Chapter 16. Archosaurs

16.1 Mesozoic Fauna251 mya - 65.5 mya-Diversification of dinosaurs occured in the late Triassic in 3 stages 231 mya, initial appearance major radiation in the mid-late Jurassic several pulses followed, most prominent in 2nd half of Cretaceous -Extinction of non-dinosaur lineages opening niches to dinosaurs vs dinosaurs outcompeting non-dinos

16.2 Diapsid Characteristics-two arches, upper and lower fenestra-lower arch has been lost in radiation, sometimes upper arch missing- increasing cranial kinesis-in archosaurs: antorbital fenestra inverted triangular shaped orbital laterally compressed teeth fourth trochanter for tail muscle attachment on leg -diapsids can be split into two lineages: Archosaurs (crocs, birds, dinosaurs, pterosaurs) Lepidosaurs (squamates and tuatara, plesiosaurs, placodonts, ichthyosaurs)

16.3 Marine LineagesPlacodonts:-least specialized-Triassic, Tethys sea -dermal bone, armor along dorsal surface-possibly herbivorous, distinct wearing on anterior teethPlesiosaurs:-Late Triassic, until end of Cretaceous - elongated necks or long heads-limbs acting as oars, hyperphalangy