Abstract

Alatoform bivalves are a polyphyletic group characterized by antero-posteriorly compressed shells and a ventro-lateral wing originating from a tight fold of the shell wall. This bizarre shell morphology is interpreted as an adaptation for algal photosymbiosis in heliophilous bivalves. The group contains the living heart cockle Corculum together with four extinct genera ranging in age from the Permian to the Jurassic. The Jurassic alatoform bivalve is Opisoma, which has an aragonitic shell that is divided into two regions, both with different functions: one for stabilization, the other for hosting symbionts. The dorsal part of the shell is massive and played the stabilization role. The ventral part has a very thin and fragile shell that permitted the transmission of light into the internal tissues harbouring photosymbionts. The morphology of this delicate ventral part has thus far remained obscure, due to lack of preservation. Accumulations of Opisoma excavatum Boehm with exquisitely preserved shells containing the fragile winged ventral parts are common within the Pliensbachian shallow-water, lagoonal carbonate succession of the Rotzo Formation of northern Italy. The wings have internal curved chambers limited by septa parallel to the wing edge. The shell of the ventral part consists of irregular fibrous prismatic and homogeneous structures which progressively infill the chambers. As the chambered wings are analogous structures among alatoform bivalves, they are no longer considered a taxonomic character. According to the observed shell orientation in the field and the consequent organization of the soft parts, Opisoma had an opisthogyrate shell.

Keywords

  1. Aberrant bivalves
  2. Early Jurassic
  3. Palaeoecology
  4. photosymbiosis
  5. Southern Alps
Alatoform (or winged) bivalves are a polyphyletic group characterized by a strongly antero-posteriorly compressed shell with a wide lateral carina originating from a tight fold of the shell wall (Yancey & Boyd ; Yancey & Stanley ; Yancey et al. ). The alatoform shell is characterized by the living heart cockle Corculum cardissa (Linnaeus), which lives in photosymbiosis, harbouring zooxanthellae (dinoflagellates) in the gills, mantle and liver (Kawaguti ). The zooxanthellae receive light through transparent windows located on the posterior surface of the shell (e.g. Kawaguti ; Cowen ; Seilacher ). The alatoform shell is thus interpreted as an adaptation for algal photosymbiosis in heliophilous bivalves (e.g. Seilacher ; Ohno et al. ; Yancey & Stanley ).
Extinct winged bivalves are represented by the Lower–Middle Permian Alatoconchoidea, the Middle Triassic Ramonalinidae (Myalinida), the Late Triassic Wallowaconchidae (Megalodontida) and by Opisoma Stoliczka, a Jurassic member of the Astartidae (Yancey & Boyd ; Yancey & Stanley ; Isozaki & Aljinovic ; Yancey et al. ). Alatoconchids and wallowaconchids show the most aberrant and largest shells reaching about one metre in length. The wallowaconchid shell is characterized by chambered wings formed by curved vertical partitions parallel to the outer carina edge. This wing partitioning has been proposed as the main diagnostic character at family level (Yancey & Stanley ; Yancey et al. ). Opisoma has been considered as the ‘least specialized alatoform’ (Yancey & Wilson ), a depiction based, however, on the incomplete knowledge of its winged region.
The Jurassic Opisoma, with a Corculum-like shell, evolved from shallow burrowing ancestors and became a secondarily semi-infaunal edgewise recliner adapted to photosymbiosis (e.g. Clari ; Seilacher ; Aberhan & von Hillebrandt ; Savazzi ). The Opisoma shell is divided into two parts, each with different functions. The dorsal or hinge region is very large and massive. Partially buried in the sediment, this region stabilized the animal on the bottom sediment surface displaying a heavyweight strategy (sensu Seilacher ). To allow for light transmission to the photosymbionts hosted in the internal tissues, the shell of the ventral region has been interpreted as being very thin (Clari ; Seilacher ; Aberhan & von Hillebrandt ), although this region is not preserved in all the Opisoma species described in the literature (e.g. Accorsi Benini ; Aberhan & von Hillebrandt ). The discovery of several completely preserved shells within the Lower Jurassic Rotzo Formation of the Southern Alps (northeast Italy) allows for the complete morphological reconstruction of the shell of the genus Opisoma. The morphological and micro-structural analysis of specimens with the ventral region still preserved permits new insights into the shell architecture and structure of ventral region, the valve orientation of Opisoma and its life position. The systematic significance of some taxonomic characters (i.e. wing partitioning) of the alatoform bivalves is also reconsidered.

