Short-term fluctuations in the diversification rate of Paleozoic marine animal genera are more strongly correlated with extinction-rate variation than with origination-rate variation. Diversity dynamics are strikingly different in the Mesozoic and Cenozoic, when variation in origination is more important than extinction. Data on the lithologic context of taxonomic occurrences in the Paleobiology Database are used to assess the substrate affinities of Paleozoic genera. The greater role of extinction-rate variation in the Paleozoic is found to characterize genera with an affinity for carbonate substrates but not those that prefer terrigenous clastic substrates. It is therefore plausible that the Paleozoic to post-Paleozoic shift in diversity dynamics is underlain in part by the secular decline in the relative areal extent of carbonate environments, and the concomitant decline in the relative diversity of carbonate- versus clastic-loving taxa.

Studies of taxonomic diversity over time commonly count and compare first- and last-appearance data (FADs and LADs) over a succession of temporal intervals, and interpret them with respect to taxon origination and extinction. Singleton taxa, which first appear and last appear in the same temporal interval, are often removed from analyses because they might result from preservational biases rather than evolutionary processes, or they might represent non-independent FADs and LADs. Should singleton taxa always be excluded? We argue that in the case of Paleozoic terebratulide brachiopods, although they may be sensitive to biases in sampling intensity, singleton genera should be included in diversity studies because they do not appear to result from more typical biases, such as Lagerstätten and temporal interval length, that arguably can result in artificially high numbers of singleton genera.

Singleton genera can be critical and effective when used to test hypotheses regarding the existence and generation of latitudinal diversity gradients. Contrary to the anti-tropical diversity pattern of modern articulated brachiopods, Paleozoic terebratulides show a latitudinal diversity gradient that peaks in the Tropics. The hypothesis that the Tropics are either a diversity source or sink can be tested by comparing FAD and LAD latitudes. For singleton genera, FAD and LAD latitudes are taken from the same data points and must be removed for statistical comparisons to be valid. We suggest that taxon age distributions can accommodate singleton data, as the taxon age metric considers origination and extinction simultaneously. We generated taxon age distributions to test the hypothesis that the observed Paleozoic diversity gradient results from a latitudinal bias in generic turnover rate. We discovered that singletons are not randomly distributed over latitude, with proportionally more singleton genera occurring in the Tropics. In this case, singleton genera may reflect rapid evolutionary turnover of taxa, rather than simply preservational bias. Methods that can accommodate singleton taxa should be used to study the diversity of Paleozoic terebratulides and possibly other well-skeletonized marine metazoans.

Short-term variations in rates of taxonomic extinction and origination in the fossil record may be the result of true changes in rates of turnover, variable rates of fossil preservation, or some combination of the two. Here, positive extinction and origination rate excursions among Phanerozoic marine animal genera are reexpressed in terms of the amount of normal, background time they represent. In addition to providing a background-adjusted calibration of rate intensities, this reexpression determines the durations of sampling gaps that would be required to explain entirely all observed rate excursions as sampling artifacts. This possibility is explored by analyzing a new compilation of the timing and duration of sedimentary hiatuses in North America.

Hiatuses spanning more than approximately one million years (Myr) in the marine sedimentary rock record have a mean duration of 73 Myr. There are two major hiatus types—those that form in response to long-duration tectonic cycles and that bound the major Sloss-scale sequences (mean duration 200 Myr), and those that form in response to shorter-duration changes in sediment accommodation space and that occur within major Sloss-scale sequences (mean duration less than 23 Myr). The latter are approximately exponentially distributed and have a mean duration that is comparable to the mean duration of intervening sedimentary rock packages.

