AN INSIGHT INTO THE SYSTEMATICS OF PLICATOSTYLIDAE (BIVALVIA), WITH A DESCRIPTION OF PACHYGERVILLIA ANGUILLAENSIS N. GEN. N. SP. FROM THE LITHIOTIS FACIES (LOWER JURASSIC) OF ITALY

To


IntroductIon
The Early Jurassic is characterized by a remarkable spread of gregarious bivalves with large and thick shells that proliferated in the tropical shallow-water marine environments of the Tethyan and Panthalassan coasts.This global bioevent is recorded by thick shell accumulations which produced large sedimentary bodies of the Lithiotis facies (e.g., De Castro 1962;Bosellini 1972;Nauss & Smith 1988;Buser & Debeljak 1995;Leinfelder et al. 2002;Fraser et al. 2004;Posenato & Masetti 2012;Posenato et al. 2018;Brame et al. 2019).The most common and peculiar bivalves of this facies are three aberrant pterioid genera known as lithiotids: Lithiotis, Cochlearites and Lithioperna, which have been studied from systematic and paleobiological points of view by several authors (e.g., Accorsi Benini & Broglio Loriga 1977;Accorsi Benini 1979;Chinzei 1982;Posenato et al. 2022 with references therein).These genera are mostly restricted to the Pliensbachian and disappeared at the onset of the early Toarcian oceanic anoxic event (e.g., Fraser et al. 2004;Posenato et al. 2018).
The Lithiotis facies contains other gregarious and rock-forming bivalves such as the pterioids Gervilleioperna and Mytiloperna, the myalinid Pseudopachymytilus, the carditid Opisoma, and the megalodontids Pachymegalodon and Protodiceras.Among these genera, only Opisoma has been recently analyzed from systematic (Accorsi Benini 1981) and morpho-functional (Posenato et al. 2013) points of view, while only preliminary data have been published on the microstructures of the other taxa (Benini & Loriga 1974;Accorsi Benini 1980;Accorsi Benini & Broglio Loriga 1982).Among the above-cited genera, only specimens from the Lithiotis facies referred to Mytiloperna were not attributed to a species.Indeed, these specimens were determined by Benini & Loriga (1974) and Accorsi Benini & Broglio Loriga (1982) in open nomenclature as Isognomon (Mytiloperna) sp.ind.or by Broglio Loriga & Posenato (1996) as Mytiloperna sp.This taxon has been recovered from the Southern Apennines (Posenato et al. 2018) and Morocco (Fraser et al. 2004;Brame et al. 2019).Considering its wide geographical distribution and the lack of a specific assignment, a systematic revision of this taxon becomes necessary.This study, based on specimens from museum collections, analyzed both shell morphology and microstructural characters of Isognomon (Mytiloperna) sp.ind.or Mytiloperna sp. to better clarify its taxonomic position and to consider its relationships with the other pterioid bivalves of the Lithiotis facies.

StratIgraphIc SettIng, provenance of the materIal and termInology
The studied material, stored in the Ferrara, Pavia and Camposilvano (Verona) museum collections, has been collected in the Rotzo Formation of the Calcari Grigi Group in northern Italy (Lower Jurassic), which is characterized by remarkable lateral and vertical facies variability.The formation was deposited in a lagoonal environment which was closed seawards by oolitic shoals and bars and bordered landwards by marshes (e.g., Bosellini & Broglio Loriga 1971;Clari 1975;Posenato & Masetti 2012;Fig. 1).Unlike in other areas, where the bivalves of the Lithiotis facies are contained in very hard limestone, the bivalves of the Rotzo Formation in some cases can be easily extracted from marlstone or claystone.Additionally, in some localities of the Trento Platform, the shells preserve their original mineralogical composition and microstructure (Posenato et al. 2022).
