FOSSILS ET AL.
Trials and Triumphs of Our Ancient Relatives
The story of Thrinaxodon and how its study reveals a lot about the past and present of the mammalian world
250 million years ago, Earth was a vastly different place compared to its current state. It had already seen the horrors of three mass extinctions, the most recent being the end-Permian mass extinction that almost wiped out everything that breathed oxygen on Earth. What was left behind was the largest piece of land ever assembled, called Pangea. The inner parts of Pangea were hot and arid, particularly around the equator. Only those plants that could adapt to these conditions through deep roots and smaller leaves, like the conifers, could survive in these places. Life was tough for flora and fauna, and these tough times called for tougher survivors, those who could power through these difficulties and contribute to the evolution of mammals — and this led to the evolution of Thrinaxodons.
A detailed description of the end -Permian mass extinction can be found in the link below.
Discovery
Thrinaxodon was one of the cynodonts that emerged during the Early Triassic period. Its fossils were originally discovered in the Karoo basin of South Africa. They were described by Harry Govier Seeley, who made a separate genus for them and called them Thrinaxodon liorhinus. Thrinaxodon literally translates to ‘trident tooth’, which highlights the presence of trident shaped cusps on its teeth, that were unique in all the cynodont fossils that were found until then. Its specific name, liorhinus translates to ‘smooth nose’.
During the second half of the 20th century, its fossils were also discovered in Antarctica, suggesting that it was spread across the Pangean landscape instead of just being limited to the modern-day Karoo desert.
Lifestyle
Thrinaxodon was a small carnivore, no bigger than a fox. Unlike its diapsid cousins, its limbs were placed directly below its body. They were mostly straight, allowing for better movement than reptiles, who had to wriggle their bodies sideways, forcing them to compress one lung and expand the other one in the process, passing stale air between lungs instead of exhaling it out completely. Its body seems to have been covered in hair, which indicates that cynodonts had begun to develop warm-blooded bodies, as hair prevents the loss of body heat by trapping it. This adaptation allowed Thrinaxodon to become nocturnal, hunting at night and staying hidden in burrows during the day to avoid larger predators.
Beneath a night sky that would have hosted unfamiliar constellations to the human eyes, this creature hunted small herbivores and insects. Its jaws were powerful, a trait it inherited from the pelycosaurs, enabling stronger bites to crush the tough armors of insects. It had also developed teeth similar to those of mammals, which were used to chew food, making it one of the earliest animals to begin digestion in the mouth. Its body also developed a hard roof of the mouth, differentiating its mouth from the nasal passage and allowing it to breathe while eating, a privilege not extended to the reptilian ancestors.
Research indicates that Thrinaxodon likely exhibited minimal parental care for its offspring, as suggested by the very low survival rate of young ones after birth. Steve Brusatte, in his book ‘The Rise and Reign of Mammals’ suggests that Thrinaxodon may have had very short lifespans, probably being born, reaching adulthood, mating and dying within one year. This hypothesis is supported by the random patterns of collagen in its bones, indicating rapid growth.
Significance
Although Thrinaxodon disappeared from Earth during the late Triassic period about 200 million years ago, its fossils have proven to be a window for paleontologists to look into the past. The studies conducted on the fossils of Thrinaxodon have revealed remarkable developments in cynodonts, many of which were polished and incorporated by mammals. For example, the burrowing habits are still used by small mammals like rabbits to hide from predators and provide a safe nursery for their young ones.
It is hypothesized that Thrinaxodon and other nocturnal cynodonts developed excellent senses of smell and hearing, as they had to depend on these to hunt and avoid predators in the dark. Their eyes were also better adapted to see in the dark, which left little room for the development of eyes to see in broad daylight. We see all these features in today’s mammals (except humans) who roam in the wild. Even in our case, the pupils in the eyes dilate involuntarily in the dark to allow more light to enter the eyes, which is why we can see better after staying in the dark for a few minutes.
By studying the fossils of cynodonts before and after the end-Permian mass extinction, paleontologists found that the size of the cynodonts reduced after the event. This was called the ‘Lilliput effect’. Though the name may sound hilarious, this phenomenon of natural selection allowed cynodonts to outlive the gorgonopsians and dicynodonts during the extinction. This reduction in size allowed them to reduce predation risk, reduce the amount of resources needed to survive, and increase the reproduction rates.
Thrinaxodon not only survived but thrived during the Triassic period. Along with its other cynodont relatives, its fossils provide valuable insight into the early evolution of mammals and their emergence as a distinct branch in the tree of life.
Published in Fossils et al. Follow to learn more about Paleontology.
