HALISAURUS STERNBERGI, A SMALL MOSASAUR WITH AN INTERCONTINENTAL DISTRIBUTION

Journal of Paleontology, Jul 2005 by Lindgren, Johan, Siverson, Mikael

ABSTRACT-Remains of Halisaurus sternbergi (Wiman, 1920) from the latest Early Campanian (sensu germanico) of the Kristianstad Basin, southern Sweden, represent the first record of this species outside of the USA. The material comprises numerous marginal tooth-crowns, a premaxilla, an incomplete pterygoid, and vertebrae. The Kristianstad Basin population of H. sternbergi was probably derived from individuals that migrated from the Mississippi Embayment in North America sometime during the Early Campanian. Even though H. sternbergi thrived in great numbers in the coastal waters of the southern part of the Baltic Shield during the latest Early Campanian, the population appears to have been short-lived. Available data indicate that H. sternbergi, along with several other species of mosasaurs, vanished from the region following an intercontinental mosasaur extinction event, or a series of events, near the Early/Late Campanian boundary.

INTRODUCTION

HALISAURUS MARSH, 1869 is one of the most basal members of the Mosasauridae (Bell, 1997). It had both a wide geographic distribution and a long stratigraphie range (see Caldwell and Bell, 1995; Holmes and Sues, 2000; and discussion below). At present, the genus includes three nominal species (Holmes and Sues, 2000; Bardet et al., 2005) H. platyspondylus Marsh, 1869, H. arambourgi Bardet et al., 2005, and H. sternbergi (Wiman, 1920), although a fourth species, H. onchognathus Merriam, 1894, has been identified from the Smoky Hill Chalk in Kansas, USA (the type, and only known, specimen of this taxon was probably destroyed during World War II; see Russell, 1967). Bell (1997) recognized (but did not formally describe) a new Halisaurus from the Mooreville Chalk of Alabama, USA, but we have been unable to identify definite characters separating it with confidence from H. sternbergi (see discussion below).

Very little attention has been paid to the morphology of vertebrae posterior to the thoracal series in Halisaurus. Brief descriptions of caudals have been given in papers by, among others, Baird (1986b), Holmes and Sues (2000), and Bardet and Pereda Suberbiola (2001). Unfortunately, these descriptions are too generalized and/or lack accompanying illustrations, and hence cannot be used to separate Halisaurus from other mosasaurs. Likewise, with the sole exception of a single tooth assigned to Pluridens walkeri Lingham-Soliar, 1998 (=Halisaurus; see discussion below) by Lingham-Soliar (1998, fig. 4), tooth-crowns of Halisaurus have never been illustrated.

The purpose of this paper is twofold. Firstly, it documents the dental morphology and vertebral centra/premaxilla anatomy of Early Campanian Halisaurus from southern Sweden and Alabama, USA. Secondly, it demonstrates the great adaptability of Halisaurus, which not only allowed it to survive an intercontinental migration attempt (despite being one of the smallest known mosasaurs) but also enabled it to compete successfully with the local mosasaur fauna of the Kristianstad Basin until the mosasaur community virtually collapsed at the Early/Late Campanian boundary.

GEOLOGY

The Cretaceous deposits of the Kristianstad Basin are demarcated to the southwest by the Linder�ds�sen and N�vlinge�sen horsts (Fig. 1). The northern limit of the basin is more diffuse with several isolated pockets of Campanian/Maastrichtian strata situated north of the irregular northern boundary of the basin. Exposures of the Cretaceous are located either in quarries along the two horsts to the southwest or in abandoned quarries/natural outcrops in the northern part of the basin. The central area of the basin has a rather thick cover of Quaternary sediments, rendering quarrying activities unprofitable. Accessible strata range in age from the Early or early Middle Santonian (Ringelesl�tt; see Christensen, 1975) to the earliest Maastrichtian (e.g., Ballingsl�v I and II; see Siverson, 1992a). Early Cretaceous sediments are known from cored subsurface sections in the central parts of the basin (Norling and Skoglund, 1977; Bergstr�m and Sundquist, 1978; Norling, 1981). The sediments deposited during the latest Early Campanian Belemnellocamax mammillatus Zone (an informal biozone correlating with the German Belemnitella mucronata senior/Gonioteuthis quad rata gracilis Zone; see Christensen, 1975) comprise a rather wide range of lithologies, reflecting a mosaic of local shallow water environments in an archipelago setting. More or less calcareous quartz sands (e.g., �sen), calcarenites (e.g., Ignaberga), and oyster banks (e.g., parts of the stratigraphic column at Iv� Klack) are the dominant sediment types. Storm-generated conglomerates (typically with crystalline pebbles, belemnite guards, and oysters) attest to a nearshore, high-energy environment (Erlstr�m and Gabrielson, 1992). For a more comprehensive description of the development of the Kristianstad Basin, see e.g., Christensen (1975), Bergstr�m and Sundquist (1978), Lidmar-Bergstr�m (1982), and Erlstr�m and Gabrielson (1986, 1992). For locality data, see Appendix.