The regeneration abilities of crinoids not only are important to understanding crinoid ecology, but also can serve as the basis for assessing the pressure exerted on crinoids by predators both in the Recent and in the geologic past. This is especially true of regenerating arms, because arm loss, and subsequent regeneration, is thought to result from interactions with predators, primarily fish. However, the commonly used regeneration-based proxy for predation pressure (proportion of individuals with regenerating arms) does not provide a measure of the rate at which those events occurred. Here we present a method for reconstructing the arm-loss rate per individual, a more direct proxy of predation pressure. This metric accounts for differences in arm length, arm number, and branching pattern, features highly variable among taxa, among environments, and through geologic time. Normalizing for those characters permits the transformation of observed proportions of regenerating arms to rates that can be compared across samples of morphologically distinct crinoids. Applying this method to a Recent crinoid (Cenometra bella) reveals that this shallow-water comatulid loses arms at a rate of about once every ten days. The same approach reveals that Mississippian shallow-water crinoids (Rhodocrinites kirbyi) experienced arm loss much less frequently, approximately once every 36 days, suggesting that predation pressure on crinoids today is greater than it was in the Mississippian.

Reconstructing patterns of macroevolution has become a central endeavor in paleobiology, because it offers insight into evolutionary models shaping the history of life. As the most diverse and abundant animals since the Cambrian period, arthropods provide copious data to elucidate the emergence of body plans in metazoan lineages. However, information provided by fossils on the tempo and mode of this phenomenon has lacked a recent synthesis. Here, I investigate macroevolutionary patterns of morphological evolution in Euarthropoda using a combined extinct and extant dataset optimized for multivariate analyses. Overall ordination patterns between the main morphogroups are consistent with another, independently coded, extant-only dataset providing molecular and morphological rates of evolution. Based on a “deep split” phylogenetic framework, total-group Mandibulata and Arachnomorpha emerge as directional morphoanatomical lineages, with basal fossil morphogroups showing heterogeneously spread-out occupations of the morphospace. In addition to a more homogeneous morphological variation, new morphogroups arose by successive reductions of translation distances; this pattern was interrupted only by terrestrialization events and the origin of pancrustaceans. A displaced optimum type of model is proposed to explain the fast assembly of canalized body plans during the Cambrian, with basal fossil morphogroups fitting intermediate fitness peaks in a moving adaptive landscape. Given time constraints imposed by the paleontological evidence, and owing to the interplay between canalization and modularity, as well as a decoupling between molecular and morphological rates, the rise of euarthropods would support the view that the swiftness of the Cambrian explosion was mostly associated with the buildup of genetic regulatory networks.

The purpose of this paper is to explore the possibility and utility of estimating multivariate selection in fossil assemblages, using naticid gastropods as a case study. We used the presence or the absence of a naticid borehole as an index of survival with respect to drilling attacks, enabling us to estimate the multivariate selection gradient exerted by this predator on the shell length and shell thickness of two bivalve genera, Astarte and Spisula. We hypothesized that naticid selection pressure would favor the survival of large, thick-shelled bivalve prey throughout the Cenozoic.

Differential survival of prey was recorded over geologic time using processed bulk assemblages from the Miocene, Pliocene, and Pleistocene Epochs. Multivariate logistic regressions were performed by time period to determine if length and thickness were important factors affecting survival. The direction and magnitude of selection on length and thickness for the two genera ranged from zero (no selection) to large positive or negative values. Only two selection coefficients were significant after sequential Bonferroni corrections: thin-shelled Astarte survived substantially better than thick-shelled Astarte during the Pleistocene (βavggrad = −1.23) and large Spisula survived slightly better than small Spisula during the Miocene Epoch (βavggrad = 0.05).

This is the first study using fossils to calculate multivariate selection gradients. It suggests that naticids were not necessarily strong agents of selection on two traits previously thought to be important to survival of drilling attacks for two of their common prey species. We also show that multivariate selection gradient estimates differ from traditional predation intensity estimates but are superior for estimating the magnitude and direction of natural selection because they use differential mortality of different prey phenotypes rather than just absolute mortality from predation. This work is especially significant for research that involves estimating the relative importance of predation (naticid or otherwise) as an evolutionary force and will be useful for fossil studies where differential survival can be recorded.

