Key morphological traits reveal changes in functional morphospace occupation of reef fish assemblages over time. We used measurements of key functional attributes (i.e., lower jaw length and orbit diameter) of 208 fossil fish species from five geological periods to create bivariate plots of functional morphological traits through time. These plots were used to examine possible function and ecological characteristics of fossil reef fish assemblages throughout the Mesozoic and Cenozoic. A previously unknown trend of increasing orbit diameter over time became apparent. The Teleostei are the principal drivers of this change. The Eocene appears to mark a dramatic increase in two previously rare feeding modes in fishes: nocturnal feeding and high-precision benthic feeding. Interestingly, members of the Pycnodontiformes had relatively large eyes since the Triassic and appear to be the ecological precursors of their later teleost counterparts and may have been among the earliest nocturnal feeding fishes. Our results highlight potential changes in the roles of fishes on coral reefs through time.

The Late Devonian extinction event was not geologically “instantaneous,” in that extinctions during the epoch are not concentrated into a single sharp pulse at the end of the Frasnian. Extinction rates are elevated for a period of at least 2 to 4 m.y. during the middle and late phases of the Frasnian, with maximum rates occurring generally 2 m.y. before the terminal Frasnian. Neither was the Late Devonian biotic crisis a “gradual” event. In the analysis of the evolution of ecosystems, it is misleading to consider the pattern of extinction rates alone. Frasnian marine ecosystems flourished during the same time interval characterized by elevated extinction rates because origination rates of new species are higher, per time interval, than corresponding extinction rates. This pattern of relative origination/extinction rates abruptly reversed during the latest Frasnian—precipitating a rapid loss of species diversity. Within limits of current stratigraphic correlation, the ecosystem collapse appears to have occurred simultaneously in such widespread geographic regions as New York State (U.S.A.) and the southern Urals (U.S.S.R.).

In viewing the Late Devonian event from an ecological perspective, the most important question is not “What triggered the elevated extinction rates?”, but rather “What was the inhibiting factor that caused the cessation of new species originations?”

Extinctions at the top of the Sunwaptan Stage (=“Ptychaspid Biomere”) near the Cambrian-Ordovician boundary eliminated about half of North American trilobite families. The families that extend from the shelf into the upper slope show significantly higher survival than those confined to the shelf. Biofacies and lithofacies distribution patterns indicate that the extinctions cannot be attributed to a shelfwide physical environmental perturbation, such as a fall in water temperature or the spread of anoxic waters. We develop a simple biogeographic model which suggests that diversity of a faunal province is influenced profoundly by changes in the number of component biofacies. This model is tested with an analysis of biofacies distribution patterns across the upper boundary of the Sunwaptan Stage. The extinctions correspond closely to lithofacies shifts in the outer shelf that indicate the initiation of major paleogeographic changes, possibly in response to a sea-level rise. The effects of these changes cascade across the entire shelf by the shoreward migration of off-shelf and shelf-margin taxa. Biofacies become reduced in number through telescoping and their environmental ranges expand during the extinction interval, suggesting an increase in the proportion of eurytopic taxa. Selective survival of wide-ranging eurytopes may have influenced the dynamics of faunal replacement by lowering speciation rates of shelf taxa. Consequently, the proportion of shelf endemics will decline and biofacies will be dominated by immigrant taxa. There are sufficient similarities in extinction patterns across the upper boundary of the Sunwaptan Stage and those at other Upper Cambrian stage boundaries to suggest that the biogeographic model developed here may have broader application.

This paper presents a method for constraining the age of a clade with the ages of the earliest fossil specimens in that clade's outgroups. Given a sufficiently deep, robust, well-resolved, and stratigraphically consistent cladogram, this method can yield useful age constraints even in the absence of specific information about the fossil preservation and recovery rates of individual taxa. The algorithm is applied to simulated data sets to demonstrate that this method can yield robust constraints of clade ages if there are sufficient fossil outgroups available and if there is a finite chance that additional outgroups may be discovered in the future. Finally, the technique is applied to actual fossil data to explore the origin of modern placental mammals. Using data from recently published cladograms, this method indicates that if all Mesozoic eutherians are regarded as outgroups of Placentalia, then the last common ancestor of modern placental mammals and their Cenozoic allies lived between 65 and 88–98 million years ago, depending on the assumed cladogram and the number of outgroups included in the analysis.

