What Is a Theropod? The Group That Never Actually Went Extinct

Theropods are one of the two main branches of saurischian dinosaurs, distinguished by a set of skeletal features rather than by diet. They are bipedal, walking on the hind limbs with the forelimbs freed for other uses. They have hollow bones with air spaces connected to the respiratory system, which made them light. They have three functional weight-bearing toes on each foot, with the first toe reduced and held clear of the ground. Their hand is reduced towards three fingers in most lineages, with a characteristic arrangement of wrist bones that permitted a folding motion, and that motion is the direct ancestor of the wing fold in birds. The group first appears in the late Triassic and includes animals from under a kilogram to many tonnes, and while the large predators dominate popular attention, the group also produced herbivores, omnivores, insectivores and filter feeders, so the association with meat eating is a generalisation rather than a definition.

The group divides into recognisable lineages with different builds and habits:

• •Coelophysoids and other early forms, slender and lightly built, the first theropods to appear
• •Ceratosaurs, a diverse group including horned forms and the abelisaurs that dominated southern continents in the Cretaceous
• •Megalosaurs and spinosaurs, the latter with crocodile-like skulls and evidence of a substantially aquatic or fish-eating life
• •Allosaurs and their relatives, the dominant large predators of the Jurassic
• •Tyrannosaurs, a group that was small and unremarkable for most of its history and produced enormous forms only near the end of the Cretaceous
• •Coelurosaurs generally, an enormous assemblage including ostrich-like forms, the herbivorous therizinosaurs, the oviraptorosaurs, the dromaeosaurs and troodontids, and the birds

That birds are theropods is among the best supported conclusions in vertebrate palaeontology and rests on many independent lines of evidence. Skeletal features shared with dromaeosaurs and troodontids are numerous and detailed, including the wishbone, the semilunate wrist bone that allows the folding motion, hollow pneumatic bones, the arrangement of the pelvis and hind limb, and the structure of the skull. Feathers are now known from a substantial number of non-avian theropods, ranging from simple filaments to fully formed flight feathers, and preserved pigment structures have allowed the colours of several to be reconstructed. Behavioural evidence includes brooding postures preserved on nests, medullary bone indicating egg-laying physiology, and sleeping postures with the head tucked as birds do. Growth rates from bone microstructure match birds rather than reptiles. The sequence in the fossil record shows the features accumulating gradually rather than appearing together, which is what an evolutionary transition looks like and which makes the boundary between bird and non-bird a matter of definition.

Very large body size evolved independently in several theropod lineages rather than once, which is visible in the fossil record and is one of the reasons the group is studied for what limits animal size. Allosaurs, megalosaurs, spinosaurs, carcharodontosaurs, abelisaurs and tyrannosaurs each produced large forms from smaller ancestors at different times and on different continents, and the largest of them approached or exceeded the size of the most famous example. Growth rings in bone show that the largest reached adult size in under two decades through a period of extremely rapid growth in adolescence, which is closer to a bird or a mammal than to a reptile. Bite force estimates for the largest tyrannosaurs exceed anything measured in a living animal, achieved through a heavily reinforced skull and thick conical teeth suited to crushing rather than only slicing, and that construction is unusual within the group, since most large theropods had lighter skulls and blade-like teeth for cutting. Forelimbs reduced repeatedly and independently as skull size increased, which suggests a functional trade rather than a coincidence.

Public attention concentrates on the largest predators and the group's interest lies substantially elsewhere. Several lineages became herbivorous independently, with therizinosaurs developing huge claws, small heads and broad guts consistent with plant eating, and ornithomimosaurs losing their teeth entirely in favour of beaks with evidence of filter feeding in some forms. Oviraptorosaurs developed strange crested skulls and powerful beaks of uncertain diet. Alvarezsaurs became small with extremely reduced forelimbs bearing a single large claw, apparently for breaking into insect nests. Dromaeosaurs include forms with four wings and evidence of gliding, indicating that flight or something like it was experimented with repeatedly rather than achieved once. Body size shrank steadily along the lineage leading to birds over tens of millions of years, which has been measured directly and is one of the more striking results from quantitative work on the group, since sustained miniaturisation over that timescale is unusual and appears to be what made flight possible.

The group is defined by skeletal features including hollow air-filled bones, three weight-bearing toes and a folding wrist, not by diet, and it produced herbivores and filter feeders alongside the famous predators. Birds sit inside it, supported by shared anatomy, feathers on many non-avian members, brooding postures on nests and matching growth rates. Body size shrank steadily along the line towards birds, which is what made flight possible.