To assess evolutionary processes in deep time, it is essential to understand the roles of development and environment, both recorded through the morphological variability of fossil assemblages. Thanks to their great abundance and the high temporal resolution of their fossil record, conodont elements are ideal to address this issue. In this paper, we present the first quantitative study of a Carnian–Norian (Late Triassic) assemblage of closely related P1 conodont elements. Using geometric morphometrics (landmarks, sliding landmarks, and elliptic Fourier analysis), we explore the main axes of phenotypic variation and relate them to classically used taxonomic characters. We show that some important taxonomic features follow laws of covariation, hence highlighting developmental constraints. Furthermore, the intraspecific variation within all considered species, either Carnian or Norian forms, is similarly restricted, emphasizing, for the first time in conodont P1 elements, a common line of least resistance to evolution, which means that similar intrinsic (developmental) factors were acting on these taxa and likely biased the evolutionary trajectories of all these taxa in a similar way. Because the evolution between Carnian and Norian forms is known to have followed a trajectory that is significantly different from the line of least resistance, strong extrinsic pressures, such as environmental disturbances, were probably at play around the Carnian/Norian boundary to counteract the effects of these intrinsic, developmental constraints.

Improvements in the perceived completeness of the fossil record may be driven both by new discoveries and by reinterpretation of known fossils, but disentangling the relative effects of these processes can be difficult. Here, we propose a new methodology for evaluating historical trends in the perceived completeness of the fossil record, demonstrate its implementation using the freely available software ASCC (version 4.0.0), and present an example using crown-group birds (Aves). Dates of discovery and recognition for the oldest fossil representatives of 75 major lineages of birds were collected for the historical period ranging from 1910 to 2010. Using a comprehensive phylogeny, we calculated minimum implied stratigraphic gaps (MIG range) across these 75 lineages. Our results show that a reduction in global MIG values of 1.35 Ga (billion years) occurred over the past century in avian paleontology. A pronounced increase in the average rate of global MIG reduction is noted in the post-1970s interval (290.5 Myr per decade) compared to the pre-1970s interval (31.9 Myr per decade). Although the majority of the improvement in the fossil record of birds has come from new discoveries, substantial improvement (∼22.5%) has resulted from restudy and phylogenetic revision of previously described fossils over the last 40 years. With a minimum estimate indicating that at least 1.34 Gyr of gaps remain to be filled between the predicted and observed first appearances of major lineages of crown Aves, there is much progress to be made. However, a notable tapering off in the rate of global MIG reduction occurs between 1990 and 2010, suggesting we may be approaching an asymptote of oldest record discoveries for birds. Only future observations can determine whether this is a real pattern or a historical anomaly. Either way, barring the discovery of fossils that substantially push back the minimum age for the origin of crown-clade Aves, new discoveries cannot continue to reduce global MIG values at the average post-1970s rate over the long term.

Amecystis Ulrich and Kirk is a rhombless rhombiferan cystoid exhibiting adaptation to an active mode of life in the bilaterally symmetrical, dorso-ventrally compressed theca and to an active mode of respiration suggested by the lack of pore rhombs and greatly enlarged periproct. Like other members of the Pleurocystitidae, Amecystis probably wriggled along the bottom, but may have adopted a temporarily sessile habit by pushing the distal stele into the substrate or by grasping. Evolution of the genus began in the Middle Ordovician, probably as a development from Pleurocystites. General trends in the stratigraphic succession of recognized morphospecies: A. raymondi Parsley, A. woodi n.sp., and A. laevis (Raymond) show a reduction in angularity of thecal outline and obsolescence of surface features on the thecal plates. The use of the periproct as a pumping organ in respiration is regarded as an advanced trend that was probably utilized by many rhomb-bearing cystoids as an accessory respiratory mode; alone, it proved to be an unsuccessful adaptation in Amecystis.

