We still have much to learn about the evolution of taxonomic diversity gradients through geologic time. For example, have latitudinal gradients always been as steep as they are now, or is this a phenomenon linked to some form of Cenozoic global climatic differentiation? The fossil record offers potential to address this sort of problem, and this study reconstructs latitudinal diversity gradients for the last (Tithonian) stage of the Jurassic period using marine bivalves. At this time of relative global warmth, bivalves were cosmopolitan in their distribution and the commonest element within macrobenthic assemblages.

Analysis of 31 regional bivalve faunas demonstrates that Tithonian latitudinal gradients were present in both hemispheres, though on a much smaller magnitude than today. The record of the Northern Hemisphere gradient is more complete and shows a steep fall in values at the tropical/temperate boundary; the Southern Hemisphere gradient exhibits a more regular decline in diversity with increasing latitude.

Tithonian latitudinal gradients were underpinned by a tropical bivalve fauna that comprises almost equal numbers of epifaunal and infaunal taxa. The epifaunal component was dominated by three pteriomorph families, the Pectinidae, Limidae and Ostreidae, that may be regarded as a long-term component of tropical bivalve diversity. Of the mixture of older and newer “heteroconch” families that formed the bulk of the infaunal component, the latter radiated spectacularly through the Late Cretaceous and Cenozoic to dominate tropical bivalve faunas at the present day. This pulse of heteroconch diversification, which was a major cause of the steepening of the bivalve latitudinal gradient, provides important evidence that rates of speciation may be negatively correlated with latitude.

Nevertheless, we cannot exclude the possibility that elevated extinction rates in the highest latitudes also contributed to the marked steepening of bivalve latitudinal gradients over the last 150 Myr. Rates of extinction within epifaunal bivalve taxa appear to have been higher in these regions through the Cretaceous period, but this was largely before any significant global climatic deterioration. Infaunal bivalve clades have had differential success over this time period in the polar regions. Whereas, in comparison with the Tropics, heteroconchs are very much reduced in numbers today, the anomalodesmatans are much better represented, and the protobranchs have positively thrived. We are beginning to appreciate that low temperature per se may not be a primary cause of elevated rates of extinction. Food supply may be an equally important control on both rates of speciation and extinction; those bivalves that have been able to adapt to the extreme seasonality of food supply have flourished in the polar regions.

Iterative evolutionary changes are of special interest because they imply that the recurring morphological changes had a cause that also repeated itself and might therefore be possible to uncover. We describe a set of iterative morphological changes in melanopsid gastropods from the ancient, long-lived Lake Pannon. First in the Pannonian Age, and again in the Pontian Age approximately three million years later, a smooth-shelled ancestor gave rise to a shouldered descendant. In both cases, the morphological change was probably coincident with a shift from habitats just outside the lake (e.g., rivers and streams) to habitats within the lake itself. Many other convergent examples exist in which a smooth-shelled river dweller is closely related to a shouldered and/or ribbed lacustrine snail. The frequency of this type of morphological change suggests that it has an adaptive basis; response to differing predators or hydrodynamic conditions seem the most plausible explanations, but the functional nature of these morphological changes remains unknown.

The late Paleocene-early Eocene transition was characterized by a long period of global warming that culminated with the highest temperatures of the Cenozoic. This interval is associated with a significant increase in plant diversity in temperate latitudes. However, data from tropical regions remain largely unknown. The record of pollen and spore diversity across the late Paleocene to the early middle Eocene of eight sections in central and eastern Colombia was analyzed. Several techniques, including range-through method, rarefaction, bootstrap, detrended correspondence analysis, and Shannon index, were used to assess the significance of the observed diversity pattern. The palynofloral record indicates that the lower to middle Eocene contains a significantly higher palynofloral diversity than the underlying upper Paleocene strata. This pattern is maintained after accounting for sample size, number of samples/time unit, lithofacies, and depositional systems. Eocene palynofloras have higher alpha and beta diversities and a higher equitability than Paleocene palynofloras. This increase in diversity is the product of a gradual increase in the rate of first appearances and a gradual decrease in the rate of last appearances. The early to middle Eocene increase in diversity, as well as the increase in spore abundance and diversity, suggests that tropical (equatorial) climate became wetter during the early to middle Eocene. This interpretation favors causes for early Eocene warming that do not involve significant increases in greenhouse gases. Samples from strata associated with the Paleocene/Eocene thermal maximum were barren for palynomorphs, and the effects of this climatic event on tropical vegetation remains uncertain.

