The Early Pliocene extinction of the mega-toothed shark Otodus megalodon: a view from the eastern North Pacific

Keywords: Otodus megalodon, Otodus, Otodontidae, Extinction, Lamniformes, Miocene, Pliocene, California, Baja California, North Pacific

Table 1. Measurements (in mm), age, and occurrence of Otodus megalodon teeth examined during this study.

Table 2. Summary of corrected ages of Otodus megalodon occurrences used in the Optimal Linear Estimation analysis.

Global Otodus megalodon occurrence

We re-evaluated the entire dataset published by Pimiento & Clements (2014: table S1; text S1; Table 2; Appendix 1 and 2). Age justifications by these authors included limited references to the peer-reviewed stratigraphic literature and in some cases relied solely upon paleontological articles without examination of more recent published geological studies providing refined geochronologic data. Paleontological studies frequently re-cite the paper first reporting a fossil occurrence without citing subsequent geological refinements, but since stratigraphy is not static, it is critical to exhaustively search for stratigraphic and geochronologic data published afterward (Parham et al., 2012). We have audited the dataset by identifying the intraformational stratigraphic position of each occurrence (if applicable) and exhaustively citing relevant, up-to-date publications with geochronologic dates, favoring microfossil age correlations and absolute dates (⁸⁷Sr/⁸⁶Sr ratios, radiometric dates from interbedded ash/tuff/basalt, etc.) whenever possible; in some cases, only member- or formation-level stratigraphic control is available. In order to preserve our reasoning for future evaluation this information is presented in Appendix 2 of this study. In addition, we excluded occurrences where any one or more of the following types of problems existed (see Appendix 2): (1) lack of sub-epochal age control (e.g., “middle Miocene-Pliocene” or “Pliocene”); (2) lack of minimum age control, (3) all available voucher specimens residing in a private collection; (4) specimens lacking clear provenance (e.g., “specimen probably collected from locality…”; (5) misidentified specimens that are not identifiable as Otodus megalodon; (6) occurrence with revised minimum date falling entirely within the Miocene; (7) occurrences where the estimated age is based on the occurrence of Otodus megalodon, leading to a case of circular reasoning; and (8) unpublished occurrence data where the stratigraphy and geochronology cannot be evaluated by the reader.

Optimal linear estimation analysis

Once we evaluated and revised a dataset of age occurrence data for Otodus megalodon, we them treated them as historic sightings for implementation of an OLE model, as performed by Pimiento & Clements (2014) in their earlier study. Our revised dataset of late occurrences of Otodus megalodon comprises 43 data points, and includes fossils from around the world (Appendix 1). OLE has been found to be an accurate way of assessing extinction times in the fossil record, as last known occurrences generally follow a Weibull extreme value distribution (Clements et al., 2013; Solow, 2005; Wang & Marshall, 2016).

To run our analysis, OLE was implemented in R, using a modified version of the same code provided by Pimiento & Clements (2014; see Appendix 3) and same overall parameters. We implemented 10,000 simulations, bootstrapping from a uniform distribution for each simulation. The value reported below in results represents the modal estimate of extinction, with the full range of extinction dates recovered also reported.

Geochronologic framework

The traditional threefold division of the Pliocene and Pliocene-Pleistocene boundary set at 1.806 Ma (Gradstein et al., 2004) has recently been modified by the inclusion of the Gelasian stage within the Pleistocene and designation of the Zanclean and Piacenzian stages as early and late Pliocene (respectively), and a new Pliocene-Pleistocene boundary at 2.566 Ma (Gibbard et al., 2009; Gradstein et al., 2012), which we follow herein. Stages of international usage are generally referred to throughout (e.g., Messinian, Zanclean, Piacenzian, Gelasian) to alleviate confusion between late Pliocene sensu lato (=Gelasian stage) and late Pliocene sensu stricto (=Piacenzian stage); references to North American Land Mammal Ages (NALMAs; e.g., Clarendonian, Hemphillian, Blancan) and local New Zealand stages (e.g., Opoitian) are also made. Note that other recent studies in Pliocene-Pleistocene marine vertebrate paleontology followed the compromise of Hilgen et al. (2012) in maintaining the Gelasian as the late Pliocene (Boessenecker, 2011b, 2013a, 2013b).

