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What are Anapsids? A Brief Introduction to the Oldest Amniotes

Anapsids are a group of animals that belong to the clade Anapsida, which includes turtles and their extinct relatives. Anapsids are characterized by having a skull that lacks any openings (fenestrae) near the temples, unlike most other amniotes, which have one or two pairs of fenestrae. Anapsids are considered to be the most primitive subclass of amniotes, the ancestral stock from which synapsids (mammals and their relatives) and diapsids (reptiles and birds) evolved.

The Origin and Evolution of Anapsids

The origin of anapsids is still uncertain, as the fossil record of Paleozoic tetrapods is poor and localized. The oldest known anapsid is Acleistorhinus, a small reptile from the Lower Permian of North America. By the Upper Permian, anapsids had diversified into several groups, such as millerettids, procolophonoids, pareiasaurs, lanthanosuchids, and nyctiphruretians. These groups had different adaptations and lifestyles, ranging from herbivorous to carnivorous, from terrestrial to aquatic.

However, most of these anapsid groups became extinct during the Permian-Triassic extinction event, the largest mass extinction in Earth’s history. Only procolophonoids and pareiasaurs survived into the Triassic, but they also died out by the end of that period. The only living anapsids today are the Testudines (turtles, tortoises, and terrapins), which have a worldwide distribution and a remarkable diversity of forms and habitats.

The Controversy over Anapsid Phylogeny

Traditionally, anapsids were thought to be a monophyletic group, meaning that they share a common ancestor that is not shared by any other group of amniotes. However, some studies have suggested that some anapsid groups may be more closely related to synapsids or diapsids than to other anapsids, making anapsids paraphyletic (meaning that they do not include all the descendants of their common ancestor).

The most controversial case is that of turtles. For a long time, turtles were considered to be anapsids based on their skull morphology. However, molecular studies have consistently shown that turtles are actually diapsids that lost their temporal fenestrae secondarily. Some molecular studies have placed turtles within Archosauria (the group that includes crocodiles and birds), while others have placed them as a sister group to extant archosaurs or lepidosaurs (lizards, snakes, and tuataras).

Morphological studies have also supported the diapsid origin of turtles, but with different hypotheses on their relationships. Some have suggested that turtles are related to sauropterygians (marine reptiles such as plesiosaurs), while others have suggested that they are related to lepidosauromorphs (the group that includes lepidosaurs and sphenodontians).

The phylogenetic position of turtles is still debated among paleontologists and biologists, as different lines of evidence may yield conflicting results. More data and analyses are needed to resolve this issue and to better understand the evolution of anapsids and their role in the history of life.