Silurian acastacean trilobites of the Americas

Abstract

Species of the superfamily Acastacea constitute a minor element of North and South American Silurian trilobite faunas. Phacopidina? obsoleta (Ulrich and Delo) (Llandovery, Tennessee) provides the sole Silurian record of the “kloucekiine” grade (Acastacea s.l.); a lectotype is designated for this species. Acaste birminghamensis Norford (mid Llandovery, Alabama) lacks synapomorphies of post-Llandovery Acastidae s.s. (=Acastinae + “Acastavinae” + Asteropyginae), and is the basis for Llandovacaste n. gen. (Acastidae s.l.). A well-preserved sample of Andinacaste cf. A. ledgrandi Eldredge and Braniša from the Catavi Formation (Ludlow–Přídolí), Bolivia, displays apomorphic characters of the genal spines and hypostome shared with Devonian calmoniids. Coaptative structures, including vincular furrow/pits and “bifurcated” thoracic pleural tips, are documented for Andinacaste; similar enrollment morphologies arose in other acastomorph calmoniids. Poorly known Llandovery acastaceans from Paraguay and Venezuela may represent Andinacaste. Acaste zerinae n. sp. from the Pembroke Formation (Přídolí) of Maine is closely comparable to British late Wenlock A. downingiae (Murchison). The Australian Gedinnian acastine placed in Phacopinae indet. longisulcata (Shergold) is designated Acaste lokii n. sp.