The late Paleozoic ice age (LPIA) had a profound effect on the biota. Despite much research having been focused on paleotropical regions or global-scale analyses, regional ecological changes have seldom been studied in ice-proximal basins. Here, I study the compositional turnover and diversity structure across the main Carboniferous glacial event recorded in western Argentina and the subsequent nonglacial interval. Brachiopod and bivalve data from western Argentina suggest that the transition from glacial to nonglacial climates caused major compositional changes. Turnover, however, was not uniform across the bathymetric gradient, being higher in deep environments. Because extirpation was concentrated in brachiopods, but immigration was similar in both clades, the taxonomic structure of the region was significantly modified. Although regional hierarchical diversity structure and occupancy distributions remained stable, dissecting the analysis in brachiopods and bivalves underscores that both clades had different responses to climate change. Brachiopods, on the one hand, show stability in the diversity structure and a very slight decrease in occupancies of intermediate genera, while bivalves show an important rise in diversity, both at the environment and regional scale, and an increase in genera with intermediate occupancies. The bathymetric diversity gradient was also modified from hump shaped with maximum diversity in the deep subtidal to a linear gradient with maximum values toward the offshore. However, relative compositional differences within environments remained stable, with maximum values at intermediate depths both in glacial and nonglacial intervals. Moreover, local-scale coexistence between brachiopods and bivalves changed in the nonglacial interval, showing significant segregation, which indicates relevant modifications in community assembly dynamics. Results from western Argentina highlight the magnitude of regional-scale ecological changes during the LPIA in ice-proximal regions, suggesting that the waxing and waning of glaciers was able to cause regional taxonomic turnover and medium-scale ecological changes even during intervals of relative macroevolutionary quiescence.

The seemingly aberrant coiling of heteromorphic ammonoids suggests that they underwent more significant changes in hydrostatic properties throughout ontogeny than their planispiral counterparts. Such changes may have been responses to different selective pressures at different life stages. The hydrostatic properties of three species of Didymoceras (D. stevensoni, D. nebrascense, and D. cheyennense) were investigated by creating virtual 3D models at several stages during growth. These models were used to compute the conditions for neutral buoyancy, hydrostatic stability, orientation during life, and thrust angles (efficiency of directional movement). These properties suggest that Didymoceras and similar heteromorphs lived low-energy lifestyles with the ability to hover above the seafloor. The resultant static orientations yielded a downward-facing aperture in the hatchling and a horizontally facing aperture throughout most of the juvenile stage, before terminating in an upward direction at maturity. Relatively high hydrostatic stabilities would not have permitted the orientation of Didymoceras to be considerably modified with active locomotion. During the helical phase, Didymoceras would have been poorly suited for horizontal movement, yet equipped to pirouette about the vertical axis. Two stages throughout growth, however, would have enhanced lateral mobility: a juvenile stage just after the formation of the first bend in the shell and the terminal stage after completion of the U-shaped hook. These two more mobile phases in ontogeny may have improved juvenile dispersal potential and mate acquisition during adulthood, respectively. In general, life orientation and hydrostatic stability change more wildly for these aberrantly coiled ammonoids than their planispiral counterparts.

Biophysical modeling and morphologic data from the fossil record were used to investigate the functional significance of changes in thallus morphology during the early evolutionary history of dasycladalean algae (dasyclads), a clade of benthic marine macroalgae. Modeling results indicate that the addition of cylindrical appendages (laterals) to an upright main axis, a key morphological innovation in the evolution of dasyclad thallus form, can provide for large gains in light interception efficiency, near-maximum gains in this regard being achieved when the ratio of total lateral surface area to main axis surface area is 4 or greater. Among the 13 early Paleozoic study taxa, all but one was found to exceed this value. Modeling of surface area to volume ratios for early Paleozoic dasyclads indicates that laterals for these forms conveyed only modest gains in this regard and, therefore, likely played little role in improving nutrient uptake. For survivorship, it appears that increasing thallus complexity by developing laterals conveyed important benefits by imparting both compartmentalization and redundancy, thereby increasing the likelihood that lateral-bearing forms would survive attacks by early mesograzers. Trends and patterns in the fossil record support such a survival-enhancing role for laterals and are consistent with the initial evolution of these structures as an early manifestation of an evolutionary arms race between macroalgae and herbivores initiated near the Proterozoic/Phanerozoic boundary.

