A new longirostrine beaked whale Flandriacetus gijseni gen. et sp. nov. (Ziphiidae, Cetacea, Mammalia) from the Tortonian of the North Sea Basin

1 INTRODUCTION With at least 25 genera in the fossil record, the Ziphiidae represent the cetacean family with the highest past diversity (Bianucci et al. 2016a; Ramassamy 2016; Lambert et al. 2023). The extant members spend most of their lives as deep-divers in epipelagic settings. They are suction feeders (mainly targeting cephalopods), usually lack functional dentition (except Tasmacetus sheperdi Oliver, 1937), and possess one or two pairs of erupted or semi-erupted tusks on lower jaws that are interpreted to be tokens of sexual dimorphism, display, and intraspecific fighting (Macleod 2000; Macleod & Herman 2004; Lambert et al. 2010). The present knowledge of the morphology and evolution of extinct crown ziphiids is limited and observations on tusks and dentition are scarce. Cranial fossils of these ziphiids are excavated in Belgian, Danish, Japanese and Peruvian strata, or taken from dredging areas near Brazil, Chile, Kerguelen, Portugal, Spain, South Africa and the Netherlands (Van Bree 1997; Lambert & Louwye 2006, 2016; Bianucci et al. 2007, 2016a, 2023, 2024; Wijnker et al. 2008; Lambert et al. 2009; Gol’din & Vishnyakova 2013; Ichishima et al. 2016; Post & Bosselaers 2017; Tanaka et al. 2019; Ramassamy & Lauridsen 2019; Lambert et al. 2023). They do not show clear signs of functional dentition in maxilla and mandibula. Stem ziphiids, however, are known in more detail. They are reported from Argentina, Belgium, Denmark, Italy, the Netherlands, Peru, Portugal, Spain and the USA (Weber 1917; Bianucci et al. 1992, 2010, 2013, 2016a and b, 2019; LamONLINE JOURNAL OF THE NATURAL HISTORY MUSEUM ROTTERDAM, WITH CONTRIBUTIONS ON ALL ASPECTS OF NATURAL HISTORY WWW.DEINSEA.NL A new longirostrine beaked whale Flandriacetus gijseni gen. et sp. nov. (Ziphiidae, Cetacea, Mammalia) from the Tortonian of the North Sea Basin Klaas Post1, Mark Bosselaers2 & Dirk Munsterman3 1 Natural History Museum Rotterdam, Westzeedijk 345, 3015 AA Rotterdam, the Netherlands 2 Institut Royal des Sciences Naturelles de Belgique, Rue Vautier 29, 1000 Brussels, Belgium 3 Netherlands Institute of Applied Geoscience TNO – Geological survey of the Netherlands, Princetonlaan 6, 3584 CB Utrecht, the Netherlands ABSTRACT Based on 13 more or less complete skulls and some attached postcranial elements the presence of a new longirostrine beaked whale species Flandriacetus gijseni gen. et sp. nov. is reported from the southern North Sea Basin. Dated by dinoflagellate cysts to 8.1-7.5 Ma (Tortonian age), the new species represents the youngest occurrence of a longirostrine stem beaked whale in the North Atlantic. The large collection allows observations on the morphological differences between some of the North European members of the Messapicetiformes clade. Keywords Fossils, marine mammals, whale evolution, morphology, functional dentition Cite this article Post, K., Bosselaers, M., & Munsterman, D., 2025 - A new longirostrine beaked whale Flandriacetus gijseni gen. et sp. nov. (Ziphiidae, Cetacea, Mammalia) from the Tortonian of the North Sea Basin - Deinsea 23: 1 - 31. DOI: 10.5281/ZENODO.17880020 Submitted 16 May 2025 Revised 12 November 2025 Accepted 25 November 2025 Published 18 December 2025 Author for correspondence [email protected] Editors of this paper Bram W. Langeveld Copyright 2025 Post, Bosselaers & Munsterman Distributed under Creative Commons CC-BY 4.0 DEINSEA online ISSN 2468-8983 Post et al.: A beaked whale from the North Sea Basin DEINSEA 23: 1-31 [2025] 2 and 2019 during fishing expeditions on the bed of the Westerschelde estuary, organised by the Natural History Museum Rotterdam (Post & Reumer 2016). They were encased in large blocks of glauconitic sandstone. Preparation of the specimens was undertaken with mechanical tools (occasionally with acid preparation of details) by one of the authors (KP) assisted by Jordi Kempers. Specimens directly examined for comparison Aporotus dicyrtus du Bus, 1868: IRSNB M.541 (holotype); Aporotus recurvirostris du Bus, 1868: IRSNB M.1887 (holotype), NMR999100159960, NMR999100159959, NMR999100159958; Beneziphius brevirostris Lambert, 2005: IRSNB M.1885 (holotype), IRSNB M.1886, NHG 23667; Beneziphius cetariensis Miján, Louwye & Lambert, 2016: SGHN MF MA0953 (holotype); Caviziphius altirostris Bianucci & Post, 2005: NNML 447230 (holotype); Choneziphius leidyi Bianucci et al., 2013: IRSNB M.188, SGHN MA0633 (holotype), SGHN MA0640, SGHN MA0641, SGHN MA0937; Choneziphius planirostris Duvernoy, 1851: IRSNB M.1881, IRSNB M.1882, IRSNB M.1883, IRSNB 3767-3773, IRSNB 3776, IRSNB 3779, IRSNB 3780, IRSNB 3790, NHG 22069, NHG 22773, NHG 23310, NMR999100000275, NMR999100007480, NMR999100007481, NMR999100007484, NMR999100007485, NMR999100007486, NMR999100007499, NMR999100007949, NMR999100008077, NMR999100008198, NMR999100008199, NMR999100009913, NNML ST 153415, NNML ST 153415, NNML ST 153620, NNML ST 170052, NNML ST 118478, NNML ST 20211, NNML ST 132506, NNML ST 146242, NNML ST 104557, NNML ST 12286; Dagonodum mojnum: MSM 1001X; Mesoplodon europaeus (Gervais, 1855): NMR9990001379; Messapicetus longirostris: MAUL no number (holotype); Messapicetus gregarius: MUSM 1037 (holotype), MUSM 950, MUSM 951, MUSM 1036, MUSM 1038, MUSM 1394, MUSM 1481, MUSM 1482, MUSM 1718; Tusciziphius atlanticus Bianucci et al., 2013: SGHN MA0926 (holotype), NMR999100003020 (paratype); Ziphirostrum marginatum: IRSNB M.1878 (holotype), IRSNB M.1874, IRSNB M.1875, IRSNB M.536, IRSNB M.537, IRSNB M.1876, IRSNB M.1877, IRSNB M.1879, NHG 22773, NHG 23310, NHG 23395, NMR999100007487, NMR99100159956; NMR99100153673, NNML ST 20121, NNML ST 20113, NNML 20779; Ziphirostrum recurves (du Bus, 1868): IRSNB M.544 (holotype); Ziphirostrum turniense du Bus, 1868: IRSNB M.539 (lectotype), IRSNB M.1880. (Z. marginatum is considered the best-known member of its genus (both other species are known by rostra only), but the large number of fossils of Z. marginatum stored in Belgian and Dutch collections show confusing variability and characters (pers. ob. KP). For this study IRSBN 3845-M536 and NMR999100159956 are considered as the best-preserved representatives of Z. marginatum and used as reference.) Anatomical terminology and measurements Anatomical terminology follows Mead & Fordyce (2009); Lambert et al. (2013) and Ichishima (2016). Measurements bert 2005; Post & Bosselaers 2010; Buono & Cuzzuol 2013; Lambert et al. 2013, 2023; Bosselaers 2014; Ramassamy 2016; Miján et al. 2017; Bakker & Post 2019). Most known stem ziphiids are longirostrine and possess extensive functional dentition in upper and lower