Evolution of dinosaurs
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- This article gives an outline and examples of dinosaur evolution. For a detailed list of interrelationships see Dinosaur classification.
Dinosaurs evolved
Evolution of dinosaurs
|This article needs additional citations for verification. (June 2009)|
- This article gives an outline and examples of dinosaur evolution. For a detailed list of interrelationships see Dinosaur classification.
Dinosaurs evolved from a single lineage of archosaurs 232-234 Ma (million years ago) in the Ladinian age, the latter part of the middle Triassic. Dinosauria is a well-supported clade, present in 98% of bootstraps. It is diagnosed by many features including loss of the postfrontal on the skull and an elongate deltopectoral crest on the humerus.
- 1 From archosaurs to dinosaurs
- 2 Earliest dinosaurs
- 3 Saurischia
- 4 Ornithischia
- 5 Fossil record
- 6 Evolutionary trends
- 7 Biogeography
- 8 Extinction
- 9 See also
- 10 Footnotes
- 11 References
- 12 External links
From archosaurs to dinosaurs
The process leading up to the Dinosauromorpha and the first true dinosaurs can be followed through fossils of the early Archosaurs such as the Proterosuchidae, Erythrosuchidae and Euparkeria which have fossils dating back to 250 Ma, through mid-Triassic archosaurs such as Ticinosuchus 232-236 Ma. Crocodiles are also descendants of mid-Triassic archosaurs.
Dinosaurs can be defined as the last common ancestor of birds (Saurischia) and Triceratops (Ornithischia) and all the descendants of that ancestor. With that definition, the pterosaurs and several species of archosaurs narrowly miss out on being classified as dinosaurs. The pterosaurs are famous for flying through the Mesozoic skies on leathery wings and reaching the largest sizes of any flying animal that ever existed. Archosaur genera that also narrowly miss out on being classified as dinosaurs include Schleromochlus 220-225 Ma, Lagerpeton 230-232 Ma and Marasuchus 230-232 Ma.
The earliest confirmed dinosaur fossils include saurischian ('lizard-hipped') dinosaurs Nyasasaurus 243 Ma, Saturnalia 225-232 Ma, Herrerasaurus 220-230 Ma, Staurikosaurus possibly 225-230 Ma, Eoraptor 220-230 Ma and Alwalkeria 220-230 Ma. Saturnalia may be a basal saurischian or a prosauropod. The others are basal saurischians.
Among the earliest ornithischian ('bird-hipped') dinosaurs is Pisanosaurus 220-230 Ma. Although Lesothosaurus comes from 195-206 Ma, skeletal features suggest that it branched from the main Ornithischia line at least as early as Pisanosaurus.
It is clear from this figure that early saurischians resembled early ornithischians, but not modern crocodiles. Saurischians are distinguished from the ornithischians by retaining the ancestral configuration of bones in the pelvis. Another difference is in the skull, the upper skull of the Ornithischia is more solid and the joint connecting the lower jaw is more flexible; both are adaptations to herbivory and both can already be seen in Lesothosaurus.
Setting aside the basal Saurischia, the rest of the Saurischia are split into the Sauropodomorpha and Theropoda. The Sauropodomorpha is split into Prosauropoda and Sauropoda. The evolutionary paths taken by the Theropoda are very complicated. The Dinosauria (2004), a major reference work on dinosaurs, splits the Theropoda into groups Ceratosauria, Basal Tetanurae, Tyrannosauroidea, Ornithomimosauria, Therizinosauroidea, Oviraptorosauria, Troodontidae, Dromaeosauridae and Basal Avialae in turn. Each group branches off the main trunk at a later date. See Dinosaur classification for the detailed interrelationships between these.
The first sauropodomorphs were prosauropods. Prosauropod fossils are known from the late Triassic to early Jurassic 227-180 Ma. They could be bipedal or quadrupedal and had developed long necks and tails and relatively small heads. They had lengths of 2.5 (8.2 ft) to 10 m (33 ft) and were primarily herbivorous. The earliest prosauropods, such as Thecodontosaurus from 205-220 Ma, still retained the ancestral bipedal stance and large head to body ratio.
These evolved into the sauropods which became gigantic quadrupedal herbivores, some of which reached lengths of at least 26 m (85 ft). Features defining this clade include a ratio of forelimb length to hindlimb length greater than 0.6. Most sauropods still had hindlimbs larger than forelimbs; one notable exception is Brachiosaurus whose long forelimbs suggest that it had evolved to feed from tall trees like a modern-day giraffe.
Sauropod fossils are found from the times of the earliest dinosaurs right up to the Cretaceous–Paleogene extinction event event, from 227 to 66 Ma. Most sauropods are known from the Jurassic, to be more precise between 227 and 121 Ma.
The Cretaceous sauropods form two groups. The Diplodocoidea lived from 121 to 66 Ma. The Titanosauriformes lived from 132 to 66 Ma. The latter clade consists of series of nested subgoups, the Titanosauria, the Titanosauridae and Saltasauridae. Both the Diplodocoidea and Titanosauriformes are descended from the Neosauropoda, the earliest of which lived in about 169 Ma.
The sauropods are famous for being the largest land animals that ever lived, and for having relatively small sk