Alycia L. Stigall

Climate refugia: joint inference from fossil records, species distribution models and phylogeography

Climate refugia, locations where taxa survive periods of regionally adverse climate, are thought to be critical for maintaining biodiversity through the glacial-interglacial climate changes of the Quaternary. A critical research need is to better integrate and reconcile the three major lines of evidence used to infer the existence of past refugia - fossil records, species distribution models and phylogeographic surveys - in order to characterize the complex spatiotemporal trajectories of species and populations in and out of refugia. Here we review the complementary strengths, limitations and new advances for these three approaches. We provide case studies to illustrate their combined application, and point the way towards new opportunities for synthesizing these disparate lines of evidence. Case studies with European beech, Qinghai spruce and Douglas-fir illustrate how the combination of these three approaches successfully resolves complex species histories not attainable from any one approach. Promising new statistical techniques can capitalize on the strengths of each method and provide a robust quantitative reconstruction of species history. Studying past refugia can help identify contemporary refugia and clarify their conservation significance, in particular by elucidating the fine-scale processes and the particular geographic locations that buffer species against rapidly changing climate.

Speciation collapse and invasive species dynamics during the Late Devonian “Mass Extinction”

The Late Devonian (Frasnian-Famennian) interval includes one of the most dramatic intervals of biotic turnover in the Phanerozoic. Statistical evaluation of diversity change reveals that the primary cause of biodiversity decline was reduced speciation during the crisis interval, not elevated extinction rates. Although various hypotheses have been proposed to explain extinction increase during the Late Devonian, potential causes for reduced speciation have previously been largely unaddressed. Recent analyses focusing on biogeographic and phylogenetic patterns of species in shallow marine ecosystems of Laurentia indicate that a dramatic increase in interbasinal species invasions, facilitated by transgressive pulses, fundamentally affected biodiversity by enabling range expansion of ecological generalists and eliminating vicariance, the primary pathway by which new species typically form. Modern species invasions may result in similar speciation loss, exacerbating the current biodiversity crisis.

Invasive Species and Biodiversity Crises: Testing the Link in the Late Devonian

During the Late Devonian Biodiversity Crisis, the primary driver of biodiversity decline was the dramatic reduction in speciation rates, not elevated extinction rates; however, the causes of speciation decline have been previously unstudied. Speciation, the formation of new species from ancestral populations, occurs by two primary allopatric mechanisms: vicariance, where the ancestral population is passively divided into two large subpopulations that later diverge and form two daughter species, and dispersal, in which a small subset of the ancestral population actively migrates then diverges to form a new species. Studies of modern and fossil clades typically document speciation by vicariance in much higher frequencies than speciation by dispersal. To assess the mechanism behind Late Devonian speciation reduction, speciation rates were calculated within stratigraphically constrained species-level phylogenetic hypotheses for three representative clades and mode of speciation at cladogenetic events was assessed across four clades in three phyla: Arthropoda, Brachiopoda, and Mollusca. In all cases, Devonian taxa exhibited a congruent reduction in speciation rate between the Middle Devonian pre-crisis interval and the Late Devonian crisis interval. Furthermore, speciation via vicariance is almost entirely absent during the crisis interval; most episodes of speciation during this time were due to dispersal. The shutdown of speciation by vicariance during this interval was related to widespread interbasinal species invasions. The lack of Late Devonian vicariance is diametrically opposed to the pattern observed in other geologic intervals, which suggests the loss of vicariant speciation attributable to species invasions during the Late Devonian was a causal factor in the biodiversity crisis. Similarly, modern ecosystems, in which invasive species are rampant, may be expected to exhibit similar shutdown of speciation by vicariance as an outcome of the modern biodiversity crisis.

Geologic Drivers of Late Ordovician Faunal Change in Laurentia: Investigating Links between Tectonics, Speciation, and Biotic Invasions

Geologic process, including tectonics and global climate change, profoundly impact the evolution of life because they have the propensity to facilitate episodes of biogeographic differentiation and influence patterns of speciation. We investigate causal links between a dramatic faunal turnover and two dominant geologic processes operating within Laurentia during the Late Ordovician: the Taconian Orogeny and GICE related global cooling. We utilize a novel approach for elucidating the relationship between biotic and geologic changes using a time-stratigraphic, species-level evolutionary framework for articulated brachiopods from North America. Phylogenetic biogeographic analyses indicate a fundamental shift in speciation mode—from a vicariance to dispersal dominated macroevolutionary regime—across the boundary between the Sandbian to Katian Stages. This boundary also corresponds to the onset of renewed intensification of tectonic activity and mountain building, the development of an upwelling zone that introduced cool, nutrient-rich waters into the epieric seas of eastern Laurentia, and the GICE isotopic excursion. The synchronicity of these dramatic geologic, oceanographic, and macroevolutionary changes supports the influence of geologic events on biological evolution. Together, the renewed tectonic activity and oceanographic changes facilitated fundamental changes in habitat structure in eastern North America that reduced opportunities for isolation and vicariance. They also facilitated regional biotic dispersal of taxa that led to the subsequent establishment of extrabasinal (=invasive) species and may have led to a suppression of speciation within Laurentian faunas. Phylogenetic biogeographic analysis further indicates that the Richmondian Invasion was a multidirectional regional invasion event that involved taxa immigrating into the Cincinnati region from basins located near the continental margins and within the continental interior.

