Paleecological data allow not only the study of trends along deep-time chronological transects but can also be used to reconstruct ecological gradients through time, which can help identify causal factors that may be strongly correlated in modern ecosystems. We have applied such an analysis to Bergmann’s rule, which posits a causal relationship between temperature and body size in mammals. Bergmann’s rule predicts that latitudinal gradients should exist during any interval of time, with larger taxa toward the poles and smaller taxa toward the equator. It also predicts that the strength of these gradients should vary with time, becoming weaker during warmer periods and stronger during colder conditions. We tested these predictions by reconstructing body-mass trends within canid and equid genera at different intervals of the Oligo-Miocene along the West Coast of North America. To allow for comparisons with modern taxa, body mass was reconstructed along the same transect for modern Canis and Odocoileus. Of the 17 fossil genera analyzed, only two showed the expected positive relationship with latitude, nor was there consistent evidence for a relationship between paleotemperature and body mass. Likewise, the strength of body-size gradients does not change predictably with climate through time. The evidence for clear gradients is ambiguous even in the modern genera analyzed. These results suggest that, counter to Bergmann’s rule, temperature alone is not a primary driver of body size and underscore the importance of regional-scale paleoecological analyses in identifying such drivers.

We investigate the association between geographic range and survival in Mesozoic marine animal genera. Previous work using data from the Paleobiology Database (paleobiodb.org) demonstrated greater survivorship overall among Phanerozoic genera that were widespread during their stage of first appearance, but this relationship did not hold during the Mesozoic. To explore this unexpected result, we consider geographic range in conjunction with temporal variation in survival and variation in survival among higher taxa. Because average range and average survival are negatively correlated among stages, for reasons that are still unclear, and because the data are heavily influenced by cephalopods, which include many wide-ranging and short-lived genera, the effect of geographic range on survival is obscured in the aggregate data. Thus, range is not a significant predictor of survival when data are analyzed in aggregate, but it does have a significant effect when variation in average range and average survival among stages and classes is taken into account. The best-fitting models combine range with both temporal and taxonomic heterogeneity as predictive factors. Moreover, when we take stage-to-stage variation into account, geographic range is an important predictor of survival within most classes. Cephalopod genera must be more widespread than genera in other classes for geographic range to significantly increase odds of survival, and factoring in survival heterogeneity of superfamilies further increases model fit, demonstrating a nested nature in the sensitivity of range and taxonomic aggregation.

Taxonomic status of several members of the family Naticidae is extremely vague because of its simple shell morphology. Conventional taxonomic classification schemes suggest that most of the morphological characters tend to be homoplastic and exhibit convergence. Such morphological convergence complicates naticid taxonomy and makes it difficult to understand the evolutionary history of this group; several unrelated taxa are often misidentified as naticids, thereby exaggerating the actual diversity of this group. Here, we employ a standard landmark-based approach to understand the pattern of morphological evolution of this family. Ordination methods such as principal components analysis and canonical variate analysis were used to create morphospaces, and disparity was quantified using variance and range. Our results reveal that when naticids are compared with their sister taxon, Ampullinidae, the two families show significant differences in their average shapes, despite their superficial resemblances. Among naticids, although the mean shapes of the individual subfamilies are different, overall, the family Naticidae has displayed extreme morphological conservatism from the Jurassic to the Holocene. Interestingly, this conservatism has been unaffected by taxonomic changes—neither the extinction of the subfamily Gyrodinae nor the appearance of the subfamily Sininae affected this morphological conservatism. Naticids have always shown strong ecological preference toward an infaunal mode of life and strict behavioral selectivity in handling and preying upon infaunal organisms, and this ecological and behavioral conservatism could have enabled them to diversify without undergoing a change in their basic Bauplan.

