Mass extinction of lizards and snakes at the Cretaceous-Paleogene boundary

Abstract

The Cretaceous-Paleogene (K-Pg) boundary is marked by a major mass extinction, yet this event is thought to have had little effect on the diversity of lizards and snakes (Squamata). A revision of fossil squamates from the Maastrichtian and Paleocene of North America shows that lizards and snakes suffered a devastating mass extinction coinciding with the Chicxulub asteroid impact. Species-level extinction was 83%, and the K-Pg event resulted in the elimination of many lizard groups and a dramatic decrease in morphological disparity. Survival was associated with small body size and perhaps large geographic range. The recovery was prolonged; diversity did not approach Cretaceous levels until 10 My after the extinction, and resulted in a dramatic change in faunal composition. The squamate fossil record shows that the end-Cretaceous mass extinction was far more severe than previously believed, and underscores the role played by mass extinctions in driving diversification.

Fig. 1.

Maastrichtian lizards and snakes. (A–H) Lizard dentaries; (I and J) snake vertebrae. (A) Socognathus brachyodon, n. sp., YPM-PU 16724 Lance Formation, western WY. (B) Obamadon gracilis, n. gen et sp., UCMP 128873, Hell Creek Formation, MT. (C) Laramie polyglyphanodont, UCM 42164, Laramie Formation, CO. (D) Cemeterius monstrosus n. gen. et sp., USNM 25870, Hell Creek Formation, MT. (E) Pariguana lancensis n. gen et sp., AMNH 22208; Lance Formation, eastern WY. (F) Lamiasaurus ferox n. gen et sp., UW 25116A, Lance Formation, southern WY. (G) Lonchisaurus trichurus n. gen et sp., AMNH 15446, Lance Formation, eastern WY. (H) Sweetwater lizard, UW 25116B, Lance Formation, southern WT. (I) Cerberophis robustus n. gen et sp., UCMP 130696, Hell Creek Formation, MT. (J) Lance alethinophidian, Lance Formation, WY.

Fig. 2.

Systematics of North American squamate assemblage. Adams consensus with alethinophidian placement following a recent study (5). Complete results are provided in SI Appendix. Species crossing the K-Pg boundary are shown in bold type.

Fig. 3.

Comparisons of late Maastrichtian and Paleocene diversity. (A) Late Maastrichtian (Lancian) and early Paleocene (Puercan-Torrejonian). (B) Diversity of lizard genera by land vertebrate age (using range-through assumption), Aquilan to Wasatchian.

Fig. 4.

Selective survivorship of squamates at the K-Pg boundary. (A) Cenograms (size data for species in rank order), with tooth diameter as a proxy for size, for K-Pg survivors and victims. (B) Logistic regression, showing decreasing survival probability at larger sizes. (C) Number of localities occupied by survivors and victims. (D) Logistic regression showing increasing survival probability with increasing geographic range (using number of localities as a proxy for range size).

Fig. 5.

Disparity of late Maastrichtian and early Paleocene squamates. (A) Cenograms showing Maastrichtian and Paleocene size disparity. (B) Comparison of shape disparity in teeth of Maastrichtian and Paleocene squamates, showing results of a principle components analysis of 2D landmark data. The Maastrichtian fauna is circumscribed by the red hull, with survivors bounded by a dark-gray hull and survivors plus immigrants bounded by a light-gray hull.

Fig. 6.

Cretaceous–Paleogene faunal succession. The Cretaceous fauna is dominated by Polyglyphanodontidae, whereas the Paleogene fauna is dominated by Anguidae and Iguanidae.