Cave bears, an extinct subgenus (Spelearctos) of Ursus, were versatile enough to inhabit large areas of the northern hemisphere during the middle and late Pleistocene, yet they had evolved a specialized dentition that emphasized grinding functions, implying a heavy dietary reliance on tough, fibrous foods (i.e., plants). Isotope studies have yielded conflicting results on cave bear diet, however, often without consideration of the provenance of the samples or the possible contradictions that taphonomic and morphologic evidence might pose to dietary interpretations. It is likely that cave bear habits varied somewhat in response to environmental circumstance, and the limits on their abilities to do so remain unknown. If the larger goal of paleontological inquiry is to reconstruct the adaptations of cave bear species, then variation and commonalities among populations must be tracked closely, and the disparate lines of evidence currently available examined together on a case by case basis. Clearly, no single analytical technique can achieve this. By way of example we present the results of a cross-disciplinary collaboration that combines osteometric, isotopic, and taphonomic approaches to studying the paleoecology of a bear assemblage from Yarimburgaz Cave in northwest Turkey. Reference information on the linkages between diet, hibernation, and population structure in modern bears provides test implications for the investigation. Osteometric techniques demonstrate the presence of two coextant middle Pleistocene bear species in the sample–Ursus (Spelearctos) deningeri, a form of cave bear, and U. arctos or brown bear–the former abundant in the sample, the latter rare. An attritional mortality pattern for the bears and the condition of their bones show that most or all of the animals died in the cave from nonviolent causes in the context of hibernation. The study also elucidates several characteristics of the cave bear population in this region. Osteometric techniques show that the adult sex ratio of the cave bears is only slightly skewed toward females. This pattern lies near one extreme of the full range of possible outcomes in modern bear species and can only reflect a strong dietary dependence on seasonally available plants and invertebrates, showing that hibernation was a crucial overwintering strategy for both sexes; the results specifically contradict the possibility of regular, heavy emphasis on large game (hunted or scavenged) as a winter food source. The nature of wear and breakage to the adult cave bear teeth indicates that food frequently was obtained from cryptic sources, requiring digging and prying, and that extensive mastication was necessary, leading to complete obliteration of some cheek tooth crowns in old individuals. The patterns of tooth damage during life corroborate the dietary implications of the adult sex ratio and also argue for a diet rich in tough, abrasive materials such as nuts, tubers, and associated grit. The carbon and oxygen isotopic compositions of cave and brown bear tooth enamel from the site are virtually identical, and there is no evidence of a strong marine signal in either species, despite the cave's proximity to a modern estuary of the Sea of Marmara; nitrogen isotope ratios could not be examined because of poor protein preservation. The isotope results suggest that both bear species were highly omnivorous in the region during the middle Pleistocene and obtained nearly all of their food from terrestrial and fresh-water habitats. Bone pathologies, usually originating from trauma, occur in some of the adult bears, testifying to long lifespans of some individuals in this fossil population. The Yarimburgaz cave bears also exhibit great size dimorphism between the sexes, based on weight-bearing carpal bone dimensions, with adult males attaining roughly twice the body mass of adult females.

Questions related to dinosaur behavior can be difficult to answer conclusively by using morphological studies alone. As a complement to these approaches, carbon and oxygen isotope ratios of tooth enamel can provide insight into habitat and dietary preferences of herbivorous dinosaurs. This approach is based on the isotopic variability in plant material and in surface waters of the past, which is in turn reflected by carbon and oxygen isotope ratios of animals that ingested the organic matter or drank the water. Thus, it has the potential to identify and characterize dietary and habitat preferences for coexisting taxa.

