A Tylosaurine Mosasauridae (Squamata) from the Late Cretaceous of the Basque-Cantabrian Region

An isolated mosasaurid tooth from the Campanian of Alava (Basque Country), previously referred to as cf. Mosasaurus sp., is here reattributed to a tylosaurine. It may belong to Tylosaurus, a nearly cosmopolitan genus known from the Santonian-Maastrichtian. This is the first occurrence of a tylosaurine mosasaurid in the Iberian Peninsula. Moreover, it corresponds to the southernmost occurrence of this clade in the northern margin of the Mediterranean Tethys. Tylosaurinae fossils are known from North America, Europe, New Zealand, Antarctica, Africa and Asia, but remain unknown from the southern Mediterranean Tethyan margin and from tropical palaeolatitudes.

In Europe, mosasaurids are represented with more than twenty species currently considered as valid (Bardet & Pereda Suberbiola, 1996;Jagt, 2005).The richest outcrops are those of the Maastrichtian of Belgium and the Netherlands, where numerous skeletons have been recovered since the last century.However, mosasaurids are very scarcely known in the Late Cretaceous of the Iberian Peninsula.Until now, their fossils have only been reported from the "Garumnian" of Portugal (Sauvage, 1897(Sauvage, -1898) ) and, recently, from the Santonian to Maastrichtian marine formations of the Basque-Cantabrian Region, where both Mosasaurinae and Plioplatecarpinae taxa have been recognised: Mosasaurus lemonnieri, Leiodon anceps, Prognathodon solvayi, Platecarpus cf.ictericus and undeterminated species of the genera Mosasaurus and Leiodon (Bardet et al., 1993(Bardet et al., , 1997(Bardet et al., , 1999)).
Here we revised a previously described tooth referred to as cf.Mosasaurus sp. from the Campanian of Alava (Bardet et al., 1993(Bardet et al., , 1997) ) and reattribute it to Tylosaurinae.

Geological context
The Basque-Cantabrian Region is a geological structure located in the northern part of the Iberian Peninsula.Marine Upper Cretaceous sediments, deposited under offshore platform environment, crop out extensively in the central part of this basin (Navarro-Cantabrian Region).The here described tooth comes from the northern flank of the Miranda-Treviño syncline, extending East-West on the Alava Province.It has been unearthed near the town of Castillo-Lasarte, in the grey-blue marls of the Vitoria Formation, which is late Campanian (H.marroti Zone) in age (Santamaría Zabala, 1996).
Description (fig.1).MCNA 1654 is the crown, about 3 cm high, of a marginal tooth.In lateral view, the crown is triangular and is moderately posteriorly recurved.The basal cross-section of the crown is oval.It bears a pronounced anterior carina clearly serrated and a lingually located slight carina which serrations remain discrete.The position of the carinae indicate that it is an anterior tooth.The buccal surface is convex and bears indistinct facets ending at two thirds to three-fourths of the crown height.The lingual surface is also convex but flatter and has more than ten poorly defined facets.At the base of the crown on both faces, there are fine striations about 5 mm high.
Comparison.Until recently, poor attention has been paid to mosasaurid teeth, which descriptions are often vague and uninformative, precluding for detailed comparisons between taxa.However, recent studies have revealed the utility of teeth -including isolated ones-in mosasaurid taxonomy and have permitted significant results (i.e.Lindgren & Siverson, 2002;Lindgren, 2005).
MCNA 1654 was previously referred to as cf.Mosasaurus sp. on the basis of the presence of facets on the buccal and lingual surfaces of the crown (Bardet et al., 1993(Bardet et al., , 1997)).On the light of new data, the Alava tooth can be safely referred to a russellosaurine mosasaurid because the crown is faceted with basal striae on the lingual and lingual surfaces (Bell, 1997).Among Russellosaurina, the plioplatecarpines bear slender teeth strongly posteriorly recurved in lateral aspect, having fine carinae without serrations, marked facets and subcircular cross-section.With regard to tylosaurines, they exhibit large broadly triangular teeth slightly recurved in lateral aspect, bearing marked carinae which are serrated or not, and an oval to compressed cross-section (see Lindgren, 2005).Following this, MCNA 1654 belongs more probably to a tylosaurine than to a plioplatecarpine mosasaurid.
Tylosaurinae includes three genera: the near-cosmopolitan Tylosaurus Marsh, 1872, the European restricted Hainosaurus Dollo, 1885, and the southern hemisphere circum-polar Taniwhasaurus Hector, 1874.Lakumasaurus Novas et al., 2002 from Antarctica is probably a junior synonym of Taniwhasaurus (M.Fernández, personal communication) but as this data remains unpublished, it is still considered here.The marginal teeth of Tylosaurus have an asymmetric oval cross-section, with a convex buccal side and a U-shape lingual one.They are moderately posteriorly recurved in lateral view, and bear carinae, facets and basal striations slightly marked.The marginal teeth of Hainosaurus are bucco-lingually compressed, with a symmetric elliptical cross-section (i.e., equally developed convex surfaces), almost straight in lateral view, with marked carinae, basal striae and shallow facets.The teeth of Taniwhasaurus and Lakumasaurus are laterally compressed, slightly faceted, and bear only one anterior obtuse carina without serrations.Based on its oval cross-section, moderately defined facets and posterior curvature, the Alava tooth resembles more those of Tylosaurus than those of other tylosaurines.
With regard to the Alava tooth, it represents the southernmost (about 35 °N) occurrence of Tylosaurinae in the northern margin of the Mediterranean Tethys, as previous European records are known from Sweden, England, Belgium, The Netherlands and Poland (about 40 °N).It confirms also the nearly cosmopolitan status of the genus Tylosaurus and enlarges its palaeobiogeographical distribution to the Iberian Peninsula.2002), Lingham-Soliar (1992), Novas et al. (2002).Map after Smith et al. (1994).