More than half of the "Mytiloperna sp." specimens of the Benini & Loriga (1974) collection from Vajo dell'Anguilla consists of articulated shells, suggesting a biogenic shell accumulation sensu Kidwell & Jablonski (1983).However, the shells are strongly abraded.In some articulated specimens, the body chamber has been almost completely removed.The outer shell ornamentation is usually preserved only on the anterior surface, which was protected from erosion because resting in the substrate.Despite the strong abrasion, the shells maintained their autochthonous position because they were tightly packed and partially buried (Accorsi Benini & Broglio Loriga 1982).
"Mytiloperna sp." has been also recognized in the Taramelli collection, stored in the Kosmos Museum of the Pavia University.The specimens are represented by an articulated and calcitized shell and by a small disarticulated right valve, both coming from the Premaloch Valley, near Asiago (Fig. 1).These specimens were assigned by Boehm (1884) to "Perna aff.Taramellii".Perna taramellii Boehm, 1884, was proposed for specimens housed in the Pavia and Verona museums, collected in the nineteenth century from Vajo del Paradiso (southern Lessini Mountains, Trento Platform), where the succession of the Rotzo Formation is about 60 m-thick.Based on the absence of Orbitopsella this succession is considered late Pliensbachian in age (Posenato & Masetti 2012).Most of the studied specimens are stored in the Museo "Piero Leonardi" of the University of Ferrara (MPL acronym).This material has been studied by Benini & Loriga (1974), Accorsi Benini &Broglio Loriga (1982), andBroglio Loriga &Posenato (1996).The other analyzed specimens are housed in the Kosmos Museum of the Pavia University (KM acronym) and in the Museo Geopaleontologico di Camposilvano (Verona; MC acronym; see the list of material in Tab. 1).We hereafter refer to these examined specimens of the Lithiotis facies, not assigned to a species, as belonging to "Mytiloperna sp." or "Mytiloperna", to distinguish them from the other species of the genus Mytiloperna von Ihering, 1903.
The bivalves of the Lithiotis facies are often characterized by aberrant shells and therefore different terminology has been used in the literature (Fig. 3).The morphological terminology of sticklike shells of Lithiotis and Cochlearites follows Posenato et al. (2022) who indicated as cardinal area the middle part of the elongated umbonal region where valves are in contact (Figs 3a1,b1).This area is anteriorly and posteriorly flanked by the feather-like or growth line areas.In the small body cavity, a myofringing crest connects the ventral extremity of the cardinal area to the muscle scar.
Since these bivalves have an unusual shell morphology, the morphological terminology adopted here (Fig. 3) needs some clarifications.The inner shell cavity has a wide median cardinal area which has been also indicated as contact or sub-ligamental area (Accorsi Benini 1979;Broglio Loriga & Posenato 1996).This surface may bear irregular, elongated and subparallel teeth.A broad and strong anterior radial carina delimits an anterior hearth-shaped flattened or concave surface representing the resting surface.This surface, here indicated as the anterior area, has been also named lunule (Broglio Loriga & Posenato 1996).However, it cannot be considered a true lunule, as the latter is defined as a depressed anterior area separated from the surrounding shell surface by an incised line or a change in sculpture and/or color (Carter et al. 2012).h, crinoids; i, Lychnothyris rotzoana (Schauroth, 1865) (Brachiopoda); j, undetermined bivalves; k, Lithioperna scutata (Dubar, 1948); l, roots.

SyStematIc paleontology
The classification below follows that of Carter et al. (2011).
Tab. 1 -Measurements of the studied specimens of Pachygervillia anguillaensis n. gen.n. sp.
of prism, as thick as the underlying nacreous tablets.
The nacre tablets decrease in thickness and increase in width towards the main part of the nacreous layer.
