Frill (parietal-squamosal frill)

The ceratopsian frill is formed by the posterior extension of two paired skull bones: the parietal, which constitutes the midline and medial portions of the frill, and the squamosal, which forms the lateral margins. These bones are sutured together along a contact that runs roughly anteroposteriorly along each side of the frill. In life, the frill would have been covered by skin and possibly keratinous integument, and the bone surface in many specimens bears vascular grooves and foramina indicative of a rich blood supply to the overlying soft tissues.

In early-diverging ceratopsians, the frill is little more than a modest posterior extension of the skull roof. In Psittacosaurus, the frill is extremely abbreviated, while in Yinlong downsi—the earliest known ceratopsian from the Late Jurassic (Oxfordian, approximately 158–155 million years ago) of China—the posterior skull margin shows only an incipient shelf. In basal neoceratopsians such as Protoceratops andrewsi, the frill is considerably larger, projecting caudally well beyond the occiput and displaying notable ontogenetic changes in size and proportions. In the large-bodied Ceratopsidae of the Late Cretaceous, the frill reaches its most extreme development, with some taxa bearing frills that exceed one meter in both length and width.

A pair of parietal fenestrae—large openings perforating the parietal portion of the frill—is present in most neoceratopsians. These openings vary considerably in size and shape among taxa. In chasmosaurines such as Chasmosaurus, the fenestrae are proportionally enormous, giving the frill a roughly heart-shaped outline when viewed dorsally, with much of the interior of the parietal reduced to a thin frame of bone. In centrosaurines like Centrosaurus, the fenestrae are also large but differently proportioned.

Notably, Triceratops possesses a solid frill lacking fenestrae, which is unusual among derived neoceratopsians. The parietal of Triceratops instead bears ventral depressions (fossae) on its underside, which were initially proposed by Scannella and Horner (2010) to represent incipient fenestrae that might develop into the large openings seen in Torosaurus. However, Longrich (2012) demonstrated that these depressions differ in position, shape, and structural context from the true parietal fenestrae of Torosaurus, arguing against the synonymy of the two genera. The Triceratops ventral fossae are bordered by a massive collar of thickened bone (approximately 30 mm thick) along the posterior frill margin, whereas the Torosaurus frill is comparatively thin (up to 20–25 mm thick). The question of whether Torosaurus represents a mature growth stage of Triceratops or a distinct genus remains debated, but evidence from ontogenetic staging analyses has identified both immature Torosaurus and fully mature Triceratops individuals, arguing against simple synonymy.

In ceratopsids, the frill margin bears a series of dermal ossifications known as epiparietals (along the parietal margin) and episquamosals (along the squamosal margin). These are osteoderms that form in association with the overlying scales and fuse to the frill during ontogeny. They take a remarkable diversity of forms across taxa, including triangular hornlets, elongate spikes, recurved hooks, and low scalloped processes.

The number and arrangement of epiparietals are taxonomically diagnostic and appear to be established early in development. Triceratops typically possesses five to seven epiparietals, while Torosaurus has ten or more. Styracosaurus albertensis is renowned for bearing at least four—and often six—long parietal spikes that project posteriorly and posterolaterally from the frill, creating one of the most visually dramatic cranial displays among dinosaurs. Kosmoceratops richardsoni, from the Campanian of Utah, bears an extraordinary array of fifteen epiossifications around the frill margin, making it the most ornamented ceratopsian known.

4a. Defense

The earliest functional interpretation of the ceratopsian frill was as a defensive shield protecting the vulnerable neck from predator attack. This hypothesis was initially plausible given the frill's position covering the neck, particularly in solid-frilled taxa like Triceratops. However, the widespread presence of large fenestrae in many ceratopsian frills significantly undermines this interpretation, as perforated bone would provide limited physical protection. Additionally, thin-frilled taxa such as Chasmosaurus, whose parietal is reduced to a skeletal frame, would have been poorly served by a defensive structure. While not entirely dismissed—especially for Triceratops, whose solid frill may have provided incidental protection—defense is now widely considered the least supported functional hypothesis for the frill as a whole.

Farke et al. (2009) found that Triceratops exhibits a significantly higher prevalence of healed lesions on the squamosal bone of the frill compared to Centrosaurus (P = 0.002), consistent with the hypothesis that Triceratops used its horns in intraspecific combat and that the solid frill in this taxon may have functioned partly as protection during such encounters. Centrosaurus, with its fenestrated and thinner frill, shows no comparable pattern of squamosal injury, suggesting different behavioral ecology.