Systematic paleontology

• Chondrichthyes Huxley, 1880

• Lamniformes Berg, 1958

• Otodontidae Glikman, 1964

• Otodus Agassiz, 1838

• Otodus megalodon Agassiz, 1843

Occurrence data

Pliocene-aged teeth of Otodus megalodon have been recovered or reported from several formations in California and Baja California (Fig. 1), including the Lomita Marl, Capistrano, Fernando, Niguel, Purisima, San Diego, San Mateo, and Tirabuzón Formations, the ages of which are summarized below. These specimens exhibit a combination of morphological characters including: a large overall size and labiolingual thickness, triangular shape, fine serrations, V-shaped chevron on the lingual surface between the crown and root, and loss of lateral cusplets at the base of the crown. These characters, when observed together, indicate that the specimens undoubtedly belong to Otodus megalodon. The only other sharks that could be confused with Otodus megalodon during the late Miocene and early Pliocene are those belonging to Carcharodon (C. hubbelli and C. carcharias), which have significantly smaller and labiolingually flatter teeth lacking V-shaped chevrons and have coarser serrations. Therefore, we are confident in assigning these specimens to Otodus megalodon. Additionally, we found that relatively few Otodus megalodon teeth from ENP Neogene sediments are present in museum collections; for example, a total of 145 teeth from lower Miocene through Pliocene west coast deposits are represented in LACM, SDNHM, and UCMP collections, primarily from California. In comparison, Purdy et al. (2001:131) referred 82 specimens in addition to “several hundred isolated teeth” from the Pungo River Limestone and Yorktown Formation at the Lee Creek mine alone, and countless additional teeth exist in other collections and from other stratigraphic units from the Atlantic coastal plain. Intense collecting at ENP localities like the Sharktooth Hill Bonebed suggests that this is not simply a case of collection bias and likely reflects genuine rarity (whether biogenic or taphonomic) of Otodus megalodon teeth from Pacific coast deposits. An alternative hypothesis is a geochronologically earlier extinction of Otodus megalodon in the Pacific basin than the Atlantic.

Purported Pleistocene and Holocene records of Otodus megalodon

The record of Otodus megalodon from the Lomita Marl (Jordan, 1922) is substantially younger than many other records from California. However, as noted by Mount (1974), numerous sharks and other marine vertebrates from the Lomita Quarry locality are only found elsewhere in middle and late Miocene localities, such as Allodesmus (Jordan & Hannibal, 1923: plate 9J) and Carcharodon hastalis (Jordan & Hannibal, 1923: plate 9E and 9F). Furthermore, shark teeth including Otodus megalodon teeth were collected by quarry manager S. M. Purple (Bailey, 1922; Mount, 1974), without accompanying stratigraphic information, and it is unclear where in the Lomita Quarry these specimens were collected. Hanna (in Jordan & Hannibal, 1923) notes that the base of the Lomita Marl within the Lomita Quarry was a glauconitic sandstone with abundant abraded whale bones, and that in addition to Miocene marine mammals and sharks, Pleistocene terrestrial mammals and a single Pleistocene pinniped were present in the quarry. This curious mixture of taxa suggests stratigraphic reworking of older fossil material; indeed, the holotype specimen of the gastropod Mediargo mediocris was considered by Wilson & Bing (1970:7) to be reworked from Pliocene sediments into the Lomita Marl. Woodring, Bramlette & Kew (1946) report that the Lomita Marl includes “beds of gravel consisting chiefly or entirely of limestone pebbles and cobbles derived from the “Monterey” Shale. Locally huge boulders of soft Miocene mudstone and Pliocene siltstone are embedded in calcareous strata.” These specimens of Otodus megalodon (RMM A597-1, A597-9A, and A597-9B) are fragmented, and strongly abraded with polished enameloid, indicating reworking. Only RMM A597-12 showed little evidence of abrasion, although taphonomic experiments on fossil teeth by Argast et al. (1987) noted that abrasion is not a guaranteed outcome of transport or reworking. Lastly, anthropogenic mixing of multiple strata during mining operations is also a possibility for seemingly older taxa in younger beds. Dynamite was used for mining in the quarry, which apparently “[brought] down bones of whales, sea lions, land animals, chipped flints, pieces of charcoal, sea shells, shark’s teeth, arrowheads, all mixed together” (Bailey, 1922). The report of Otodus megalodon from the Pleistocene Lomita Marl could be due to reworking from the “Monterey” Formation, anthropogenic mixing from mining operations, collection from underlying strata, poor record keeping, or any combination of the above. In this context, teeth of Otodus megalodon from the Lomita Marl are considered to be allochthonous (either by sedimentologic or anthropogenic reworking) and thus not relevant to the consideration of the timing of the extinction of the species.

