Biodiversity has two principal components: richness (the number of taxa) and evenness (the distribution of individuals among taxa). Both of these attributes are critical in defining community composition and structure, but evenness may be particularly important for ecological reasons, especially in the context of a possible evolutionary increase in richness. Here I examine abundance data in well-preserved Cambrian and Ordovician benthic marine assemblages from mixed carbonate-shale and shale lithofacies deposited below normal wave base in North America. Evenness increases significantly from the Cambrian to the Ordovician in these assemblage data. The increase cannot easily be attributed to differences in sample size, lithology, water depth, or other sample characteristics. There is relatively little variation in evenness among stages within the Cambrian and Ordovician periods. Much of the within-period variance in evenness appears to arise from environmental and / or taphonomic differences that may exist between stratigraphic formations, but some variance may also be ascribed to the degree of taxonomic and / or ecological overlap among dominant taxa. Specifically, assemblages that are co-dominated by taxa from the same class or order tend to have lower evenness than assemblages dominated by genera from different higher taxa, but the effect is not strong in these data.

The Cambrian and Paleozoic evolutionary faunas show similar, but out-of-phase, patterns of evenness within assemblages. Both faunas have comparably low evenness early in their history but then increase to similar, higher evenness values. In the case of the Cambrian fauna, the increase occurs in the Late Cambrian or Early Ordovician. In the Paleozoic fauna, the increase appears to occur after the lower Arenigian.

We present a quantitative analysis of megafloral turnover across the Cretaceous/Paleogene boundary (K/T) based on the most complete record, which comes from the Williston Basin in southwestern North Dakota. More than 22,000 specimens of 353 species have been recovered from 161 localities in a stratigraphic section that is continuous across and temporally calibrated to the K/T and two paleomagnetic reversals. Floral composition changes dynamically during the Cretaceous, shifts sharply at the K/T, and is virtually static during the Paleocene. The K/T is associated with the loss of nearly all dominant species, a significant drop in species richness, and no subsequent recovery. Only 29 of 130 Cretaceous species that appear in more than one stratigraphic level (non-singletons) cross the K/T. Only 11 non-singletons appear first during the Paleocene. The survivors, most of which were minor elements of Cretaceous floras, dominate the impoverished Paleocene floras. Confidence intervals show that the range terminations of most Cretaceous plant taxa are well sampled. We infer that nearly all species with last appearances more than about 5 m below (approximately 70 Kyr before) the K/T truly disappeared before the boundary because of normal turnover dynamics and climate changes; these species should not be counted as K/T victims. Maxima of last appearances occur from 5 to 3 m below the K/T. Interpretation of these last appearances at a fine stratigraphic scale is problematic because of local facies changes, and megafloral data alone, even with confidence intervals, are not sufficient for precise location of an extinction horizon. For this purpose, we rely on high-resolution palynological data previously recovered from continuous facies in the same sections; these place a major plant extinction event precisely at the K/T impact horizon. Accordingly, we interpret the significant cluster of last appearances less than 5 m below the K/T as the signal of a real extinction at the K/T that is recorded slightly down section. A maximum estimate of plant extinction, based on species lost that were present in the uppermost 5 m of Cretaceous strata, is 57%. Palynological data, with higher stratigraphic but lower taxonomic resolution than the megafloral results, provide a minimum estimate of a 30% extinction. The 57% estimate is significantly lower than previous megafloral observations, but these were based on a larger thickness of latest Cretaceous strata, including most of a globally warm interval, and were less sensitive to turnover before the K/T. The loss of one-third to three-fifths of plant species supports a scenario of sudden ecosystem collapse, presumably caused by the Chicxulub impact.

Ammonoid taxonomic history is a well-documented series of diversity “booms and busts”, but the effect of this taxonomic pattern on morphological evolution has not received as much attention. We know particularly little about the effects of the Permo-Triassic mass extinction on the subsequent morphological evolution of the ammonoids.

