An exceptionally preserved upogebiid (Decapoda: Reptantia) from the Eocene of California

摘要

Construction excavation within member "B" of the middle Eocene-aged Santiago Formation at Bressi Ranch in the southern part of the City of Carlsbad, California, USA, have produced exceptionally preserved upogebiid fossils. While most fossil upogebiids are only known fragmentarily, the specimens described here are preserved as relatively complete articulated specimens. Preserved structures include: the cephalothoracic shield with a short rostrum, a well-developed cervical groove and anterior coarse tuberculation; the pleon, with a characteristic trapezoidal first tergite and the second tergite representing the largest of the series; the appendages including (fragmentary) maxillipeds two and three, and the five walking limbs; the tail fan with uropods with both sub-triangular rami possessing bulging anterior edges and one (endopod) or two (exopod) keels running in parallel to the anterior bulging edge, the exopod lacking a diaresis, and the telson being sub-rectangular with a median suture. Exceptional minute details preserved are the bases of setae on the uropods and muscles in pleomere six. These muscles show fiber bundles about 80 μm in diameter, and individual fibers about 10 μm in diameter. The specimens were documented with up-to-date imaging techniques, including stereo photography or depth-map-based surface reconstructions. Due to the exceptional preservation, the fossils can be recognized as an upogebiid of the species Upogebia aronae sp. nov. As numerous specimens have been found at that locality, this discovery indicates similarly dense populations as seen in modern fauna.