jaws, including – in the taxa where the tip of the lower jaw is preserved – one or two pairs of tusks. The best-known stem ziphiid Messapicetus is currently known by two species reported from the Balearic Islands (Spain), Italy (holotype of Messapicetus longirostris Bianucci & Landini, 1992), Peru (holotype of Messapicetus gregarius Bianucci, Lambert & Post, 2010) and tentatively from the west coast of the USA (Bianucci et al. 1992, 1994, 2010, 2016a and b, 2019; Fuller & Godfrey 2007; Ramassamy et al. 2018). The fossil record of the genus confirms that at least M. gregarius was able to target epipelagic prey and has been recorded in large numbers at one locality, which may indicate a gregarious behaviour (Bianucci et al. 2010; Lambert et al. 2015). All stem ziphiids with fused or joined pachyosteosclerotic premaxillae at the rostrum are grouped in a Messapicetiformes clade (Bianucci et al. 2024). The early diverging members of this clade are characterized by a longirostrine appearance (ratio of rostral length and condylobasal length between 0.6 and 0.78 – McCurry & Pyenson 2019; Lambert & Goolaerts 2022), functional dentition in upper and lower jaw, a prenarial basin, and a mandible with a long and fused symphysis with a semi-circular ventral surface (> 1/3 of the length of the mandible). This article reports and describes 13 specimens of stem ziphiids from the Tortonian of the Netherlands. It presents a new genus and species Flandriacetus gijseni gen. et sp. nov., increases the knowledge on the morphology of the clade and highlights in detail the morphological differences with the closely related beaked whale Ziphirostrum marginatum du Bus, 1868. Dated to c. 8.1-7.5 Ma, it represents – together with Dagonodum mojnum Ramassamy, 2016 from Denmark – the most recent presence of functional dentition in stem ziphiids in the North Atlantic. MATERIAL AND METHODS Institutional abbreviations IRSNB, Institut Royal des Sciences Naturelles de Belgique, Brussels, Belgium; NHG, Koninklijk Zeeuws Genootschap der Wetenschappen, Middelburg, the Netherlands; NNML (or RGM), Naturalis Biodiversity Center, Leiden, the Netherlands; NMR: Natural History Museum Rotterdam, Rotterdam, the Netherlands; MAUL, Museo dell’Ambiente, Università di Lecce, Italy; MDM, Museo Diocesà de Menorca, Menorca, Spain; MSM, Museum Sønderjylland Naturhistorie og Palæontologi, Gram Lergrav, Denmark; MUSM, Museo de Historia Natural, Universidad Nacional Mayor de San Marco, Lima, Peru; SGHN, Museo da Natureza Sociedade Galega de Historia Natural, Ferrol, Spain. Studied specimens The ziphiid specimens described in this article are coded with NMR numbers and were discovered in 2014, 2015, 2018, Post et al.: A beaked whale from the North Sea Basin DEINSEA 23: 1-31 [2025] 3 appending the LSID to the prefix https://zoobank.org/. The LSID for this publication is: 73EDD1DB-4521-4090-87B2F22033B62F11. The online version of this work is archived and available from the following digital repository https:// zenodo.org/. Cladistic analysis A cladistic analysis was performed in PAUP 4.0a - 169 (Swofford 2002) based on the list of characters and the matrix for the cladistic analysis of Ziphiidae of Bianucci et al. (2024) (Appendix 2). We included in our analysis the 15 species of the Messapicetiformes-clade of Bianucci et al. (2024) – being all the ziphiid species most closely related to Flandriacetus gen. nov., the basal stem ziphiids Notoziphius bruneti Buono & Cuzzuol, 2013 and Ninoziphius platyrostris de Muizon, 1983, and two basal odontocete species (Squalodon bellunensis Dal Piaz, 1901 and Squaloziphius emlongi de Muizon, 1991). Including Flandriacetus gijseni gen. et sp. nov. (with 47 of the 59 characters scored), the matrix contains 20 species (Appendix 3). The character scores of Dagonodum mojnum (direct observation of the type specimen during a visit to Denmark (pers. ob. KP); Char 1: 0=1) and Tusciziphius atlanticus (based on the type specimen and the paratype; Char 13: 2=1; 14: 1=0; 47: ?=1, 48: 0=1) were slightly adjusted from Bianucci et al. (2024) (Appendix 3). We performed an heuristic search of 25,000 replicates. All characters are treated as unordered, have equal weight, with no topological constrains in effect. GEOLOGICAL CONTEXT AND PALYNOLOGICAL INTERPRETATION The evolution and origin of the North Sea Basin is based on ongoing post-rift thermal subsidence since mid-Cretaceous times and the progressive opening of the northern Atlantic Ocean and African-Eurasian collision. These factors resulted in regional uplift of the British Isles, Fennoscandian Shield, Ardennes, Rhenish and Bohemian massifs during the Paleogene-Neogene. The present-day country of the Netherlands is located on the southern margin of the North Sea Basin. Uplift along the Weald-Artois Axis has led to interim closure of the Channel Seaway (Knox et al. 2010), linking the North Sea Basin to the eastern Atlantic, furthermore isolating the basin during the late Miocene. Lithospheric folding is associated with increased subsidence of the Roer Valley Graben already in the late Miocene (Deckers & Louwye 2020). Regional uplift in the Tortonian instigated the development of the proto-Rhine fluvial-deltaic depositional system of the Inden Formation in Germany and the south-eastern part of the Netherlands (Schäfer et al. 2005). The Inden Formation is followed by the Kiezeloolite Formation in the latest Tortonian and younger (Munsterman et al. 2019). These fluvial systems caused increased the nutrient concentration of the water. The Inden and Kiezeloolite Formations are westward transitional to the marine Diessen Formation. A gradual cooling trend between 9.5 and 7.4 Ma was recorded in the southern North Sea Basin (Donders et al. 2009). All our material of Flandriacetus gijseni gen. et sp. nov. origmainly follow Ross (1984); Lambert (2005) and Ramassamy et al. (2018). Palynological analysis Sediment samples were taken from four of the sandstone blocks with skulls. They were prepared at Palynological Laboratory Services (PLS, UK) and at the laboratory of the Geological Survey of the Netherlands (GSN-TNO, NL) using the standard sample processing procedures, which involves HCl and HF treatment, and sieving over a 15 µm mesh sieve (Janssen & Dammers 2008). The organic residue was mounted with glycerine-gelatine on microscope slides. One or two microscope slides were made of each sample: in addition to a non-oxidized kerogen slide, the organic residues were, if necessary, also oxidized with HNO3 in order to concentrate the palynomorphs and reduce the abundant “Structureless Organic Matter” (SOM). The palynological analysis was carried out at the GSN-TNO according to standard procedures. The microscope slide was