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Journal of Paleontology 92(5):751–767
Eurytreta is in the Cordillera Oriental of Argentina, on the margin of Gondwana (Mergl et al., 2015). Although the species of Schizambon documented in Texas
are apparently endemic to Laurentia, the oldest reported occurrences of the genus are from Gondwana terranes in strata coeval with the Laurentian Cambrian upper Steptoean Stage (Shergold et al., 1976; Henderson et al., 1992; González- Gómez, 2005) or Sunwaptan Stage (Mergl et al., 2015). Schizambon has its lowest appearance in lower Skullrockian strata in the Great Basin (Popov et al., 2002), the same stratigraphic level as in Texas, suggesting migration of species of the genus from Gondwana to Laurentia during or after the Sunwaptan–Skullrockian faunal turnover. Migration from Gondwana to Laurentia is again suggested by the coeval lowest occurrence of siphonotretids with spines in both this study and in the Shirgest Formation of Iran (Popov et al., 2009a), and the potentially slightly older occurrence of this morphology in the Mila Formation of Iran (Popov et al., 2009b). The appearance of the Gondwana-/Kazakhstan-linked
Skullrockian fauna implies that biofacies shifts did play more of a role in the recovery of linguliform brachiopod faunas after the Sunwaptan–Skullrockian extinction. Nonetheless, this biofacies shift was not a shift of the Laurentian shelf margin biofacies shoreward because the two faunas are virtually identical in upper Sunwaptan strata. The appearance of Eurytreta, whose lowest occurrence is in the seamount margin facies of the Malyi Karatau Range of Kazakhstan (Cook et al., 1991) reveals that migration of Eurytreta sublata was from Kazakhstan to the Laurentian shelf/slope.
Extinctions and recoveries as separate events.—We suggest that working exclusively with trilobite faunas, which show consistent post-extinction replacement by deep-water cosmo- politan faunas, has led to conflating the process of extinction with the process of recovery while considering the potential causes of the repeating late Cambrian–earliest Ordovician crises. The Sunwaptan–Skullrockian extinction event appears to coincide with a sequence boundary, while the appearance of the globally distributed recovery fauna appears to coincide with a subsequent transgression. Recoveries from extinction may certainly be influenced by the events that caused extinction, but recoveries are complex phenomena and other factors may play a role (e.g., Erwin, 2001). As research into the nature of the biomere events continues, greater emphasis should be placed on understanding the changing nature of the larger ecosystem through both extinction and recovery phases by integrating multiple data sets (e.g., Saltzman et al., 2015).
Conclusions
The mid to upper Sunwaptan strata of central Texas record similarity between linguliform faunas of the Laurentian platform and those of the shelf margin of the Great Basin. Sunwaptan–Skullrockian trilobite and conodont extinction events apparently coincide with complete turnover of linguli- form brachiopod taxa at the genus level, which may be related to a sequence boundary. After these events, the replacement linguliform brachiopod fauna demonstrates global affinities. The timing of appearance of these taxa in Laurentia as compared
to Kazakhstan and Gondwana suggests migration from these areas to Laurentia during transgression. The extinction event and the event that facilitated the repopulation of the Laurentian shelf are separate events.
Acknowledgments
The Welge, Lange, and Crenwelge families of Doss, Texas generously provided hospitality and access to their ranches. Freeman’s research was supported by the W. Kent McWilliams Geological Research Fund of Tulane University as well as by a CRAA grant from the University of Kentucky’s College of Arts and Sciences. Miller’s research was funded by National Science Foundation Grants EAR 8108621, EAR 8407281, and EAR 8804352, and by Faculty Research Grants from Missouri State University. Parts of this work were carried out in the Argast Family Instrumentation and Analysis Lab at Indiana University Purdue University Fort Wayne. J. Loch, J. Taylor, and K. Evans gave invaluable assistance with fieldwork. We thank two anonymous reviewers for thoughtful reviews that improved the paper. This paper is a contribution to IGCP 653: the onset of the Great Ordovician Biodiversification Event.
Accessibility of supplemental data
Data available from the Dryad Digital Repository:
http://doi.org/ 10.5061/dryad.64td7
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