Growth-increment and isotopic studies of shells of the marine bivalve Glycymeris americana are a potential source of information bearing on its life history and preferred environment over the late Cenozoic on the U.S. eastern seaboard. We demonstrate that the ages of shells can be determined from growth bands and ontogenetic profiles of oxygen isotope (δ¹⁸O) composition, and that shell aragonite is deposited in oxygen isotopic equilibrium with seawater, enabling calculation of ambient temperatures by means of a generic transfer function. Modern specimens from North Carolina rarely reach the large size commonly attained by modern forms from Florida and Early Pleistocene forms from both states, and modern populations from North Carolina probably include fewer old individuals, the most certain disparity being with Early Pleistocene populations from the state. The temporal change in age structure in North Carolina may be an effect of recent scallop trawling but earlier non-anthropogenic environmental change cannot be ruled out as the cause. Maximum and minimum temperatures calculated from the δ¹⁸O profiles of Early Pleistocene shells indicate a larger seasonal range than now in both Florida and North Carolina, due to cooler winters. This may reflect greater southward penetration of cool northern waters, with transport along the shelf supplemented by upwelling of water brought south at depth.

Meiofaunal traces in cored Pleistocene to Holocene silt-clay glaciogenic rhythmites consist of four types: clay-rich fill (type A) representing feeding structures, and a silt-rich fill with a clay-rich lining (type B) representing dwelling structures. A third burrow type (type C) is filled with framboidal pyrite suggestive of microbially mediated early diagenesis under anoxic and circumneutral pH conditions. A variant of type C is filled with framboidal pyrite overgrown by poorly crystalline pyrite indicating at least two stages of iron sulphide growth within earlier formed burrows. Higher in the cored succession a bed thickness increase to about ∼ 14 cm is marked by an absence of meiofaunal traces. Since each bed represents deposition during a single year, it suggests that sustained sedimentation rates of ∼ 14 cm yr^–1 represents a threshold for endobenthic activity. A fourth burrow type (type D) is filled mostly with clay and a lining with a distinctive parallel-aligned fabric of clay and dispersed silt grains, which occurs in slump blocks resedimented downslope from a shallower water setting. The absence of type A, B, or C burrows in these silt-rich slump blocks suggests that prior to slope failure, the silt-rich sediment substrate, and likely brackish pore fluid conditions, were not conducive to these tracemakers and/or trace-making behaviors. These differences illustrate the first-order control of physico-chemical factors, specifically, sedimentation rate, salinity, and substrate composition on the behavior of burrowing meiofauna in a glaciomarine basin plus the significance of meiobenthic activity on controlling early, shallow burial diagenesis.

Archaeocyaths, the first metazoan reef builders of the Phanerozoic, faced their ultimate demise during early Cambrian Age 4 but the exact timing of their local extinctions varied globally. In this study, we report archaeocyaths in strata that overlie the last robust archaeocyathan reefs of the western United States (Laurentian Cordillera). These are found in small microbial mounds in the upper unit of the Mule Spring Limestone near Split Mountain, Clayton Ridge, Nevada, as well as in storm beds in the Thimble Limestone Member of the Carrara Formation in Echo Canyon of the Funeral Mountains, Death Valley, California. Thin-section analysis revealed the presence of modular archaeocyaths, with Archaeocyathus being the only genus present. The small microbial mounds of the lowermost upper unit of the Mule Spring Limestone preserve frame-building Archaeocyathus in situ, a few meters above the well-known Bristolia beds of the Mule Spring Limestone. As some of the youngest known archaeocyaths of the western US, these occurrences represent the last gasp of archaeocyaths in the early Cambrian of Laurentia and one of the latest occurrences globally of archaeocyaths. We thus interpret Archaeocyathus in these units as an example of a dead clade walking—some of the last true archaeocyaths that locally persisted into the later Age 4 Cambrian following the disappearance of diverse archaeocyath reefs in western Laurentia. These last archaeocyath communities exhibited low diversity and disparity before the total extirpation of this reef-building hypercalcified sponge and their ecosystem.