Pyritonema tigris, Wisshak, 2017

Wisshak, Max, 2017, Taming an ichnotaxonomical Pandora’s box: revision of dendritic and rosetted microborings (ichnofamily: Dendrinidae), European Journal of Taxonomy 390, pp. 1-99 : 76-77

Rhopalondendrina tigris igen. et isp. nov.

urn:lsid:zoobank.org:act:F4317101-DB41-4575-A819-43426AF9BC33

Fig. 30

Sponge, Form 1 – Günther 1990: 238, pl. 56, figs 4–7.

Echinoid form – Radtke 1993: 90, pl. 17, fig. 5. — Glaub 2004: 72, fig. 4i.

Diagnosis

Strongly arcuate entry tunnel, occasionally ornamented with a ridge of vertically oriented tapering protrusions, arches back towards the substrate surface, leading to a flat, palmate cavity with short, round or pointed, finger-like protrusions. Trace connected to the substrate surface by short rhizoidal appendages. Texture irregular to hairy.

Etymology

From the ancient Greek ‘τίγρις’ (tigris), tiger, referring to the paw-shaped morphology of these dendrinid traces.

Type material, locality and horizon

The holotype ( Fig. 30D ) and two paratypes ( Fig. 30C , E–F) are found in the same epoxy resin cast from an experimental bivalve shell ( Callista ) that was bioeroded at a water depth of 15 m in Pioneer Bay,

Orpheus Island, Great Barrier Reef, Australia. The cast bearing the holotype and paratypes is deposited in the trace fossil collection of the Senckenberg Institute in Frankfurt , Germany ( SMF XXX 872 ).

Description

The trace originates at a single, circular to oval entrance, from which an arcuate entry tunnel enters the substrate sub-vertically, arches back towards the surface and forms a wide prostrate fan with short, radiating, finger-like, round or pointed protrusions, giving the trace a distinct paw-like appearance ( Fig. 30 A–F). The surface texture of the entrance tunnel, and particularly of the palmate cavity, is irregular and often bears hairy or cone-shaped protrusions. Along the entrance tunnel, these may form a vertical cockscomb-shaped ridge (e.g., Fig. 30A , D–E). The trace is connected to the substrate surface, at least at the periphery of the trace, by short rhizoidal appendages of only a few micrometres in diameter ( Fig. 30 C–F).

For morphometrical data, four semi-mature to mature specimens (including the types) were measured. The length of these traces varied from 125 to 179 µm (mean = 150 ± 27 µm) and the maximum width of the trace measured 53 to 111 µm (mean = 85 ± 30 µm). The diameter of the proximal entrance tunnel measured 21 to 33 µm (mean = 27 ± 5 µm) and the maximum depth of penetration was quantified with a range from 83 to 141 µm (mean = 109 ± 26 µm).

Remarks

This trace was first reported in samples from Cozumel, Yucatan ( Mexico) by Günther (1990) under the informal name ‘Sponge, Form 1’, and closely reminiscent traces were later reported by Radtke (1993) from mollusc shells sampled at Lee Stocking Island, Bahamas, informally addressed as ‘Echinoid form’. This informal name was also adopted by Glaub (2004), who found the trace in samples from depths of 41 to 68 m on the continental shelf of Mauritania.

All of the three previous records and the new record (from Australia’s Great Barrier Reef) are from modern seas, and there is no unequivocal fossil material to choose type material from. However, since this peculiar bioerosion trace is quite distinctive, it is herein nevertheless established as a new ichnospecies, based on the new material from Australia. This is in line with the interpretation of the fossilisation barrier in bioerosion trace fossils, as discussed recently by Bromley & Nielsen (2015). Since no definition of the fossilisation barrier is given in the ICZN, this practice does not violate the Code.

This ichnospecies is clearly distinguished from the other three ichnospecies of Rhopalondendrina igen. nov. by the strongly arching entry tunnel and the overall paw-shaped appearance of the trace, with only short prostrate appendages.

Figures

Fig. 30. Rhopalondendrina tigris igen. et isp. nov. A–B. Planar and lateral views of two specimens illustrating the diagnostic arcuate entrance tunnel and paw-shaped main chamber; SEM of epoxy casts from a bivalve shell sampled off Mauritania. C–D. SEM of specimens in an initial (C, paratype) and a mature (D, holotype) ontogenetic stage, cast in epoxy from experimental bivalve substrates that were deployed at the Great Barrier Reef, Australia. E–F. Lateral and planar views of another specimen (paratype) from the type locality.