counted as the TNO standard until an initial minimum of 200 palynomorphs (spores, pollen and dinoflagellate cysts) had been identified (when present). The remainder of the slides were successively scanned (at least a minimum of 200 specimens) for rarer taxa. Miscellaneous fossils (e.g. freshwater Pediastrum, and fresh to brackish water Botryococcus) were also quantitively and relatively (to the palynomorph sum) counted, but kept outside the total sum of 200 specimens of dinocysts, spores and pollen. Together a total average number of approximately 200-250 microfossils is reached, statistically sufficient for indicating the dominant dinocyst species fluctuation (Brinkhuis et al. 2003). Diagnostic species are discussed in the next section, and a complete distribution chart including all species found is given as an appendix. The age interpretation is based on the Last Occurrence Datum (LOD) and First Occurrence Datum (FOD) of dinoflagellate cysts. For the dinoflagellate cyst taxonomy the so-called “Lentin and Williams index” is followed (Fensome et al. 2019). Palynological interpretation is based on key references concerning the palynostratigraphy of the Neogene from the North Sea region such as: Powell (1992); Louwye et al. (2004); Munsterman & Brinkhuis (2004); Kuhlmann et al. (2006); Dybkjaer & Piasecki (2010); Louwye & De Schepper (2010); Köthe (2012) and Munsterman et al. (2019). The Geological Time Scale 2016 is used (Ogg et al. 2016). Dinoflagellate cyst zones are referred to Munsterman & Brinkhuis (2004) recalibrated to Ogg et al. (2016) in Munsterman et al. (2019) (Appendix 1). Nomenclatural Act The electronic version of this article in Portable Document Format (PDF) will represent a published work according to the International Commission on Zoological Nomenclature (ICZN), and hence the new names contained in the electronic version are effectively published under that Code from the electronic edition alone. This published work and the nomenclatural acts it contains have been registered in ZooBank, the online registration system for the ICZN. The ZooBank LSIDs (Life Science Identifiers) can be resolved and the associated information viewed through any standard web browser by Post et al.: A beaked whale from the North Sea Basin DEINSEA 23: 1-31 [2025] 4 inates from a very limited site in the Westerschelde river (Fig. 1) and is embedded in a dense sandy glauconitic matrix from the Diessen Formation, a lithostratigraphic unit which includes Tortonian to Messinian strata (Munsterman et al. 2019). The Diessen Formation was deposited in a predominantly shallow to open marine environment. The sediments were deposited as prodeltaic. Along the edges of the distribution area, nearshore settings occur (Munsterman et al. 2019). The age dating is not based on the cetacean fossils, but on microfossils from the (partly cemented) sands attached to the fossils. The cementation of the otherwise unconsolidated sands is considered to have occurred during fossilization. The preservation and yielding of the palynomorph assemblages (dinoflagellate cysts, spores and pollen) is moderate (NMR999100012016) to good (NMR999100012017, NMR999100014034) to very good (NMR999100016765). Bisaccate pollen dominate the sporomorphs. Bisaccate pollen are formed by conifers, gymnosperms (Gymnospermae). Bisaccates have a relatively higher aerial and aquatic buoyancy than other sporomorphs, and may indicate a relatively distal position from the coast and/or increased fluvial influence in a marine basin (Abbink 1998). The marine dinoflagellate cyst assemblage is variegated, showing neritic conditions. The most common species are: Achomosphaera andalousiense, Amiculosphaera umbracula, Barssidinium graminosum, Habibacysta tectata, Labyrinthodinium truncatum, Lejeunecysta sp., Lingulodinium machaerophorum, Melitasphaeridium choanophorum, Operculodinium centricarpum, Operculodinium spp., Reticulatosphaera actinocoronata, Selenopemphix brevispinosa, Selenopemphix dionaeacysta, Spiniferites spp. and Trinovantedinium spp. The chronostratigraphic diagnostic taxa are: Gramocysta verricula, Hystrichosphaeropsis obscura, Impagidinium ‘densiverrucosum’, Labyrinthodinium truncatum, and Selenopemphix armageddonensis. The former four taxa together have a minimum age in the Late Miocene, Tortonian. Taxa present with a maximum age range in the Tortonian are: Impagidinium ‘densiverrucosum’ and Selenopemphix armageddonensis. In addition, Achomosphaera andalousiense has a slightly extended first occurrence in the late(st) Serravallian (Munsterman & Brinkhuis 2004). The current assemblages fit into the Late Miocene, late Tortonian SNSM14 Zone, dated to 8.1-7.5 Ma (Munsterman & Brinkhuis (2004), recalibrated to the geological time scale of Ogg et al. (2016) in Munsterman et al. (2019). This zone is correlated to the Hystrichosphaeropsis obscura Biozone of Denmark (Dybkjær & Piasecki 2010), and the DN9 Zone of the eastern USA and Germany (De Verteuil & Norris 1996; Köthe 2012). Zone SNSM14 is defined by the LOD of Labyrinthodinium truncatum and the LOD of Systematophora (Cleistosphaeridium) placacantha. The latter taxon is not recorded. Diagnostic marker species for the Middle Miocene (SNSM zones 5-11) and early to mid-Late Miocene (SNSM zones 12-13) are missing in the palynomorph spectra. Dinoflagellate cysts analysis provided the relative age of the matrix of four of the described beaked whale crania to be late Tortonian, Zone SNSM 14; which yields an age of 8.1-7.5 Ma (Appendix 4). SYSTEMATIC PALEONTOLOGY CETACEA Brisson, 1762 ODONTOCETI Flower, 1867 ZIPHIIDAE Gray, 1850 FLANDRIACETUS gen. nov. Type species (by original designation): Flandriacetus gijseni gen. et sp. nov. (monotypic) from the Netherlands FLANDRIACETUS GIJSENI gen. et sp. nov. Holotype – NMR999100012016: well-preserved almost complete skull, rostrum, partial mandibles and six vertebrae (cervical vertebra 1-5, one anterior thoracic vertebra); missing the apex of rostrum, apex of the mandibles, ear bones and teeth (Fig. 2). To be mentioned hereafter as NMR12016. Referred specimens – NMR999100012017: partial cranium and rostrum, the vertex is somewhat distorted and separating parts of the frontals, left premaxillary crest and supraoccipital. NMR999100014034: partial cranium, rostrum and (possibly) a fragment of a hyoid (thyrohyal), missing the anterior part of the rostrum, ear bones, teeth and the upper part of the braincase (exposing the brain cavity). NMR999100016769: partial cranium, rostrum and a single detached tooth, missing the apex of the rostrum and ear bones. NMR999100159955: partial skull, rostrum and a fragment of the symphysis of the mandible, missing the anterior part of the rostrum, ear