Fenestra

The vertebrate skull exhibits several distinct categories of fenestrae, each differing in position, bounding bones, and functional significance.

Temporal Fenestrae are openings in the temporal region of the skull, posterior to the orbit. They are the most taxonomically significant fenestrae in amniote evolution. The supratemporal (dorsotemporal) fenestra is bounded dorsally by the parietal bone, anteriorly by the postorbital bone, and posteriorly by the squamosal bone. The infratemporal (lateral temporal) fenestra lies below it, typically bordered by the jugal, postorbital, and squamosal bones. These openings provide pathways and attachment surfaces for the jaw adductor musculature. The configuration of temporal fenestrae formed the basis for the classical amniote classification system established by Osborn (1903) and later expanded by Williston (1917).

The Antorbital Fenestra is an opening in the snout between the external naris and the orbit, formed primarily between the maxilla and lacrimal bones. It is a defining synapomorphy of Archosauriformes and is closely associated with the antorbital cavity, a depression on the lateral surface of the snout that houses the antorbital sinus. Witmer (1997), in a comprehensive monograph published as Society of Vertebrate Paleontology Memoir 3, demonstrated through extant phylogenetic bracketing that the antorbital cavity in all archosaurs housed a paranasal pneumatic diverticulum (air sinus), rejecting earlier hypotheses that it contained a gland or a muscle. Among living archosaurs, birds retain the antorbital fenestra, while crocodylians have secondarily lost the external opening although a vestigial antorbital cavity persists in some species.

The Mandibular Fenestra is an opening through the lower jaw, formed between the dentary, surangular, and angular bones. It is another synapomorphy of archosaurs or near-archosaurs, contributing to the reduction of skull weight and potentially housing neurovascular structures. According to the University of California Museum of Paleontology, the mandibular fenestra and the antorbital fenestra together represent part of the broader tetrapod trend of reducing skull bone mass through fusion and fenestration.

The Orbit is the socket for the eyeball and is present in all craniate vertebrates. While not a fenestra in the strict sense used for taxonomic classification, it is the most prominent cranial opening in most vertebrate skulls and is often discussed alongside fenestrae in comparative osteology.

Cranial fenestrae serve multiple overlapping functions that have been debated and refined over more than a century of study.

Weight Reduction: By replacing solid bone with open space, fenestrae significantly reduce skull mass. This is particularly important in large-skulled animals such as theropod dinosaurs. The skull of Tyrannosaurus rex, approximately 1.5 meters in length, was rendered manageable in weight through extensive fenestration, including large temporal fenestrae, antorbital fenestrae, and mandibular fenestrae. The National Park Service notes that dinosaur skulls are among the rarest fossils precisely because the thin bones connecting fenestrae make skulls structurally fragile and prone to crushing during fossilization.

Jaw Muscle Attachment and Expansion: The edges of temporal fenestrae provide origin sites for jaw adductor muscles. Werneburg (2019) explains that the rounded edges of fenestrae offer greater structural stability than flat bone surfaces, making them mechanically advantageous for muscle attachment. In the excavated morphotype seen in derived synapsids (including mammals), the temporal fenestra expanded dorsally, allowing the temporalis muscle to pass through the opening and attach to the external skull roof, producing longer muscle fibers and greater bite force. The zygomatic arch, formed ventral to the fenestra, became the origin for the newly differentiated masseter muscle, enabling the sophisticated chewing motions characteristic of mammals. Similarly, in diapsids, the two temporal openings allowed a more complex and finely differentiated arrangement of adductor muscles, facilitating diverse feeding strategies from insectivory to herbivory to high-force carnivory.

Pneumatic Sinuses and Air Sacs: The antorbital fenestra in archosaurs houses the antorbital sinus, a paranasal air-filled diverticulum connected to the nasal cavity. Witmer (1997) established through detailed comparative analysis that this pneumatic sinus is universally present in archosaurs with an antorbital cavity. In theropod dinosaurs, additional pneumatic openings (accessory antorbital fenestrae) are common, and the pneumatic system extended into the braincase and postcranial skeleton in many lineages. These pneumatic structures are homologous with the elaborate air sac system of modern birds, which plays a role in unidirectional airflow ventilation.

Thermoregulation: A 2019 study by Holliday, Porter, Vliet, and Witmer published in The Anatomical Record demonstrated that the dorsotemporal fenestra in archosaurs houses not only jaw muscle but also a pad of blood vessels and adipose tissue within a structure called the frontoparietal fossa. Using thermal imaging of American alligators at the St. Augustine Alligator Farm, the researchers showed that this vascular network exhibits temperature differentials relative to body surface temperature throughout the day, being relatively warm in cool morning conditions and relatively cool in hot afternoon conditions. This suggests a thermoregulatory function, shunting blood to warm or cool the brain as needed. Fossil dinosaur skulls, including those of Tyrannosaurus and Velociraptor, show the same anatomical hallmarks (a distinct frontoparietal fossa within the dorsotemporal fenestra), strongly implying that non-avian dinosaurs possessed similar cranial thermoregulatory systems.

The systematic use of temporal fenestrae for amniote classification has a long history. Henry Fairfield Osborn (1903) first divided amniotes into Synapsida and Diapsida based on the number of temporal openings. Williston (1917) expanded the scheme, formally recognizing Anapsida (no temporal openings), Synapsida (one pair, the infratemporal fenestra), Diapsida (two pairs), and Euryapsida (one upper opening, as in ichthyosaurs and plesiosaurs). This fourfold classification became a standard framework in vertebrate paleontology textbooks for most of the twentieth century.

