Semimammalians

Semimammalians is a term sometimes used informally to refer to extinct synapsid reptiles that exhibit traits considered transitional between reptiles and mammals. These animals are part of the Pelycosauria and Therapsida orders, which are the evolutionary ancestors of mammals. They are characterized by a gradual increase in skeletal and physiological features that are now considered distinctly mammalian. For example, some later pelycosaurs and early therapsids developed more upright limbs, suggesting improved locomotion and potentially higher metabolic rates.

A key feature observed in this lineage is the development of differentiated teeth, with incisors, canines, and molars becoming more specialized, a hallmark of mammalian dentition. The evolution of a more complex jaw articulation, moving away from the multiple bones found in reptiles towards the single dentary bone characteristic of mammals, is another significant transitional feature. Furthermore, evidence suggests the gradual development of a secondary palate, allowing animals to breathe while eating, a crucial adaptation for more active lifestyles.

While not true mammals, semimammalians represent critical steps in the evolutionary journey from early amniotes to the diverse mammalian class we know today. Their fossil record provides invaluable insights into the gradual acquisition of traits that define mammals, including aspects of their posture, feeding, and thermoregulation, although direct evidence for endothermy (warm-bloodedness) in many of these forms remains debated. The study of these "semimammalian" forms is central to understanding the deep evolutionary history of mammals.