The North Alpine Foreland Basin or “Molasse Basin’’ is part of the Paratethys realm and was formed during the early Oligocene to late Miocene by the Alpine Orogeny (Lemcke 1988; Hofmayer et al. 2019). The sediments of the Molasse Basin are divided into marine, brackish and freshwater Molasse units, which were formed as a result of two transgressive-regressive megacycles. The first megacycle includes the Lower Marine Molasse, the Lower Brackish Molasse and the Lower Freshwater Molasse, whereas the second megacycle comprises the Upper Marine Molasse, the Upper Brackish Molasse and the Upper Freshwater Molasse (Janssen et al. 2018; Hofmayer et al. 2019). In the Ottnangian, the marine transgression of the western Paratethys started from the Southwest. In Baden-Württemberg, the sediments of the Upper Marine Molasse consist of approximately 50 m thick glauconitic sands of the Heidenlöcher Beds (Doppler et al. 2005), which are overlain by 20 to 70 m thick sediments of the Kalkofen Formation. The Kalkofen Formation consists of glauconitic sands and marls (Heckeberg et al. 2010) and is overlain by the Baltringen Formation and the fossil-rich Baltringer Horizon (Fig. 1).

Figure 1. 

Stratigraphic positions of European specimens of Megalolamna according to their country of discovery described in this paper.

The syntype of Otodus serotinus described by Probst (1879) was found at the site of the now closed Kodelsberg quarry east of Baltringen (near Mietingen, district of Biberach, 48.176593, 9.895192; Baltringen Formation, middle Ottnangian, middle Burdigalian) (Pollerspöck and Unger in press).

The tooth described by Barthelt et al. (1991) comes from the Walbertsweiler sand pit (between the towns of Meßkirch and Pfullendorf, district of Sigmaringen, 47.95145, 9.1572; Kalkofen Formation, Lower Ottnangian, Middle Burdigalian: Fig. 1), which is no longer accessible and has been recultivated. Organic debris and bedding planes with ripple marks indicate a nearshore, interior neritic to shallow marine environment. The water depth is estimated to be less than 50 m based on the abundance of shallow water foraminifera (Barthelt et al. 1991; Heckeberg et al. 2010; Pippèrr et al. 2011). The former coastline was located to the north, less than 25 km away.

In the former sand pits near Plesching, the deposits of the “Linzer Sande” (Egerian) and the phosphorite sands (Ottnangian) were exposed. The “Linzer Sande” of the Linz-Melk-Formation overlie the gneiss of the Bohemian Massif. The sand is whitish-grey in colour. While bivalves and gastropods are mostly distributed in the sand, a concentrated occurrence of oysters can be found at the base of the so-called oyster bank. The “Linzer Sande”, which dip at about 20° to the NNE, are unconformably overlain by the phosphorite sands of the Plesching Formation. A hiatus created by a transgression in this part of the Molasse Sea comprises the complete Eggenburgian. The coarse to fine-grained phosphorite sands are green-brownish in color due to the glauconite and phosphorite content. Beach block heaps indicate a steep rocky coast (Reiter and Berning 2012). The sands were intensively collected, especially due to the relatively frequent occurrence of shark teeth (Schultz 1968). The previously undescribed tooth was found around 1985 by Dietmar Stadlhuber (Engerwitzdorf/Mittertreffling, Austria) in the phosphorite sands near Plesching (Plesching Formation, middle Ottnangian, 48.324547, 14.342461; Fig. 1).

The Mazan site is located in Provence (Southeastern France, 44.029044, 5.157797), the southern part of the Mormoiron Basin (Vaucluse), which is part of a succession of basins that subsided between the rising Alps and the Rhodian trough. The Malemort-du-Comtat quarry, still in operation today where gypsum (Blauvac Complex Formation, Eocene) is quarried comprises sediments dating from the Miocene to the Eocene (Besson 2005; Vialle et al. 2011; Maridet et al. 2013). While the Paleogene sediments have been lithostratigraphically studied (Blauvac Complex Formation, Patis Formation, Brotia laurae and Tympanotonos labyrinthus Formation; Triat et al. 1971), the Neogene sediments lack such studies. Vialle et al. (2011) described a species-rich selachian fauna from the younger sandstone layers of the middle Miocene with predominantly deep-sea-dwelling species. The tooth found here comes from a layer that was deposited directly above the clearly recognisable Paleogene-Neogene boundary (Fig. 1). This layer can be dated to the lowest Burdigalian.

The Monferrato succession can be considered the north-western end of the Appenninic chain. It is composed of a predominantly terrigenous sequence of Eocene and Miocene age, deposited in complex internal basins (Mancin et al. 2003) and resting unconformably on the Upper Cretaceous–Eocene Ligurian Flysch. According to recent stratigraphic and structural studies, the Monferrato area consists of two stratigraphic successions that emerge in the western and eastern parts, respectively. The western sequence of the Monferrato, that consists of coarse Oligocene to Early Miocene terrigenous sediments deposited both in strongly subsiding basins and on adjacent structural uplifts, includes (from base to top): the Ranzano Sandstones, the Antognola Marls and finally the Pteropod Marls and their lateral equivalents. The eastern succession of the Monferrato, which consists of Oligocene to Early Miocene terrigenous deposits of different depths (Clari et al. 1987) and is followed by unconformable carbonates of the shelf, comprises (from bottom to top): the “Cardona Unit”, the Antognola Marls and finally the “Pietra da Cantoni Group” (Mancin et al. 2003). The tooth described by Alessandri (1897) and two other specimens are housed in the Collection of the Earth Sciences Department of the University of Turin, Italy. All come from Rosignano (La Colma), which are assigned to the Pietra da Cantoni Group (Globigerinoides trilobus zone, INF 4a, lower Burdigalian, Fig. 1).

