more rapid evolutionary changes in theropods than their other dinosaurian contemporaries. Furthermore, this rapidity may have been coupled with:
1 higher extinction rates of theropod species;
2 higher extinction rates of organisms interacting with theropods; or
3 faster changes in gene frequencies in association with environmentally-related selection pressures.
Rapid diversification in lineages is often a result of high reproductive success and rapid growth rates combined with environmental changes that encouraged the selection of specific and overlapping inheritable traits (Chapter 6). Extinctions of major archosaur groups, as recorded in Upper Triassic rocks, may have opened niches for theropods in the ecosystems of those times and thus contributed to their evolution.
Greater numbers and species of dinosaurs corresponded with the demise of other archosaurs by the beginning of the Jurassic Period, with theropods a prime example of this ascendancy (Chapter 6). Herrerasaurids of the Late Triassic were joined by abundant ceratosaurs, such as Coelophysis, and these were succeeded by larger ceratosaurs (Dilophosaurus, Syntarsus, and Ceratosaurus) by the Early and Late Jurassic. The Middle Jurassic saw the arrival of even larger and presumably more specialized carnivores, among them the first tetanurans (Megalosaurus). Although they never reached the enormous sizes of sauropods (Chapter 10), theropods trended toward increased body size by the Late Jurassic, an apparent example of Cope's Rule (Chapter 6). One example was Allosaurus, which probably weighed as much as 3 metric tons. However, one of its contemporaries, Compsognathus, was among the smallest of all known dinosaurs, weighing only about 3 kg.
By the Late Jurassic, the first avians evolved from theropod ancestors, represented by what is widely acknowledged by paleontologists as the earliest known bird in the geologic record, Archaeopteryx (Chapter 15). Following this, the Cretaceous saw numerous small birds co-existing in terrestrial environments with feathered non-avian theropods. However, some theropod lineages evolved in one important way, that is, their descendants became much larger. The largest land carnivores that ever lived are from the Early Cretaceous (Giganotosaurus and Carcharodontosaurus) and Late Cretaceous (Albertosaurus and Tyrannosaurus). Of course, not all Late Cretaceous theropods were enormous. Some of the most behaviorally interesting theropods were relatively small, such as Velociraptor, Troodon, and Oviraptor. Late Cretaceous theropods, large or small, feathered or scaled, toothed or non-toothed, comprised the greatest diversity of dinosaurs known.
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