Stratigraphical setting

The Rotzo Formation is a Lower Jurassic (Pliensbachian) shallow-water carbonate succession of the Trento Platform, a palaeogeographic unit in the Southern Alps. This formation was deposited within a tropical lagoon environment which was protected by oolitic shoals and bars from the open deep sea located to the east (Belluno Basin) and towards the west (Lombardia Basin, Fig. ). The thickness of the succession varies from 250 m in the depocentre (Altopiano di Tonezza – Folgaria) to less than 100 m towards the east (e.g. Bosellini & Broglio Loriga ; Winterer & Bosellini ; Masetti et al. ). The Rotzo Formation overlies the Loppio Oolite Limestone (Sinemurian) and is overlain and/or laterally replaced by various formations such as the Massone Oolite Limestone (Pliensbachian), the Tenno Formation (lower Toarcian) and the San Vigilio Oolite (upper Toarcian – Aalenian). The Rotzo Formation can also be abruptly followed by the pelagic and condensed succession of the Rosso Ammonitico (Aalenian – Tithonian) (e.g. Masetti et al. ; Martire et al. ).
Fig. 1. Geographic and stratigraphic settings of the Opisoma-bearing beds on the Trento Platform (Southern Alps, northern Italy). 1, Ponte dell'Anguillara, Vaio dell'Anguilla; 2, Monte Toraro, Tonezza del Cimone; 3, Peri, Adige Valley. In the Vaio dell'Anguilla, the Rotzo Formation is 75 m thick and the Opisoma bed occurs, near Ponte dell'Anguillara, about 20 m above the formational base. In the Monte Toraro area, where the formation is ca. 220 m thick, the studied Opisoma beds are located at about 100 m above the base of the formation. Palaeogeographic map modified from Posenato & Masetti. Abbreviations: MOL, Massone Oolite Limestone; RAV, Rosso Ammonitico Veronese; SVO, San Vigilio Oolite.
The Rotzo Formation contains very rich micro- and macrobenthic assemblages, which record a remarkable lateral and vertical facies variability ranging from fully marine to marsh and terrestrial ecosystems (Bosellini & Broglio Loriga ; Clari ). The formation is renowned for the presence of aberrant bivalves of the Lithiotis fauna, which represents a cosmopolitan event of reef building by bivalves (e.g. Broglio Loriga & Neri ; Chinzei ; Posenato & Masetti ). In the Rotzo Formation, the Lithiotis fauna is mostly represented by Lithiotis, Cochlearites and Lithioperna, while Opisoma, the subject of this study, is rare and itself was never found in thick shell accumulations.
Opisoma shells with the ventral region still preserved have been collected from the Monte Toraro succession (Altopiano di Tonezza del Cimone; Fig. ), which is located at the formation depocentre. Opisoma occurs in the middle part of the Rotzo Formation within the upper Orbitopsella Zone (lower Margaritatus Zone, upper Pliensbachian; Sarti & Ferrari ). In this area, the Opisoma beds show the acme of fully marine taxa including articulate brachiopods (Lychnothyris), larger foraminifera (Orbitopsella) and solitary corals (Fig. ). Micro-facies are mainly represented by peloidal packstones and bioclastic packstones. Very fine peloidal packstones and locally peloidal grainstones can also be present. The dominant biogenic components are peloids up to 1 mm in size. Benthic foraminifera such as Duotaxix metula, Siphovalvulina, sp. Glomospirella and Planiinvoluta are common. Larger foraminifera occur only in the sample TO29 with the megalospheric forms of Orbitopsella cf. primaeva. Dasycladalean algae are rare and represented by Thaumatoporella parvovesiculifera and Cayeuxia. Ostracods can be common. Micritization and fragmentation are common taphonomic signatures. The packstone matrix usually completely fills the spaces between bioclasts, but shelter cavities do occur, especially under bivalve shells oriented convex down. Opisoma is mostly represented by scattered articulated shells, which are easily detectable in the field when they are sectioned through the massive hinge region. Opisoma is relatively common within a unit of about 10 m thickness, composed predominantly of wackestone/packstone layers, 10–50 cm thick, alternating with thin marly intercalations. This unit is located at ca. 100 m above the base of the formation.
In the Monte Toraro outcrops, well-preserved shells are common within a 20-cm thick bioclastic packstone which overlies, with an erosional contact, a lower, thin (0–2 cm thick), discontinuous and barren mudstone (bed TO 30; Figs , G). The chaotic shell orientation, the preservation of the thin ventral region and the erosional contact with the lower barren mudstone are all indications of rapid burial caused by a high-energy event (event concentration sensu Kidwell ).
Fig. 2. Opisoma excavatum Boehm, Pliensbachian, Monte Toraro, Altopiano di Tonezza del Cimone, northeastern Italy. A–I, random sections of selected specimens occurring in the hard cemented limestone constituting the Opisoma beds. A, B, D, shells from bed TO 30. C, E, articulated shells from bed TO 28. E, F, H, I, articulated shells from bed TO 31. G, detail of bed TO 30 (Fig. ) showing the erosional contact (white line) between the mudstone and the overlying high-energy shell accumulation. Scale bars = 20 mm.
Opisoma shells with the original aragonitic shell preserved have also been recovered near the Ponte dell'Anguillara (Vaio dell'Anguilla, southern Trento Platform) again within the lower Orbitopsella Zone, ca. 20 m above the base of the Rotzo Formation (Clari ; Posenato & Masetti ; Fig. ). At this locality, Opisoma occurs in grey silty marlstones with abundant plant remains between two Lithioperna scutata accumulations (Fig. ). Most of the specimens used in the literature to describe the shell structures of Lower Jurassic bivalves such as Gervilleioperna, Lithioperna, Opisoma and Pseudopachymytilus were collected from this locality (Accorsi Benini ; Accorsi Benini & Broglio Loriga ).

