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.

**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.

**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.