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

Resumen

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.