Rhopalondendrina contra, Wisskah, 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 : 74-75

Rhopalondendrina contra igen. et isp. nov.

urn:lsid:zoobank.org:act:93D4CFAE-BABA-4D6C-9DE8-827C4E760446

Fig. 29

Spinate Microborings – (?) Edwards & Perkins 1974: 1133, fig. 11.

Diagnosis

Entrance tunnel prostrate or crossing the substrate vertically, leading to a plexus of strongly anastomosing galleries on the same or opposing substrate surface. Plexus covered with short spiny protrusions, some of which connect to the substrate surface.

Etymology

From the Latin ‘contra’, across from, referring to the fact that the initial entrance and plexus of the trace may develop on opposing substrate surfaces.

Type material, locality and horizon

The holotype ( Fig. 29 A–E), two paratypes ( Fig. 29 F–H) and several further specimens are cast in epoxy resin of a bivalve shell, sampled in the Balcome Clay, Fyansford Formation, Middle Miocene strata at Fossil Beach, Balcombe Bay, ~ 3 km S of Mornington, Victoria, Australia. The epoxy cast is deposited in the trace fossil collection of the Senckenberg Institute in Frankfurt , Germany ( SMF XXX 871 ).

Description

From the initial point of entry, an entrance tunnel of roughly constant diameter leads to a plexus of strongly anastomosing galleries that may fuse to form wide lobes ( Fig. 29 A–H). From the base of the entrance tunnel, further short tunnels may radiate and terminate bluntly ( Fig. 29C, E ). The entrance tunnel has a smooth surface texture and either runs closely prostrate ( Fig. 29C , E–F), or enters and crosses the substrate vertically ( Fig. 29 G–H). Consequently, in thin substrate shells, the plexus may develop on the same or on the opposite surface of the substrate as the primary entrance. The plexus is oriented closely parallel to the substrate surface and is densely covered with short, spiny protrusions that lead vertically into the substrate or connect the plexus with the substrate surface ( Fig. 29 G–H).

Morphometric data were derived from a cast with four complete specimens (including the types). The length of the traces varies from 362 to 734 µm (mean = 578 ± 168 µm), with a similar width of

363 to 761 µm (mean = 606 ± 175 µm). The initial gallery of the traces measures 156 to 203 µm (mean = 178 ± 20 µm) in length, with a diameter of 17 to 31 µm (mean = 24 ± 6 µm).

Remarks

This is the only ichnospecies of Rhopalondendrina igen. nov. that was occasionally found traversing the substrate to form the plexus on the opposite shell surface as the initial point of entry. It is furthermore distinguished by showing a very high degree of anastomosis and fusion in the plexus.

Figures

Fig. 29. Rhopalondendrina contra igen. et isp. nov. A–E. Overviews and close-ups of the holotype, illustrating the morphology of the entrance tunnel (arrows in A and D; close-ups in C and E) and the anastomosing plexus, covered with short and spiny protrusions (close-up in B). SEM of epoxy cast of a bivalve shell sampled in Middle Miocene strata at Balcome Bay, Victoria, Australia (same for F–H). F. Juvenile specimen (paratype) with clearly visible entrance tunnel (arrow indicating initial point of entry). G–H. Larger specimen (paratype) with the plexus developed on the opposite surface of the bivalve shell from the initial point of entry (arrow in G).