none 10.1666/09023.1 Cambridge University Press (CUP) Cambridge University Press 56 48386293 48518 1 10.1017 2026-02-10T16:12:36Z 2011-06-08T16:51:05Z 46 Paleobiology Paleobiology 0094-8373 1938-5331 2011 37 3 Johan Lindgren Michael J. Polcyn Bruce A. Young Incremental stages of major evolutionary transitions within a single animal lineage are rarely observed in the fossil record. However, the extraordinarily complete sequence of well preserved material spanning the 27-Myr existence of the marine squamate subfamily Mosasaurinae provides a unique exception. By comparison with extant and extinct analogs, the tail morphology of four mosasaurine genera is examined, revealing a pattern of evolution that begins with the generalized varanoid anatomy and culminates in a high-aspect-ratio fin, similar to that of sharks. However, unlike the epicercal caudal fluke of selachians in which the tail bends dorsocaudally, derived mosasaurs develop a hypocercal tail with a ventrocaudal bend. Progressive caudal regionalization, reduced intervertebral mobility, increased tail depth due to a marked downturn of the posterior caudal segment, and the development of finlike paired appendages reveal a pattern of adaptation toward an optimized marine existence. This change in morphology reflects a transition from anguilliform or sub-carangiform locomotion to carangiform locomotion, and indicates a progressive shift from nearshore dwellers to pelagic cruisers—a change in foraging habitat independently corroborated by paleobiogeographic, stable isotope, osteohistological, and paleopathological data. Evolutionary patterns similar to those observed in mosasaurine mosasaurs are seen in other secondarily aquatically adapted amniotes, notably metriorhynchid crocodyliforms, cetaceans, and ichthyosaurs, and may be explained by developmental modularity governing the observed phenotypic expression. 04 08 2016 2011 445 469 S0094837300024970 https://www.cambridge.org/core/terms 10.1666/09023.1 https://www.cambridge.org/core/product/identifier/S0094837300024970/type/journal_article https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0094837300024970 http://www.bioone.org/doi/10.1666/09023.1 10.1007/s00285-006-0036-8 Zoologica The locomotion of fishes 4 159 1926 Breder 10.1098/rsbl.2009.0097 10.1111/j.0022-1112.2004.00568.x 10.1163/156853896X00162 Proceedings of the Indian Academy of Sciences Studies on the comparative anatomy of the tail in Sauria and Rhynchocephalia. Part I. 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