Three teeth of Otodus megalodon (LACM 10141, 11194, and 159028) are questionably recorded from the upper Pleistocene Palos Verdes Sand and unnamed strata at Newport Bay Mesa (Fig. 10). LACM 11194 is now missing, but was found by an unknown collector prior to 1915 from the North Pacific Avenue and Bonita Avenue intersection in northern San Pedro, California. The locality is now built over, but was mapped as Palos Verdes Sand by Woodring, Bramlette & Kew (1946). LACM 10141, is a fragmentary tip of a tooth with longitudinally cracked enameloid and abraded serrations (Figs. 10C and 10D), and was collected from unnamed strata along the Newport Bay Mesa formerly considered to belong to the Palos Verdes Sand (collector and collection date unknown); it is alternatively possible that this specimen was collected from an exposure (now covered) of the Pliocene Fernando Formation (see Mount, 1969). LACM 159028 (Figs. 10A and 10B) possesses the following dubious locality information: “Rosecranz Ave. Long Beach, Orange Co.?” We note that Rosecrans Avenue is far from the Palos Verdes Hills and from Long Beach, and that both Rosecrans Avenue and Long Beach are located within Los Angeles County. It is also possible that this specimen is reworked from the underlying Puente Formation (L.G. Barnes, 2015, personal communication). Alternatively, some Pliocene rocks are known in the Coyote Hills near Rosecrans Avenue (Powell & Stevens, 2000), and the specimen may have been collected there. It is not possible to unambiguously recognize any of these specimens as genuine Pleistocene records of Otodus megalodon given the lack of provenance. We also note the similarity in preservation (chiefly color) between LACM 159028 and teeth of Otodus megalodon from some localities at Sharktooth Hill (middle Miocene “Temblor” Formation, Kern County). Kanakoff (1956) only listed C. carcharias from this unit. Furthermore, a comprehensive study of the ichthyofauna by Fitch (1970) only recorded C. carcharias. We hypothesize that LACM 11194 was a misidentified or mistranscribed specimen of C. carcharias and that the other two specimens originated from a separate locality. Therefore, we conclude that no reliable records of Otodus megalodon exist for Pleistocene deposits in the Los Angeles Basin.

Several studies have reported teeth of Otodus megalodon dredged from the seafloor and considered to be Pleistocene or even Holocene in age (Tschernezky, 1959; Seret, 1987; Roux & Geistdoerfer, 1988). Dredged specimens from the south Pacific were reported by Tschernezky (1959) and Seret (1987), whereas Roux & Geistdoerfer (1988) reported numerous specimens from the Indian Ocean seafloor off the coast of Madagascar. Tschernezky (1959) and Roux & Geistdoerfer (1988) both attempted to determine the age of the teeth by measuring the thickness of adhering manganese dioxide nodules and applying published rates of MnO₂ nodule growth. Tschernezky (1959) reported a range of 24,406–11,333 years for the MnO₂ nodule formation for these teeth, and Roux & Geistdoerfer (1988) reported specimens with nodules with the equivalent of 60–15 Ka of MnO₂ growth. However, both studies assumed a constant rate of nodule growth and interpreted these dates as indicating a latest Pleistocene extinction of Otodus megalodon (Tschernezky, 1959; Roux & Geistdoerfer, 1988). Tschernezky (1959) argued that even if Otodus megalodon went extinct during the Middle Pleistocene ca. 500 Ka, his dredged Otodus megalodon teeth should have had MnO₂ coatings approximately 75 mm thick. Because conditions favoring the formation and growth of MnO₂ nodules are not necessarily constant over geologic time (Purdy et al., 2001), these dates can only indicate when these teeth were exposed to seawater and do not reflect geochronologic age. It is probable that these specimens were concentrated on the seafloor through submarine erosion, winnowing, depositional hiatus, or a combination thereof. Collections of numerous resistant vertebrate hardparts from these dredgings (e.g., shark teeth and cetacean ear bones) support this suggestion. A more parsimonious scenario is that these specimens are Pliocene (or Miocene) in age and were deposited in areas of slow sedimentation with intermittent erosion, concentrating nodules and resistant marine vertebrate skeletal elements (typically teeth and cetacean skull fragments) on the seafloor. Intermittent periods of favorable chemistry fostered the formation and growth of MnO₂ nodules and coatings, and it is possible that these specimens have experienced numerous burial-exhumation cycles (Boessenecker, Perry & Schmitt, 2014). Lastly, because no extrinsic absolute or biostratigraphic age data exist for these specimens, the maximum age of these specimens is ultimately unknown and cannot be considered to represent post-Pliocene occurrences (Applegate & Espinosa-Arrubarrena, 1996; Purdy et al., 2001).

