Theropoda

Theropoda (from the Ancient Greek therion, "wild beast," and pous, "foot," coined by Othniel Charles Marsh in 1881) is the suborder and clade of bipedal saurischian dinosaurs (Dinosauria) that encompasses all Mesozoic carnivorous dinosaurs—including Coelophysis, Ceratosaurus, Spinosaurus, Allosaurus, Giganotosaurus, Tyrannosaurus rex, and Velociraptor—as well as several secondarily herbivorous and omnivorous Mesozoic clades (Therizinosauria, Ornithomimosauria, and Oviraptorosauria) and all ~11,000 living species of birds (Aves). Because modern birds are nested deeply inside Maniraptora (specifically within Paraves alongside dromaeosaurids and troodontids), Theropoda is the only clade of dinosaurs that survived the 66 Ma Cretaceous–Paleogene (K–Pg) extinction event and remains one of the most speciose vertebrate lineages on Earth today.

Appearing in the Late Triassic (Carnian–Norian, ~231–215 million years ago) as slender, 1- to 3-meter cursorial bipeds (Eodromaeus, Coelophysis bauri), non-avian theropods rapidly ascended to become the sole large-bodied terrestrial apex predators on every continent throughout the Jurassic and Cretaceous Periods (~201 to 66 Ma).

Long before the origin of powered flight in Archaeopteryx and Aves, non-avian theropod dinosaurs assembled almost the entire suite of skeletal, respiratory, and integumentary characters once thought to be uniquely avian:

1. Hollow, Thin-Walled Bones and Postcranial Skeletal Pneumaticity: All theropods possess hollow long bones (femur, tibia, humerus) with thin, dense cortical bone walls surrounding a marrow cavity. Furthermore, across Neotheropoda and Tetanurae, epithelial diverticula of the cervical and abdominal pulmonary air sacs invaded the cervical, dorsal, and sacral vertebrae (via lateral openings called pleurocoels), demonstrating that unidirectional, flow-through avian-style air-sac lungs (O'Connor & Claessens, 2005) evolved in bipedal predatory theropods to sustain high aerobic metabolic scopes (endothermy).
2. Obligate Bipedalism and the Tridactyl (Three-Toed) Mesaxonic Pes: Unlike Sauropodomorpha and many Ornithischia (which reverted to quadrupedal postures), every known theropod in 230 million years of history—from a 12-ton Tyrannosaurus rex to a 2-gram hummingbird—remained an obligate biped! The hindfoot (pes) is functionally tridactyl (three-toed) and digitigrade: body weight is carried on digits II, III, and IV (with the central digit III longest), while the fifth toe (digit V) is reduced to a tiny splint at the ankle and the first toe (hallux, digit I) is shortened and elevated off the ground (before rotating backward to become a grasping perching hallux in arboreal Avialae / Aves!).
3. The Furcula (Wishbone), Three-Fingered Manus, and Semilunate Carpal:
   • Furcula: Once thought to exist only in birds, a fused median wishbone (furcula)—formed by the midline fusion of the left and right clavicles and acting as an elastic biomechanical spring between the shoulders during forelimb predatory striking—is present across nearly all theropods from Coelophysis and Allosaurus to Tyrannosaurus and Velociraptor (Nesbitt et al., 2009)!
   • Progressive Digit Reduction in the Hand (Manus): Basal theropods (Herrerasaurus, Coelophysis) had 4 or 5 fingers with digits IV and V reduced; Tetanurae (Allosaurus, Velociraptor, Archaeopteryx) stabilized on a three-fingered grasping hand armed with curved recurved claws; and Tyrannosauridae (Tyrannosaurus, Gorgosaurus) reduced the hand to two functional fingers (or one finger in the insectivorous alvarezsaurid Mononykus and Linhenykus).
   • Semilunate Carpal: In Maniraptora (Oviraptor, Deinonychus, Velociraptor, and birds), the distal wrist bones fuse into a half-moon-shaped pulley block—the semilunate carpal—allowing the hand to fold sideways against the forearm (a motion evolved originally for raptorial folding and protecting long feathered forelimbs before being co-opted into the avian wing-folding stroke!).

