Abstract
Sinemurian and Pliensbachian Exogyrinae from northern Chile described herein as Nanogyra? cf. auricularis are considered the oldest members of the subfamily known to date. Comparisons with a large number of Jurassic Nanogyrini and aberrant Gryphaea species suggest that they are most closely related to Nanogyra? auricularis (Munster in Goldfuss) from the Toarcian of France, but probably not identical. Morphotypes of several Chilean specimens are transitional between gryphaeate and exogyrate forms, and their assignment to either Gryphaeinae or Exogyrinae appears arbitrary. In order to solve this taxonomic problem, the microstructures of Nanogyra? cf. auricularis and co-occurring typical Gryphaea species were examined and compared to a large database from previous studies. The results suggest that shell chambering is a feature common to all Exogyrinae but not to Gryphaea sensu stricto. Complex cross foliation and branching cross foliation are characteristic of both groups. However, they are more abundant in Gryphaea and apparently become phylogenetically reduced in the Exogyrinae. The transitory morphotypes and microstructures, especially of the Sinemurian specimens, are further evidence for the evolution of the Exogyrinae from the Gryphaeinae. Obviously, the Chilean specimens are very close in time and space to the origin of the subfamily. Thus, the lowermost Jurassic Andean back-arc basin of western South America is considered a plausible place of origin of the Exogyrinae.
