Paraostenia cf. ambatolokobensis ( Arduini, 1990 )

Paraostenia cf. ambatolokobensis ( Arduini, 1990)

Fig. 7.

Material.—NHMW 2024/0201/00012 from the Reingraben formation, Austrotrachyceras austriacum Zone , Austrotrachyceras minor Biohorizon, Julian 2 Ib, Carnian, Upper Triassic; Polzberg near Lunz am See, Austria.

Dimensions. —See Table 1.

Description.—Shield morphology: Curved hexagonalshaped shield in lateral view, longer than large ( Fig. 7A 1 View Fig , A 2 View Fig ). Maximal height reached around mid-length. Anterior margin made of a well-developed, symmetric optic notch, occupying the entire margin. Anterodorsal corner not preserved. Convex dorsal midline with a large fin-like carina ( Fig. 7A 3 View Fig ). Rounded posterodorsal corner. Short, concave posterior margin. Rounded posteroventral corner. Ventral margin made of a horizontal anterior part and a slightly concave posterior one, anteroventrally tilted; both parts having approximately the same length ( Fig. 7A 1 View Fig , A 2 View Fig ). Rounded anteroventral corner. Marginal fold delimited by a shallow groove near anterior and ventral margin ( Fig. 7A 4 View Fig ).

Shield macro-ornamentation: Shield with a comma-shaped ventrolateral furrow, being most probably a muscle scar ( Fig. 7A 1 View Fig , A 2 View Fig ). Long dorsolateral carina running along almost the entire length of the shield. Shield covered of short, transversal ridges. Ridges having a comma-like shape below the dorsal carina and near the anterodorsal and anteroventral corners ( Fig. 7A 3 View Fig ), and a scale-like shape on the rest of the shield ( Fig. 7A 4 View Fig ).

Posterior trunk: Posterior trunk made of at least four segments ( Fig. 7A 1 View Fig , A 2 View Fig ).

Gills: Set of eight gills located in the central part of the shield. Gills with an elliptic shape, becoming arcuate near the dorsal midline ( Fig. 7A 1 View Fig , A 2 View Fig ).

Remarks.—The specimen described above have key features of Paraostenia Secrétan, 1985 : a large optic notch, a dorsal carina, a concave posterior margin, rounded posterodorsal and posteroventral corners and a horizontal anterior part of the ventral margin. It differs from the other Paraostenia specimens from Polzberg by the presence of a dorsal carina and by its pattern of macro-ornamentation (scale-like ridges vs long, transverse ridges). In terms of size, the specimen is a bit longer and taller than Paraostenia striata and the Hmax/Ls ratios are quite similar (see Table 1). The specimen resembles the specimens of Paraostenia ambatolokobensis from the Lower Triassic of Madagascar ( Arduini 1990). Indeed, they both have a dorsal carina and a dorsolateral carina but most importantly they share a unique pattern of macro-ornamentation among species of Paraostenia : the presence of short transverse comma-like ridges on the dorsal area of the shield, and the presence of scale-like ridges on the rest of the shield (see Fig. 7B for P. ambatolokobensis ). This pattern has not been described in any other Paraostenia species. It is therefore most probable that this specimen belongs to P. ambatolokobensis . However, due to the poor preservation of some morphological features (e.g., anterodorsal corner) in our specimens and to the fact that only one specimen of this taxon has been recovered from the Polzberg Lagerstätte, we refrain to assign this specimen to a particular species and let it in open nomenclature.