FOSSIL VERTEBRATES/Dinosaurs 495 carnivores include the hunting and killing of prey in their behavioural repertoire, and it is most unlikely that theropods did not The strong build of their skulls and their grasping hands seem adapted for grappling with powerful struggling prey, supporting this idea One clade of derived theropods, the Coelurosauria, produced a range of divergent body plans This radiation probably began in the Middle Jurassic, but coelurosaurs did not become abundant until the Cretaceous The group included gigantic carnivores (tyrannosaurids), toothless presumed herbivores (ornithomimosaurs and oviraptorosaurs), and long-necked herbivores (therizinosaurs) The radiation also produced the sickle-clawed deinonychosaurs and their close relatives the flying dinosaurs or birds Bird Origins Dinosaurs did not become entirely extinct at the end of the Cretaceous One highly specialized group of theropods, the birds, survived and are still a major part of modern vertebrate faunas (see Fossil Vertebrates: Birds) The great preponderance of evidence from skeletal anatomy, eggshell microstructure, and integumentary structures indicates that the origin of birds lies deep within the theropod clade, with the Deinonychosauria being their closest relatives These two groups share a number of significant similarities, including a decoupling of the tail from the muscular system driving the hind legs (shown by the extreme mobility of the articulations of the proximal tail vertebrae and the reduction of the bony attachment sites for the caudofemoralis muscle), retroversion of the pubis (similar to the condition in Ornithischia), and a laterally facing shoulder socket that allows a flapping motion of the forelimb The caudofemoralis muscle is the main leg-retracting muscle in modern reptiles and extends from the posterior surface of the femur to the side of the tail, keeping the two organs tightly coupled during locomotion The most compelling evidence for the dinosaur ancestry of birds is the discovery of numerous species of non-avian coelurosaur that are preserved with a covering of feathers in the Early Cretaceous rocks of the Liaoning Province, China These range from simple filaments in the primitive Sinosauropteryx to complex vaned feathers (with a central rachis and lateral barbs) in more derived species such as Caudipteryx, Protarchaeopteryx, Sinornithosaurus, and Microraptor The environment in which birds evolved and the possible behaviours and selection pressures that led to the origin of flight are more difficult to elucidate Critics of the dinosaur hypothesis for the origin of birds point to the improbability of flight evolving in a ground-dwelling runner Nevertheless there is no logical reason why some dinosaurs could not have adopted a scansorial (climbing) existence Indeed, some of the newly discovered small deinonychosaurs from Liaoning seem to show some scansorial adaptations Physiology No other area of investigation into dinosaur biology has produced as much controversy as the issue of their metabolic and thermal physiology Although the debate is often portrayed as a simple dichotomy between ‘warm-blooded’ and ‘cold-blooded’ dinosaurs, the issue is more complicated Few doubt that dinosaurs were capable of sustained aerobic exercise and that they maintained at least some degree of homeothermy (stable body temperatures); these are not typical characteristics of modern ectotherms (animals that require an external heat source to reach their optimum body temperature) Evidence for this comes from their erect gaits, their frequent cursorial adaptations, and the fibrolamellar microstructure of their bones (indicating fast growth) The question remains, did they achieve homeothermy through an elevated resting metabolic rate (tachymetabolism) and the internal heat that it generates (endothermy), or did they use other means, such as the thermal inertia resulting from their large size and the fairly equable climate of the Mesozoic? Most modern ectotherms cannot maintain homeothermy (a condition known as poikilothermy) but there are exceptions, such as the leatherback turtle (Dermochelys) Most of the evidence that has been used to support endothermy in non-avian dinosaurs either implies homeothermy, but not necessarily endothermy, or simply cannot be trusted owing to the distortions caused by the processes of taphonomy and diagenesis (e.g predator–prey ratios and stable isotope signatures in bone) The evidence for ectothermy is also weak, although it has been claimed by some researchers that the absence of nasal passages large enough to house respiratory turbinate bones is the ‘Rosetta Stone’ that demonstrates dinosaurian ectothermy However, the recent discovery of small coelurosaurs that were insulated by feathers means that it is almost certain that at least these dinosaurs were endotherms Since coelurosaurs are included amongst the dinosaurs that have been claimed to be ectotherms by virtue of their small nasal passages, it would now seem that nasal-passage size and thermal physiology are not as tightly correlated as the proponents of this hypothesis contend