The influence of denitrifying seawater on graptolite extinction and diversification during the Hirnantian (latest Ordovician) mass extinction event

Abstract

2007: The influence of denitrifying seawater on graptolite extinction and diversification during the Hirnantian (latest Ordovician) mass extinction event. Lethaia , Vol. 40, A continuous trench exposure within the uppermost type Vinini Formation at Vinini Creek, Roberts Mountains, Nevada, provides an unparalleled opportunity to examine the fate of graptolites, prominent Paleozoic zooplankton, during most of the Hirnantian mass extinction event. On the basis of a detailed biostratigraphic and sedimentological dataset, the relatively complete extinction record is examined in the context of ecological constraints, and it is found to reflect an ecological collapse driven by glacioeustatic sea-level fall and associated changes in oceanic circulation. Diverse graptolite populations of the Dicranograptidae-Diplograptidae-Orthograptidae (DDO) fauna, which flourished in denitrifying waters within the oceanic oxygen-minimum zone (OMZ) during sea-level highstand, largely vanished with the loss of these conditions during glacio-eustatic sea-level fall. However, populations of one clade, the normalograptids, which inhabited the oxygenated waters of the photic zone, not only survived but diversified. These survivors gave rise to rapid recolonization and diversification with re-establishment of the oxygen-minimum and denitrifying conditions during post-Hirnantian sea-level rise. This ecological model also applies globally to other welldocumented coeval stratigraphic intervals, representing both oceanic and platform sea settings.

Graptoloid diversity and disparity became decoupled during the Ordovician mass extinction

“…Although these tests do not reject the possibility that selective extinction may have favored specific morphological characters found among the Neograptina, it is not clear that any of the uniquely neograptine traits could be the basis for their unlikely survival. The more likely alternative is that the survival of the Neograptina was the result of selection for nonmorphological traits, such as larger biogeographic ranges or clade-specific physiological preferences for environmental conditions prevalent during the Hirnantian (7,23,30).…”

“…First, we use resampling simulations to test whether morphological selectivity alone can explain the preferential survival of the Neograptina during the two extinction episodes of the HME. Chen et al (7) established that the survival and proliferation of the Neograptina during the Hirnantian cannot be explained by stochastic nonselective processes, but it is unclear whether this pattern results from selection for the relatively simple morphology of Late Ordovician neograptines or unobserved variation in another set of traits, such as physiological tolerances (23).…”

Graptolite community responses to global climate change and the Late Ordovician mass extinction

“…After death, all these graptolites settled to form mixed assemblages on the sea floor, with epipelagic graptolites present at relatively onshore as well as at more offshore localities, whereas mesopelagic species occur only in strata deposited near the shelf margin and beyond. Changes in deep-ocean circulation and oxygenation driven by climate change contributed to the Ordovician mass extinction (12,13,15,21,22), which suggests that the mesopelagic biotope may have been the most vulnerable and should exhibit disproportional extinction.…”

“…The loss of graptolite biodiversity in the LOME, accompanied by the wholesale extinction of the previously dominant Diplograptina (taxonomic use follows ref. 11) and their replacement by the previously marginal, high-latitude Neograptina (12)(13)(14)(15)(16), provides an opportunity to study the impact of climate change on a macroplanktonic invertebrate fauna over several million years during an interval of unusual species turnover (17,18). A focus on climate change over geological timescales as a driver of extinction dynamics leads us to ask whether there is evidence of ecological community changes in the interval leading up to mass extinction.…”

“…S2 and Datasets S1 and S2). The Vinini Creek site (Nevada; hereafter VC) records deposition in a bathyal continental slope to ocean floor setting, slightly south of the paleoequator (15,31). The Blackstone River site (Ogilvie Mountains, Yukon, Canada; hereafter BR) was a shallower site located ∼3,000 km away, on the continental shelf, north of the paleoequator (32,33).…”