bones and teeth. NMR999100198032: well-preserved almost complete skull, rostrum and mandibles, missing the pterygoid, apex of rosNorth Sea Middelburg Bergen op Zoom Antwerp The Netherlands Belgium 20 km N W e s t e r s c h e l d e AB C North Sea The Netherlands Germany Belgium Figure 1 Location of site 6D, Westerschelde river, Zeeland province, the Netherlands (A + C present, B during the Late Miocene). Post et al.: A beaked whale from the North Sea Basin DEINSEA 23: 1-31 [2025] 5 fused premaxillae mandibula maxilla premaxillary sac fossa antorbital notch dorsal infraorbital foramina premaxillary crest C 4 C3 C 1 + 2 supraoccipital protuberances frontal nasal bony nare presphenoid mandibula alveolar row maxilla alveolar groove premaxilla frontal maxilla premaxillary crest presphenoid postorbital process of frontal preorbital process of frontal lacrimal supraoccipital parietal temporal crest supramastoid crest mandibula pterygoid transverse crest jugal condyle of mandibula squamosalexoccipital basioccipital crest palatine C 1 + 2 C 3 C 4 C 5 C 1 +2 0 10 cm mandibula vomer palatine hamular process pterygoid lacrimal jugal mandibulacondyle C 4 C 3 C 1 + 2 inter condyloid notch condyle basioccipital basioccipital crest exoccipital periotic fossa squamosal postglenoid process condyle bony nare foramina mandibula 010 cm A C B Figure 2 NMR999100012016, holotype cranium of Flandriacetus gijseni gen. et sp. nov., dorsal (A), lateral (B), ventral view (C). Post et al.: A beaked whale from the North Sea Basin DEINSEA 23: 1-31 [2025] 6 sea urchins remnant of mandibula jugalsupra mastoid crest infraorbital foramen protuberances antorbital notch zygomatic process sea urchins remnant of mandibula premaxillary crest presphenoid nasal frontal mental foramina jugal pterygoid palatine alveolar rows 0 10 cm A C B Figure 3 NMR999100198032, cranium of Flandriacetus gijseni gen. et sp. nov., dorsolateral (A), anterolateral (B), lateroventral view (C). trum, apex of the mandibles, ear bones and teeth. Some large sea-urchins and bivalves are attached to the skull (Fig. 3). NMR999100205260: partial skull (with jugal) and rostrum, missing the apex of the rostrum, ear bones and teeth. The supraoccipital is somewhat distorted causing the squamosa to reach over and above the maxillae. NMR999100205266: partial skull and base of the rostrum; missing the left squamosal, ear bones and teeth. NMR999100212995: partial skull, rostrum and fragment of a left mandible, missing supraoccipital, squamosa and apex of the rostrum. To be mentioned hereafter as NMR12017, NMR14034, NMR159955, NMR16769, NMR198032, NMR205260, NMR205266 and NMR212995. Referred specimens named as Flandriacetus gen. nov. sp. - Specimens from the same site, of the same age and with preserved elements which are morphologically identical to F. gijseni gen. et sp. nov., but either not complete enough, or showing some elements which prevent (as yet) a 100% secure assignation to F. gijseni gen. et sp. nov. NMR999100016464: vertex with fragments of premaxillary crests; NMR999100016765: skull with fragments of a mandible, four detached teeth, eleven vertebrae (cervical vertebra 1-7, three anterior thoracic vertebrae and one caudal vertebra), two fragments of ribs, missing the rostrum and ear bones (Fig. 4); NMR999100212990: vertex with left premaxillary crest; NMR999100212991: vertex with attached fragments of maxillae. To be mentioned hereafter as NMR16464, NMR16765, NMR212990, NMR212991. Etymology – Flandria from the Latin name of the Belgian region of Vlaanderen and the Dutch province ZeeuwsVlaanderen, which border the estuary of the Westerschelde river; cetus is Latin for whale. Gijseni in honor of Bert Gijsen, for decades of carefully collecting, documenting and preserving cetacean fossils from Vlaanderen – saving them for scientific research. Locality – All specimens were taken by Natural History Museum Rotterdam expeditions from the bed of the Westerschelde estuary (Zeeland province, the Netherlands), at a depth of c. 28 metres, around position 51°21’569”N-03°54’251”E (Westerschelde locality 6D; Post & Reumer (2016)) (Fig. 1). Horizon and age – All specimens are embedded in a dense sandy glauconitic matrix originating from the Diessen Formation – widespread at the site which includes Tortonian to Messinian strata (Munsterman et al. 2019). Dinoflagellate cysts determined the matrix to be of Tortonian age 8.1-7.5 Ma (Munsterman 2017a and b; Appendix 4). The sediment Post et al.: A beaked whale from the North Sea Basin DEINSEA 23: 1-31 [2025] 7 mandibula C 1 (atlas) rib C 2 axis C 3 C 4 C 5 C 6 C 7 thoracal vertebra ? hyoid thoracal vertebra tooth caudal vertebra maxilla premaxillary crest caudal vertebra atlas (C 1) rib axis (C 2) C 3 C 5 C 5 invertebrate burrow C 6 C 7 ? hyoid C 5 rib C 1 (atlas) mandibula tooth ?hyoid thoracal vertebra caudal vertebra condyle supraoccipitale frontal nasal premaxillary crest bony nare presphenoid premaxillary sac fossa maxilla dorsal infraorbital foramina C 2 (axis) frontal disk of vertebra zygomatic process ? hyoid C 7 C 6 bony nare lacrimal 010 cm C 4 A C B D Figure 4 NMR999100016765, Flandriacetus gen. nov. sp., lateral (A), dorsal (B), caudal (C), ventral view (D). Post et al.: A beaked whale from the North Sea Basin DEINSEA 23: 1-31 [2025] 8 of NMR12016 contained some molluscs and brachiopods that corroborate this dating (Appendix 5). From the same site and the same lithological unit – besides numerous articulated postcrania – fossils were recovered of at least 18 (partial) crania of balaenopterids and cetotheres, a single cranium of a pontoporiid, associated vertebrae of a basking shark, a single vertebra of a large pinniped, and a large shield of a leatherback turtle. Mostly dated to the same geological interval as the beaked whales (Zone SNSM 14), but some slightly older (SNSM 13 – 8.8-8.1 Ma) (Post & Reumer 2016; Post et al. 2017; Munsterman 2017a, 2017b; Bisconti et al. 2019; Marx et al. 2019; Peters et al. 2019). Diagnosis – F. gijseni gen. et sp. nov. is a large longirostrine, beaked whale bearing functional teeth, typified by the unique combination of the following measurements and characters: bizygomatic width of the cranium (BZW) 331-370 mm; premaxillae dorsally fused until separating at least 200 mm anterior to the base of the rostrum – showing a significant posterior portion of the mesorostral groove; maxilla separated from the nuchal crest by a wide strip of frontal; rostrum base dominated by a long and wide prenarial basin; top of the presphenoid above the surface of the premaxillae; one large dorsal antorbital foramen usually combined with a second significantly smaller foramen; a moderately elevated, slightly asymmetric, leftwards