Anapsida: The ancestral amniote skull condition, with the temporal region fully roofed by dermal bone. Early reptiles such as captorhinids and pareiasaurs display this morphology. Turtles were traditionally placed in Anapsida, but molecular phylogenetic studies consistently nest turtles within Diapsida, implying that their anapsid-like skull is a secondary condition. Werneburg (2015) proposed that secondary temporal closure in turtles evolved as a response to the mechanical stresses of neck retraction.

Synapsida: Characterized by a single temporal fenestra on each side of the skull, homologous with the infratemporal fenestra of diapsids, bordered ventrally by the zygomatic arch (composed of the jugal and squamosal). This condition first appeared approximately 318 million years ago in the Late Carboniferous. The synapsid lineage includes pelycosaurs (e.g., Dimetrodon), therapsids, and ultimately mammals. In mammals, the temporal fenestra expanded enormously through evolution, merging with the orbit in many taxa and allowing the jaw musculature to extend onto the external surface of the braincase.

Diapsida: Defined by the presence of both an upper (supratemporal) and lower (infratemporal) temporal fenestra on each side of the skull. This condition is found in the vast majority of reptiles, including dinosaurs, pterosaurs, crocodylians, lizards, snakes, tuataras, and birds. Within Diapsida, extensive secondary modification occurred: lizards lost the lower temporal bar (the katapsid condition), snakes lost both bars, marine reptiles such as ichthyosaurs and plesiosaurs modified fenestrae in various ways (leading to the now-abandoned categories Euryapsida and Parapsida), and birds fused the temporal openings with the enlarged orbit.

Modern phylogenetic consensus, as reviewed by Werneburg (2019) and Müller (2003), holds that temporal fenestra patterns are only weakly informative for higher-level taxonomy because convergent evolution has produced similar fenestral configurations in unrelated lineages. The same morphofunctional category (e.g., a single opening) can arise independently in synapsids and certain parareptiles. Nevertheless, at lower taxonomic levels, fenestral morphology remains a useful diagnostic character.

Theropoda: Large theropods such as Tyrannosaurus rex possess a suite of prominent fenestrae including paired temporal fenestrae (supratemporal and infratemporal), large antorbital fenestrae, mandibular fenestrae, and orbits. The combination of massive jaw muscles passing through the temporal fenestrae and extensive pneumatization of the antorbital region produced skulls that were both powerful and relatively lightweight. Holliday et al. (2019) demonstrated that the dorsotemporal fenestra of tyrannosaurs and other theropods contained vascular thermoregulatory structures, adding a physiological dimension beyond the purely mechanical functions of fenestration.

Ceratopsia: Ceratopsian dinosaurs such as Triceratops and Torosaurus possess large parietal-squamosal frills that may themselves be fenestrated. In Torosaurus, large parietal fenestrae perforate the frill, likely reducing weight. The presence or absence of frill fenestrae varies among ceratopsian taxa and has been used as a diagnostic character at the genus and species level. Some researchers have suggested that bone resorption processes contributed to the formation of extracranial frill fenestrae.

Sauropoda: Despite their enormous body size, sauropods had relatively small and lightly built skulls with well-developed antorbital fenestrae, temporal fenestrae, and orbits that reduced bone mass to a minimum. This lightweight cranial architecture was consistent with their long-necked body plan, in which a heavy skull at the end of a long neck would have been biomechanically disadvantageous.

Beyond the large cranial openings discussed above, the term fenestra is used in other anatomical contexts. The fenestra vestibuli (fenestra ovalis, oval window) is the opening in the medial wall of the middle ear through which the footplate of the stapes transmits sound vibrations to the perilymph of the inner ear. The fenestra cochleae (fenestra rotunda, round window) is a membrane-covered opening at the base of the cochlea that allows pressure equalization during sound transmission. According to Britannica, the oval window is a critical interface between the middle and inner ear in all terrestrial vertebrates. These uses of fenestra retain the original Latin meaning of 'window' — a small opening connecting two spaces — but refer to structures at a much smaller scale than the cranial fenestrae of paleontological significance.

Werneburg (2019) offered a developmental explanation for the evolutionary origin of temporal fenestrae in amniotes. In non-amniote vertebrates (fish, amphibians), larvae hatch early and begin feeding while the skull is still developing. Jaw muscles initially attach to the cartilaginous chondrocranium, and dermal bones develop later, eventually forming a complete, closed temporal armor supported in part by opercular bones. In amniotes, however, the larval stage is eliminated because development occurs entirely within the amniotic egg (or secondarily within the uterus). By the time of hatching, the jaw musculature is already well differentiated and spatially arranged, but the temporal dermal bones remain small and incompletely fused. This developmental condition — what Tarsitano et al. (2001) termed 'the embryological failure to close sutures' — creates the developmental substrate for fenestral formation. Once fenestrae formed, natural selection could act on their shape and size according to the functional demands of jaw musculature and feeding ecology, leading to the diverse fenestral morphologies observed across amniote evolution.

This ontogenetic framework explains why temporal fenestrae in different amniote lineages (synapsids, parareptiles, diapsids) are not strictly homologous to one another: they represent parallel outcomes of the same developmental predisposition, shaped independently by functional demands in each lineage. As Werneburg (2019) noted, the 'holes' in the dermatocranium are the result of developmental plasticity driven by functional adaptation, and temporal fenestrations can only be reliably informative for phylogenetics at certain taxonomic levels.