Cell division is a key biological process in growth, morphogenesis, and reproduction. Despite our improved understanding of the genetics and dynamics of cell division in all major groups of living organisms, paleontological evidence for cell division is largely restricted to silicified (and some carbonaceous) algae and vascular plants where three-dimensional observation is possible. Animal cell division has been documented in the fossil record to a lesser extent; however, such knowledge is highly desirable in the recently revitalized field of evolutionary developmental biology. Two fundamentally different mitotic cell division topologies are preserved in late Neoproterozoic Doushantuo phosphorites (ca. 550–600 million years old) in South China. Doushantuo algal cells (∼20 μm in diameter) are successively cleaved by mitotic division planes that are offset but not deformed by subsequent cytokinesis. Mitotic division planes in successively cleaving Doushantuo animal embryos (several hundred microns in diameter) are also offset. However, in sharp contrast to Doushantuo algae, Doushantuo animal blastomeres repeatedly shift to mechanically stable configurations by disturbing preexisting division planes. This divergence reflects the underlying cytological and developmental differences between algae and animals. Specifically, the presence/absence of rigid cell walls and different cytokinetic mechanisms, coupled with mechanics at mitotic offsets, contribute to the diverging mitotic topologies in Doushantuo algae and animal embryos. These findings not only confirm previous interpretation of Doushantuo fossils but also provide direct paleontological evidence of cell movement in the development of these early animal fossils.

Cruising speeds of Mesozoic marine reptiles have been estimated in the past by using a mathematical model of energetic equilibrium during steady swimming. This method suffered from a significant tendency to overestimate speeds of extant cetaceans for no clear reason, which raised questions about the validity of the approach itself. The present study identifies the factors that caused this shortcoming and proposes corrections and some additional modifications. These include the use of more accurate body shape models, updated metabolic rate models, and optimal rather than critical swimming speeds. The amended method successfully approximates published optimal speeds of several extant marine vertebrates, including cetaceans, showing that the basic framework of the energetic approach is valid. With this confirmation, the method was applied to Mesozoic marine reptiles, by assuming three different metabolic rate categories known in extant swimming vertebrates (i.e., average ectothermic, raised ectothermic, and marine endothermic levels). The results support previous inferences about the relative cruising capabilities of Mesozoic marine reptiles (i.e., ichthyosaurs > plesiosaurs > mosasaurs). Stenopterygius, a thunniform ichthyosaur, was probably capable of cruising at a speed at least comparable to those reported for some extant thunniform teleosts with similar diets (∼1 m/second).

Multiple regression models were developed for seasonal test inputs to, and preservation of, marsh foraminiferal assemblages for a two-year period at Bombay Hook National Wildlife Refuge (BHNWR; Smyrna, Delaware). Seasonal assemblages were quite variable and yielded poor regression models. However, signal/noise ratios were amplified using artificially time-averaged (ATA) assemblages, in which separate dead and live abundances of the most abundant species were summed for all seasons. Regression models that used ATA species abundances to retrodict original sample depths accounted for up to ∼99% (p < 0.0001) and ∼91% (p < 0.023) of the variation of dead and live ATA assemblages, respectively, and usually retrodicted sample depths within 2–3 cm of actual depths.

Artificially time-averaged assemblages were also used to extract multidecadal- to centennial-scale sea-level signals from near-surface assemblages at BHNWR formed during the past few centuries. The BHNWR sea-level curve closely resembles one previously published for marshes in Clinton, Connecticut (also based on foraminifera). The technique of artificial time-averaging therefore links the temporal scales of ecology and paleobiology by extracting high-resolution paleoenvironmental signals preserved in the fossil record.

Understanding of evolutionary changes in the vertebrate skull is greatly influenced by the knowledge of ontogeny. Extant amphibians are an outstanding example in this field, because their life cycles are complex and have been intensively studied. At the same time, fossil material of Paleozoic amphibians has become available that sheds light on the ontogeny of a long-extinct clade, prompting comparison with recent forms. In this paper, the formation of the skull of a Paleozoic amphibian (the branchiosaurid temnospondyl Apateon) is analyzed in comparison with that of an extant salamander (the hynobiid Ranodon). Both temporal patterns (sequence of ossification) and spatial patterns (morphological changes) are described. The general results are that (1) the sequence of ossification is similar in many aspects, and (2) most dermal bones share fundamental similarities in morphogenesis, although sometimes the morphological result in adults may differ conspicuously.

The comparison reveals that the parasphenoid, premaxillae, maxillae, frontals, parietals, squamosals, and prefrontals are strikingly similar in their mode of growth. In particular, the appearance of the earliest primordia and the subsequent stages of morphological transformation are almost identical. The development of the pterygoids and nasals is different in the earliest stages, but the ontogenetic trajectories converge in later stages. In Ranodon and other transforming urodeles, the pterygoids and vomers experience extensive resorption during metamorphic climax, whereas in branchiosaurids the morphology of this region remains stable throughout ontogeny.

In the sequence of cranial ossifications, the early appearance of the premaxilla and tooth-bearing elements of the palate characterize both genera, but the maxilla forms much later in Ranodon. The ectopterygoid, absent in all salamanders, is the last palatal element to appear in branchiosaurids. In the skull roof, the parietals, frontals, and squamosals are the first bones to form in both groups. Conversely, the circumorbital elements and tabular are among the last ossifications in branchiosaurids, and the prefrontal and lacrimal (the only circumorbital bones present in salamanders) form within the same interval in urodele ontogeny. The septomaxilla is the last dermal element to ossify in both groups. Comparison with caecilians and anurans indicates that salamanders are much more similar to Paleozoic branchiosaurids than to other extant lissamphibians. A major difference between salamanders and branchiosaurids is that the morphology of the latter is much less affected by developmental perturbations, such as larval specializations and metamorphosis.