Internal anatomy of a brachyuran crab from a Late Cretaceous methane seep and an overview of internal soft tissues in fossil decapod crustaceans

Cretaceous-aged methane seep carbonates in the Western Interior Seaway, USA, have yielded a relatively high diversity and concentration of fossils, including decapod crustaceans, compared to their surrounding, contemporaneous sediments. With tech-nological advances in non-destructive imaging techniques, the internal anatomy of decapods can be studied in remarkable detail. Here, we present a brachyuran crab specimen from an upper Campanian methane seep carbonate in South Dakota, USA. While the external morphology of the crab is insufficient for species identification (Secretanella sp.), remarkable details of the internal morphology are preserved includ-ing soft tissues. Four phyllobranchiate gills are visible due to a broken cuticle. A mu CT-scan revealed parts of the digestive tract including the esophagus and the cardiac stomach (foregut), and possible anterior gastric muscles. An esophagus has not been reported previously in a fossil decapod. We also found mandibles and their apodemes. Because of soft tissue preservation, the South Dakotan locality may be a candidate Konservat-Lagerstatte. To our knowledge, the studied crab contains the first preserved animal soft tissue reported from an ancient methane seep. Our new overview of inter-nal soft tissues preserved in fossil decapod crustaceans shows that, besides muscles reported previously, gills and remains of the digestive tract are most often reported (8 and 13 occurrences, respectively). Reproductive organs, nerve chords, and the heart are only known from one or two occurrences. Although reports of soft internal anatomy preservation remain relatively rare, continued application of mu CT-scans to fossil deca-pods has the potential to illuminate our knowledge of the evolution of internal decapod anatomy.