The study of tracks is often regarded as a fringe subdiscipline outside the scientific mainstream. Papers dealing with footprints traditionally appear either in popular magazines like Natural History (Bird 1939, 1944; Brown 1938), or in very obscure publications (Sarjeant 1974). Usually it is only the spectacular discoveries, which are directly applicable to topical debates, that rouse much scientific interest. For example, the discovery of Pliocene hominid tracks (Leakey and Hay 1979) provided unequivocal evidence for the antiquity of bipedalism (cf., Napier 1967). Similarly the tracks of a running theropod allowed for direct estimates of the speed attained by dinosaurs (Farlow 1981), and the tracks of a herd of running theropods fueled debate about gregariousness and stampede behavior (Thulborn and Wade 1984).

Graphoglyptids are biogenic structures commonly found in deep-sea flysch deposits and occasionally detected on the modern deep-sea floor. They extend principally horizontally and take a variety of geometric patterns, whose functional morphology remains an enigma in ichnology and paleoceanography. Based on published materials from 1850 to 2017 (79 ichnotaxa from 28 ichnogenera of graphoglyptids) and systematic observations of one of the largest deep-sea trace fossil collections in the world, this paper proposes that topological analysis is an important ingredient in the taxonomy and functional interpretation of graphoglyptids. Accordingly, graphoglyptids are classified into line, tree, and net forms by their key topological architecture, and are further attributed to 19 topological prototypes by detailed secondary topological features. Line graphoglyptids are single-connected structures with uniform tunnel width, representing primarily the feeding patterns of solitary animals. Tree graphoglyptids, the most diverse architectural group of graphoglyptids, are ascribed to 11 topological prototypes according to the connectivity features of burrow segments and the number and distributional pattern of the branching points. Net graphoglyptids are subdivided into three topological prototypes on the basis of the connectivity features and/or the regularity of the meshes. Multiconnected net forms are considered as a continuous morphological spectrum with different levels of complexity in the net formation. The various connected components in multiconnected tree and net graphoglyptids generally exhibit small and uniform tunnel diameter in a given structure (suggesting a tiny trace maker[s]). The whole structure shows relatively extensive linear or surface coverage and overall good preservation, indicating sustained processes of burrow construction. It is highly probable that certain multiconnected tree and net graphoglyptids represent some emergent patterns from self-organized collective behaviors of conspecific animals. Graphoglyptids thus provide us with a new perspective on the study of solitary and collective behaviors of macrobenthos in the deep-sea environment.

Evolutionary inferences from fossil data often require accurately reconstructing differences in richness and morphological disparity between fossil sites across space and time. Biases such as sampling and rock availability are commonly accounted for in large-scale studies; however, preservation bias is usually dealt with only in smaller, more focused studies. Birds represent a diverse, but taphonomically fragile, group commonly used to infer environmental conditions in recent (Pleistocene and later) fossil assemblages, and their relative scarcity in the fossil record has led to controversy over the timing of their radiation. Here, I use simulations to show how even weak taphonomic biases can distort estimates of richness, and render variance sensitive to sample size. I then apply an ecology-based filtering model to recent bird assemblages to quantify the distortion induced by taphonomy. Certain deposit types, such as caves, show less evidence of taphonomic distortion than others, such as fluvial and lacustrine deposits. Archaeological middens unsurprisingly show some of the strongest evidence for taphonomic bias, and they should be avoided when reconstructing Pleistocene and early Holocene environments. Further, these results support previously suggested methods for detecting fossil assemblages that are relatively faithfully preserved (e.g., presence of difficult-to-preserve taxa), and I use these results to recommend that future large-scale studies include facies diversity along with metrics such as rock volume, or compare only sites with similar taphonomic histories.