By digitally imaging colonies with more than a hundred cells, the distributions of cell size and shape are determined for four examples of 2-Ga microfossils: bacillus-shaped Eosynechococcus moorei and three dyads or diplococci (Sphaerophycus parvum and two forms of Eoentophysalis belcherensis). By assuming that each colony obeys steady-state growth, the measured distributions can be inverted to infer the time evolution of the individual cell shape. The time evolution can also be predicted analytically from rate-based models of cell growth, permitting the data to distinguish among different postulates for the physical principles governing growth. The cell cycles are found to be best described by the exponential growth of cell volume, although linear volume growth is not ruled out. However, the measured dyad cycles are inconsistent with several growth models based on surface area or the behavior of the septum at the division plane. Where they have been measured, modern bacilli obey exponential growth whereas eukaryotics obey linear growth, which implies that these 2-Ga microfossils are likely prokaryotic.

Bryozoans are active non-phototrophic biomineralizers that precipitate their calcareous skeletons in seawater. Carbonate saturation states varied temporally and spatially in Paleozoic oceans, and we used the Bryozoan Skeletal Index (BSI) to investigate whether bryozoan calcification was controlled by seawater chemistry in Paleozoic trepostome and cryptostome bryozoans. Our results show that cryptostome bryozoan genera were influenced by ocean chemistry throughout the Paleozoic and precipitated the most calcite at lower latitudes, where carbonate saturation states are generally higher, and less in midlatitudes, where carbonate will be relatively undersaturated. Trepostome bryozoan genera show a similar but weaker trend for the Ordovician to Devonian, suggesting that, like the cryptostomes, they were unable to metabolically overcome falling saturation states and simply precipitated less robust skeletons at higher latitudes. Carboniferous to Triassic trepostomes differ, however, and show a trend toward increased calcification at higher latitudes, indicating an ability to overcome unfavorable carbonate saturation states. Analysis of Permian trepostomes at the species level indicates this is most pronounced in the Southern Hemisphere, where calcification is matched by increased feeding capacity. It is proposed that this increased feeding capacity allowed trepostomes to metabolically overcome unfavorable carbonate saturation states. The differing responses of trepostome and cryptostome bryozoans to carbonate saturation states suggest that bryozoans should not be considered as a single group in marine extinctions linked to ocean chemistry changes. Likewise, it would suggest that modern stenolaemate and gymnolaemate bryozoans should be treated separately when considering their response to modern ocean chemistry changes.

Although provinces are widely used to delimit large-scale variations in biotic composition, it is unknown to what extent such variations simply reflect large-scale gradients, much as has been shown at smaller scales for communities. We examine here whether four previously described Middle and Late Ordovician provinces on Laurentia are best described as distinct provinces or as biotic gradients through a combination of the Paleobiology Database and new field data. Both data sets indicate considerable overlap in faunal composition, with spatial patterns in Jaccard similarity, quantified Jaccard similarity, and nonmetric multidimensional scaling ordination structure that correspond to variations in substrate type, specifically from carbonate-dominated strata in western Laurentia to mixed carbonate–siliciclastic strata in the midcontinent to siliciclastic-dominated rocks in easternmost Laurentia. Because sampling was limited to shallow-subtidal settings, this gradient cannot be attributed to variations in water depth. Likewise, geographic distance accounts for only a quarter of the variation in faunal composition. This cross-continent faunal gradient increases in strength into the early Late Ordovician, and appears to represent increased siliciclastic influx into eastern Laurentia during the Taconic orogeny. These results raise the question of whether biogeographic provinces may be in general better interpreted and analyzed as biotic gradients rather than as discrete entities.

Rapid shell closure in articulate brachiopods occurs by a twitch contraction of the “quick” adductor muscles. Rapid accelerations and decelerations of the valves induce large accelerations of the surrounding fluid, placing constraints on the speeds of closure; inertial forces govern the mechanics of the closing mechanism. The position, size, and physiological properties of the muscles, and the size and shape of the shell, govern the properties of the shell closing system. A numerical model predicts that, for given muscle and shell characteristics, there is a location of the adductor muscle which maximizes the shell's closing velocity, and that this position shifts allometrically with size. Positive allometry in the muscle's moment arm length is observed in both living and fossil brachiopods, although the observed coefficients are less than those predicted by the model.

