Hornestheria, Morphotype

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 : 485-487

Hornestheria ? Morphotype 2

( Figures 10 , 11 )

Material. DA21/03/04/04a, DA21/03/04/06, DA21/03/04/07-1b, DA21/03/04/07-2b, DA21/03/04/08, DA21/03/04/09.

Description. Carapace valves very large and elongated oval to oval in shape; dorsal margin slightly curved to curved, very short to long; larval carapace valve very small; position of the umbo submedial and marginal to supramarginal; anterior and posterior margins sharply curved to very sharply curved; points of maximum curvature of the anterior and posterior margins located median or slightly median-dorsal, and in the ventral margin median-anterior; the ventral margin is concavely indented in the median-posterior region of the carapace valve.

Size. Height (H) of the carapace valve between 3.2–4.9 mm; length (L) of the valve between 5.3–7.5 mm; 0.57<H/L<0.71; dorsal margin length (l) between 3.0–4.0 mm; 0.48<l/L<0.70; larval carapace valve height (h) between 0.2–0.9 mm; 0.06<h/H<0.2; often between 25–30 growth lines; all measurements based on 6 valves.

Occurrence. Punta Negra 2 site; uppermost part of Unit C, Aegean ( Diez et al. 2010), lower Anisian (lower Middle Triassic).

Remarks. The genus name is written with a question mark, because neither its characteristic radial sculpture in the umbonal area nor microstructures (ornamentation) of the carapace valves could be clearly identified, possibly due to the imperfection of their preservation. Instead of a formal species name, open nomenclature is applied to this distinctive carapace valve morphology. This morphotype co-occurs with the above described Hornestheria ? Morphotype 1 ( Figures 8 , 9 ). The latter mentioned morphotype reaches greater total carapace valve height values than Hornestheria ? Morphotype 2 ( Figures 10 , 11 ). It also differs from Hornestheria ? Morphotype 2 in having a more rounded shape.

Hornestheria ? Morphotype 2 is of larger size than both H. sollingensis sensu stricto and Hornestheria sp. aff. H. sollingensis ( Figures 2–6 ). The large carapace valves of Hornestheria ? Morphotype 2 resemble similarly-sized clam shrimps, such as “ Isaura minuta albertii ” (see Reible 1962: plate 7-fig. 7) or E. albertii albertii (see Kozur et al. 1993: fig. 3), but differ from them in both its concavely recurved ventral margin and the morphology of the umbo.

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 8. Light microscopic photographs and line drawings of Hornestheria? Morphotype 1 from Punta Negra 2 site; uppermost part of Unit C, lower Anisian. A–B, compacted carapace in lateral view (DA21/03/04/11). C–D, external view on a right carapace valve (DA21/03/04/07-1a). E–F, internal side of a right carapace valve, appearing optically inverted through artificial shallow-angle illumination (DA21/03/04/07-2a).

FIGURE 9. Light microscopic photographs and line drawings of Hornestheria? Morphotype 1 from Punta Negra 2 site; uppermost part of Unit C, lower Anisian. A–B, compacted carapace in lateral view; rock surface was covered with adhesive (DA21/03/04/07-1d). C–D, external side of a left carapace valve (DA21/03/04/12-1). E–F, external side of a fragmented right carapace valve (DA21/03/04/13a).

FIGURE 10. Light microscopic photographs and line drawings of Hornestheria? Morphotype 2 from Punta Negra 2 site, showing more or less developed concave bending of the ventral margin in the median-posterior area of the carapace valve; uppermost part of Unit C, lower Anisian. A–B, Right carapace valve, almost completely preserved as a steinkern (DA21/03/04/07-1b). C–D, imprint of a right carapace valve, appearing optically inverted through shallow illumination (DA21/03/04/07-2b). E–F, external side of a left carapace valve, overlying the internal side of the corresponding right carapace valve; both carapace valves are shifted relative to each other (DA21/03/04/08).

FIGURE 11. A–B, light microscopic photograph and line drawing of external view on a right carapace valve (DA21/03/04/09), showing the positions of the ESEM images. C–D, ESEM images of Hornestheria? Morphotype 2 from Punta Negra 2 site; uppermost part of Unit C, lower Anisian; irregularly (not smoothly) preserved surface, the ornamentation is not clearly detectable even under high magnification.