Species abundance changes during mass extinction and the inverse Signor-Lipps effect: Apparent abrupt graptolite mass extinction as an artifact of sampling

The Late Ordovician mass extinction had a significant impact on marine life. Roughly 85% of species went extinct. Graptolites underwent a large species turnover during the extinction event. Late Ordovician graptolites in low latitude regions were dominated in terms of species diversity and specimen abundance by taxa of the Diplograptidae, Dicranograptidae and Orthograptidae (DDO). During the multiphased extinction event, fossil evidence in most low latitude locations indicated an abrupt extinction of members of the DDO taxa. The abrupt disappearance of DDO taxa coincided with major sea level fall and the sharp positive carbon isotopic excursion that can be associated with the Gondwana glaciation. The other major fauna, the Normalograptidae, then radiated during the Hirnantian and filled up ecospace left open by the abrupt disappearance of the DDO taxa. This pattern holds true for every low latitude region except for the Yangtze Platform region in South China. Here, there is extended survivorship of the DDO Fauna into the Hirnantian as well as an unusually high diversity of graptolites. It had been previously suggested that the Yangtze Platform region served as a refugium for late Ordovician graptolites and promoted endemism. Exceptionally complete sections, thorough sampling techniques and large sample sizes employed in the Yangtze Platform region has led to the question of whether or not the high diversity and extended survivorship of the DDO taxa are an artifact of sampling, or a true reflection of favorable conditions during the extinction event. Resampling of other low latitude locations using sampling efforts that more closely matched those employed in the Yangtze Platform region was necessary to test the hypotheses.; The Vinini Creek Formation in Nevada USA was selected for resampling based on the completeness of the succession, relative abundance and diversity of graptolites and well-preserved carbon isotope profiles. Abundance counts revealed that large sample sizes from Vinini Creek yielded similar results to the Yangtze Platform region with extended survivorship of the DDO taxa into the Hirnantian as well as a higher overall diversity, though diversity in the Yangtze Platform region still remains higher. Counts of species abundance through the mass extinction interval have revealed that DDO taxa make up 100% of the taxa present in the late Katian. Early Hirnantian samples, defined both by graptolite biozones and geochemical data show a dramatic reversal in abundances. DDO taxa constitute less than 5% of total specimens present in the Hirnantian samples, whereas normalograptid specimens become dominant and make up approximately 95-100% of the fauna. The collections from Vinini Creek and the Yangtze Platform region both show an extended period of survivorship and overlap between the DDO taxa and the normalograptids, whereas collections from other low latitudes that are smaller or have more limited preservation, the pattern recovered represents the abrupt disappearance of the DDO fauna and sudden replacement by normalograptids. New data from Vinini Creek contradict the refugium hypothesis associated with the Yangtze Platform region. Results have also led to the speculation that a pair of extinction events during the latest Ordovician may have resulted in the extinction and turnover patterns recovered. The first extinction event would have resulted in the loss of the majority of DDO fauna, followed by a second extinction event during which, the remaining DDO species were eliminated.