The irregular fibrous prismatic microstructure and its gradual transition to the nacre microstructure could represent the ancestral stage from which the highly specialized FISphP microstructure (Posenato et al. 2022) of the family Plicatostylidae originated.This microstructural type, associated with a nacreous middle layer and an outer calcitic simple prismatic layer, is here proposed as the most important taxonomical character of the emended family Plicatostylidae which contains both the lithiotids (Cochlearites and Lithiotis) and the other pterioid bivalves of the Lithiotis facies (Lithioperna, "Mytiloperna" and Gervilleioperna).According to Bieler et al. (2014) bivalve taxa have different shell layers combined with different microstructures, whose differences are believed to be of phylogenetic and adaptive biomechanical significance.Indeed, microstructural information has been widely used as an important character in phylogenetic analyses, being also a feature that can be easily The internal septum of the Cassianellidae, placed posteriorly to the anterior muscle scar, corresponds to a myofringing ridge (Carter et al. 2012).The ridge in the monospecific genus Cochlearites is instead a myofringing crest because it is located on the posterior muscle scar.The anterior or posterior position of this crest with respect to the muscle scar depends on the shell orientation which is controversial because only very rare specimens preserve portions of the body cavity, therefore the systematic value of these ridges is questionable.
The most important character linking the Cassianellidae to Plicatostylidae seems to be the shell microstructure.The former family usually has an outer calcitic prismatic shell layer and middle and inner nacreous layers (Carter 1990).Unlike the other cassianellid species, Carter (1990) noted that Cassianella decussata (Goldfuss, 1836) has an outer shell layer with an aragonitic irregular fibrous prismatic microstructure.Prisms are 35 -40 μm-thick and 0.5 -2.0 μm-wide and show a gradual transition to the middle nacreous layer with the appearance of horizontal subdivisions inspected in fossil taxa for phylogenetic placement (e.g., Bieler et al. 2014).The FISphP microstructure occurs in "Mytiloperna sp." and Gervilleioperna (Accorsi Benini & Broglio Loriga 1982;Broglio Loriga & Posenato 1996).These two multivincular genera were included by Cox (1969) in the family Isognomonidae, which is now considered a subfamily (Isognomoninae) of Malleidae (superfamily Pterioidea) (Carter et al. 2011)."Mytiloperna sp." and Gervilleioperna ombonii (Negri, 1891), two species occurring in the Lithiotis facies, have a thick inner shell layer with an aragonitic "fibrous prismatic" microstructure (Accorsi Benini & Broglio Loriga 1982).The microstructure of the first species has been here re-examined (see below) and corresponds to the FISphP microstructure, recently recognized in Lithiotis and Cochlearites (Posenato et al. 2022).The same microstructure is present in G. ombonii and, although with shorter prisms, in Lithioperna (Benini & Broglio Loriga 1982;Broglio Loriga & Posenato 1996).The FISphP microstructure can be therefore used as a taxonomic character at family level which links Lithiotis and Cochlearites, already included in the fam.Plicatostylidae, to Lithioperna, Gervilleioperna and "Mytiloperna".These pterioid taxa have quite different shell morphologies due to their gregarious and sessile (byssate or cemented) life habits.However, they share thick and large monomyarian shells with a small body cavity and a large median cardinal area.
