Glossary

A **cranial crest** is a bony protrusion atop the skull of certain dinosaurs—most prominently lambeosaurine hadrosaurids and various theropods—formed primarily by dorsal expansions of the premaxillae and nasals. In lambeosaurines, the crest is hollow, housing elaborate convolutions of the nasal passage including s-loops, a common median chamber, and lateral diverticula; these internal airways functioned as resonating chambers capable of producing low-frequency vocalizations, as demonstrated by acoustic analyses (Weishampel, 1981) and computer-based sound reconstructions (Diegert & Williamson, 1998). In theropods such as *Dilophosaurus* and oviraptorids, the crest is solid and served primarily as a visual display structure for species recognition and sexual selection. Crest morphology is highly diagnostic at the genus and species level and undergoes dramatic allometric change during ontogeny, making it a critical but potentially misleading character in taxonomy when growth stage is not accounted for. A 2014 discovery of a mummified *Edmontosaurus regalis* specimen bearing a fleshy, cock's-comb-like soft-tissue crest (Bell et al., *Current Biology*) revealed that cranial display structures extended beyond ossified elements and may have been far more widespread among dinosaurs than the skeletal record alone suggests. Cranial crests thus served multifunctional roles—acoustic signaling, visual display, and possibly structural reinforcement—and represent one of the most striking examples of socio-sexual ornamentation in the fossil record.

The frill is a bony platform that extends posteriorly and posterolaterally from the rear of the skull in ceratopsian dinosaurs, formed primarily by expansions of the parietal bone along the midline and the squamosal bones along the lateral margins. It is a neomorphic structure unique to Ceratopsia among archosaurs, overhanging the occiput and, in many taxa, projecting well beyond the back of the skull to cover the neck region dorsally. In small-bodied, early-diverging ceratopsians such as Psittacosaurus and Yinlong, the frill is relatively short and narrow, but in large-bodied ceratopsids (exceeding 1,000 kg), it can surpass one meter in length and width, constituting more than half the total skull length. Most neoceratopsians possess a pair of parietal fenestrae—large openings that perforate the frill—although some taxa, notably Triceratops, have solid, unfenestrated frills. In the derived Ceratopsidae, the frill margin is adorned with epiparietal and episquamosal ossifications that form a spectacular variety of spikes, hooks, and scalloped processes, with each species displaying a distinctive configuration of these ornaments. The function of the frill has been debated since the discovery of Triceratops in the late nineteenth century. While a defensive role was long assumed, many frills are perforated or composed of thin bone that would have provided limited protection. Oxygen isotope analysis of Triceratops frill bone by Barrick and Stoskopf (1998) demonstrated high and uniform heat flow through the parietal, with mean frill temperatures only 0–4°C below the body core, suggesting a possible thermoregulatory function. Jaw muscle attachment was proposed by Ostrom (1966), who argued that the frill provided an expanded surface for the origin of the external adductor musculature, conferring greater bite force. However, the most strongly supported hypothesis is that the frill served as a socio-sexual signalling structure. Studies on Protoceratops andrewsi have shown that the frill displays positive allometry, modularity, significantly higher rates of ontogenetic size and shape change, and greater morphological variance than other skull regions—all patterns consistent with socio-sexual selection. Display characters in ceratopsians diverge more rapidly than internal or non-display characters across the clade, further supporting a signalling role. The frill is one of the defining features of Ceratopsia and, together with the horns, has been central to understanding ceratopsian taxonomy, evolution, and palaeobiology.

An osteoderm is a mineralized skeletal element embedded within the dermis of a vertebrate, composed primarily of osseous tissue (bone) along with variable amounts of mineralized and unmineralized fibrous connective tissue. Osteoderms form directly in the skin at or adjacent to the stratum superficiale of the dermis, without requiring a cartilaginous precursor. Their development typically involves metaplastic ossification—the direct transformation of pre-existing connective tissue into bone—followed by remodeling through standard osteoblastic osteogenesis. Osteoderms range enormously in size and shape, from minute granular elements less than a millimeter across in some geckos to massive plates exceeding one meter in height in stegosaurian dinosaurs. They are widely but discontinuously distributed across tetrapod phylogeny, occurring in representatives of most major lineages: various amphibians (certain frogs and extinct temnospondyls), lepidosaurs (many lizard families, and recently confirmed in sand boas), archosaurs (crocodilians, many non-avian dinosaur lineages, aetosaurs, and phytosaurs), turtles, some synapsids (armadillos, extinct ground sloths, and the spiny mouse Acomys), and extinct marine reptiles such as placodonts. This irregular phylogenetic distribution has led researchers to propose that osteoderms represent a case of deep homology—a latent but ancestral capacity of the dermis to produce bone, which can be repeatedly activated or suppressed across evolutionary time. Functionally, osteoderms have been associated with physical protection from predators and conspecific attacks, thermoregulation via vascularized bone facilitating heat exchange, mineral (calcium) storage and mobilization during reproduction, biomechanical reinforcement of the body during locomotion, and visual signaling for species recognition or display.

The **thumb spike** is a conical ungual phalanx borne on the first digit (pollex) of the hand in *Iguanodon* and related iguanodontian ornithopod dinosaurs. In *Iguanodon bernissartensis*, the spike takes the form of a large, curved, conical spine that articulates freely against the fused carpo-metacarpal block, projecting laterally away from the three central weight-bearing digits. The bony core alone measures approximately 14 cm or more in adult specimens, but in life the spike was sheathed in keratin, making it considerably larger and sharper than the fossilized bone suggests. The structure is a shared derived character (synapomorphy) of the clade Ankylopollexia, though its size, shape, and degree of fusion to the carpus vary markedly among genera. Its function remains one of the longest-running debates in dinosaur palaeontology: proposed roles include defense against predators, foraging assistance such as stripping foliage or breaking into seeds, and intraspecific combat or display. None of these hypotheses has been conclusively supported by direct evidence. The thumb spike is also one of the most celebrated examples of misinterpretation in palaeontological history: first described by Gideon Mantell in the 1820s as a nasal horn, it was correctly identified as a manual digit by Louis Dollo following the 1878 discovery of articulated skeletons in the coal mines of Bernissart, Belgium.