Mirabairdia longispinosa Kristan-Tollmann, 1978

Forel, Marie-Béatrice, Kolar-Jurkovšek, Tea & Jurkovšek, Bogdan, 2020, Ostracods from the ‘ Raibl Beds’ (Carnian, Late Triassic) of Belca section in Karavanke Mountains, northwestern Slovenia, Geodiversitas 42 (21), pp. 377-407 : 386-388

Mirabairdia longispinosa Kristan-Tollmann, 1978 ( Fig. 6 E-H)

Mirabairdia longispinosa Kristan-Tollmann, 1978: 94-96 , pl. 3, figs 1-3; pl. 7, fig. 5. — Forel et al. 2018: 10, figs 4-20, 4-21.

EXAMINED MATERIAL. — Eleven left valves, 18 right valves and several fragments.

OCCURRENCE. — Cordovelian, early Carnian, Late Triassic, Ruones- Wiesen, Pralongia, Dolomites, Italy (Kristan-Tollmann 1978); Spongotortilispinus moixi radiolarian Zone, early Tuvalian, late Carnian, Late Triassic, Tavusçayiri Block, Sorgun Ophiolitic Mélange, southern Turkey, Huğlu Tuffite (Forel et al. 2018); Nicoraella ? budaensis conodont zone, late Julian-?earliest Tuvalian, Carnian, Late Triassic, samples BE2, 17, 18, 21, 23/1 ( Fig. 3 ), Belca section, ‘Raibl Beds’, Karavanke Mountains, Slovenia (this work).

DIMENSIONS. — Fig. 5D.

DISCUSSION

Mirabairdia longispinosa Kristan-Tollmann, 1978 has been found in all samples of the Belca section where is not abundant except in sample BE18 ( Fig. 3 ). In the current state of our knowledge, this species is restricted to the Carnian of the western tethyan realm. The present record is the third known occurrence of M. longispinosa worldwide: it was described from the Cordovelian, early Carnian, Late Triassic, of Italy (Kristan-Tollmann 1978), and was more recently documented from the Tuvalian, late Carnian, Late Triassic, of Turkey (Forel et al. 2018). The present assemblage is unique in providing numerous specimens, making it possible to discuss the ontogenetic development of this species

( Fig. 5D ). It first appears that the holotype of M. longispinosa (left valve; Kristan-Tollmann 1978) is larger than all other known specimens. Therefore, if the holotype is assumed adult, then all specimens recovered from the Carnian layers of Belca and Turkey should be juveniles. However, until inner structures are available for this species, this hypothesis is difficult to confirm. Because of the important size gap between the holotype and the largest specimens in Belca, which is 800 µm in length, we decide not to attribute the successive scatter-points observable in Fig. 5D to any given ontogenetic stage. We rather discuss the successive ontogenetic groups relatively to the surrounding ones, without assum - ing precisely the stage they might correspond to. The right and left valves obtained in Belca gather into four successive scatter plots ( Fig. 5D ). The ontogeny of M. longispinosa is marked by the strengthening of the lateral sculpture of the valves, with short and frail spines in the smallest left valve developing into robust and thick spines in the largest left valve ( Fig. 5D ). As a rule, the left valves are more rounded and plump than the more elongate right valves: this pattern is observed quite early in the development of the species, although the smallest specimens are here only represented by left valves.

Figures

FIG. 6. — Ostracods from the Belca section,Karavanke Mountains,northwestern Slovenia,Carnian,Late Triassic.All specimens are deposited in the micropalaeontology collections of the Geological Survey of Slovenia, under the numbers GeoZS6132 to GeoZS6239,preceded by sample number:A-C, Isobythocypris? sp. 1; A, lateral view of a right valve,BE2 (GeoZS6153);B, lateral view of a right valve,BE2 (GeoZS6154);C, lateral view of a right valve,BE2 (GeoZS6155);D, Isobythocypris sp. 2, lateral view of a right valve, BE18 (GeoZS6156); E-H, Mirabairdia longispinosa Kristan-Tollmann, 1978; E, lateral view of a right valve, BE21 (GeoZS6157); F, lateral view of a right valve, BE18 (GeoZS6158); G, lateral view of a right valve, BE18 (GeoZS6159); H, lateral view of a left valve, BE17 (GeoZS6160); I, Bythocypris? sp. 1, lateral view of a right valve, BE18 (GeoZS6161); J-L, Bythocypris? sp. 2; J, lateral view of a right valve, BE18 (GeoZS6162); K, lateral view of a left valve, BE18 (GeoZS6163); L, internal view of a right valve, BE18 (GeoZS6164); M-O, Bythocypris? sp. 3; M, right lateral view of a complete carapace, BE17 (GeoZS6165); N, lateral view of a left valve, BE18 (GeoZS6166); O, lateral view of a right valve, BE18 (GeoZS6167); P, Paracypris sp., lateral view of a left valve, BE18 (GeoZS6168); Q, Pontocyprella sp., lateral view of a left valve, BE18 (GeoZS6169); R-U, Pontocypris? karavankensis Forel, n. sp.; R, Paratype, right lateral view of a complete carapace,BE18 (GeoZS6170); S, Holotype, right lateral view of a complete carapace,BE18 (GeoZS6171); T, lateral view of a left valve, BE18 (GeoZS6172); U, lateral view of a left valve, BE18 (GeoZS6173). Scale bars: 100 μm.

FIG. 3. — Distribution of ostracod species through the Belca section, Karavanke Mountains, northwestern Slovenia.

FIG. 5. — Height/Length scatter plots of species recovered from the Belca section, Karavanke Mountains, northwestern Slovenia, Carnian, Late Triassic. In all diagrams, the dimensions of right and left valves of complete carapaces are linked. A, Bairdia balatonica Méhes, 1911; B, Bairdia cassiana (Reuss, 1869); C, Dicerobairdia buseri Forel, n. sp., all specimens are from the Belca section (this work); D, Mirabairdia longispinosa Kristan-Tollmann, 1978; E, Pontocypris? karavankensis Forel, n. sp. all specimens are from the Belca section (this work). Scale bars: 100 μm.