BULLETIN OF THE GEOLOGICAL SOCIETY OF DENMARK · VOL. 69 · 2021 A tale from the middle Paleocene of Denmark: A tube- dwelling predator documented by the ichnofossil Lepidenteron mortenseni n. isp. and its predominant prey, Bobbitichthys n. gen. rosenkrantzi (Macrouridae, Teleostei) WERNER SCHWARZHANS, JESPER MILÀN & GIORGIO CARNEVALE Schwarzhans, W., Milàn, J. & Carnevale, G. 2021. A tale from the middle Paleocene of Denmark: A tube-dwelling predator documented by the ichnofossil Lepidenteron mortenseni n. isp. and its predominant prey, Bobbitichthys n. gen. rosenkrantzi (Macroridae, Teleostei). Bulletin of the Geological Society of Denmark, vol. 69, pp. 35–52. ISSN 2245-7070. https://doi.org/10.37570/bgsd-2021-69-02 Geological Society of Denmark The ichnofossil Lepidenteron provides a unique taphonomic window into the life https://2dgf.dk habits of a tube-dwelling predator, probably an eunicid polychaete, and its fish prey. Here we describe a new tube-like ichnofossil Lepidenteron mortenseni n. isp. from the Received 2 November 2020 Kerteminde Marl (100–150 m palaeo-water depth) from the Gundstrup gravel pit Accepted in revised form near Odense, Fyn, Denmark. 110 individual tubes were examined which contain fish 27 January 2021 Published online remains, including a variety of disarticulated bones and otoliths, by far dominated 23 February 2021 by a single gadiform taxon referred herein to as Bobbitichthys n. gen. The isolated otoliths here associated with disarticulated gadiform bones have previously been © 2021 the authors. Re-use of material is described, from the time equivalent Lellinge Greensand exposed in the Copen- permitted, provided this work is cited. hagen area, as Hymenocephalus rosenkrantzi, a grenadier fish (family Macrouridae). Creative Commons License CC BY: The abundance of associated bones and otoliths in the examined tubes allowed us https://creativecommons.org/licenses/by/4.0/ to reconstruct part of the cranial configuration of Bobbitichthys rosenkrantzi and to tentatively interpret it as a stem macrourid. Bobbitichthys rosenkrantzi represents the earliest grenadier known in the fossil record. Additional, although considerably less abundant, skeletal remains and otoliths have been tentatively referred to a long-fin bonefish (family Pterothrissidae, Pterothrissus? conchaeformis), a viviparous brotula (family Bythitidae, Bidenichthys? lapierrei), a conger eel (family Congridae, possibly belonging to Rhynchoconger angulosus), and another unidentified gadiform. Keywords: Predatory polychaete; Macrouridae; Lepidenteron tube; otolith; osteology; Kerteminde Marl; Paleocene; Selandian. Werner Schwarzhans [[email protected]], Ahrensburger Weg 103, D-22359 Hamburg, Germany; also Natural History Museum of Denmark, Universitetsparken 15, DK-2100 Copenhagen Ø, Denmark. http://orcid.org/0000-0003-4842-7989. Jesper Milàn [jesperm@ oesm.dk], Geomuseum Faxe, Østsjællands Museum, Rådhusvej 2, DK-4640 Faxe, Den- mark. https://orcid.org/0000-0002-9556-3177. Giorgio Carnevale [giorgio.carnevale@ unito.it], Dipartimento di Scienze della Terra, Università degli Studi di Torino, Via Valperga Caluso 35, I-10125 Torino, Italy. http://orcid.org/0000-0002-3433-4127. Corresponding author: Jesper Milàn Skeletal elements documenting predator–prey inter- Lepidenteron from the middle Paleocene (Selandian) actions offer a unique opportunity to study specific Kerteminde Marl (Clemmensen & Thomsen 2005; palaeoecological relationships in deep time (McAl- Schnetler & Nielsen 2018), coming from the gravel lister 2003). Here we investigate fish skeletal remains pit at Gundstrup, north of Odense, Fyn, Denmark found in the tubular trace fossils of the ichnogenus (Fig. 1). The Lepidenteron tubes of the Kerteminde A tube-dwelling predator and its prey from the middle Paleocene of Denmark · 35 Marl, which represent a new ichnospecies described with the fossil bones, to test two alternative phylo- herein, are remarkable for their abundance (a total genetic attributions that have been discussed in past of 110 tubes have been retrieved) and the contained literature. Only a few other fish taxa were identified skeletal elements which mostly belong to a single from the Lepidenteron tubes of the Kerteminde Marl, gadiform species hitherto known only from isolated including (in order of abundance) the viviparous otoliths and originally referred to as Hymenocephalus brotula Bidenichthys? lappierrei, the longfin-bonefish rosenkrantzi Schwarzhans 2003, a grenadier fish, from Pterothrissus? conchaeformis, a second unidentifiable the time-equivalent Lellinge Greensand exposed in gadiform, a single otolith of a conger eel possibly the Copenhagen area. belonging to Rhynchoconger angulosus and, finally, The associated otoliths and bones of the fishes a