Biogeographic control of trilobite mass extinction at an Upper Cambrian “biomere” boundary

Abstract

Extinctions at the top of the Sunwaptan Stage (=“Ptychaspid Biomere”) near the Cambrian-Ordovician boundary eliminated about half of North American trilobite families. The families that extend from the shelf into the upper slope show significantly higher survival than those confined to the shelf. Biofacies and lithofacies distribution patterns indicate that the extinctions cannot be attributed to a shelfwide physical environmental perturbation, such as a fall in water temperature or the spread of anoxic waters. We develop a simple biogeographic model which suggests that diversity of a faunal province is influenced profoundly by changes in the number of component biofacies. This model is tested with an analysis of biofacies distribution patterns across the upper boundary of the Sunwaptan Stage. The extinctions correspond closely to lithofacies shifts in the outer shelf that indicate the initiation of major paleogeographic changes, possibly in response to a sea-level rise. The effects of these changes cascade across the entire shelf by the shoreward migration of off-shelf and shelf-margin taxa. Biofacies become reduced in number through telescoping and their environmental ranges expand during the extinction interval, suggesting an increase in the proportion of eurytopic taxa. Selective survival of wide-ranging eurytopes may have influenced the dynamics of faunal replacement by lowering speciation rates of shelf taxa. Consequently, the proportion of shelf endemics will decline and biofacies will be dominated by immigrant taxa. There are sufficient similarities in extinction patterns across the upper boundary of the Sunwaptan Stage and those at other Upper Cambrian stage boundaries to suggest that the biogeographic model developed here may have broader application.

Effects of the Mesozoic Marine Revolution on the taxonomic, morphologic, and biogeographic evolution of a group: aporrhaid gastropods during the Mesozoic

“…6; Aporrhais, Maa = 4, Pal = 4), whereas two However, the shape of the distribution were narrow ranging (i.e., occupying three changed markedly following the Maastricht-or fewer geographic divisions: Dicroloma, Maa ian extinction, with the Paleocene fauna con-= 1, Pal = 2; Strufhiochenopus, Maa = 1, Pal = taining roughly equal proportions of broad-1). No geographic data were available for the (Bretsky 1973;Fortey 1983;Jablonski 1986Jablonski , 1989Westrop & Ludvigsen 1987), the difference between widespread and restricted genera is not statistically significant (G-test, 0.1 > p > 0.05). However, given the small number of genera involved in the present study, the power of statistical tests is suspect and hence the survivorship pattern documented should be interpreted with caution.…”

Persistent oceanic anoxia and elevated extinction rates separate the Cambrian and Ordovician radiations

“…The base Stairsian extinction likely resulted from ecological factors triggered in part by spread of anoxic and/or dysoxic water and possibly sea-level-related habitat destruction in the outer shelf (Westrop and Ludvigsen, 1987). Consistent with the observation that the positive d 13 C excursion was initiated just below the base Stairsian sequence boundary ( 3), multiple carbon cycle drivers in addition to sea level are likely superimposed to produce the complex signal (e.g., changes in ocean circulation and ventilation or weathering fluxes); this could also explain why some large Ordovician sea-level changes (e.g., basal Stonehenge transgression; Miller et al, 2003;Taylor et al, 2012;Landing et al, 2012) are not associated with isotopic excursions or extinctions.…”

“…Extinction events at stage or "biomere" (biostratigraphic unit) boundaries (Stitt, 1983;Palmer, 1984;Westrop and Ludvigsen, 1987;Taylor et al, 2012) occurred at least 5 times over the ~20 m.y. of the Furongian to mid-Early Ordovician (Tremadocian Stage) (Adrain et al, 2009).…”

Biogeographic shifts in a transgressive succession: the Cambrian (Furongian, Jiangshanian; Latest Steptoean–earliest Sunwaptan) agnostoid arthropodsKormagnostellaRomanenko andBiciragnostusErgaliev in North America

“…They provide further evidence for shifts in biogeographic and environmental distribution during Cambrian trilobite and agnostoid extinction events (e.g., Ludvigsen and Westrop, 1983;Westrop and Ludvigsen, 1987).…”