Although morphological variation is known to influence the evolutionary fates of species, the relationship between morphological variation and survivorship in the face of extinction-inducing perturbations is poorly understood. Here, we investigate this relationship for veneroid bivalves in association with the Plio-Pleistocene extinction in Florida. Fourteen pairs of related species were selected for analysis, with each pair including one species that survived the Plio-Pleistocene extinction and another that became extinct during the interval. Morphological landmark data were acquired for more than 1500 museum specimens, representing 19 localities that encompass four well-known Plio-Pleistocene units in the study region. Procrustes superimposition was applied to each sample, and overall multivariate variation was calculated as the mean squared partial Procrustes distance between specimens and their mean form. Morphological variation was calculated at three geographic scales for each species, and differences in variation between survivors and victims were examined within each species pair. Results indicate that species surviving the Plio-Pleistocene extinction were significantly more variable morphologically than victims. Greater morphological variation may promote survivorship by directly enhancing species adaptations to changing conditions or by permitting the occupation of a larger geographic range. Alternatively, high morphological variation and survivorship may both be mediated by a third variable, such as large geographic range.

The evolutionary history of shell geometry of Early Jurassic ammonoids during the Pliensbachian–Toarcian second-order mass extinction is explored at both adult and ontogenetic levels. The ontogenetic approach builds on the concept of allometric space to get insights into the developmental aspects of morphological evolution. Investigation of the deployment of taxa in adult morphospace and allometric space allows the appraisal of the temporal evolution of morphological and allometric disparities. Curves of taxonomic diversity, adult morphological disparity, allometric disparity, and average adult size are contrasted. Results show that during the Pliensbachian–Toarcian interval, ammonoids underwent two successive and drastic declines in taxonomic diversity. Patterns of morphospace and allometric space occupancy suggest nonselective extinction at both morphological and developmental levels. Another measure of allometric disparity suggests the occurrence of two heterochronic trends, a peramorphocline followed by a paedomorphocline, during the Toarcian. These trends are concomitant with changes in average adult size that compensate for the heterochronic effects and explain the striking stability of morphological disparity despite changes in diversity. The results also emphasize the existence of two contrasted evolutionary dynamics in Pliensbachian and Toarcian ammonoids. Methodologically, the allometric disparity approach appears to be a fruitful tool to analyze the rather understudied clade-wide ontogenetic aspects of morphological evolution. Combining multiple approaches to describe clade morphological dynamics leads to a better characterization and understanding of the diversity-disparities relationships and a better distinction of the potential processes driving these macroevolutionary patterns.

We use Fourier analysis to investigate evolutionary dynamics related to periodicities in marine fossil biodiversity. Coherent periodic fluctuation in both origination and extinction of “short-lived” genera (those that survive <45 Myr) is the source of an observed ∼62 million year periodicity (which we confirmed in an earlier paper and also found to be ubiquitous in global compilations of marine diversity). We also show that the evolutionary dynamics of “long-lived” genera (those that survive >45 Myr) do not participate in the periodic fluctuation in diversity and differ from those of “short-lived” genera. The difference between the evolutionary dynamics of long- and short-lived genera indicates that the periodic pattern is not simply an artifact of variation in quality of the geologic record. Also, the interplay of these two previously undifferentiated systems, together with the secular increase in abundance of “long-lived” genera, is probably the source of observed but heretofore unexplained differences in evolutionary dynamics between the Paleozoic and post-Paleozoic as reported by others. Testing for cycles similar to the 62-Myr cycle in fossil biodiversity superimposed on the long-term trends of the Phanerozoic as described in Paper I, we find a significant (but weaker) signal in sedimentary rock packages, particularly carbonates, which suggests a connection. The presence of a periodic pattern in evolutionary dynamics of the more vulnerable “short-lived” component of the marine fauna demonstrates that a long-term periodic fluctuation in environmental conditions capable of affecting evolution in the marine realm characterizes our planet's history. Coincidence in timing is more consistent with a common cause than with sampling bias. A previously identified set of mass extinctions preferentially occur during the declining phase of the 62-Myr periodicity, supporting the idea that the periodicity relates to variation in biotically important stresses. Further work should focus on finding links to physical phenomena that might reveal the causal system or systems.

Delayed biotic recovery from the end-Permian mass extinction has long been interpreted to result from environmental inhibition. Recently, evidence of more rapid recovery has begun to emerge, suggesting the role of environmental inhibition was previously overestimated. However, there have been few high-resolution taxonomic and ecological studies spanning the full Early and Middle Triassic recovery interval, leaving the precise pattern of recovery and underlying mechanisms poorly constrained. In this study, we document Early and Middle Triassic trends in taxonomic diversity, assemblage evenness, and size distribution of benthic foraminifers on an exceptionally exposed carbonate platform in south China. We observe gradual increases in all metrics through Early Triassic and earliest Middle Triassic time, with stable values reached early in the Anisian. There is little support in our data set for a substantial Early Triassic lag interval during the recovery of foraminifers or for a stepwise recovery pattern. The recovery pattern of foraminifers on the GBG corresponds well with available global data for this taxon and appears to parallel that of many benthic invertebrate clades. Early Triassic diversity increase in foraminifers was more gradual than in ammonoids and conodonts. However, foraminifers continued to increase in diversity, size, and evenness into Middle Triassic time, whereas diversity of ammonoids and conodonts declined. These contrasts suggest decoupling of recovery between benthic and pelagic environments; it is unclear whether these discrepancies reflect inherent contrasts in their evolutionary dynamics or the differential impact of Early Triassic ocean anoxia or associated environmental parameters on benthic ecosystems.

