Rhopalondendrina avis, 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 : 69-71

Rhopalondendrina avis igen. et isp. nov.

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Fig. 27

J-Form F-4 – Glaub 1988: 154, fig. 3D.

Rhopalondendrina-Form – Glaub 1994: 132, pl. 12, figs 1–2 .

Rhopalodendrina form – (?) Vogel & Marincovich 2004: 12, fig. 4.1.

Without name – Thuy et al. 2014: 2, fig. 1i .

Diagnosis

Entry tunnel straight or arched, leading to a mainly dichotomously and rarely trichotomously ramifying plexus of prostrate tunnels. Increase in tunnel diameter and lateral widening at branching points or terminations towards the periphery of the trace. Converging tunnels fuse to form anastomoses or evade and develop deeper tiers.

Etymology

Based on the Latin ‘avis’, bird, acknowledging Prof. Dr. Klaus Vogel (Vogel, German = bird) and his team (including Ingrid Glaub, referred to in the etymology of this new ichnogenus) in Frankfurt, who contributed immensely to the understanding of microbioerosion processes.

Type material, locality and horizon

The trace was recorded in epoxy resin casts of unidentified belemnite rostra from marls of the Kehlbach and Scheck Members within the Adnet Formation, Sinemurian to Pliensbachian, Early Jurassic, exposed in the Glasenbach Gorge, south of Salzburg in the northern Calcareous Alps, Austria. For further details concerning the type locality and its ichnological record, see Thuy et al. (2014). The holotype ( Fig. 27 E ) and four paratypes ( Fig. 27 A–D, F) are preserved in three epoxy resin casts, together with many other traces of the new ichnospecies, and are deposited in the trace fossil collection of the Senckenberg Institute in Frankfurt, Germany ( SMF XXX 867 , including the holotype, SMF XXX 868 , including one paratype and SMF XXX 869 , including three paratypes).

Description

From a single point of entry, a short and nearly straight or slightly arched gallery ( Fig. 27F ) leads to a first dichotomous ramification ( Fig. 27A ). The two resulting tunnels repeatedly ramify (or end blindly), typically dichotomously, occasionally trichotomously, with a total number of up to six subsequent branching points ( Fig. 27 B–F). The nearly bilaterally symmetrical system of galleries runs closely parallel to, or directly below, the substrate surface. In more developed ontogenetic stages with converging galleries, these may fuse and form an anastomosing pattern ( Fig. 27 D–E), or they may evade and form deeper tiers ( Fig. 27E ). The diameter of the tunnels increases from the initial gallery towards the periphery of the trace, and terminations are laterally slightly widened. In late ontogenetic stages, the lateral widening of the gallery terminations and ramification points is most pronounced ( Fig. 27E ).

Among the observed ~ 50 specimens from the type locality, 12 traces 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 418 to 1057 µm in maximum length (mean = 653 ± 188 µm) and 425 to 1570 µm in maximum width (mean = 803 ± 359 µm). The initial gallery was found to be 144 to 294 µm in length (mean = 217 ± 44 µm) and 29 to 45 µm in diameter (mean = 34 ± 5 µm). The angle of dichotomous bifurcations varied from 55° to 136°, with a mean of 90° ± 15° (n = 89) and the internodes measured 56 to 263 µm in distance, with a mean of 123 ± 36 µm (n = 98). The diameter of the tunnels, measured halfway along the internodes, varied from 28 to 99 µm, with a mean of 52 ± 14 µm (n = 180) and the widened terminations were 30 to 110 µm in width, resulting in a mean of 61 ± 15 µm (n = 112). The traces are also very common in Upper Cretaceous belemnite rostra, where they show a comparable branching pattern and similar dimensions ( Figs 27 G–K).

Remarks

The traces partly exhibit a limonitic infill that was either impregnated within the epoxy resin or provided a “natural cast” when the belemnite substrate was dissolved in hydrochloric acid. Both in Jurassic as well as Cretaceous belemnites, R. avis igen. et isp. nov. often occurs in clusters of up to several tenths of individuals ( Fig. 27I ).

Early ontogenetic stages closely resemble Abeliella riccioides in morphology, which is, however, by definition restricted to osteic substrates. Similarity is also observed with Fascichnus bellafurcus comb. nov., but this ichnospecies does not exhibit the diagnostic increase in tunnel diameter towards the periphery and it never forms anastomoses where tunnels converge.

Figures

Fig. 27 (page 70). Rhopalondendrina avis igen. et isp. nov. A–E. Series of paratypes (A–D) and the holotype (E) showing the development from early ontogenetic stages to advanced stages with several orders of ramification and rosette-shaped plexus with typical increase of tunnel diameter and widening towards the periphery of the trace. SEM of epoxy casts of Lower Jurassic belemnites from the Glasenbach Gorge, Austria. F. Oblique view of C, showing arched nature of initial tunnel (arrow at left). G. SEM of an early ontogenetic stage of a specimen in an Upper Campanian belemnite from Kronsmoor, Germany. H. SEM of a more irregularly branching specimen in a belemnite from the Lower Maastrichtian of Rügen, Germany. I–K. Overview and two close-ups of a typical cluster in a belemnite from the Lower Maastrichtian of Rügen, Germany.

Fig. 1 (page 7). Morphological characters of dendrinid microborings with respect to overall shape and symmetry of the trace, its vertical profile, openings to the substrate surface, orientation of entrance tunnel (where present), branching pattern and surface texture (in order of decreasing relevance as ichnotaxobase).

Fig. 2. Dendrina dendrina (Morris, 1851) comb. nov. A. Original illustration of belemnite with various traces, including D. dendrina (arrows; reproduced from Quenstedt 1849: pl. 30, fig. 36); provenance unresolved. B. Original amalgam, including D. dendrina (arrows and label “d”; reproduced from Quenstedt 1885: pl. 38, fig. 39). C. Re-illustration of original Quenstedt Dendrina (reproduced from Plewes 1996: pl. 22, fig. 5). D. Original illustrations of Talpina dendrina (= D. dendrina; reproduced from Morris 1851: pl. IV, figs 4–7; 5 = Talpina ramosa; 6a = Trypanites solitarius). E. Morris’ original belemnite from the Upper Cretaceous of Norfolk, UK, including the lectotype of D. dendrina (encircled). F–G. Close-up of lectotype under incipient and transparent light; the inlet canal is clearly visible. H–I. Three paralectotypes in the same belemnite. J–K. Planar and oblique views of the holotype of junior synonym D. fluensis Hofmann, 1996, illustrating the typical multi-tier occurrence, irregular branching, and meandering galleries; SEM of epoxy cast of a belemnite from the lower Maastrichtian at Kronsmoor, Germany. L. Holotype of junior synonym D. crassa Hofmann, 1996, within the morphological range of D. dendrina; SEM of epoxy cast of a belemnite from the lower Maastrichtian at Kronsmoor, Germany.