Material and methods

The shells from the Monte Toraro are completely replaced by calcite. The massive hinge regions are frequently dissolved, and the shell wall is empty or infilled by sparry calcite (e.g. Fig. C, F). This type of preservation, together with the hard cemented limestone, precludes isolating specimens from the matrix. The reconstruction of shell morphology of Opisoma was thus obtained through serial sectioning of some of the limestone blocks (with a total volume of ca. 10,000 cm3) originating from bed TO30 (Fig. ). The limestone blocks were sectioned with a diamond saw in parallel slabs about 10 mm thick, which were then polished. Acetate peels were made from the slab surfaces, which were also scanned by an optical scanner at 1200 dpi. In these blocks, four articulated shells and 18 disarticulated valves are present, many of which are fragmented, but never deformed (Figs 4, 5). The shell sizes range from 3 to 15 cm in height. In many specimens, the exact dimensions are impossible to determine because the shells are incomplete and sections are randomly oriented. Reconstruction of the shell morphology is based mostly on these serial sections, although random sections observed in the field are also considered (Fig. ).

Museum collection

The analysed museum collection of Opisoma consists of the specimens studied by Accorsi Benini. The collection is housed in the ‘Piero Leonardi’ Museum of the University of Ferrara (under the acronym PLM). It consists of more than a hundred individuals, many of which were collected from the Opisoma bed of the Ponte dell'Anguillara, Monti Lessini (Fig. ). Opisoma shells from this locality have been herein analysed for new SEM observations.
The studied material collected from the Monte Toraro outcrops is labelled as MPL TO (Toraro) and deposited in the ‘Piero Leonardi’ Museum of the University of Ferrara. The specimens belonging to the Accorsi Benini collection are labelled with the acronym MPL LVS (Accorsi Benini ).
The unusual shell morphology, the problematic orientation of the valves and the uncertain life position of Opisoma (e.g. Aberhan & von Hillebrandt ) make it impossible to apply a priori the conventional morphological terms of bivalve shell, in particular to determine the anterior–posterior axis. For these reasons, the terminology for the edgewise recliners proposed by Yancey et al. is partially adopted (Fig. ). In particular, the flattened side is named the basal surface. The opposite side, containing the crest and dorsal heart-shaped depression is named the apical surface. Differences in the terminology proposed here concern the beak and the ventral edges. The terms ‘anterior’ and ‘postero-ventral’ of Yancey et al. (, text-fig. ) are herein replaced by ‘dorsal’ and ‘ventral’, respectively. The controversy concerning the valve orientation of Opisoma (e.g. Aberhan & von Hillebrandt ) and the relationship between the herein adopted terminology and the biological orientation of Opisoma are discussed below. The terminology of Carter et al. is used for the description of the shell micro-structure.
Fig. 3. Terminology and measured parameters adopted in this study. The terminology for edgewise recliners proposed by Yancey et al. is partially adopted.

