Paleobiology

enlarge figure

Figure 1. Causes of barren marine sediment (excluding dissolution). Arrows indicate direction of forcing, but not magnitude or nature of interaction. Shelf physiography subsumes tectonic context of shelves, bathymetric profiles, geometry, geographic position, and characteristics of adjacent oceanic water masses and currents. Climate includes both global and local effects. Circulation includes exchange of water between the shelf and open ocean as well as wind-driven mixing. Oval boxes list some of the general signal types that may be unique to each proximal cause of barren sediment. Strat., stratification of the water column; O₂ sol., oxygen solubility

enlarge figure

Figure 2. Number of marine sedimentary rock units described as “unfossiliferous” and “fossiliferous” in a Georef search versus geologic time. A, Number of unfossiliferous units binned by period and measured per million years. Percent continental flooding from Ronov (1994) shown on same axis in gray. B, Number of fossiliferous units binned by period and measured per million years

enlarge figure

Figure 3. Total number of Paleozoic (Pz, Ordovician– Devonian, solid line) and Neogene (Ng, dashed line) fossil assemblages sampled from different depth zones. A, Assemblage data from Bush and Bambach (2004). Depth zones given by Bush and Bambach (2004) are (e1) shorelines, (e2) nearshore shelf, (e3) open shelf, (e4) distal open shelf, (e5) outer shelf margin. Estimated position of average storm wave base (swb) is shown by dashed line. B, Percent collections in the PBDB Marine Invertebrate Working Group for the Pz (4742 total collections) and Ng (1198 total collections). Environmental bin e1 corresponds to PBDB marginal marine, coastal, and foreshore collections, e2 corresponds to PBDB shallow subtidal and shoreface collections, e3 corresponds to PBDB deep subtidal and lower shoreface collections, e4 corresponds to PBDB offshore collections, and e5 corresponds to PBDB slope and basinal collections

enlarge figure

Figure 4. Number of Paleobiology Database (PBDB) collections from deep water and marginal marine environments versus geologic time. A, Absolute time series. Solid line labeled “deep” includes collections identified as deep subtidal, transition zone/lower shoreface, offshore, basinal, slope, and submarine fan. Dashed line labeled “marginal” includes collections designated as coastal, marginal marine, estuary/bay, paralic, lagoonal, peritidal, and foreshore. Compare the deep curve to the curve for continental flooding (Fig. 2A) and compare the marginal curve to the number of fossiliferous Georef references (Fig. 2B). B, Ratio of marginal to deep PDBD collections over time

enlarge figure

Figure 5. Paleogeographic reconstructions of North America in the Late Ordovician and early Miocene by Ron Blakey (http://jan.ucc.nau.edu/rcb7/). Gray scale in ocean represents depth, with the lightest areas being epicontinental seas (on continental crust) that are generally much less than 200 m in depth. Dark areas represent open ocean (on oceanic crust). High sea levels establish widespread, shallow epeiric seas extending more than 1000 km inboard from the open ocean. By contrast, relatively low sea level restricts epicontinental marine environments to narrow shelves (generally less 200 km wide) located along continental margins. Here, the terms “epeiric” and “epicontinental” are used interchangeably and include all seaways located on continental crust that are separated by large distances from the open ocean. Foreland and cratonic basins may both be covered by epeiric seas and are here lumped under one heading for the sake of brevity, despite the very different environmental conditions that would prevail in each. All water-mass properties arising as a result of the very different shelf physiographies illustrated here are here collectively referred to as the “Epeiric Sea Effect.”

enlarge figure

Figure 6. Flexicalymene aggregation of four articulated individuals preserved in pyrite-bearing gray mudstone from the type Cincinnatian Series in Ohio. It is here hypothesized that episodic bouts of hypoxia resulted in the death of trilobites and other benthic animals in many epicontinental settings. Subsequent burial by modest quantities of sediment preserved the dead animals in fantastic death poses (shown here), modified death poses, or as scattered carcasses in various states of decay and disarticulation. See text for further explanation and for discussion of similar hypoxia-related enrollment among modern horseshoe crabs (Fisher 1977). Individual specimen at lower left side of photo has been removed from the matrix and turned upside down in order to reveal the enrolled specimen located directly beneath the partially prone individual. Specimen and photo courtesy of Kenneth D. Karns