Aragonitic prismatic microstructures also occur in other genera of the Lithiotis facies such as the myalinid Pseudopachymytilus (family Myalinidae, order Myalinida), the carditid Opisoma (family Astartidae, order Carditida), and the megalodontid Pachymegalodon (family Pachyrismatidae, order Megalodontida).However, these prismatic microstructures are not associated with nacre but occur with the complex crossed-lamellar microstructure which is missing in the emended Plicatostylidae.Also, the prisms of the above-cited genera are shorter and thinner (myostracal type prisms of Accorsi Benini & Broglio Loriga 1982) than those of the FISphP microstructure and originate in Opisoma from a modification of the 2 nd order lamellae which assume a columnar arrangement (Accorsi Benini 1980).Subfamily Plicatostylinae Lupher & Packard, 1929 Emended diagnosis: Shells large, thick, very elongated dorsoventrally, compressed, slightly to strongly inequivalve, attached by the thicker valve; umbonal region very elongated, with a median cardinal area flanked by growth line areas; body cavity small with myofringing crest, hinge edentulous.Remarks.The emended diagnosis of the subfamily is based on the description of the family ?Lithiotidae by Cox (1971).The subfam.Plicatostylidae is composed of three monospecific genera: Plicatostylus, Cochlearites, and Lithiotis.Accorsi Benini & Broglio Loriga (1977) considered Plicatostylus as a junior synonym of Lithiotis.The morphological differences between Plicatostylus gregarious Lupher & Packard, 1929 and Lithiotis problematica Gümbel, 1871 could justify only a separation at species level.However, Accorsi Benini & Broglio Loriga (1977) left the specific question unresolved due to the scarcity of material at their disposal.Cochlearites loppianus (Tausch, 1890) and Lithiotis problematica Gümbel, 1871 have been carefully described by Accorsi Benini & Broglio Loriga (1977) and Chinzei (1982).These bivalves are characterized by very large and aberrant shells whose orientation is debated.Reis (1903) interpreted both the attached valves of Lithiotis and Cochlearites as right valves.Accorsi Benini & Broglio Loriga (1977), orienting the shell on the base of the usually posterior setting of the muscle scar in monomyarian bivalves, considered both the larger and attached valves of Cochlearites and Lithiotis as the left ones, although they did not exclude that Lithiotis could also attach with the right valve.Chinzei (1982), based on the more expanded anterior space for gills accommodation in monomyarian bivalves, considered the attached val-Tab. 2 -Changes of classification at suprageneric level of the pteriod genera included in the emended fam.Plicatostylidae.
ve of Lithiotis as the right one but that of Cochlearites as the left.In the latter genus the myofringing crest is not as raised as figured by Reis (1903, figs 1, 2), but it is represented by weak undulations which may occur on both sides of the muscle scar (Reis 1923;Accorsi Benini & Broglio Loriga 1977;Chinzei 1982).In the living Isognomon isognomum (Linnaeus, 1758) and I. ephippium (Linnaeus, 1758) the muscle scar is reniform with the convex side oriented postero-ventrally (Yonge 1968: figs 7, 8).Following this criterion, the attached valves of Lithiotis with the preserved muscle scar (e.g., Accorsi Benini & Broglio Loriga 1977: pl. 1, fig.1a) can be considered as the right valve (Fig. 3a1).The attached valves of Cochlearites (Reis 1923: fig. 1;Accorsi Benini & Broglio Loriga 1977: pl. 1, fig.1a) have the muscle scar oriented in the opposite direction (Fig. 3b1); therefore, they represent left valves.This shell orientation, in agreement with that proposed by Chinzei (1982), is confirmed by the muscle scar orientation occurring in other allied genera (e.g., Lithioperna and Pachygervillia n. gen., Figs 3c1, d1), the shell orientation of which follows the conventional criteria used for the bivalves (e.g., Carter et al. 2012).
The most significant difference between Lithiotis and Cochlearites is the morphology of the elongated cardinal area occurring in the middle umbonal region and the degree to which the valves are unequal (Figs 3a3,b3).The shell of Lithiotis is strongly inequivalve.It has a laminar, few mm-thick left valve, while the right valve is 1-3 cm-thick.Cochlearites is slightly inequivalve; the left valve is slightly thicker than the right one.In Lithiotis, the cardinal area is furrowed by narrow and parallel ligament grooves (cardinal area or furrowed plate) suggesting an origin from a multivincular bivalve (Savazzi 1996).In Cochlearites, the middle umbonal region is smooth and with an elongated groove in the left valve corresponding to a bulge in the right valve.It has only a few and irregular ligament grooves in the early stages of growth (Figs 3b1-b3).In the Rotzo Formation Cochlearites occurs stratigraphically above Lithiotis and the genus is very common in the upper part of the formation, in the Lituosepta compressa Zone (upper Pliensbachian; Posenato & Masetti 2012), while L. problematica is common in the upper Orbitopsella Zone (lower Pliensbachian).This latter species is also recorded in the lower Toarcian of South America (Peru and Chile; Hillebrandt 1982).Plicatostylinae, as well as most part of the large bivalves of the Lithiotis facies, have been affected by the early Toarcian mass extinction (Fraser et al. 2004;Posenato et al. 2018).