The monospecific genus Megalolamna with the species paradoxodon was described by Shimada et al. (2017) based on Miocene teeth from the USA, Japan and Peru. However, the present study reveals that there were already two cases of historical findings of this species that have so far remained unnoticed or unrecognized in the literature. The first case is represented by the work of Probst (1879), who described the species Otodus serotinus from the Baltringen Formation (lower Miocene, Burdigalian, regional stage Ottnangian) on the basis of four specimens that represent the syntypes of the species (Probst 1879: pl. 2 fig. 82–85). During a recent revision of the fossil shark fauna described by Probst (1879) it was determined that the syntype under fig. 84 is conspecific and belongs to Megalolamna paradoxodon Shimada, Chandler, Lam, Tanaka & Ward, 2017 (Pollerspöck and Unger in press). The second case is represented by the study of Alessandri (1897), who described the species Lamna bassanii from Pietra da Cantoni, Eastern Monferrato (Miocene, Burdigalian, G. trilobus Zone (INF 4) and illustrated two teeth (pl. 1, fig. 16, 16a). Based on the description and illustration by Alessandri (1897) and the visible morphological characters, these two teeth also belong to the genus Megalolamna.

While both species names, ‘O. serotinus’ and ‘L. bassanii’, were published under the conditions of Article 10 of the International Code of Zoological Nomenclature (ICZN 1999) and are available, the taxonomic position, as described in Systematic Paleontology above, must be clarified, in particular, which species name is valid on the basis of the ICZN Article 23 (Principle of Priority).

ICZN Article 23 provides that “the valid name of a taxon is the oldest available name applied to it, unless that name has been invalidated or another name is given precedence by any provision of the Code or by any ruling of the Commission”. Exceptions to this principle are only possible under the conditions of Article 23.9 (Reversal of precedence). In this case, both of the following two conditions must be cumulatively fulfilled:

1. the senior synonym or homonym has not been used as a valid name after 1899 (Article 23.9.1.1.), and
2. the junior synonym or homonym has been used for a particular taxon, as its presumed valid name, in at least 25 works, published by at least 10 authors in the immediately preceding 50 years and encompassing a span of not less than 10 years (Article 23.9.1.2.)

An extensive literature search revealed that the species names O. serotinus and/or L. bassanii were used as valid names after 1899 in the following works: Joleaud (1907: Lamna serotina p. 172); D’Erasmo (1924: Lamna bassanii, p. 24); Fowler (1966: Otodus serotina, p.135; Lamna bassanii, p. 131); Cappetta (2006: ?Cretolamna bassanii, p. 31). It follows that the species names bassanii and paradoxodon are to be regarded here as junior synonyms of O. serotinus, representing the only known species within the genus Megalolamna.

Prior to this study, the genus Megalolamna was known from the upper Oligocene–Miocene deposits (but mostly Miocene) in only the following five countries: USA, Colombia, Ecuador, Peru, and Japan (Shimada et al. in press, and references therein). Therefore, the speci­mens reported in this paper nearly doubles the known occurrences of the genus by country, which now also include Austria, France, Germany, and Italy (a total of nine countries: Fig. 4). Ocean-wise, the distribution of Megalolamna consists of the eastern and western margins of the Pacific Ocean, western Atlantic Ocean, and the Caribbean Ocean, and the present study adds the Mediterranean and Paratethys seas. Latitudinally, the occurrence of Megalolamna is distributed from the tropical to mid-latitudinal zone in both the Northern and Southern hemispheres.

Figure 4. 

Geographic distribution of previous records (circle) and newly described European records (star) of Megalolamna (Carrillo-Briceño et al. 2016, 2020; Shimada et al. 2017, in press) using early Miocene (Aquitanian–Burdigalian) paleogeographic map (after Smith et al. 1994, p. 27).

Since the original description in 2017, Megalolamna has been characterized as an ‘elusive’ extinct lamniform shark due to its spotty but geographically widespread occurrences even though its existence was chronologically quite constrained, primarily to the Miocene Epoch (Shimada et al. 2017, in press). The new occurrences from Europe reported here further demonstrates its elusiveness as teeth of Megalolamna are never in abundance at any single locality, even though some areas have been heavily exploited palaeontologically, particularly for fossil shark teeth (e.g., Probst 1878, 1879; Schultz 1968; Caretto 1972; Barthelt et al. 1991; Brisswalter 2009; Höltke et al. 2020). In fact, its spotty but geographically widespread occurrences is reminiscent to a few present-day lamniform sharks, such as the goblin shark (Mitsukurina owstoni), megamouth shark (Megachasma pelagios), and the most elusive of all, the bigeye sandtiger shark (Odontaspis noronhai) (Ebert et al. 2021; Ng et al. 2022). Given that these extant taxa are sparsely but broadly distributed, it is quite possible that teeth of Megalolamna may be discovered or recognized in the future at other Miocene localities not presently recorded, possibly along the Atlantic coasts of Africa and South America as well as along the Indian Ocean and southwestern Pacific Ocean in the Oceania region.