The brachymetopid trilobite Radnoria in the Silurian (Wenlock) of New York State and Arctic Canada

Abstract

Radnoria bretti n. sp., from the Wenlock (Sheinwoodian) Rochester Formation of western New York State, is the best preserved and most completely known member of its genus. It provides the first definitive information on Radnoria's hourglass-shaped rostral plate, the first known hypostome of a Silurian member of the genus, demonstrates that Radnoria engaged in sphaeroidal enrollment, and reveals that early holaspid individuals had tubercles on the posterior thoracic axes and pygidial axial rings that were effaced with maturity. Three new species from the Wenlock of the Cape Phillips Formation of Nunavut are known from sparse material and are reported in open nomenclature. Together, the species greatly increase knowledge of Laurentian Silurian brachymetopids, which have until now been known from a single cranidium from the Wenlock of Arkansas.

The oldest brachymetopid trilobite record from the European peri-Gondwana

“…simplex from the lower part of the Portixeddu Formation, suggesting that their stronger caecal sculpture and larger L1 compared to the specimens from higher in the formation are probably the result of deformation. We agree with Adrain & Tetreault (2005) that these and other morphological differences indicate the presence of two distinct species of Radnoria in the Portixeddu Formation. However, a complete specimen collected by us at locality Gon 1 of Hammann & Leone (horizon TH2b, the lower part of the Portixeddu Formation) suggests that the two species are not separated stratigraphically but occur together, at least in the lower part of the formation.…”

“…2). We do not agree with Adrain & Tetreault (2005) that R. elongata could be an early member of Cordania, because it lacks the typically pronounced epiborder furrow and the anterior facial suture is significantly less divergent.…”

“…Sandford & Holloway (2006) from the other species assigned to Radnoria in the configuration of the preglabellar area and the lack of a very broad cephalic doublure, and on the basis of these features we retain the species in the Aulacopleuridae. Adrain & Tetreault (2005) questioned the generic assignment of Radnoria elongata from the Ludlow of Australia because of its pygidial features (large number of axial rings and distinct marginal rim) and perceived differences in the cephalon (isolated L1 and broader glabella). However, the type-species R. syrphetodes and R. humillima from the Wenlock of the Czech Republic have a similar number of pygidial axial rings (10-12) and we do not think that the existence of a marginal rim is a reliable difference (compare Sun 1990, pl.…”

“…specimen from the Darriwilian of Canada to assign it unequivocally to Radnoria. Adrain & Tetreault (2005, p. 2089 assigned to Radnoria some librigenae from the Wenlock of Arctic Canada that they described as Radnoria sp. 1, R. sp.…”

“…Radnoria bretti Adrain & Tetreault, 2005; Rochester Formation, Wenlock, New York, USA. Owens & Hammann, 1990; Cystoid Limestone, Katian (Kralodvorian of the Mediterranean regional chronostratigraphic chart), Iberian Chains, Aragon, Spain.…”