Type genus: Pachygervillia n. gen.Derivation of name: From Pachygervillia n. gen.Diagnosis: Shell medium-sized to large, from subquadrate, mytiliform to ensiform, flattened to strongly inflated; subequivalve to highly inequivalve with left valve more convex than right valve; posterior wing undifferentiated to well-defined, anterior auricle small or missing; outer ornamentation concentrically lamellose; commissure plane flat or undulating; outer multivincular ligament area high and with numerous pits; with or without byssal gape affecting both valves; cardinal area large and smooth or with irregular and elongated teeth in adult stage.
Remarks.This new subfamily contains Gervilleioperna Krumbeck, 1923, Lithioperna Accorsi Benini, 1979, and Pachygervillia n. gen., whose specimens were previously known in the literature as Isognomon (Mytiloperna) sp. or Mytiloperna sp.(Benini & Loriga 1974;Accorsi Benini & Broglio Loriga 1982;Broglio Loriga & Posenato 1996).The former genus has been included in the family Isognomonidae by Cox (1969) (Crampton, 1988).However, the occurrence of teeth at the adult stage in Gervilleioperna prompted some authors (e.g., Damborenea 1987) to place it in the fam.Bakevellidae King, 1850, according to the Cox (1969)  Bakevellidae, to the superfamily Pterioidea Gray, 1847.Taxa of the latter superfamily are characterized in both valves by an outer shell layer composed of regular prismatic calcite and by nacreous middle and inner shell layers.The middle and inner layers are separated by a thin pallial myostracum (Taylor et al. 1969).According to Cox (1969) Isognomonidae (now subfam.Isognomoninae) probably evolved from the Bakevellidae (Cox 1969;Stanley 1972) losing the Pteria-like form.They have a similar shell microstructure, but they differ in the microstructure of the outer calcitic layer, which can be regular simple prismatic, irregular prismatic or homogeneous in the Bakevellidae, whereas in Isognomonidae (now Isognomoninae) it can be only regular simple prismatic (Carter 1990).
Gervilleioperna and Lithioperna are here moved to the subfamily Pachygervilliinae nov., which comprises also Pachygervillia n. gen., based on the occurrence in these genera of the peculiar combination of FISphP and nacre microstructures, which do not occur in other genera of the Bakevellidae and Isognomoninae.Gervilleioperna, Lithioperna and Pachygervillia n. gen.are characterized by an inner large median cardinal area (area of contact of Accorsi Benini 1979), separated from the anterior margin by a conical appendix of the body cavity and, occasionally, from the posterior margin by a flattened appendix (Fig. 3).The dorsal apex of both the appendices reaches the ventral extremity of the ligamental area.A dorso-ventrally elongated cardinal platform acted as a fulcrum for shell opening and closing.This mechanism was based on the flexibility of the ventral margin predominantly composed of a thin outer calcitic prismatic layer rich of intercrystalline organic matrix as already described for the Plicatostylinae (Posenato et al. 2022).The cardinal area may bear elongated and irregularly distributed teeth obliquely inclined with respect to the ventral margin of the ligamental area; they are more numerous in adult and large shells.The teeth are common in Pachygervillia n. gen., less common in Gervilleioperna, while they are rare in Lithioperna (e.g., Broglio Loriga & Posenato 1996, pl. 4, fig.1; Fig. 3).The occurrence or lack of teeth, their different length and number can be considered as intraspecific variability, therefore this character has little systematic value.