Morphological data from 322 Triassic ammonoid genera (322 species) were combined with previously published data for Late Paleozoic ammonoids. This data set of 601 specimens was subjected to PCA to assess (1) the effects of the Permo-Triassic taxonomic bottleneck on ammonoid morphological evolution, by comparing the strength and sign of correlations in the Paleozoic and in the Triassic, and (2) whether the Triassic ammonoids immediately shifted to Jurassic morphologies, retained Paleozoic morphologies, or evolved in a more “mosaic” fashion.

The Triassic ammonoids recapitulate the late Paleozoic W-D distribution, but in S-D space their distribution closely foreshadows that of the Lower Jurassic ammonites. Given these findings it is clear that at even the level of shell geometry the Triassic ammonoids evolved in a mosaic fashion

The Triassic ammonoids reoccupy, and extend, the volume of morphospace occupied by the Paleozoic ammonoids. Overall, Triassic correlations between pairs of characters are weaker, but not significantly so.

Recent molecular analyses indicate that many reef coral species belong to hybridizing species complexes or “syngameons.” Such complexes consist of numerous genetically distinct species or lineages, which periodically split and/or fuse as they extend through time. During splitting and fusion, morphologic intermediates form and species overlap. Here we focus on processes associated with lineage fusion, specifically introgressive hybridization, and the recognition of such hybridization in the fossil record. Our approach involves comparing patterns of ecologic and morphologic overlap in genetically characterized modern species with fossil representatives of the same or closely related species. We similarly consider the long-term consequences of past hybridization on the structure of modern-day species boundaries.

Our study involves the species complex Montastraea annularis s.l. and is based in the Bahamas, where, unlike other Caribbean locations, two of the three members of the complex today are not genetically distinct. We measured and collected colonies along linear transects across Pleistocene reef terraces of last interglacial age (approximately 125 Ka) on the islands of San Salvador, Andros, and Great Inagua. We performed quantitative ecologic and morphologic analyses of the fossil data, and compared patterns of overlap among species with data from modern localities where species are and are not genetically distinct.

Ecologic and morphologic analyses reveal “moderate” overlap (>10%, but statistically significant differences) and sometimes “high” overlap (no statistically significant differences) among Pleistocene growth forms (= “species”). Ecologic analyses show that three species (massive, column, organ-pipe) co-occurred. Although organ-pipes had higher abundances in patch reef environments, columnar and massive species exhibited broad, completely overlapping distributions and had abundances that were not related to reef environment. For morphometric analyses, we used multivariate discriminant analysis on landmark data and linear measurements. The results show that columnar species overlap “moderately” with organ-pipe and massive species. Comparisons with genetically characterized colonies from Panama show that the Pleistocene Bahamas species have intermediate morphologies, and that the observed “moderate” overlap differs from the morphologic separation among the three modern species. In contrast, massive and columnar species from the Pleistocene of the Dominican Republic comprise distinct morphologic clusters, similar to the modern species; organ-pipe species exhibit “low” overlap (<10%, only at species margins) with columnar and massive species.

Assuming that “moderate” overlap implies hybridization and “high” overlap implies more complete lineage fusion, these results support the hypothesis of hybridization among species within the complex in the Bahamas during the Pleistocene. Hybridization involved introgression of three distinct evolutionary lineages, in association with Pleistocene sea level and temperature fluctuations, and appears to have been limited geographically primarily to the Bahamas and the northern Caribbean. Thus, not only does the structure of species boundaries within the complex vary geographically, but these geographic differences may have persisted since the Pleistocene.

