Editors' choice Centrichnus eccentricus revisited: A new view on anomiid bivalve bioerosion CHRISTIAN NEUMANN, MAX WISSHAK, MARTIN ABERHAN, PETER GIROD, THOMAS RÖSNER, and RICHARD G. BROMLEY Neumann, C., Wisshak, M., Aberhan, M., Girod, P., Rösner, T., and Bromley, R.G. 2015. Centrichnus eccentricus re- visited: A new view on anomiid bivalve bioerosion. Acta Palaeontologica Polonica 60 (3): 539–549. Saddle oysters (Anomiidae) attach themselves to calcareous hard substrates by means of a calcified byssus that etches an attachment structure, referred to as ichnospecies Centrichnus eccentricus. Examination of rich material from the Late Cretaceous of central Europe extends the fossil record of this ichnotaxon and revealed a set of previously unrecognised morphological features which appear to be typical for this time period and the respective anomiid trace maker. Excellent preservation of a large number of trace fossil specimens with a complete set of morphological characters allowed a biometrical analysis and additional observations indicating a distinct substrate preference for belemnite rostra, a strong intra- and interspecific competition for settlement space, as well as interactions with durophaguous predators. Further implications for anomiid palaeobiology and palaeoecology arise from allometric shell growth and an etched outline suture in the substrate along the dorsal, lateral and ventral shell margins. These features enhanced a firm attachment and increased shear resistance, and thus are interpreted as an effective defence mechanism against shell-crushing enemies under the intensified predation pressure in marine environments in the Late Cretaceous. Key words: Mollusca, Anomiidae, Centrichnus eccentricus, belemnite, trace fossil, attachment bioerosion, Creta- ceous, Europe. Christian Neumann [[email protected]] and Martin Aberhan [[email protected]], Museum für Naturkunde, Leibniz Institute for Evolution and Biodiversity Science, 10115 Berlin, Germany. Max Wisshak [[email protected]], Senckenberg am Meer, Marine Research Department, 26382 Wilhelms- haven, Germany. Peter Girod [[email protected]], Holteistrasse 2, D-10245 Berlin, Germany. Thomas Rösner, Elbestrasse 16, D-12045 Berlin, Germany. Richard G. Bromley [[email protected]], Rønnevej 97, 3720 Aakirkeby, Denmark. Received 18 March 2014, accepted 5 September 2014, available online 22 September 2014. Copyright © 2015 C. Neumann et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. only indication of anomiid presence in many depositional Introduction systems. The record of anomiid body fossils and remains of calcified byssi dates back to the Jurassic (Fürsich and Palmer Bivalves of the family Anomiidae (“saddle oysters”) live 1982), whereas their etching traces do not appear before the permanently attached to hard substrates and secrete an often Late Cretaceous (Bromley 2004; this study). The original calcified byssus that serves as an attachment organ (Brom- description of C. eccentricus elaborates solely on the etch- ley and Martinell 1991; Bromley and Heinberg 2006). Such ing of the byssus. Here, we show that C. eccentricus from a calcified byssus, which protrudes through a byssus gape the Late Cretaceous of central Europe (France, Germany, in the right valve, is unique among the Bivalvia. Equally Denmark, and Belgium) is more complex. Depending on the unique is the etching trace produced by the byssus which has shell morphology of its peculiar anomiid tracemaker and the been described as the trace fossil Centrichnus eccentricus by geometry of the substrate, we observe conspicuous imprints Bromley and Martinell (1991) (Fig. 1). Since anomiid body on the substrate along the dorsal, lateral, and ventral valve fossils are rarely found attached to their original substrate, margins. Our interpretation of these new morphological fea- and because the inconspicuous and brittle valves often suffer tures, in concert with a biometrical analysis based on several from destructive taphonomical processes, their attachment hundred traces, permit clues to the poorly known anomiid traces deserve special attention because these are often the palaeobiology and palaeoecology, as well as on a potential Acta Palaeontol. Pol. 60 (3): 539–549, 2015 http://dx.doi.org/10.4202/app.00079.2014 540 ACTA PALAEONTOLOGICA POLONICA 60 (3), 2015 A 10 mm B 10 mm Fig. 1. Holotype (MGUH 19742) of Centrichnus eccentricus Bromley and Martinell, 1991, anomiid bivalve attachment trace, from Pleistocene deposits off Palamós, NE Iberian Peninsula, Spain. A. Valve of Arctica islandica bearing the holotype. B. Detail of the holotype; white arrows indicate grooves not included in the original diagnosis, black arrow indicates the main imprint of the byssus. taxonomic control implied by the presence and high abun- locality “Jasmund” (Maastrichtian, Rügen Island, Germa- dance of these new characters in strata deposited in the late ny) has yielded the richest material (428 specimens). The Cretaceous chalk sea. oldest occurrence is from the Turonian of Haute-Normandy Institutional abbreviations.