Taphonomy of lacustrine interbeds in the Kirkpatrick Basalt (Jurassic), Antarctica

Abstract The Kirkpatrick Basalt (Jurassic) of South Victoria Land and the Central Transantarctic Mountains, Antarctica, includes sedimentary interbeds representing shallow lakes and ephemeral ponds (some with microbial mat accumulations), deep permanent lakes, and lake-margin areas, especially vegetated wetlands. Fossil assemblages in these sedimentary interbeds are dominated by spinicaudatans (conchostracans), but ostracodes, insect nymphs, actinopterygian fish, and plants are locally abundant. Similar biotas in contrasting contemporaneous deposits allow the taphonomy of these organisms to be compared across lacustrine depositional settings. Spinicaudatan carapaces and fish remains are preserved primarily in calcium phosphate, whereas ostracode carapaces are preserved in calcium carbonate, reflecting the original skeletal composition of the animals. Where microbial mats are present, silica replacement of spinicaudatan carapaces occurs more extensively than in other deposits; microbial processes may have enhanced silicification. This study is the first well-documented example of microbial mat influence on preservation in high-latitude lacustrine systems.

Modern Nautilus (Cephalopoda) taphonomy in a subtidal to backshore environment, Lifou (Loyalty Islands)

Abstract Thirty-two samples of submerged Nautilus macromphalus shells were recovered in 2008 from Lifou, Loyalty Islands in the South Pacific Ocean. Specimens were collected from carbonate-dominated sediment in water depths of 1–3 m. Some specimens were partly buried, whereas others rested on the seafloor. The majority of the specimens (66%) were recovered in a horizontal position, whereas 34% of the specimens were oriented vertically. Some specimens were pristine, with sharp color stripes and little encrustation by algae, cyanobacteria, or epizoans. The majority of specimens have substantial algal and cyanobacterial overcoats with some epizoans. In some specimens, the overcoats also trapped substantial amounts of carbonate sediment. Comparison of the 2008 collection of subtidal specimens to 43 beached Nautilus shells collected in 2002 from the same location reveals that the nearshore taphonomic pathways for drift cephalopod shells can be more complicated than published theoretical models suggest. Nautilus shells may or may not float directly to the beach. Shells not immediately deposited on the beach sink in the shallow water in a vertical position. Weight added by attached organisms and water infiltration, causes the submerged shells to eventually assume a horizontal position. Waves, currents, and bioturbation can then flip the shells over from side to side. Eventually submerged shells are buried, broken apart, or transported onto the beach. Beached shells that follow this taphonomic pathway have conspicuous algal coatings compared to those that simply float to shore. The Lifou subtidal population represents the first substantial modern externally shelled cephalopod collection from a shallow water environment to be analyzed to determine its taphonomic pathways. Conclusions from this analysis can be applied to nearshore deposits that contain externally shelled, fossilized cephalopods.

Species-level phylogenetic revision of the Ordovician orthide brachiopod Glyptorthis from North America

Species of the orthide brachiopod genus Glyptorthis occur as common constituents of Late Ordovician benthic marine faunas, particularly in Laurentia. A phylogenetic analysis of 23 North American species of Glyptorthis was conducted to inform a systematic revision and ascertain evolutionary relationships amongst species. Both discrete and continuous characters, from which character states were determined using statistical methods, were utilized within a maximum parsimony approach. Glyptorthis maquoketensis is recognized as a distinct species of Glyptorthis rather than a subspecies of G. insculpta. Glyptorthis virginica is herein synonymized with G. uniformis. Several species of Glyptorthis became widely distributed via dispersal events during the evolution of the clade. The revised phylogenetic topology provides structure to interpret biogeographical patterns within the genus. For example, Glyptorthis insculpta participated in the Late Ordovician Richmondian Invasion into the Cincinnati Basin. The recovered phylogenetic topology suggests that G. insculpta may have invaded the Cincinnati Basin from a low latitude subtropical region of Laurentia, such as the American mid-Continent.

