REEVALUATING CLAWED ARTHROPODA FROM UPPER DEVONIAN BLACK SHALES: IS THE “FIRST SHRIMP” REALLY A SHRIMP?

Elongate-clawed arthropods that have been reported from Upper Devonian (Famennian) black shales include claws once assigned to Eurypterida, claws and other body parts from the Woodruff Formation of Nevada and the Holy Cross Mountains of Poland as well as other sites in Europe currently assigned to the genus Angustidontus, and the presumed oldest shrimp Aciculopoda from the Woodford Shale of Oklahoma. (Schramidontus, a form from Belgium closely related to Angustidontus, has been described from continental rocks.) Studies of these forms have been hampered by partial preservation (mainly claws) and the generally poor preservation of rare, more complete organisms, allowing for alternative interpretations of the morphology and affinity of these forms. This is further complicated by differences in preservation and orientation of more complete fossils. A review of previous work, done in an attempt to identify material from the Cleveland Member of the Ohio Shale, reveals similarities between Angustidontus, Shramidontus, and Aciculopoda. Both Angustidontus and Aciculopoda are generally similar in size and in body proportions. These two genera also have similarly proportioned abdominal segments and a tapering telson. Both also have an elongate anterior appendage terminating with a single elongate claw lined with alternating short and long teeth set perpendicular to its long axis, a claw like those that have been referred to Angustidontus. The claw is preceded by two stout segments, the more proximal of which is longer and stouter. The terminal claw of Aciculopoda is probably not likely the merus of a chelate appendage as originally described. This clawed appendage is instead best interpreted as an anterior maxilliped. It is also likely that the taxon is not as laterally compressed as described but instead preserved on its side due to the flexed position of the maxillipeds. Nor is the carapace rotated as much as first described. This reinterpretation suggests that the Aciculopoda and Angustidonus are closely related, which is interesting as the genotypes of both taxa are from the Woodford Shale. Aciculopoda is shrimplike, but probably not a member of the Penaeoidea, the superfamily containing true shrimp.