Gryphaeate Oysters with Radial Ornamentation in the Middle Jurassic of Chile and Argentina

TOMO 1 - Análisis de Cuencas GRYPHAEATE OYSTERS WITH RADIAL ORNAMENTATION IN THE MIDDLE JURASSIC OF CHILE AND ARGENTINA: SUPRASPECIFIC AFFINITIES AND POSSIBLE TAXONOMIC IMPLICATIONS Alfonso Rubilar R. Servicio Nacional de Geología y Minería, [email protected] Most of the Gryphaea species (Family Gryphaeidae, Subfamily Gryphaeinae) from the Middle Jurassic of Chile and Argentina (many not yet described) have radial structures on the right and especially the left valve, either raised ribs or striae (few or many; thick or fine; irregular or uniform) or wide and more or less scattered grooves (Fig. 1). Additionally, the left valve generally shows two main morphotypes, identified at first sight by a sub-ovoid or sub-trigonal outline, where the highest convexity is located in the middle zone of the valve or next to its posterior margin, respectively. In addition, the posterior flange generally has a retarded growth in all of these species, being little or not visible in the dorsal half of the shell in external view, particularly in specimens with a sub-trigonal outline (Fig. 1). On the contrary, the posterior flange (and sulcus) is visible in external view especially near the ventral margin, where it can be either well detached or not in different members of the same species. These oysters can be divided into two main morphological groups (A and B, Fig. 1), considering the type of radial structures and the early growth of the shell (see Table 1). AFFINITIES WITH COETANEOUS OYSTERS FROM NORTH AMERICA AND EUROPE It is noteworthy that most of the Middle Jurassic gryphaeate oysters from North America and Europe have in common with the South American species the aforementioned general features (e.g., Hallam and Gould, 1975, Figs. 10, 11; Bayer et al., 1985, Figs. 1, 3, 5; Johnson and Lennon, 1990, Text-fig. 1, Pl. 2, 3; Johnson, 1993, Figs. 5, 6). As a whole, the South American oysters of the Group A and Subgroup B2 (Fig. 1; Table 1) are closely related to species present in North America with equivalent ages (Table 2). In addition, similar anagenetic changes seem to have occurred in taxa of the Group A in both regions, showing different types of parallelism between North and South America. 121 XI CONGRESO GEOLOGICO CHILENO 122 TOMO 1 - Análisis de Cuencas Table 1. Groups and subgroups of South American Middle Jurassic gryphaeate oysters with radial ornamentation. Group A (Fig. 1; Toarcian - Callovian) - Radial ornamentation. It is present generally only on one of the two valves and especially on the right one, where consists in few, short, uniform and isolated ribs (Fig. 1, specimen 6c). In the left valve there are scarce (Fig. 1, specimen 2a) or abundant (G. neuquensis Weaver, 1931) thick and irregular radial ribs, or just scattered grooves (Fig. 1, specimen 3a, 4a). - Convexity. Early the left valve has a generally globoid (isometric) and very wide convexity (Fig. 1, specimens 3c, 6c), which involves the zone next to the anterior margin and establishes the whole convexity of the adult shell. Such early high convexity can be more or less delayed in the ontogeny (Fig. 1, specimen 5) or take place allometrically in more than one occasion (Fig. 1, specimen 2). - Form and growth of the beak. It is pointed and short (in some species can be atrophied by a big attachment area). The beak appears to be lengthened (with a reduced increasing of the width) where the early high convexity of the shell is delayed in the ontogeny (Fig. 1, specimen 5b). - Umbonal curvature. It tends to be low, either gentle (Fig. 1, specimen 2b) or moderate (Fig. 1, specimen 6b), but it can be absent (Fig. 1, specimen 5b). The beak overpasses very little or not the commissural plain. - Sub-trigonal morphotype. It is frequent (e.g., Fig. 1, specimen 4) but generally not well defined. Group B (Fig. 1; Bajocian - Callovian) - Radial ornamentation. It consists in many, fine and uniform striae that cover the umbonal zone of the left valve (Fig.1, specimen 14a). - Convexity. From the apex, the left valve increases its convexity in a regular form. Several species tend to have a prominent anterior margen (or even an anterior flange) and the shell reaches the commissural border by a tilted and almost flat surface. - Form and growth of the beak. It is generally long and growths in a regular form. " Umbonal curvature. It is variable, generally moderate (Fig. 1, specimens 11b, 18b) or strong (Fig. 1, specimen 9b), but it can be gentle (Fig. 1, specimens 7b, 21b) or absent (Fig. 1, specimen 10b). The beak generally overpasses the commissural plain. - Sub-trigonal morphotype. It is very frequent and well defined (Fig. 1, specimens 10a, 11a, 13a, 15a, 18a, 20a). Subgroup B1. The earliest convexity of the left valve is marked by an anterior (and posterior) narrow margin (Rubilar, 2005). The sub-trigonal morphotype is well defined in some species. Although the posterior flange is generally well visible in external view, its