Klaas Post 1 & Mark Bosselaers 2 1 Natuurhistorisch Museum Rotterdam 2 Berchem, Antwerpen Late Pliocene occurrence of Hemisyntrachelus (Odontoceti, Delphinidae) in the southern North Sea Post, K. & Bosselaers, M., 2005 - Late Pliocene occurrence of Hemisyntrachelus (Odontoceti, Delphinidae) in the southern North Sea - DEINSEA 11: 29-45 [ISSN 0923-9308]. Published 29 December 2005 Late Pliocene Hemisyntrachelus is reported from the southern North Sea, marking the youngest and northernmost occurrence of the genus. The mandibular morphology of the North Sea fossils is compared to Pliocene Hemisyntrachelus from Italy, the status and characteristics of the genus are discussed and arguments for the inclusion of Early Pliocene Tursiops oligodon from Peru in this genus are presented. Keywords: Late Pliocene, Hemisyntrachelus, Tursiops oligodon, Odontoceti, Delphinidae, North Sea Correspondence: K. Post, Natural History Museum Rotterdam, P.O. Box 23452, 3001 KL Rotterdam, the Netherlands, e-mail [email protected]; M. Bosselaers, Lode van Berckenlaan 90, 2600 Berchem, Antwerpen, Belgium, e-mail [email protected] INTRODUCTION ribs and thoracic vertebrae and the morphology The fossil dolphin genus Hemisyntrachelus of the lumbar vertebrae being different from Brandt, 1873 is based on material from the those in Tursiops. Although Slijper was fol- Pliocene of Italy, with Delphinus cortesii lowed by some authors (e.g. Simpson 1945), Fischer, 1829 designated as the type spe- others continued to disagree (e.g. Pilleri 1979) cies. It was placed in the Delphinapteridae or expressed reservations and the need for fur- by Brandt (1873) without further explanation ther study (Barnes et al. 1985, Barnes 1990). or systematic arguments. Previous to Brandt, Bianucci (1996) presented reasons to maintain Cuvier (1823) had already noted that some Hemisyntrachelus at the genus-level, such as fossil Tursiops-like crania from Italy showed a rostrum that is broader at the base, a more characters ‘intermediate’ between Tursiops and robust antorbital process, a smaller number of Orcinus. larger teeth, a shorter tooth row and a pecu- The close resemblance of Hemisyntrachelus liar mandibular symphysis in comparison to and Tursiops caused controversy about the Tursiops. He included Hemisyntrachelus in the validity of the genus and the two have often Delphinidae on the basis of two apomorphies: been considered synonymous (cf. Capellini a wide mesethmoid in dorsal view and the 1882, Portis 1885). Nevertheless, Slijper reduction of the posterior ascendant branch of (1936) resurrected Hemisyntrachelus and the premaxilla. Slijper (1936) and Bianucci coined the family Hemisyntrachelidae for it. (1996) thought Hemisyntrachelus and Tursiops Slijper’s decision was based on morphological to be contemporaneous because they consid- considerations, in particular the articulation of ered Tursiops ossenae simonelli, 1911 from 29 DEINSEA 11, 2005 the Pliocene of Italy to be a ‘genuine’ Tursiops. genus. The North Sea fossils are compared In an attempt to a phylogenetic analysis, with Italian Hemisyntrachelus specimens, Bianucci (1996) compared Hemisyntrachelus as well as with some Recent Delphinidae, to with Tursiops, while at the same time making reconsider the validity of the genus and to remarks on features perceived ‘intermedi- establish distinctive characters for it. We also ate’ between Hemisyntrachelus, Tursiops, summarize the Late Pliocene marine mam- Orcinus and Pseudorca. He presented three mal fauna of the southern North Sea of which hypotheses on the phylogenetic relation- Hemisyntrachelus was a member. ships of Hemisyntrachelus, of which the one considered most parsimonious by MATERIAL AND METHODS him shows Hemisyntrachelus as the sister All important fossils of Hemisyntrachelus in taxon of Pseudorca, Orcinus, and ‘other Italian collections were studied during the Globicephalinae’. However, a recent study summer of 2003. Extant delphinids in the col- based on full cytochrome b sequences (LeDuc lections of KBIN, NNM and ZMA were used et al. 1999) showed that Tursiops is para- for comparison. For measurement methodol- phyletic, with Tursiops aduncus sharing a more ogy, see Fig. 1. Special attention was given to recent common ancestor with Delphinus and the following specimens: some species of Stenella than with Tursiops Hemisyntrachelus cortesii, cast of holotype truncatus. Moreover, LeDuc et al. (1999) (MP); found the Delphininae to be not closely related Hemisyntrachelus pisanus, holotype (MB to Orcinus and Pseudorca, with Orcinus being 1coc14); outside the Globicephaline clade to which Tursiops oligodon, holotype (SMNK pal. Pseudorca belongs. Clearly, further investiga- 3841); tions are needed in order to resolve the phylo- Tursiops ossenae, holotype (MB 8561); genetic position of Hemisyntrachelus. Orcinus citoniensis, holotype (MB 1coc11); Apart from Delphinus cortesii, other nomi- Tursiops capellinii (= Hemisyntrachelus nal taxa that have at times been included cortesii) (MPP 48-48); in Hemisyntrachelus are Tursiops brochii Hemisyntrachelus sp. (NMR 9991-01756, capellini, 1863; Tursiops capellinii del NMR 9991-01757, Y-1409, A-3231). prato, 1897; Tursiops astensis sacco, 1891; Tursiops cortesii var. pedemontana sacco, Collection acronyms 1891 and Hemisyntrachelus pisanus Bianucci, A - private collection of mr L. Anthonis of 1996, but - in his review of the genus - only Bouwel, Belgium cortesii and pisanus were considered valid by IFG - Museo di Geologia e Paleontologia Bianucci (1996), who relegated brochii, capel- dell’Università di Firenze, Florence, Italy linii, astensis and pedemontana to the syn- KBIN - Koninklijk Belgisch Instituut voor onymy of cortesii. Natuurwetenschappen, Brussels, Belgium Pilleri (1979) speculated that taxa tradition- MB - Museo Giovanni Cappellini ally included in Hemisyntrachelus originated dell’Università di Bologna, Italy from an exclusively Italian radiation, but this MP - Museo di Storia Naturale e del Territorio view finds little support in the dubious taxo- Certosa di Calci dell’Università di Pisa, nomic status of some of the nominal taxa and Calci, Italy the controversial nature of the genus itself. MPP - Museo di Paleontologia dell’Università Here, we present data on recently discovered di Parma, Italy specimens, assigned to Hemisyntrachelus, NMR - Natuurhistorisch Museum Rotterdam, from the latest Pliocene of the southern North the Netherlands Sea, thereby considerably extending the known NNM - Nationaal Natuurhistorisch Museum geographical and stratigraphical range of the Naturalis, Leiden, the Netherlands 30 POST & BOSSELAERS: Hemisyntrachelus in the North Sea Figure 1 Measurements taken on delphinid mandibula in this study. NWHCH - Norfolk Museums and Archaeology antorbital process of the frontal, periotic with Service, Norwich, UK robust anterior process and low pars coch- SMNK - Staatliches Museum für Naturkunde, learis with wide external auditory window, Karlsruhe, Germany broad tympanic - in ventral view - with a nar- Y - private collection of mr C. van Hooydonk row medial lobe, short dental rows with 14-16 of Rucphen, the Netherlands robust teeth and short mandibular symphysis. ZMA - Zoölogisch Museum, Universiteit van Moreover Bianucci (1997) considers the robust Amsterdam, the Netherlands anterior process of the periotic as distinctive for the genus. Most of the postcranial and cra- CHARACTERISTICS OF nial characters cannot be recognized unambig- HEMISYNTRACHELUS uously in fragmented fossils, but mandibular Slijper (1936) stated that Hemisyntrachelus characters in Hemisyntrachelus are remarkably differs from other delphinid genera (such as constant and easy to quantify, as well as well- Tursiops and Orcinus) by the shape of the lum- differentiated from other delphinid genera (Fig. bar vertebrae and the articulation of the ribs. In 2), to the extent that even fragments of man- his emended diagnosis of the genus Bianucci dibulae can be identified unambiguously. (1996, 1997) compared Hemisyntrachelus to Tursiops and listed the following distinc- The mandibula of Hemisyntrachelus differs tive characters: sturdy vertebrae, atlas with a from all other Delphinidae by the combination dorso-ventrally compressed neural arch and of the following characters (see Table 1): pointed transverse processes, broad base of a - low length/height ratio of the symphysis long rostrum, not narrowing premaxilla on the (< 1.6); apical portion of the rostrum, deep and narrow - presence of a prominent ventral keel on the antorbital notch, square-shaped neurocranium symphysis; in dorsal view, anterior location of the robust - few labial foramina mentis (< 4); 31 DEINSEA 11, 2005 Figure 2 Mandibular characters of Hemisyntrachelus (bottom) compared with those of other delphinid genera. 32 POST & BOSSELAERS: Hemisyntrachelus in the North Sea - downward sloping mandibular tip; - size of teeth (Ø of alveolae > 12 mm); - less than 17 teeth per tooth row; - estimated tooth count of 12. - teeth robust, round and pointed (Ø of alveolae > 8 mm) Unfortunately, so far no complete mandibula has been found in the southern North Sea, The Hemisyntrachelus specimens from the but based on the diameter of the teeth versus southern North Sea show all the diagnostic the length of the ramus it is likely that the characters of the genus and can be distin- North Sea Hemisyntrachelus had 12 (certainly guished from the known species of the genus < 14) teeth in the dental row. This assump- by the following characters (see Table 1): tion is perhaps corroborated