Biogeography and Mass Extinction: Extirpation and re-invasion of Normalograptus species (Graptolithina) in the Late Ordovician Palaeotropics

Abstract

SUMMARY 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) evolutionary origin probably 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: (1) competitive exclusion; or (2) 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.

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

“…S3). Neograptine species invade the paleotropics immediately before the start of the Hirnantian Age (14), and, thus, only four of these species occur within the latest Katian at the study sites (Dataset S2); two emerge as epipelagic, but the other two are too rare to classify.…”

“…At both sites, the replacement of Diplograptina by Neograptina occurs during the early phase of relative sea level fall. After being absent from the paleotropics throughout the late Katian (14), the Neograptina invaded at the onset of the glacial epoch and immediately became the dominant species: Neograptines comprise 90-99% of specimens recovered in this interval at both sites, despite their different depositional settings. At VC, the appearance of the neograptine community is associated with hopane−sterane ratios that suggest the phytoplankton community shifted from cyanobacteria in O4 to dominantly green algae in O5.…”

“…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.…”

Graptoloid diversity and disparity became decoupled during the Ordovician mass extinction

“…Absolute time scale for the zones is taken from Sadler et al (39). Note that the lack of neograptine species in the uncinatus Zone reflects the rarity of normalograptid taxa in that interval rather than a poorly sampled record (45).…”

Greenhouse−icehouse transition in the Late Ordovician marks a step change in extinction regime in the marine plankton

“…It was associated with major positive excursions in the carbon isotope (δ 13 C carb ) ratio (25,29,42), global continental glaciation (43,44), changes in oceanic circulation, water mass properties, and microphytoplankton populations (42,45), and the deep-water graptoloid biotope was severely degraded or destroyed (13,29). The main surviving group after the LOME, the cold-adapted normalograptids (46), diversified rapidly in the early Silurian, driving a rapid recovery in species richness of the clade. Our findings suggest that the LOME was a unique event that marks the transition to a new regime in which frequent extinction episodes disturbed the age structure of the entire clade, prevented the accumulation of long-lived species in the Silurian, and reduced the median species duration to half its Ordovician value (Fig.…”