In this study, stable isotope ratios from coexisting hadrosaurian and ceratopsian dinosaurs of the Hell Creek Formation of North Dakota are compared for four different stratigraphic levels. Isotopic offsets between tooth enamel and tooth dentine, as well as taxonomic differences in means and in patterns of isotopic data among taxa, indicate that primary paleoecological information is preserved. The existence of taxonomic offsets also provides the first direct evidence for dietary niche partitioning among these herbivorous dinosaur taxa. Of particular interest is the observation that the nature of this partitioning changes over time: for some localities ceratopsian dinosaurs have higher carbon and oxygen isotope ratios than hadrosaurs, indicating a preference for plants living in open settings near the coast, whereas for other localities isotope ratios are lower, indicating a preference for plants in the understory of forests. In most cases the isotope ratios among hadrosaurs are similar and are interpreted to represent a dietary preference for plants of the forest canopy. The inferred differences in ceratopsian behavior are suggested to represent a change in vegetation cover and hence habitat availability in response to sea level change or to the position of river distributaries. Given our current lack of taxonomic resolution, it is not possible to determine if dietary and habitat preferences inferred from stable isotope data are associated with single, or multiple, species of hadrosaurian/ceratopsian dinosaurs.

Many questions have emerged recently regarding the importance and methodology of analysis of adaptation. Divergent views reflect both problems of definition and more substantive issues of interpretation. Defining the state of adaptation in terms of its contribution to current fitness, rather than origin by natural selection, is essential if natural selection is to be considered an explanation of adaptation. The context dependency and relativity of fitness apply also to adaptation. Design criteria are essential components of adaptation, but only to the extent that they are subsumed as elements of the causal interactions determining relative reproductive potential. The local, relational, contingent character of adaptation supports only limited reference to improvement. Most long-term patterns of change can be better described as diffusion within a structured design-space than as progressive improvement of design. The analysis of adaptation is part of a broader inquiry into the processes and constraints that control form and the history of changing form. It offers one perspective on how organisms operate on ecological time scales and how their configurations might be maintained or transformed over evolutionary time. Hypotheses concerning adaptation are sometimes tested by reference to predictions concerning the central tendency or trend of some aspect of an anatomical-behavioral system. These can be interpreted with minimal reference to assumptions of optimality if the analysis is viewed in terms of Bayesian inference. However, an alternative and frequently preferable approach to testing relies on limit-oriented predictions. Analysis of adaptation can be visualized as inferring the pattern and nature of interactions comprising the causal plexus that determines fitness. A comprehensive understanding of form and form-change requires that this be integrated with the perspective offered by studies of development, genetics, phylogenetic history, and external perturbations acting on the system.

A record of late Paleozoic foraminiferal diversity, origination and extinction frequencies, and provincialism at million-year temporal resolution and species-level taxonomic resolution has been achieved by analyzing composite standard databases. Foraminiferal species diversity increased throughout Mississippian and Pennsylvanian time leading up to its peak at the Pennsylvanian/Permian boundary. Foraminifers then experienced a steep decline in diversity during the Early Permian. Frequencies of origination and extinction broadly tracked changes in global diversity. From Late Mississippian time onward, patterns in total foraminiferal diversity were dominated by fusulinoideans. There is no clear relationship between rates of foraminiferal evolution and the alternating glacial and nonglacial intervals that characterized the late Paleozoic ice age. Rather, high rates of origination and extinction might reflect instability of neritic environments as a consequence of high-frequency, high-amplitude base-level fluctuations (cyclothemic deposition). Further, the advent of algal symbiosis in fusulinoideans was a physiologic innovation that promoted diversification as these symbiont-bearing taxa experimented with morphologic adaptations for partitioning the low-nutrient environments to which they were specialized. Growth to large size and delayed maturation in fusulinoideans might have been enabled by the late Paleozoic hyperoxic atmosphere and the widespread development of oligotrophic, carbonate platform and shelf environments. The late Paleozoic history of foraminiferal diversification was influenced also by closure of the Rheic Ocean beginning in Late Mississippian time. Foraminiferal associations on opposite sides of Pangea exhibited relatively high similarity prior to the closure, but then similarity decreased steadily after destruction of the subequatorial marine corridor. Arctic-Eurasian and North American associations were nearly isolated from one another throughout the main burst of fusulinoidean diversification, so that parallel lineages developed independently in the two regions, resulting in many instances of convergence.

