622 Assessing temporal trends
Potentially more important than assessing the extent of a bias is exploring how it might alter perceived trends. To explore this, we conducted two analyses at two different temporal scales: within a stratigraphic sequence and through geologic time.
Stratigraphic model.—A generalized stratigraphic sequence containing several distinct depositional environments was pro- duced to simulate the effects of differential preservation on crinoid disparity. Four intervals within this theoretical sequence, representing settings characterized by dissimilar suites of paleoenvironmental processes and therefore capable of produ- cing distinctive taphofacies (sensu Speyer and Brett, 1986), were selected as datums. Using the sequence stratigraphic terminology of Catuneanu (2006), these intervals comprise a carbonate interval within the transgressive systems tract, a siliciclastic mudstone interval within the early highstand systems tract, an impure mudstone interval within the late highstand systems tract, and a calcareous siltstone interval within the falling stage systems tract. All lithologic character- istics, sequence stratigraphic interpretations, and taphofacies descriptions strongly agree with direct field observations as well as published data (e.g., Brett, 1995; McLaughlin et al., 2008). The carbonate environment represents deposition during a
phase of relatively rapid base-level rise, resulting in siliciclastic sediment starvation in distal environments and a corresponding increase in the amount of time a skeleton would remain in the taphonomically active zone. Consequently, total disarticulation is assumed for all echinoderm skeletons. Characters related to isolated ossicles and features that would otherwise be concealed but are revealed through disarticulation control the disparity in this interval. The siliciclastic mudstone-dominated intervals represent
deposition following waning base-level rise followed by the transition into stillstand, which, in turn, transitions into gradual base-level fall, eventually progressing into increasingly rapid base-level fall. Hence, the transition between transgressive and early highstand systems tracts (i.e., the maximum flooding surface) is generally not a particularly prominent surface. During early highstand, the relatively slow rate of base-level fall resulted in generally low background sedimentation rates; in distal environments, sedimentation is dominated by siliciclastic clays. Given the slow background sedimentation rate, episodic rapid burial events become stacked in thin intervals where the primary mechanism for sediment accumulation is in the form of obrution events. Repeated rapid burial of crinoid populations by fine-grained sediment results in genesis of a taphofacies characterized by an abundance of completely articulated individuals; thus, complete skeletal articulation is assumed for the early highstand interval. By contrast, the comparatively elevated background sedimentation associated with increasing base-level fall produces a late highstand taphofacies where articulation is not as common. Although it may initially appear paradoxical that increased sedimentation rate would result in a decreased frequency of articulated crinoids, the overall sedi- mentation rate is only moderate compared to other phases of base-level change but is nevertheless sufficient to separate
Journal of Paleontology 91(4):618–632
(‘unstack’) episodic rapid burial horizons, splaying apart obrution horizons with greater thicknesses of background sediment; hence, the general taphofacies is dominated by partially articulated or disarticulated specimens buried moder- ately quickly to somewhat slowly during background intervals. The fine grain size of these two taphofacies would preclude loss of surface features via abrasion, even in isolated ossicles. The calcareous siltstone interval represents deposition
during a relatively rapid phase of base-level fall. This resulted in significantly increased influx of coarser-grained siliciclastic sediment into distal settings and a shift to more energetic environments. As with the late highstand taphofacies described in the preceding, an increased rate of background sedimentation does not correlate with increased preservational quality as the higher energy settings are associated with elevated rates of abrasion on exposed skeletal material as well as frequent erosion and exhumation of crinoid carcasses even if initially buried relatively rapidly. Hence, total disarticulation is assumed for all crinoids, as is loss of surface details of isolated ossicles.
Stratigraphic and taphonomic assumptions.—Certain assump- tions of the model used in this study require some justification from stratigraphic and taphonomic perspectives as any number of stratigraphic scenarios and taphonomic interpretations can be employed in order to test theoretical relationships. Specifically, the following are worth brief commentary: (1) the use of a mixed carbonate-siliciclastic sequence for our modeled stratigraphic setting, (2) the use of crinoids to the exclusion of other echino- derm groups in modeling disparity changes through a strati- graphic sequence, (3) the degree to which the modeled distribution of crinoids realistically reflects crinoid occurrences, (4) the use of the specific crinoid taxa selected for the model, and (5) the basis for interpreting which features are likely to be lost or retained within each analyzed portion of the theoretical stra- tigraphic sequence. The stratigraphic model used to study disparity through
changing depositional environments is based on a mixed carbonate-siliciclastic system in an epeiric setting. This was selected because the interplay between siliciclastic influx and carbonate production in the absence of siliciclastic influx results in strong contrasts in sediment character and paleoenviron- mental processes between different, discrete phases of base- level change. These relationships are more subtle, and therefore more difficult to model clearly, in pure siliciclastic and pure carbonate systems (e.g., Catuneanu, 2006). Moreover, a mixed carbonate-siliciclastic system is characteristic of the Ordovician through Devonian of the greater eastern midcontinent and northern Appalachian Basin of North America, a region where numerous high-resolution stratigraphic and paleontologic stu- dies have produced an unparalleled database for echinoderm occurrences with regard to sequence stratigraphic setting (for details of eastern North American middle Paleozoic sequence stratigraphy see, e.g., Holland and Patzkowsky, 1996; Brett et al., 2004, 2012; McLaughlin et al., 2008; Ettensohn et al., 2013).
Crinoids were selected as an ideal group to test changes in
disparity through a theoretical depositional sequence. Crinoid taxa were analyzed rather than other echinoderm groups because they represent Type 2 morphologies, making them
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