Hungarella koessenensis

Hungarella koessenensis (Mette & Mohtat-Aghai, 1999)

Fig. 12 N–Q

Ogmoconcha koessenensis Mette & Mohtat-Aghai, 1999: 52–53, pl. 2, figs 1–4.

? Ogmoconcha sp. – Sohn 1987: C18, pl. 10, figs 12–14, pl. 11, figs 13–16, 20–24.

Material examined

ROMANIA • 1 complete carapace; Black Sea, Romanian Continental Shelf, borehole 817LV, sample CM31B; Rhaetian, Upper Triassic; MNHN.F. F63309 • 1 LV; same collection data as for preceding; Rhaetian, Upper Triassic; MNHN.F. F63310 • 1 LV; same collection data as for preceding; Rhaetian, Upper Triassic; MNHN.F. F63311 • 1 RV; same collection data as for preceding; Rhaetian, Upper Triassic; MNHN.F. F63312 • 3 complete carapaces, 14 LV, 8 RV; same locality as for preceding but samples CM31A, CM31B and CM31C; Rhaetian, Upper Triassic; MNHN.F. F63363.

Occurrence

Eiberg Member, upper K̂ssen Formation, Waidringer Setinplatte, Northern Calcareous Alps, Tyrol, Austria, upper Rhaetian, Upper Triassic (Mette & Mohtat-Aghai 1999); Romanian Continental Shelf, Black Sea, Rhaetian, Upper Triassic (this paper).

Dimensions

Fig. 4B.

Remarks

This species was ascribed to Ogmoconcha Triebel, 1941 by Mette & Mohtat-Aghai (1999) and it is here re-attributed to Hungarella as explained above. Mette & Mohtat-Aghai (1999) noted the closeness of H. koessenensis to Hungarella sp. from the Carnian of Hungary (identified as Ogmoconcha sp. in Sohn 1987) but could not confirm their conspecificity because of low preservation and low abundance of their material. In spite of the relatively good preservation of the present material, apart from the inner structures, the illustrations of Ogmoconcha sp. in Sohn 1987 do not allow us to ascertain the conspecificity of the specimens.

The dimensions of all known valves and carapaces are shown in Fig. 4B, where LV and RV are distinguished. The delimitation of successive ontogenetic stages is here hardened by the relatively important overlap of LV over RV, resulting in the overlap of their distributions on the H/L diagram (Fig. 4B). The three carapaces known (e.g., Fig. 12N) serve as anchor-points and appear to document a relative increase in the thickness of the overlap through the ontogeny of Hungarella koessenensis. The H/L diagram furthermore documents two distinct groups of valves that are not visible in the smallest specimens: a first group of small valves and a second group of larger valves, the three carapaces belonging to the first one. These two groups may represent sexually dimorphic specimens, the larger ones being wider posteriorly, with PB close to vertical, and possibly related to females. Such a pattern of size and morphology has also been observed for H. subtera Zheng, 1976 from the Carnian of South China (Forel et al. 2019b). Owing to these observations, we consider that six ontogenetic stages, A-5 to adult, occur for H. koessenensis (Fig. 4B). A-5 is represented by a unique RV while A-3 is represented by only two LV. The ontogeny of this species appear to be mainly related to size increase and the apparition of sexual dimorphism in A-3.