SPECIES ABUNDANCE CHANGES DURING MASS EXTINCTION AND THE INVERSE SIGNOR-LIPPS EFFECT: APPARENTLY ABRUPT GRAPTOLITE MASS EXTINCTION AS AN ARTIFACT OF SAMPLING

The probability of sampling species in the fossil record is directly proportional to the abundance of preserved specimens. Variable abundance can cause an abrupt mass extinction to appear gradual (the Signor-Lipps effect) by the backward displacement of the apparent time of extinction of the rarer taxa. Species abundance changes associated with graptolite species turnover during the Hirnantian mass extinction at the end of the Ordovician suggests a related sampling artifact can cause a gradual mass extinction to appear more abrupt than was originally the case: an “inverse Signor-Lipps effect.” Late Ordovician graptolite faunas of the tropical realm were dominated, both in species diversity and specimen abundance, by taxa of the Diplograptidae, Dicranograptidae, and Orthograptidae (DDO). Normalograptid species were few and were generally minor components of the tropical biotas. During the extinction DDO taxa collected in most regions (e.g., Kazakhstan, Siberia, Scotland, and the Canadian arctic) disappear abruptly, coincident with a sharp positive carbon isotopic excursion associated with major sea level fall and Gondwanan continental glaciation. Hirnantian faunas thereafter are exclusively normalograptid. However, previous data from China and our new data from Vinini Creek in Nevada, show a pattern of more extended survivorship of DDO taxa, including several Lazarus taxa in late Hirnantian strata. Counts of species abundance through the mass extinction based on intensive sampling reveal that individual DDO taxa exhibit variable abundance in the pre-extinction interval but are collectively greater than 95% of the fauna. Early Hirnantian samples (defined both on graptolite zones and geochemical data) show a complete reversal of abundances: specimens of DDO taxa rapidly decline to less than 5% of the total fauna whereas normalograptid specimens become numerous and make up 95% or more. Species ranges based on samples large enough to recover these rare DDO's are effected less by this abrupt change in specimen abundance. Thus, the Vinini and Yangtze collections show an extended period of overlap between DDO and new Hirnantian normalograptid species whereas samples from sites where collections are smaller or preservation more limiting, exhibit a pattern of abrupt truncation and sudden replacement.