BIOGEOGRAPHY AND MASS EXTINCTION: EXTIRPATION AND RE-INVASION OF NORMALOGRAPTUS SPECIES (GRAPTOLITHINA) IN THE LATE ORDOVICIAN PALAEOTROPICS

Normalograptids constitute a distinctive but not easily identified group of derived axonophorans. A new study of Normalograptus morphology indicates that many Late Ordovician taxa previously identified as Normalograptus are actually referable to Styracograptus and related climacograptoids. These re-identifications have led to the recognition of a complex biogeographic history among normalograptids and their descendants, including an interesting pattern that had previously been overlooked – a late Katian disappearance of Normalograptus from the palaeotropics. Normalograptid palaeogeographic distribution in the Ordovician appears to comprise 5 main phases - 1. probable evolutionary origin in relatively high palaeolatitudes in the early to mid Darriwilian; 2. spread to global distribution in later Darriwilian time; 3. retreat from most low palaeolatitude regions during the early Katian, with perhaps a slightly delayed retreat from Laurentia; 4. a complete extirpation from the low palaeolatitudes by the mid Katian (Ea4); 5. reinvasion of the low palaeolatitudes that accompanies the remarkable ecological and evolutionary replacement of the Diplograptina by species of Normalograptus and their descendants during the latest Katian-early Hirnantian. We propose two sets of hypotheses to explain Late Ordovician Normalograptus distribution. First, given the low species diversity of Normalograptus, we suggest as a null hypothesis that the disappearance of normalograptids from low palaeolatitudes during the late Sandbian and early Katian as simply a stochastic outcome - a small clade simply went extinct by chance alone. The alternative is that the loss of Normalograptus species reflects some form of extinction for cause, either a) competitive exclusion or b) an inability to adapt to changing environmental conditions. In the case of the competitive exclusion hypothesis, the most likely competitors are species of the Diplograptina.