Timing of the extinction of Otodus megalodon in the eastern North Pacific

Although numerically less abundant than in deposits of the Atlantic Coastal Plain, fossil teeth of Otodus megalodon have been reported from numerous middle Miocene localities in California and Baja California (Jordan & Hannibal, 1923; Mitchell, 1966; Deméré et al., 1984). Late Miocene occurrences of Otodus megalodon include the Almejas (Barnes, 1992), Monterey (Barnes, 1978), “lower” San Mateo (Domning & Deméré, 1984), Capistrano (Barboza et al., 2017; this study), and Purisima formations (Boessenecker, 2016; this study), Santa Cruz Mudstone (Jordan & Hannibal, 1923; this study), and Santa Margarita Sandstone (Barnes, 1978; Domning, 1978). Pliocene occurrences in California (reviewed above) are restricted to the Capistrano, Fernando, “upper” San Mateo, basal San Diego, and Tirabuzón formations (Fig. 11). In the context of dubious provenance or clear evidence of reworking for specimens younger than these, we do not consider any post-early Pliocene records of Otodus megalodon to be reliable and putative Quaternary specimens are particularly dubious. Several specimens of Otodus megalodon are now recorded from the basal San Diego Formation, which is as old as 4.2 Ma (Wagner et al., 2001; Vendrasco et al., 2012), and we interpret these records as earliest Pliocene (Zanclean equivalent; Fig. 11). The lack of Otodus megalodon specimens and abundant Carcharodon carcharias teeth in younger sections of the San Diego Formation is paralleled in the Purisima Formation at Santa Cruz. Although Carcharodon carcharias teeth are common within well-sampled bonebeds, no teeth of Otodus megalodon have been discovered from the Pliocene section of either unit. However, teeth of Otodus megalodon are rare within established Miocene marine vertebrate collections relative to C. hastalis or C. carcharias (e.g., within the Sharktooth Hill Bonebed, approximately 80+ specimens of C. hastalis are recorded vs. nine teeth of Otodus megalodon in UCMP collections; accessed October 23, 2018). In summary, specimens discussed herein are entirely latest Miocene or earliest Pliocene in age (Messinian-Zanclean equivalent; Fig. 11). Qualitative assessment of the reliable occurrences of Otodus megalodon in California and Baja California suggests extinction of this taxon during the early Pliocene, perhaps during the Zanclean stage or near the Zanclean-Piacenzian boundary (ca. 4–3 Ma; Fig. 11).

A worldwide view of Otodus megalodon extinction

The fossil record of Otodus megalodon in other regions lends support to an early Pliocene (Zanclean) extinction (Figs. 9 and 11). Previously described records of Pliocene age possibly relevant to temporally constraining the extinction of Otodus megalodon include occurrences from the eastern USA, Japan, Australia, New Zealand, western Europe (Belgium, Spain, UK, Denmark), southern Europe (Italy), Africa (Libya), and South America (Chile, Ecuador, Peru, Venezuela).

In deposits around the North Sea, Otodus megalodon has been reported from the Miocene, Pliocene, and Pleistocene (Bendix-Almgreen, 1983; Donovan, 1988). A tooth from the upper Miocene Gram Formation of Denmark was interpreted by Bendix-Almgreen (1983:23–24) as representing the youngest record of Otodus megalodon from the eastern North Atlantic. A tooth of Otodus megalodon from the Pliocene to Pleistocene Red Crag Formation of eastern England was mentioned by Donovan (1988), although the majority of marine vertebrate remains—marine mammals in particular—show evidence of reworking including abrasion, polish, and phosphatization and furthermore typically consist of dense elements with relatively high preservation potential (e.g., cetacean tympanoperiotics, teeth and tusks, and osteosclerotic beaked whale rostra; Owen, 1844, 1870; Lydekker, 1887). This evidence suggests that marine vertebrate material has been reworked from preexisting strata predating the Red Crag Formation; indeed, the Red Crag unconformably overlies the Eocene London Clay and the lower Pliocene Coralline Crag Formation (Zalasiewicz et al., 1988), and marine vertebrate remains may date to the Eocene-Pliocene depositional hiatus (or erosional lacuna) between the London Clay and overlying Red Crag Formation, or may have been reworked from the Coralline Crag Formation. A single record from the Piacenzian of France is cited by Cappetta (2012) from Gervais (1852), but no geographic or stratigraphic information is given by Gervais (1852:173) and this record cannot be evaluated.