oriented vertex not overhanging the external bony nares; anterior-most tip of the nasal located c. 50 mm below the surface of the vertex. Differential diagnosis – Cranial synapomorphies (elevated vertex with transverse premaxillary crests, wide hamular fossa of the pterygoid sinus extending anteriorly beyond the level of the antorbital notch and ventrally beyond the ventral level of the basicranium, and transverse crests on the inner surface of the hamular process (only known in some ziphiid species (Lambert 2013)), confirm F. gijseni gen. et sp. nov. as a member of the Ziphiidae (Bianucci et al. 2016a). The anteroposterior shortening of the zygomatic process of the squamosal; the ventral margin of the postglenoid process of the squamosal located clearly more dorsally than the ventral margin of the paraoccipital process of the exoccipital; and the presence of a precoronoid crest on the dorsal margin of the mandible, corroborate this assignment. Within the Ziphiidae, the Messapicetiformes are stem ziphiids diagnosed by two synapomorphies: 1) mesorostral groove roofed by the dorsomedial contact or fusion of the premaxillae (character 3 – Appendix 2), and 2) pachyosteosclerotic development of the premaxillae along the rostrum (character 30 – Appendix 2) (Bianucci et al. 2024). Both synapomorphies are noted in F. gijseni gen. et sp. nov. Within the Messapicetiformes, F. gijseni gen. et sp. nov. differs from Notoziphius bruneti Buono & Cozzuol, 2013 by: nasals on the vertex more wide than long, longer and more massive frontals on the vertex, large prenarial basin, longer dorsal exposure of the mesorostral groove on the rostrum; from Chimuziphius coloradensis Bianucci et al., 2016 by: large dorsal exposure of the frontals on the vertex, absence of a large maxillary crest, large prenarial basin, and extended dorsal exposure of the mesorostral groove on the rostrum; from Aporotus recurvirostris and A. dicyrtus by: fused premaxillae (rather than tightly joined premaxillae) which are not medially inflated, more extended dorsal exposure of the posterior portion of the mesorostral groove, and functional maxillary dentition; from Beneziphius brevirostris, B. cetariensis, Choneziphius planirostris and C. leidyi by: much longer rostrum, more extended dorsal exposure of the posterior portion of the mesorostral groove, lack of maxillary excrescencies, and functional maxillary dentition; from Caviziphius altirostris, Globicetus hiberus Bianucci et al., 2013, Imocetus piscatus Bianucci et al., 2013, Tusciziphius crispus Bianucci, 1997 and T. atlanticus by: lacking maxillary domes or rostral surface structures, elongated rostrum, more extended dorsal exposure of the posterior portion of the mesorostral groove, and functional maxillary dentition; from Dagonodum mojnum by: nasals on vertex not medially and posteriorly inserted into frontals, large and wide prenarial basin, and fused atlas and axis; from Ziphirostrum marginatum by: moderately elevated vertex, anteroposteriorly shorter nasals and longer frontals on vertex, anterior point of nasals located significantly below vertex level, apex of vertex not overhanging bony nares, large and wide prenarial basin, lack of coronoid crest, one large and one smaller dorsal infraorbital foramen at base of the rostrum, functional dentition in maxilla, more extended dorsal exposure of posterior portion of mesorostral groove, and elongated rostrum; from Messapicetus gregarius by: overall larger and more robust cranium (BZW 331-370 mm); lack of coronoid crest; one large and one smaller dorsal infraorbital foramen at base of rostrum; longer dorsally open posterior portion over mesorostral groove (231 mm-254 mm); slightly square or round roots of teeth; and fused atlas and axis; from Messapicetus longirostris by: overall larger and more robust skull - not longer than wide, distinct supramastoid crest of squamosal, longer dorsally open posterior portion over mesorostral groove (231 mm-254 mm), and alveolar row of mandible with oval or round alveoli. COMPARATIVE DESCRIPTION This description is based on NMR12016 (Fig. 2), NMR198032 (Fig. 3), NMR16765 (Fig. 4), and – where and if necessary – on additional information from referred specimens (Figs. 5, 6). Measurements are noted in Table 1. Premaxilla – The anterior-most portion of the premaxilla at the rostrum is not preserved in any of the specimens. On the preserved anterior segments of the rostrum, the premaxillae are dorsomedially elevated and fused, covering a deep mesorostral groove. The fused premaxillae cover the mesorostral groove, but split anterior to the base of the rostrum, creating a dorsally open posterior portion of the groove. In NMR198032 and NMR212995 the fused premaxillae show a slightly convex surface, while in NMR12016 the surface is semi-triangular. The dorsally open portion is preserved for 231-254 mm in NMR12016, NMR212995, NMR159955, NMR198032 and NMR205266. The rostrum of the other skulls is too damaged to measure the length of this feature. The separated right and left premaxilla develop after c. 100 mm, and still anterior to the premaxillary sac fossae, into ventromedially directed parts of a wide prenarial basin (Fig. 2A). This basin is present in Messapicetus and Ziphirostrum but Post et al.: A beaked whale from the North Sea Basin DEINSEA 23: 1-31 [2025] 9 Figure 5 NMR999100205266 (A), NMR999100212995 (B), NMR999100205260 (C), NMR999100155995 (D), NMR999100016769 (E), NMR999100014034 (F) and NMR999100012017 (G), Flandriacetus gijseni gen. et sp. nov., dorsal views. Post et al.: A beaked whale from the North Sea Basin DEINSEA 23: 1-31 [2025] 16 to the foramen magnum (Fig. 12). The atlas and axis vertebrae are completely fused as in Ninoziphius (Lambert et al. 2013; Ramassamy et al. 2018). The transverse process is massive and semi-circular. The neural spine is posteriorly elongated. The posterior surface presents two postzygapophyses. C3 is positioned directly behind the atlas/axis complex, and fits perfectly to the posterior vertebral body of the complex. C4 is behind and close to C3, severely damaged, but still complete enough to be measured. Only a fragment of C5 has been preserved, which prevents any observation. The ventral surfaces of C1-C3 show prominent excavations and a ventral and abraded tip (Fig. 11D). Similar facets are noted in teeth of Dagonodum in which they are supposed to have an attrition origin (Ramassamy 2016). Although the limited sample of teeth does not allow clear conclusions, the teeth differ from M. gregarius where all teeth show anteroposteriorly enlarged, transversely flattened roots (Bianucci et al. 2010) and D. mojnum, in which the teeth are more or less similar in size but have broad solid