Fossil samples almost always accumulate on timescales much longer than the life spans of individual organisms. This time-averaging has the potential to inflate the variability of fossil samples because phenotypic changes may occur during the interval of sample accumulation. Although many have realized that this effect may increase the variance of fossil samples, only qualitative predictions have been possible thus far. In this paper, I assume a simple but general Markovian model of evolution to derive expressions that predict the effects of time-averaging on trait variance and covariance. For lineages evolving as an unbiased random walk, phenotypic variance in samples increases linearly with the duration of time-averaging, at a slope that is proportional to the evolutionary rate. Although based on a very simple model of specimen input into time-averaged samples, the expressions relating variance, time-averaging, and evolutionary rate prove to be robust or adaptable to more realistic assumptions.

The theoretical findings are applied to analyze variation in a set of samples of the deep-sea ostracode Poseidonamicus miocenicus that vary greatly in temporal acuity. The relationship between duration of time-averaging and morphological variance is used to estimate evolutionary rates of two morphological traits in these ostracodes. These rate estimates are similar to those calculated independently from differences between presumed ancestor-descendant pairs of populations. Consistent with other studies of variability and time-averaging, these data suggest that phenotypic variance tends to increase rather slowly with the duration of time-averaging, indicating that greatly inflated variance is expected only in samples that have accumulated over many tens of thousands of years.

Morphometric analyses of sauropod limbs have the potential to illuminate functional aspects of sauropod locomotion and paleobiology. However, analyses of sauropod limb dimensions typically show few discernible morphological trends because of large size differences among the individuals in a sample. For sauropods, combined analyses of both limb dimension and shape may be more desirable. Numerous humeri and femora from Apatosaurus, Diplodocus, and Camarasaurus provide an opportunity to explore and compare limb morphology in contemporaneous, sympatric sauropods. Thin-plate splines were used to analyze landmark-based shape differences in combination with traditional morphometrics. The aims of the analysis were (1) to determine if humerus and femur shape were significantly different among the genera; (2) to determine where shape changes occurred; and (3) to infer the basic functional implications of the shape differences using an Extant Phylogenetic Bracket approach. Few differences were detected among the genera using traditional morphometric analyses, and linear regression revealed a predominantly isometric relationship between most measurement variables and element size. Thin-plate splines revealed significant shape differences among the taxa. Apatosaurus humeri and femora were the most robust, with expanded regions for muscle insertion and more distally placed deltopectoral and caudofemoral landmarks. Diplodocus humeri and femora were gracile, with more proximally located landmarks of muscular insertion. Camarasaurus humeri were surprisingly gracile, with a less extensive deltopectoral crest, but had more robust femora similar to those of Apatosaurus. Few differences distinguished juvenile from adult specimens. These data suggest some locomotor differences were present among the three genera.

Distributional patterns of eye lens variation in different morphs of the phacopine trilobite Acuticryphops acuticeps (Kayser 1889) are investigated. The specimens were collected from the latest Frasnian preceding the Upper Kellwasser (UKW) global extinction event in the Frasnian-Famennian (Late Devonian) stratotype section at Coumiac, southern France. In six successive populations a gradual reduction in the mean number of lenses occurs within the short time span of a single conodont Zone. This morphological change cannot be imputed either to the size of individual specimens or to variation in cephalic morphology. Thus morphs with different numbers of eye lenses are considered intraspecific. However, the intrapopulational percentage relation between morphs does not remain constant, as the coefficient of variation in lens number continuously increases from one population to the next. Cases of individuals with asymmetric eyes appear in the two latest assemblages prior to the UKW level. The fact that the total variation of the shape of the cephalon is not affected in these assemblages suggests that the morphological changes observed in the visual complex may account for the relaxation of selective pressures on this trait. Such a change in the regime of selection would have been accompanied by a lessening of the processes that control the development of this trait. As the phenomenon of eye reduction is not restrained by local conditions at Coumiac (Montagne Noire) but occurs contemporaneously to various extents in other crustal blocks such as Rhenish Slate Mountains (Avalonia), Thuringia (Armorica), and Morocco (Northern Gondwana), it is considered as constituting an adaptation to global eustatic deepening that occurred in the terminal Frasnian just before the global extinction event.