In a review of the stratigraphic range of Pliocene to Pleistocene elasmobranchs from Italy, Marsili (2008) indicated that Otodus megalodon disappeared from the record during the Zanclean (∼4 Ma) and that no Piacenzian records existed, contra Pimiento & Clements (2014: table S1). In their discussion of the shark fauna of Malta, Ward & Bonavia (2001) considered Otodus megalodon to have become extinct in the early Pliocene (but without further comment). Other early Pliocene (Zanclean equivalent) records of Otodus megalodon from western Europe and the Mediterranean region include the Huelva Formation of Spain (Garcia et al., 2009) and unnamed strata in the Sabratah Basin of northwestern Libya (Pawellek et al., 2012). Elsewhere in Africa, Otodus megalodon is recorded from the early Pliocene of Angola (Antunes, 1978).

In a summary of Mesozoic and Cenozoic ichthyofaunas from Japan, Yabumoto & Uyeno (1995) reported that Otodus megalodon is widely known from Miocene strata and occurs in the lower Pliocene, but not from younger upper Pliocene and Pleistocene rocks. Subsequently, a review by Yabe, Goto & Kaneko (2004) reported widespread occurrences of Otodus megalodon in the earliest Pliocene (Zanclean) and a few late early Pliocene records (Piacenzian), and considered Otodus megalodon to have gone extinct in the late early Pliocene or late Pliocene. Three post-Zanclean occurrences were listed by Yabe, Goto & Kaneko (2004): one is uncertainly Piacenzian, another Zanclean or Piacenzian, and one strictly Piacenzian in age. However, these specimens were not figured by Yabe, Goto & Kaneko (2004) and it is unclear whether or not they are reworked.

An early Pliocene (Zanclean or Piacenzian) extinction of Otodus megalodon seems to be reflected in the fossil record of Australia and New Zealand. Late Miocene occurrences of Otodus megalodon are common from both landmasses (Keyes, 1972; Kemp, 1991; Fitzgerald, 2004). Several early Pliocene records of Otodus megalodon have been reported from Australia (Kemp, 1991; Fitzgerald, 2004), including a single specimen from the lower Pliocene Cameron Inlet Formation (Zanclean-Piacenzian correlative; Kemp, 1991; Fitzgerald, 2004). However, judging from Kemp’s (1991: plate 30C) illustration, this specimen is a misidentified Carcharodon carcharias tooth owing to its small size, lack of a preserved chevron, and relatively large serrations. Keyes (1972) reported several specimens ranging in age from early Pliocene to Pleistocene age, but many of them have tenuous provenance. For example, one such specimen (included in the analysis by Pimiento & Clements, 2014) can only be pinpointed to a 180 km section of coastline. Only a single published Pliocene tooth of Otodus megalodon from New Zealand has reliable provenance, a specimen collected from Patutahi Quarry on the North Island. According to Keyes (1972), strata at the quarry correspond to the local New Zealand Opoitian Stage (5.33–3.6 Ma); accordingly, this tooth represents the youngest reliable record of Otodus megalodon from New Zealand.

In South America, Otodus megalodon is known continuously from at least the middle Miocene to the lowermost Pliocene in the Pisco Basin of Peru (Muizon & De Vries, 1985; Ehret et al., 2012). However, owing to the absence of well-sampled younger marine vertebrate assemblages, it is unclear if this simply reflects an artifact of preservation. Otodus megalodon has also been reported from the latest Miocene-early Pliocene of Ecuador (Longbottom, 1979). Although Otodus megalodon has been reported from the well-sampled uppermost Miocene to lower Pliocene Bahia Inglesa Formation of Chile (Long, 1993), the exact age of this occurrence is imprecisely known (Walsh & Hume, 2001; Walsh & Naish, 2002). On the Caribbean coast of South America, Otodus megalodon is continuously known from middle Miocene through lower Pliocene deposits, with the youngest specimens occurring in the lowermost Pliocene (Zanclean-correlative; Aguilera, Garcia & Cozzuol, 2004).

