Warren D. Allmon

Diversity of Atlantic Coastal Plain Mollusks Since the Pliocene

About 70 percent of tropical western Atlantic mollusk species have become extinct since the Pliocene, which has led to perceptions of a corresponding decline in diversity. However, a compilation of gastropod species from Plio-Pleistocene faunas of the United States Atlantic coastal plain and from Recent western Atlantic faunas indicates that regional diversity has not changed since the Pliocene. Gastropod diversity in the Pliocene Pinecrest Beds in Florida approximates that seen today on either coast of Florida. Gastropod diversity is not demonstrably different in the Recent tropical western Atlantic than in the Recent tropical eastern Pacific. High extinction rates must have been balanced by high origination rates.

Nutrients, temperature, disturbance, and evolution: a model for the late Cenozoic marine record of the western Atlantic

Abstract Major changes in the marine biota of the western Atlantic region occurred over the last five million years, but the causes of these changes, and especially the relative roles of changes in temperature and nutrients in affecting them, have been controversial. The resolution of this issue has implications beyond this particular time and region because they include two environmental perturbations (the formation of the Central American Isthmus and the initiation of northern hemisphere glaciation) of global significance. Analysis of the western Atlantic late Neogene may also offer insights of broader significance into how environmental disturbance affects evolution, especially the interaction between the processes of extinction and speciation. Review of available data on paleoenvironments and biotas (including mollusks, corals, foraminifera, ostracodes, marine mammals, sea birds, and sea grasses) within the context of an explicit theoretical framework for the process of allopatric speciation allows construction of a model for evolution in the region during this time. The framework breaks speciation down into formation, persistence, and differentiation of isolated populations. The model for the western Atlantic proposes that many of the environmental and changes observed in the record of this region over the past five million years can be connected via this framework, especially around the role of environmental disturbance; disturbance connects extinction and speciation because the event of speciation may lie on a continuum of disturbance between population fluctuation and extinction. Although changes in temperature (and perhaps other factors) may have had significant effects, it was change in nutrient conditions — which most likely created conditions of habitat disturbance conducive to both enhanced speciation and extinction — that played the dominant role in causing the observed patterns of origination and extinction in the Plio-Pleistocene of the western Atlantic.

Levels of selection and macroevolutionary patterns in the turritellid gastropods

This analysis examines the evolution of the greater diversity of species with non-plank- tonic larval types relative to species with planktonic larval types in the turritellid gastropods. This sort of trend has been documented in both the fossil and recent biota of several gastropod families. Two mechanisms for generating diversity gradients in larval types have been proposed in the literature. The first, species selection, focuses on the population biology of larval types. The second proposes that factors in development that are mediated by organismal adaptation are responsible. Turritellids have been cited as a classic example of species selection. In order to examine the relevance of these two proposed mechanisms, a phylogenetic analysis of the turritellids using molecular sequence data was performed to determine the evolution of larval types in this clade. The resultant phylogeny suggests that species selection is not the only process driving the trend toward increasing numbers of non-planktonic species through time. Developmental processes, apart from those in- volving organismal adaptation (except in the trivial sense), are implicated as playing a role in this trend. In particular, these processes may involve changes in the timing of germ-line sequestration in organisms. Germ-line sequestration governs how accessible organisms are to heritable variation during ontogeny. Embryological evidence from gastropods suggests that non-planktonic species have early germ-line sequestration relative to planktonic species, making them more resistant to developmental change. Thus, non-planktonic lineages will only rarely revert to a planktonic larval mode.

Late Neogene Oceanographic Change along Florida's West Coast: Evidence and Mechanisms

Evidence from vertebrate and invertebrate fossil assemblages and isotopic analyses supports the hypothesis that during the Pliocene biological productivity in the eastern Gulf of Mexico was considerably higher than during the Pleistocene and Recent. Late Pliocene faunal changes in the eastern Gulf, Western Atlantic, and possibly elsewhere may have resulted, at least in part, from this shift in productivity conditions. Even if marine temperatures declined, paleontological and isotopic data appear to require a change in productivity in the Late Pliocene. This putative productivity decline may have been caused by some combination of causes at three geographic scales: (1) globally-marine productivity may have fallen due to changes in continental weathering; (2) regionally-North Atlantic productivity may have fallen as a result of initiation of North Atlantic Deep Water formation (possibly a consequence of formation of the Central American Isthmus, CAI) and resulting net transfer of nutrients to the Pacific; (3) locally-productivity may have fallen only in the eastern Gulf, due to circulation changes assisted with the formation of the CAI, and an accompanying decline in upwelling. The relative importance of processes at these three geographic scales remains unclear. The probable role of the formation of the CAI in two of the three, however, points to the importance of further investigation of the paleoceanographic consequences of this event for Late Cenozoic biological communities of the region.

