Halicyne agnota von Meyer 1844
Creators: name: Pieroni, Vittorio
Publisher: name: Zenodo
Year: 2024
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Maxillopoda
Cyclida
Cyclidae
Halicyne

description: Halicyne cf. agnota (von Meyer, 1838) Fig. 3a–d.
cf. 1971 Halicyne agnota (von Meyer, 1838): Zorn, p. 80, pl. 15, fig. 36, (morphotype 2).
Material examined a single small specimen MSNVI-SOST-01 (repository: MSNVI, Museo di Storia Naturale “A. Stoppani” Venegono Inferiore, VA, Italy). Other fossils were not found in the same level.
Description Carapace ovate, slightly longer than wide, narrowing posteriorly, smooth to more or less finely granular. Frontal margin overall straight, with concavities at distal extremes that may be orbital notches. Marginal rim flat, beginning at outer-orbital angle, extended at the second lateral lobe and laterally reduced, terminating in a strong posterior spine. Marginal rim positioned below remainder of carapace surface, rim with smooth surface and smooth margin. Axial region long, narrow, extending more than 75 percent length of carapace. Posterior axial lobe elongate, slightly constricted at about mid-length; first axial lobe polygonal, weakly differentiated from posterior axial lobe, second axial lobe elongate, flattened, wider at mid-region; first lateral lobes elongated, narrow; second lateral lobes oblique, strongly inflated, slightly reniform, elongate, terminating anteriorly just posterior to orbital notch. Third lateral lobes subdivided posteriorly towards the first axial lobe. Inner lyrate keel weak, short, separated from long outer keel extending from posterior of second lateral lobe, arcing axially crossing axis.
Measurements (carapace) length 5 mm; width 4.3 mm; maximum thickness 1.7 mm; total width of the lateral lobes area 3.7 mm; total length of the axial region 3.8 mm.
Remarks Zorn (1971) described three specimens from Monte San Salvatore (uppermost Middle San Salvatore Dolomite, Gervillienhorizont, Early Ladinian, Curionii Zone, 70 m E from P. 856). He described also other specimens from the localities near Kaisten (Canton Aargau, CH, Trigonodusdolomit) and Edlisberg (near Waldenburg, Canton Baselland, CH, Middle Muschelkalk). Among these specimens, Zorn identified three morphotypes. The first one has a carapace wider than long (two specimens from Monte San Salvatore); the second one has a carapace longer than wide, with a reduced marginal rim and well-developed anterior lateral lobes (one specimen from Monte San Salvatore); the third one based on juvenile specimens, with carapace longer than wide and large keel region (from other localities). The present specimen fits very well with the second morphotype.
Occurrence The present specimen was found in the San Salvatore Formation of Sostegno (BI, Italy), probably in a Lower Ladinian bed.
Other occurrences The species H. agnota was previously documented from the epicontinental Middle Triassic of Germany (Baden-Württemberg, Rottweil Formation, Muschelkalk) and northern Switzerland (Trigonodusdolomit and Middle Muschelkalk), from the Middle Triassic of the Eastern Alps in Switzerland (Ducan-Landwasser area, Silveretta Nappe, Prosanto Formation) and from the Middle Triassic of the Southern Alps in Switzerland (see Bürgin et al. 1991; Furrer, 2019; Schweitzer et al. 2020, and the varieties described by Zorn, 1971 from Monte San Salvatore).
Paleoecology and environment
According to Zorn (1971), H. agnota could have lived in a hypersaline environment corresponding to the intertidal environment of the Lower Ladinian Gervillienhorizont at Monte San Salvatore. The specimens described by Zorn from Monte San Salvatore were collected with only two species of bivalves: Modiolus salzstettensis Hohenstein, 1913 and Bakevellia costata (von Schlotheim, 1820), one fragment of ammonoid (Monophyllites sp.) and a reptile remain (vertebra). H. agnota was documented also in the upper Prosanto Formation (Early Ladinian) of the south-eastern Swiss Alps (Ducan and Landwasser region near Davos, Canton Graubünden, Austroalpine Silvretta Nappe), with a paleoenvironment well comparable to the Besano/Monte San Giorgio Basin (Bürgin et al., et al. 1991; Furrer, 2019), characterized by anoxic to dysoxic bottom water conditions. Rare Cyclida were cited by Stockar and Garassino (2013) from the Sceltrich beds (Early Ladinian, lowermost part of Upper Meride Limestone, Monte San Giorgio, Ticino, Switzerland) and also by Furrer and Vandelli (2014: fig. at p. 89) from the lower Meride Limestone (Early Ladinian) (two unpublished specimens in the Cava superiore beds near Meride: personal information by Heinz Furrer, 21/10/2023). The neighboring partly time-equivalent San Salvatore Formation, Besano Formation, Meride Limestone, and San Giorgio Dolomite were deposited mainly between Lugano and Varese (Monte San Giorgio/ Besano Basin). However, the geographical distribution of the San Salvatore Formation is wider (Fig. 1b) and the here-described locality represents the southwestern region. Therefore, the geographical distribution of H. agnota (and its varieties) extended from the Muschelkalk basin of southwestern Germany (Baden-Württemberg) to the Tethys realm of the western part of the Eastern Alps (Ducan-Landwasser area, Canton Graubünden) to the western part of the Southern Alps (Monte San Giorgio/ Besano and also Biellese area).
Nothing definitive is known about the mode of life of Cyclida. Gall and Grauvogel (1967) suggested a benthic, predatory lifestyle of the Triassic Halicyne whereas Müller (1955) tentatively supposed a parasitic lifestyle, having lived on fishes. The latter interpretation would be supported by the abundance of fishes and other nektonic vertebrates in the finding horizons, where benthic organisms are almost lacking. Schram et al. (1997) suggested that the frequent association of many Paleozoic Cyclida with plant material might reflect a herbivorous or detritus-eating habit. There are several cyclidan species known from transitional marine or brackish environments to lake conditions (Schweigert, 2007; Schweitzer et al. 2020). For example, Halicyne plana (von Seebach, 1857) was documented from the Ladinian Erfurt Formation in Thuringia (Germany), where different kinds of fish, amphibians, and archosauriforms have been found, suggesting a lake or brackish environment (Schweitzer et al. 2020).
description: Published as part of Pieroni, Vittorio, 2024, The first Cyclida from the Triassic of Italy, pp. 1-8 in Swiss Journal of Palaeontology (9) (9) 143 (1) on pages 3-6, DOI: 10.1186/s13358-024-00306-9, http://zenodo.org/record/12006703