CONSTRAINTS ON EXAMINING SPECIATION IN THE FOSSIL RECORD: THE ROLE OF SEA LEVEL

By necessity, data for examining the nature of speciation – documenting the pattern of tempo and mode in the fossil record – is derived from the stratigraphic record. The shortcomings of this record have been documented over a spectrum of scales ranging from micro- to macrostratigraphic components and over a range of different depositional settings, but rarely has the focus been on relating sea-level reconstructions to the nature of the record being examined. Furthermore, although considerable efforts have been devoted to examining the sedimentation rates and relative completeness of various depositional environments and specific sequences, more limited efforts have concentrated on reconstructing how the sedimentary record of relatively shallow marine deposits reflects variation in sea level. Here, we compare the stratigraphic record of the past ~160 Ma using Haq et al’s (1988) and Miller et al’s (2005) sea-level curves in combination with data on subsidence rates to estimate what the broad-scale completeness of the records along passive margins and within foreland basins might be and how this potentially influences our ability to reconstruct the evolutionary history of speciation in relatively shallow-water settings. Preliminary results suggest that the stark differences in the sea-level curves overprints the entire results. The Haq et al curve has substantially higher sea levels throughout the study interval with very few lowstands, whereas not only are the highstands siginificantly dampened in the Miller et al curve, but sea level is reconstructed as much more volatile with numerous lowstands. In terms of the overall stratigraphic completeness, the former curve suggests that the record should be quite complete in all shallow-water settings, whereas the latter suggests a much more fragmented record with considerable gaps. At a finer scale, the pronounced, rapid, asymmetric sea-level changes associated with icehouse conditions results in a much less complete record with only maximum highstands recorded.