Big-headed marine crocodyliforms and why we must be cautious when using extant species as body length proxies for long-extinct relatives

Body size is commonly used as a key variable for estimating ecomorphological trends at a macroevolutionary scale, making reliable body length estimates of fossil taxa critically important. Crocodylomorphs (extant crocodylians and their extinct relatives) evolved numerous 'aberrant' body-plans during their ~230 million-year history, ranging from ‘hooved’ terrestrial species to dolphin-like pelagic species. Such clades evolved distinct cranial and femoral scaling ratios (compared to total body length), thereby making extant taxa unsuitable proxies for estimating their body lengths. Here we illustrate that the fossil clade Teleosauridae also fits into this category. Teleosaurids were a predominately shallow marine clade that had a global distribution during the Jurassic. Known to have evolved a wide range of body lengths (2–5 m based on complete skeletons), there is currently no way of reliably estimating the size of incomplete specimens. This is surprising, as some teleosaurids have been considered very large (9–10 m in total length), thus making Teleosauridae the largest bodied clade during the first 100 million years of crocodylomorph evolution. Our examination and regression analyses of the best preserved teleosaurid skeletons demonstrates that: they were smaller than previously thought, with no known specimen exceeding 7.2 m in length; and that they had proportionally large skulls, and proportionally short femora, when compared to body length. Therefore, while many teleosaurid species evolved a cranial length of ≥1 m, these taxa would not necessarily have been larger than species living today. We advise caution when estimating body length for extinct taxa, especially for those outside of the crown group. Mark T. Young. School of GeoSciences, The King’s Buildings, University of Edinburgh, James Hutton Road, Edinburgh, EH9 3FE. myoung5@staffmail.ed.ac.uk Márton Rabi. (1) Department of Earth Sciences, University of Turin, Via Valperga Caluso 35, 10125, Turin, Italy. (2) Institut für Geowissenschaften, University of Tübingen, Hölderlinstraße 12, 72074 Tübingen, Germany. iszkenderun@gmail.com Mark A. Bell. Department of Earth Sciences, Kathleen-Lonsdale Building, University College London, Gower Street, London, WCIE 6BT. mark.bell521@gmail.com Davide Foffa. School of GeoSciences, The King’s Buildings, University of Edinburgh, James Hutton Road, Edinburgh, EH9 3FE. davidefoffa@gmail.com Young, Mark T. , Rabi, Márto, Bell, Mark A. , Foffa, Davide, Steel, Lorna, Sachs, Sven, and Peyer, Karin. 2016. Big-headed marine crocodyliforms and why we must be cautious when using extant species as body length proxies for long-extinct relatives. Palaeontologia Electronica 19.3. 30A: 1-14 palaeo-electronica.org/content/2016/1554-teleosaurid-size-estimation Copyright: Palaeontological Association September 2016 YOUNG ET AL.: TELEOSAURID SIZE ESTIMATION Lorna Steel. Department of Earth Sciences, Natural History Museum, Cromwell Road, London, SW7 5BD, United Kingdom. l.steel@nhm.ac.uk Sven Sachs. Naturkundemuseum Bielefeld, Abteilung Geowissenschaften, Adenauerplatz 2, 33602 Bielefeld, Germany and Im Hof 9, 51766 Engelskirchen, Germany. sachs.pal@gmail.com Karin Peyer. Département Histoire de la Terre, Muséum National d'Histoire Naturelle, Paris, 8 rue Buffon, CP 38, F-75005, France. karin_peyer@yahoo.fr