4b. Thermoregulation

The thermoregulatory hypothesis proposes that the frill functioned as a heat exchange surface, analogous to the dorsal plates of Stegosaurus. The bone surface of many frills bears extensive vascular grooves and foramina, indicating a rich blood supply to the overlying skin. Barrick and Stoskopf (1998) measured oxygen isotope ratios in the bone phosphate of a Triceratops frill and found very high and uniform heat flow through the parietal, with mean frill temperatures maintained at only 0–4°C below the body core temperature. This is consistent with the frill functioning as a radiator or heat-exchange surface. However, whether thermoregulation was a primary function or merely a secondary consequence of a richly vascularized display surface remains unclear.

4c. Jaw Muscle Attachment

Ostrom (1966) proposed that the ceratopsian frill evolved primarily as an expanded attachment site for the jaw adductor musculature. In early ceratopsians and basal neoceratopsians, the jaw muscles (particularly the musculus adductor mandibulae externus) originate at the base of the frill and along the squamosal, with the frill expansion providing a larger surface area for muscle origin and thus conferring greater mechanical advantage and bite force. This hypothesis is well supported for early-diverging ceratopsians, where the frill is relatively small and closely associated with the jaw musculature. However, Prieto-Márquez et al. (2020) found evidence that in more derived ceratopsids, the frill became decoupled from the jaw musculature, evolving as a semi-independent module (supported by modularity analyses showing the frill as a distinct evolvable module). This decoupling may have freed the frill from functional constraints, allowing the explosive diversification of frill shapes and ornamentation seen in Late Cretaceous ceratopsids.

4d. Socio-Sexual Signalling

The socio-sexual signalling hypothesis is the most strongly supported explanation for the diversity and elaboration of ceratopsian frills. Multiple lines of evidence converge on this interpretation. Hone et al. (2016) demonstrated positive allometry for frill size in Protoceratops andrewsi, meaning that the frill grows disproportionately larger relative to body size—a pattern typical of sexually selected traits in extant animals. Knapp et al. (2021) extended this work using three-dimensional geometric morphometrics on 65 Protoceratops skulls and found that the frill forms a distinct phenotypic module, exhibits significantly higher rates of ontogenetic size and shape change than any other skull region, and shows greater morphological variance independent of size. Three of four predictions of the socio-sexual signalling hypothesis were supported; only sexual dimorphism in shape was not detected, possibly due to mutual mate choice or socio-sexual selection operating similarly on both sexes.

Knapp et al. (2018) showed that display characters (frills, horns, bosses) diverge more rapidly between ceratopsian species than internal or non-display characters, regardless of phylogenetic distance. Importantly, sympatric species were not found to differ significantly more in ornamental disparity than non-sympatric species, which argues against species recognition as the primary driver of frill diversity and favors intraspecific socio-sexual selection.

The ceratopsian frill exhibits extraordinary morphological diversity across the clade, and frill shape is among the most important features for distinguishing species.

Triceratops: Characterized by a relatively short, solid frill lacking parietal fenestrae. The posterior margin bears five to seven low epiparietals. The frill bone is thick (up to 30 mm at the posterior margin), and the squamosal is flat without a medial bar. Triceratops skulls show ontogenetic change in frill proportions, with juveniles having short, boxy frills with deeply scalloped margins, and adults having broader, smoother-margined frills.

Chasmosaurus: The frill is extremely elongate with very large parietal fenestrae, giving it an open, frame-like appearance. The fenestrae are bordered by relatively thin struts of bone. The overall outline is roughly triangular to heart-shaped. Low episquamosals line the lateral margins.

Styracosaurus albertensis: Distinguished by a dramatic array of long parietal spikes—at least four, and in some individuals six or more—projecting posteriorly and posterolaterally from the frill margin. A tall, straight nasal horn complements the frill ornamentation. The frill bears large parietal fenestrae between the spike bases.

Pentaceratops sternbergii: Possesses one of the largest known skulls of any land animal, with total skull length reaching approximately 2.3 meters in the largest specimens (though the specimen once considered the largest, OMNH 10165, has since been referred to the separate genus Titanoceratops). The frill of Pentaceratops is very long, with large parietal fenestrae and a margin lined with triangular epiparietals.

Kosmoceratops richardsoni: Bearing fifteen cranial horns and hornlets, including ten epiparietals arranged along the posterior frill margin (with the medial epiparietals curving anteriorly in a distinctive hook), Kosmoceratops has been described as the most ornamented dinosaur known.