Paralleling the record in Venezuela, abundant Miocene records of Otodus megalodon exist in the western North Atlantic and Caribbean, with the youngest specimens consistently being earliest Pliocene in age (Flemming & McFarlane, 1998; Purdy et al., 2001; Ward, 2008). In deposits of the Atlantic coastal plain of the United States, teeth of Otodus megalodon are abundant within the lower Pliocene Sunken Meadow Member of the Yorktown Formation (Purdy et al., 2001; Ward, 2008), but absent from the upper Pliocene Rushmere and Moore House members of the Yorktown Formation (Ward, 2008). The extinction of Otodus megalodon was interpreted by Ward (2008) to have occurred during the time recorded by the unconformity and depositional hiatus of uncertain duration between the Sunken Meadow and Rushmere members. A number of possible Pleistocene occurrences of Otodus megalodon from Florida are present in FLMNH collections, but originate from temporally mixed fossil assemblages and quarry spoil piles (D.J. Ehret, 2015, personal observation).

We interpret the absence of Otodus megalodon in the Rushmere and Moore House members of the Yorktown Formation, upper San Diego Formation, and “upper” parts of the Purisima Formation to be biochronologically real and reflect the genuine absence of this taxon. Given the intense collecting of these localities by amateur and professional paleontologists alike, collection bias is not likely a factor in determining the stratigraphic occurrence of Otodus megalodon.

Results of the OLE analysis based upon our revised version of the dataset (Table 2; Appendix 1) incorporating current stratigraphic and geochronologic data indicate that Otodus megalodon was most likely extinct by 3.6 Ma and perhaps even as early as 4.1 Ma (maximum) and certainly no later than 3.2 Ma (minimum; Figs. 9 and 11), strongly indicating an extinction during the Zanclean stage or close to the Zanclean-Piacenzian boundary (3.6 Ma). This global estimate compares well with the qualitative approach for Otodus megalodon occurrence data in the ENP, and consistent identification of minimum ages near the Zanclean-Piacenzian boundaries is consistent with a globally synchronous extinction at or around 3.6 Ma. We note that the separation between the maximum and minimum inferred extinction dates (∼970,000 years; 4.1–3.2 Ma) from the OLE are substantially narrower than those results (3.66 million years; 3.5 Ma–160 ky in the future) reported by Pimiento & Clements (2014:2, Fig. 1). Pimiento and Clements, in their results, focus on range of dates that result in a cumulative 50% probability of extinction, which give a range of extinction dates from 3.5 to 2.6 Ma. If we follow this reasoning, and focus on the cumulative 50% probability of extinction found in our study, we still get a narrower range of possible extinction times, between 4.1 and 3.6 Ma, or a range of 500,000 years, vs. the 900,000 of Pimiento & Clements (2014).

Perhaps more critical than the narrower range of modeled extinction dates is the earlier shift in modal extinction date by nearly one million years relative to Pimiento & Clements (2014). Geologically speaking, one million years seems trivial, but the difference between the two reconstructed extinction dates is critical given that this extinction occurred quite recently; an earlier date now poses problems for the supposed synchronicity of Plio-Pleistocene marine mammal extinctions and/or faunal turnover (Pimiento et al., 2017; but see Boessenecker, 2013a). Recently, Wang & Marshall (2016:4) noted that the “poorly resolved ages of many of the fossil occurrences” of the Pimiento & Clements (2014) dataset led a wide confidence interval in their OLE. Indeed, the results of macroevolutionary studies of extinction timing are sensitive to the quality of available dates (Price et al., 2018). Our finer resolution highlights the importance of carefully vetting the provenance of each reported occurrence and thoroughly exploring the geological literature for such fossil occurrences—critical for any study of biochronology (Price et al., 2018) as well as selecting fossil calibrations for molecular clock dating (Parham et al., 2012).