Convincing hypotheses of the origin of major invertebrate groups are difficult to make in the absence of phylogenetic analyses. In spite of this, several scenarios exist for the origin of the unusual echinoid order Clypeasteroida. I expand upon the most probable of these models by performing a phylogenetic analysis on three clypeasteroid suborders, the enigmatic fossil genus Togocyamus, and the extinct Oligopygoida. This analysis shows that the oligopygoids are the sister group of the Clypeasteroida plus Togocyamus. The latter is here considered a plesion (extinct sister group) to the crown group Clypeasteroida. Within that order, the suborder Clypeasterina is the sister group to the Laganina plus Scutellina. A new classification of all these taxa is presented. The phylogeny is based on 47 characters and incorporates data on external appendages, Aristotle's lantern anatomy, and test structure of irregular echinoids, as well as new information on the morphology of Togocyamus. The earliest clypeasteroids had a lantern similar to that of adult oligopygoids, which in turn inherited their lantern from a cassiduloid-like ancestor that retained the lantern into adulthood. This lantern is absent in adult cassiduloids. Subsequent changes, including modification of the lantern into a crushing mill, extreme flattening of the test, and proliferation of food-gathering tube feet have allowed clypeasteroids to become epifaunal inhabitants of environments characterized by fine, shifting substrates, a habitat previously inaccessible to most other irregular echinoids.

Models of “ideal” orthoconic shells having simple concave septa with minimal weight and maximal strength and analysis of 72 species of fossil orthocones and cyrtocones yield important insights into the physical principles underlying cephalopod shell design. The ideal septum is a spherical cap weighing only 77% of a hemispherical septum of equal strength. The septa of most longicones approximate this ideal shape while those of brevicones are less curved, probably owing to buoyancy problems. Increase in septal strength leads to weight increase unless the shell becomes more logiconic or septal spacing increases or both. However, increased spacing requires more cameral liquid for septum formation, thus reducing buoyancy. In ideal longicones, septal spacing resembles the cone radius for thick, strong septa but declines to half of the cone radius for thin, weak septa. In ideal intermediates and brevicones, spacings are respectively reduced by factors of about 2 and 4, with similar additional dependence on septal thickness. Most real septa resemble these ideal models.

The relative length of the body chamber to the phragmocone varies greatly between about 0.2 and 1.5, depending mainly on the wall thickness and to a lesser degree on the septal thickness, apical angle and body density. Removal of cameral liquid in the adult must be compensated for by additional growth to retain neutral buoyancy. The conditions for neutral equilibrium calculated for longicones with different “counterweights” indicate that: (1) cameral liquid only is least feasible; (2) half-and-half calcium carbonate and liquid results in one-third length and one-quarter volume reduction of the body chamber; (3) with calcium carbonate only, body chamber reduction is minimal. Real ‘counterweights’ appear to be intermediate between (2) and (3), providing the animal with horizontal stability, which is missing in (3). Most uncalcified siphuncles reduce the body chamber only slightly although they improve horizontal stability. If the wall attains full thickness only at the apical end of body chamber, the liquid-only ‘counterweight’ becomes feasible.

The published fossil record has significant bias in favor of common and biostratigraphically important taxa when compared with data obtained from a thorough examination of several hundred collections from the Western Interior of North America. Overall species diversity is underestimated by a factor of 3 to 4, and bivalve and gastropod diversity by a factor of 5. The proportion of bivalves increased from 40 to 56% of the fauna, and the proportion of ammonites decreased from 28 to 18%. Thirteen published reports listed 65 species from 203 reported occurrences. Data from all sources showed 170 species for 1050 occurrences. By using abundance data and assuming a log-normal distribution, as many as 200 fossilizable mollusc species may have inhabited the Western Interior during the uppermost biozone of the Cenomanian. The importance of this study is that it quantifies the bias in the published fossil record relative to the potential fossil record for an unusually well studied interval of geologic time. The bias would be greater for less well studied strata.