The brachiopod closing mechanism provides a model for understanding the biomechanical interrelationships between components of dynamic skeletal systems. Dynamic systems, where the muscles induce resisting inertial reactions, are characterized by mechanical feedback loops, or couplings. Mechanical couplings may represent an important class of constraints on the evolution of skeleto-muscular architecture.

No one needs reminding that we are well into a revolutionary phase in the study of evolution, systematics, and the interrelationships of organisms. To belabor the metaphor a little, to the thesis of Darwinian evolution and Mayrian systematics has been added a new cladistic antithesis which says that the search for ancestors is a fool's errand, that all we can do is determine sister group relationships of monophyletic taxa based on the analysis of derived characters. Consequently, our attention should be focused upon patterns which can be delineated, discussed, and modified in the light of new data, rather than upon processes which may only be the subject of untestable hypotheses. It is a change in approach that is not easy to accept for, in a sense, it runs counter to what we have all been taught, and it is particularly difficult for the paleontologist to accept because a major feature of the approach is a concentration upon living rather than on fossil forms. In fact, fossils are, in general, either difficult to use or irrelevant, or both, in terms of new attempts to set out the relationships of the major groups of organisms. The subject of this brief notice is a critically important new addition to the literature on this growing, dare I say evolving, subject.

We use a Gaussian logistic regression model to characterize epoch-to-epoch and stage-tostage changes in the latitudinal response curves of Cenozoic marine bivalve and gastropod genera along the global latitudinal gradient, and analyze these changes to understand the mode and tempo of changes in latitudinal distribution. A ubiquitous ‘‘hollow curve’’ pattern is apparent, wherein smaller changes in response-curve parameters are much more common than larger changes. Curves are strikingly consistent in terms of the average level of change exhibited, despite the many unique environmental and biological changes documented between each of these intervals. This implies that the pace and magnitude of changes in the latitudinal distribution of marine mollusks are not controlled, in aggregate, by time-period-specific conditions. Additionally, we find no evidence for long-term migration from tropical to extratropical latitudes. Our results instead favor a model of either equatorward migration or no general trend. This likely reflects the tendency of genera to maintain their highest concentrations in the tropics even if their ranges become extended out of the tropics over time.

The ecogeographic rule known as Bergmann's rule suggests that there is a positive relationship between body size and latitude when comparing closely related taxa. The underlying mechanism or mechanisms to explain this pattern vary as widely as the taxa that seem to follow it, which has led to skepticism over whether Bergmann's rule should be considered a rule at all. Despite this, Bergmann's rule is widespread among modern birds, mammals, beetles, and some amphibians, but far fewer extinct taxa have been subjected to tests of Bergmann's rule. To examine whether Bergmann's rule is detected in extinct taxa, we compared body-size proxies in Lystrosaurus recovered from Early Triassic–aged strata in Antarctica, South Africa, India, and China. Our results reveal that average body size is largest at mid-northern paleolatitudes (~45°N) instead of the highest southern paleolatitudes (~70°S). Additionally, maximum body size is consistent across the Northern and Southern Hemispheres, indicating that Bergmann's rule did not apply for Lystrosaurus during the Early Triassic. To test potential sample size biases in our results, we used rarefaction and subsampling to show that only the Karoo Basin is well sampled and that large individuals are exceedingly rare, except in the Turpan-Junggar Basin of Xinjiang, China. Taken together, our results suggest that Lystrosaurus had the potential to reach large body sizes in each of the latitudinally widespread geologic basins studied here, but that local conditions may have allowed individuals at mid-northern paleolatitudes a greater chance of reaching a large size compared with southern congeners that suffered increased mortality when young or at a small size.

