Non-technical Summary
This study analyzes previously undescribed fossils of a new species within the genus Opisoma, a marine bivalve from the Early Jurassic found in west-central Argentina. These organisms lived in warm seas associated with bivalve reef communities. The fossils were examined in hand specimens and via computed tomography, which allows observation of internal structures in articulated valves without the need to use destructive techniques. This provided new evidence for solving the longstanding controversy regarding valve orientation in Opisoma. Wing-shaped bivalves such as Opisoma have shells that are compressed and heart-shaped. This form appeared multiple times and can still be seen today in some bivalves that live in symbiosis with microscopic algae. This discovery in the Neuquén Basin represents the southernmost record known for the genus and expands its geographic distribution.
Introduction
The genus Opisoma was established by Stoliczka in 1871 and was included within the family Astartidae d’Orbigny, Reference d’Orbigny1844, specifically in the subfamily Opinae Chavan, Reference Chavan1952. This is a small subfamily of Mesozoic astartid bivalves that inhabited primarily the Tethyan seaway. The hinges, which are narrow and vertically elongated in both valves, have robust and expanded cardinal teeth with a trigonal morphology, while the lateral teeth are either absent or reduced. Opisoma is alatoform (wing-shaped) with an external ligament like other opisthodetic astartids. Unlike the infaunal lifestyle typical of astartids, Opisoma developed an epifaunal or semi-infaunal mode of life, with the commissural plane perpendicular to the substrate. This ecological shift was likely driven by a photosymbiotic strategy (Kirkendale and Paulay, Reference Kirkendale and Paulay2017).
Alatoform bivalves are a polyphyletic group characterized by shells compressed in the anteroposterior direction. This homeomorphic convergence can be recognized in four main extinct groups: the Alatoconchoidea from the early to middle Permian, the Ramonalinidae from the Middle Triassic, the Wallowaconchidae from the Late Triassic, and the genus Opisoma (Astartidae) from the Early Jurassic (Yancey and Boyd, Reference Yancey and Boyd1983; Yancey and Stanley, Reference Yancey and Stanley1999; Isozaki and Aljinović, Reference Isozaki and Aljinovic2009; Yancey et al., Reference Yancey, Wilson and Mione2009; Posenato et al., Reference Posenato, Bassi and Nebelsick2013).
In the shallow marine and lagoonal settings of Tethys and Panthalassa, the Early Jurassic interval was characterized by the appearance and spread of reefs built by large, gregarious bivalves known as the Lithiotis fauna. Although to date the Lithiotis fauna has not been recognized in the Neuquén Basin, in other regions, Opisoma was a subordinate component of this fauna (Bassi et al., Reference Bassi, Posenato, Nebelsick, Owada, Domenicali and Iryu2017). Lithiotids played a major role as carbonate producers in shallow marine environments due to their large size (Posenato et al., Reference Posenato, Bassi and Nebelsick2013). Among extinct wing-shaped bivalves, Opisoma has been considered the “least specialized alatoform” (Yancey and Wilson, Reference Yancey, Wilson, Malchus and Pons2006). Photosymbiosis in Opisoma has been repeatedly proposed in the literature. Clari (Reference Clari1975) interpreted this bivalve as an organism capable of hosting algal symbionts, comparing its strategy to that of Tridacna Bruguière, Reference Bruguière1797. Later, Seilacher (Reference Seilacher1990) further supported this hypothesis from a functional–morphological perspective. In addition, Aberhan and von Hillebrandt (Reference Aberhan and von Hillebrandt1999) suggested the possibility of photosymbiosis in the genus. Finally, microstructural analyses by Posenato et al. (Reference Posenato, Bassi and Nebelsick2013) demonstrated that certain regions of the shell were suitable for light transmission, pointing toward a strategy comparable to that observed in Corculum (Röding, Reference Röding1798).
Position of the ligament and lunule in relation to the umbo has been variably interpreted, making identification of right and left valves problematic. As a consequence, opposing hinge dentition formulas and adductor muscle insertion arrangements have been proposed. This has led to considerable controversy regarding the orientation of the valves in Opisoma.
The aims of this work are to provide new data on this longstanding debate and to document the southernmost known record of the genus. A discussion on its systematic affinities and relationships within the family Astartidae is also provided, contributing to the discussion about valve orientation and the possible presence of a transposed hinge. Finally, we intend to analyze the anatomical changes and the functional morphology associated with the adoption of an epifaunal mode of life in an originally shallow infaunal group, the life habit of Opisoma, and its possible relationship with photosymbiosis.
Geologic setting
The Neuquén Basin formed during the Late Triassic as the result of intraplate continental extension (Vergani et al., Reference Vergani, Tankard, Belotti and Welsink1995; Howell et al., Reference Howell, Schwarz, Spalletti, Veiga, Veiga, Spalletti, Howell and Schwarz2005) and was filled by mostly marine sediments during the Jurassic and Cretaceous. Two main depocenters, a large one to the north and a smaller one to the south (Fig. 1.1), were separated by a structural elongated high called Huincul High (= “Dorsal de Huincul”, De Ferrariis, Reference De Ferrariis1947). From the lithostratigraphic point of view, two main sedimentary cycles are recognized during the Jurassic: the Cuyo and Lotena groups.
Location: (1) general location of the Neuquén Basin in western Argentina; (2) geological sketch map of Cerro Granito area, central Neuquén Province, with indication of the Opisoma locality (star) and main lithostratigraphic units; (3) sketch of geological section showing the beds bearing the Opisoma romeroi n. sp. specimens described here.
Figure 1. Long description
The top left panel is a regional map of western Argentina, with the Neuquén Basin shaded in blue, bounded by Chile to the west and the Sierra Pintada and North Patagonian Massifs. The inset shows Argentina with the basin’s location marked. The top right panel is a geological sketch map of the Cerro Granito area, oriented with north at the top. Distinct colors represent lithostratigraphic units: alluvial deposits, Neuquén Group, Mendoza Group, Lotena Group, Cuyo Group, porphyries, and granodiorite. The Opisoma locality is marked by a red star near the center-north. Puesto Romero is labeled to the west. The bottom panel is a stratigraphic section running north to south, showing Holocene cover, Los Molles Formation, and Liassic deposits, with the Opisoma level indicated by an oval. Lithologies are differentiated by patterns: highly fossiliferous level, sandy shales and siltstones, fossiliferous ruditic limestones, immature calcareous sandstones, gravels, coarse-grained calcarenite, poorly stratified conglomerates, and the Chachil Plutonic Complex to the south. The section includes a horizontal scale bar (0 to 10 meters) and marks the position of fossil beds.
All the material described here was collected at the northwest slope of Cerro Granito, Neuquén (Fig. 1.2). This hill is located on the central-western sector of the Huincul High. The granitoids that form the core of Cerro Granito are covered by Early and Middle Jurassic sedimentary rocks that crop out surrounding the hill. The Early Jurassic sediments at that locality were first mentioned by Groeber (Reference Groeber1918, p. 47–48) and Keidel (Reference Keidel1925, p. 25, pl. 2), who indicated the presence of limestones at the base of a transgressive sequence, which were dated then by the presence of Weyla alata (von Buch, Reference von Buch1838). Weaver (Reference Weaver1931, p. 22, 26) described Early Jurassic sandy limestones lying directly on old granites all around the hill, followed by Middle Jurassic fossiliferous beds on the northeast slope. The area was first mapped by Suero (Reference Suero1942, with more detail in 1951), who also mentioned the basal limestones with Weyla alata , rhynchonellid brachiopods, and corals, followed by beds with late Pliensbachian ammonites (Fig. 1.3).
Recently, the structural cinematics of this western sector of the Dorsal de Huincul were analyzed by Rossello and López de Luchi (Reference Rossello and López de Luchi2022), and the granitoids of the Cerro Granito core were studied by López de Luchi et al. (Reference López de Luchi, Rossello, Martínez Dopico and Ruffet2023). These authors also mapped the overlying Jurassic sediments of the Cuyo Group. Suero (Reference Suero1942, pl. 6, fig. 1) and Rossello and López de Luchi (Reference Rossello and López de Luchi2022, p. 481) recorded that the Early Jurassic fossiliferous limestones fill not only irregular cavities of the granitic surface but also fractures (as clastic dikes).
Field observations by one of us (A.C.G.) show a strong lateral change in thickness of the Early Jurassic deposits of Cerro Granito due to the development of a marked paleorelief carved on plutonic rocks (Chachil Plutonic Complex, late Permian). In the fossiliferous sector (Fig. 1.3), the Jurassic strata dip about 80° due to strong regional deformation. Field measurements revealed that the “Liassic deposits" (Suero, Reference Suero1951) are 17.4 m thick; they were interpreted as deposited in a shallow-water high-energy environment. These deposits comprise four distinctive lithological units (Fig. 1.3):
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(1) 6.6 m of poorly stratified conglomerates and sandy conglomerates of monomictic composition, integrated mainly by angular granodioritic clasts derived from the underlying plutonic substrate;
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(2) 0.2 m of a massive tabular bed of ochraceous coarse-grained calcarenites;
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(3) 8.7 m of texturally immature calcareous massive sandstones of variable grain size, showing occasionally small gravelly lenses (in general terms, this section of the sequence shows a normal grading, with the presence of large fossil remains [mollusks] in the upper levels of the succession, where the Opisoma specimens were recovered);
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(4) 1.9 m of highly fossiliferous ruditic limestones, with variable epiclastic content. This level contains abundant specimens of Weyla alata alongside brachiopods and coral remains.
Toward the southwest slope of Cerro Granito near Puesto Romero, in 1973, Riccardi, Manceñido, and Damborenea collected, in laterally equivalent levels, bivalves and brachiopods that were referred to a latest Pliensbachian–earliest Toarcian age (see section in Damborenea, Reference Damborenea1987, p. 45, text-fig. 4). The bivalves were described by Damborenea (Reference Damborenea1987, Reference Damborenea2002) and Echevarría et al. (Reference Echevarría, Damborenea and Manceñido2021), and the brachiopods by Manceñido (Reference Manceñido, MacKinnon, Lee and Campbell1990). Throughout the south of Neuquén Basin, close to Cerro Granito, coral-dominated communities developed, including an extensive coral meadow pointing to rather uniform paleoenvironmental conditions of probably low energy, shallow water depth, and well-illuminated substrates (Echevarría et al., Reference Echevarría, Hoqui, Damborenea, Manceñido, Garberoglio and Harguindeguy2024).
Materials and methods
Ten specimens were collected from the northwest slope of Cerro Granito (Neuquén, Argentina). Most specimens are housed in the Provincial Museum of Natural Sciences “Prof. Dr. Juan Olsacher” in Zapala, Neuquén Province, and one specimen is housed in the Repository of the Faculty of Exact and Natural Sciences of Buenos Aires University, cast in La Plata Museum.
Specimens MOZ-PI 4163, 4202, and CPBA 20515 were analyzed using a combination of traditional morphological methods, including macroscopic observation, photography, and caliper measurements. In addition, computed tomography (CT) analyses were performed, a non-invasive procedure that poses no risk to the integrity of the material. Both right and left valves were preserved in the studied material. When a valve is absent, the teeth of the missing valve remain adhered to those of the preserved one, making separation nearly impossible without compromising the structure. This observation is consistent with Accorsi Benini (Reference Accorsi Benini1981) and Posenato et al. (Reference Posenato, Bassi and Nebelsick2013) and prompted the examination by non-destructive methods, such as CT scan. The tomographic study was conducted at San Martín Hospital, in La Plata, Argentina. The scans were acquired using a Canon Prime SP Aquilon scanner. All scans were reconstructed using the software Mimics 21.0.
We follow Carter et al. (Reference Carter, Harries, Malchus, Sartori and Anderson2012) for descriptive morphological terms such as corselet, as well as to describe the anatomical orientation of Opisoma (i.e., dorsal, ventral, anterior, posterior) (Fig. 2.1). Posenato et al. (Reference Posenato, Bassi and Nebelsick2013) used “basal area” for what is here considered the anterior surface limited by the right and left anterior carinae, while their “apical area” corresponds to the corselet.
Opisoma romeroi n. sp. (1, 2) Sketches of shell exterior showing general morphology and measurements: (1) right valve, external lateral view; (2) both valves, posterior view. (3, 4) Sketches of shell inner views showing hinge and ligamental characters: (3) left valve; (4) right valve. Black arrows point to the shell anterior region.
Figure 2. Long description
Panel 1, top left, shows the right valve in external lateral view. The dorsal-ventral and posterior-anterior axes are marked with double-headed arrows. The umbo is at the dorsal-posterior corner. The corslet is a curved band running from the umbo toward the anterior carina. The area is the main shell surface. Posterior and anterior carina are labeled at the shell margins. Panel 2, top right, shows both valves in posterior view. Width and height are marked with double-headed arrows. The umbo is at the dorsal midline, with the nymph just below. The corslet forms a broad arc across the shell. The area is central. Posterior and anterior carina are at the lateral margins. The commissure is at the ventral edge. Panel 3, bottom left, shows the left valve in internal view. The umbo and nymph are at the dorsal edge. The corslet is a band near the dorsal margin. The lunule is a small area near the umbo. Teeth (T) are aligned along the hinge. The anterior adductor scar and internal sulcus are shaded regions at the ventral margin. Panel 4, bottom right, shows the right valve in internal view, mirroring panel 3. All features are similarly labeled. Black arrows in all panels point to the anterior region of the shell.
Repositories and institutional abbreviations
Type, figured, and other specimens examined in this study are deposited in the following institutions: Provincial Museum of Natural Sciences “Prof. Dr. Juan Olsacher” (MOZ), Zapala, Neuquén, Argentina; Faculty of Exact and Natural Sciences, University of Buenos Aires (CPBA), Buenos Aires, Argentina; La Plata Museum (MLP), La Plata, Argentina; Mollusk Fossil Collection, Muséum National d´Histoire Naturelle, Paris (MNHN, Paris, France).
Systematic paleontology
Subclass Archiheterodonta Giribet in Taylor et al., Reference Taylor, Williams, Glover and Dyal2007
Order Carditida Dall, Reference Dall1889
Superfamily Crassatelloidea de Férussac, Reference de Férussac1822
Family Astartidae d’Orbigny, Reference d’Orbigny1844
Subfamily Opinae Chavan, Reference Chavan1952
Genus Opisoma Stoliczka, Reference Stoliczka1871
Type species
Cardium paradoxum Buvignier, Reference Buvignier1843, p. 230, pl. 4, figs. 3, 4 (= Opis paradoxa Buvignier, Reference Buvignier1852, p. 17, pl. 13, figs. 37–42). Late Jurassic (Oxfordian), Moselle Department, France (Fig. 3).
Opisoma paradoxum (Buvignier, Reference Buvignier1843), type species of the genus Opisoma, specimens figured by Buvignier (Reference Buvignier1852, pl. 13). (1, 2) Right valve, MNHN.F.R 07429: (1) external view, cf. fig. 37 in Buvignier (Reference Buvignier1852); (2) internal view, cf. fig. 39 in Buvignier (Reference Buvignier1852). (3) MNHN.F.R 07383, both valves in posterior view. Photos downloaded from Mollusk Fossil Collection, Muséum National d´Histoire Naturelle, Paris. Scale bars: =10 mm.
Figure 3. Long description
From left to right, panel one displays the right valve of specimen M N H N dot F dot R 07429 in external view, with the number R dot 7429 written in black ink near the top edge. The surface is triangular with subtle growth lines and a pale brown coloration. Panel two shows the same specimen in internal view, revealing a deep cavity with prominent ridges radiating from the hinge area and a small rectangular label reading ‘FIGURÉ’ at the base. Panel three presents specimen M N H N dot F dot R 07383 in posterior view, showing both valves together. The number R dot 7383 is marked in black ink along the upper edge. The valves are tightly joined, forming a pronounced central ridge. All panels include a black scale bar at the bottom, each representing 10 millimeters.
Other species
See discussion in Accorsi Benini (Reference Accorsi Benini1981, p. 201)
Amended diagnosis
Shell large, thick, ovate to cordiform posterior outline, wing-shaped and antero-posteriorly compressed. Equivalve and inequilateral. Umbones opisthogyrate to orthogyrate. Shell with posterior and anterior carinae; area between carinae deeply concave to flat. Posterior carina meeting commissure at about one- to two-thirds of shell height from umbo. Anterior region externally convex, flat, or even concave. Anterior adductor muscle scar oval to circular, sometimes positioned on a distinct platform. Hinge with strong, trigonally expanded teeth, which are parallel to each other or slightly curved. Left valve with three teeth, right valve with two or three teeth. Deep nymph, parallel to last posterior tooth.
Remarks
There is considerable controversy regarding the shell orientation of Opisoma. Some authors regard Opisoma as opisthogyrate (Buvignier, Reference Buvignier1852; Dubar, Reference Dubar1948; Chavan, Reference Chavan, Cox, Newell, Boyd, Branson and Casey1969; Clari, Reference Clari1975; Accorsi Benini, Reference Accorsi Benini1980; Posenato et al., Reference Posenato, Bassi and Nebelsick2013), whereas others consider it as prosogyrate (Boehm, Reference Boehm1884; Accorsi Benini, Reference Accorsi Benini1981; Aberhan and von Hillebrandt, Reference Aberhan and von Hillebrandt1999; Li et al., Reference Li, Ye, Rao, Han and Wang2026). These divergent interpretations arise from differing assessments of the position of the ligament and lunule, the identification of the left and right valves, the hinge formula, and the placement of the adductor muscle scars. Stoliczka (Reference Stoliczka1871) noted that some species of Opisoma closely resemble others of Opis in the general shell shape but differ markedly in hinge structure, showing a considerably higher number of cardinal teeth than Opis. A more recent extensive review of the subfamily Opinae (Squires and Saul, Reference Squires and Saul2009) redefined its diagnostic characters and proposed that opines can be distinguished from the other three astartid subfamilies in having obsolete or no lateral teeth and commonly a carinate shell.
The dentition pattern is one of the most useful characters for generic distinction, not only in veneroid bivalves but also in most heterodont bivalves (Jaitly and Mishra, Reference Jaitly and Mishra2009). However, caution is necessary as various authors such as Popenoe and Findlay (Reference Popenoe and Findlay1933) and Matsukuma (Reference Matsukuma1996) have reported the occurrence of hinge transposition in Crassatelloidea, both fossil and extant. From the analysis of the Argentine material and bibliographic references, we conclude that the genus Opisoma has three teeth on the left valve and two on the right valve, except for O. menchikoffi Dubar, Reference Dubar1948, which has three teeth on both valves.
Opisoma romeroi new species
Opisoma romeroi n. sp. Holotype MOZ-PI 4163. (1) Both valves, posterior view, box indicates the area shown in (2). (2) Posterior view of right valve, detail of nymph and umbo. (3) Right valve, internal view, box indicates the area shown in detail in (6). (4) Left valve showing striated tooth. (5) Left valve, internal view. (6) Right valve, detail of nymph, umbo, and anterior adductor muscle scar. (7) Both valves, left lateral external view. Scale bars = 50 mm.
Figure 4. Long description
Panel 1 at top left displays both valves in posterior view, with a rectangular box marking a region on the right valve. Panel 2, to the right, is a close-up of the boxed area from panel 1, showing the posterior of the right valve with visible nymph and umbo. Panel 3, below panel 1, presents the right valve in internal view, with a box highlighting a region for further detail. Panel 4, to the right of panel 3, shows the left valve in lateral view, emphasizing the striated tooth along the hinge. Panel 5, to the right of panel 4, displays the left valve in internal view, revealing muscle scars and hinge structures. Panel 6, below panel 3, is a close-up of the boxed area from panel 3, detailing the nymph, umbo, and anterior adductor muscle scar on the right valve. Panel 7, at bottom right, presents both valves in left lateral external view. All panels include horizontal scale bars labeled as 50 mm, providing dimensional context for each view.
Opisoma romeroi n. sp. (1–3) Paratype CPBA N 20515; (1) both valves, anterior view; (2) both valves, posterior view; (3) left lateral external view. (4) Paratype MOZ-PI 4205, left valve, internal view. (5) MOZ-PI 4202, left valve anterior view, box indicates the area shown in detail in (6). (6) Left valve with internal layer exposed. (1–5) Scale bars =100 mm; (6) scale bar = 10 mm.