Integrated taphonomy of an avian death assemblage in marine sediments from the late Pliocene of Florida

We integrate taphonomic data on vertebrate and invertebrate paleontology, micropaleontology and palynology to explain the formation of a late Pliocene death assemblage of marine birds and fish in the “Pinecrest Sand”, Gulf Coastal Florida. Stereonet plots of orientation data on over 1500 cormorant (Phalacrocoracidae: Phalacrocorax) bones indicate that this fossil assemblage formed first from gradual accumulation of bone, shell and sediments on a barrier island beach, and second by rapid sedimentation in a quiet, back-beach setting associated with multiple episodes of breaching of the barrier. This latter event resulted in the preservation of 137 partial and complete cormorant skeletons and thousands of isolated bones that show a high angle of dip and a preferred orientation to the northeast. Invertebrate fossils exhibit taphonomic signatures characteristic of high-energy reworking with a large percentage of abraded shell fragments similar to beach deposits. Moreover, these data indicate that more than a single depositional episode caused the formation of the deposits referred to as the bird layers. Palynological evidence supports this conclusion. The pollen recovered from the deposits is highly abraded and broken and does not represent an in situ vegetational environment as compared to other deposits of this age in Florida and Georgia. The large number of cormorant and other seabird and fish remains in the bird layers appears to have been caused by a series of toxic red tides that occurred on the Gulf Coast of Florida. These events today cause die offs of large flocks of cormorants and bottom-dwelling fish similar to those recovered from the site. Palynological analysis of the sediments revealed abundant cysts of a dinoflagellate species known to produce toxic red tides. The most likely cause of the death of the cormorants and other vertebrates at this site is a toxic bloom of a variety of Pyrodinium bahamense, the thecate form of the dinoflagellate cyst Polysphaeridium zoharyi, which has a stratigraphic range from the lower Eocene to the Holocene.

The Generification of the Fossil Record

Abstract Many modern paleobiological analyses are conducted at the generic level, a practice predicated on the validity of genera as meaningful proxies for species. Uncritical application of genera in such analyses, however, has led—perhaps inadvertently—to the unjustified reification of genera in an evolutionary context. While the utility of genera as proxies for species in evolutionary studies should be evaluated as an empirical issue, in practice it is increasingly assumed (rather than demonstrated) that genera are suitable proxies for species. This is problematic on both ontological and epistemological grounds. Genera are arbitrarily circumscribed, non-equivalent, often paraphyletic, and sometimes polyphyletic collections of species. They are useful tools for communication but have no theoretical or biological reality of their own and, whether monophyletic or not, cannot themselves operate in the evolutionary process. Attributes considered important for understanding macroevolution—e.g., geographic ranges, niche breadths, and taxon durations—are frequently variable among species within genera and will be inflated at the generic level, especially in species-rich genera. Consequently, the meaning(s) of results attained at the generic level may not “trickle down” in any obvious way that elucidates our understanding of evolution at the species level. Ideally, then, evolutionary studies that are actually about species should be pursued using species-level data rather than proxy data tabulated using genera. Where genera are used, greater critical attention should be focused on the degree to which attributes tabulated at the generic level reflect biological properties and processes at the species level.

Seafood through time revisited: the Phanerozoic increase in marine trophic resources and its macroevolutionary consequences

Abstract We review and synthesize multiple biotic and abiotic proxies for marine nutrient and food availability, primary productivity, and food quality (stoichiometry) and propose what their relationships may have been to macroevolutionary processes, especially speciation. This review confirms earlier suggestions that there has been an overall increase in marine primary productivity over the Phanerozoic, but indicates that the increase has been irregular and that present levels may not be the peak. We integrate these indicators into a new estimate of relative primary productivity in the global ocean through the Phanerozoic. We then combine multiple, frequently conflicting ecological-evolutionary hypotheses into a general model for how primary production may affect speciation over geological time scales. This model, an elaboration and extension of the “speciation cycle” previously proposed by Grant and Grant, attempts to explain why an increase in food supply sometimes is associated with decreased diversity, and at other times with increased diversification. We propose some simple tests for the application of this model to the fossil record.