Species selection has received a great deal of theoretical attention but it has rarely been empirically tested. It is important to determine the level of selection that operated during a particular extinction event because it can help distinguish between traits that were actually responsible for extinction and those that were merely correlated with it. Here, we present a test that can help distinguish between organismal and species-level selection, which we demonstrate using the high-resolution fossil record of planktonic foraminifera species recorded in deep-sea sediment cores. Our test examines the fate of survivors and victims during the Cretaceous/Paleogene (K/Pg) mass extinction within single geographic regions, where all individuals experience the same selection pressures. Selection at the organismal level implies that individual members of surviving species are more fit than those of victimized species, and therefore should be more likely to survive in affected areas; conversely, selection at the species level implies individuals will suffer equally within an affected area. We find that survivors of the mass extinction suffered very high extirpation rates in cores where the overall extinction rate was high, indicating that individual members of the surviving species were generally no more fit than individual members of extinct species. Rather, these species were able to survive because they possessed advantageous species-level traits, such as larger geographic ranges and greater abundances than victimized species. This geographic pattern of extirpation suggests that selection operated at the species, rather than organismal, level during the K/Pg mass extinction of planktonic foraminifera.

The geographic distribution of 293 Modern bivalve genera has been analyzed and found to be statistically correlated with distance. In particular, a least-squares regression analysis of the data indicates that the distance between faunal realms (D) in kilometers can be estimated using the equation D = (ln(d) + 0.4233)/−0.00013, where d is the Dice coefficient of faunal similarity. Analysis of 59 genera of Late Ordovician bivalves indicates that the above equation also describes their biogeographic distribution.

Using this formula, the distance between Laurentia and Scotland/Northwest Ireland was estimated to be 5500 kilometers. This is consistent with the reconstruction of a connection among these areas during the Late Ordovician based on brachiopod and graptolite biogeographic data.

Paleomagnetic and paleoclimatic data also suggest that Avalonia, Baltica, and Laurentia were at tropical latitudes. Distances between these paleocontinents can therefore be used to estimate paleolongitudes. If the location of England on the eastern side of Avalonia is used as zero degrees paleolongitude for the Late Ordovician as it is today, the paleolongitude for South America, Laurentia, Scotland and northwest Ireland, and Baltica would be 125°W, 45°W, 10°W, and 15°E, respectively. Because of drifting of the Avalonia plate, these paleolongitudes probably do not coincide with the longitudinal grid used today. The paleolongitudes indicate only the relative spacing between continents in the past. The methodology in this study should be useful for improving the accuracy of paleogeographic reconstructions for the Late Ordovician throughout the Cenozoic, and especially the Paleozoic periods for which magnetic seafloor anomaly data are not available.

Incremental stages of major evolutionary transitions within a single animal lineage are rarely observed in the fossil record. However, the extraordinarily complete sequence of well preserved material spanning the 27-Myr existence of the marine squamate subfamily Mosasaurinae provides a unique exception. By comparison with extant and extinct analogs, the tail morphology of four mosasaurine genera is examined, revealing a pattern of evolution that begins with the generalized varanoid anatomy and culminates in a high-aspect-ratio fin, similar to that of sharks. However, unlike the epicercal caudal fluke of selachians in which the tail bends dorsocaudally, derived mosasaurs develop a hypocercal tail with a ventrocaudal bend. Progressive caudal regionalization, reduced intervertebral mobility, increased tail depth due to a marked downturn of the posterior caudal segment, and the development of finlike paired appendages reveal a pattern of adaptation toward an optimized marine existence. This change in morphology reflects a transition from anguilliform or sub-carangiform locomotion to carangiform locomotion, and indicates a progressive shift from nearshore dwellers to pelagic cruisers—a change in foraging habitat independently corroborated by paleobiogeographic, stable isotope, osteohistological, and paleopathological data. Evolutionary patterns similar to those observed in mosasaurine mosasaurs are seen in other secondarily aquatically adapted amniotes, notably metriorhynchid crocodyliforms, cetaceans, and ichthyosaurs, and may be explained by developmental modularity governing the observed phenotypic expression.