Beginning in 1996 with the discovery of Sinosauropteryx prima in the Early Cretaceous Jehol Biota (Yixian and Jiufotang Formations, ~125–120 Ma) of Liaoning, China, paleontologists have recovered over 50 species of non-avian dinosaurs preserved with carbonized feathers and melanosomes, confirming step-by-step the Richard O. Prum & Alan H. Brush (2002) developmental model showing that feathers evolved first for thermal insulation, brooding, and visual signaling—millions of years before being co-opted for flight:

1. Stage I — Single Hollow Cylindrical Filaments ("Protofeathers"): Unbranched tubular filaments produced by a cylindrical epidermal follicle collar, preserved along the back and tail of the basal coelurosaur Sinosauropteryx prima (whose melanosomes were reconstructed by Zhang et al., 2010 and Smithwick et al., 2017 as a ginger/chestnut-orange countershaded coat with a raccoon-like bandit mask and a striped tail!).
2. Stage II — Radially Branched Downy Tuft Plumules: Multiple barbs fused at their base to a short calamus without a central shaft (rachis), forming an insulating pelt across the body of Sinornithosaurus and the 9-meter, 1.4-metric-ton early tyrannosauroid Yutyrannus huali (Xu et al., 2012)—the largest known fully feathered animal!
3. Stage III — Central Rachis with Unhooked Planar Barbs: Barbs fuse along a central shaft (rachis) to form a flat, symmetrical vane lacking hooked barbules, preserved in therizinosaurians (Beipiaosaurus).
4. Stage IV — Closed Pennaceous Vanes with Hooked Barbules (Symmetrical): Distal and proximal barbules evolve interlocking hooklets (barbicels), zipping the barbs into a closed, planar pennaceous feather attached to the arms ( forming quill knobs on the ulna of Velociraptor; Turner et al., 2007) and tail fan of non-flying Oviraptorosauria (Caudipteryx, Similicaudipteryx) and Ornithomimosauria (Ornithomimus, Zelenitsky et al., 2012)—used to shade and brood nests of eggs and for courtship displays!
5. Stage V — Asymmetric Aerodynamic Flight Feathers: The leading-edge vane narrows relative to the trailing-edge vane to resist aerodynamic twisting during wing downstrokes, preserved on both the forelimbs and hindlimbs of the four-winged paravian Microraptor gui (Xu et al., 2003, covered in iridescent glossy black feathers; Li et al., 2012), Archaeopteryx, and Crown Aves.

How fast did Tyrannosaurus rex grow, and could an adult T. rex actually run?

Histological counting of annual Lines of Arrested Growth (LAGs) in cross-sections of Tyrannosaurus rex leg bones by Gregory M. Erickson, Peter J. Makovicky, Philip J. Currie, and colleagues (2004) revealed that T. rex lived a "live fast, die young" life history: juveniles remained relatively small and slender (<1,000–1,800 kg) until age 13–14, and then underwent an rapid teenage growth spurt between ages 14 and 18, packing on 2.1 kilograms (4.6 pounds) of body mass every single day for four years before plateauing near adult size (~8,000+ kg) at age ~20 and rarely living past age 28 to 33! While slender juvenile T. rex (formerly misclassified as "*Nanotyrannus*") were agile high-speed runners, multi-body biomechanical simulations incorporating bone bending stress limits by John R. Hutchinson and Mariano Garcia (2002) and William I. Sellers et al. (2017) proved that an 8-ton adult T. rex could not execute an aerial running phase without shattering its own tibia and femur—reaching a maximum fast-walking speed of ~20 to 27 km/h (12–17 mph), which was still faster than its multi-ton Triceratops and Edmontosaurus prey!

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