Results

Opisoma from the Monte Toraro

In the Monte Toraro outcrops, Opisoma shells with the preservation of the thin ventral and lateral wings are common. The wings are characterized by septa and partitions (Fig. A, B). The alatoform and chambered shells show (1) transversal sections through the hinge region with the typical moustache-like outline and (2) the hinge consisting of two teeth in the LV (LV, left valve) and three in the RV (right valve). These two morphological characters are diagnostic of Opisoma as circumscribed by Accorsi Benini and Aberhan & von Hillebrandt. Four representative specimens are described herein: MPL TO-A-1, MPL TO-A-2, MPL TO-B3-7 and MPL TO-D-10.
The smaller shell (Block A, specimen MPL TO-A-1; Fig. A) shows sections oriented almost perpendicularly to the commissure and bedding planes. The shell is articulated and ca. 40 mm long, 25 mm wide and 12 mm high. The thickness of the wing edge ranges from 1.6 mm near the ventral edge (Fig. A1) to 0.5 mm in the umbonal region (Fig. A5). The shell wall near the ventral edge is 0.2 mm thick and thins towards the commissure. In the ventral extremity of the wing (Fig. A1), two septa, about 0.4 mm thick, are present on each valve. The septa define sub-squared chambers, about 1 mm wide, which are dorsally infilled by a shelly deposit (Fig. A4). The ventral chambers are therefore dorsally closed and the wings become massive, apart from a small chamber, which opens near the main body chamber (Fig. A3). On the crest slopes (Fig. A4), the shell is very thin (about 0.3 mm thick) while it is about 0.5–0.6 mm on the basal side. The hinge region is very flat with the typical moustache-like outline (Fig. A5). The hinge teeth are not well distinguishable.
Fig. 4. Selected polished parallel sections and morphological shell interpretation of Opisoma excavatum Boehm, Pliensbachian, Monte Toraro, Altopiano di Tonezza del Cimone, northeastern Italy. A, specimen MPL TO-A-1 (bed TO 30). B, specimen MPL TO-A-2 (bed TO 30). Symbols: S, septa; other symbols as in Fig. . Scale bars = 5 mm in A, 10 mm in B.
On the same slab of the above described specimen, another articulated shell is present. The sections are strongly oblique to the commissural plane (specimen MPL TO-A-2; Fig. B). A portion of the hinge region is missing due to present-day erosion. The inferred size of this specimen is about double that of the specimen MPL TO-A-1. The ventral wing is about one-fourth of the total shell length. The wing chambers are curved, parallel to the wing edge and asymmetrically distributed between the two valves. The septa are very thin, about 0.3 mm thick, and thinner than the shell walls. The ventral chambers are regularly spaced and sub-rectangular in outline (Fig. B2–5). The ventral wing edge is about 5 mm thick, while the wing thickness at a distance of 10 mm from edge is reduced to about 2 mm (Fig. B3, 4). In the wing, the shell of the basal side is slightly thicker than that of the apical side (0.3 mm vs. 0.5 mm). The thickness is reversed in the body chamber where the shell of crest slopes is thinner than that of the basal side (0.9 mm vs. 1.5 mm; Fig. B2). The maximum shell thickness occurs on the apical ridge, although here it is affected by the strong inclination of the section (Fig. B2).
The connection between the ventral partitioning and the hinge structure, characteristic of Opisoma, is shown in the specimen MPL TO-B3-7 (Fig. A). In this specimen, sectioned both through the wing and the hinge region, the crest and part of the wing are broken. The chambered part of the wing is broad and extends for about half of the valve length. In transversal section, the ventral winged region is bell shaped. The basal side is concave, and the wing edge slightly curved towards the opposite side (Fig. A3–5). The hinge structure is detectable in the specimen MPL TO-D-10 (Fig. B1, 2), which has the greatest size (ca. 150 mm long) among the sectioned specimens. Three teeth on the left side and two on the right side can be easily distinguished (Fig. B1). According to the interpretation of Aberhan & von Hillebrandt, the cavity located between the hinge and the basal surface (Fig. B2) corresponds to the site of the posterior adductor muscle.
Fig. 5. Selected polished parallel sections and morphological shell interpretation of Opisoma excavatum Boehm, Pliensbachian, Monte Toraro, Altopiano di Tonezza del Cimone, northeastern Italy. A, specimen MPL TO-B3-7 (bed TO 30). B, specimen MPL TO- D-10 (bed TO 30). Scale bars = 10 mm.
In the field, about 40 specimens were observed and photographed. Some shells were sectioned through the ventral partitioned wings characterized by the typical broad bell-like shape (Fig. A, B). Other specimens are sectioned towards the ventral extremity of the apical depression and crest top (Fig. E). The shell of the basal side of these specimens is very thick, while that of the apical side is thin and incomplete due to damage. Other articulated specimens are sectioned through the hinge region (Fig. F, H, I). The largest articulated shell (Fig. I) was up to ca. 25–30 cm in length.

Accorsi Benini's collection of Opisoma excavatum Boehm

