Pyritonema arctica Wisshak 2017, isp. nov.

Pyrodendrina arctica isp. nov.

urn:lsid:zoobank.org:act: A41E7E41-748C-4DE6-B32F-6383C3852036

Fig. 24

‘Sponge form VI’ – (?) Wisshak et al. 2005a: 991, fig. 7F.

‘Microsponge-form 6’ – (?) Wisshak 2006: 86, fig. 28H.

Prostrate galleries with a high degree of anastomosis and a decrease in diameter toward the periphery. From the central node of trace, ramifying, sub-vertical tunnels with tapering terminations extend deeper into the substrate.

Latinised from the geographic name Arctica, making reference to the known occurrence of this trace being limited to the polar waters off Svalbard and Jan Mayen (as well as one uncertain record from coldtemperate waters off Sweden).

The holotype (Fig. 24 A–B) and one paratype (Fig. 24 C–D), together with several further specimens, are cast in epoxy resin from a subfossil (possibly Late Pleistocene to probably Holocene) shell of the bivalve Chlamys islandica (O.F. Müller, 1776), sampled in 75 to 85 m water depth at Sørkappbanken, SW Svalbard Shelf (76°23′ N, 15°57′ E; station 15/161-AGT of the ARK VI/1-4 Expedition of the RV Polarstern in 1989; see Krause et al. 1991 for details). Deposited in the trace fossil collection of the Senckenberg Institute in Frankfurt, Germany (SMF XXX 864, including the holotype, and SMF XXX 865, including the paratype).

This species describes relatively large dendrinids with a basal network of anastomosing galleries, running closely parallel to the substrate surface and emerging from an indistinct central cavity (Fig. 24 A–E). At least near the centre of the trace, vertically-oriented tunnels reach deeper into the substrate (e.g., Fig. 24 A–B), with rapidly ramifying, tapering, and pointed terminations. The overall appearance of the trace in side view resembles a blazing campfire – in accordance with the ichnogenus name (Fig. 24A).

Measured complete specimens range from 1067 to 2936 µm in maximum diameter (mean = 1779 ± 564 µm; n = 180), at a width of 824 to 2301 µm (mean = 1405 ± 447 µm; n = 17).

Since no deep-time fossil material is available and neither the exact age of the substrate (Pleistocene to Holocene) nor that of the boring is known, the type material is herein regarded as fossil. This is in concord with the view that bioerosion traces in skeletal or lithic hard substrates can be regarded as “ready-made fossils” with a fossilisation barrier most practically defined as the death of the borer (Bromley & Nielsen 2015). Since no definition of the fossilisation barrier is given in the ICZN, this practice does not violate the Code.

This large Pyrodendrina is distinguished from other ichnospecies in that ichnogenus by the high degree of anastomosis in the basal rosette, and the higher number and density of vertical tunnels. In contrast to P. belua isp. nov. and P. villosa isp. nov., the surface texture is smooth.