Histriabairdia Forel 2020, gen. nov.

Genus Histriabairdia Forel gen. nov.

urn:lsid:zoobank.org:act: E3BC5C1B-2B5E-4A14-AF20-94748EC55F14

Type species

Histriabairdia pontuseuxinusensis Forel gen. et sp. nov.

Diagnosis

A moderately calcified bairdiid genus, subrectangular, with antero-ventral and postero-ventral margins laterally compressed and bordered by marginal denticles at both valves; anterior margin large; surface smooth, reticulate to spinose.

Etymology

This genus is named after the Histria Depression, where the studied borehole was drilled.

Species included

• Anchistrocheles sp. A in Bolz 1971 from the Rhaetian of Austria (Bolz 1971a, 1971b)

• Anchistrocheles? spinosa Sheppard in Brand, 1990 from the Bathonian, Middle Jurassic, of France (Brand 1990)

• Anchistrocheles sp. A from the Bathonian, Middle Jurassic, of India (Khosla & Jakhar 1999; Khosla et al. 2009)

• Anchistrocheles? tuningensis Beher, 2004 from the Sinemurian, Early Jurassic, of Germany and Switzerland (Beher 2004)

• Anchistrocheles sp. B from the Bajocian–Bathonian, Middle Jurassic, of India (Khosla et al. 2006, 2009)

• “ Anchistrocheles ” gemmellaroi Crasquin in Crasquin et al., 2018 from the Carnian of Sicily (Crasquin et al. 2018)

• Histriabairdia pontuseuxinusensis Forel gen. et sp. nov. from the Rhaetian of the Black Sea (this paper)

• Histriabairdia gen. nov. sp. 1 from the Rhaetian of the Romanian Black Sea shelf (this paper)

• Histriabairdia gen. nov. sp. 2 from the Rhaetian of the Romanian Black Sea shelf (this paper)

Questionable species

• Pustulobairdia subparalella Kristan-Tollmann, 1970 from the Ladinian, Middle Triassic, of the Dolomites (Kristan-Tollmann 1970): the general outline of this species is in line with the diagnositic features of Histriabairdia gen. nov. but the antero-ventral and postero-ventral compressions are not mentioned in its description and not visible on the drawings provided by Kristan-Tollmann (1970)

• Anchistrocheles? sp. in Beher 2004 from the Sinemurian, Early Jurassic, of Germany and Switzerland (Beher 2004) is questionably included because this species is only shown in inner view

• Anchistrocheles sp. 1 in Forel et al. 2019 from the Carnian of Turkey (Forel et al. 2019a): anteroventral and postero-ventral compressions are lacking, but the overall morphology is reminiscent of that of Histriabairdia gen. nov.; the size of the unique LV (L = 463 µm; H = 275 µm, excluding marginal spines) and the very narrow calcified inner lamella indicate that it might be an immature instar, which could explain the absence of a lateral compression as seen in the taxa described in the present contribution.