—MfN, Museum für Naturkunde (France), whereas the youngest record stems from the upper- Leibniz Institute for Evolution and Biodiversity Science most Maastrichtian of Stevns Klint (Denmark). All sampled localities, their stratigraphy, sample size, substrate type and Berlin, Germany; MGUH, Geologisk Museum Copenhagen, facies are summarised in Table 1. Denmark. The recognition of Centrichnus trace fossils is not trivial and requires special observation techniques. Bulk samples of belemnite rostra and other macrofossils have been care- Material and methods fully examined under low angle light using a head lens with a threefold magnification. Each finding has been numbered Detailed examination of macrofossils acting as potential and the metric characters have been measured to an accuracy hard substrates for Centrichnus eccentricus resulted in the of 0.1 mm using a slide calliper. For biometric analyses, only recognition of 468 specimens of C. eccentricus from five specimens exposing a complete set of characters from the different sedimentary basins across western and central Eu- Jasmund locality (n = 367) have been selected. The terms rope. The sampled localities cover the Anglo-Paris Basin used to describe anomiid morphology and the trace fossil (Haute-Normandy), the Aachen-Limburg Cretaceous Basin C. eccentricus are shown in Fig. 2A–D, and the measured (Lixhe, Belgium), the Lower Saxony Basin (Hannover area, biometric characters are shown in Fig. 2C. Additionally, the Germany), and the North German and Danish Cretaceous type of hard substrate, the growth direction and evidence for subbasins of the North Sea Basin (Kronsmoor, Jasmund, predation or competition with other epibionts was quantified. Møns Klint and Stevns Klint). With the exception of the All biometrical analyses were undertaken in MS EXCEL. Hannover area, where marly limestone occurs, the predomi- Photographic documentation was carried out with a NIKON nant facies in which C. eccentricus was found is white chalk Coolpix 4500 and ring light NIKON SL-1 after coating with (i.e., weakly cemented calcareous nannoplankton ooze). The ammonium chloride in order to reveal finest details of mi- Table 1. Localities, stratigraphy, facies, sample size, and substrate types of examined Centrichnus eccentricus attachment traces. Locality Stratigraphy Facies n Substrate (occurrences) Haute-Normandy (France) Turonian chalk 2 echinoid (2) Hannover area (Germany) Campanian marly limestone 10 belemnite (9), oyster (1) Kronsmoor (Germany) Early Maastrichtian chalk 4 belemnite (4) Lixhe (Belgium) Campanian–Maastrichtian chalk 10 belemnite (5), echinoid (4), oyster (1) belemnite (369), oyster (32), brachiopod (16), echinoid (7), Jasmund, Rügen (Germany) Early Maastrichtian chalk 428 inoceramid (4) Møn (Denmark) Early Maastrichtian chalk 13 belemnite (9), oyster (1), brachiopod (3) Stevns Klint (Denmark) Late Maastrichtian chalk 1 belemnite (1) NEUMANN ET AL.—ANOMIID BIVALVE BIOEROSION 541 left valve byssal gape right valve muscle scar ligament dorsal ventral A B calcified byssus remains byssal trace shell margin trace retractor muscle calcified byssus adductor muscle left valve erior t ligament right valve n a anterior margin width length width width posterior etched traces C length D Fig. 2. Sketches illustrating anomiid and trace morphology. A. Shell exterior of left valve. B. Shell interior of right valve. C. Morphological features and measured dimensions of Centrichnus eccentricus. D. Schematic cross section of a living anomiid attached to the substrate illustrating the position of etching traces (modified after Yamaguchi 1998). crotopography (see Fig. 3). Samples for scanning electron Centrichnus eccentricus Bromley and Martinell, 1991 microscopy (SEM) were sputter-coated with gold and visu- Figs. 1–7. alised with a TESCAN Vega II. 1991 Centrichnus eccentricus isp. n.; Bromley and Martinell 1991: Apart from the holotype of Centrichnus eccentricus 248–249, figs. 5, 6. (MGUH 19742), which is stored in the collections of the 1999 Centrichnus eccentricus Bromley and Martinell; Bromley 1999: Geologisk Museum Copenhagen (Denmark), all figured 176, figs. 2, 3. 2002 Centrichnus eccentricus Bromley and Martinell; Taddei Ruggiero specimens (MB.W 3074–3093) are stored in