Although sedimentary hiatuses are generally long enough in duration to accommodate the hypothesis that short-term variations in rates of genus origination and extinction are artifacts of sampling failures at major unconformities (“Unconformity Bias” hypothesis), the observed evolutionary rates are not correlated with hiatus durations. Moreover, hiatuses that follow the major mass extinctions are not long in comparison to most other non–mass extinction intervals. These results do not support the hypothesis that hiatuses at major unconformities alone have artificially clustered genus first and last occurrences, thereby causing many of the documented statistical similarities between the temporal structure of the sedimentary rock record and macroevolutionary patterns. Instead, environmental changes related to the expansion and contraction of marine environments may have been the primary forcers of both biological turnover and the spatio-temporal pattern of sediment accumulation. Fully testing this “Common Cause” hypothesis requires quantifying and overcoming lingering taxonomic, biostratigraphic, facies, and numerous other biases that are both inherent in geologic data and imposed by imperfect knowledge of the fossil record.

Most studies of extinction selectivity have focused on mass extinctions. Here we analyze the patterns of susceptibility to extinction during the late Pliocene and Pleistocene of California in the family Pectinidae. The Pectinidae declined in diversity from a high of 32 species in the late Pliocene to the current level of 11 species living in the California region, indicating that the composition of the living fauna has been shaped primarily by extinction in the last several million years. Extinction appears to have occurred in one large pulse, but because of uneven sampling we were unable to resolve the timing further and have analyzed the patterns of extinction treating the late Pliocene through middle Pleistocene as a single period of elevated extinction. Extinctions were not random with regard to taxonomic relationships. Species-level extinctions were higher in more speciose genera, but these genera were buffered against genus-level extinctions. This resulted in a disproportionately large number of monospecific genera in the living fauna. In addition, extinctions were not random with regard to body size, with large species preferentially surviving. This selectivity pattern is evident only when clade membership is taken into account; when analyzed across the entire family, no pattern of size selectivity was apparent. Our results suggest that (1) patterns of extinction selectivity at the genus level may be a poor proxy for species-level patterns, and (2) whole-fauna analyses may not uncover strong selectivity within lineages.

This study examines the morphology and function of hindlimb plumage in Archaeopteryx lithographica. Feathers cover the legs of the Berlin specimen, extending from the cranial surface of the tibia and the caudal margins of both tibia and femur. These feathers exhibit features of flight feathers rather than contour feathers, including vane asymmetry, curved shafts, and a self-stabilizing overlap pattern. Many of these features facilitate lift generation in the wings and tail of birds, suggesting that the hindlimbs acted as airfoils. A new reconstruction of Archaeopteryx is presented, in which the hindlimbs form approximately 12% of total airfoil area. Depending upon their orientation, the hindlimbs could have reduced stall speed by up to 6% and turning radius by up to 12%. Presence of the “four-winged” planform in both Archaeopteryx and basal Dromaeosauridae indicates that their common ancestor used fore- and hindlimbs to generate lift. This finding suggests that arboreal parachuting and gliding preceded the evolution of avian flight.

Cutan, a resistant non-hydrolyzable aliphatic biopolymer, was first reported in the cuticle of Agave americana and has generally been considered ubiquitous in leaf cuticles along with the structural biopolyester cutin. Because leaves and cuticles in the fossil record almost always have an aliphatic composition, it was argued that selective preservation of cutan played an important role in leaf preservation. However, the analysis of leaves using chemical degradation techniques involving hydrolysis to test for the presence of cutan reveals that it is absent in 16 of 19 taxa (angiosperm and gymnosperm), including many previously reported to contain cutan on the basis of pyrolysis data. Cutan is clearly much less widespread in leaves than previously thought, and its presence or absence does not exert any major bias on the preservation of leaves in the fossil record. In the absence of cutan, other constituents—cutin, plant waxes, and internal plant lipids—are incorporated into the geomacromolecule and contribute to the formation of a resistant aliphatic polymer by in situ polymerization during diagenesis.

