Dinosaurs are a diverse group of animals of the clade
Dinosauria. They first appeared during the Triassic period, approximately 230
million years ago, and were the dominant terrestrial vertebrates for 135
million years, from the beginning of the Jurassic until the end of the
Cretaceous, when the Cretaceous–Paleogene extinction event led to the
extinction of most dinosaur groups at the close of the Mesozoic Era. The fossil
record indicates that birds evolved from theropod dinosaurs during the Jurassic
Period and, consequently, they are considered a subgroup of dinosaurs by most
paleontologists.
Some birds survived
the extinction event that occurred 66 million years ago, and their descendants
continue the dinosaur lineage to the present day. Dinosaurs are a varied group
of animals from taxonomic, morphological and ecological standpoints. Birds, at
over 9,000 living species, are the most diverse group of vertebrates besides
perciform fish. Using fossil evidence, paleontologists have identified over 500
distinct genera and more than 1,000 different species of non-avian dinosaurs.
Dinosaurs are represented on every continent by both extant species and fossil
remains. Some are herbivorous, others carnivorous. While dinosaurs were
ancestrally bipedal, many extinct groups included quadrupedal species, and some
were able to shift between these stances.
Elaborate display structures such as horns or crests are
common to all dinosaur groups, and some extinct groups developed skeletal
modifications such as bony armor and spines. Evidence suggests that egg laying
and nest building are additional traits shared by all dinosaurs. While modern
birds are generally small due to the constraints of flight, many prehistoric
dinosaurs were large-bodied the largest sauropod dinosaurs may have achieved
lengths of 58 meters and heights of 9.25 meters. Still, the idea that non-avian
dinosaurs were uniformly gigantic is a misconception based on preservation
bias, as large, sturdy bones are more likely to last until they are fossilized.
