Theropsida

Therapsida is a group of synapsids that includes mammals and their ancestors. Many of the traits today found in mammals originated within this group. Therapsids first appeared in the Permian period, about 275 million years ago, and were the dominant land animals by the end of the Permian. However, their numbers were greatly reduced by the Permian-Triassic extinction event. During the subsequent Triassic period, therapsids were supplanted by archosaurs (ancestors to crocodiles, birds, and dinosaurs) in most terrestrial ecosystems, but one group of therapsids, the cynodonts, survived through the Triassic and eventually gave rise to mammals in the Jurassic period.

Evolution and Characteristics

Therapsids evolved from pelycosaurs, specifically from a group called Sphenacodontidae, which also included the famous Dimetrodon. Early therapsids were carnivorous, but the group quickly diversified to include herbivores and omnivores. This diversification was accompanied by significant changes in body form, including the development of a more upright limb posture, which allowed for more efficient locomotion and breathing.

One of the defining characteristics of therapsids is the structure of their skulls. Unlike their pelycosaur ancestors, therapsids have a single temporal fenestra (an opening in the skull behind the eye) that is larger and positioned more dorsally. This feature is associated with the expansion of jaw muscles, allowing for a more powerful bite. Other mammalian features that appeared in therapsids include differentiated teeth (incisors, canines, and molars) and a secondary palate, which allows for breathing while chewing.

Classification

Therapsida is divided into several major groups, including the Biarmosuchia, the earliest and most primitive therapsids; the Dinocephalia, large herbivores and carnivores that were among the dominant terrestrial vertebrates of the Middle Permian; the Anomodontia, which includes the Dicynodontia, herbivores that survived the Permian-Triassic extinction and were common in the Triassic; the Therocephalia, carnivorous therapsids that were especially diverse in the Late Permian; and the Cynodontia, which includes both carnivorous and herbivorous species and is the group from which mammals eventually evolved.

Extinction and Legacy

The majority of therapsid groups did not survive the Permian-Triassic extinction event, the largest mass extinction in Earth's history. The causes of this extinction are still debated, but it likely involved massive volcanic eruptions, climate change, and changes in ocean chemistry. The cynodonts, however, managed to survive and continued to evolve throughout the Triassic, gradually developing more mammal-like features. By the end of the Triassic, the first true mammals appeared, marking the beginning of mammalian dominance in the Mesozoic era.

In Popular Culture

Therapsids, particularly the more mammal-like forms such as Dimetrodon (which, while often associated with dinosaurs, is actually a pelycosaur, not a therapsid) and Dicynodonts, have made appearances in various forms of popular culture, including books, television shows, and movies. They are often depicted as strange, prehistoric creatures that provide a glimpse into the diverse life forms that existed before the age of dinosaurs.