Major Features of Lizard Evolution
Gauthier, Jacques A.
Department of Geology and Geophysics
Yale Peabody Museum of Natural History
New Haven, CT
The composition and distribution of lizards alive today cannot be understood except in light of the fossil record. The divergence between sphenodonts and squamates (= lizards) likely occurred immediately after the Permo-Triassic Mass Extinction. Durable jawbones of stem sphenodonts are common in the early Mesozoic, but no stem lizards are known from that time. The disparity in relative abundance persists for the next 60 million years; stem sphenodonts are still abundant, and even though lizard backbone clades diverged, lizards remain relatively rare in the Late Jurassic. Everything changed after the Early Cretaceous Terrestrial Revolution after which lizards rose to dominance. The stem lizard from that interval, Huehuecuetzpalli mixtecus, is very primitive, but reveals a number of distinctly lizard-like characters, including cranial kinesis and modifications for running and climbing. Crown lizards radiated in the Hot House climates that prevailed for the next 100 million years from the Early Cretaceous to the Early Oligocene, yet everything changed radically in the middle of that interval as a rock the size of Manhattan Island collided with the Earth. Marine lizards were wiped out, as were 83% of terrestrial species inhabiting the American Western Interior, with only a few secretive anguimorphs and xantusiids surviving. Survivors spread throughout Laurasia during the early Paleogene. The transition from Hot House to Ice House climates restricted megathermal lizards to lower latitudes. Care should be taken when interpreting lizards currently in South America and Africa as being Gondwanan. If they are not also in Australia, they may, like amphisbaenians or snakes, have originated in Laurasia and subsequently invaded Gondwanan landmasses. The final formative event was the ‘Fall’ from 12-15Ma to modern climates, unique in Phanerozoic history for glaciers on both poles. Arid-land lizards such as phrynosomtines and diurnal snakes such thamnophiines rose to dominance in North America at that time.