Figure 5. Long description
There are six photo panels arranged in two rows. Top row, from left to right: Panel 1 shows both valves of paratype C P B A N 20515 in anterior view, displaying a heart-shaped outline with a central suture and fine surface striations. Panel 2 presents the same specimen in posterior view, highlighting the convexity and surface texture. Panel 3 shows a left lateral external view of the same specimen, emphasizing the thickness and curvature of the valves. Bottom row, from left to right: Panel 4 displays paratype M O Z dash P I 4205, left valve in internal view, revealing muscle scar impressions and internal ridges. Panel 5 shows M O Z dash P I 4202, left valve in anterior view, with a rectangular box marking the area magnified in panel 6. Panel 6 is a close-up of the boxed region from panel 5, showing the internal layer exposed with fine lamination. Scale bars in panels 1 to 5 represent 100 millimeters, while panel 6 uses a 10 millimeter scale bar. All panels are in grayscale and focus on fossil morphology.
Opisoma romeroi n. sp. CT scan images. (1–3, 5, 6) Holotype MOZ-PI 4163: (1) both valves showing the striated tooth in coronal view from a 2D tomographic slice; (2) coronal 2D tomographic slice of both valves, section perpendicular to commissural plane; (3) same view as in (2) with tooth contact highlighted (white dashed line); (5) 3D reconstruction of the right valve, internal view; (6) 3D reconstruction of the left valve, internal view. (4) MOZ-PI 4202: 2D tomographic slice of the entire left valve with adjoined teeth of the right valve. Scale bars = 50 mm.
Figure 6. Long description
Panel 1 at top left shows a coronal 2D tomographic slice of both valves, with RV (right valve) and LV (left valve) labeled, revealing the striated tooth structure. Panel 2, right of panel 1, presents a coronal 2D slice perpendicular to the commissural plane, both valves labeled RV and LV. Panel 3, right of panel 2, repeats the view of panel 2 but highlights the tooth contact with a white dashed line. Panel 4, bottom left, displays a 2D tomographic slice of the entire left valve with the adjoined teeth of the right valve. Panel 5, bottom center, is a 3D reconstruction of the right valve in internal view, showing surface texture and internal ridges. Panel 6, bottom right, is a 3D reconstruction of the left valve in internal view, mirroring panel 5. All panels include a scale bar equal to 50 mm.
Measurements (mm) of Opisoma romeroi n. sp., including height, width, and length. 1V = one valve; 2V = two valves.
Table 1. Long description
The table has four columns: Specimen, Height, Width, and Length. The first row lists MOZ dash P I 4164 with height 151.9 millimeters, width 137.5 millimeters two valves, length 80.1 millimeters. The second row is MOZ dash P I 11203 slash 2 with height 153.3 millimeters, width 119 millimeters two valves, length 79.9 millimeters. The third row is MOZ dash P I 11203 slash 1 with height 132.2 millimeters, width 107.2 millimeters two valves, length 95 millimeters. The fourth row is MOZ dash P I 11210 with height 129.1 millimeters, width 57.6 millimeters one valve, length 71.2 millimeters. The fifth row is MOZ dash P I 4202 with height 189.6 millimeters, width 102 millimeters one valve, length 85.5 millimeters. The sixth row is MOZ dash P I 4205 with height 166 millimeters, width 71.4 millimeters one valve, length 60.6 millimeters. The seventh row is MOZ dash P I 4162 dash a with height 147 millimeters, width 71.4 millimeters one valve, length 79 millimeters. The eighth row is MOZ dash P I 4162 dash b with height 160 millimeters, width 61 millimeters one valve, length 53.4 millimeters. The ninth row is C P B A number 20515 with height 211 millimeters, width 173 millimeters two valves, length 85 millimeters.
Reference Damborenea, Echevarría and Ros-Franch2013 Opisoma sp. Damborenea et al., fig. 3.4.a.
Holotype
A nearly complete shell with both valves, MOZ-PI 4163 (Figs. 4, 6.1–6.3, 6.5-6.6) Cerro Granito, Zapala Department, Neuquén, Argentina, latest Pliensbachian/earliest Toarcian.
Other specimens
Paratypes, CPBA 20515 (=MLP 18460, cast) (Fig. 5.1–5.3) and MOZ-PI 4205 (Fig. 5.4). Other materials, MOZ-PI 4162-a, b, 4164, 4202 (Figs. 5.5, 5.6, 6.4), 11203/1, 2 and 11210.
Diagnosis
Opisoma with large, thick shell, ovate to cordiform posterior outline, crescentic longitudinal profile. Umbones opisthogyrate. Both carinae well developed; area between them concave. Posterior carina meeting commissure at about two-thirds of shell height from the umbo. External parivincular ligament in a deep nymph. Anterior adductor scar small and circular, excavated, deep, on a distinct platform adjacent to the hinge plate. Corselet plane to slightly concave. Hinge robust, with strong, nearly straight, obliquely striated, trigonal expanded teeth parallel to each other; left valve with three teeth, right valve with two. Regular commarginal lirae on external shell surface.
Occurrence
Cerro Granito, Zapala Department, Neuquén, Argentina. Latest Pliensbachian/ earliest Toarcian.
Description
The shell is very large, robust, equivalve, and very inequilateral. It is thick and antero-posteriorly compressed, resulting in an ovate to cordiform outline in anterior and posterior views. The longitudinal section is typically crescentic. The umbones are opisthogyrate, and both carinae (anterior and posterior) originate from it (Fig. 4.1, 4.2, 4.7), dividing the shell into three different regions. The shell has a planar commissural plane. The posterior carina meets the commissure at about two-thirds of the shell height from the umbo, forming a prominent rounded dorsal crest (Figs. 2, 4.1). The shell thickens toward the anterior region (Fig. 4.3, 4.5, 4.7). The anterior carina is sharp, acute, and externally projected (Fig. 5.3). The area between carinae is concave (Figs. 4.7, 5.3). The corselet is concave (Figs. 2, 4.1, 4.3).
There is a deep nymph (Figs. 2.3, 2.4, 4.2) parallel to the last posterior tooth (Figs. 2.3, 2.4, 4.5). The lunule is poorly defined. The hinge is robust, with strong teeth, trigonally expanded and parallel to each other. The left valve has three teeth (Figs. 2.3, 4.5, 5.4), a posterior one weak and parallel to the nymph. The right valve has two strong teeth (Figs. 2.4, 4.3, 6.3, 6.5) All teeth are obliquely striated (Fig. 4.4); the striations possibly represent growth increments and are clearly visible in both the CT scans (Fig. 6.1) and the hand specimens. The anterior adductor muscle scar is small, circular, deeply impressed, adjacent and anterior to the anterior tooth, never placed ventrally to it (Figs. 2.3, 2.4, 4.3, 4.5, 4.6, 5.4). A well-defined lateral longitudinal groove develops from below the hinge platform to the posterior tip of the shell (Figs. 2.3, 2.4, 4.3, 4.5, 5.4). This groove corresponds to what Accorsi Benini called “solco interno longitudinale” (=longitudinal internal sulcus) and is flanked by a “costa interna longitudinale” (=longitudinal inner costa), posterior to the sulcus, and an anterior ridge. The shell is naturally abraded, leaving the internal layer exposed showing clear radial lines (Fig. 5.5, 5.6) as indicated by Accorsi Benini and Broglio Loriga (Reference Accorsi Benini and Broglio Loriga1982).