Paleobiological and paleoecological interpretations rely on constraining the temporal resolution of the fossil record. The taphonomic clock, that is, a correlation between the alteration of skeletal material and its age, is an approach for quantifying time-averaging scales. We test the taphonomic clock hypothesis for marine demersal and pelagic fish otoliths from a 10–40 m depth transect on the Mediterranean siliciclastic Israeli shelf by radiocarbon dating and taphonomic scoring. Otolith ages span the last ~8000 yr, with considerable variation in median and range along the transect. Severely altered otoliths, contrary to pristine otoliths, are likely to be older than 1000 yr. For pelagic fish otoliths, at 30 m depth, taphonomic degradation correlates positively with postmortem age. In contrast, no correlation occurs for demersal fishes at 10 and 30 m depth, mostly because of the paucity of very young pristine (<150 yr) otoliths, possibly due to a drop in production over the last few centuries. Contrary to molluscan and brachiopod shells, young otoliths at these depths are little affected and do not show a broad spectrum of taphonomic damage, because those that derive from predation are excreted in calcium- and phosphate-rich feces forming an insoluble crystallic matrix that increases their preservation potential. At 40 m depth, all dated otoliths are very young but rather damaged because of locally chemically aggressive sediments, thus showing no correlation between taphonomic grade and postmortem age. Our results show that local conditions and the target species population dynamics must be considered when testing the taphonomic clock hypothesis.

The Permian/Triassic mass extinction marks a permanent phylogenetic shift in the composition of the sessile benthos, from one largely dominated by articulate brachiopods to one dominated by mollusks. Widespread evidence of oceanic hypoxia and anoxia at this time provides a possible selective kill mechanism that could help explain the large taxonomic losses in brachiopods compared to the morphologically and ecologically similar bivalve molluscs. Our study compared the oxygen consumption of an articulate brachiopod, Terebratalia transversa, with that of two pteriomorph bivalves, Glycymeris septentrionalis and Mytilus trossulus, under normoxia and hypoxia, as well as their tolerance to anoxia, to gain insight into the relative metabolic characteristics of each group. We found no significant difference in the oxygen consumption of the three species when normalized to the same dry-tissue mass. However, when calculated for animals of the same external linear dimensions, bivalve oxygen consumption was two to three times greater than that of brachiopods. Our results also showed no significant decrease in the oxygen consumption of the three species until measured at a partial pressure of oxygen ∼10% of normoxic values. Finally, T. transversa and M. trossulus showed no significant difference in their tolerance to complete anoxia, but both showed a much lower tolerance than another bivalve, Acila castrensis. Findings from this study suggest that oxygen limitation is unlikely to account for the observed selective extinction of brachiopods during the Permian/Triassic mass extinction. Results may provide valuable information for assessing hypotheses put forth to explain why articulate brachiopods continue to remain a relatively minor group in marine environments.

Morphological gradients in graptoloid colonies are explained by the production and diffusion of a morphogen from the sicula distalwards. Size of the thecae is inversely related to the amount of morphogen present. The graduate decrease of morphogen is given by a set of algebraic equations, and its effect on zooid growth is described by a modified Michaelis-Menten relationship. Changes in size of thecae computed on the basis of these equations fit the changes observed in graptolite colonies. Sets of differential equations are also given to describe the rate of morphogen diffusion and some other processes possibly involved in the development of uniaxiate graptoloid colonies. The suggested basic model of these colonies can be completed by a number of additional assumptions; however, computer experiments reveal that such assumptions do not affect the main properties of the model, namely the appearance of graded series of thecae. New evidence for regeneration of the sicular portion of the broken rhabdosome is presented, providing arguments that morphogen was produced by the siculozooid as a single bolus of secretion. Some local exclusions from the regular size gradient are discussed, and tentative explanations are suggested.

Barnacles living along rocky shores provide the classic example of competitive dominance in the marine ecosystem: by means of firm attachment and rapid growth, balanoid barnacles commonly undercut and overgrow the genus Chthamalus, which is thereby restricted to the upper fringe of the intertidal zone, where balanoids are physiologically incapable of living. Today, after perhaps less than 50 Myr of evolution, balanoid barnacles are in the midst of rampant adaptive radiation, being represented by about 273 species, of which about half are free-living species of intertidal or shallow subtidal habitats. Chthamaloid barnacles, in contrast, are on the decline, having originated at least 70 Myr ago but today comprising only about 53 living species, approximately 40 of which occupy the uppermost intertidal. The remainder persist as localized, relict, and often disjunct populations. Through competitive exclusion, balanoid barnacles have apparently caused the ecological restriction and decline of the chthamaloids. The balanoids have an advanced feeding mechanism, but the most important adaptive breakthrough leading to their competitive success was probably the origin of a tubiferous wall structure, which affords rapid skeletal growth for the efficient monopolization of free space and for the destruction of chthamaloids.