The view that interspecific competition with balanoid barnacles has caused the evolutionary decline of chthamaloids (Stanley and Newman 1980), is challenged on 3 grounds. First, as a phyletic stock, chthamaloid size, as indicated by the mean adult basal diameter of extant taxa, has been decreasing, probably since the end of the Cretaceous. Small organisms will lose in interspecific competition to larger ones. However small body size reduces the susceptibility to predation and various forms of disturbance, and therefore allows chthamaloids to inhabit intertidal areas denied to balanoids. Second, although many species of Chthamalus occupy a high intertidal “refuge,” there is little evidence that competition is the primary ecological force maintaining them there. Predation is the most significant influence in tropical areas. In many temperate zone communities characterized by intense invertebrate predation or physical disturbance, chthamaloids achieve their maximum density lower in the intertidal than do balanoids or coronuloids. Last, the fossil evidence supporting a decline in chthamaloids is missing. The difficulty of seeking a single mechanism to explain global, multispecific phenomena is discussed.

The concept of coordinated stasis, manifest as a pattern of long intervals of concurrent taxonomic and ecologic persistence separated by comparatively abrupt periods of biotic change, has been challenged in recent studies that claim a lack of prolonged persistence of taxa and associations. A key problem has been the difficulty of distinguishing faunal change owing to localized, short-term environmental fluctuation or patchiness from that indicating regionally pervasive, long-term evolutionary or ecological change. Here, we use an extensive database from the Middle Devonian Hamilton Group of the Appalachian Basin to test for taxonomic and ecologic persistence within this ecological-evolutionary subunit, a succession of purported relative stability. Replicate samples collected from many localities and stratigraphic horizons over a wide geographic area allow us to address the effects of small-scale environmental variation and localized faunal patchiness while exploring basin-scale variation in faunal composition within and between the formations of the Hamilton Group.

Observed stratigraphic distributions of fossils are consistent with a scenario in which all taxa are present from bottom to top of the Hamilton Group, and absences result only from sampling failure. Although small-scale variation in faunal composition indeed does occur, there is no more variation among formations than occurs within them. Assemblages from different formations, whether they are defined by taxonomic or ecologic composition, are statistically indistinguishable according to several independent metrics, including ANOSIM and a maximum likelihood estimation that evaluates stratigraphic turnover using Bayesian “Information Criterion.” Simulated data sets indicate that test results are most consistent with species-level extinction of 2.6% per Myr within the Hamilton Group, far lower than the Givetian rate of 11.5% per Myr generic extinction derived from a global database. Such faunal persistence over the ~5.5 Myr encompassed by this unit is consistent with the pattern of coordinated stasis. Earlier studies showing greater amounts of temporal turnover in Hamilton Group faunas are likely influenced by their smaller geographic scale of analysis, suggesting that regional studies done elsewhere may yield similar results.

Although only a few studies have explicitly evaluated live-dead agreement of species and community responses to environmental and spatial gradients, paleoecological analyses implicitly assume that death assemblages capture these gradients accurately. We use nine data sets from modern, relatively undisturbed coastal study areas to evaluate how the response of living molluscan assemblages to environmental gradients (water depth and seafloor type; “environmental component” of a gradient) and geographic separation (“spatial component”) is captured by their death assemblages. We find that:

1. Living assemblages vary in composition either in response to environmental gradients alone (consistent with a species-sorting model) or in response to a combination of environmental and spatial gradients (mass-effect model). None of the living assemblages support the neutral model (or the patch-dynamic model), in which variation in species abundance is related to the spatial configuration of stations alone. These findings also support assumptions that mollusk species consistently differ in responses to environmental gradients, and suggest that in the absence of postmortem bias, environmental gradients might be accurately captured by variation in species composition among death assemblages. Death assemblages do in fact respond uniquely to environmental gradients, and show a stronger response when abundances are square-root transformed to downplay the impact of numerically abundant species and increase the effect of rare species.

2. Species' niche positions (position of maximum abundance) along bathymetric and sedimentary gradients in death assemblages show significantly positive rank correlations to species positions in living assemblages in seven of nine data sets (both square-root-transformed and presence-absence data).