Etymology
The species epithet romeroi is dedicated to Francisco Ramón Romero, first director of the “Carmen Funes” Museum of Plaza Huincul (Neuquén Province), who collected the first specimens in the Cerro Granito area 40 years ago. The name appears in labels written by the late Dr. H. Camacho, indicating the recognition of the new species and his intention to name it after its first collector.
Remarks
Opisoma specimens from the Toarcian of Chile were referred to O. excavatum Boehm by Aberhan and von Hillebrandt (Reference Aberhan and von Hillebrandt1999). Chilean and Argentine specimens exhibit remarkable morphological similarity and might be conspecific, but they differ in certain characteristics of the anterior adductor muscle scars and teeth. The Chilean specimens show a larger and oval (not circular) adductor muscle scar and have much more arcuate cardinal teeth than the Argentinian specimens.
The shells here described from Argentina differ from Italian specimens of O. excavatum Boehm (Reference Boehm1884, p. 771, pl. 23, figs. 1, 2, pl. 25, figs. 1–3; Accorsi Benini, Reference Accorsi Benini1981, figs. 2, 4, 8a, 9, 11b, 13, 16, 17, tav. 1, figs. 2–4, tav. 4, figs. 4, 6, 7; Posenato et al., Reference Posenato, Bassi and Nebelsick2013) in the area between the two carinae, which is flat in O. excavatum but concave in O. romeroi n. sp. Most of the specimens of O. excavatum reported by Posenato et al. (Reference Posenato, Bassi and Nebelsick2013) from the Rotzo Formation are smaller than the Argentine ones, although some individuals reach comparable sizes. The anterior (= basal) region of the valves is externally convex in Opisoma romeroi n. sp., whereas in O. excavatum it is flat or even concave in cross section (Posenato et al., Reference Posenato, Bassi and Nebelsick2013). In addition, in O. excavatum the posterior carina meets the commissure at about one-third of the shell height from the umbo, while in O. romeroi n. sp. it extends further ventrally, meeting the commissure at about two-thirds of the shell height from the umbo (Figs. 4.3, 5.2; compare with Posenato et al., Reference Posenato, Bassi and Nebelsick2013, fig. 3). In general, the Argentine specimens also exceed in size all the widely known species from the Veneto studied by Accorsi Benini (Reference Accorsi Benini1981, Reference Accorsi Benini1985) in the Lithiotis facies.
Opisoma paradoxum (Buvignier, Reference Buvignier1843), the type species of the genus (Fig. 3), shows certain similarities to the Argentine specimens, such as the nearly straight cardinal teeth. However, notable differences are observed in its smaller size and in the anterior adductor muscle scar, which is oval and located ventral to the hinge platform (Buvignier, Reference Buvignier1843, pl. 4, figs. 3, 4, 1852, pl. 13, figs. 37–42; Péron, Reference Péron1906, pl. 1, fig. 6; Fig. 3.2 here). Furthermore, the area between the two carinae is flat in Opisoma paradoxum (Fig. 3), whereas in the Argentine species it is distinctly concave (Fig. 5.3). Opisoma romeroi n. sp. is comparable in size to some of the Moroccan species described by Dubar (Reference Dubar1948, p. 182, pl. 18, figs. 5–7, pl. 19, figs. 1–4, pl. 20, figs. 2–5, pl. 21, figs. 1–3, pl. 22, figs. 1, 2, text-figs. 63–66), such as Opisoma menchikoffi (see also Accorsi Benini, Reference Accorsi Benini1981, fig. 7, table 2), which exhibits an extremely elongate and strongly opisthogyrate shell, as well as a deeply concave area between the two carinae. By contrast, O. romeroi n. sp. does not display the markedly crescent-shaped outline of O. menchikoffi, and the concavity between the carinae is considerably less pronounced.
All specimens of O. romeroi n. sp. in which the hinge could be observed have three teeth on the left valve and two teeth on the right valve. Most commonly, astartids bear three cardinal teeth on the right valve and two on the left valve (Boyd and Newell, Reference Boyd and Newell1968). The reversed formula found in Opisoma suggests a case of transposed hinges, i.e., those exhibiting in the right valve hinge elements that normally occur in the left valve and vice versa (Popenoe and Findlay, Reference Popenoe and Findlay1933). This situation has been recorded in both extant and extinct representatives of the family Astartidae, as well as in representatives of the subfamily Trigonopinae.
Discussion
Change from infaunal to epifaunal life habit
Unlike the infaunal lifestyle typical of astartids, Opisoma developed an epifaunal or semi-infaunal mode of life. This life habit change, probably triggered by a photosymbiotic habit, involved several structural modifications on the shell. All authors agree on an orthothetic epifaunal mode of life for the taxon (Accorsi Benini, Reference Accorsi Benini1981; Seilacher, Reference Seilacher1984, Reference Seilacher1990; Savazzi, Reference Savazzi1985; Squires and Saul, Reference Squires and Saul2009), varying the interpretation about which surface was against the substrate: the posterior or the anterior one.
Anatomically, Accorsi Benini (Reference Accorsi Benini1981) interpreted the nymph as a tubular lunule, probably due to the separation of the anterior tips of both halves, so her interpretation is the reverse of the one considered here. A similar interpretation was made by Aberhan and von Hillebrandt (Reference Aberhan and von Hillebrandt1999) although based on hinge structure, which here is considered as transposed. By contrast, Posenato et al. (Reference Posenato, Bassi and Nebelsick2013) reconstructed the shell as opisthogyrate; they also proposed a morphologic terminology independent from anatomical reconstruction but related to their morphofunctional interpretation, with a basal surface (what we recognize as anterior) and an apical surface (our posterior; Fig. 2).
Seilacher (Reference Seilacher1990) and Aberhan and von Hillebrandt (Reference Aberhan and von Hillebrandt1999) reconstructed the life mode of Opisoma species with the posterior (apical) surface against the substrate. By contrast, Posenato et al. (Reference Posenato, Bassi and Nebelsick2013) considered that Opisoma lived with the anterior (basal) surface against the substrate.
A similar evolutionary trend toward semiinfaunal and epifaunal life habits is present in other bivalve lineages, such as the Fragiinae within the Cardiidae. Fragum Röding, Reference Röding1798 may represent a first step toward this “epifaunalization” with a semi-infaunal to infaunal life habit, but spreading the posterior mantle margins, with zooxanthellae, on the surrounding sediment. It has antero-posteriorly compressed shells (relatively short and high), although not as much as Opisoma, and a high posterior margin. Corculum is fully epifaunal, with the posterior slope completely exposed above the sediment. Semi-transparent windows in the shell allow the passage of sunlight to the mantle. The antero-posterior flattened shell is an adaptation to offer a large posterior surface to sunlight, as well as to offer more stability on a soft substrate with a wide anterior surface (see Discussion).
The particular shell shape of Opisoma is included among the group of alatoform bivalves, such as the Alatoconchoidea (middle Permian), the Ramonalinidae (Middle Triassic), and the Wallowaconchidae (Late Triassic). All members of this polyphyletic morphologic group share antero-posteriorly compressed and laterally expanded shells, and all of them are reconstructed as edgewise recliners, with a flattened anterior surface oriented downward and the posterior margin directed upward. Some Permian Alatoconchidae from Japan have been interpreted as byssate forms (Asato et al., Reference Asato, Kase, Ono, Sashida and Agematsu2017).