Ammonoid cephalopods were Earth's most abundant oceanic carnivores for hundreds of millions of years, yet their probable range of swimming capabilities is poorly constrained. We investigate potential hydrodynamic costs and advantages provided by different conch geometries using computational fluid dynamics simulations. Simulations of raw drag demonstrate expected increases with velocity and conch inflation, consistent with published experimental data. Analysis at different scales of water turbulence (via Reynolds number) reveals dynamic trade-offs between conch shape, size, and velocity. Among compressed shells, the cost of umbilical exposure makes little difference at small sizes (and/or low velocity) but is profound at large sizes (and/or high velocity). We estimate that small ammonoids could travel one to three diameters per second (i.e., a typical ammonoid with a 5-cm-diameter shell could travel 5–15 cm/s), but that large ammonoids faced greater discrepancies (a 10 cm serpenticone likely traveled <30 cm/s, while a 10 cm oxycone might achieve >40 cm/s). All of these velocities are proposed only for short bursts of jet propulsion, lasting only a few seconds, in the service of dodging a predator or conspecific rival. These analyses do not include phylogeny, taxonomy, second-order conch architecture (ribs, ornament, etc.), or hydrostatic consequences of internal anatomy (soft body, suture complexity). For specific paleoecological context, we consider how these results inform our reconstruction of Jurassic ammonite recovery from the end-Triassic mass extinction. Greater refinements will come with additional simulations that measure how added mass is influenced by individual shape-trait variations, ornament, and subtle body extensions during a single jet motion.

The Asteropyginae Delo, 1935 is a group of phacopid trilobites in the family Acastidae Delo, 1935 that has served as the focus for several studies due to their distinctive morphologies and diversity. However, despite an interest in these characteristic morphologies, there have been no studies that have examined this group using morphometric techniques. Our investigation utilized both geometric morphometric and elliptical Fourier methods to quantify the morphology of cephalic sclerites of asteropyginid specimens representing wide taxonomic sampling of the clade. We constructed a phylomorphospace that shows temporal and spatial patterns of phenotypic evolution within the framework of a novel tip-dated phylogenetic tree generated using Bayesian inference. We recovered similar patterns in disparity regardless of the morphometric approach. Both analyses illustrated a marked expansion into morphospace throughout the temporal range of the clade, peaking in disparity in the Emsian and with European taxa exhibiting the highest disparity in glabellar morphospace. Additionally, glabellar shape showed low phylogenetic signal and no major patterns in phylomorphospace. This study highlights the utility of employing different methodologies to quantitatively explore the disparity of fossil taxa. It also illustrates some of the patterns of morphological change occurring during one of the final and major evolutionary radiations within Phacopida.

Developmental plasticity, where traits change state in response to environmental cues, is well studied in modern populations. It is also suspected to play a role in macroevolutionary dynamics, but due to a lack of long-term records, the frequency of plasticity-led evolution in deep time remains unknown. Populations are dynamic entities, yet their representation in the fossil record is a static snapshot of often isolated individuals. Here, we apply for the first time contemporary integral projection models (IPMs) to fossil data to link individual development with expected population variation. IPMs describe the effects of individual growth in discrete steps on long-term population dynamics. We parameterize the models using modern and fossil data of the planktonic foraminifer Trilobatus sacculifer. Foraminifera grow by adding chambers in discrete stages and die at reproduction, making them excellent case studies for IPMs. Our results predict that somatic growth rates have almost twice as much influence on population dynamics than survival and more than eight times more influence than reproduction, suggesting that selection would primarily target somatic growth as the major determinant of fitness. As numerous paleobiological systems record growth rate increments in single genetic individuals and imaging technologies are increasingly available, our results open up the possibility of evidence-based inference of developmental plasticity spanning macroevolutionary dynamics. Given the centrality of ecology in paleobiological thinking, our model is one approach to help bridge eco-evolutionary scales while directing attention toward the most relevant life-history traits to measure.