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 : 479-484

Hornestheria ? Morphotype 1

( Figures 8 , 9 )

Material. DA21/03/04/01, DA21/03/04/02, DA21/03/04/07-1a, DA21/03/04/07-2a, DA21/03/04/07-1c, DA21/03/04/07-2c, DA21/03/04/07-1d, DA21/03/04/10, DA21/03/04/11, DA21/03/04/12-1, DA21/03/04/12-2, DA21/03/04/13a, DA21/03/04/13b.

Description. Carapace valves very large and round in shape; dorsal margin straight, very short to short; larval carapace valve very small; position of the umbo anterior to slightly submedial and inframarginal to slightly marginal; anterior and posterior margins sharply curved to very sharply curved; points of maximum curvature of the anterior margin located median-dorsal to median-ventral, in the posterior margin median-ventral, and in the ventral margin median-anterior; ventral margin is often indented in its median-posterior area.

Size. Height (H) of the carapace valve between 4.9–7.8 mm; length (L) of the carapace valve between 6.0– 9.9 mm; 0.76<H/L<0.87; dorsal margin length (l) between 2.7–4.6 mm; 0.38<l/L<0.56; larval carapace valve height (h) between 0.3–0.9 mm; 0.05<h/H<0.16; up to 55 growth lines, 5–7 growth lines/ 1 mm towards growth direction; all measurements based on 10 valves plus counterparts.

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

Remarks. The use of open nomenclature at the species level is preferred for this set of specimens, due to the small sample available for study. This careful practice follows previous clam shrimp studies by Schneider et al. (2005: p. 11) and Schneider & Scholze (2018: p. 369), in order to prevent the creation of new definitions for what may prove to be synonymous species. A formal species diagnosis for Hornestheria ? Morphotype 1 should be made only when more congeneric material from a wider study area is available.

Large clam shrimps identified as “ Dictyonatella sp. ” ( Żyła et al. 2013: figs. 5A, B) and “cf. Dictyonatella ” ( Soussi et al. 2017: figs. 13A, B) from the upper Lower Triassic–Middle Triassic of Poland and Tunisia, respectively, have a rounded shape similar to Hornestheria ? Morphotype 1. According to Kozur (1982), the genus Dictyonatella has, among other characteristics, an ornamentation on the growth bands represented by a coarse reticulation and a recurvature of posterior growth lines below the dorsal margin. These two characteristics clearly differ from the generally small reticulated ornaments of Hornestheria sensu lato and its non-concave recurvature in the posterodorsal margin of the carapace valve. Notably, in Hornestheria ? Morphotype 1, a recurved posterodorsal margin is absent.

Shen et al. (2002) described “ Euestheria (Magniestheria) truempyi ” from the basal Olenekian (basal upper Lower Triassic), which has a large size and a rounded carapace valve shape similar to the Hornestheria ? Morpho- type 1. Additionally, a minute reticulated ornamentation noted by Shen et al. (2002: plate 2, fig. 2) resembles that of Hornestheria sp. aff. H. sollingensis ( Figure 7 ). “ Euestheria (Magniestheria) truempyi ” of Shen et al. (2002), however, shows pronounced concentric ribs in the umbonal and central areas of the carapace valves, whereas the growth bands in all the specimens of the present study are both narrower and their convex curvature is less intense, lacking pronounced concentric ribs.

More recently, Scholze et al. (2020: fig. 9E) described a Lower Triassic clam shrimp fauna from the Jialingjiang Formation in southwestern China, which also contained a species designated preliminarily as Magniestheria sp. aff. Magniestheria subcircularis ( Chernyshev, 1934) . Its size, shape, and the position of points of maximal curvature of the outer margins of the carapace valve are similar to those of Hornestheria ? Morphotype 1 ( Figures 8 , 9 ), but the umbo of Magniestheria sp. aff. M. subcircularis is located in a more submedial position. Additionally, the strongest convexity of the carapace valves, in lateral view, is located in the umbonal area in the case of Hornestheria ? Morphotype 1, whereas in Magniestheria sp. aff. M. subcircularis it is located in the central area of the carapace valve.

Figures

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.

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).