From a morphogenetic point of view, to obtain subcircular shells offering a wide stabilizing surface, the whorl expansion rate must decrease, but as a consequence, umbones tend to touch each other, hindering shell opening (Savazzi, Reference Savazzi1985). There are three main potential solutions for this problem: (1) offsetting of the umbones, like in Corculum, (2) the development of a prosogyrate or an opisthogyrate shell, generating a more elongate morphology, arcuate in lateral view, and (3) the spreading apart of umbones. The offsetting of the umbones is barely developed in a few specimens of O. romeroi n. sp. The main solutions found in this species are the development of an opisthogyrate shell (Figs. 4, 5.3, 5.4) and the spreading apart of the umbones (Fig. 5.2). The latter has also the effect of stretching apart the opisthodetic ligament on its umbo-proximal tip (Figs. 5.2, 7.3). This results in an unusual morphology of the nymph, which probably led some authors to interpret it as a tubular lunule (Accorsi Benini, Reference Accorsi Benini1981).
Opisoma romeroi n. sp., functional morphology sketches. (1, 2) Reconstruction with marginal line of both nymphae halves in contact: (1) valves closed due to adductor muscle contraction pulling them together (black arrows), resulting in tensile stress on ligament outer lamellar layer; (2) muscle adductor relaxation releases the tensional stress of the outer lamellar layer, pulling the valves open (black arrows) and resulting in a rotational axis parallel to nymphae margin (dashed line). (3, 4) Reconstruction with umbones spreading apart during ontogeny: (3) valves closed due to adductor muscle contraction pulling them together (black arrows), resulting in tensile stress on umbo-proximal ligamental tip; (4) muscle adductor relaxation releases the tensional stress of the umbo-proximal ligamental tip, pulling the valves open (black arrows), resulting in a rotational axis normal to nymphae margin (dashed line).
Figure 7. Long description
Panel 1, top-left, shows both nymphae halves in contact with the adductor muscle (brown, central) contracted, pulling the valves closed as indicated by inward black arrows. The ligament outer lamellar layer is under tensile stress. Panel 2, top-right, shows the adductor muscle relaxed, releasing tension in the outer lamellar layer, and the valves opening outward, with black arrows pointing away from the muscle and a dashed line marking a rotational axis parallel to the nymphae margin. Panel 3, bottom-left, depicts umbones spreading apart during ontogeny, with the adductor muscle contracted and black arrows pulling the valves closed, creating tensile stress at the umbo-proximal ligamental tip. Panel 4, bottom-right, shows the adductor muscle relaxed, releasing tension at the umbo-proximal ligamental tip, opening the valves with black arrows pointing outward and a dashed line indicating a rotational axis normal to the nymphae margin. Each panel highlights the anatomical structures involved and the direction of force application.
In bivalves with regularly developed nymphae (with the whole marginal line of both halves in contact) the opisthodetic parivincular ligament has an outer lamellar layer subjected to tensile stress and an inner fibrous layer compressed, with the pivotal axis (representing the fulcrum of a lever system) located between them (Fig. 7.1, 7.2; Trueman, Reference Trueman1949). As a result, shell gape is maximum at the margin opposing the ligament line (Fig. 7.2).
The spreading apart of the umbones in Opisoma results in the umbo-proximal stretch of the opisthodetic ligament (Figs. 5.2, 7.3). This stretching developed during ontogeny and probably resulted in tensile stress pulling together the umbones (Fig. 7.3, 7.4) and decreasing toward the umbo-distal tip of the ligament. The adductor muscle closed the valves during contraction, like in most bivalves. During muscle relaxation, the tensile stress in the umbo-proximal tip of the ligament pulled the valves open, but with a new rotational axis perpendicular to the main length of the ligament and located at the umbo-distal tip of the nymph (Fig. 7.4). Hence, the umbo-distal tip of the ligament acted as fulcrum, experiencing little or no stress. As a consequence, shell gape would be maximum at the ventral tip of the shell and the umbo-distal ends of the carinae (Fig. 7.4). When the shell thickness is analyzed, we see that a large amount of shell material is concentrated on the hinge, providing an apical balance for the wedge-shaped shell (Fig. 8.1). In reclining pleurothetic bivalves, the thicker valve rests against the substrate, providing much stability to the shell (Seilacher, Reference Seilacher1984). In Opisoma, the apex clearly fulfils part of the stabilizing function. In addition, when considering both faces (basal and apical) we see that the main keel (anterior carina) delimiting the basal surface is very thick. This results in a much more stable position with the anterior surface toward the substrate (Fig. 8.2). Even more, given the wider basal surface, a resting position with the apical surface against the substrate would produce a downward pull of the shell keels (Fig. 8.3), constantly opening the valves and forcing the animal to make an active effort to maintain them in position.
Opisoma romeroi n. sp., functional morphology sketches. (1) Schematic representation of the shell in left lateral view, with the approximate position of the gravity center (black dot with downward arrow). (2) Reconstruction on ventral view with the anterior (basal) surface toward the substrate. (3) Reconstruction on ventral view with the posterior (apical) surface toward the substrate; gray dots with downward arrows indicate the gravitational pulling of valve sides. Gray dashed line: approximate position of visceral cavity.
So the combination of morphogenetic oddities and drawbacks of Opisoma shells may produce a highly adaptive morphologic result. The anterior stretching of the ligament may result in a change of the articulation axis of the valves (Fig. 7). As a result, the widest opening of the shell is at the ventral instead of the anterior margin. Coherent with this interpretation are the large teeth of these shells, which are parallel to the nymph. If the parivincular ligament was to work in the regular way, the teeth in that position (parallel to the articulation axis) would hinder valve opening (they should be laterally curved, with an upward concave surface). If the articulation axis were perpendicular to the nymph, then the teeth would be in a more functional position.
In addition, the opisthogyrate shell growth that may reduce the tensile stress on the ligament provides a convex basal surface, and the weight of the hinge results in an upwardly directed posterior margin of the shell and the keel slightly above the sediment (Fig. 8.1). This can be observed in empty shells that tend to remain in that position as the most stable one.
A potential mutualistic association
Bivalve lineages that shifted from infaunal to epifaunal lifestyle have been considered “primary suspects” for photosymbiosis as this type of association appears to be the only ecological advantage that would justify abandoning a protected life within the sediment (Seilacher, Reference Seilacher1990). The underlying principle in all these cases is that the organism must, in some way, maximize the amount of light reaching the soft tissues housing symbionts.