3. The proportion of compositional variation explained by environmental gradients in death assemblages is similar to that of counterpart living assemblages. Death assemblages thus show the same ability to capture environmental gradients as do living assemblages. In some instances compositional dissimilarities in death assemblages show higher rank correlation with spatial distances than with environmental gradients, but spatial structure in community composition is mainly driven by spatially structured environmental gradients.

4. Death assemblages correctly identify the dominance of niche metacommunity models in mollusk communities, as revealed by counterpart living assemblages. This analysis of the environmental resolution of death assemblages thus supports fine-scale niche and paleoenvironmental analyses using molluscan fossil records. In spite of taphonomic processes and time-averaging effects that modify community composition, death assemblages largely capture the response of living communities to environmental gradients, partly because of redundancy in community structure that is inherently associated with multispecies assemblages. The molluscan data sets show some degree of redundancy as evidenced by the presence of at least two mutually exclusive subsets of species that replicate the community structure, and simple simulations show that between-sample relationships can be preserved and remain significant even when a large proportion of species is randomly removed from data sets.

The frequency of sublethal damage was examined in two ophiuroid species, Palaeocoma milleri and ?Ophioderma tenuibrachiata (Ophiodermatidae), from a shallow-water Jurassic “starfish bed.” None of 60 specimens was regenerating arms, a result that agrees with four previous studies of Paleozoic and early Mesozoic ophiuroids. By contrast, 66.1 percent of a living Ophioderma brevispinum population from Belize were regenerating one or more arms. For living populations of Ophiothrix oerstedi, the natural injury frequency was high at sites where the field mortality of tethered individuals was high. All predators that have been observed feeding on ophiuroids cause sublethal injuries, which would appear as regenerating arms in the fossil record. These results support the hypothesis that predation pressure on ancient ophiuroid populations was low and increased after the Jurassic.

Recent molecular phylogenetic and molecular clock data both suggest a pre-Mesozoic age for the divergence of the angiosperm lineage from other seed plants, greatly predating the confirmed fossil record of the angiosperm crown group. In addition, molecular phylogenetic studies have not supported the morphologically based conclusion that gnetophytes are the extant sister group to angiosperms. We examine these relationships and divergence ages by using a novel approach of examining the presence of oleanane. This includes the development of methods using zeolites to preferentially reduce hopanes that can co-elute with oleanane. The presence of this molecular fossil strongly correlates with angiosperm diversification; in its functionalized form, along with its triterpenoid precursors, it is found in many living angiosperms. Our data show that among non-angiosperm seed plants examined thus far, oleanane is found only in fossil Cretaceous Bennettitales and Permian Gigantopteridales, both of which share characteristics with angiosperms. Previous morphological phylogenetic results indicate Bennettitales could be a sister group to or member of the angiosperm stem lineage, and results of our preliminary phylogenetic analysis including the Gigantopteridales suggests the same. Our data, based on a new pyrolysis method to treat living species, support previous research indicating that oleanane and its precursors are absent in living gnetophytes. If oleanane originated once in seed plants then the angiosperm stem lineage would have diverged from other seed plant lineages by the late Paleozoic.

Developmental constraints presumably operate by influencing patterns of variability: when development causes some features to vary more than others and when the level of variability is correlated with evolutionary change, then development can be said to constrain evolution. This idea was tested by examining the relationship between tooth variation and three other factors: developmental processes, tooth function, and evolutionary change. Data came from two lineages of viverravid carnivorans (Viverravidae, Carnivora) from the Paleogene of North America. Variability in cusp position was significantly correlated with position in the developmental cascade, with the amount of intercusp growth (when growth is relatively greater in some cusps than others), and with amount of evolutionary change. This indicates that tooth development exerts a local constraint on phenotypic variability and on the evolutionary response to functional selection, but comparative data suggest that the developmental constraint itself may evolve. Intense directional or stabilizing selection may modify the developmental cascade so that the constraint is either removed or modified to permit new evolutionary patterns. Thus development does not constrain evolution in an absolute sense, but rather introduces modifiable patterns of covariance among crown features. Both development and function seem to play important, intertwined roles in coordinating evolutionary change in mammalian molars.

