Hornestheria sp.

Scholze, Frank & Matamales-Andreu, Rafel, 2021, Triassic clam shrimps (“ Conchostraca ”; Branchiopoda: Diplostraca) from Mallorca: Taxonomic description and interregional comparisons, Zootaxa 4964 (3), pp. 471-496 : 476

Hornestheria sp. aff. Hornestheria sollingensis Kozur et Lepper (in Kozur & Weems 2010)

( Figures 2–7 )

Material. DA21/03/02/58, DA21/03/02/68, DA21/03/02/72, DA21/03/02/75, DA21/03/02/77, DA21/03/02/78, DA21/03/02/80, DA21/03/02/81.

Description. Carapace valve medium-sized to large, elongated-oval to oval shape; dorsal margin straight, short to long; larval carapace valve very small; umbo located submedial and marginal to inframarginal; anterior and posterior margins sharply curved to very sharply curved; points of maximum curvature along the anterior and posterior margins located in the median-dorsal to median-ventral region, and along the ventral margin in the median to median-posterior region; ornaments fine, indistinct or irregularly reticulated.

Size. Height (H) of the carapace valve between 1.6–3.9 mm; length (L) of the valve between 2.5–3.9 mm; 0.57<H/L<0.7; dorsal margin length (l) between 1.8–2.3 mm; 0.51<l/L<0.70; larval carapace valve height (h) between 0.1–0.5 mm; 0.04<h/H<0.19; up to 20 growth lines; all measurements based on 8 valves.

Occurrence. Pedra Alta site; upper part of Unit B; lower Anisian (lower Middle Triassic), dated as Aegean by Diez et al. (2010).

Remarks. H.W. Kozur and J. Lepper (in Kozur & Weems 2010) first defined Hornestheria sollingensis as a new species, providing the following parameters: total carapace valve length between 3.0– 3.8 mm, total height between 1.95–2.85 mm, length/height ratio values between 1.3–1.66 ( Kozur & Weems 2010). The morphotype of the specimens in the present study identified as Hornestheria sp. aff. H. sollingensis ( Figures 2–6 ) differ from H. sollingensis Kozur et Lepper in their larger size and wider range of the height/length ratio values. Additionally, the larval carapace valve (=“free umbonal area” in Kozur & Weems 2010: p. 397) of H. sollingensis is smaller than that of Hornestheria sp. aff. H. sollingensis . In accordance to the general description of the genus Hornestheria by Kozur & Weems (2010), the presence of a radial element on the larval carapace valve of Hornestheria sp. aff. H. sollingensis cannot be clearly observed, except for two individuals (collection numbers DA21/03/02/80, DA21/03/02/81; Figure 2B–E ). Problematically, in most specimens the small larval carapace valve seems to be poorly preserved and the umbonal area has been almost without exception affected by deformation (i.e., sedimentary compaction). Generally, large sample sets, preferably including layers of mass occurrence (see Figure 2A ), need to be analysed in order to better understand the full spectrum of preservation variability found in specimens attributable to Hornestheria .

When the radial sculpture cannot be clearly seen on the larval carapace valves, both H. sollingensis Kozur et Lepper (in Kozur & Weems 2010) and Hornestheria sp. aff. H. sollingensis (the present study) are morphologically reminiscent of both Euestheria albertii albertii (in the sense of Kozur et al. 1993; formerly designated as “ Isaura minuta albertii ( Voltz, 1835) ” by Reible 1962) and Euestheria albertii mahlerselli Kozur et Lepper (in Kozur & Weems 2010). By comparison with the literature, E. albertii albertii (figured by Kozur et al. 1993: figs. 3.1–3.2) shows a more rounded shape, whereas E. albertii mahlerselli (figured by Kozur et al. 1993: figs. 3.4, 3.6; for references to further figures see the synonymy list in Kozur & Weems 2010: p. 391) shows a more elongated shape than Hornestheria sp. aff. H. sollingensis . Additionally, E. albertii albertii and E. albertii mahlerselli have the strongest carapace valve convexity in the umbonal area, whereas in Hornestheria sp. aff. H. sollingensis it is more or less at the centre of the carapace valve.

Figures

FIGURE 2. Clam shrimps and plants from Pedra Alta site; upper part of Unit B, lower Anisian. A, mass-occurrence of Hornestheria sp. aff. Hornestheria sollingensis and coalified plant remains (marked with yellow arrows); DA21/03/02/80. B, external view on a right carapace valve of Hornestheria sp. aff. Hornestheria sollingensis. C, radial sculpture on the larval carapace valve (marked by red arrow), which is a diagnostic character for the genus Hornestheria; white arrows mark concentric growth lines in the umbonal area of the carapace valve; DA21/03/02/81. D, detail framed in A, external view on a left carapace valve of Hornestheria sp. aff. Hornestheria sollingensis; DA21/03/02/80. E, detail framed in D, showing an indistinct radial sculpture on the larval carapace valve (red arrow) and earliest growth lines (white arrows).

FIGURE 3. Light microscopic photographs (Figs. A, C), line drawings, and ESEM images (D, E) of Hornestheria sp. aff. Hornestheria sollingensis from Pedra Alta site; upper part of Unit B, lower Anisian. A–B, external view on a right carapace valve (DA21/03/02/75). C–E, details of the same individual showing finely reticulated ornaments (highlighted by yellow dashed lines) on the growth bands.

FIGURE 4. Light microscopic photographs and line drawing of Hornestheria sp. aff. Hornestheria sollingensis from Pedra Alta site; upper part of Unit B, lower Anisian. A–B, external view on a right carapace valve (DA21/03/02/72). C, details of the same individual showing indistinct fine ornaments on the growth bands.

FIGURE 5. Light microscopic photographs and line drawing of Hornestheria sp. aff. Hornestheria sollingensis from Pedra Alta site; upper part of Unit B, lower Anisian. A–B, external view on a left carapace valve, slightly shifted against the underlying internal side of the right carapace valve (DA21/03/02/77). C–D, details of the same individual showing indistinct fine ornaments on the growth bands.

FIGURE 6. Light microscopic photographs and line drawings of Hornestheria sp. aff. Hornestheria sollingensis from Pedra Alta site; upper part of Unit B, lower Anisian. A–B, external view on a right carapace valve (DA21/03/02/58). C–D, external view on a left carapace valve (DA21/03/02/78). E–F, external view on a left carapace valve (DA21/03/02/68).

FIGURE 7. ESEM image of the ornamentation on the external side of a right carapace valve of Hornestheria sp. aff. Hornestheria sollingensis from the Pedra Alta site; upper part of Unit B, lowerAnisian; collection number DA21/03/02/72.A, image taken in the central area of the carapace valve; concentric growth lines are marked by white arrows. B, detail showing irregularly reticulated ornaments on the growth band.