The middle Eocene Pachyperna laverdana Oppenheim, 1900 (type species of Pachyperna Oppenheim, 1900) is a large and multivincular bivalve (Figs 3f1, f2) having a shell morphology and microstructure closely related to the genera included into the new subfamily Pachygervilliinae.This species has a boulder shape and an extremely thick shell.At the juvenile stage it had an epibyssate, pleurothetic lifestyle which became reclining orthothetic in the adult stage.The body cavity is very small, with a large cardinal plate occasionally carrying teeth in large shells (Posenato 1995).In addition to these morphological characters, Pachyperna shares with the above-described Jurassic bivalves also the same aragonitic FISphP microstructure of the inner shell layer, although the nacreous middle layer has not yet been documented.The FISphP microstructure consists of extremely elongated, fibrous and fusiform prisms with a highly variable outline in crosssection (Posenato 1995, fig.11d-e).The prisms are several mm-long and about 30-40 μm in width.They are arranged in bundles with opposite orientations, as in Pachygervillia n. gen.(see below).Therefore, we propose to move this genus to the Pachygervilliinae nov.subfam.Further analyses are required to check the occurrence of the middle nacreous layer confirming or not the present attribution.
All the genera included in this new subfamily show a close correspondence in the morphological and microstructural characters of the shell, supporting the proposal to include them in the same systematic group and to reject the hypothesis that the proposed group is based on analogous characters and represents adaptive convergence.
In the Trento Platform, the appearance of Pachygervillinae is recorded by Gervilleioperna buchii (De Zigno, 1870) which occurs in the Monte Zugna Formation, at the transition between the turneri and  Pachygervillia n. gen.
Derivation of name: the name is a combination of two terms: Pachys (Greek) means thick and Gervillia is a genus of the fam.Bakevellidae.
Diagnosis: Shell subrectangular to mytiliform, subequivalve, large and thick with anterior carina running from beak to anteroventral margin and separating a large anterior area from the rest of the shell; posterior wing undifferentiated; ligament area flat with numerous irregularly spaced ligament grooves; cardinal area large with elongated, usually oblique and irregular teeth.
Remarks.In the past, the majority of the studied shells have been determined as Isognomon (Mytiloperna) sp.ind.(Benini & Loriga 1974;Accorsi Benini & Broglio Loriga 1982) or Mytiloperna sp.(Seilacher 1984;Broglio Loriga & Posenato 1996;Fraser et al. 2004).The genus Mytiloperna von Ihering, 1903 (type species Perna americana Forbes in Darwin 1841 from the Jurassic of Chile) is characterized by equivalve and mytiliform shells with undifferentiated wings, strong anterior carinas, few and widely spaced ligamental grooves and by lack-ing a byssal notch and gape.As discussed above, the different microstructure of the inner shell layer of "Mytiloperna" (= Pachygervillia n. gen.) suggests a different position of these specimens at the family level than previously proposed in the literature.The occurrence of a large and toothed cardinal platform at the adult stage and the thick shell represent other remarkable differences to distinguish the specimens assigned to this new genus from those of Mytiloperna, which occasionally have only small and few teeth (e.g., Mytiloperna patchamensis Cox, 1940;Jaitly et al. 1995: text-fig. 19).

Derivation of name:
The name derives from Anguilla, which is the name of the valley (Vajo) of the Lessini Mountains (Verona Province), which is the type locality.
Diagnosis: Pachygervillia with subrectangular to ensiform outline; shell thick as inflated with strong anterior carina, umbo terminal and slightly twisted, occasionally with a short and blunt anterior wing; anterior area large with flattened or concave surface, byssal gape inconspicuous.
Description.Shell large, subequivalve and thick with a rather variable outline ranging from subrectangular to elongated mytiloid; beak terminal and slightly twisted; shell obliquity (angle between hinge margin and anterior carina) ranging from ca. 30° to 80°; occasionally with a short and blunt anterior wing.Anterior carina large and representing the maximum shell thickness, running from the umbo to the anteroventral margin and delimiting a broad and high anterior area ranging from flat to concave, inward inclined or perpendicular to commissural plane and furrowed by an oblique byssal groove; shell thickness gradually decreasing in ventral and posterior directions; anterior margin S-shaped, dorsal margin straight to gently convex, posterior margin straight to slightly convex, ventral margin not preserved; no distinct byssal gape.Ornamentation consisting of dense and irregularly distributed growth scales usually better preserved in the anterior area.Ligamental area multivincular, high and extending ca.1/5 or less of the preserved valve height; ligamental grooves large, usually about ten in adult individuals and, in most cases, as wide as crests or irregularly spaced.Cardinal platform large, irregular in outline, with subparallel and irregularly distributed teeth; their inclination corresponds to the shell obliquity; adductor scar reniform and located in the anterior-median part of the inner surface; anterior part of body cavity conical in shape with its apex directed anterior-dorsally, posterior part of body cavity, behind the cardinal platform, narrow and flattened.