If your undergraduate experience in stratigraphy was anything like mine, it was overwhelmingly dull. It was all about nomenclature and classification and endless lists of formation names. There were apparently no questions to be asked and the goal of stratigraphy was to devise boxes into which to place all types of geologic data. Notwithstanding the important work of correlation, the perception was that it was the sedimentologists who did the real science, science that focused on the processes of sediment accumulation and their controlling factors. Since then, stratigraphy has undergone a conceptual revolution, and it is necessary to examine how the field has changed and what those advances mean for paleobiology.

Drillholes made by naticid and muricid gastropods are frequently used in evolutionary and ecological studies because they provide direct, preservable evidence of predation. The muricid Ecphora is common in many Neogene Atlantic Coastal Plain assemblages in the United States, but is frequently ignored in studies of naticid predation. We used a combination of Pliocene fossil, modern beach, and experimentally derived samples to evaluate the hypothesis that Ecphora was an important source of drillholes in infaunal bivalve prey shared with naticids. We focused on the large, thick-shelled venerid, Mercenaria, which is commonly drilled by naticids today. Laboratory experiments, modern beach samples, and the published literature confirm that naticids preferentially drill near the umbo (significant clumping of holes), show a significant correlation between prey size and predator size (estimated by outer borehole diameter), and prefer Mercenaria <50 mm antero-posterior width when other prey are present. Fossil samples containing Ecphora (with or without other large muricids) show no drillhole site stereotypy (no significant clumping, greater variability in placement), no significant predator: prey size correlation, drilled prey shells larger than the largest modern naticids could produce in an experimental setting, and drillholes larger in diameter than those estimated for the largest Pliocene naticids, thus supporting our hypothesis. Substantial overlap in the placement of holes drilled by naticids and muricids, however, made identifying predators from drillhole position problematic. The lack of overlapping ranges of prey shell thickness between fossil and other samples precluded the use of drillhole morphology to establish predator identity (e.g., ratio of inner borehole diameter to outer borehole diameter, drillhole angle). Whereas the difficulty in determining predator identity from drillholes limits the types of analyses that can be reliably performed in mixed-predator assemblages, recognizing Ecphora as a prominent drilling predator creates the opportunity to investigate previously unrecognized questions.

A large sample of postembryonic specimens of Dalmanitina proaeva elfrida and D. socialis from the Upper Ordovician (Sandbian to Katian) Prague Basin allows for the first reasonably complete ontogenetic sequence of Dalmanitoidea (Phacopina). The material provides an abundance of morphological information, including well-preserved marginal spines in protaspides and meraspides, and hypostome external surfaces throughout. The development of D. proaeva elfrida is unusual due to variability in timing of the first trunk articulation. This broadens our developmental understanding of Phacopina, a diverse group of phacopid trilobites, and also allows us to study the evolution of their specializations in exoskeletal molting behavior. Adult phacopines, unlike most other trilobites, had fused facial sutures. This means that rather than molting through the sutural gape mode, characterized by opening of the facial sutures and separation of the librigenae, they disarticulated the entire cephalon in Salter’s mode of molting. For other phacopine clades (Phacopoidea) the transition to Salter’s mode occurs during the meraspid period or at the onset of holaspis, and its developmental timing is intraspecifically fixed. However, owing to the large sample size, we can see that facial suture fusion likely occurred later in Dalmanitina, usually during the holaspid period, and was intraspecifically variable with holaspides of varying sizes showing unfused sutures. Further, D. proaeva elfrida specimens showed an initial librigenal–rostral plate fusion event, where the librigenae began as separate entities but appear fused with the rostral plate as one structure (the “lower cephalic unit”) from M1, and are discarded as such during molting. Dalmanitoidea is considered to represent the first phacopine divergence, occurring earliest in the fossil record. This material therefore provides insight into how linked morphologies and behaviors evolved, potentially suggesting the timing of facial suture fusion in Phacopina moved earlier during development and became more intraspecifically fixed over geological time.

