Abstract

The Figueroa sulfide deposit located in Franciscan Complex rocks in the San Rafael Mountains, California, contains the only known Jurassic hydrothermal vent community. Based on radiolarian biostratigraphy it is Pliensbachian (early Jurassic) in age. The Figueroa fossil organisms lived at a deepwater, high temperature vent site located on a mid-ocean ridge or seamount at an equatorial latitude. The vent site was then translated northeastward by the motion of the Farallon Plate and was subsequently accreted to its present location. The vent fossils are preserved as molds of pyrite and there is no remaining shell or tube material. The fossil assemblage is specimen rich, but of low diversity, and comprises, in order of decreasing abundance, vestimentiferan worm tubes, rhynchonellide brachiopods (Anarhynchia cf. gabbi), and trochoidean gastropods (Francisciconcha maslennikovi new genus and species). These fossils represent only primary consuming organisms, some of which may have had chemosynthetic microbial endosymbionts, like many modern dominant vent animals. The Figueroa vent assemblage shares vestimentiferan tube worms and gastropods with other fossil and modern vent communities, but is unique in having rhynchonellide brachiopods. It shares this feature with contemporary Mesozoic cold seep communities. Many other taxonomic groups found at modern vent sites are missing from the Figueroa assemblage. The presence of vestimentiferan tube worm fossils in the Figueroa deposit is at odds with the supposed time of origin of the modern vestimentiferans (∼100 Ma), based on molecular data.

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