single otolith of Centroberyx integer, which was found in the tubes of the new Lepidenteron ichnospe- found outside of a Lepidenteron tube. The majority cies from the Kerteminde Marl provide an oppor- of the otoliths of Bidenichthys? lappierrei were found tunity to review the systematic position of Hymeno- in two discrete tubes and their associated bones did cephalus rosenkrantzi, leading to the establishment of not yield any useful taxonomic information. In the the fossil genus Bobbitichthys n. gen. A specimen each case of Pterothrissus? conchaeformis a rare instance of of the extant Hymenocephalus italicus and Euclichthys a partially articulate oral jaw was found in a single polynemus were dissected for a direct comparison tube and a large maxilla in another. Middle Upper Miocene Deep Shallow Lower Miocene Oligocene Chrono- stratigraphy Eocene Middle Upper Paleocene Age Lithology and lithostratigraphy (Southeast Denmark) Lower Paleocene (Danian) Epoch W E Upper Cretaceous Æbelø Fm Sorgenfrei–Tornquist ZoneLower Cretaceous and older Kerteminde Marl Jylland Selandian DENMARK Copenhagen Ringkøbing–Fyn High Gundstrup Sjælland Lellinge Greensand Fyn Paleocene Danian limestone late Danian A B Fig. 1. A: Pre-Quaternary map of Denmark indicating the location of the Gundstrup gravel pit in the middle upper Paleocene deposits on the island of Fyn (55.56°N, 10.35°E). Modified from Håkansson & Pedersen (1992). B: Schematic representation of the Upper Danian – Selandian stratigraphy of south-eastern Denmark. Modified from Clemmensen & Thomsen (2005). 36 · Bulletin of the Geological Society of Denmark Geological setting and location gravel pit, have been examined together with two otoliths found outside of tubes. Overall, 529 otoliths The Paleocene sedimentation in the Danish Basin have been recognized, of which 361 are identifiable. started as carbonate dominated during the Danian. Of the identified otoliths, 320 belong to Bobbitichthys n. The carbonate deposition ended at the end of the gen. rosenkrantzi, the remainder to five different spe- Danian as a consequence of a major regression that cies (see below for details). Considering the abundance took place at about 61.6 Ma (Vandenberghe et al. 2012), of otoliths referred to a single species, it is reasonable resulting in an extensive erosional unconformity at to attribute a large part of the isolated fish bones to the boundary to the overlying Selandian deposits (e.g. the same taxon, i.e., Bobbitichthys rosenkrantzi. In fact, Thomsen & Heilmann-Clausen 1985; Clemmensen & many of the isolated identifiable fish bones are readily Thomsen 2005). The succeeding Selandian transgres- recognizable as belonging to a single gadiform spe- sion resulted in a different depositional regime domi- cies. Also, a quantitative evaluation of the measured nated by siliciclastic sedimentation, as documented otoliths shows that most of the fish remains located by the Kerteminde Marl and its lateral equivalent the in the tubes are derived from specimens of similar Lellinge Greensand (Clemmensen & Thomsen 2005). size. Based on these observations we felt able to reli- While the Danian carbonates were deposited in a ably reconstruct an idealized portion of the skull of subtidal shelf palaeoenvironment situated at a depth Bobbitichthys rosenkrantzi from the individual bones. down to a few hundred metres, the Selandian Kerte- The specimens were studied and drawn with a minde Marl is interpreted as being deposited in an stereo-microscope equipped with a camera lucida offshore shelf environment at about 100–150 m depth drawing tube. Photographs were taken with a digital (Clemmensen & Thomsen 2005; Heilmann-Clausen camera adapted to a Wild M400 photomacroscope and & Surlyk 2006). The overlying Selandian – Thanetian remotely controlled from a computer. Sets of photo- Æbelø Formation is predominantly clay and repre- graphs of differing fields of depth of individual speci- sents a progressively deeper depositional environ- mens were stacked using the HeliconFocus software ment (Clemmensen & Thomsen 2005). The thickness of HeliconSoft and were then digitally retouched with of the Kerteminde Marl is up to 150 m, thinnest over Adobe Photoshop for sand grains or minor inconsist- the Ringkøbing–Fyn High and thickest northward encies, as far as doing so did not alter the morphol- towards the Sorgenfrei–Tornquist Zone (Sorgenfrei ogy of the photographed specimens. Mirror imaged & Buch 1964; Clausen & Huuse 1999; Clemmensen figures are indicated in the captions as ‘reversed’. All & Thomsen 2005). The Selandian depocentre of the the investigated and figured specimens are housed Kerteminde Marl is located on western Sjælland at the collection of Geomuseum Faxe, Østsjællands (Clemmensen & Thomsen 2005). Museum (OESM)