Accorsi Benini published a detailed morphological analysis and taxonomic revision of Opisoma examining historical collections and more than 100 specimens from the Rotzo Formation, most of which were collected from two localities: Peri (Adige Valley) and Ponte dell'Anguillara (Monti Lessini; Fig. ). The latter is the type locality of the lectotype of Opisoma excavatum Boehm, which is considered the sole Opisoma species occurring in the Southern Alps (Accorsi Benini ).
Knowledge of O. excavatum is mostly restricted to the massive cardinal region, which is characterized by a high morphological variability regarding the shape and depth of the apical depression, sharpness of the apical ridge and the umbonal morphology (Accorsi Benini ; Aberhan & von Hillebrandt ). Opisoma had a strong hinge and, therefore, preserved articulated shells are common, while the fragile ventral region is generally incomplete, because it was exposed above the sediment–water interface and commonly affected by fragmentation and abrasion (Aberhan & von Hillebrandt ). The Accorsi Benini collection contains unfigured articulated specimens that have considerable remains of the ventral region. In particular, specimen MPL LVS-118 can be estimated to reach 13 cm in length, 8 cm in width and 4 cm in height (Fig. ). The shell is moderately elongated with an impressed apical depression limited by a rounded and large apical ridge. The basal side is concave with an inverted V-shaped profile in transversal section. The basal valve surfaces, although slightly deformed, are clearly converging towards the commissural plane (Fig. E). This specimen has the original mineralogy preserved in a thin (0.5–2 mm) outer band, while the internal aragonite is fully replaced by calcite. The shell consists of sub-layers showing cyclical growth (Fig. D).
Fig. 6. Opisoma excavatum Boehm, Pliensbachian, Ponte dell'Anguillara, specimen MPL LVS-118, Accorsi Benini collection, ‘Piero Leonardi’ Museum, University of Ferrara. Apical (A), lateral (B) and basal views (C) of the studied specimen with the position of the parallel slab sections (E). D, detail of the polished section 6 showing the interpreted arrangement of the shell sub-layers in the septa (S) and in the wing walls.
In the most ventrally situated cross-section (Figs A6, E6), the wings have one or two septa each. The septa delimit sub-rectangular chambers in cross-section, which are almost completely infilled by secondary laminated shell deposits (Fig. D). These chambers are closed dorsally, after ca. 10 mm (compare Fig. E5 and E6); thus, they were open only ventrally where they communicated with the main body cavity. The septum is initially very thin, about 0.2 mm thick, and later reinforced by secondary shell sub-layers (Fig. D). The orientation of sub-layers in the apical side of the wing is different from that of the basal side. In the apical shell, the sub-layers are thin and very slightly inclined (about 10°) with respect to the inner surface. In the basal shell, the sub-layers are decidedly more inclined forming an angle of about 35°(Fig. D). The different inclination of the sub-layers indicates that the apical shell towards the commissure remained thinner for a shell sector decidedly wider than that of the opposite side. The body chamber is narrow, mostly located below the ventral part of the apical depression and the crest and ventrally indented by few and short chambers (Figs A, ). These latter have an antero-posteriorly flattened and curved sub-pyramidal shape.
Fig. 7. Opisoma excavatum Boehm, Pliensbachian, Rotzo Formation. A1, basal view of the specimen MPL LVS-117 (Accorsi Benini collection, ‘Piero Leonardi’ Museum, University of Ferrara) affected by compaction which produced, on the outer shell surface, ribs and grooves connected with the internal septa and chambers. A2, reconstruction of the main body cavity and wing chamber disposition of the same specimen. B, morphological shell reconstruction of O. excavatum mostly based on specimen MPL LVS-118 (see Fig. ).
Specimen MPL LVS-117 (Fig. A) consists of a small, articulated shell (ca. 50 mm long and 35 mm wide) with its apical side fractured. The shell has been replaced predominantly by calcite, with only a very thin aragonitic outer rim preserved. This specimen displays extraordinary preservation of the very thin (ca. 0.3 mm thick) shell of the basal side. Sediment compaction has compressed the chambers and raised the septa resulting in grooves and crests parallel to the wing edge (Fig. A). This sculpturing of diagenetic origin allows the shape of the internal chambers to be reconstructed. Each wing contains at least four curved chambers, decidedly longer than those occurring in other specimens, running parallel to the wedge wing and dorsally closed.
In summary, specimens MPL LVS-117 and MPL LVS-118 were collected at the type locality of O. excavatum and ascribed to this species by Accorsi Benini. Both specimens are characterized by wings with ventro-lateral chambers separated by curved septa. The chambers are connected to the main body cavity along the median region, while they are progressively closed dorsally by secondary shell material. The chambered alate Opisoma specimens from the Monte Toraro can be therefore ascribed to O. excavatum.

