Hornestheria Kozur et Lepper

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 : 475-476

Genus Hornestheria Kozur et Lepper in Kozur et Weems, 2010

Type species. Hornestheria sollingensis Kozur et Lepper (in Kozur & Weems, 2010); from the Frohrieper Berg Quarry (East of the town of Lauenförde, Lower Saxony, Germany), Solling Formation (Middle Buntsandstein Subgroup, Buntsandstein Group), uppermost Spathian (uppermost Olenekian, uppermost Lower Triassic) ( Kozur & Weems 2010: p. 395). Hornestheria sollingensis was used in stratigraphic charts (e.g., Bachmann & Kozur 2004) before the taxonomic characteristics for its genus and species levels were formally published by Kozur & Weems (2010: appendix 1).

Remarks. A diagnosis of this genus was proposed by H.W. Kozur and J. Lepper (in Kozur & Weems 2010), but metric values for total carapace valve length and height were not provided. Besides a low convexity of the carapace valves and their almost straight dorsal margin, other characteristics of Hornestheria included the location of points of maximum curvature of the anterior margin in the horizontal midline of the carapace valve and of the posterior margin below the horizontal midline ( Kozur & Weems 2010). The larval carapace valve (=umbonal area of the valve free of growth lines) bore a rather enigmatic radial element, which was either “often quite distinct” ( Kozur & Weems 2010: p. 395) or unrecognizable, depending on the respective preservation state of individual carapace valves.

Based on the description of Hornestheria in Kozur & Weems (2010), the radial element on the larval carapace valves of Hornestheria most closely resembles that of Lioestheria Depéret et Mazeran, 1912 , which has both nodular and radial sculptures on its larval carapace valves (e.g., Kozur et al. 1981). In contrast, however, the size of the larval carapace valve in adult individuals of Lioestheria is greater than 1/3 of the total carapace valve length ( Holub & Kozur 1981), and this is much larger relatively than the size of the larval valve seen in the carapace of the genus Hornestheria . This difference provides a clear distinction between Hornestheria and Lioestheria sensu Holub & Kozur (1981) .

Prominent sculptures are also found on the larval carapace valves of the clam shrimp genus Cornia Lutkevich, 1937 , which is widely distributed in the Lower Triassic (e.g., Kozur & Seidel 1983a; Kozur & Mock 1993; Tassi et al. 2013; Scholze et al. 2016). This genus is characterized by an umbonal spine with a circular base located on the larval carapace valve. The spine can be variably preserved (Kozur 1983), being either perpendicular or obliquely embedded in the sediment or being broken, due to variable deformation (e.g., sedimentary compaction) of the rock. According to the description for Hornestheria provided by Kozur & Weems (2010), the generally radial shape of its larval sculpture makes it consistently different from the circular shape of the base of the umbonal spine in Cornia .

A further characteristic of Hornestheria in the sense of H.W. Kozur and J. Lepper (in Kozur & Weems 2010) is the presence of certain types of microsculpture (ornamentation) located on the growth bands of the carapace valves. The ornamentation of the genus Hornestheria was originally described as a dense, fine reticulation, which became replaced by fine, short, radial lines in the outer margins in adult individuals ( Kozur & Weems 2010). However, it should be noted that the figures of H. sollingensis in Kozur & Weems (2010: fig. A2) show only indistinct reticulation.

In lateral view, the carapace valves of Hornestheria are morphologically similar to Euestheria Depéret et Mazeran, 1912 , and Magniestheria Kozur, 1982 . However, in contrast to Hornestheria , the carapace valves of Euestheria have a more strongly convex shape with its highest convexity in the umbonal area. The highest convexity in carapace valves of Magniestheria is located in its central area. Both Euestheria and Magniestheria differ in their ornamentation from Hornestheria . The ornamentation of Euestheria is either finely dotted to weakly reticulated ( Kozur & Seidel 1983a) or very finely pitted, having pits of round to round-oval shapes with diameters of about 10 μm ( Sell 2018), that appear generally to be more regular and smoother than the ornamentation seen in Hornestheria . The ornamentation of Magniestheria is indistinct, varying between coarse pits and very short, irregular, radial lines ( Kozur & Seidel 1983a); their expression is less intense than in Hornestheria .

Occurrence. Uppermost Spathian (uppermost Olenekian; upper Lower Triassic) and Anisian (lower Middle Triassic) in Germany ( Kozur & Weems 2010), China (see discussions on “ Protomonocarina ” in Kozur & Weems 2010: p. 395), and Mallorca (the present study).