Remarks

All species from the literature re-attributed to Histriabairdia gen. nov. were originally attributed, tentatively or not, to Anchistrocheles Brady & Norman, 1889 emend. Maddocks 1976 (type species Anchistrocheles fumata Brady & Norman, 1889 subsequently designated by Brady 1890). Recent species of Anchistrocheles are known for instance from the North Sea, Irish Sea and English Channel (Brady & Norman 1889), Madagascar (e.g., Brady 1890; Maddocks 1969), Bermuda (Maddocks 1976), Mozambique (Maddocks 1969) and the Mediterranean Sea (e.g., Breman 1975; Aiello et al. 1996). Fossil species are for instance known from the Neogene of Hawaii (Brady 1890), the Plio–Pleistocene of the Mediterranean Basin (Aiello et al. 1996; Sciuto & Pugliese 2013), the Pleistocene of Italy (e.g., Colalongo 1965; Greco et al. 1974; Sciuto 2012, 2015), Rhodes (Mostafawi 1989) and Japan (Yajima 1987), the Miocene of Australia (Whatley & Downing 1984; Warne 1990), the Eocene of the USA (Swain 2000) and the Late Cretaceous of Poland (Herrig 1992). Older species from the Triassic–Jurassic time interval have been tentatively attributed to the genus Anchistrocheles in the Bathonian, Middle Jurassic, of France (Brand 1990) and India (Khosla & Jakhar 1999; Khosla et al. 2009), the Bathonian-Bajocian, Middle Jurassic, of India (Khosla et al. 2006, 2009), the Sinemurian, Early Jurassic, of Germany and Switzerland (Beher et al. 2001; Beher 2004), the Rhaetian, Late Triassic, of Austria (Bolz 1971a, 1971b), and the Carnian, Late Triassic, of Sicily (Crasquin et al. 2018) and Turkey (Forel et al. 2019a). The carapace of Anchistrocheles is small, delicate, mirror-smooth, laterally compressed, streamlined, reniform in outline, with extremely compressed margins, but this genus is mainly diagnosed by characters of the inner part of their valves (broad fused marginal zone, abundant straight radial pore canals, bythocyprid adductor muscle-scar pattern) and appendages (Maddocks 1969; Maddocks & Illife 1991). Anchistrocheles has an overall juvenile aspect as compared with Bairdiinae (Maddocks 1969). Warne (1990) furthermore considered Anchistrocheles s. str. as including forms with a reniform to subreniform outline, narrow compression along the anterior, posterior and ventral margins, and a very broad inner lamella except in the middle portion of the dorsal margin. Inner structures are only partly accessible for Histriabairdia gen. nov., but the type species displays an inner lamella that does not extend as far dorsally as in the conception of Warne (1990), excluding it from Anchistrocheles. Histriabairdia spinosa (Sheppard in Brand, 1990) gen. et comb. nov. from the Bathonian, Middle Jurassic, of France (Brand 1990) has muscle scars described has “typical bythocyprid-type with anterior row of 3 horizontal oval scars and 1 posteroventral scar”. The inner structures of these Triassic–Jurassic forms are visible in Beher (2004), but their preservation is poor and only the extension of the inner lamella is visible.

The subfamilial position of Anchistrocheles has been unstable and it is now placed in the Pussellinae: they are medium to small-size Bairdiidae with a smooth or weakly punctate, elongate, laterally compressed, thin-walled carapace, conspicuous ventral indentation and anteroventral angle, normal pores funnelshaped or with low, flat wall, width of fused zone and number of distinct radial pore canals proportional to size of carapace, bythocyprid muscle-scar pattern (5 scars, adductor scars arranged in an anterior row of three horizontal scars plus one posteroventral scar, all scars may be divided but not separated; Maddocks 1969) and an adont hinge (Danielopol in Maddocks 1976). Pussellinae are adapted to phytal, interstitial and possibly cryptic habitats, mostly in tropical reefs, as shown by the morphological adaptations of their antennae, posterior legs and the laterally compressed, streamlined, mirror-smooth, fragile carapace with broadened marginal zone of concrescence (Maddocks 1976; Maddocks & Illife 1991). Conversely, the Bairdiinae are ovate to subhexagonal in lateral view, with LV larger than RV and overlapping it dorsally and ventrally, usually of different shapes; the RV hinge is a simple bar with a dorsal groove and the LV hinge is an incised groove with a dorsal shelf and ventro-terminal triangular sloping platforms; the adductor muscle-scar pattern is characteristically composed of 8–10 scars arranged in three anterior plus one postero-ventral rows of 2–3 scars each (Maddocks 1969). According to the emended diagnoses of the Pussellinae Danielopol in Maddocks, 1976 and of Anchistrocheles (Maddocks 1976), they are characterized by a smooth or weakly punctate carapace. This character clearly excludes most of the present species from the actual genus Anchistrocheles and from the Pussellinae. The spinose anteroventral and postero-ventral margins are reminiscent of recent Bairdiidae (e.g., Maddocks 1969).

Histriabairdia gen. nov. differs from Bairdia McCoy, 1844 by its conspicuous AV and PV lateral compressions on both valves, overall rectangular outline of the carapace and surface ornamentation ranging from smooth to reticulate and spinose. Pustulobairdia Sohn, 1960 differs from Bairdia by having its surface covered with pustules. However, Pustulobairdia also lacks the AV and PV lateral compressions and rows of spines that are characteristic of Histriabairdia gen. nov. Furthermore, the visible pustules expressed at the surface of the type species are specific characters, contrary to the interpretation of Sohn (1960) for Pustulobairdia. Lobobairdia Kollmann, 1963 is diagnosed as having AV and PV laterally compressed into deep furrows. However, Lobobairdia has a highly convex dorsal margin with a much lower anterior margin.