SYSTEMATICS, AFFINITY, AND BIOSTRATIGRAPHIC UTILITY OF THE EARLY ORDOVICIAN TRILOBITE AMBLYCRANIUM ROSS, FROM THE GREAT BASIN, WESTERN USA

Amblycranium has been known from two species described over 60 years ago on the basis of a total of eight illustrated sclerites, all either cranidia or librigenae. Recent fieldwork in the Fillmore Formation, western Utah, and Garden City Formation, southeastern Idaho, has yielded abundant material of both species, including all exoskeletal parts, as well as rich samples representing five newly discovered species. The genus is now known to occur through most of the Stairsian Stage (upper Tremadocian) of northern Laurentia (in Ordovician terms). Possibly because it has been so poorly known, several other species from Laurentia and South China have subsequently been questionably assigned to the genus; in light of new knowledge of the anatomy of the type and other species of Amblycranium, it is clear that none of these are related.

Most previous authors have assigned Amblycranium to the Family Hystricuridae, but prior to the current study no thoracic or pygidial material has been available. New collections reveal that most species of the genus had thoraces with prominent pleural and axial spines and small pygidia lacking a border, but with triangular pleural spines on each segment. Stratigraphically low and phylogenetically basal species lack these features, but have small pygidia morphologically similar to contemporaneous members of the Family Dimeropygidae, including species of Heckethornia and Bearriverops. Hence we reinterpret the genus as a dimeropygid, and hypothesize that the spinose morphology of more derived species arose via paedomorphosis, as it resembles earlier stages in the documented ontogenies of other dimeropygids. A hypothesis of phylogenetic relationship based on cladistic parsimony analysis indicates close agreement with sampled stratigraphic order.

New collections reveal that unique species of Amblycranium occur in all but one of the recently established Stairsian trilobite zones from the lower Stairsian Hystricurus zanderi Zone to the penultimate Pseudoclelandia cornupsittaca Zone, and that no species ranges between any zones. Combined with striking and easily identifiable morphology, this pattern of species occurrence indicates considerable utility in the correlation of northern Laurentian Stairsian successions.