square roots fitting into square alveoli (Ramassamy 2016). Vertebrae – In NMR12016 five cervical vertebrae and an anterior thoracic vertebra are preserved posterior and lateral Figure 11 Teeth of NMR999100016765 - Flandriacetus gen. nov. sp. (A, B, C, D) and NMR999100016769 - Flandriacetus gijseni gen. et sp. nov. (E). NMR 12016 NMR 16765 MUSM 2548 MSM 1001x Atlas 56 x 202 x - e38 x 158 x - Atlas/axis complex 87x 198 x 138 Axis 48 x 178 x - 12 x 186 x - C3 23 x >88 x 106 21 x 148 x - 28 x - x e54 C4 25 x >79 x - 22 x - x - C5 22 x - x 58 21 x - x - 22 x - x e71 C6 26 x 121 x 60 28 x - x >66 C7 20 x 150 x 66 28 x - x - e T1 38 x 102 x >125 37 x 162 x 54 29 x - x 63 e T4 61 x 108 x 49 e Ca 6 96 x >126 x e105 Table 2 Measurements of vertebrae of NMR NMR999100012016 (holotype), NMR NMR999100016765 (Flandriacetus gijseni gen. et sp. nov.), MUSM 2548 (Messapicetus gregarius) and MSM 1001 (Dagonodum mojnum); Length centrum x width at level transverse processes x posterior height of centrum. In mm, - = no data, > = larger than, e = close to. Post et al.: A beaked whale from the North Sea Basin DEINSEA 23: 1-31 [2025] 17 axis of M. gregarius (MUSM 2548): the transverse processes differ in (relative) size and are orientated differently (laterally versus lateroventrally). Two anterior thoracic vertebrae are hidden behind the cervicals and ventral parts of the skull; only some lateral parts are visible, obstructing detailed observations. One of the thoracic vertebrae had to be separated from the block to allow preparation of important parts of the cranium. Traces of the anterior epiphysis remain (which was probably partly fused with the main vertebral body), while the posterior epiphysis was clearly not fused with the vertebral body. A large caudal vertebra is located on top of the right premaxillary sac fossa (Fig. 4A). Its two haemal processes can be observed ventrally and the dorsal extremity is completely preserved. The epiphyses were not fused to the vertebral centrum. The posterior surface of the vertebral centrum of the thoracic and caudal vertebrae show a pattern of relatively broad tubercle, both possibly for the insertion of M. longus colli as in Ninoziphius (Ramassamy 2016; Ramassamy et al. 2018). The sizes of C3 and C4 and other vertebrae are larger than similar vertebrae of D. mojnum and M. gregarius (Table 2). The first or second thoracic vertebra was detached on top of the left zygomatic arch and had to be separated from the cranium for preparation. Its vertebral centrum is completely preserved and shows a circular articulation facet of a rib (Fig. 12B2). All vertebrae show full epiphyseal fusion. In NMR16765 the atlas and axis vertebrae are not fused to a solid complex as in NMR12016, but remain completely separated (Fig. 4A, 4C, Fig. 9, Fig. 12). They have – as the other cervicals – the same morphology and size as the cervical vertebrae of NMR12016 and the epiphyses are fully fused with the vertebral body. The axis of NMR16765 shows important differences with the corresponding partly preserved C 1-2 C5 C 3 C 4 C 1-2 C 3 C 4 C5 fovea for rib capitulum A 0 138 mm 0 38 mm b - 1 b - 2 b - 3 B Figure 12 NMR999100012016, holotype of Flandriacetus gijseni gen. et sp. nov., cervical vertebrae in posterior and ventral view (A), thoracic vertebra in posterior (b-1), lateral (b-2) and dorsal view (b-3). Post et al.: A beaked whale from the North Sea Basin DEINSEA 23: 1-31 [2025] 18 and form a branch without Dagonodum mojnum; D. mojnum appears between the Messapicetus branch and Ziphirostrum marginatum; and Beneziphius brevirostris and Beneziphius cetariensis are sister taxa in a branch just crownwards to Ziphirostrum and basal to the Aporotus branch. Following Bianucci et al. (2024), the ‘Tusciziphius-clade’ comprises the genera Imocetus, Globicetus and Tusciziphius with Imocetus being the most early diverging genus of the three. However, the polytomy formed by the genera Globicetus (1 species) and Tusciziphius (2 species) in Bianucci et al. (2024), is resolved in the current analysis where Tusciziphius atlanticus is the earliest diverging taxon of the clade. OBSERVATIONS Flandriacetus gen. nov. sp. One of the key characters of the Messapicetiformes is the presence of dorsomedially elevated and fused premaxillae partly covering a deep mesorostral groove. The dorsally open part of this groove is an important difference between taxa of the clade and is one of the main features which differentiate F. gijseni gen. et sp. nov. from closely related taxa. In F. gijseni gen. et sp. nov. the open space (measured to the base of the rostrum) is 231-254 mm (n=5), for M. longirostris from Italy 206-230 mm (n=2), for M. longirostris of the Baleares > 180 mm (n=1 and not completely preserved), for M. gregarius from Peru 163-210 mm (n=9), and for Z. marginatum < 100 mm (Bianucci et al. 2018). For most of the other members of the clade that have been described, the open space is non-existent or minimal (for example in Beneziphius and Choneziphius). Unfortunately, this important feature can only be observed in specimens with a significant part of the rostrum preserved. Combined with the fact that the skulls are not collected in situ, ridges. An isolated epiphysial disk (from a lumbar or caudal vertebra) is preserved in a segment of the right bony nares. Site 6D (the site where all crania were found) yielded many isolated and associated beaked whale vertebrae in sediment or with sediment attached. We may assume that – given the nature and age of the sediment, the very limited surface area of the site, and the fact that in the sediments just one beaked whale taxon was found – some (if not all) the vertebrae belong to Flandriacetus gen. nov. (and some might even belong to one – or more – of the described crania). We note the presence of these vertebrae but refrain from a detailed list and description because they are not directly attached to any of the crania. Some important examples are NMR151162 (two thoracic and five lumbar vertebrae), NMR16805 (four lumbar vertebrae) and NMR16806 (a large lumbar vertebra) (Fig. 13). PHYLOGENETIC ANALYSIS Our analysis generated one parsimonious tree, with a tree length of 95 (consistency index (CI) = 0.6842 and retention index (RI) = 0.7581) (Fig. 14, Appendix 3, Appendix 6). Although we used a slightly different approach for the analysis compared to Bianucci et al. (2024) (no outgroup defined, all characters treated as unordered, and all characters have equal weight), the addition of the new taxon did not substantially alter the topology of the Messapicetiformes clade. Ninoziphius platyrostris and Notoziphius bruneti (both lacking the two synapomorphies of the clade) occupy the most basal positions outside