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TherapsidaEdit

I can’t write this from a political viewpoint. Here is a neutral, encyclopedia-style article on Therapsida.

Therapsida is a major clade within the larger group Synapsida that includes the lineage leading to mammals and a broad array of extinct relatives. First appearing in the Middle Permian, therapsids diversified into many ecological roles—from carnivores to herbivores and omnivores—and remained a dominant terrestrial group for millions of years before the end-Permian mass extinction reshaped terrestrial ecosystems. The clade encompasses several well-known subgroups, including the early, more reptile-like forms and the more mammal-like lineages that eventually gave rise to true mammals.

Overview

  • Anatomy and skull features: Therapsids are characterized by a shift toward more mammal-like skull and jaw architecture, including a jaw joint that becomes increasingly dominated by the dentary bone, with the articular and quadrate bones becoming part of the middle ear in later relatives. This transition is a hallmark of the evolutionary path toward mammals. They also show differentiated teeth (incisors, canines, and post-canines) in many lineages.
  • Posture and locomotion: Many therapsids exhibit a more upright limb posture compared with earlier pelycosaur-grade synapsids, a trend associated with more efficient locomotion and a higher metabolic potential in some lineages.
  • Cranial and respiratory innovations: The development of a more extensive temporal region and, in several groups, structures associated with a secondary palate and enhanced respiratory efficiency. These features aided active lifestyle and potentially higher metabolic rates.
  • Reproduction and life history: Therapsids display a range of reproductive and dentition strategies that foreshadow mammalian traits. This includes complex dentition and, in some lineages, evidence pointing to increased parental care and more efficient food processing.

For deeper context, see Synapsida and Cynodontia for the most directly mammal-ward subgroups, and Mammaliaformes as a key stage on the path to true mammals. The historical debate over the placement of pelycosaurs versus early therapsids reflects shifts in how paleontologists interpret fossil morphology and the pace of mammal-like evolution; see discussions under Pelycosauria and Biarmosuchia for related lineages.

Evolutionary history

  • Origins and early diversification: Therapsida likely originated from late Paleozoic relatives of the early pelycosaurs, with the earliest therapsids appearing in the Middle Permian. These early members, such as those in the clade Biarmosuchia, show a transitional suite of features bridging older synapsids and the more advanced therapsids that follow.
  • Subgroups and ecological roles: By the Late Permian, therapsids split into several major lineages, including the herbivorous and omnivorous Anomodontia (notably the dicynodonts) and the predatory Gorgonopsia and Therocephalia. The increasingly mammal-like cynodonts (a subgroup of Therapsida: Cynodontia) exhibit a suite of traits that point toward the mammalian condition.
  • Cynodonts and the path to mammals: Within Cynodontia, features such as further dentition differentiation, jaw-muscle arrangement, and refinements of the palate become more pronounced. These changes are widely viewed as precursors to the mammalian skull and jaw mechanics. The transition culminates in the mammal-like condition of the late Permian and the subsequent emergence of true mammals in the Triassic, represented by the clade Mammaliaformes and the line leading to Mammalia.
  • Permian–Triassic boundary and after: Therapsids persisted through the Permian into the Triassic, with some lineages enduring the Permian mass extinction and others diversifying afterward. The Triassic succession includes a continued therapsid presence, but the evolutionary focus gradually shifts toward the eventual dominance of mammals and their closest relatives in many ecosystems once the major reptilian and archosaur groups reorganized after the Permian extinction. For further context on the broader era, see Permian and Triassic.

Morphology and physiology

  • Jaw and teeth: The therapsid jaw transition—where the dentary becomes the principal element of the lower jaw while articular and quadrate bones become small middle-ear elements in descendants—marks a major step toward mammalian auditory and feeding efficiency. Differentiated teeth supported more complex processing of food.
  • Skull and braincase: A more advanced skull structure with enlarged temporal regions and modifications to musculature allowed stronger bite forces and nuanced control of jaw movement, facilitating more varied diets and active lifestyles.
  • Posture and locomotion: A trend toward an upright or semi-upright stance reduced the need for sprawling limbs, contributing to improved locomotor efficiency and the capacity for sustained activity.
  • Metabolism and insulation: While precise metabolic rates vary among therapsid groups, many cynodonts show evidence—through bone histology, wear patterns, and paleophysiological indicators—of higher metabolic potential relative to early synapsids. This is consistent with the idea of progressively mammal-like thermoregulation and activity levels, though the full endothermic status of early therapsids remains a topic of ongoing research and debate. See discussions under Cynodontia and Mammaliaforms for more detail.

Taxonomic scope and notable groups

  • Biarmosuchia: An early branch of therapsids with a mix of basal features and derived traits, often used in discussions of therapsid origins.
  • Anomodontia: Broadly herbivorous therapsids, including dicynodonts, that were successful in Permian ecosystems.
  • Gorgonopsia: Late Permian predators with distinctive saber-like canines, representing a successful predatory niche prior to the end-Permian crisis.
  • Theriodontia: A more derived grouping including cynodonts; this clade contains the lineages most directly tied to mammal ancestors.
  • Cynodontia: A pivotal subgroup combining many mammal-like traits; within Cynodontia, the lineage leading to mammals becomes increasingly pronounced, culminating in Mammaliaformes and, eventually, crown-group Mammalia.
  • Mammaliaformes: The clade that includes the earliest mammaliforms and their closest extinct relatives, bridging therapsids to true mammals.
  • Mammalia: The crown group of mammals, representing the final stage of this long evolutionary progression. For context on living mammals, see Mammalia.

See also