The rates and patterns of phyletic body size increase and decrease are discussed on the assumption that size change proceeds toward some limit due to selective advantage. Because the selection pressure must decrease as the average body size of an evolving population approaches the limit, some sigmoidal curve should be regarded as more appropriate for the model of size increase than an exponential curve as in the case of population growth. For phyletic size decrease, the predicted pattern is similar to a radiometric decay curve. The conventional unit of morphological rate, darwin, has been used on the assumption that the change is exponential, but actual lineages in the fossil record may represent only fractions of such sigmoidal curves. Some actual data on size increases in ceratopsian dinosaurs and Jurassic bivalves are examined, and it is concluded that sigmoidal size increase seems also empirically to be a more widespread pattern than exponential.

Investigation of the evolution of calcareous nannoplankton is hindered by the extremely small size (5-10 μm) of their fossils. We introduce new technology, a scanning electron microscope connected to an image analysis system (SEM-IAS), which allows this field to be explored as never before. This system enables a host of morphocharacters to be measured and included in studies of phylogeny and evolutionary dynamics. We have applied the SEM-IAS to study the evolution of Paleocene coccolith genera Cruciplacolithus, Chiasmolithus, and Sullivania. A variety of detailed measurements have been made on over 4000 coccoliths from Deep Sea Drilling Project Site 384 in the temperate North Atlantic and Ocean Drilling Program Site 690 in the Weddell Sea off Antarctica. Our results indicate no simple relationships between morphocharacters, the shapes of all three genera are both complex and highly variable. Although most morphocharacters possess little phylogenetic significance, the areas of different shield cycles show gradual divergence between Chiasmolithus and Sullivania through the Paleocene. Change of most other morphocharacters occurs at variable rates and reversals in trends are common. Minimal correlation exists between the trends and oscillatory shape changes observed at the two sites. We conclude that these trends and oscillations represent local, transitory ecophenotypic variation of the complex form. There is little stasis in the ten-million-year record studied.

Nonparametric randomization tests were used to determine whether fossil marine families classified in different higher taxa are significantly different in their durations, their rates of origination and extinction, and their proportion of originations. For each of these properties, significant differences among orders and classes were found that could not be explained by differences at other taxonomic ranks, but no such differences were found among phyla. Thus, orders and classes appear to possess an evolutionary coherence that phyla lack.

Persistence in the structure of ecological communities can be predicted both by deterministic and by stochastic theory. Evaluating ecological patterns against the neutral theory of biodiversity provides an appropriate methodology for differentiating between these alternatives. We traced the history of benthic foraminiferal communities from the Huon Peninsula, Papua New Guinea. From the well-preserved uplifted reef terrace at Bonah River we reconstructed the benthic foraminiferal communities during a 2200-year period (9000–6800 yr B.P.) of reef building during the Holocene transgressive sea-level rise. We found that the similarity of foraminiferal communities was consistently above 60%, even when comparing communities on either side of a massive volcanic eruption that smothered the existing reef system with ash. Similarly, species diversity and rank dominance were unchanged through time. However, similarity dropped dramatically in the final stages of reef growth, when accommodation space was reduced as sea-level rise slowed. We compared the community inertia index (CII) computed from the observed species abundances with that predicted from neutral theory. Despite the differences in foraminiferal community composition in the younger part of the reef sequence, we found an overall greater degree of community inertia with less variance in observed communities than was predicted from neutral theory, regardless of foraminiferal community size or species migration rate. Thus, persistent species assemblages could not be ascribed to neutral predictions. Ecological incumbency of established foraminiferal species likely prevented stochastic increases in both migrant and rare taxa at the Bonah River site. Regardless of the structuring mechanisms, our reconstruction of Holocene foraminiferal assemblages provides historical context for the management and potential restoration of degraded species assemblages.

Prominent extinction “events” have been recognized from statistical analyses of marine invertebrate genera represented in Mesozoic and Cenozoic assemblages, contrasting with relatively low “background” extinction intensities measured in terms of a “percentage extinction” index. On a logarithmic scale, the slope of the relationship between time and extinction intensity for background extinctions is shown to be parallel to the slope obtained for most extinction events, characterized by intensities 100.35 above prevailing background levels. Although extinction intensities are variable, this study suggests that the magnitude of the factor(s) primarily associated with most mass extinctions in a 260-m.y. period (N = 9) need not necessarily have been very different from one event to another, an exception being the mass extinction at the end of the Cretaceous.

