First occurrence of Orthotheca Novák, 1886 (Hyolitha, Early Devonian) in North America

Abstract

Family Orthothecidae Sysoev, 1958 is herein emended to encompass only hyoliths that possess a very shallow longitudinal ventral concavity and a tightly rounded dorsum, resulting in a near-triangular to only slightly kidney-shaped cross section. Orthotheca Novak, 1886 is restricted to orthothecids that possess longitudinal ornamentation on the dorsum, with Cryptocaris suavis Barrande, 1872 now recognized as the type species of that genus. Based on the revised generic concept, orthothecids from the Lower Devonian (Emsian) Shriver Chert of northeastern Pennsylvania are here included within the genus as Orthotheca shriveri n. sp., making this the first occurrence of the genus in North America. The Orthothecida were a group of sessile, benthic hyoliths that ranged from Early Cambrian to Early Devonian. Their life habit appeared to have included filter-feeding. Their ability to retract the operculum into the shell when closed probably aided in preventing sediment influx, and the ventral furrow may have been an adaptation to differing degrees of firmness of the underlying substrate. Despite faunal linkages between the Devonian paleocontinents, the majority of orthothecid genera seem to be largely restricted to their type areas, although Orthotheca documented herein has eroded slightly their endemic character.