Glossary

Bipedalism is a form of terrestrial locomotion in which an animal moves by means of its two hind limbs (or lower limbs). It encompasses walking, running, and hopping gaits and is categorized into obligate bipedalism, where an animal moves exclusively on two legs, and facultative bipedalism, where an animal switches between bipedal and quadrupedal movement depending on context. Within the dinosaur lineage, bipedalism is regarded as the ancestral condition. The earliest dinosauriforms of the Middle Triassic (c. 235–230 Ma) already exhibited bipedal or strongly bipedal-tending body plans, a trait linked to the well-developed caudofemoralis longus muscle that transmitted powerful propulsive force from the tail to the hindlimb, conferring a cursorial advantage. By freeing the forelimbs from a locomotor role, bipedalism enabled their co-option for prey capture, manipulation, and display, and it is widely considered a key innovation underlying the ecological rise of dinosaurs during the Triassic. Bipedalism is also a defining trait of the human lineage among primates; however, human upright (orthograde) bipedalism differs fundamentally from the horizontal (pronograde), tail-counterbalanced bipedalism of non-avian dinosaurs. In both lineages, bipedalism profoundly restructured the skeleton, musculature, and biomechanics, making it one of the most consequential locomotor transitions in vertebrate evolutionary history.

Cheek teeth are the teeth located posterior to the canines in the dental arcade, positioned along the inner surface of the cheeks. In mammals, the term collectively refers to the premolars and molars—teeth characterised by complex occlusal surfaces bearing cusps, ridges, and basins specialised for grinding, shearing, and crushing food. Premolars are distinguished from molars ontogenetically: premolars are replaced once during diphyodont development (having deciduous precursors), whereas molars erupt only as permanent teeth. In non-mammalian vertebrates, the term is applied more broadly to any posterior jaw teeth that perform analogous food-processing functions. Cheek teeth attained their most elaborate development in herbivorous ornithischian dinosaurs. Hadrosaurids (duck-billed dinosaurs) evolved dental batteries containing up to approximately 300 teeth per jaw ramus stacked in 60 tooth positions, forming a constantly replenished grinding surface for processing tough plant fibre. Ceratopsians such as Triceratops independently evolved dental batteries with a distinct slicing function, their cheek teeth composed of five different dental tissue layers that self-sharpened through differential wear to create blade-like cutting edges. The morphology of cheek teeth is tightly correlated with diet across vertebrate lineages. In palaeontology, cheek tooth form, occlusal wear patterns, and dental microwear provide primary evidence for reconstructing the dietary ecology and feeding behaviour of extinct animals, making cheek teeth among the most informative anatomical structures in the vertebrate fossil record.

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.

**Digitigrade** is a form of terrestrial locomotion in which an animal stands and walks on its digits (phalanges), with the metatarsals and heel (calcaneum) elevated above the ground. This foot posture characterizes a wide range of vertebrates, including dogs, cats, most non-human cursorial mammals, the majority of dinosaurs (including all theropods), and all extant birds. By restricting ground contact to the distal phalanges, digitigrade posture effectively increases functional limb length, which in turn lengthens stride and enhances running speed. The reduced mass concentrated at the distal limb also permits higher stride frequencies and more efficient storage and release of elastic strain energy in the tendon–muscle complexes of the ankle extensors. Digitigrade locomotion occupies an intermediate position between plantigrade posture (in which the entire sole contacts the ground, as in humans and bears) and unguligrade posture (in which only the tips of the digits, typically encased in hooves, touch the ground, as in horses and deer). The concept was formalized as a comparative anatomical category by Georges Cuvier in 1817 in *Le Règne Animal*, where he distinguished digitigrade carnivores (e.g., canids, felids) from plantigrade carnivores (e.g., ursids). In paleontology, digitigrade foot posture is inferred from fossil trackways in which only digit impressions appear without metatarsal or heel marks, providing critical evidence for reconstructing the locomotion and body size of extinct animals.

A **fenestra** (plural: fenestrae) is an opening or window-like aperture in the skull of vertebrates, particularly amniotes. Cranial fenestrae form where sutures between dermal bones fail to close or where bone is reduced during development, resulting in distinct openings of varying size and position. The principal types include temporal fenestrae (behind the orbit), the antorbital fenestra (between the naris and orbit), the mandibular fenestra (in the lower jaw), and the orbit itself. Functionally, fenestrae reduce skull weight, provide attachment surfaces and expansion room for jaw adductor musculature, house paranasal air sinuses and pneumatic diverticula, and may facilitate cranial thermoregulation via vascular networks. The number and arrangement of temporal fenestrae have historically served as a key criterion for classifying amniotes into Anapsida (no temporal fenestrae), Synapsida (one pair), and Diapsida (two pairs). The antorbital fenestra is a defining synapomorphy of Archosauriformes, first appearing in the Triassic and retained in extant birds. Overall, the morphology and distribution of cranial fenestrae are fundamental anatomical markers for understanding vertebrate evolutionary diversification.

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.