Deep-sea benthic ostracod assemblages covering the last 2 Myr were investigated in Integrated Ocean Drilling Program Site U1426 (at 903 m water depth) in the southern Sea of Japan. Results show that (1) orbital-scale faunal variability has been influenced by eustatic sea-level fluctuations and oxygen variability and (2) secular-scale faunal transitions are likely associated with the mid-Brunhes event (MBE, ~0.43 Ma) and the onset of the Tsushima Warm Current (TWC, ~1.7 Ma). Krithe, Robertsonites, and Acanthocythereis are the three most abundant genera throughout the core, accounting for 78.5% of total specimens. Multiple-regression tree analysis indicated that the TWC, the MBE, and oxygen content are the significant controlling factors of ostracod dominance. Changes in assemblages exhibit decline and recovery patterns corresponding to orbital-scale cyclicity of sea-level changes. In the Sea of Japan marginal ocean setting, this cyclicity shows a close relationship with bottom-water oxygen variability since the onset of the TWC influx. The MBE amplified the influence of the TWC and oxygen variability to the deep-sea ecosystem through larger sea-level fluctuations. Acanthocythereis dunelmensis, a circumpolar species, dominates before the TWC onset. After the TWC onset and during the mid-Pleistocene transition (MPT, ~1.2–0.7 Ma) Krithe spp., known for their low-oxygen tolerance, substantially increase under moderate oxygen depletion. At the end of the MPT, Krithe dominance diminishes and is replaced by Robertsonites hanaii and Propontocypris spp. after the MBE. The post-MBE assemblage, characterized by R. hanaii, suggests a slightly warmer environment under the development of the TWC. In addition, the post-MBE high-amplitude climate system may have caused the increased abundance of active-swimming Propontocypris spp. due to their superior migration ability. Benthic ecosystems in marginal seas are sensitive and vulnerable to both short- and long-term climatic changes, and the MBE is suggested to be a global biotic event affecting benthic ecosystems substantially.

A hierarchical birth-death process is often used as a model for the diversification of species and higher taxa. We evaluate the validity of this null hypothesis by introducing a new measure: the distribution of species within higher taxa. The proportion of monotypic taxa and the number of species in the largest taxon generated by 1000 simulations are compared with extant mammalian data. The initial model is amended to include two sampling patterns, polyphyletic origins as well as ecological and genetic constraints on higher taxon origination rates. Simulated results are extremely variable. For genera, they are found to predict the true distribution of species quite well. For families, however, the null hypothesis is rejected in all its forms. Simulated distributions have both too many monotypic taxa and too large a dominant taxon. The latter finding stands in contrast to previous claims. Interestingly, although polyphyly and constraints slightly improve the fit of the simulations to the data, sampling does not. Finally, Smith and Patterson's dismissal of monotypic taxa is reviewed in light of this model. We argue that while some apparently monotypic taxa are sampling artifacts, this observation has no bearing on the true proportion of monotypic taxa.

Long-term persistence of patterns in geographic variation within species is an interesting and puzzling phenomenon. I present a well-defined natural experiment in the land snail Cerion agassizi from the islands of Great Bahama Bank. C. agassizi is the best-known fossil of the ca. 120,000 years BP dunes of New Providence, Cat and Eleuthera Islands; populations have survived on Cat and Eleuthera. During the Wisconsin glacial advance, all these islands joined together in an emergent bank. Presence of the same species on two islands at two times permits a test for both time signatures (does change occur in the same manner on both islands) and island signatures (do aspects of shell phenotypes remain constant on each island through time).

Factor and discriminant analyses establish morphological separations among fossil populations of the three islands. These differences occur along pathways specified by well-known covariance sets in the complex allometric ontogeny of Cerion. By these routes, small variations in the geometry of growth may be magnified to large differences in external appearance. I found a time signature, probably attributable to introgression of modern populations by Cerion glans on both Cat and Eleuthera. Despite the intermediate period of emergence and joining of all islands, I also found an island signature in the preservation through time, on both Cat and Eleuthera, of the differentia that separate fossil populations. The basic distinctions of the two islands, expressed as patterns of covariance in growth, have been stable for at least 120,000 years.