Cases of convergent evolution, particularly within ecomorphological contexts, are instructive in identifying universally adaptive morphological features across clades. Tracing of evolutionary pathways by which ecomorphological convergence takes place can further reveal mechanisms of adaptation, which may be strongly influenced by phylogeny. Ecomorphologies of carnivorous mammals represent some of the most outstanding cases of convergent evolution in the Cenozoic radiation of mammals. This study examined patterns of cranial shape change in the dog (Canidae) and hyena (Hyaenidae) families, in order to compare the evolutionary pathways that led to the independent specialization of bone-cracking hypercarnivores within each clade. Geometric morphometrics analyses of cranial shape in fossil hyaenids and borophagine canids provided evidence for deep-time convergence in morphological pathways toward the independent evolution of derived bone-crackers. Both clades contained stem members with plesiomorphic generalist/omnivore cranial shapes, which evolved into doglike species along parallel pathways of shape change. The evolution of specialized bone-crackers from these doglike forms, however, continued under the constraint of a full cheek dentition and restriction on rostrum length reduction in canids, but not hyaenids. Functionally, phylogenetic constraint may have limited borophagine canids to crack bones principally with their carnassial instead of the third premolar as in hyaenids, but other cranial shape changes associated with durophagy nevertheless evolved in parallel in the two lineages. Size allometry was not a major factor in cranial shape evolution in either lineage, supporting the interpretation of functional demands as drivers for the observed convergence. The comparison between borophagines and hyaenids showed that differential effects of alternative functional “solutions” that arise during morphological evolution may be multiplied with processes of the “macroevolutionary ratchet” already in place to further limit the evolutionary pathways available to specialized lineages.

The evolutionary history of the Order Carnivora is marked by episodes of iterative evolution. Although this pattern is widely reported in different carnivoran families, the mechanisms driving the evolution of carnivoran skull morphology remain largely unexplored. In this study we use coordinate-point extended eigenshape analysis (CP-EES) to summarize aspects of skull shape in large fissiped carnivores. Results of these comparisons enable the evaluation of the role of different factors constraining the evolution of carnivoran skull design. Empirical morphospaces derived from mandible anatomy show that all hypercarnivores (i.e., those species with a diet that consists almost entirely of vertebrate flesh) share a set of traits involved in a functional compromise between bite force and gape angle, which is reflected in a strong pattern of morphological convergence. Although the paths followed by different taxa to reach this “hypercarnivore shape-space” differ because of phylogenetic constraints, the morphological signature of hypercarnivory in the mandible is remarkably narrow and well constrained. In contrast, CP-EES of cranial morphology does not reveal a similar pattern of shape convergence among hypercarnivores. This suggests a lesser degree of morphological plasticity in the cranium compared to the mandible, which probably results from a compromise between different functional demands in the cranium (e.g., feeding, vision, olfactory sense, and brain processing) whereas the mandible is only involved in food acquisition and processing. Combined analysis of theoretical and empirical morphospaces for these skull data also show the lower anatomical disparity of felids and hyaenids compared to canids and ursids. This indicates that increasing specialization within the hypercarnivorous niche may constrain subsequent morphological and ecological flexibility. During the Cenozoic, similar skull traits appeared in different carnivoran lineages, generated by similar selection pressures (e.g., toward hypercarnivory) and shared developmental pathways. These pathways were likely the proximate source of constraints on the degree of variation associated with carnivoran skull evolution and on its direction.

Taxonomic membership frequencies exhibit distributions in which groups with few numbers of subtaxa are much more common in a clade than those with more subtaxa. Here, a “broken plate” model is developed to describe such taxonomic memberships; some higher taxonomic group (the plate) is randomly subdivided into intermediate taxonomic units (plate fragments), whose sizes are dependent on the number of taxonomic subunits that they each contain. Theoretical distributions of membership frequencies produced by this model yield a superior fit to data from both modern and fossil groups, as illustrated by classifications for primarily fossil brachiopods and entirely modern mammals. The nature of these distributions is consistent with the contention that Linnaean membership frequencies result from the random partitioning of taxonomic/morphologic space. Moreover, numbers of taxa contained within hierarchically equivalent groups are unrelated, as are membership numbers at taxonomically higher and lower levels of consideration. Agreement between observed taxonomic memberships and those anticipated from the random partitioning of diversity as described by the “broken plate” model bears directly on a number of fundamental questions including the significance of extreme polytypy and inferred causes of adaptive radiation within many taxonomic groups.

Species abundance data are of vital importance in paleontology, but fossil accumulations invariably represent a biased subset of original source communities. Efforts to quantify taphonomic biases are typically prevented by a lack of independent data on the ecological composition of prehistoric faunas. However, analysis of the continental Holocene record can provide a rare opportunity for independent calibration of fossil abundance patterns. We analyzed a comprehensive data set available for the Holocene avifauna of Sweden to investigate the relationship between species abundance in the recent fossil and zooarchaeological records and in prehistoric source communities, and to characterize the importance of different ecological factors in determining terrestrial vertebrate fossil abundances. The number of assemblages in which species occurred was compared with modern-day species abundance, annual residence, body mass, and ecological realm. Modern-day abundance is only one of several significant predictors of fossil abundance; the strongest predictor is body mass, and Holocene species abundance can be interpreted as a measure of species abundance in source communities for a given size class only. Our study represents one of the only direct attempts to quantify species abundance biases between fossil faunas and source communities, and has general applicability for a wide range of terrestrial vertebrate faunas.