Shell microstructure.The shell of Pachygervillia anguillaensis n. gen.n. sp.consists of three layers (Pl.5).The outer layer, not preserved in the studied specimens, is made up of calcitic simple prisms (Accorsi Benini & Broglio Loriga 1982).The middle layer is composed of nacre which, in specimen MPL8660-M2, is preserved on the growth scales occurring in the anterior and ligamental areas where it interdigitates with the prisms of the inner layer (Pl.5, figs 9-13).The nacreous tablets of the ligamental area, about 20-30 μm wide and 2.5-5 μm thick, show a gradual transition to the aragonitic prisms of the inner layer.The prisms are very elongated, several mm-long and fusiform in shape with the long axis slightly inclined with respect to the inner shell surface (Pl.5, figs 1-4).They are up to 200 μm wide and show a very irregular outline in horizontal section (Pl. 5,figs 5,6), forming bundles of prisms with opposite orientations (Pl. 5,figs 1,14,15), as those occurring in Pachyperna laverdana Oppenheim (Posenato 1995: fig. 11).The lateral surface of each prism shows slightly incli- ned ridges (Pl.5, figs 3, 4) which interdigitate with adjacent prisms.These oblique ridges suggest a prism growth pattern like that of Lithiotis problematica and Cochlearites loppianus (FISphP microstructure) in which the large and elongated first-order prisms are built by lath-type second-order prisms, with helical development, raising from a nucleation site located at the apex of the first-order prisms (Posenato et al. 2022).This latter microstructure, which represents a very peculiar character of these bivalve taxa, has been extensively described, illustrated and discussed in Posenato et al. (2022), to which we refer.The myostracal prisms are very narrow and elongated, about 1 μm wide and more than 100 μm long (Pl. 5,figs 7,8).
The shell was covered by growth lines, but in most of the analyzed specimens, they are preserved only along the anterior region; on the rest of the shell surface, they are not visible as the outer calcitic layer is not preserved due to the high amount of organic matter occurring between the prisms which decomposed after the animal death causing prism disruption (e.g., Posenato et al. 2022).However, the scaly growth lines can be observed in the specimen MC1208 (Pl. 1, where they indicate the shape of the ventral margin. The two specimens from the Taramelli collection determined by Boehm (1884) as "Perna aff.
Taramellii" are here attributed to Pachygervillia anguillaensis n. gen.n. sp.They have been collected in the Premaloch Valley (Asiago, Vicenza Province) and consist of an articulated shell with an obliquity of ca.50° and a twisted left beak (KM16815a; Pl. 4, figs 7-11), and of a small right valve with an obliquity angle of ca.40° (KM16815b; Pl. 7, fig.11).They display the most important morphological taxonomic characters of P. anguillaensis n. gen.n. sp.consisting of a very thick shell wall, large cardinal platform, and twisted beaks.
Paleoecology.The mode of life of Pachygervillia anguillaensis n. gen.n. sp. has been discussed by several authors (Accorsi Benini & Broglio Loriga 1982;Seilacher 1984;Broglio Loriga & Posenato 1996;Fraser et al. 2004).It was a byssate and gregarious bivalve which formed clusters in shallow subtidal settings, transitional to swamps and emerged land (Accorsi Benini & Broglio Loriga 1982).The occurrence of a shallow byssal groove on the anterior surface, but of an inconspicuous byssal gape, suggests a weak byssal attachment.The bottom stabilization was therefore also guaranteed by a thick and heavy shell (edgewise recliner with a heavy-weight strategy of bottom stabilization sensu Seilacher 1984;Broglio Loriga & Posenato 1996;Fig. 4).