Among extant bivalves, the Cardiidae have apparently developed a photosymbiotic relationship twice in their evolution, appearing independently in the subfamilies Tridacninae and Fraginae. However, the phylogenetic relationship between the two subfamilies is not yet fully resolved (Schneider, Reference Schneider1998; Herrera et al., Reference Herrera, ter Poorten, Bieler, Mikkelsen, Strong, Jablonski and Steppan2015; Kirkendale and Paulay, Reference Kirkendale and Paulay2017). The Fraginae appear morphologically convergent with Opisoma. The species within Fragum temporarily open their valves to expose mantle tissue and, in some cases, extend it outside the shell. Fragum fragum (Linnaeus, Reference Linnaeus1758) has thin and translucent valves that open only slightly and lack thickening or expansion of the siphonal mantle. By contrast, F. unedo (Linnaeus, Reference Linnaeus1758) possesses thick, opaque valves capable of opening widely, covered by large and highly expanded siphonal mantles (Kawaguti, Reference Kawaguti1983). In general, species of the genus Fragum exhibit only a limited extension of the mantle beyond the shell margin. Some species show minimal modifications, retaining a morphology similar to non-photosymbiotic species in shell shape and size, as observed in F. erugatum (Tate, Reference Tate1889) and F. fragum (see Ohno et al., Reference Ohno, Katoh and Yamasu1995). The heart cockles (Corculum) remain closed as light passes through transparent windows present in their shells (McCoy et al., Reference McCoy, Burns, Klopfer, Herndon, Ogunlade, Dionne and Johnsen2024). Tridacninae clams solve the light-capturing problem by opening their valves and extending the mantle with photosymbionts outside them to illuminate it directly. The extension of the mantle over the shell is not, in itself, sufficient evidence for photosymbiosis as it is prevalent in some aposymbiotic lineages. However, certain bivalve morphologies facilitate light capture and may indicate photosymbiosis, although none of these features alone provides conclusive evidence. These include a wide gape between the valves in the up-facing position, often accompanied by a greatly thickened and heavily weighted umbonal, anterior, or ventral side of the shell, undulating shell margins, mantles extended beyond the valve margins, and thin, translucent shells with microstructures suitable for transmitting light (Seilacher, Reference Seilacher1990; Vermeij, Reference Vermeij2013; Kirkendale and Paulay, Reference Kirkendale and Paulay2017). None of these modifications is found in O. romeroi n. sp. (except for the weighted umbonal side of the shell, which is related mostly to a reclining habit).
Rapid shell mineralization, high growth rates, and gigantism have also been used to infer photosymbiosis in fossil taxa as they are found in living Tridacna. This is not a mandatory condition, nonetheless, as symbiosis with algae does not always imply gigantism of skeletal parts (e.g., the genera Corculum and Fragum have small shells). Conversely, large bivalves from the Lithiotis facies such as Lithiotis Gümbel, Reference Gümbel1871, Cochlearites Reis, Reference Reis1903, and Lithioperna Accorsi Benini, Reference Accorsi Benini1980 exhibit a high longevity rather than a mutualistic relationship with photosymbionts according to Accorsi Benini (Reference Accorsi Benini1985). Increases in calcification rate may result in excessive growth of skeletal parts without necessarily implying parallel growth of soft tissues. Opisoma romeroi n. sp. has a heavily calcified shell with a relatively small cavity for soft tissues (Figs. 4.3, 6.3, 6.5, 8.1).
The search for a potential record of a photosymbiotic phenomenon in the shell can be conducted on a microstructural basis, provided fossils with original microstructure are available. Unfortunately, the recrystallized shells of O. romeroi n. sp. do not allow for such analysis. The microstructural description of O. excavatum by Posenato et al. (Reference Posenato, Bassi and Nebelsick2013) shows that the basal shell wall was very thick, exhibiting a complex crossed-lamellar structure, which would limit efficient light transmission. However, the authors noted that this complex external layer was not observed in the apical region, favoring shell transparency. Therefore, O. excavatum was inferred to have developed a strategy similar to that of Corculum, allowing light to penetrate the tissues hosting the symbionts. The septa present in O. excavatum could not be observed in our material.
So the evidence for a photosymbiotic life habit in Opisoma romeroi n. sp. is not conclusive. It shares the general shell morphology of other lineages traditionally interpreted as (or even known to be) photosymbiotic, but at the same time, it lacks the main characters associated with photosymbiosis identified in other Opisoma species. Although not associated with the typical bivalves of the Lithiotis facies, it certainly inhabited communities of tropical latitudes, as indicated by the thriving coral communities in the area (Echevarria et al., 2024) and the paleomagnetic studies (Iglesia-Llanos et al., Reference Iglesia-Llanos, Riccardi and Singer2006). The heavily calcified shell may support a photosymbiotic habit, but it is not conclusive. In any case, if a photosymbiotic association was achieved, it was most likely maintained by exposing mantle tissues through the posterior margin of the shell (Fig. 9), similar to Tridacna or Fragum although with an epifaunal life habit. The maximum shell gape on ventral margin (7.4) would support this interpretation.
Opisoma romeroi n. sp., Early Toarcian, Cerro Granito, Neuquén, Argentina. Reconstruction of living habit, drawn by Samanta Vanesa Faiad, La Plata Natural Sciences Museum. (1) Paleoenvironmental reconstruction in block diagram showing Opisoma in life position within a shallow marine setting, associated with ammonites of the genus Dactylioceras. (2) Posterior dorsal view of Opisoma; the bright green area represents extruded mantle. (3) Oblique lateral view of Opisoma; the bright green area represents extruded mantle.
Figure 9. Long description
Panel 1 at the top left presents a block diagram of a shallow marine environment with sandy substrate at the base, water column above, and several Opisoma romeroi individuals partially buried in sediment. Ammonites of the genus Dactylioceras, depicted as coiled shells, are distributed throughout the water column and near the substrate. Opisoma are shown in life position with their shells slightly open and a bright green mantle extruded from the dorsal side. Panel 2 at the bottom left displays a posterior dorsal view of a single Opisoma, emphasizing the symmetrical, fan-shaped shell and the bright green extruded mantle emerging from the dorsal midline. Panel 3 at the bottom right provides an oblique lateral view, showing the shell’s curvature and the bright green mantle extending outward. The green mantle is consistently highlighted in both anatomical views to indicate its functional or morphological significance.
Paleogeographic distribution of Opisoma through time
Early Jurassic Opisoma is regarded as a member of the Lithiotis facies, which is a remarkable example of a short-lived assemblage, latitudinally constrained to tropical shallow seas (Geyer, Reference Geyer1977, fig. 1; Hallam, Reference Hallam1977; Accorsi Benini, Reference Accorsi Benini1981, fig. 1; Nauss and Smith, Reference Nauss and Smith1988, fig. 13; Fraser et al., Reference Fraser, Bottjer and Fischer2004; Krobicki and Golonka, Reference Krobicki and Golonka2009, fig. 1, Reference Krobicki and Golonka2014; Franceschi et al., Reference Franceschi, Dal Corso, Posenato, Roghi, Masetti and Jenkyns2014, fig. 1; Damborenea, Reference Damborenea2017, fig. 1; Li et al., Reference Li, Ye, Rao, Han and Wang2026, fig. 8; Posenato et al., Reference Posenato, Bassi, Parente, Takayanagi, Cipriani, Garbelli, Montanaro, Asahara and Iryu2026). In this context, it is appropriate to review the available data on the distribution of Opisoma species through time (Fig. 10).
