Therapsid

Therapsida 3

Pristeroognathus DB

Raranimus dashankouensis

Euchambersia DB

Bonacynodon witout visible ears

Biarmosuchus BW

Therapsids are a group of synapsids that includes mammals and their ancestors. Many of the traits today found in mammals originated in therapsids; including traits such as differentiated teeth (incisors, canines, molars) and probably warm-bloodedness. The first therapsids appeared in the Middle Permian period, around 275 million years ago, and thrived until the end of the Permian period, when the Permian-Triassic extinction event occurred, approximately 252 million years ago. This event led to the decline of therapsids during the Triassic period, with the last non-mammalian therapsids disappearing in the early Jurassic.

Evolution and Characteristics

Therapsids evolved from pelycosaurs, specifically from a group called sphenacodonts, which were apex predators during the early Permian. The transition from pelycosaurs to therapsids is marked by significant changes in the skull and postcranial skeleton, which indicate a more active and possibly endothermic (warm-blooded) lifestyle. Therapsids' skulls were larger, with bigger braincases and more differentiated teeth compared to their ancestors. These adaptations likely helped them become more efficient predators and herbivores.

The therapsids are divided into several major groups, including the Dinocephalia, Anomodontia, Therocephalia, and Cynodontia. The Cynodontia, in particular, gave rise to the first mammals in the Late Triassic, around 225 million years ago.

Decline and Extinction

The dominance of therapsids in terrestrial ecosystems came to a halt at the end of the Permian period, with the Permian-Triassic extinction event. This event, the most significant extinction event in Earth's history, wiped out approximately 90% of all species, including many therapsids. The causes of the extinction are still debated, but likely include massive volcanic eruptions, climate change, and changes in ocean chemistry.

Following the extinction, therapsids were largely supplanted by archosaurs, including the ancestors of dinosaurs, crocodiles, and birds, during the Triassic period. However, a branch of therapsids, the cynodonts, survived and eventually gave rise to mammals.

Legacy

Therapsids are crucial for understanding the evolution of mammals and mammalian characteristics. Studies of therapsid fossils have provided insights into the transition from reptile-like reproduction to the complex reproductive systems of mammals, including the development of the mammary glands, live birth, and parental care.

See Also

• Synapsid
• Permian-Triassic extinction event
• Cynodont
• Evolution of mammals