Synchronous diversification of Laurentian and Baltic rhynchonelliform brachiopods: Implications for regional versus global triggers of the Great Ordovician Biodiversification Event

The profound global impact of marine radiations during the Great Ordovician Biodiversification Event (GOBE) is widely appreciated; however, diversification varied among paleocontinents and these individual trajectories are less understood. Here we present a new species-level diversity curve for rhynchonelliform brachiopods from midcontinental Laurentia based on bed-by-bed analysis of the Simpson Group of Oklahoma (USA). Diversity and abundance data span the Dapingian through Sandbian Stages, which encompass the interval of maximum global diversification. A rapid, statistically significant increase in brachiopod diversity was observed in the early Darriwilian Histiodella holodentata Biozone. We interpret this as a biological signal because the increase cannot be explained by sampling intensity, facies types, or position along depositional gradient. Diversifications on Laurentia and Baltica were temporally synchronous at the biozone level, and cumulative diversity curves for the regions show similar patterns, suggesting a global driver for the radiations. The taxonomic composition of the brachiopod faunas, however, differs substantially, highlighting the importance of regional controls on diversification. Thus both global and local factors controlled diversity increase during the GOBE.

Phylogenetic Revision of the Late Ordovician Orthid Brachiopod Genera Plaesiomys and Hebertella from Laurentia

Abstract The orthidine brachiopod genera Plaesiomys and Hebertella are significant constituents of Late Ordovician benthic marine communities throughout Laurentia. Species-level phylogenetic analyses were conducted on both genera to inform systematic revisions and document evolutionary relationships. Phylogenetic analyses combined discrete and continuous characters, from which character states were determined using a statistical approach, and utilized both cladistic and Bayesian methodologies. Plaesiomys cutterensis, P. idahoensis, and P. occidentalis are herein recognized as distinct species rather than subspecies of P. subquadratus. Similarly, Hebertella montoyensis and H. prestonensis are recognized as distinct species separate from H. occidentalis, and H. richmondensis is recognized as a distinct species rather than a geographical variant of H. alveata. Hebertella subjugata is removed from its tentative synonymy with H. occidentalis and revalidated. The development of species-level evolutionary hypotheses for Plaesiomys and Hebertella provides a detailed framework for assessing evolutionary and paleobiogeographic patterns of Late Ordovician brachiopods from Laurentia. The geographic range of Hebertella expanded throughout Laurentia during the Richmondian into both intracratonic and marginal basins. Plaesiomys subquadratus participated in the Late Ordovician Richmondian Invasion. The recovered phylogenetic topology for Plaesiomys suggests that P. subquadratus may have migrated into the Cincinnati region from a basin situated to the paleo-northeast.

Controls on niche stability in geologic time: congruent responses to biotic and abiotic environmental changes among Cincinnatian (Late Ordovician) marine invertebrates

Abstract The set of environmental conditions under which a taxon can survive and maintain viable populations, known as the ecological niche, is a fundamental determinant of a taxons distribution. Because of the central importance of ecological niches, they have been assumed to remain relatively stable during intervals of morphological stasis. However, the assumption of niche stability has rarely been tested directly with fossil data spanning multiple temporal intervals. Thus, the conditions under which this assumption is likely to be accurate are not well understood. In this study, we use ecological niche modeling (ENM) to reconstruct the ecological niche for 11 genera of marine benthos (crinoids, trilobites, molluscs, bryozoans, and corals) from the Type Cincinnatian Series (Late Ordovician, Katian Stage) across nine temporal intervals spanning approximately three million years. This interval includes both abiotic environmental change (gradual sea-level fall) and biotic change (rapid pulses of the Richmondian Invasion), thus allowing the relative effect of different environmental perturbations to be constrained. A previous symmetrical analysis of niche stability of brachiopod species recovered an increase in niche evolution following the Richmondian Invasion. Herein we test the generality of the brachiopod pattern within the community. Niche stability was evaluated in geographic space, ecological space, and niche parameter space. Niche stability varied through time; during the Pre-Invasion interval, taxa exhibited niche stability during gradual shallowing of sea level in the basin, whereas niche evolution became more common during the Richmondian Invasion. Taxa adjusted to the increased competition by altering aspects of their niche. Notably, surviving taxa contracted their niche into a subset of their previous niche parameters. This represents an adaptive response to increased competition for resources with the newly established invader taxa, and it was employed most successfully by generalist taxa. Patterns of niche evolution were congruent between clades, among feeding styles, and across taxonomic levels.