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.