Sharks have a long and rich fossil record that consists predominantly of isolated teeth due to the poorly mineralized cartilaginous skeleton. Tiger sharks (Galeocerdo), which represent apex predators in modern oceans, have a known fossil record extending back into the early Eocene (ca. 56 Ma) and comprise 22 recognized extinct and one extant species to date. However, many of the fossil species remain dubious, resulting in a still unresolved evolutionary history of the tiger shark genus. Here, we present a revision of the fossil record of Galeocerdo by examining the morphological diversity and disparity of teeth in deep time. We use landmark-based geometric morphometrics to quantify tooth shapes and qualitative morphological characters for species discrimination. Employing this combined approach on fossil and extant tiger shark teeth, our results only support six species to represent valid taxa. Furthermore, the disparity analysis revealed that diversity and disparity are not implicitly correlated and that Galeocerdo retained a relatively high dental disparity since the Miocene despite its decrease from four to one species. With this study, we demonstrate that the combined approach of quantitative geometric morphometric techniques and qualitative morphological comparisons on isolated shark teeth provides a useful tool to distinguish between species with highly similar tooth morphologies.

The emergence of shell forms in the growth of foraminifera is an essential problem in the morphogenesis of these microorganisms. We present a model of foraminiferal shells that applies a moving-reference system. Previous models have referred to fixed-reference axes and have neglected apertures. Our model focuses on real morphologic characteristics and follows stepwise natural biological processes. It introduces apertures based on minimization of the local communication path and applies three parameters, which are either predetermined or selected at random from given ranges. Expression of stochastic parameters mimics phenotypic variability of a shell. We also present a detailed description of the method with examples of simulated shells and the first step toward analyses of the theoretical morphospace. The morphospace is divided into certain regions (phases) separated by transitional planes (phase transitions). Further prospects for foraminiferal modeling, which should focus on more in-depth models based on realistic intracellular dynamics, are also presented.

Estimates of taxonomic richness and abundance are complicated by sampling biases. The failure to sample rare taxa is most often attributed to inadequate sampling and to removal during the process of sample-size standardization. Here I present two methods for unveiling rare diversity by integrating species presence/absence data from museum collections and the literature with quantitative estimates of species richness and abundance gathered from field-based bulk samples. Combining museum, literature, and field data can provide a more comprehensive estimate of taxonomic richness and abundance without substantial increase in current sampling effort. First, in a given bulk sample, the lowest proportional abundance value observed can be used to estimate the maximum abundance of rare species known to have occurred at the locality at least once but not recorded in the current sample. Second, a model-selection approach can be used, in which a set of relative abundance distribution models are fit to the bulk-sample abundance data and the parameter estimates for the best model used to calculate the abundance distribution for all species known from the locality. The Paleogene marine fossil record of the U.S. Gulf Coastal Plain is suitable for applying these methods, because (1) the molluscan fauna is well represented in museum collections and the literature, (2) the molluscan fauna has been taxonomically standardized, and (3) many classic localities remain accessible for standardized bulk sampling. I introduce these methods by applying them to a single locality and then, using the faunas of the Gosport, Moodys Branch, and Red Bluff Formations, I demonstrate how the model-fitting approach can be used to compare taxonomic richness among multiple localities. A substantial fraction of the molluscan richness known from each locality is not captured in bulk samples and much of this unobserved richness may be attributed to the rarity of species. The multiple-locality comparison suggests that the greatest Paleogene decline in standing richness occurred in the middle Eocene and that the recovery of richness following the Eocene-Oligocene extinction was quite rapid despite substantial loss of taxa. These analyses underscore the magnitude of veiled diversity in marine fossil assemblages and the potential of existing sources of data to unveil rare taxa, allowing them to be incorporated into quantitative diversity studies.

The morphological and ecological diversity of lemurs and lorisiformes once rivaled that of the rest of the primate order. Here, we assemble a dataset of 3D models representing the second mandibular molars of a wide range of extant and fossil strepsirrhines encompassing this diversity. We use these models to distill quantitative descriptors of tooth form and then analyze these data using new analytical methods. We employ a recently developed dental topography metric (ariaDNE), which is less sensitive to details of random error in 3D model quality than previously used metrics (e.g., DNE); Bayesian multinomial modeling with metrics designed to measure overfitting risk; and a tooth segmentation algorithm that allows the shapes of disaggregated tooth surface features to be quantified using dental topography metrics. This approach is successful at reclassifying extant strepsirrhine primates to known dietary ecology and indicates that the averaging of morphological information across the tooth surface does not interfere with the ability of dental topography metrics to predict dietary adaptation. When the most informative combination of dental topography metrics is applied to extinct species, many subfossil lemurs and the most basal fossil strepsirrhines are predicted to have been primarily frugivorous or gummivorous. This supports an ecological contraction among the extant lemurs and the importance of frugivory in the origins of crown Strepsirrhini, potentially to avoid competition with more insectivorous and folivorous members of Paleogene Afro-Arabian primate faunas.