Paleogeographic distribution of the genus Opisoma (circles). (1) During the Early Jurassic, with indication of general coincidence with Lithiotis facies (stars). (2) Middle and Late Jurassic occurrences of Opisoma. Base maps modified from Scotese (Reference Scotese2014).
Figure 10. Long description
The top panel displays the Early Jurassic world map with yellow circles for Opisoma and red stars for Lithiotis facies. Opisoma sites are concentrated in western North America, northwestern Africa, and the Caribbean. Lithiotis facies stars cluster along the western Tethys margin, from western North America through northern Africa to Madagascar. Some yellow circles overlap with red stars, indicating co-occurrence. The bottom panel shows the Middle-Late Jurassic map. Middle Jurassic Opisoma sites are marked by orange circles, Late Jurassic by red circles. These are mainly in Europe, with a few in the Caribbean and South America. The legend in the lower right clarifies symbol meanings. Both panels use Mollweide projection and paleogeographic reconstructions from Scotese 2014.
The oldest records of Opisoma were reported from the Sinemurian of Spain (Geyer, Reference Geyer1977), and the genus had already achieved a wide distribution during the Pliensbachian: Spain (Rey et al., Reference Rey, García-Hernández, Martín-Algarra and Vera1990), Slovenia (Buser and Debeljak, Reference Buser and Debeljak1995; Dozet, Reference Dozet2009), France (Boehm, Reference Boehm1888), Italian Alps and Apennines (Boehm, Reference Boehm1884; Posenato et al., Reference Posenato, Bassi and Nebelsick2013 and references therein), Algeria (Benhamou et al., Reference Benhamou, Elmi and Alméras2000), Morocco (Dubar, Reference Dubar1948; Lee, Reference Lee1983; Fraser et al., Reference Fraser, Bottjer and Fischer2004; Li et al., Reference Li, Ye, Rao, Han and Wang2026), Oman (Glennie et al., Reference Glennie, Boeuf, Clarke, Moody-Stuart, Pilaar and Reinfardt1974), Tibet (Li et al., Reference Li, Ye, Rao, Han and Wang2026), and Timor (Krumbeck, Reference Krumbeck1923). It has previously been recorded as reaching western South America during the late Pliensbachian/early Toarcian (von Hillebrandt, Reference von Hillebrandt1973; Aberhan and von Hillebrandt, Reference Aberhan and von Hillebrandt1999). This record slightly extends southward the range of Opisoma in South America.
During the Early Jurassic, Opisoma was frequently but not always associated with Lithiotis reefs (being absent, for example, in North America). The material described here shows that at least in South America Opisoma occurred at higher latitudes than Lithiotis (Fig. 10.1). This record also adds new evidence for subtropical weather conditions in the Neuquén Basin during the late Early Jurassic, already suggested by the presence of coral reefs even farther south (Echevarría et al., Reference Echevarría, Hoqui, Damborenea, Manceñido, Garberoglio and Harguindeguy2024). This is also consistent with a proposed paleolatitude of ~25°S for the region at that time (Iglesia Llanos et al., Reference Iglesia-Llanos, Riccardi and Singer2006).
It is interesting to point out that, unlike most members of the Lithiotis fauna (see Posenato et al., Reference Posenato, Bassi, Trecalli and Parente2018), Opisoma survived the early Toarcian extinction event, and the genus had a longer stratigraphical span (Fig. 10.2). Nevertheless, the genus was severely affected by this event, and although some species were referred to Opisoma (some doubtfully) from the Bathonian–Bajocian of France (Bigot, Reference Bigot1895) and the Callovian–Oxfordian of France (Cossmann, Reference Cossmann1924) and Somalia (Stefanini, Reference Stefanini1939), they all are small-sized and have much thinner shells. Some late Oxfordian species have again large thick shells, as recorded from the Oxfordian of France (Buvignier, Reference Buvignier1843, Reference Buvignier1852; Péron, Reference Péron1906), England (d’Orbigny, Reference d’Orbigny1850; Arkell, Reference Arkell1936), Romania (Barbulescu, Reference Barbulescu1976), and central Russia (Lahusen, Reference Lahusen1883) and from the Kimmeridgian of Germany (Boehm, Reference Boehm1881), France (Cottreau, Reference Cottreau1929), and Romania (Roniewicz, Reference Roniewicz1976).
Records from the Cretaceous and even Paleocene of India and North America (Stoliczka, Reference Stoliczka1871; Whiteaves, Reference Whiteaves1879; Anderson, Reference Anderson1958) are currently referred to other Opinae genera (Kendrick and Vartak, Reference Kendrick and Vartak2007; Squires and Saul, Reference Squires and Saul2009, p. 1319).
Conclusion
Opisoma romeroi n. sp. was a member of the Opinae that lived in the Paleopacific. It was a highly specialized alatoform bivalve, characterized by a large and aberrant shell. There has been longstanding controversy regarding the orientation of the shell in Opisoma. In this study, Opisoma is interpreted as a bivalve with an anterior adductor scar and opisthogyrate umbo. Transposed hinges were identified in the specimens examined, a phenomenon widely documented among astartids.
Opisoma romeroi n. sp. exhibits a combination of morphological features consistent with, but not unequivocally associated with, a photosymbiotic lifestyle. Unlike the typical infaunal habit of astartids, this species adopted an epifaunal to semi-infaunal mode of life, with the commissural plane perpendicular to the substrate and the anterior region resting on it, partially emerging from the sediment. Opisoma romeroi n. sp. also reached exceptionally large sizes compared with other astartids, suggesting that it could have developed a mechanism for light exposure similar to that of extant Tridacna, or possibly like some Fragum species.
Opisoma was a subordinate component of the Lithiotis fauna, suggesting that its occurrence in the Neuquén Basin, Argentina, represents an outpost of this assemblage. The Opisoma romeroi n. sp. specimens found at Cerro Granito represent the southernmost occurrence known for the genus. This record indicates that Opisoma had a broader paleolatitudinal range than previously acknowledged. The occurrence of Opisoma romeroi n. sp. at these latitudes supports the existence of faunas of tropical affinity, which would be consistent with a subtropical climate.
Acknowledgments
The assistance of B. Boilini, M. Tanuz, D. Lazo, and C. Noyès during the visit/access of the different collections is deeply appreciated. We thank J.A. Muiño for aiding with the CT scan and the San Martin Hospital (La Plata) for allowing the use of the equipment. We are thankful to J. Posik for helping with the cleaning and preparation of the material and to S.V. Faiad for the illustrations of life habit reconstructions. We are also grateful to F.M. Irazoqui and C.M. Pérez for the assistance with the CT software. We thank M. Hautmann, R. Knight, and an anonymous reviewer for their constructive and productive comments, which helped improve the manuscript. V.C. is indebted to Williams Foundation and Asociación Paleontológica Argentina for a scholarship to attend a meeting to show preliminary results, and to CONICET for a doctoral scholarship.
Competing interests
The authors declare none.