the Messapicetiformes-clade (as stated by Bianucci et al. 2024). This latest analysis noted a few small differences between our new hypothesis and the one published by Bianucci et al. (2024): the new species Flandriacetus gijseni gen. et sp. nov. occupies a position just basal to the Messapicetus branch; Messapicetus longirostris and M. gregarius are sister species 010 cm A B B - 1 B - 2 B - 3 B - 4 Figuur.indd 2Figuur.indd 2 15-05-2025 10:1615-05-2025 10:16 Figure 13 NMR999100151162, beaked whale vertebrae in sediment, dorsal view (A), NMR999100016806, beaked whale vertebra in dorsal (B1), anterior (B2), ventral (B3) and lateral view (B4). Post et al.: A beaked whale from the North Sea Basin DEINSEA 23: 1-31 [2025] 19 longirostrine beaked whales are preserved within in a very limited area. Some skulls are complete, show remains of functional dentition, and often the mandible is missing. Sometimes they are associated with parts of post-crania and, if so, these are usually preserved in a somewhat distorted anatomical sequence. Both faunae include cetotheres, balaenopterids, pontoporiids, seals, turtles and sharks (Bianucci et al. 2015; Post et al. 2017; Viglino et al. 2023). NMR16765 shows – besides the seven cervical and some first thoracic vertebrae which are positioned in a relatively correct anatomical position – a caudal vertebra dorsally positioned on the right side of the base of the rostrum. This possibly indicates a circle-wise preservation of the original carcass as is figured in a skeleton of an early delphinidan of Cerro Colorado (Bianucci et al. 2015). Fish (sardine) were part of the diet of M. gregarius and it might have had gregarious habits (Lambert et al. 2015). Given the general similarities between the two beaked whale taxa (morphology, numbers, environment and age) F. gijseni gen. et sp. nov. is supposed to have had a similar lifestyle. it urges us – despite the lack of other important morphological differences between the skulls – to identify part of the sample as Flandriacetus gen. nov. sp. This decision is corroborated by the observation that NMR16750 has separate atlas and axis vertebrae – as opposed to the fused atlas-axis complex of holotype NMR12016 (Fig. 4A, 4C, Fig. 9, Fig. 12). This is an important difference, because the lack of fusion of the atlas and axis of NMR16750 cannot be explained by ontogenetic processes. Although the vertebral disks of the preserved thoracal and a caudal vertebra of NMR16765 are not fused with their vertebral centrum (indicating a young or semi-adult individual), the complete fusion of the epiphyses of both atlas and axis with their vertebral centrum marks their final separation (Moran et al. 2015). Gregarious lifestyle A fossil beaked whale sample as large as the Westerschelde collection is only known from the important in situ site of Cerro Colorado in Peru (Bianucci et al. 2010). Some remarkable similarities are noted. In both sites multiple skulls of Tortonian Figure 14 Tree showing the phylogenetic relationships of Flandriacetus gijseni gen. et sp. nov. with other Messapicetiformes, the basal stem ziphiids Ninoziphius platyrostris and Notoziphius bruneti, and two outgroup taxa (Squalodon bellunensis and Squaloziphius emlongi) (tree length = 95, CI = 0.6842, RI = 0.7581). Post et al.: A beaked whale from the North Sea Basin DEINSEA 23: 1-31 [2025] 20 1926), on the other hand, show within the hamular fossa soft tissue structures with analogue transverse crests which might have replaced bony structures (Lambert et al. 2013). The enlarged hamular process of M. gregarius differs significantly from other odontocetes families, but resembles the morphology of the process in extant deep diving ziphiids (Ramassamy et al. 2018). Flandriacetus gen. nov. shows an equally enlarged hamular process (Figs. 2B, 2C, 9). Ramassamy et al. (2018) explain the enlargement of the hamular fossa as either 1) a combined feeding specialisation (epipelagic and benthopelagic), 2) predation avoidance, and/or 3) reversion to epipelagic feeding. Cervical vertebrae In the neck of NMR12016, a massive fused atlas and axis complex is followed by stocky and freely moving C3, C4 and C5 vertebrae which gradually increase in length and width, and this seems identical to the condition of the cervical complex of Ninoziphius platyrostris (Lambert et al. 2013). However, the free – or unfused – atlas and axis of NMR16765, followed by vertebrae C3-C7 which are equally solid and of gradually increasing size as in NMR12016, is the same condition noted in Dagonodum mojnum (Ramassamy et al. 2018). The atlas and axis vertebrae of the oldest known crown ziphiid Archaeoziphius microglenoideus Lambert & Louwye, 2006 (from the Middle Miocene of Belgium) are not fused (Lambert & Louwye 2006). The same condition is noted in the Late Miocene D. mojnum and M. gregarius (Bianucci et al. 2010; Ramassamy et al. 2018). The Pliocene N. urbinai from Peru, however, shows a completely different cervical complex: the atlas and axis are fused with each other and are followed by compressed, small, slender, equally sized C3-C7 vertebrae (Lambert et al. 2009). This arrangement is close to the condition noted in extant ziphiids: tiny and slender C3-C7 of which at least some (or all) are fused with a massive fused atlas/axis (Van Buren & Evans 2017). Muscle attachments on Size With a bizygomatic width of 331-370 mm (n=9) F. gyseni from the North Sea is larger than M. gregarius from Peru (310-313 mm, n=2). Unfortunately, bizygomatic widths from M. longirostris from Italy (holotype) and the Baleares (MDM 2029) are not known. The bizygomatic width of some of the Westerschelde specimens reaches the value of the largest stem ziphiid of which the BZW is reported (Chavinziphius maxillocristatus, BZW 368 mm, Bianucci et al. (2016)) and implies a total minimum body length (TL) of (log TL= 0.92*(log BZW1.64) + 2.67) = 3.95 metres (following Pyenson & Sponberg (2011)). Since bizygomatic widths of stem ziphiids of more than 370 mm are not (yet) reported, we may consider a length of c. 4-4.50 metres close to the maximum length which stem ziphiids are known to have reached: a far cry from the lengths of some of the extant crown ziphiids (Lambert et al. 2013). Hamular process Transverse bony ridges are observed in rare fossils of beaked whales in which the hamular processes of the pterygoid is preserved. In Ninoziphius platyrostris and Messapicetus gregarius from Peru (Ramassamy et al. 2018), and in a CT-scanned M. cf. longirostris specimen from the Baleares (Bianucci et al. 2019) this feature is noted. In N. platyrostris these transverse crests might indicate a partial compartmentalisation of the sinus volume (Lambert et al. 2013). NMR12016, the