Simulations are playing an increasingly important role in paleobiology. When designing a simulation study, many decisions have to be made and common challenges will be encountered along the way. Here, we outline seven rules for executing a good simulation study. We cover topics including the choice of study question, the empirical data used as a basis for the study, statistical and methodological concerns, how to validate the study, and how to ensure it can be reproduced and extended by others. We hope that these rules and the accompanying examples will guide paleobiologists when using simulation tools to address fundamental questions about the evolution of life.

Historical biogeographers are divided over the major process whereby extant species, or their ancestors, arrived in their present geographical locations. One faction promotes the idea of discrete “centers” of evolutionary radiation, from which species have been supplied to other areas by long-distance dispersal. A second faction advances the notion of widespread ancestral biotas, which have been fractionated into descendant biotas over time (vicariance). A seemingly endless polemic has elevated these rather simple alternatives to causes célèbres, and many workers have responded by dismissing virtually the entire vicariance literature (see Pielou 1981). We believe that the important distinction between vicariance and center-of-origin/long-distance-dispersal can be made clear to those interested in questions of paleobiogeography. In our opinion, the recent contribution by Briggs (1981) in this journal did not do so, and we now take the opportunity to try.

Consider a one-dimensional sequence (temporal or spatial) of samples from a many-species community. The sequence may exhibit no steady change, or gradual, steady change, or one or more abrupt, stepwise changes. The pattern is graphically displayed in a similarity matrix, whose elements are the pairwise similarities between every possible pair of samples.

Statistical study of the patterns of similarity matrices will be shown to be informative. A criterion, Q, is proposed for measuring the “disarray” of a similarity matrix; matrices constructed from “perfect” sequences, exhibiting uninterrupted steady change, have Q = 0. The probability distribution of Q in “random” similarity matrices (constructed from sequences in which the ordering is wholly haphazard) is examined.

Several applications are illustrated, using data on foraminiferal assemblages in marine sediment cores as examples. Use of the criterion permits the following: (1) Objective recognition of regions in which stratigraphy is especially well preserved. (2) Selection for study of cores in which the sequence of change is especially clear, and rejection of cores in which the natural sequence has been obscured by reworking. (3) Judgment as to whether, in sequences exhibiting stepwise changes, gradual change between the steps occurs as well. (4) Discrimination between species-sets that track gradual environmental change and sets that switch abruptly. (5) Objective choice of the best similarity index for measuring pairwise similarities in a given body of data.

The end-Cretaceous (K/Pg) mass extinction event is the most recent and well-understood of the “big five” and triggered establishment of modern terrestrial ecosystem structure. Despite the depth of research into this event, our knowledge of upper Maastrichtian terrestrial deposits globally relies primarily on assemblage-level data limited to a few well-sampled formations in North America, the Hell Creek and Lance Formations. These assemblages disproportionally affect our interpretations of this important interval. Multiple investigations have quantified diversity patterns within these assemblages, but the potential effect of formation-level size-dependent taphonomic biases and their implications on extinction dynamics remains unexplored. Here, the relationship between taphonomy and body size of the Hell Creek Formation and Lance Formation dinosaurs and mammals are quantitatively analyzed. Small-bodied dinosaur taxa (<70 kg) are consistently less complete, unlikely to be articulated, and delayed in their description relative to their large-bodied counterparts. Family-level abundance (particularly skeletons) is strongly tied to body mass, and the relative abundance of juveniles of large-bodied taxa similarly is underrepresented. Mammals show similar but nonsignificant trends. The results are remarkably similar to those from the Campanian-aged Dinosaur Park Formation, suggesting a widespread strong taphonomic bias against the preservation of small taxa, which will result in their seemingly depauperate diversity within the assemblage. This taphonomically skewed view of diversity and abundance of small-bodied taxa amid our best late Maastrichtian samples has significant implications for understanding speciation and extinction dynamics (e.g., size-dependent extinction selectivity) across the K/Pg boundary.

