Thylacocephala

Thylacocephala Pinna et al., 1982

Genus Atropicaris Arduini & Brasca, 1984

Type species: Atropicaris rostrata Arduini & Brasca, 1984 , by original designation— Argilliti di Riva di Solto Formation , Sevatian , Norian , Upper Triassic ; Ponte Giurino, Valle Imagna, Italy .

Included species: Atropicaris lintveri Laville et al., 2024 , Anisian, Middle Triassic, Slovenia; Atropicaris aff. lintveri, Anisian, Middle Triassic , Slovenia ( Laville et al. 2024); Atropicaris rostrata Arduini & Brasca, 1984 , Upper Triassic, Italy; Atropicaris ? sp., Anisian, Middle Triassic, China ( Feldmann et al. 2015); Atropicaris ? sp., Carnian, Upper Triassic, Austria (this study); Atropicaris sp. , Ladinian, Middle Triassic, Switzerland ( Bürgin et al. 1991); Atropicaris sp. , Carnian, Upper Triassic, Slovenia ( Laville et al. 2024).

Diagnosis (after Laville et al. 2024). — Thylacocephala with a triangular rostrum ending in a spatulate tip, a serrate dorsal midline, a concave posterior margin, a spiny posteroventral corner, and dorsal and ventral intercalary ridges.

Stratigraphic and geographic range.—Anisian to Carnian, Middle to Late Triassic; Asia ( China) and Europe ( Austria, Italy, Slovenia, Switzerland).

Atropicaris ? sp. Fig. 2 View Fig .

Material.— 24 specimens: NHMW 1910/0015/0033, 0035; NHMW 2021/0123/0059, 0767, 0769, 0776, 0777, 0779, 0780, 0782–0784, 0787, 0788, 0790, 0793, 0795; NHMW 2021/0123/0078, 0101–0106. All from Reingraben formation, Austrotrachyceras austriacum Zone , Austrotrachyceras minor Biohorizon, Julian 2 Ib, Carnian, Late Triassic; Polzberg near Lunz am See, Austria.

Dimensions. —Due to the deformation and distortion of the specimens, we only provide rough estimates for the length and maximal height of the shield: Ls> 15.5 mm; Hmax ≈ 8.5 mm.

Description.—Shield morphology: Trapezoidal shield in lateral view, longer than high ( Fig. 2 View Fig ). Maximal height reached around mid-length. Straight anterior margin slightly inclined posteroventrally ( Fig. 2A, B View Fig ). Anterodorsal corner not preserved. Convex dorsal midline, poorly preserved. Posterodorsal corner, posterior margin and posteroventral corner not preserved. V-shaped ventral margin consisting in an anterior part and a posterior one of equal length. Rounded anteroventral corner ( Fig. 2B View Fig ). Marginal fold delimited by a shallow groove at least near the ventral margin ( Fig. 2B View Fig 1 View Fig ).

Shield macro-ornamentation: Shield covered of at least 57 transverse sigmoid ridges, posteroventrally oriented with at least 49 primary ridges, three dorsal intercalary ridges and five ventral intercalary ridges ( Fig. 2 View Fig ).

Other structures: Appendages, posterior trunk and soft parts not preserved.

Remarks.—Some of the specimens described here were previously assigned to Austriocaris striata (now Paraostenia striata comb. nov.; see below). However, their morphology differs from the one of the type material (and thus from Paraostenia ). Even though all specimens possess transversal ridges, the specimens described above have a straight and anterior margin slightly inclined posteroventrally, without any optic notch, an anterior part of the ventral margin anterodorsally tilted and a straight posterior part of the ventral margin while the type material of P. striata has an anterior margin formed of a large optic notch, occupying the entire margin, a ventral margin with a horizontal anterior part and a slightly concave posterior part. Therefore, those specimens do not belong to P. striata .

One of the main features of our specimens is the presence of transverse ridges. This kind of ornamentation is well known in multiple thylacocephalans taxa such as species of Ankitokazocaris Arduini, 1990 , Atropicaris , Concavicaris Rolfe, 1961 , Ferrecaris Calzada and Mañé, 1993 , Harrycaris Briggs and Rolfe, 1983 , Kamnikaris Laville et al., 2024, Keelicaris Teruzzi & Charbonnier in Charbonnier et al., 2017, Microcaris Pinna, 1974 , Paraconcavicaris Rak et al., 2018, and Thylacocephalus Lange et al., 2001 .

The species described here differ from those of Ankitokazocaris , Concavicaris , Harrycaris , and Paraconcavicaris by the absence of an optic notch. Additionally, species of Paraconcavicaris has a ventral margin with a horizontal anterior part, longer than the posterior one. Our specimens have similarities with species of Kamnikaris, including a convex dorsal midline and a straight anterior margin, slightly tilted posteroventrally. However, they differ from this taxon by their pattern of macro-ornamentation: species of Kamnikaris have vertical arcuate ridges on its anterior part, and sigmoid ridges on its median and posterior parts while the studied specimens only has sigmoid ridges on the entire surface of its shield. Moreover, species of Kamnikaris have a ventral margin, which has an anterior part much shorter than the posterior one, while it is of equal length in our specimens. This is also the case for species of Thylacocephalus and Microcaris . Moreover, these of Microcaris are characterised by a straight dorsal midline and a horizontal anterior part of the ventral margin. The specimens described herein also differ from species of Thylacocephalus by the absence of spines on the anterior margin, of a dorsal carina and of numerous pores on the lateral surface of the shield. The morphology of species of Keelicaris is also quite different from the one of our specimen. In Keelicaris, the shield is keel-shaped with straight transverse ridges anteroventrally tilted, the anterior margin is made of an optic notch and the ventral margin is formed of a concave posterior part and a horizontal ventral part.

Species of Atropicaris and Ferrecaris display several similarities with our specimens: a straight anterior margin, slightly tilted posteroventrally, convex dorsal midline, v-shaped ventral margin with straight posterior and anterior parts of equal lengths. It is therefore most likely that our specimens belong to one of these taxa. Atropicaris and Ferrecaris differ by their rostrum morphology (spatulate tip for Atropicaris ; sharp, triangular tip for Ferrecaris ), the ornamentation of their dorsal midline (serrate vs. smooth), the morphology of their posterior margin (concave surrounded by a posterodorsal and a posteroventral spine vs. straight with rounded posterodorsal and posterodorsal corners). Unfortunately, the rostrum and the posterior part are not preserved in our specimens and the dorsal midline is only poorly preserved, not allowing the observation of the ornamentation. It is therefore almost impossible to assign to one or the other taxon. As it seems that only species of Atropicaris show ventral intercalary ridges, this feature could be useful to accurately identify our specimens. Thus, our specimens most probably belong to a species of Atropicaris . Nevertheless, in view of the poor preservation of the type material of Ferrecaris , the latter character should be taken with caution as it would be possible for such ridges to be observed if better preserved material was discovered. Based on those comparisons, we leave our specimens in open nomenclature, although probably belonging to a species of Atropicaris .