The end-Permian mass extinction is commonly portrayed not only as a massive biodiversity crisis but also as the time when marine benthic faunas changed from the Paleozoic Fauna, dominated by rhynchonelliform brachiopod taxa, to the Modern Fauna, dominated by gastropod and bivalve taxa. After the end-Permian mass extinction, scenarios involving the Mesozoic Marine Revolution portray a steady increase in numerical dominance by these benthic molluscs as largely due to the evolutionary effects of an “arms race.” We report here a new global paleoecological database from study of shell beds that shows a dramatic geologically sudden earliest Triassic takeover by bivalves as numerical dominants in level-bottom benthic marine communities, which continued through the Early Triassic. Three bivalve genera were responsible for this switch, none of which has any particular morphological features to distinguish it from many typical Paleozoic bivalve genera. The numerical success of these Early Triassic bivalves cannot be attributed to any of the well-known morphological evolutionary innovations of post-Paleozoic bivalves that characterize the Mesozoic Marine Revolution. Rather, their ability to mount this takeover most likely was due to the large extinction of rhynchonelliform brachiopods during the end-Permian mass extinction and aided by their environmental distribution and physiological characteristics that enabled them to thrive during periods of oceanic and atmospheric stress during the Permian/Triassic transition.

An extreme, dorsally hyperextended posture of the spine (opisthotonus), characterized by the skull and neck recurved over the back, and with strong extension of the tail, is observed in many well-preserved, articulated amniote skeletons (birds and other dinosaurs, pterosaurs, and at least placental mammals). Postmortem water transport may explain some cases of spinal curvature in fossil tetrapods, but we show how these can be distinguished from causes of the opisthotonic posture, which is a biotic syndrome. Traditional biotic explanations nearly all involve postmortem causes, and have included rigor mortis, desiccation, and contraction of tendons and ligaments. However, examination of the process of rigor mortis and experimental observations of drying and salinity in carcasses of extant animals show that these explanations of the “dead bird” (opisthotonic) posture account for few or no cases. Differential contraction of cervical ligaments after death also does not produce the opisthotonic posture. It is not postmortem contraction but perimortem muscle spasms resulting from various afflictions of the central nervous system that cause these extreme postures. That is, the opisthotonic posture is the result of “death throes,” not postmortem processes, and individuals so afflicted assumed the posture before death, not afterward. The clinical literature has long recognized that such afflicted individuals perish from asphyxiation, lack of nourishment or essential nutrients, environmental toxins, or viral infections, among other causes. Accepting the actual causes of the opisthotonic posture as perimortem and not postmortem provides insights into the causes of death of fossilized specimens, and also revises interpretations of paleoenvironmental conditions of many fossil deposits. The opisthotonic posture tells us more about the circumstances surrounding death than about what happened after death. Finally, the opisthotonic posture appears to have a phylogenetic signal: it is so far reported entirely in ornithodiran archosaurs (dinosaurs and pterosaurs) and in crown-group placentals, though the distribution in mammals may expand with further study. It seems important that the opisthotonic posture has been observed extensively only in clades of animals that are known or thought to have high basal metabolic rates: hypoxia and related diseases would be most likely to affect animals with high oxygen use rates.

The biogeography of differences in average shape of the modern planktonic foraminifer Globorotalia truncatulinoides (d'Orbigny) exhibits systematic relationships to changes in the ocean's surface environment. Comparison of these shape changes, as they exist today in the Southern Hemisphere, with fossil shapes preserved in a Late Pleistocene record from the South Atlantic Ocean, shows that the biogeography of G. truncatulinoides ecophenotypes has changed markedly through time. Beginning at least 700,000 yr ago and continuing up to the present, there has been a gradual but clear migration of certain morphotypes of G. truncatulinoides toward lower latitudes. The history of this migration bears no simple relationship to the cyclic climatic changes that characterize the Late Pleistocene. We conclude that either (1) phenotypic variants of Gr. truncatulinoides reflect some previously unmeasured, gradually changing aspect of Late Pleistocene oceans, or (2) we are witnessing a gradual evolution of the environment preferences of G. truncatulinoides.