The Carcharocles megalodon is the largest macropredatory shark to have ever lived. However, the fossil record of this predator is restricted to isolated teeth and vertebral centra due the cartilaginous skeleton of most of the sharks. The most complete specimen is an associated column of around 150 vertebral centra from the Miocene of the Antwerp Basin, Belgium (Leriche, 1926). The record of C. megalodon teeth ranges from 17 to 2 Ma (middle Miocene to Pleistocene) (Gottfried et al., 1996; Purdy, 1996; Pimiento et al., 2010).

Based on tooth size, C. megalodon reached a maximum inferred total length around 16 m (Randall, 1973; Gottfried et al., 1996), more than twice the reported maximum length of the white shark Carcharodon carcharias with 6.4 m (Bigelow & Schroeder, 1948; Randall, 1973; Compagno, 1984). The largest tooth of C. megalodon measures 168 mm (specimen PF 1168, Field Museum of Natural History of Chicago).

Data on C. megalodon from the Iberian Peninsula are relatively scarce, sometimes reported only as news in the daily press, without scientific revision. Descriptions of remains of C. megalodon from the Iberian Peninsula are those of Areitio (1877) and Meseguer Pardo (1924) from Messinian diatomites of the Lorca Basin (Southeastern Spain); Balbino (1995) from uppermost Miocene of the Alvalade Basin (Portugal); and García et al. (2009) from the Arenas Fm (Huelva, Spain) in the western Guadalquivir Basin.

In this work a complete tooth of C. megalodon from the Eastern Guadalquivir Basin (South Spain) is reported and examined in detail, including comparisons with other shark teeth of C. megalodon and C. carcharias described in the literature.

The Guadalquivir Basin is the foreland basin of the Betic Cordillera and it opens directly to the Atlantic to the WSW (Gulf of Cádiz). Autochthonous units to the north and allochthonous units to the south may be differentiated in this basin (Fig. 1A). The allochthonous units are composed by olistostrome deposits and olistoliths, located at the frontal part of the Subbetic nappes, which are mixed with Miocene clays and marls.

The shark tooth was collected in diatomaceous marl blocks obtained from the old quarry of San Félix, located 2 km south of Porcuna (coord. 37° 51′ 21.4″ N; 4° 11′ 21.6″ W) (Fig. 1B). The diatomaceous marls are white to yellowish, sometimes laminated, and they alternate locally with greyish marls without visible stratification. These deposits belong to the highest part of the Valenzuela Formation (Aquitanian-Torttonian in age, Tjalsma, 1971), whose stratotype was established in the San Félix Quarry. The marls contain abundant diatoms and subordinately sponge spicules, silicoflagellates and radiolarians. The sediments of the type section (San Felix Quarry area) have a planktic foraminiferal assemblage with Globorotalia menardii, Globigerina globorotaloidea, Globigerina nepenthes and Globigerinoides obliquus , among others (Tjalsma, 1971). This foraminiferal assemblage belong to the zone N16 of planktic foraminifera (Tortonian) equivalent with most of the middle part of the stratotype of the Tortonian. The main diatom assemblages consist mostly of planktic (Thalassionema, Thalassiotrix, Thalassiosira) and scarce benthic (Delphineis) forms typical of cold oceanic upwelling waters (Molina et al., 1987; López-García & Bustillo, 1994). The outcrops belong to the third and last episode of diatomitic sedimentation defined in this area by Bustillo & López-García (1997) of the Thalassiosira yabei Zone (Middle Tortonian).

In this area of the North Betic Strait, which connected the Atlantic Ocean to the palaeo-Mediterranean sea, high concentrations of diatomaceous deposits were produced because it was more oceanographically dynamic than the Atlantic. The Guadalquivir Basin was an Atlantic basin connecting with the palaeo-Mediterranean through small basins (e.g. Granada Basin and Guadix Basin) to the south by straits. Flood-tidal currents as well as temperature and salinity gradients favoured movement of sea-water (e.g. Reolid et al., 2012; García-García et al., 2014; Martín et al., 2014). The interaction of the cold nutrient-rich Atlantic waters with the gulfs, bays, small basins and bottom topography of the North Betic Strait caused upwelling, and high productivity of biogenic silica (Molina et al., 1987). The narrowing of the strait from the Langhian to the Tortonian could have caused the progressive upwelling deduced from the study of the diatomites. This narrowing could also explain the evolution of the upwelling from open sea conditions to near shore conditions (Bustillo & López-García, 1997).

The youngest part of the Valenzuela Fm, in which the area of the San Felix Quarry is included, has to be considered parautochthonous of the syn-olistostromic sequence of the middle-late Tortonian times (Riaza & Martínez del Olmo, 1996), while the older parts of the Valenzuela Fm belong no doubt to the allochthonous basin filling that have travelled longer distances.

The tooth analysed, coming from diatomites of the Eastern Guadalquivir Basin, is now displayed in the Museo Obulco (Porcuna, Jaén province). This tooth is described following traditional measurements used by previous authors working on C. megalodon and C. carcharias (e.g. Applegate & Espinosa-Arrubarrena, 1996; Hubbell, 1996; Mollet et al., 1996).