Possible causes for the extinction of Otodus megalodon

Determination of the timing of the extinction of Otodus megalodon is a necessary step in identifying potential causal factors contributing to its demise (Pimiento & Clements, 2014; Pimiento et al., 2016). Although testing various hypotheses in a quantitative manner is beyond the scope of this article, some comments regarding potential biotic and physical drivers are appropriate given the revised extinction date presented herein. Abiotic drivers such as changes in climate, upwelling, currents, sea level, and paleogeography are possible determinants in the decline of the otodontid lineage (Pimiento et al., 2016). Physical events coincident with a “mid”-Pliocene extinction include: (1) a decrease in upwelling in the ENP (Barron, 1998), (2) increased seasonality of marine climates (Hall, 2002); (3) a period of climatic warming and permanent El-Niño like conditions in the equatorial Pacific (Wara, Ravelo & Delaney, 2005; Fedorov et al., 2013), (4) followed by late Pliocene global cooling (Zachos et al., 2001), (5) closure of the Panama seaway and restriction of currents and east-west dispersal among marine organisms (Collins et al., 1996; Haug et al., 2001; O’Dea et al., 2016; Jaramillo et al., 2017), and (6) stable eustatic sea level during the early Pliocene, (7) followed by eustatic sea level fall related to initial glaciation during the late Pliocene (Miller et al., 2005). Some of these changes in oceanic circulation and upwelling were regional, and therefore do not represent likely causes in the extinction of Otodus megalodon (if the extinction was indeed globally synchronous; e.g., Pimiento & Clements, 2014); however, these events may have been, in part, responsible for range fragmentation. Long term cooling following the middle Miocene Climatic Optimum (Zachos et al., 2001) may have reduced the geographic range of this species (Purdy, 1996; Dickson & Graham, 2004; but see Pimiento et al., 2016; Ferrón, 2017). However, a robust analysis of worldwide geographic distribution in Otodus megalodon found no change in the latitudinal distribution coincident with changes in global climate (Pimiento et al., 2016).

The lack of evidence for a climatic or geographic driver of Otodus megalodon extinction suggests that a biotic driver is probably responsible (Pimiento et al., 2016). Within the ENP, many “archaic” marine mammal taxa became extinct during the early Pleistocene (Gelasian stage, ∼2 Ma; Boessenecker, 2013a, 2013b), but the revised extinction of Otodus megalodon (this study) seems to have pre-dated this (∼3.6 Ma). The appearance of the modern marine mammal fauna appears to have occurred by the early Pliocene in the North Atlantic and western South Pacific (Whitmore, 1994; Fitzgerald, 2005; Boessenecker, 2013a), suggesting globally asymmetric origination of modern marine mammal genera and species (Boessenecker, 2013a), in contrast with an apparently synchronous extinction of Otodus megalodon (Pimiento & Clements, 2014; this study). Many extant genera of cetaceans first appeared during the Pliocene (Fordyce & Muizon, 2001), apparently temporally coincident with the extinction of Otodus megalodon, but with uncertain relevance. Other biotic effects have been hypothesized to have affected or been driven by Otodus megalodon. Recently described macrophagous sperm whales appear to have been diverse worldwide in the middle and late Miocene, were similar in size to Otodus megalodon, and were likely competing apex predators (Lambert et al., 2010). A high diversity of small-bodied baleen whales during the middle Miocene is implicated in supporting such an assemblage of gigantic predators (Lambert et al., 2010; Collareta et al., 2017). Similarly, Lindberg & Pyenson (2006) noted that the extinction of Otodus megalodon is roughly contemporaneous with the earliest fossil occurrences of killer whales (Orcinus) in the fossil record, and perhaps competition with killer whales during the Pliocene could have acted as a driver in the extinction of Otodus megalodon. However, the Neogene fossil record of Orcinus is limited to two occurrences: an isolated tooth from Japan (Kohno & Tomida, 1993), and the well-preserved skull and skeleton of Orcinus citoniensis from the late Pliocene of Italy (Capellini, 1883). Furthermore, Orcinus citoniensis was small in comparison to extant Orcinus orca (est. four m body length: Heyning & Dahlheim, 1988) and possessed a higher number of relatively smaller teeth and narrower rostrum (Bianucci, 1996), and was probably not an analogous macrophagous predator. Because fossils of Orcinus are not widespread during the Pliocene, claims of competition between Otodus megalodon and Orcinus are problematic. Furthermore, the decline and loss of cosmopolitan macrophagous physeteroids (Tortonian-Messinian; Lambert et al., 2010) appears to have predated the early Pliocene extinction of Otodus megalodon by several million years.