Shell micro-structure

Current knowledge of shell micro-structure in fossil alatoform bivalves is focussed on the outer calcitic layer, when preserved. The alatoconchid shell has an outer layer of simple calcitic prisms, which are about 3 mm long and oriented perpendicularly to the shell surface. The inner shell layer was composed of aragonitic micro-structures that are recrystallized to calcite in fossil material (Yancey & Boyd ). The shell structure of the ramonalinids is similar to that of alatoconchids, differing in having the inner layers characterized by both recrystallized and original calcitic prisms (Yancey et al. ). No information is available on the micro-structures of the wallowaconchids, which is interpreted as having a fully aragonitic shell that is now completely recrystallized (Yancey & Stanley ).
Original aragonitic composition and micro-structures are known only in Opisoma, as far as fossil winged bivalves are concerned. The Lower Jurassic specimens studied were collected from the Ponte dell'Anguillara locality (Vaio dell'Anguilla, Monti Lessini; Fig. ). The detailed description by Accorsi Benini was mostly concerned with the massive hinge region, while no data are available for the thin ventral region. According to Accorsi Benini, the Opisoma shell was aragonitic throughout. Its outer layer has a complex crossed-lamellar structure with its maximum thickness in the umbonal region (up to 400 μm), thinning ventrally. The inner layer consists of cyclically arranged sub-layers formed by alternations of myostracal prismatic, homogeneous and complex crossed-lamellar sub-layers. In the umbonal region, the complex crossed-lamellar sub-layers remain in the myostracal prismatic sub-layers (up to 80 μm thick). In its central part, the shell mostly consists of thick myostracal prismatic sub-layers (up to 600 μm thick) usually separated by thin homogeneous sub-layers (up to 33 μm thick). These also occur at the transition between the myostracal prismatic and crossed-lamellar sub-layers.
New micro-structural SEM analyses have been carried out on a figured specimen of the Accorsi Benini collection (Accorsi Benini ; pl. 1, Fig. ). It is an articulated and almost complete shell (about 60 mm in length) that preserves the original aragonitic composition and micro-structure, as well as conspicuous parts of the ventral region (Fig. ). The analysis shows the occurrence of the complex crossed-lamellar structure in the middle part of the basal surface (Fig. D). This structure, representing the outer layer, is found immediately below the valve surface, which is in turn covered by a thin crust of calcite cement. Towards the commissure, at the central part of basal surface, the outer layer is missing. Here, sub-layers of irregular fibrous prismatic structure (‘myostracal prismatic’ of Accorsi Benini ; up to 250 μm thick) and complex crossed-lamellar structure (up to 350 μm thick) alternate (Fig. E). The shell of the apical ridge is mostly built by sub-layers of irregular fibrous prisms separated by the homogeneous structure (Fig. B).
Fig. 8. Opisoma excavatum Boehm, Pliensbachian, Ponte dell'Anguillara, specimen MPL LVS-48, Accorsi Benini, pl. 1, fig. ) collection, ‘Piero Leonardi’ Museum, University of Ferrara. In the centre, apical (left) and basal (right) shell views show the locations of the SEM analysed surfaces and sections. A, horizontal section of irregular fibrous prismatic micro-structure, ventral wing. B, irregular fibrous prismatic and homogeneous micro-structure, radial section through the apical ridge. C, tangential section of the lateral wing with location of the irregular fibrous prismatic (C1, C3) and homogeneous (C2) micro-structures. D, complex crossed-lamellar micro-structure of a broken outer shell surface. E, complex crossed-lamellar and irregular fibrous prismatic sub-layers of the inner shell layer observed in planar (horizontal) view near the commissure. Scale bars = 20 μm, otherwise as indicated.
The wing has been observed in an oblique radial section located in the middle-ventral part of valve, near the outer wing edge (Fig. C). On the apical side, the outer, preserved layer consists of a thick (ca. 500 μm) sub-layer of irregular fibrous prisms (Fig. C, C1). Inwards, this sub-layer is followed by a second sub-layer, which has the same micro-structure of the outer layer and a thickness ranging from 150 to 200 μm. The chamber is completely infilled by a chalky deposit with a homogeneous structure (Fig. C, C2). The basal side is composed by two sub-layers of irregular fibrous prisms which have different thickness: 400 μm in the outer sub-layer and 300 μm in the inner sub-layer. The crossed-lamellar structure that characterizes the outer shell layer has not been observed in the wing. It is likely that this outer layer was too thin to be preserved in this specimen. In horizontal section, the irregular fibrous prismatic structure shows an irregular outline of the prisms, ranging from 2 to 3 μm in diameter (Fig. A).
As already noted in the literature (e.g. Accorsi Benini ; Aberhan & von Hillebrandt ), the shell structure of Opisoma corresponds to that of living Astartidae. Taylor et al. described and figured a radial section of Astarte borealis (Schumacher), which has an outer layer made up of crossed-lamellar structure and an inner layer of myostracal prisms. The prismatic layer of A. borealis, however, thins in a ventral direction and beyond the pallial line. Here, the shell is completely constructed by the crossed-lamellar structure, in contrast to that occurring in the Opisoma shell. This difference in micro-structure between Astarte and Opisoma is likely related to different life strategies and functionality.

