Turnover from mollusk-dominated Depauperate Zone (late Ordovician) to brachiopod-dominated (early Silurian) faunas in central North America

Abstract

The Depauperate Zone from the base of the Ordovician (Katian) Maquoketa shale in Dubuque, Iowa, features a range of diminutive infaunal, epifaunal, nektonic and planktonic members with multiple growth stages. Fossils are preserved as phosphatized internal molds. In rare cases, shell replacement conserves growth lines. Sixteen species were recovered, including three-dimensional graptolites (Rectograptus peosta). The bivalves Palaeoneilo fecunda and Nuculites neglectus are most abundant, but the orthid brachiopod Plaesiomys subquadrata also is common. Bivariate analyses were conducted on the bivalve and brachiopod species, two gastropods (Liospira micula and Cyrtolites carinatus), a reputed sponge (Hindia sphaeroidalis) and the graptolite. All mollusks follow an isometric growth pattern and the same is especially characteristic of the orthid brachiopod. Winnowing by waves or bottom currents was minimal. Pene-contemporaneous replacement of fine details was due to concentrations of P2O5 in seawater intermittently brought from the shelf margin through upwelling. Paedomorphosis was likely, because the brachiopod P. subquadrata shows a record of individuals 150% larger in adult size elsewhere in North America. In contrast, populations of Silurian Pentamerus oblongus preserve a full range of immature to adult individuals with a curvi-linear growth pattern. Clear-water deposition under conditions of good marine circulation prevailed during much of the Silurian, although secondary silicification of original CaCO3 shells was common. The muddy Maquoketa substrate vanished due to sea-level rises that flooded all clastic sediment sources.