A population of shells of the Cretaceous oyster Agerostrea mesenterica was obtained from a single locality. The age at death for each shell could be determined, and the population was deemed to be essentially undisturbed and capable of providing a mortality record of the local population averaged over a span of time. We measured four morphometric characters which, on the basis of a functional morphologic model, could be expected to have adaptive value. The frequency distribution of each character was analyzed separately for the three year old and for the six year and older specimens in order to determine which individuals died young and which survived to old age. Ontogenetic effects were separated from those of differential mortality by independent means. The results indicated that there was (1) directed selection for maximum curvature, (2) centripetal selection for a plica number of about 8, and (3) a net centripetal effect on both arc length and plica height, composed of selection against largest values coupled with a possible selection against smaller values that cannot be separated from ontogenetic effects. Only the selection for maximum curvature and optimal plica number can be reconciled with the predictions of the model.

The origin of the Neogene planktonic foraminifer Globorotalia (Globoconella) pliozea in the subtropical southwest Pacific has been attributed to its isolation resulting from intensification of the Subtropical Divergence (Tasman Front). Oxygen isotopic analyses suggest that, although the Subtropical Divergence may have played a role, the evolution of Gr. (G.) pliozea was facilitated by depth segregation of Gr. (G.) conomiozea morphotypes (low and high conical) during an interval of near-surface warming and increasing thermal gradient. Oxygen isotopic analyses suggest that low conical morphotypes of Gr. (G.) conomiozea inhabited greater depths than high conical morphotypes. Low conical forms of Gr. (G.) conomiozea are considered ancestral to the low conical species, Gr. (G.) pliozea. Oxygen isotopes indicate that Gr. (G.) pliozea inhabited greater depths than its ancestor, Gr. (G.) conomiozea.

These data are consistent with depth-parapatric and depth-allopatric models, but not with a sympatric model of speciation. In the allopatric model, reproduction at different water depths acts as a barrier between morphotypes. In the parapatric model, clinal variation along a depth gradient acts as a barrier between morphotypes living at the limits of the gradient. Depth segregation in both models results in genetic isolation and evolutionary divergence. Our data support a correlation between morphological evolution and habitat changes in the Globoconella clade, implying separation of populations as a driving force for morphological evolution.

Ecological segregation of morphotypes and species may be related to morphology (height of the conical angle), based on the data from Gr. (G.) conomiozea and Gr. (G.) pliozea. However, morphological differences alone do not necessarily produce depth differences. Large morphological differences between Gr. (G.) pliozea and closely related Gr. (G.) puncticulata did not result in isotopic and therefore depth differences between these species. These species coexisted at the same water depths for nearly 1 m.y. Thus, it is unlikely that the extinction of Gr. (G.) pliozea in the middle Pliocene resulted from competition with Gr. (G.) puncticulata, as previously suggested.

As The Blob, of Steve McQueen's greatest triumph, so amply demonstrated, the more you encompass the more formless you become. Orzack accuses me of construing the modern synthesis too narrowly in describing a version championed only by Mayr and Fisher—a pair of unlikely bedfellows, I would have thought. Yet his version is so broad that he wins his own argument by internal definition, thus rendering it meaningless. As “the basis of the modern synthesis,” Orzack cites “the great evolutionary idea” — “all evolution can be explained in principle by an examination of the properties of individuals and of populations.” But what else is there, except perhaps Schindewolf's supernovae and Teilhard's ever-watchful Jesus, drawing organic activity towards Omega. If the synthesis only excludes some fringing finalists and unrepentant Lamarckians, then we are all inevitably in it, and we might as well recast the term as a synonym for evolution itself.