Three new proetide trilobite larvae from the Lower Ordovician Garden City Formation in southern Idaho

Abstract

Larvae of three proetide trilobites are described from the Garden City Formation: Proetide A, Goniophrys prima, and Licnocephala cavigladius. The larvae of Proetide A share a spindle-shaped axis and natant hypostomal condition with other proetide larvae as synapomorphies. The forward-tapering glabella and slender posterior fixigenal area of post-larval cranidia of Proetide A indicate a close affinity to the earliest proetide family (Hystricuridae). The primitive nature of the larvae of Proetide A is further supported by the possession of plesiomorphic features, including four glabellar lobes and bilobed axial lobes (L1/L2/L3). Larval features of Goniophrys prima (a telephinid) such as an elliptical outline and smooth exoskeleton are not consistent with the hypothesis that the genus is the ancestor to younger telephinids. The larvae of Licnocephala cavigladius are typical of the Bathyuridae in having a smooth, ovoid shield and less distinct axial furrows.