Solichnus

Wisshak, Max, Neumann, Christian, Sanna, Giovanni, Nielsen, Kurt S. S. & Milàn, Jesper, 2023, Suspected foraminiferan parasitism on a Late Cretaceous echinoid host recorded by the new attachment trace fossil Solichnus aestheticus, Acta Palaeontologica Polonica 68 (1), pp. 13-22 : 14-16

Ichnogenus Solichnus nov.

Zoobank LSID: urn:lsid:zoobank.org:act:C60A3E00-F5B7-4021-ACB9-E13898CA1DF1

Type ichnospecies: Solichnus aestheticus isp. nov., monoichnospecific; see below.

Etymology: Compound of the Latin sol, sun and ichnus (the Latinised form of Ancient Greek ἴχνος, ichnos), foot print, track, trace; referring to the distinctly sun-shaped morphology of the trace. An additional connotation is that given by Sól being the sun personified in Germanic mythology, thus referring to the Scandinavian type locality of the new trace fossil.

Diagnosis.—Circumradial surface etchings with a central depression and radiating open canals in calcareous skeletal substrates.

Ethological category.—Fixichnia ( Gibert et al. 2004; attachment traces).

Remarks.—The new ichnogenus deemed necessary to clearly separate the morphology of Solichnus with its diagnostic radiating canals from the sole circular depressions of various kinds in echinoderm skeletons comprised within the ichnogenus Tremichnus Brett, 1985 . While the ichnogenera Entobia Bronn, 1837 , and Planobola Schmidt, 1992 , include some ichnospecies with a vague similarity to Solichnus aestheticus (see below), they would not have been an appropriate parent ichnogenus, because the former concerns interconnected chambers below the substrate surface and the latter addresses bowl-shaped microborings that usually lack radiating grooves.

Solichnus aestheticus isp. nov.

Figs. 2 , 3 .

Zoobank LSID: urn:lsid:zoobank.org:act:82AE4741-8B0E-41D2-9E02-0B2151E32219

Etymology: Adopts the Latin aestheticus , aesthetic; emphasizing the delicate nature and aesthetical value of the trace fossil.

Type material: The Echinocorys jaekeli test carrying the type specimens of Solichnus aestheticus igen. et isp. nov. ( Fig. 1 ) MGUH 34117. The holotype of Solichnus aestheticus igen. et isp. nov. is registered as MGUH 34117a, and the seven paratypes as MGUH 34117b–h ( Table 1). Type locality: Hvideklint (54.930658°N, 12.272882°E) at the southern shore of the island Møn, Denmark.

Type horizon: Uppermost Campanian of the Boesdal Member, Mandehoved Formation, Upper Cretaceous (see Surlyk et al. 2013 for detail).

Diagnosis.—Bowl-shaped circular depression, wider than deep, from which numerous open canals radiate in a meandering fashion, ramify, and thin out.

Description.—A bowl-shaped circular depression, 0.8–1.5 mm in diameter (mean = 1.2± 0.3 mm; n = 8) and wider than deep, marks the centre of the trace. From this excavation 17– 23 canals (mean = 20.5±2.3; n = 6) radiate with a meandering course and repeatedly bifurcate in random intervals. These open canals are wider than deep, proximally about 0.1 mm wide (0.05–0.15 mm), and taper towards the indistinct periphery of the trace. The maximum diameter of the trace reaches 13.2–32.3 mm in diameter (mean = 19.5± 5.7 mm; n = 8). There is no correlation between the diameter of the central depression and the trace as a whole (r 2 of linear regression = 0.046). See Table 1 for detailed morphometrical data.

Remarks.—The morphology of Solichnus aestheticus igen. et isp. nov. is unlike any other ichnospecies for attachment etchings or other bioerosion traces previously reported in the literature. The only traces with a vague similarity, albeit much larger in dimension, are some unicamerate sponge boring ichnospecies in the ichnogenus Entobia Bronn, 1837 , including Entobia devonica ( Clarke, 1921) , E. solaris Mikuláš, 1992 , E. astrologica Mikuláš, 1992 , E. cracoviensis Bromley and Uchman in Bromley et al., 2009, and E. resinensis Santos, Mayoral, and Bromley, 2011 . However, in the centre of these traces there is a globular chamber rather than a bowl-shaped depression, but hardground erosion can unroof this structure and also expose the radiating tunnels, which only then appear like open canals. There is also some similarity to one of the ichnospecies of the microboring ichnogenus Planobola Schmidt, 1992 , namely P. radicata Schmidt, 1992 , which is much smaller in dimension and has a central cavity of only 15 to 30 µm in diameter with a latitudinal contact to the substrate surface, from which thin tunnels, not canals, radiate. None of the ichnospecies within the related ichnogenus Tremichnus Brett, 1985 , shows the radiating canals diagnostic for Solichnus aestheticus .

It shall be noted that there is some superficial resemblance to composite traces of borings (various ichnospecies) overprinted by the grazing/predation activity of echinoids Gnathichnus pentax ), such as those illustrated by Bromley 1970: fig. 6 and pl. 3/4) or Wisshak (2006: fig. 23D). These composite traces had erroneously led to the establishment of Asteriastoma Breton, 1992 , and its type ichnospecies A. cretaceum Breton, 1992 , both consequently rejected by Wisshak et al. (2019).

All specimens are loosely clustered on the lateral aboral surface of the same Echinocorys host echinoid, six on its anterior and two on the posterior part of the test ( Fig. 2 ). All specimens show regeneration tissue (newly grown primary and secondary tubercles) formed by the echinoid ( Fig. 3A 1 – A 3 , B 1 , B 2 , C 1 , C 2 ). The canals seem to largely circumvent the areoles of the primary tubercles but cross plate sutures without any interference ( Fig. 3B 3, C 3 ). They interfere, however, with canals of neighbouring specimens, in which case they turn before intersecting, forming a mutual avoidance pattern ( Fig. 3B 1 , B 2 , C 1 , C 2 ).

Geographic and stratigraphic range.— Type locality and horizon only.

Figures

Fig. 2. Holasteroid echinoid Echinocorys jaekeli Nietsch, 1921 (MGUH 34117) from the upper Campanian of Hvideklint, Møn, Denmark; carrying the type series of the new foraminiferan attachment trace fossil Solichnus aestheticus igen. et isp. nov. Anterior (A1) and posterior (A2) views of the original specimen and the respective views (A3, A4) of a textured 3D digital surface model with the positions of the holotype (h; MGUH 34117a) and the seven paratypes (p1–7; MGUH 34117b–h) of Solichnus aestheticus igen. et isp. nov.; an interactive viewer with this digitype can be accessed online via Sketchfab at https://skfb.ly/oAEIA.

Fig. 3. Type specimens of the new foraminiferan attachment trace fossil Solichnus aestheticus igen. et isp. nov. from the upper Campanian of Hvideklint, Møn, Denmark. A. The holotype trace (MGUH 34117a), photographed after (A1) and before (A2) coating with ammonium chloride, showing the extent of the diagnostic radiating canals and their interference with those of neighbouring paratypes. Close-up of the central depression of the holotype (A3) with echinoid regeneration texture (newly formed tubercles). Backscatter electron SEM image of the central depression of the holotype (A4); note that →

Fig. 1. Location and stratigraphy. A. Hvideklint is located on the southern shore of the island of Møn in eastern Denmark. B. Schematic representation of the Campanian to Maastrichtian stratigraphy of eastern Denmark (modified after Surlyk et al. 2013).