Functional morphology: life habit and photosymbiosis

The life habit of Opisoma hippponix Boehm (O. excavatum Boehm) was analysed by Clari using material originating from the Ponte dell'Anguillara section, Monti Lessini (Figs ). Clari concluded the following: (1) Opisoma was an epifaunal sedentary bivalve with a Corculum-like shell; (2) it rested on the flattened surface (basal surface) with the commissure perpendicular to the bottom; (3) the flattening and thickening of the basal region had a stabilization function; (4) the apical surface was alate, and thus the inhalant and exhalant apertures was raised above the sediment surface; and that (5) it was adapted to photosymbiosis with a strategy similar to Tridacna, spreading out the mantle from the shell cavity as the shell was not transparent because its great thickness.
Fig. 9. Opisoma excavatum Boehm, Pliensbachian, Ponte dell'Anguillara, Accorsi Benini collection, ‘Piero Leonardi’ Museum, University of Ferrara. A, specimen MPL LVS-108. B, specimen MPL LVS-109. These individuals, originating from the same bed, show the high phenotypic variability of the crest. For reconstruction of their life position see Fig. . Basal (1), apical (2), dorsal (3) and lateral (4) views. Scale bar = 10 mm.
Photosymbiosis was also suggested by Seilacher, who interpreted the pronounced antero-posterior compressed and laterally expanded shell of Opisoma as a broad light receptor. The functional morphology and trophic strategy of O. excavatum from Chile were studied and discussed by Aberhan & von Hillebrandt who remarked the following: (1) the hinge region was partially buried and had a stabilizing function; (2) the ventral region, never preserved in the Chilean and other specimens, was probably very thin, and placed outside the sediment; (3) Opisoma was adapted to photosymbiosis and light entered through the thin and transparent ventral region; and (4) the photosymbiosis allowed for a high calcification rate which produced large and thick shells.
The orientation of Opisoma shell in relation to the substrate is controversial and related to the problematic orientation of the valves. The valve orientation of an edgewise recliner bivalve has significant implication about its life position because it is conditioned by the location of inhalant and exhalant apertures to maintain an efficient filter-feeding activity. Some authors consider Opisoma as opisthogyrate (Buvignier ; Dubar ; Chavan ; Clari ; Accorsi Benini ), while others consider it prosogyrate (e.g. Boehm ; Accorsi Benini ; Aberhan & von Hillebrandt ). These opposite orientations are based on different interpretations regarding the position of ligament, muscle scars and hinge structure. Considering Opisoma as prosogyrate, mainly on the basis of the hinge structure, Aberhan & von Hillebrandt argued that the basal side (their posterior area) was not used as a supporting surface, because the occurrence on this side of the inhalant and exhalant apertures. They suggested that Opisoma reclined on the apical surface (the anterior surface) as already proposed by Seilacher, Fig. ; Fig. A). Aberhan & von Hillebrandt did not observe any specimen displaying this life position. Their interpretation was supported by ‘rare epibionts (serpulid worm tubes, encrusting bryozoans)’ (Aberhan & von Hillebrandt, p. 159) restricted to the posterior shell surface.
Fig. 10. Two different modes of life for Opisoma have been proposed in the literature. A, the hinge region, having a stabilizing function, was partially buried, and the flattened basal surface was exposed outside the sediment and directed upwards (Seilacher ; Aberhan & von Hillebrandt ). Arrows indicate the resulting position of inhalant and exhalant currents; triangles indicate the position of the right and left shell sections. The question mark indicates sediment accumulation along the ventral commissure which is detrimental to filter feeding. B, the majority of the articulated shells occurring in the Monte Toraro Opisoma beds, with the exclusion of those occurring in high-energy accumulations, indicate a life position with the apical surface upwards. In this position, the inhalant and exhalant apertures were located on the apical ridge, which consequently indicates the posterior region of an opisthogyrate shell. Individuals with a low median apical ridge (B1) suggest a low sedimentation rate (reconstruction based on the specimen MPL LVS-109 of Fig. B); individuals with high a median apical ridge and short shells (B2) suggest a higher sedimentation rate (reconstruction based on specimen MPL LVS-108 of Fig. A). Even in this latter life position, the very thin ventral region was exposed outside the sediment allowing the light to penetrate inside the shell where the symbionts were hosted in the chambered wings and main body cavity.
In the Opisoma-bearing outcrops from the Monte Toraro succession, articulated shells seen in section are relatively common (Fig. ). The shells occur both in high-energy accumulations, such as in bed TO30 where shells are chaotically oriented (Fig. G) and in prevailing bioclastic packstones of a low-energy environment (TO 28–TO32; Fig. C, E, F, H–I). In this environment, located below the fair-weather wave-base, the shells are sparse and their overturning (e.g. Fig. F), in absence of clear evidence of higher energy events, could have been caused by predation or bioturbation.
In the outcrops studied, on a total of 23 articulated shells were found in beds that showed little significant evidence of hydraulic reworking, and 20 specimens were observed with their commissure planes oriented vertically to the bedding, the flattened surface downwards (Fig. ). This prevalent shell orientation suggests that Opisoma lived with the flattened surface facing the bottom (Fig. B), and this orientation implies an opisthogyrate shell.
The shell orientation of Opisoma as observed in the field and as proposed in this study supports that proposed for the other three fossil alatoform bivalve groups, namely the Alatoconchoidea, the Ramonalinidae and the Wallowaconchidae. These have all been reconstructed oriented with the flattened anterior surface facing downwards and with the crest facing upwards to raise the mantle apertures above the sediment surface (Yancey & Stanley ; Yancey et al. ). The burial of the dorsal shell region requires the fusion of the mantle lobes along the commissure located inside the apical depression.
The morphology of the apical depression was mostly controlled by the growth of the apical ridge where the mantle apertures were located along the commissure. The specimens from the Opisoma bed of the Ponte dell'Anguillara locality show a remarkable variability of the development of the apical ridge (Fig. ). This variation probably reflects phenotypic differences due to different burial or bottom sediment conditions (Fig. B). In specimens with a high apical ridge, the shell is short and the ventral region is scarcely developed (Figs A, 10B2). The shortening of the ventral region would, however, have reduced the amount of tissues hosting potential photosymbionts resulting in a reduction in photosymbiosis and corresponding calcification and growth rates. An increase in the number and length of chambers could have balanced the shortening of the ventral wing. Nevertheless, the apical ridge (Fig. A) indicates that the animal was able to maintain an efficient filter-feeding activity, although it probably had small gills as suggested by the reduced volume of the body cavity.
In the grey silty marls of the Ponte dell'Anguillara location, individuals with strongly raised apical ridge are common. The shells have a very high keel-like apical ridge (e.g. MPL LVS-108, Fig. A; Clari, pl. 1, fig. 14), which reaches about 70% of the shell length (Fig. A). The specimen MPL LVS-108 has an apical keel that shows a discontinuous growth, perhaps reflecting intermittent sedimentation events (Fig. A). These specimens have also well-developed numerous ventro-lateral chambers (e.g. MPL LVS-117; Fig. A), which probably indicate a necessity to increase the amount of photosymbionts under poorly illuminated conditions.
The shell orientation proposed here is further supported by observation of different thickening of the basal shell wall with respect to the apical shell wall (e.g. Figs B2, 5A, 6D). The consequent difference in weight indicates that the gravity centre was shifted towards the basal side. Moreover, in adult individuals, the basal surface is concave (Figs ) and the valve surfaces are converging inwards, towards the commissure. If the animals were oriented with this side up, the risk of fouling the mantle cavity and gills would be very high (Fig. A).
Algal symbiosis in Opisoma has been analysed and proposed by several authors (e.g., Clari ; Seilacher ; Ohno et al. ). The most detailed analysis was conducted by Aberhan & von Hillebrandt on Toarcian material from northern Chile. These authors recognized almost all the criteria needed to consider Opisoma as a candidate for photosymbiosis including high calcification rates, a specialized and bizarre morphology and a shallow-water palaeoenvironmental setting (Cowen ). Aberhan & von Hillebrandt had limited doubts originating from their proposed shell orientation. Following the Accorsi Benini micro-structure description, they noted that the basal shell wall is too thick and has a complex crossed-lamellar structure, factors that would have hindered efficient light transmission. The shell orientation proposed here, together with the predominantly vertical fibrous prismatic micro-structure of the ventro-lateral wings, guarantees a sufficient illumination of the bivalve thus ensuring that the symbionts in the mantle and gill tissues received enough light.
As proposed for the wallowaconchids (Yancey & Stanley ), the internal septa of Opisoma had mostly a mechanical function, similar to the ‘strutwork within aeroplane wings or the corrugation in cardboard box material’ (Yancey & Stanley, p. 16) to reinforce the thin ventro-lateral shell fold. Other proposed functions of the internal septa are as a barrier to the body cavity if wings were damaged and to increase the surface and volume for culturing photosymbionts (Yancey & Stanley ). During ontogeny, the chambers of Opisoma were reinforced by secondary layers of fibrous prisms. At this stage, light probably was still transmitted to the chamber. A chalky deposit finally infilled the chambers during later growth stages. This deposit consists of a rapidly secreted porous material which infills the abandoned body cavity of oysters and of other bivalves of the Lithiotis facies (Chinzei ).