growth is more or less retarded depending on the occurrence of different heterochronical processes (Rubilar, 2005). Subgroup B2. The earliest convexity of the left valve is rounded. The sub-trigonal morphotype is well defined. The posterior flange is generally visible near to the ventral margin (Fig. 1, specimens 13a, 16a) and tends to be kept in a tilted surface visible only in posterior view (Fig. 1, specimen 13a, b). Some specimens have a sub-rectangular outline (Fig. 1, specimens 12a, 17a), where the posterior flange is visible only in posterior view. Table 2. Some North American and European gryphaeate oysters placed in the morphological groups here recognized (Table 1), based on their features. Ages are mainly referencial. Group A North America: Gryphaea planoconvexa fraterna Imlay (Upper Bajocian); G. planoconvexa planoconvexa Whitfield (Upper Bajocian); G. impressimarginata McLearn (Upper Bathonian - Lower Callovian). Europe: Gryphaea pictaviensis Hébert (Upper Toarcian); G. bilobata Sowerby (Upper Toarcian - Bathonian); G. calceola Quenstedt (Middle Aalenian - Lower Bajocian); G. sublobata Deshayes (Lower Bajocian); G. dilobotes Duff (Lower Callovian); G. lituola Lamarck (Upper Callovian); G. dilatata Sowerby (Lower Oxfordian); G. bullata Sowerby (Lower Kimmeridgian). Subgroup B2 North America: Gryphaea nebrascensis Meek and Hayden (Upper Bathonian? - Lower Callovian); G. culebra Imlay (Lower Callovian); G. mexicana Félix (Oxfordian). On the other hand, the oysters of the Group A (Fig. 1; Table 1) are related to all the Upper Lower and Middle Jurassic Gryphaea from Europe (Table 2), characterized as having either irregular and generally thick radial ribs (abundant or scarce) or just grooves on the left valve, and/or few and isolated radial ribs on the right valve. Likewise, in these oysters the left valve generally has a high and very rounded (globoid) convexity early and/or late in the ontogeny, and the frequent sub-trigonal morphotype tends to be not well differentiated (e.g., Johnson and Lennon, 1990; Johnson, 1993). To these affinities could be added the common small size of the beak, and the reduced curvature of the umbo. 123 XI CONGRESO GEOLOGICO CHILENO CONCLUSIONS Most of the Middle Jurassic gryphaeate oysters from the west margin of the Pacific and Europe differs from the typical Lower Jurassic Gryphaea in having some type of radial structure, a retarded growth of the posterior flange and a strong tendency to develop a sub-trigonal outline (inversely correlated with a continuous posterior flange). This distinctive and recurrent morphological pattern permit to propose their inclusion in a new taxonomical category (e.g., at a Subfamily level), recorded from the upper Lower Jurassic. These oysters would have had a strong endemism in both regions, with main evolutionary changes apparently centralized in the mentioned and other features (mosaic evolution). In this taxonomical context, the Group A and its relatives from North America and Europe would represent a different genus, whose denomination requires to redefine the subgenus G. (Bilobissa) Stenzel (1971). Furthermore, the North and South American oysters of the Group B and Subgroup B2 would belong to a new genus and subgenus, respectively. Finally, the radial ribs and frequent sub- trigonal outline observed in Catinula, Africogryphaea and Praeexogyra would make them form part of the Subfamily stated here. These observations suggest at least the decline of Gryphaea (currently recorded from the Upper Triassic to Upper Jurassic) at the end of the Lower Jurassic or beginning of the Middle Jurassic, or even its possible extinction in the last period, if the gryphaeate oysters from this time (and younger) are more related to the Subfamily here sketched. REFERENCES Bayer, U.; Johnson, A.; Brannan, J. 1985. Gryphaea: the relationship between form and environment. In: Bayer, U.; Seilacher, A. (Eds.). Sedimentary and evolutionary cycles, Lecture Notes in Earth Sciences 1: 436- 463. Hallam, A.; Gould, S. 1975. The evolution of British and American Middle and Upper Jurassic Gryphaea: a biometric study. Proceedings of the Royal Society of London B189: 511-542. Johnson, A. 1993. Punctuated equilibria vs. Phyletic gradualism in European Jurassic Gryphaea evolution. Proceedings of the Geologists Association 104: 209-222. Johnson, A.; Lennon, C. 1990. Evolution of gryphaeate oysters in the mid-Jurassic of western Europe. Palaeontology 33: 453-485. Rubilar, A. 2005. Heterochrony in Middle Jurassic species of Gryphaea (Ostreoidea, Gryphaeidae) from southern South America. Geologica Acta 13 (2): 185-203. Stenzel, H. 1971. Oysters. In: Moore, R. (ed.). Treatise on Invertebrate Paleontology. Part N, Bivalvia 3. The Geological Society of America and the University of Kansas, University of Kansas Press, N953-N1224. Weaver, C. 1931. Paleontology of the Jurassic and Cretaceous of West Central Argentina. University of Washington, Memoir 1: 1-469. 124 .

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