The main measurements for describing the tooth and estimating the total length of the shark are the crown height and the crown width (Fig. 2). According to Gottfried et al. (1996) and Hubbell (1996), the crown height or tooth height is the vertical distance between a straight line touching the lower extensions of the enamel adjacent to the root at the base of the crown and a parallel line touching the tip of the enamel. For these authors, the crown width is the widest part of the enamel at the base of the crown (Fig. 2).

The Recent white shark Carcharodon carcharias is the closest living relative of C. megalodon, and therefore several authors have used skeletal material of C. carcharias as the primary basis for comparison with C. megalodon and to infer lifestyle (Uyeno et al., 1989; Applegate, 1991; Gottfried et al., 1992, 1996; Applegate & Espinosa-Arrubarrena, 1996; Adnet et al., 2010; Pimiento et al., 2010). In order to better approximate the total length of the specimen of C. megalodon from Porcuna, we considered data on crown height and crown width of upper anterior teeth from 102 specimens of C. carcharias (85 specimens) and C. megalodon (17 specimens) available in the literature (Table 1).

This tooth presents the diagnostic characters of C. megalodon such as large size, triangular shape, fine serration on the cutting edges, a convex lingual face, a slightly convex to flat labial face, and a large v-shaped neck (Fig. 3). Two root lobes are well-differentiated and rounded on the basal root edge. The crown height is 92.2 mm and the crown width is 81.3 mm. The total height of the tooth measured perpendicular from the apex of the tooth to the baseline of the root is 123.96 mm.

The tooth is symmetrical and nearly perpendicular, only slightly slanted (2.1°). For this reason, the mesial crown edge length is 102.0 mm while the distal crown edge length is 94.8 mm. The size of the teeth varies within the jaw, anterior teeth being large and symmetrical, whereas the large posterior teeth are asymmetrical and feature slanted crowns (Applegate & Espinosa-Arrubarrena, 1996; Hubbell, 1996; Pimiento et al., 2010). Taking into account the symmetry and the size, this tooth probably corresponds to the upper anterior (A2 or A3 s. Hubbell, 1996; or I or III s. Applegate & Espinosa-Arrubarrena, 1996) or the second upper lateral position.

The cutting edge presents 135 serrations in the mesial crown edge and 126 serrations in the distal crown edge (Fig. 3D). There are approximately 13 serrations per cm, and they are smaller and denser close to the root and the apex. The points of basal serrations are oriented 90° with respect to the cutting edge.

Due to the fact that the cartilaginous skeleton of chondrichthyans is only preserved in rare instances (e.g. Uyeno et al., 1990; Shimada, 1997; Ehret et al., 2009a; Kriwet et al., 2014), the lack of more complete fossil specimens has led to conflicting interpretations about taxonomy. The taxonomic assignment of this species has long been debated and there are two main interpretations:

A) The megalodon shark is placed in the genus Carcharocles (Family Otodontidae) (Casier, 1960; Gluckman, 1964; Capetta, 1987; Ward & Bonavia, 2001; Nyberg et al., 2006; Ehret et al., 2009a, 2012; Pimiento et al., 2010).

B) The genus of megalodon shark is congeneric with the living white shark Carcharodon carcharias (Family Lamnidae) (Uyeno et al., 1989, Applegate, 1991; Applegate & Espinosa-Arrubarrena, 1996; Gottfried et al., 1992, 1996; Purdy, 1996).

We follow the first hypothesis, where C. megalodon is separated from genus Carcharodon on the basis of tooth features such as large size, absence of lateral denticles, and fine serrations—as opposed to a small size, large serrations and the presence of lateral denticles in C. carcharias. However, Carcharocles and Carcharodon are included in the Order Lamniformes, and in the absence of living members of Otodontidae, Carcharodon carcharias is the most analogous species available.

A large shark tooth is recorded in Tortonian diatomites of the Eastern Guadalquivir Basin (Porcuna, Jaén). The large size (92.2 mm crown height), triangular shape, broad serrated crown, lingual face convex, labial face flat, large neck, and robust, thick angled root determine that this specimen corresponds to Carcharocles megalodon. The symmetry with a low slant indicates this is an upper anterior tooth. The total length estimated from the tooth crown height is around 11 m.

In the North Betic Strait, which connected the Atlantic Ocean to the palaeo-Mediterranean sea, the interaction of the cold nutrient-rich Atlantic waters with the palaeogeography and the bottom topography caused upwelling, and a high productivity of biogenic silica (diatomites). High nutrients and productivity favoured the presence of large vertebrates such as mysticetid whales, pinnipeds and small sharks (Isurus) recorded in the Upper Miocene deposits of this area. These large vertebrates constitute potential prey of C. megalodon. The palaeogeography of this basin, as an Atlantic branch connecting with the Mediterranean Sea, was probably well suited to the hunting techniques of C. megalodon.

The research activity of M. Reolid was supported by Projects RYC-2009-04316 (Ramón y Cajal Program) and P11-RNM-7408 (Junta de Andalucía). The research of J.M. Molina was sustained by RNM-200 Research Group. The authors are indebted to Luis Emilio Vallejo Delgado (Curator of Museo Obulco, Porcuna) for helping during the work in the museum. Catalina Pimiento (University of Florida and Smithsonian Tropical Research Institute) helped with the literature and first stages of classifying shark teeth from the Eastern Guadalquivir Basin. The authors are indebted to the reviewers Carlos Martínez (University of Bristol) and an anonymous reviewer for their valuable comments. We thank Jean Sanders for reviewing the grammar.