What Is an Anapsid? Temporal Openings and Turtle Evolution

An anapsid is any amniote (a land vertebrate that lays eggs on land or retains them internally) whose skull has no openings in the temporal region behind the eye socket. The term comes from the Greek for “without arch,” referring to the solid sheet of bone where other lineages developed one or two holes. For most of the twentieth century, “Anapsida” was treated as a formal taxonomic group, but that classification has largely fallen apart. The skull shape turns out to be a poor guide to evolutionary relationships, and modern research treats “anapsid” as a description of anatomy rather than a branch on the tree of life.

Three Skull Shapes and What They Mean

Reptilian skulls have long been sorted into three broad categories based on the number and position of temporal fenestrae, which are openings in the bone behind the eye. An anapsid skull has zero such openings. A synapsid skull has one opening low on the side (this is the condition found in the lineage leading to mammals). A diapsid skull has two openings, one above the other, and characterizes lizards, snakes, crocodilians, birds, and their relatives.¹ These categories were first described in the early 1900s, and for decades they were treated as reliable markers of deep evolutionary kinship. Animals that shared a skull type were assumed to be closely related.

The logic was intuitive: a hole in the skull is a conspicuous feature, and it seemed unlikely that the same hole would evolve independently in unrelated lineages. But that assumption turned out to be wrong in important ways.

Why “Anapsida” Stopped Being a Real Group

The classification scheme based on temporal fenestrae was elegant, but as more fossils were discovered and analytical methods improved, problems accumulated. Many of the groups erected on the basis of their temporal-region anatomy are no longer recognized as natural groupings, and the shape of the temporal region is not necessarily an adequate trait for reconstructing evolutionary relationships among amniotes.² In other words, having no temporal opening does not guarantee that two animals share a recent common ancestor. The “anapsid” condition can arise because a lineage never evolved openings, or because it once had them and later closed them up.

This matters because it means “Anapsida” as a taxonomic bin lumps together animals that may not be closely related at all. Turtles, for example, have fully roofed skulls that look classically anapsid. But genetic evidence places them firmly among the diapsids, meaning their ancestors had two temporal openings that were subsequently lost. So putting turtles in Anapsida alongside genuinely ancient lineages that never had openings creates a misleading picture of evolutionary history. Modern systematists generally avoid using “Anapsida” as a formal clade name, preferring instead to talk about specific lineages like Parareptilia or Eureptilia and to treat the anapsid skull condition as a morphological description, not a family tree address.

Parareptiles and the Animals Once Called Anapsids

The group most commonly associated with the anapsid skull type is Parareptilia, an extinct clade of amniotes that lived from the late Carboniferous through the end of the Triassic. Parareptilia was notable for the wide array of body forms its members took on and for their success in many different ecological roles.³ Some were small insectivores the size of a modern gecko. Others were heavy-bodied herbivores that rivalled a cow in bulk.

Pareiasaurs, for instance, were a diverse group of parareptilian herbivores that thrived during the middle and late Permian period, roughly 270 to 252 million years ago. These stocky animals had broad skulls covered in bony knobs and flanges, giving them an armored appearance. Bone tissue studies reveal that pareiasaurs grew relatively rapidly early in life, then shifted to a slower but sustained growth pattern that continued for several years into adulthood.⁴ This growth strategy is reminiscent of some modern large reptiles but not identical to any living species, making pareiasaurs difficult to compare directly to anything alive today.

Defining Parareptilia precisely has its own complications. A formal phylogenetic definition of both Parareptilia and its sister clade Eureptilia was put forward to help stabilize early amniote classification, but the relationships among early amniote groups remain actively debated.⁵ One of the difficulties is that some animals placed in Parareptilia did not have perfectly “anapsid” skulls. The early Permian mesosaurid Mesosaurus, for example, appears to have possessed a lower temporal opening, shaped and bounded by bones in a way that resembles the configuration seen in some early synapsids and a handful of other reptiles.⁶ This blurs the tidy line between “anapsid” and “non-anapsid” even within the group historically defined by having no openings.

Turtles and the Closed-Skull Puzzle

No animal has done more to destabilize the anapsid classification than the turtle. Living turtles have fully roofed skulls with no temporal fenestrae, which by the old system makes them textbook anapsids. For much of the twentieth century, this is exactly where they were placed. Many paleontologists assumed turtles were surviving members of an ancient anapsid lineage, the last holdouts of a group that predated the evolution of temporal openings.

Molecular phylogenetics upended this view. Analyses using both gene-sequence data and whole-genome comparisons consistently place turtles as the sister group to archosaurs, meaning their closest living relatives are crocodilians and birds.⁷ Both crocodilians and birds are diapsids, with skulls that originally had two temporal openings. This means turtles descend from ancestors that had those openings and secondarily closed them. The turtle skull is useful precisely because of this history: it demonstrates that temporal fenestrae can be lost as well as gained, making it a powerful model for understanding skull diversity.⁸

The fossil record has gradually caught up with the genetic evidence. Stem turtles from the Triassic show intermediate skull configurations that bridge the gap between a typical diapsid skull and the sealed-over condition seen in modern species. These fossils provide physical evidence that turtles evolved their closed skulls over millions of years, not that they inherited them from some primordial anapsid ancestor.

How Turtles Sealed Their Skulls Shut

Developmental biology has started to reveal the mechanism behind the turtle’s unusual skull. Researchers studying gene activity in reptile embryos found that two genes involved in early bone formation, Runx2 and Msx2, are expressed in an unusually broad pattern across the temporal region of turtle embryo heads. In species that develop temporal openings, these genes are more restricted in where they are active. The wider zone of bone-promoting gene expression in turtle embryos corresponds to the region where other reptiles form fenestrae, suggesting that turtles fill in those would-be openings with bone during development.⁹ Comparisons with crocodile and snake embryos show that each lineage has a distinct spatial pattern of these genes, and the patterns correlate with the eventual skull shape of the adult animal.

