Rhopalondendrina acanthina, 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 : 72-74

Rhopalondendrina acanthina igen. et isp. nov.

urn:lsid:zoobank.org:act:1C10C48B-2B51-49AB-9D6E-DFA88BDECE92

Fig. 28

Spinate boring form – (?) Zeff & Perkins 1979: 185, fig. 15.

Sponge from B – (?) Budd & Perkins 1980: 889, fig. 8D.

Dendroid-Form III – (?) Schmidt 1992: 91, pl. 11, fig. 5. — (?) Glaub & Schmidt 1994: 106, pl. 3, fig. 2. Dendroid form 2 – Wisshak et al. 2011: 505, fig. 7K.

Diagnosis

A very short, sub-vertical entrance gallery gives way to a closely prostrate system of irregularly bifurcating galleries, with spiny surface texture and tapering or blunt terminations. Converging tunnels evade and cross, or run parallel to each other.

Etymology

Latinised from the ancient Greek ‘ἀκάνθινος’ (acanthos), thorny, referring to the common spiny surface texture of this trace.

Type material, locality and horizon

The holotype ( Fig. 28A ) and the paratype ( Fig. 28 B–C) are found in the same epoxy resin cast from a shell of the bivalve Acanthocardia echinata (Linnaeus, 1758) , sampled at the northern flank of an isolated hill one km SW of Lardos village, Rhodes, Greece, in a rhodolith bed determined as Marine Isotope Stage 12, Lindos Bay Formation , Middle Pleistocene. For further details on the type locality and its ichnological record see Titschack et al. (2013) and Wisshak (2008). The epoxy cast is deposited in the trace fossil collection of the Senckenberg Institute in Frankfurt , Germany ( SMF XXX 870 ).

Description

From a circular point of entry, a very short vertical gallery enters the substrate, from which the main gallery of the closely prostrate tunnel system emerges laterally and increases in thickness until the first point of ramification ( Fig. 28 A–G). The tunnels commonly bifurcate irregularly at various angles between 45° and 160° and with internodes of varying length, forming a trace oriented to one side of the initial entry point. The diameter of the galleries has a moderate variability and distal galleries taper or end bluntly ( Fig. 28 E–F). The tunnels always feature a dense cover of spiny protrusions, part of which probably connect to the substrate surface ( Fig. 28G ). Converging tunnels evade and cross, or they run parallel to pre-existing tunnels; true anastomosis has not been observed ( Fig. 28 E–F).

Among the observed specimens from the Azores experiment ( Wisshak et al. 2011; therein addressed as ‘Dendroid form 2’), 16 specimens were complete (including a wide range of ontogenetic stages) and were documented by SEM images in planar view, allowing for accurate morphometrical measurements. These traces ranged from 120 to 1020 µm in maximum length (mean = 401 ± 264 µm) and 71 to 1371 µm in maximum width (mean = 430 ± 370 µm). The initial gallery before reaching the first bifurcation was found to be 10 to 116 µm in length (mean = 60 ± 31 µm) and 10 to 18 µm in diameter (mean = 13 ± 2 µm). The angle of bifurcations varies from 45° to 161°, with a mean of 87° ± 19° (n = 121) and the internodes measure 16 to 266 µm in distance, with a mean of 73 ± 53 µm (n = 120). The diameter of the tunnels varies from 17 to 43 µm, with a mean of 26 ± 6 µm (n = 229).

Remarks

In contrast to R. avis igen. et isp. nov., this ichnospecies never forms true anastomoses, always exhibits a spiny surface texture and has a characteristic, very short and vertical entrance tunnel from which the system of prostrate galleries emerges laterally. It shares the convergent morphological character of the spiny protrusions with the tubular microboring Orthogonum spinosum Radtke, 1991, but is clearly distinguished from it by the formation of a system of irregularly branched and densely spaced galleries rather than straight galleries with alternating and perpendicular short side branches.

A morphologically related and previously undescribed dendrinid microboring was found in the same substrates from the Azores experiment that yielded many Recent R. acanthina igen. et isp. nov. This form shows a collar around the short vertical entrance tunnel, the surface texture is very smooth and it does form true anastomoses ( Fig. 28 H–I). However, an ichnotaxonomical treatment is postponed until more (preferentially fossil) material becomes available.

Figures

Fig. 28. Rhopalondendrina acanthina igen. et isp. nov. A. SEM of holotype, partly obscured by other microborings, with point of entry at the lower left, in an epoxy cast of a Middle Pleistocene bivalve shell from the Island of Rhodes, Greece. B–C. Planar and oblique views of the paratype, illustrating an early ontogenetic stage with short vertical entrance tunnel; same epoxy cast as the holotype. D–F. SEM of three different ontogenetic stages (arrows indicating initial points of entry), in epoxy casts from experimental bivalve shells deployed at Faial Island in the Azores (same for G–I). G. Oblique view of a specimen with vertical entrance tunnel and short spiny protrusions connecting the trace to the substrate surface. H–I. Two specimens of a related dendrinid from the Azores experiment, awaiting ichnotaxonomical treatment as soon as more material becomes available.