to mammals.

The prozonstrodontians form part of the larger group of therapsids, known as the probainognathians. This is a group of dog-like therapsids that evolved a large cerebellum and lost the parietal foramen for the first eye, which is an organ on the skull roof that detects light for monitoring thermoregulation and daily rhythms.

Research done on mutant mice shows that these two mammalian features are controlled by the same gene, MSX2, which is also what controls mammary gland development and body hair maintenance. Basically, this is the gene that makes a mammal, a mammal.

Based on the CT based anatomical observations in probainognathians, it seems that the MSX2 gene went through some very significant changes around 240 million years ago. This triggered the evolution of many typical mammalian traits, such as hair, whiskers, larger cerebellum, complete ossification of the skull roof and the mammary glands, all which define mammals today.

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

The full study is published in Scientific Reports journal.