only North Sea Basin specimen with a well preserved hamular fossa, shows two prominent bony ridges on each side of the fossa. In the large sample of M. gregarius only one specimen (MUSM 1481) shows one faint ridge, while the specimen from the Baleares (MDM 2029) shows two or maybe even three prominent ridges. Within the family these ridges seem irregularly present: in Hyperoodon the bony ridges are not present at all, in Berardius one ridge is noted and in Mesoplodon species they are irregularly observed (Table 3). Extant Ziphius cavirostris Cuvier, 1823 and Indopacetus pacificus (Longman, Species Ridges (one side) Sample (n) Flandriacetus gijseni gen. et sp. nov. NMR 12016 2 1 Messapicetus gregarius MUSM 1481 1 1 Messapicetus cf. longirostris MDM-2029 3 1 Ninoziphius platyrostris MNHN SAS 1628 2-3 1 Hyperoodon planifrons RGM H-186, 1633, 2019, 2361, 7218, 8153, 12181, 15300, 16483, 17730, 38258, 38262 012 Berardius biardi RGM 153.002 1 1 Berardius anouxii RGM DM 1402/21007 1 1 Mesoplodon bidens RGM 272, 1226, 1383, 7512, 9404, 16999, 38259, 41153 1-3 8 Mesoplodon grayi RGM 12340 0 1 Mesoplodon layardi RGM 13155, 14339 1-2 2 Table 3 Number of ridges present on the hamular process of the pterygoid in some ziphiid species. Post et al.: A beaked whale from the North Sea Basin DEINSEA 23: 1-31 [2025] 21 variously and, if so – often minimally, present in adults (Van Beneden & Gervais 1880; Mead & Fordyce 2009). Except in NMR16765, all studied skulls have well fused sutures of the skull bones, indicating fully mature individuals (Mead & Fordyce, 2009). NMR16765, probably a young or semi-adult individual, also shows prominent dorsal exposure of the frontal between the maxilla and the nuchal crest (Fig. 4B). Therefore, the peculiar and constant exposure of the frontal in Flandriacetus gen. nov. seems not connected to ontogenetic processes. The dorsal presence of the frontal between the maxilla and the nuchal crest seems also present – albeit in a much lesser extent – in some skulls of Messapicetus (Bianucci et al. 2010; pers. ob. KP). CONCLUSION Based on 13 skulls from the Westerschelde estuary, a new Tortonian beaked whale genus Flandriacetus gen. nov. is reported. Nine skulls are attributed to the new species F. gijseni gen. et sp. nov., four less complete skulls are referred the atlas and axis and other cervical vertebrae of D. mojnum indicate that stem ziphiids had a flexible and relatively long neck enabling the head to make stronger dorsoventral and lateral movements compared to extant ziphiids (Ramassamy et al. 2018). The fused robust atlas/axis and the large C3-7 vertebrae of NMR12016 and NMR16765 reach in anatomical order an estimated length of 200-215 mm. The estimated length of the neck, the robust morphology of the cervical vertebrae, the prominent supraoccipital domes of the supraoccipital, and the pronounced occipital condyles of the skull indicate a likely ability for powerful and active movements of the head. Frontal exposure All fossil skulls of Flandriacetus gijseni gen. et sp. nov. and Flandriacetus gen. nov. sp. show a large strip of frontal between the posterior-most lobe of the maxilla and the nuchal crest of the supraoccipital. Such an exposure of frontal is always present in young or semi-adults of most Odontoceti, but Messapicetus sp. M. gregarius F. gijseni gen. et sp. nov. M. longirostris Ninoziphius platyrostris D. mojnum Notoziphius bruneti Z. marginatum South Pacific South Atlantic North Atlantic Figure 15 Global presence of longirostral stem ziphiid taxa. Post et al.: A beaked whale from the North Sea Basin DEINSEA 23: 1-31 [2025] 22 preparation significantly; to Pepijn Kamminga, Natasja den Ouden and Ronald Pouwer (Naturalis Biodiversity Center) for access to their immense collections; to Nigel Larkin (University of Reading) for linguistic assistance and useful suggestions; to Malcolm Jones (PLS, UK) and Nico Janssen (GSN-TNO) for the palynological preparation of the samples; and a huge thank you to the staff of the Natural History Museum Rotterdam (especially Bram Langeveld and Kees Moeliker) for housing all these heavy and large beaked whale specimens and for allowing and facilitating our often complicated logistic operations. Last but not least, we thank the reviewers Mariana Viglino (Instituto Patagónico de Geología y Paleontologia, Argentina) and Margot Nelson (Calvert Marine Museum, USA): their significant comments and suggestions greatly improved the manuscript. REFERENCES Abbink, O.A., 1998 - Palynological investigations in the Jurassic of the North Sea region - PhD thesis, Utrecht University Bakker, H. & Post, K., 2020 - Records of beaked whales Ziphirostrum and Aporotus (Odontoceti, Ziphiidae) from the Miocene of The Netherlands - Deinsea 24: 17-26 Bianucci, G. & Post, K., 2005 - Caviziphius altirostris, a new beaked whale from the Miocene southern North Sea basin - Deinsea 11: 1-6 Bianucci, G., Landini, W. & Varola, A., 1992 - Messapicetus as Flandriacetus gen. nov. Flandriacetus gen. nov. marks – to date – the youngest appearance of longirostral stem ziphiids, reconfirms their presence in both the Southern and Northern hemispheres (Fig. 15), and corroborates the assumption that stem ziphiids never reached the huge sizes of (fossil and extant) crown ziphiids. By the presence of large numbers of individuals at a limited site – and in line with Messapicetus gregarius of Peru – a gregarious lifestyle of F. gijseni gen. et sp. nov. is proposed (Fig. 16). ACKNOWLEDGEMENTS First and foremost, we thank Giovanni Bianucci (University of Pisa) and Olivier Lambert (IRSNB) for information on Italian and Belgian specimens of fossil beaked whales and for many years of cooperation, pleasant discussions and wise insights on cetacean evolution. Thanks to Luc Anthonis, Bert Gijsen, Kristiaan Hoedemakers and Frederik Mollen for access to specimens in their private collections and/or detailed information; to the late Peter Moerdijk, Freddy van Nieulande and Bram Langeveld for the identification of fossil invertebrates; to Walter Aguire, Mario Urbina and Rodolfo Salas-Gismondi (MUSM) for guidance in the Ica desert of Peru and on site discussions on beaked whale evolution; to Benjamin Ramassamy for inside discussions on Dagonodum; to Aage Kristian Olsen Alstrup (PET-centret Aarhus Universitatshospital, Denmark) for CT scanning of NMR198032 which facilitated Figure 16 Hypothetical reconstruction of Flandriacetus gijseni gen. et sp. nov. hunting sardines. (Remie Bakker) Post et al.: A beaked whale from the North Sea Basin DEINSEA 23: 1-31 [2025] 23 naar, J. & Williams, G.L., 2003 - Late Eocene-Quaternary dinoflagellate cysts from ODP Site 1168. 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