The trilobites Smoothcanyonops gen. nov. and Ceeops gen. nov. from the Early Ordovician (mid-Tremadocian; Stairsian) of the Great Basin, western USA

 

Authors: Adrain JM

Published in: Bulletin of Geosciences, volume 101, issue 1; pages: 49 - 64; Received 12 October 2025; Accepted in revised form 24 December 2025; Online 3 May 2026

Keywords: Trilobita, Lower Ordovician, Tremadocian, Laurentia, silicified, taxonomy,

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Abstract

Extensive field-based sampling of the shallow water trilobite faunas of the northern Laurentian palaeo-margin (present day western North America) has yielded an abundance of well preserved, secondarily silicified, faunas spanning the Lower Ordovician. As sampling has accumulated, some very rare species have been revealed. It is unusual to encounter new post-Cambrian trilobites whose family-group affinity is obscure, but two new Stairsian (mid-Tremadocian) genera contain reasonably well known species with effaced morphologies that have no obvious close comparisons among other Ordovician taxa. Smoothcanyonops gen. nov. (type species S. smoothcanyonensis sp. nov.) includes species with dorsally smooth, considerably effaced morphology, apparently with yoked librigenae. A second assigned species is S. middlensis sp. nov. Ceeops gen. nov. (type and only species C. housensis sp. nov.) is also mostly unsculptured and effaced, but has a greatly anteriorly expanded cranidium, broad librigenae, and very broad cephalic doublure. While their thin cuticles, general lack of sculpture, and weakly expressed dorsal furrows indicate they could be representatives of the same family-group taxon, there are also considerable morphological differences between them. Among other trilobites, they both have some resemblances, particularly in librigenal morphology, to bathyurids. However they are older than the oldest widely agreed species of bathyurids and they lack many bathyurid features. At present their affinity is difficult to assess with any confidence.