EVOLUTIONARY TEMPO AND MODE IN CAMBRIAN UPPER MISSISSIPPI VALLEY DIKELOCEPHALID TRILOBITES II: OSCEOLIA AND WALCOTTASPIS

Late Cambrian strata of the upper Mississippi Valley provide opportunity to explore evolutionary tempo and mode in an ancient nearshore setting within a well-represented trilobite clade, the Dikelocephalidae. A relatively small sized dikelocephalid genus, Osceolia, appears immediately above the maximum flooding surface in the early part of the falling stage systems tract, endured regionally for approximately 0.5 Ma, and is commonly found co-occurring with its relative Dikelocephalus. Its abrupt regional appearance may be an artifact of accumulation history. Osceolia contains two species, one a new species known only from offshore, carbonate-dominated facies. In the nearer shore siliciclastic-dominated environment, larger individuals of O. osceola (Hall, 1863) apparently preferred distal environments while the younger stages dominate nearer shore settings. Osceolia osceola shows marked morphological variation with significant intracollectional variation of partly ontogenetic and partly uncertain biotic origin, but no obvious signal of morphological evolution emerges, and its preserved record suggests net evolutionary stasis. Walcottaspis vanhornei is sister taxon to early falling stage Dikelocephalus species but bears a distinct pygidium. It both appears and disappears abruptly within the falling stage systems tract, where it is recorded for perhaps as little as 50,000 years within the regional temporal range of Dikelocephalus, which it displaced locally. Its close phylogenetic position, similar maximum size and cephalic morphology all suggest that it may have occupied an ecological niche comparable to that of Dikelocephalus. Dikelocephalus evidently endured elsewhere and later replaced W. vanhornei, presumably when conditions were more advantageous to it. The abrupt appearance of Walcottaspis is likely to be of biotic, as opposed to stratigraphic, origin. Regional dikelocephalid evolutionary history contrasts that of localized ephemeral species W. vanhornei with those of more widespread and enduring species of Dikelocephalus and Osceolia. As in Dikelocephalus, complex patterns of morphological variation within both Osceolia and W. vanhornei likely include microevolutionary changes but these were not the main driver of species-level evolution within the group.