Abstract: The taxonomic origin of the white shark, Carcharodon , is a highly debated subject. New fossil evidence presented in this study suggests that the genus is derived from the broad‐toothed ‘mako’, Carcharodon ( Cosmopolitodus ) hastalis , and includes the new species C . hubbelli sp. nov. – a taxon that demonstrates a transition between C . hastalis and Carcharodon carcharias . Specimens from the Pisco Formation clearly demonstrate an evolutionary mosaic of characters of both recent C. carcharias and fossil C. hastalis . Characters diagnostic to C. carcharias include the presence tooth serrations and a symmetrical first upper anterior tooth that is the largest in the tooth row, while those indicative of C. hastalis include a mesially slanted third anterior (intermediate) tooth. We also provide a recalibration of critical fossil horizons within the Pisco Formation, Peru using zircon U‐Pb dating and strontium‐ratio isotopic analysis. The recalibration of the absolute dates suggests that Carcharodon hubbelli sp. nov. is Late Miocene (6–8 Ma) in age. This research revises and elucidates lamnid shark evolution based on the calibration of the Neogene Pisco Formation.

Methods and materials Numerous studies indicate the Miocene Epoch was characterized by rapidly increasing 87Sr/86Sr in the global ocean; therefore, it is especially amenable to dating and correlating marine sediments using strontium isotope chemostratigraphy (Hodell et al. 1991; Miller et al. 1991, Hodell and Woodruff 1994, Oslick et al. 1994, Miller and Sugarman 1995, Martin et al. 1999, McArthur et al. 2001). We analysed three fossil marine mollusc shells from each of five localities to determine the ratio of 87Sr/86Sr in the shell calcium carbonate. When compared with the global seawater reference curve, these data allow us to estimate the age of the fossil molluscs for each locality (Table 1). Table 1. Strontium chemostratigraphic analyses of fossil marine mollusc shells from the Pisco Formation. Fossil Horizon Mean 87Sr/86Sr Age estimate (Ma) 95% CI (Ma) El Jahuay 0.7089424 7.46 9.03–6.51 Montemar 0.7089468 7.30 8.70–6.45 Sud Sacaco (West) 0.7089659 6.59 10.77–2.50 Sud Sacaco (West) 0.7089978 5.93 6.35–5.47 Sacaco 0.7090005 5.89 6.76–4.86 For isotopic analyses, we first ground off a portion of the surface layer of each shell specimen to reduce possible contamination. Areas showing chalkiness or other signs of diagenetic alteration were avoided. Approximately 0.01–0.03 g of aragonite or low‐magnesium calcite powder was recovered from each fossil sample. The powdered samples were dissolved in 100 μl of 3.5 N HNO 3 and then loaded onto cation exchange columns packed with strontium‐selective crown ether resin (Eichrom Technologies, Inc., Lisle, IL, USA) to separate Sr from other ions (Pin and Bassin 1992). Sr isotope analyses were performed on a Micromass Sector 54 thermal ionization mass spectrometer equipped with seven Faraday collectors and one Daly detector in the Department of Geological Sciences, University of Florida. Sr was loaded onto oxidized tungsten single filaments and run in triple collector dynamic mode. Data were acquired at a beam intensity of about 1.5 V for 88Sr, with corrections for instrumental discrimination made assuming 86Sr/88Sr = 0.1194. Errors in measured 87Sr/86Sr are better than ±0.00002 (2σ), based on long‐term reproducibility of NIST 987 (87Sr/86Sr = 0.71024). Age estimates were determined using the Miocene portion of Look‐Up Table Version 4:08/03 associated with the strontium isotopic age model of McArthur et al. (2001). Zircons were extracted from samples using standard crushing, density separation and magnetic separation techniques. The zircons were hand picked, mounted in epoxy plugs along with the reference zircon FC‐1 (Paces and Miller 1993) and analysed using laser ablation multi‐collector inductively coupled plasma mass spectrometry (LA‐MC‐ICP‐MS). We used a Nu Plasma mass spectrometer fitted with a U‐Pb collector array at the Department of Geological Sciences, University of Florida. 238U and 235U abundances were measured on Faraday collectors and 207Pb, 206Pb and 204Pb abundances on ion counters. The Nu Plasma mass spectrometer is coupled with a New Wave 213 nm ultraviolet laser for ablating 30–60 μm spots within zircon grains. Laser ablation was carried out in the presence of a helium carrier gas, which was mixed with argon gas just prior to introduction to the plasma torch. Isotopic data were acquired during the analyses using Time Resolved Analysis software from Nu Instruments. Before the ablation of each zircon, a 30 s peak zero was determined on the blank He and Ar gases with closed laser shutter. This zero was used for online correction for isobaric interferences, particularly from 204Hg. Following blank acquisitions, individual zircons underwent ablation and analysis for c. 30–60 s. The analyses of unknown zircons were bracketed by analysing an FC‐1 standard zircon.

Conclusions The recalibration of fossil horizons within the Pisco Formation finds ages are older than previously published (Muizon and Bellon 1980; Muizon and DeVries 1985; Muizon and Bellon 1986). While these changes are not exceptionally large, it does directly relate to the evolutionary history of the genus Carcharodon. The discovery and description of an outstanding specimen from the Pisco Formation further elucidate the taxonomy and palaeobiology of the white sharks. The hypothesis that I. escheri is a sister taxon of C. carcharias is refuted based on the Miocene and Pliocene distribution of Carcharodon fossils from the Pacific Basin and tooth morphology. The genus name Carcharodon is proposed for the species hastalis, hubbelli and carcharias based on dental characters shared between the taxa discussed above, and our interpretation of the C. hastalis‐hubbelli‐carcharias transition as an example of chronospecies. Palaeobiological information from UF 226255 reveals that this specimen grew at a rate comparatively slower than modern white sharks. MUSM 1470 confirms that the diet of C. hubbelli was at least partially comprised of marine mammals as early as the Late Miocene. Continued research of these specimens and newly discovered materials from the Pisco Formation and other fossil localities will only further advance our knowledge of the fossil lamnid sharks.

Acknowledgments Acknowledgements. This research is supported by National Science Foundation grants EAR 0418042 and 0735554. We especially thank Gordon Hubbell from Jaws International, Gainesville, Florida, for donating UF 226255 to the Florida Museum of Natural History as well as access to his collection and his wealth of knowledge of fossil sharks. We thank M. Stucchi from the Museo de Historia Natural of Lima, Peru, who provided assistance in the field in 2007. We also thank M. Siverson from the Western Australian Museum, Welshpool, Western Australia, and J. Bourdon of http://www.elasmo.com for discussions and constructive suggestions for the improvement of this manuscript. Thanks to E. Mavrodiev from the Florida Museum of Natural History for assistance with Russian translations. We also appreciate the comments from J. Bloch, R. Hulbert and J. Bourque from the Florida Museum of Natural History and two anonymous reviewers towards the improvement of this manuscript. This is University of Florida Contribution to Paleobiology 627. Any opinions, findings, conclusions or recommendations expressed in this paper are those of the author and do not necessarily reflect the views of the National Science Foundation. Editor. Adriana López‐Arbarello