Graptoloid diversity and disparity became decoupled during the Ordovician mass extinction

Abstract

The morphological study of extinct taxa allows for analysis of a diverse set of macroevolutionary hypotheses, including testing for change in the magnitude of morphological divergence, extinction selectivity on form, and the ecological context of radiations. Late Ordovician graptoloids experienced a phylogenetic bottleneck at the Hirnantian mass extinction (∼445 Ma), when a major clade of graptoloids was driven to extinction while another clade simultaneously radiated. In this study, we developed a dataset of 49 ecologically relevant characters for 183 species with which we tested two main hypotheses: (i) could the biased survival of one graptoloid clade over another have resulted from morphological selectivity alone and (ii) are the temporal patterns of morphological disparity and innovation during the recovery consistent with an interpretation as an adaptive radiation resulting from ecological release? We find that a general model of morphological selectivity has a low probability of producing the observed pattern of taxonomic selectivity. Contrary to predictions from theory on adaptive radiations and ecological speciation, changes in disparity and species richness are uncoupled. We also find that the early recovery is unexpectedly characterized by relatively low morphological disparity and innovation, despite also being an interval of elevated speciation. Because it is necessary to invoke factors other than ecology to explain the graptoloid recovery, more complex models may be needed to explain recovery dynamics after mass extinctions.M ass extinction events are defined by their effect on taxonomic diversity, but they also have profound impacts on the biotic diversity of morphology and ecology (1-3). Quantitative assessments of morphological diversity, i.e., disparity, can shed light on the selectivity of extinction and add to our understanding of the ecological context of recovery patterns after extinction events (4,5).In this paper, we quantify the morphological dynamics of the Graptoloidea during the end-Ordovician mass extinction, which began ∼445 million years ago. This event is composed of two separate extinction episodes during the Hirnantian Stage (6) and is commonly referred to as the Hirnantian mass extinction (HME). These episodes of increased extinction rate are associated with the initiation and termination of a global cooling period in the first half of the Hirnantian, during which sea-surface temperatures dropped ∼5°C (7, 8).Graptoloids are an extinct clade of colonial zooplankton (Hemichordata) noted for their well-sampled fossil record and high rate of species turnover (9-11). Previous work on the effect of the HME on the Graptoloidea has revealed a tumultuous and complex pattern. The graptoloids experienced some of the highest extinction intensities among marine invertebrates during the HME (12), and the surviving taxa were a limited sample of pre-HME lineages. Before the mass extinction event, graptoloid lineages can be divided into two monophyletic clades that diverged more than 20 million...

Trait-based diversification shifts reflect differential extinction among fossil taxa

“…However, extinction resistance/selectivity does not predict unusually high anagenetic HSC: The mechanism for paraclade extinction does not coincide with the appearance of derived traits. Moreover, we have empirical examples of extinction resistance associated with primitive traits (23,56) as well as many cases in which there is no obvious selectivity at all (57). These considerations make extinction resistance/selectivity a less reliable and less powerful explanation; however, we cannot discount it entirely.…”

Clades reach highest morphological disparity early in their evolution

“…However, clades terminating at a mass extinction event might be similarly truncated and are likely to have higher CGs for similar reasons. Mass extinctions have undoubtedly influenced the manner in which clades have explored morphospaces (42), but this phenomenon received little attention until recently (37,(43)(44)(45)(46)(47). Moreover, only one of these studies (44) focused on extinction selectivity per se; all others investigated the subsequent evolution of extinction survivors.…”

“…Several of the analyses that originally spurred the debate (10,(21)(22)(23)35) used discrete character matrices to compare anatomically very disparate forms. Many studies have recently followed similar protocols (27,(36)(37)(38), and we adopted these methods here as a unifying approach. Where discrete and continuous character data have been compared for the same sets of taxa (39), relative estimates of disparity have been similar.…”

Graptolite community responses to global climate change and the Late Ordovician mass 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.…”