Centrichnus Bromley and Martinell, 1991

Neumann, Christian, Wisshak, Max, Aberhan, Martin, Girod, Peter, Rösner, Thomas & Bromley, Richard G., 2015, Centrichnus eccentricus revisited: A new view on anomiid bivalve bioerosion, Acta Palaeontologica Polonica 60 (3), pp. 539-549 : 541-542

Ichnogenus Centrichnus Bromley and Martinell, 1991

Type ichnospecies: Centrichnus eccentricus, Pleistocene of Palamós ( Spain) .

Centrichnus eccentricus Bromley and Martinell, 1991 Figs. 1–7 .

1991 Centrichnus eccentricus isp. n.; Bromley and Martinell 1991: 248–249, figs. 5, 6.

1999 Centrichnus eccentricus Bromley and Martinell ; Bromley 1999: 176, figs. 2, 3.

2002 Centrichnus eccentricus Bromley and Martinell ; Taddei Ruggiero and Annunziata 2002: 48, pl. 2.

2005 Centrichnus eccentricus Bromley and Martinell ; Wisshak et al. 2005: 108, fig. 14F.

2005 Centrichnus eccentricus Bromley and Martinell ; Bromley 2005: 903, fig. 8A, B.

2006 Centrichnus eccentricus Bromley and Martinell ; Bromley and Heinberg 2006: 141–142, fig. 10.

2006 Centrichnus eccentricus Bromley and Martinell ; Santos and Mayoral 2006: 731, pl. 3: 10.

2006 Centrichnus eccentricus Bromley and Martinell ; Wisshak 2006: 289, fig. 23E.

2007 Centrichnus eccentricus Bromley and Martinell ; Beuck et al. 2007: 164, fig. 4a.

2007 Centrichnus cf. eccentricus Bromley and Martinell ; Gibert et al. 2007: 792, fig. 9A, B.

2007 Centrichnus eccentricus Bromley and Martinell ; Olszewska-Nejbert 2007: pl. 32: 4.

2007 Centrichnus eccentricus Bromley and Martinell ; Taddei Ruggiero and Bitner 2007: 370, fig. 4.

2007 Centrichnus eccentricus Bromley and Martinell ; Blisset and Pickerill 2007, fig. 2A, B.

2008 Centrichnus eccentricus Bromley and Martinell ; Wisshak 2008: 216, fig. 2H.

2008 Centrichnus cf. eccentricus Bromley and Martinell ; Zamora et al. 2008: 21, pl. 3: 8.

2011 Centrichnus eccentricus Bromley and Martinell ; Wisshak et al. 2011: 508, fig. 9F.

2013 Centrichnus eccentricus Bromley and Martinell ; Girod and Rösner 2013: 283, fig. 17.

Emended diagnosis.—Teardrop-shaped Centrichnus comprising a bundle of bow-shaped grooves, concave toward the pointed anterior of the trace. A facultative lateral and posterior feature are faint to progressively more deeply carved eccentric grooves with the outermost and often longest and most deeply carved groove marking the margin of the trace. The anterior delineation may be formed by two linear series of pits, sometimes fused to elongate grooves, deepening outwards on both sides of the trace’s longitudinal axis.

Description.—The classical Centrichnus eccentricus consists of a teardrop-shaped bundle of grooves, curved around a centre that is at or beyond the narrow end of the bundle, and is found on a variety of biogenic hard substrates ( Fig. 4A and see below). In 115 specimens of C. eccentricus from Jasmund belemnite rostra, the bundle length ranged from 2.8 to 8.9 mm (mean 5.0 ± 1.2 mm) and the width ranged from 1.2 to 4.2 mm (mean 2.9 ± 0.6 mm, see Fig. 4B ). One or two further components may be present, the first one of which consists of faint ( Fig. 5C) to progressively more deeply carved grooves ( Fig. 5B), which frame the teardrop-shaped bundle of grooves and form the lateral and posterior margin of the trace. The second component forms the anterior delineation of the trace and consists of a pair of linear and outwards deepening series of pits which are often fused to elongate grooves. They extend on either side of the longitudinal axis of the trace. The length of the pair of grooves ranges from 2.8 to 8.9 mm (mean 5.1 ± 1.1 mm). The length of the complete trace ( Fig. 4B–C ) extends from 8.3 to 22.0 mm (mean 13.7 ± 2.9 mm) and its width ranges from 6.2 to 13.8 mm (mean 9.7 ± 1.6 mm). Size frequency analyses of total length shows a normal distribution ( Fig. 4C ).

Remarks.—The original diagnosis and description of C. eccentricus is restricted to the byssus imprint, although the holotype (from the Pleistocene off Palamós, NE Iberian Peninsula) and one of the paratypes (from the Lower Campanian of Misburg, Germany) also exhibit groove imprints of the valve margin ( Fig. 1 ). This renders the establishment of a new ichnospecies unfeasible even though the presence of anteriorly positioned imprints cannot be verified in the holotype because the respective area is overgrown by a bryozoan colony ( Fig. 1B ).

Stratigraphic and geographic range.—According to Bromley (2004) the fossil record of C. eccentricus extends from the Campanian to the Recent from tropical to high-latitude (palaeo-) environments. Today it has a broad bathymetric range from the shallow subtidal to bathyal depths where it has been reported for instance from cold-water coral reefs in a depths down to 1039 metres ( Beuck et al. 2007). The present data extend the first occurrence back to the Turonian of Haute-Normandy where two specimens with complete morphology have been observed on the tests of the echinoid Echinocorys . The youngest of the specimens studied herein comes from the uppermost Maastrichtian chalk of Stevns Klint ( Denmark) where it has been recorded on a belemnite rostrum ( Belemnella kasimirovensis ) just below the Cretaceous–Palaeogene boundary. According to the current fossil record, C. eccentricus exposing the complete morphology was dominant in the Late Cretaceous. It has been found in a wide geographical range across western and central Europe, comprising several sedimentary basins. It is most common in the hemipelagic chalk facies but also occurs rarely (10 records) in the more neritic marly limestone of the Lower Saxony Basin (Hannover). The majority of our records are from the Early Maastrichtian white chalk of Jasmund. Although this picture may be biased by a collection artefact (this locality is the one which has been most extensively sampled), a bulk sample from this locality demonstrated that 50 out of 500 belemnites (10%) exposed the trace fossil C. eccentricus , a percentage that is probably not observed in the other sampled localities.

Figures

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.

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).

Fig. 3. Belemnite (MB.W3074) from Lower Maastrichtian, Jasmund (Rügen, Germany), exhibiting dendrinid boring traces with superimposed Centrichnus eccentricus Bromley and Martinell, 1991 anomiid bivalve attachment trace.In the uncoated belemnite, only the dendrinid boring traces are visible (A, colour contrast), with ammonium chloride coating, the C. eccentricus trace is clearly visible (B, relief contrast).

Fig. 4. Statistics and biometry of anomiid bivalve attachment trace C. eccentricus. All records (A), complete traces on belemnites (B–F) from the Lower Maastrichtian, Jasmund (Rügen, Germany). A. Substrate types utilised for attachment. B. Box plot of biometric measurements. C. Size frequency distribution of trace length. D. Number of traces per belemnite. E. Site selectivity on belemnite. F. Trace orientation on belemnite.