Conclusions

The Lower Jurassic astartid O. excavatum is a highly specialized alatoform bivalve with a large and aberrant shell, which reflects a photosymbiotic trophic strategy. This study provides new data to elucidate the morphology and autecology of Opisoma mostly regarding: (1) the fragile and thus far undescribed ventral part of the shell; (2) the chambered wings; and (3) the micro-structure.
The Opisoma specimens from the Rotzo Formation thrived in a tropical oligotrophic shallow-marine environmental setting as suggested by associated corals, sponges and larger foraminifera. Opisoma excavatum had a Corculum-like strategy to allow sunlight to penetrate through aragonitic fibrous prisms oriented perpendicularly to the inner and outer shell surfaces. The outer shell layer, formed by complex crossed-lamellar micro-structure, is scarcely efficient for light transmission, but it has not been detected in the ventral region. If present, it was very thin (few tens of microns) and did not significantly hinder the transparency of the shell. The light illuminated the tissues harbouring symbionts, mostly located in the median ventral part of the body cavity and inside curved chambers occurring in the ventro-lateral shell folds (wings). These chambers were open towards the sagittal plane and connected with the main body cavity. The chambers were progressively covered by secondary sub-layers of fibrous prisms and later completely infilled the homogeneous structure which strengthened the wing dorsally. As the chambered wings are analogous structures among alatoform bivalves, they are no longer considered a diagnostic character for the Wallowaconchidae (Megalodontida) as proposed in the literature.
Specimens observed in the field show a life position similar to that of the other alatoform bivalves. Opisoma lay on the basal shell surface with the commissure plane perpendicular to the bottom surface. Such a shell orientation reopens the debate concerning the systematic value of the hinge structure. The phylogenetic interpretation of a bivalve hinge is questionable when its ontogenetic development is unknown (e.g. Boyd & Newell ). According to the observed shell orientation in the field and the consequent organization of the soft parts, the Opisoma shell is considered to be an opisthogyrate bivalve. Specimens with low and high median apical ridge occur together in the field. Individuals with keeled apical ridge considerably raised the inhalant and exhalant apertures above the sediment. This latter morphotype indicates that the animal did not abandon the filter-feeding mechanism for food uptake.
Opisoma contains other Jurassic species, such as O. menchikoffi Dubar and O. scalprum Dubar from the Pliensbachian of Morocco and O. paradoxum (Buvignier) from the Late Jurassic of northern France. Further studies are needed to identify the possible occurrence of chambers inside the shell wings of these species and to reveal the missing chapters in the evolutionary history of this Jurassic photosymbiotic bivalve group.

Acknowledgements

We thank Franz T. Fürsich and an anonymous reviewer for their constructive suggestions. We are grateful to B. Sala and R. Ferri of the ‘P. Leonardi’ Museum and Lisa Volpe, of the Ferrara University, respectively, for the loan of the Accorsi Benini collection and SEM photographs. We thank Renzo Tamoni for the serial sectioning and polishing of the limestone blocks. This research has been financed by local research funds at the University of Ferrara (FAR 2009–2012).

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Information & Authors

Information

Published In

Volume 46Number 41 October 2013
Pages: 424437

History

Received: 17 October 2012
Accepted: 1 February 2013
Published online: 29 August 2013
Issue date: 1 October 2013

Authors

Affiliations

Renato Posenato [email protected]
Dipartimento di Fisica e Scienze della Terra, Università di Ferrara Polo Scientifico-Tecnologico, Via G. Saragat 1, Ferrara 44122, Italy;
Davide Bassi [email protected]
Dipartimento di Fisica e Scienze della Terra, Università di Ferrara Polo Scientifico-Tecnologico, Via G. Saragat 1, Ferrara 44122, Italy;
James H. Nebelsick [email protected]
Institute of Geosciences, University of Tübingen, Sigwartstrasse 10, Tübingen 72076, Germany;

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