This finding is significant because it shows that the difference between an “anapsid” skull and a “diapsid” skull can come down to where certain bone-forming genes are switched on during a brief window of embryonic development. It does not require a fundamentally different body plan or a vast evolutionary distance. A relatively subtle shift in gene regulation can close or open a temporal fenestra, which helps explain why skull morphology in this region has been so unstable across evolutionary history.

What Temporal Openings Are Actually For

If temporal fenestrae come and go over evolutionary time, a natural question is what purpose they serve. The traditional explanation is that they lighten the skull and provide attachment points for jaw muscles, allowing them to bulge outward during biting. This is partly true, but recent biomechanical analysis paints a more nuanced picture.

The forces acting on the skull during feeding play a major role in shaping the temporal region. A strong bite concentrated at the front of the jaw tends to favor retaining a bony bar along the lower margin of the temporal region, the infratemporal bar. A powerful bite focused further back in the jaw instead favors an upper temporal arcade. Side-to-side forces, including inertia from struggling prey and neck posture, also influence which parts of the temporal region get reinforced with bone and which get thinned out or lost.¹⁰ Under this framework, the anapsid condition is not simply “primitive.” It can be actively maintained or re-evolved whenever the mechanical demands on the skull favor a complete bony covering, as appears to have happened in turtles.

This biomechanical perspective also explains why animals within the same family can vary in their temporal-region anatomy. Feeding ecology can shift over relatively short evolutionary timescales, and the skull responds. A lineage that evolves to eat tougher food, or to capture larger prey, may reinforce or reduce the temporal bars accordingly. The skull is not a static badge of identity; it is a structure under constant selective pressure from the physics of feeding.

Parareptiles Through the Great Dying

The end-Permian mass extinction, roughly 252 million years ago, wiped out an estimated 90 percent of marine species and about 70 percent of land vertebrates. For parareptiles, however, the picture is more complicated than simple devastation. Available fossil data do not fit a model of sudden catastrophic decline at the Permian-Triassic boundary. Instead, the record suggests a rapid alternation of originations and extinctions in several parareptile groups, both before and after the boundary.¹¹ Some lineages were already declining before the extinction pulse, while others were diversifying.

Procolophonoids, a subgroup of parareptiles that included small, lizard-like animals, came through the extinction event in remarkably good shape. Phylogenetic analysis calibrated against the fossil record indicates that four of the six procolophonoid lineages that originated in the Permian extended into the Triassic, a survival rate of about 67 percent. This strongly suggests that procolophonoids were barely affected by the mass extinction that ended the Paleozoic era.¹² After the extinction, procolophonoids diversified substantially during the Triassic. In fact, the only statistically significant burst of diversification detected across all of Parareptilia occurred among Triassic procolophonoids.¹³

The success of procolophonoids stands in contrast to pareiasaurs, which vanished entirely at or near the Permian-Triassic boundary. Why one “anapsid” group sailed through the worst mass extinction in Earth’s history while another was obliterated remains an open question. Body size, diet, geographic range, and metabolic flexibility have all been proposed as factors, but the fossil record is spotty enough that firm conclusions are elusive. What is clear is that having an anapsid skull did not uniformly doom or protect these animals. Their fates were shaped by ecology, not cranial architecture.

Millerettids and the Problem of “Primitive” Looks

Among the animals historically grouped as anapsids, the millerettids offer a particularly instructive case. These small, late-Permian reptiles from southern Africa were long considered primitive parareptiles based on their skull shape and overall proportions. More recent work using high-resolution scanning of millerettid skulls has challenged that placement. A detailed reconstruction of the skull of Milleropsis pricei, one of the most basal (earliest-branching) millerettids, revealed anatomical features shared with Sauria, the crown group that includes lizards, snakes, crocodilians, birds, and turtles.¹⁴ This suggests that millerettids may belong closer to the crown of the reptile family tree than previous studies assumed.

If millerettids are indeed more closely related to modern reptiles than to other parareptiles, then their “anapsid” skulls are yet another case of the closed-skull condition appearing in a lineage that sits within a broader group of diapsids, or at least closer to diapsids than anyone suspected. The recurring theme across all of these discoveries is that a solid temporal region is not a reliable indicator of deep ancestry. It is a feature that has appeared, disappeared, and reappeared in multiple lineages, driven by developmental tweaks and ecological pressures rather than by some fundamental constraint on body plan evolution.

When Textbook Categories Outlive Their Usefulness

The anapsid-synapsid-diapsid scheme persists in introductory biology and paleontology courses because it is easy to teach and easy to remember. Three skull types, three groups of animals, clean diagrams with arrows. The trouble is that this clean picture was already fraying by the 1990s, and two decades of molecular and developmental research have left it in tatters. “Anapsid” remains a perfectly good anatomical descriptor: if a skull has no temporal fenestrae, calling it anapsid tells you something real about its shape. But using it as a taxonomic label, to imply that all animals with this skull type form a single evolutionary branch, is misleading.

Part of the problem is that convergent evolution in skull shape is far more common than early researchers assumed. When two unrelated lineages face similar mechanical demands on their skulls, they can independently arrive at similar solutions, including sealing up temporal openings. Turtles did it. Millerettids may have done something analogous in a different context. Mesosaurids went the other direction and opened a fenestra that their relatives lacked. The temporal region of the skull is not a passive inheritance; it is a highly responsive structure shaped by the biomechanics of feeding, the physics of head movement, and the regulatory genes that lay down bone during embryonic development. Treating it as a fixed evolutionary stamp led to decades of misclassification, and unpicking those errors remains an active area of research in vertebrate paleontology.