Protoceratops andrewsi: A basal neoceratopsian with a proportionally large frill relative to its body size. The frill of Protoceratops lacks the epiparietal and episquamosal ornamentation of ceratopsids but shows pronounced positive allometry, growing disproportionately large during ontogeny. Parietal fenestrae are present but relatively smaller than in ceratopsids.

The frill undergoes dramatic changes during growth in ceratopsian dinosaurs. Studies on Triceratops by Horner and Goodwin (2006) identified a sequence of ontogenetic transformations: juveniles possess short, boxy frills with deeply scalloped posterior margins and immature bone surface texture (striated, indicating rapid growth); subadults develop elongate, broader frills with fusing skull elements and the beginning of rugose bone texture; adults display fully fused epiparietals and episquamosals, smooth frill margins, and heavily gnarled, vascularized bone surfaces.

In Protoceratops andrewsi, the frill shows the highest rate of both size and shape change during ontogeny of any skull region. Hatchlings have minimal frills that progressively expand caudally and laterally as the animal matures. The allometric slope for frill size against skull size is significantly steeper than for any other cranial module. Shape changes are concentrated in the caudal and caudolateral margins of the frill, which expand disproportionately.

The frill is a synapomorphy of Ceratopsia. Its earliest expression is seen in Yinlong downsi from the Late Jurassic (approximately 158–155 Ma) of Xinjiang, China, where the posterior skull margin shows only a modest incipient shelf. Through the Early and Late Cretaceous, the frill progressively enlarged across successive ceratopsian lineages. Psittacosaurus, from the Early Cretaceous, retains only a rudimentary posterior shelf with small jugal horns. Basal neoceratopsians (Liaoceratops, Archaeoceratops) show increasing frill development, and by the time of Protoceratops and its relatives (Late Cretaceous, Campanian–Maastrichtian), the frill is a prominent structure overhanging the neck.

The most dramatic frill diversification occurred within the Ceratopsidae during the Campanian and Maastrichtian stages of the Late Cretaceous (approximately 83–66 Ma), particularly in western North America. The two major ceratopsid subfamilies—Centrosaurinae and Chasmosaurinae—evolved distinct frill morphologies. Centrosaurines typically bear dramatic parietal ornaments (long spikes, recurved hooks) with relatively shorter frills, while chasmosaurines tend toward longer frills with variably sized fenestrae and more subdued episquamosal ornamentation. Prieto-Márquez et al. (2020) found that frill evolution was characterized by peramorphosis (overdevelopment relative to the ancestral condition), with acceleration operating early in neoceratopsian evolution followed by progenesis (truncated development) in later-diverging ceratopsids. This evolutionary trajectory was likely facilitated by the decoupling of the frill from the jaw musculature, which relaxed functional constraints and allowed more rapid morphological diversification.

All non-avian ceratopsians, and thus all frilled dinosaurs, became extinct at the Cretaceous–Paleogene boundary approximately 66 million years ago.

Although direct evidence of frill integument is rarely preserved, the extensive vascular grooves on the bone surface indicate that the frill was covered by richly vascularized skin in life. In some exceptionally preserved specimens of Psittacosaurus, squamous (scaly) skin is preserved, suggesting that at least early ceratopsians bore scales over the cranial region. By analogy with extant reptiles that use richly vascularized skin for display (such as the frill of the frill-necked lizard Chlamydosaurus kingii or the dewlap of Anolis lizards), it is plausible that ceratopsian frills bore vivid coloration or patterns that enhanced their visual signalling function. The presence of extensive blood vessel traces suggests that color change through flushing blood to the frill surface was also possible, similar to the display behavior observed in some extant lizards. However, these inferences remain speculative, as fossil pigment preservation has not been documented for ceratopsian frills.

The ceratopsian frill occupies an analogous functional niche to other elaborate cranial display structures in dinosaurs, including the bony cranial crests of lambeosaurine hadrosaurs (e.g., Parasaurolophus, Corythosaurus), the dome-shaped frontoparietal thickenings of pachycephalosaurs, and the casque-like structures of theropods such as oviraptorids. All of these structures have been interpreted as socio-sexual signals, and many show similar patterns of positive allometry, species-specific diversity, and absence of clear sexual dimorphism. The ceratopsian frill is unique, however, in the degree to which it has been modified and elaborated across the clade, producing a range of morphologies unmatched by any other dinosaurian cranial structure.