Evolution of dinosaurs
This article gives an outline and examples of dinosaur
evolution. For a detailed list of interrelationships see
Dinosaur classification.
Evolution of dinosaurs
Dinosaurs evolved from the 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
The process leading up the first dinosaurs can be fol-
lowed 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.[1]
The Dinosaurs can be defined as the last common an-
cestor of birds (Saurischia) and Triceratops (Ornithischia)
and all the descendants of that ancestor. With that
definition, the pterosaurs and several species of archo-
saurs narrowly miss out on being classed as Dinosaurs.
The pterosaurs are famous for flying through the Meso-
zoic skies on leathery wings. Archosaur species that nar-
rowly miss out on being classed as dinosaurs include
Schleromochlus 220-225 Ma, Lagerpeton 230-232 Ma and
Marasuchus 230-232 Ma.
Earliest dinosaurs
The first known dinosaurs were bipedal predators that
were one to two metres long.
Spondylosoma may or may not be a dinosaur, the
fossils
(all
postcranial)
are
tentatively dated at
230-232 Ma.[1]
The earliest confirmed dinosaur fossils include the
Saurischia
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 bas-
al Saurischians.
The earliest Ornithischia
includes Pisanosaurus
220-230 Ma.
Although
Lesothosaurus
comes
from
195-206 Ma, skeletal features suggest that it branched
fr