Evolutionary interactions with baleen whales have also been implicated for the Otodus lineage (Collareta et al., 2017). Lambert et al. (2010) and Lambert, Bianucci & De Muizon (2016) suggested that higher diversity of small-bodied mysticetes during the middle Miocene drove the evolution of killer sperm whales; similarly, this could have driven body size increases in Otodus megalodon. Cetacean diversity peaked in the middle Miocene and began to decrease in the late Miocene (Lambert et al., 2010; Marx & Uhen, 2010), and maximum body length amongst fossil mysticetes increased during the late Miocene and Pliocene (Lambert et al., 2010), heralding the appearance of modern giants such as Balaena, Balaenoptera, Eschrichtius, Eubalaena, and Megaptera. Despite the increase in maximum body size among mysticetes and apparently coincidental extinction of Otodus megalodon during the Pliocene, numerous small-bodied archaic mysticetes persisted into the Pliocene (Bouetel & Muizon, 2006; Whitmore & Barnes, 2008; Collareta et al., 2017) and even Pleistocene (Boessenecker, 2013a), complicating this relationship. A modal extinction date of 3.6 for Otodus megalodon pre-dates the extinction of certain dwarf mysticetes such as Balaenula (Piacenzian-Gelasian; Barnes, 1977), Herpetocetus (Calabrian-Ionian; Boessenecker, 2013b) and various dwarf balaenopterids (Deméré, 1986; Boessenecker, 2013a). Indeed, further study of rare late Pliocene marine mammals is necessary to further elucidate potential competition with Otodus megalodon, extinctions, and faunal dynamics (Pimiento et al., 2017).

Another potential biotic factor in the extinction of Otodus megalodon is the evolution of the modern great white shark, Carcharodon carcharias (Pimiento et al., 2016). It gradually evolved from the non-serrated Carcharodon hastalis during the late Miocene, transitioning first into the finely serrated Carcharodon hubbelli approximately 8–7 Ma, then evolved into the coarsely serrated C. carcharias approximately 6–5 Ma (Ehret, Hubbell & MacFadden, 2009a; Ehret et al., 2012; Boessenecker, 2011b; Long, Boessenecker & Ehret, 2014). However, in the western North Atlantic, C. carcharias is absent in the early Pliocene Sunken Meadow Member of the Yorktown Formation (Purdy et al., 2001; Ward, 2008), and in its place is C. hastalis (=I. hastalis and I. xiphodon in Purdy et al., 2001). Carcharodon carcharias instead occurs higher in the Rushmere Member of the Yorktown Formation (Müller, 1999). This suggests that the appearance of C. carcharias in the Atlantic may have been delayed relative to the Pacific. Pawellek et al. (2012) reported an earliest Pliocene fish assemblage on the Mediterranean coast of Libya that included C. carcharias and Otodus megalodon; clarifying the timing of first appearance of C. carcharias in ocean basins outside the Pacific is necessary, but beyond the scope of this study. Nevertheless, the timing of Otodus megalodon extinction appears to overlap with the final widespread global occurrence of C. carcharias in the early Pliocene. It is critical to note that a single putative tooth of C. carcharias has been reported from the middle Miocene Calvert Formation and has been identified as evidence supposedly disproving the C. hastalis–C. hubbelli–C. carcharias transition (Purdy, 1996; Gottfried & Fordyce, 2001), although Ehret et al. (2012) indicated this specimen is a misidentified juvenile Otodus megalodon tooth.

The development of serrations in Carcharodon hubbelli suggests a refined ability to prey upon warm-blooded prey relative to other large lamnid and carcharhinid sharks (Frazzetta, 1988; Ehret, Hubbell & MacFadden, 2009a; Ehret, MacFadden & Salas-Gismondi, 2009b; Ehret et al., 2012). Perhaps trophic competition with the newly evolved C. carcharias contributed to the extinction of Otodus megalodon, in which adult C. carcharias would have been in the same size range and likely would have competed with juvenile Otodus megalodon. Owing to its global scope, the first appearance of modern C. carcharias during the early Pliocene is a likely candidate for the driver behind the extinction of Otodus megalodon. Further investigations regarding body size trends in the Otodus and Carcharodon lineages, the C. hastalis–C. hubbelli–C. carcharias anagenetic lineage in the Pacific basin and elsewhere (Fig. 11), and the timing of C. carcharias first appearances and Otodus megalodon last appearances in the Atlantic and other ocean basins are necessary to evaluating these hypotheses of extinction drivers of Otodus megalodon.

Competing Interests

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Supplementary Materials

Data Availability Statement

The following information was supplied regarding data availability:

The raw data/code is included in the article (Table 1, Table 2, and Appendices). All specimens examined during this study are permanently curated at the following museums: CAS, California Academy of Sciences, San Francisco, California, USA; LACM, Natural History Museum of Los Angeles County, Los Angeles, California, USA; RMM, Riverside Municipal Museum, Riverside, California, USA; SDNHM, San Diego Natural History Museum, San Diego, California, USA; UCMP, University of California Museum of Paleontology, Berkeley, California, USA.