Pachygervillia taramellii
Description.Shell large, subequivalve, subrectangular to elongated mytiloid in outline, beak terminal and pointed; shell obliquity of about 60 -75°; shell inflation about twice of wall thickness, anterior and posterior wings inconspicuous; anterior carina low, rounded and delimiting an anterior area with an oblique byssal groove emerging from a rounded gape along the mid anterior margin; anterior margin S-shaped, dorsal margin straight to gently convex, posterior margin straight, ventral margin weakly convex.Ornamentation consisting of irregularly distributed growth scales.Ligamental area flat with irregularly spaced and narrow pits, at least 8 in number in adult individuals; juveniles with elongated cardinal platform bearing transversally elongated teeth; adductor scar not preserved.
Remarks.The description and discussion of this species are based on figured and non-figured syntypes of "Perna Taramellii", some of which belong to the Taramelli collection kept in the Kosmos Museum of the Pavia University.All the syntypes have been collected from the Rotzo Formation from Vajo del Paradiso (Lessini Mountains, Verona Province).In the Kosmos Museum, "Perna Taramellii" is recorded by two specimens: a juvenile right valve, and an articulated subequivalve mature shell with detached valves (Pl.4, figs 1-4), proposed herein as lectotype.None of these specimens seems to be those figured by Boehm (1884).It cannot be excluded that the published drawings may not faithfully reproduce the studied specimens.The figured specimen of the Nicolis collection, Verona Museum of Natural History (Boehm 1884: pl. 18, fig. 4) consists of an umbonal fragment of a left valve in which the anterior area has a deep anterior byssal groove emerging from a rounded byssal gape."Perna Taramellii" is included in the new genus Pachygervillia because of a very large and toothed cardinal platform in adult individuals and subequivalve shells.It differs from Pachygervillia anguillaensis n. gen.n. sp. by a more pointed and non-twisted beak, a lesser shell thickness, and by the occurrence of a byssal groove and gape on both valves.It differs from Gervilleioperna because the latter genus has an inequivalve shell, an inconspicuous byssal gape, and a well-developed posterior wing.Pachygervillia taramellii occurs in younger layers than P. anguillaensis n. gen.n. sp. as the type-locality (Vajo del Paradiso succession, Lessini Mountains) and is late Pliensbachian in age (Posenato & Masetti 2012).
The syntypes of P. taramellii from the "Durgahorizonte" of Vajo del Paradiso are embedded in a marly rocky matrix and were deformed during diagenetic processes, thus the anterior area is usually folded inward (e.g., Pl. 4, figs 1-4).Therefore, the shell thickness is reduced, and the original morphology (e.g., shell inflation) is not completely detectable in the available fossil material.According to Dames (1891) Perna taramellii is characterized by a much higher intraspecific variability than according to Boehm's description, but this variability could also be related to deformation, caused by sediment compaction.Perna taramellii is cited by Geyer (1977: 314) as Isognomon taramellii (Boehm).However, Geyer (1977) assigned to Boehm's species the specimens figured by Berti Cavicchi et al. (1971: pl. 3, figs. 1-3) which were later classified by Accorsi Benini (1979) as Lithioperna scutata (Dubar, 1948).
The last occurrence of this species is probably recorded by "Mytiloperna sp." specimens occurring in the upper part (early Toarcian) of the Lithiotis Member of the Apennine Carbonate Platform (Posenato et al. 2018, fig. 9).A precise specific determination of these specimens is impossible because they are embedded in hard limestone.However, the greater dimensions of the pallial cavity and the lower shell thickness make them close to P. taramellii.

Fig. 4 -
Fig. 4 -Reconstruction of the life habit of Pachygervillia anguillaensis n. gen.n. sp.(from Broglio Loriga & Posenato 1996, mod.).a) buried dead shells; b) living shell in life position; c) position of the antero-posterior sections sketched in the upper part of the figure.