A **metacarpal** is any of the tubular bones situated between the carpal (wrist) bones and the phalanges (finger bones) in the forelimb of a land vertebrate, collectively forming the metacarpus — the skeletal framework of the palm or forefoot. In humans, five metacarpals are present, each classified as a long bone consisting of a proximal base, a shaft, and a distal head. The bases articulate with the distal carpal row at the carpometacarpal joints, while the heads articulate with the proximal phalanges at the metacarpophalangeal joints to form the knuckles. These bones create both longitudinal and transverse arches in the hand, enabling the precise manipulation and powerful grip characteristic of the human hand. The metacarpals are among the most evolutionarily labile elements of the vertebrate skeleton, undergoing dramatic modifications across lineages in response to functional demands. In theropod dinosaurs, the metacarpals were elongated and flexible to facilitate prey capture, and their progressive reduction from five to three digits — accompanied by the evolution of the semilunate carpal — is central to the dinosaur-to-bird transition. In sauropod dinosaurs, the metacarpals were arranged vertically in a unique semi-tubular to tubular configuration that distributed enormous body weight through columnar forelimbs. In pterosaurs, the fourth metacarpal was massively elongated to support the wing membrane used for powered flight. In modern birds, the metacarpals are fused with carpal bones to form the carpometacarpus, a rigid platform for the attachment of primary flight feathers. Among mammals, the horse lineage provides the most extreme example of metacarpal reduction: from four functional metacarpals in the Eocene Hyracotherium, through three in the Oligocene Mesohippus, to a single dominant third metacarpal (the cannon bone) flanked by vestigial splint bones in modern Equus.

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.

**Pneumatic bones** are skeletal elements that contain air-filled internal cavities (pneumatic chambers) formed through the invasion of pneumatic diverticula—epithelial outgrowths of the pulmonary air sac system—into bone tissue. Among extant terrestrial vertebrates, postcranial skeletal pneumaticity (PSP) is unique to birds, where air sac diverticula penetrate and remodel bones throughout the axial and appendicular skeleton, connecting to the exterior via pneumatic foramina. The internal architecture of pneumatized bones ranges from large, regularly branching chambers (camerae) to dense honeycomb-like networks of small cavities (camellae), providing structural reinforcement while substantially reducing skeletal mass. In the fossil record, unambiguous evidence of PSP has been documented in three distinct clades of bird-line archosaurs (Ornithodira): non-avian theropod dinosaurs, sauropodomorph dinosaurs, and pterosaurs, with the earliest clear occurrences dating to the Late Triassic (approximately 210 million years ago). The presence of pneumatic bones in these extinct taxa constitutes one of the primary lines of evidence for inferring that they possessed bird-like respiratory systems featuring air sacs and potentially unidirectional pulmonary ventilation. This adaptation was critical for enabling the evolution of extreme body sizes in sauropods—where individual vertebrae could reach 89% air by volume—and for supporting the metabolically demanding lifestyles of active theropod predators and flying pterosaurs.

**Quadrupedalism** is a form of terrestrial locomotion in which an animal uses all four limbs to bear weight and move. It represents the ancestral locomotor condition for fully terrestrial tetrapods, and the vast majority of living and extinct land vertebrates are quadrupeds. Within Dinosauria, quadrupedalism carries a distinctive evolutionary significance. Because the earliest known members of all major dinosaur lineages were bipedal, every instance of quadrupedal locomotion in dinosaurs represents a secondary reversion from bipedal ancestry—a transition known as **secondary quadrupedality**. This reversion is exceptionally rare among tetrapods, yet it occurred convergently at least four times within dinosaurs: once in Sauropodomorpha and at least three times in Ornithischia (in Thyreophora, Ceratopsia, and Hadrosauriformes). Outside of Dinosauriformes, no tetrapod lineage is known to have reverted from bipedality to quadrupedality. The transition to quadrupedal locomotion fundamentally transformed forelimb function—from roles in foraging and grasping to primary weight-bearing—and enabled the evolution of multi-tonne body masses, broad ecological diversification, and the restructuring of terrestrial ecosystems throughout the Mesozoic.

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.

A **toothless beak** is a cranial feeding structure in which the jaw bones are entirely devoid of teeth (edentulous) and are instead covered by a keratinous sheath known as a rhamphotheca. The rhamphotheca envelops both the outer (rostral) and part of the inner (oral) surfaces of the jawbones, functionally replacing teeth for food acquisition and manipulation. Within theropod dinosaurs alone, fully edentulous beaks evolved independently at least seven times, appearing in lineages such as Oviraptorosauria, Ornithomimosauria, Therizinosauria, Ceratosauria (notably Limusaurus), and multiple clades of Mesozoic birds. Ornithischian dinosaurs, including ceratopsians and hadrosaurs, also possessed beaks, though typically in combination with posterior dentition. Biomechanical analyses using finite element modeling have demonstrated that keratinous beaks reduce stress and strain in the rostral skull, enhancing structural stability during feeding. The repeated convergent evolution of toothless beaks across Dinosauria reflects a complex interplay of selective pressures, including dietary shifts toward herbivory or omnivory, weight reduction, enhanced cranial stability, and possibly shorter incubation periods linked to the elimination of embryonic tooth development.