Natural History of Turritelline Gastropods (Cerithiodea: Turritellidae): A Status Report

ABSTRACT A new review of available information, published and unpublished, on the biology and paleobiology of turhtelline gastropods expands our knowledge of this important group, which is among the most abundant and widespread marine gastropod clades of the past 130 million years. It also highlights many areas in which additional research is needed. Living turhtellines are mainly sessile, semi-infaunal suspension feeders, in shallow waters of full-marine salinity and temperatures below 20°C, but they can occasionally be more active and diverse in their habits, crawling on the surface and thriving in a variety of depths, salinities, and temperatures. They are eaten by a surprising diversity of predators, and infected by numerous parasites, especially trematodes, but little is known about interactions with predators, parasites, competitors, or commensals in nature. Their reproductive patterns (broadcast spawning and spermatophores) appear to depend on high abundance, which may help explain why they are the dominant species in many marine communities. The larvae of most species float (or swim) and feed in the plankton for no more than 2 weeks, but nothing is known about settlement. Several fossils and at least one modern species are brooders. Spermatozoa are dimorphic, and frequently paired and/or multiflagellate. Soft anatomy is known for only a few species, and ranges of variation remain largely undocumented. Radulae in particular have been strangely neglected, and this paper presents only the second published SEM image of a turritelline radula. Shell growth rates are variable but can be relatively rapid. Most species appear to live less than three years. Little is known about shell function or development. Although there is strong evidence for a positive correlation of abundance with available nutrients and/or primary productivity, this has not been examined quantitatively or experimentally. Gathering such natural history information on marine gastropod groups should be as high a priority as systematic and phylogenetic analyses. Information scattered in the literature is now much more available via internet search tools, and this approach should be used for other groups.

Morphological Variation in Turritellid Gastropods from the Pleistocene to Recent of Chile: Association with Upwelling Intensity

Abstract Environmental change, such as variation in upwelling intensity and consequent variation in marine primary productivity, may have profound effects on organisms. In the fossil record, morphological variation within lineages may be explained by environmental changes, although it is often difficult to establish a causal relationship between morphological and environmental change. Size and shape change were quantified in suspension-feeding turritellid gastropods from the Pleistocene and Recent of Chile, wherein both temporal and geographic patterns of variation were found. Upwelling and productivity conditions were reconstructed from a variety of published sources, including information on diatom, foraminiferan, and coccolithophore diversity and abundance, organic carbon burial, percent carbonate, and phosphorite deposition. In addition, carbon and oxygen stable isotope ratios from turritellid shell material were analyzed in order to determine temperature and productivity conditions. Comparison of results from the morphological analysis to upwelling/productivity reconstructions indicates that wider whorl shape and larger shell size are associated with more intense upwelling. Increases in the amount of shell material secreted may be explained by increased availability of food associated with stronger upwelling. This interpretation is consistent with observations of increased shell growth recorded for modern, non-turritellid mollusks. This type of analysis may help to establish causation for certain kinds of morphological evolution observed in the fossil record of turritellids and other suspension-feeding gastropods.

Cretaceous Marine Nutrients, Greenhouse Carbonates, and the Abundance of Turritelline Gastropods

Modern marine carbonate sediments accumulate where carbonate‐producing organisms are abundant and siliciclastic input is low. Such accumulations occur today in two main environments and may be characterized as warm, low‐nutrient (WLN; also known as tropical or chlorozoan) carbonates or cool, high‐nutrient (CHN; also known as cool‐water, temperate, nontropical, foramol, or heterozoan) carbonates. Few carbonates form today in warm, high‐nutrient (WHN) conditions because such environments are very rare. Modern WLN environments occur mainly in low latitudes, where nutrients from upwelling or terrestrial runoff are low. Modern CHN environments occur mainly in middle to high latitudes, where nutrients, mostly from upwelling, are abundant. However, WHN depositional environments may have been widespread during the Cretaceous and Paleogene. During these times, upwelled nutrient‐rich waters would, in general, have been warmer than they are today. Nutrients in these waters supported diverse carbonate‐producing biotas that are not equivalent to either modern chlorozoan or heterozoan biotas. These conclusions derive from analysis of a growing data set of more than 40 turritelline gastropod‐dominated fossil assemblages (TDAs), a term herein redefined, ranging in age from Lower Cretaceous to Pleistocene. Today TDAs occur mainly in CHN conditions. They were widespread in both carbonate and siliciclastic facies in the Cretaceous and Paleogene but (with only a single known exception in the Pliocene of Chile) occur in the Neogene only in siliciclastic sediments. This change in environmental distribution can be explained by positing changes in (1) modal environmental preferences of turritellines, from WHN to CHN, and (2) carbonate depositional environments, from \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape