THE LATE CAMBRIAN DIKELOCEPHALID TRILOBITE WALCOTTASPIS VANHORNEI AND ITS EVOLUTION IN THE CONTEXT OF THE SEQUENCE STRATIGRAPHIC ARCHITECTURE OF THE UPPER MISSISSIPPI VALLEY

Nearshore environments are commonly invoked as the sites of major morphological innovation, and paleontological onshore-offshore diversity trends confirm the later spread of such innovations to offshore regions. However, the pattern of morphological evolution is commonly discontinuous in the nearshore settings, making it challenging to resolve primary fluctuations in the rate of morphological change through time from artifactual patterns induced by the episodic nature of sediment accumulation in such environments. The dikelocephalid trilobite fossil record of the Upper Mississippi Valley (UMV) offers an important opportunity to explore the patterns of morphological variation in the context of well-resolved sediment accumulation history. This nearshore basin contains a condensed yet comprehensive record of sedimentation. Initial phylogenetic assessment of dikelocephalid trilobites suggests that members of the genera Dikelocephalus, Osceolia and Walcottaspis form a derived clade within dikelocephaline trilobites, of which at least one species of Dikelocephalus was widespread across Laurentia and endured for more than 1.5 million years. Walcottaspis vanhornei appears to be sister taxon to a derived Dikelocephalus species and is similarly large. Although its cephalon resembles that of Dikelocephalus its trunk region shows notable modification, consistent with it being distinguished at generic level. Walcottaspis vanhornei occurs within at least one parasequence of the St. Lawrence Formation that represents the early part of a falling stage systems tract. Stratigraphic architecture suggests a high fidelity record of deposition at this time in the region. The occurrence of W. vanhornei at several localities suggest that the species was locally dominant over a limited temporal span, possibly in the order of 100,000 years. It is not known to co-occur with Dikelocephalus, but at Hokah, Minnesota specimens of that genus are abundant in a parasequence almost immediately above that containing common W. vanhornei. Derived Dikelocephalus encompassed the temporal range of W. vanhornei and apparently resumed their position within the local ecosystem as W. vanhornei was extirpated. Walcottaspis vanhornei appears as a fleeting and distinctive “start up species” that failed to dislodge Dikelocephalus.