For well over 50 years, the telome theory of Walter Zimmermann has been extremely influential in interpreting the evolutionary history of land plant architecture. Using the “telome/ mesome” distinction, and the concept of universal “elementary processes” underlying the change in form in all plants, the theory was an ambitious synthesis based on the proposition that evolutionary change might be understood by a simple set of developmental or evolutionary rules. However, a major problem resides in deciding exactly how assertions of change are to span both developmental and evolutionary domains simultaneously, and, we argue, the theory critically fails testability as a scientific theory. Thus, despite continued popularity for the descriptive terms derived from the theory in evolutionary studies of early land plants, time has come to replace it with a more explicit, testable approach. Presented here is an attempt to clarify perhaps the most important issue raised by the telome theory—whether simple changes in basic developmental processes suffice to describe much of early land plant evolution. Considering the morphology of Silurian–Devonian fossil members, it is hypothesized that early land plants possessed a common set of developmental processes centered on primary growth of shoot apical meristems. Among these were (1) the capacity to monitor and act upon internal physiological status here modeled as “apex strength,” (2) a mechanism for allocation of apex strength in a context-dependent way at each point of branching, (3) a rule for context-dependent apex angle for branches, (4) a largely invariant phyllotaxis unrelated to physiological status, and (5) a simple switch for terminating primary growth, based in part on genetics. Implemented as a set of developmental rules within a simple L-system model, these aspects of primary development in plants determine a sizable range of resultant morphologies, some of which are highly reminiscent of the early fossils. Thus, some support is found, perhaps, for Zimmermann's intuition. However, traditional concepts of growth patterns in plants, including the contrast between epidogenesis and apoxogenesis, require updating. In our reformulation, developmental processes, stated as rules of developmental dynamics, together constitute what we term the plant's developmental state. Using a hypothetico-deductive format, one may hypothesize intrinsic (or genetic) developmental processes that play out as realized developmental activity in specific spatial/temporal contexts, as modified by multiple context factors. The resultant plant morphology is highly dependent on multiple and simultaneous pathway ontogenetic trajectories. Within a likely set of developmental rules reasonably inferred from plant development, some of Zimmermann's elementary processes are perhaps recognizable whereas others are not. Progressively “overtopped” morphologies are easily produced by modifying intrinsic branch allocation. However, even so, the other developmental rules have a profound effect on final architectures. Planate architectures and circination vernation, often treated as special cases by plant morphologists, are perhaps better understood in terms of recurrent or iterative developmental relationships. Much analytic work remains before a completely specified system of rules will emerge. A well-articulated relationship between ontogeny and phylogeny remains fundamentally important in assessing evolutionary change. Fossil and living plants make it abundantly clear that current evolutionary concepts involving modification of a single ontogenetic trajectory from ancestor to descendant need to be greatly expanded into consideration of the entire logical geometry of causation in development. A mechanism for testing is also required that need not wait for complete elucidation at the molecular level. The relative simplicity of plant development, combined wit

The analysis of morphological disparity and of morphospace occupation through the macroevolutionary history of clades is now a major research program in paleobiology, and increasingly so in organismal and comparative biology. Most studies have focused on the relationship between taxonomic diversity and morphological disparity, and on ecological or developmental controls. However, the geographic context of diversification has remained understudied. Here we address geography quantitatively. Diversity, disparity, and paleogeographic dispersion are used to describe the evolutionary history of an extinct echinoderm clade, the class Stylophora (cornutes, mitrates), from the Middle Cambrian to the Middle Devonian (about 128 Myr subdivided into 12 stratigraphic intervals). Taxonomic diversity is estimated from a representative sample including 73.3% of described species and 92.4% of described genera. Stylophoran morphology is quantified on the basis of seven morphometric parameters derived from image analysis of homologous skeletal regions. Three separate principal coordinates analyses (PCO) are performed for thecal outlines, plates from the lower thecal surface, and plates from the upper thecal surface, respectively. PCO scores from these three separate analyses are then used as variables for a single, global, meta-PCO. For each time interval, disparity is calculated as the sum of variance in the multidimensional morphospace defined by the meta-PCO axes. For each time interval, a semiquantitative index of paleogeographic dispersion is calculated, reflecting both global (continental) and local (regional) aspects of dispersion.

Morphospace occupation of cornutes and mitrates is partly overlapping, suggesting some morphologic convergences between the two main stylophoran clades, probably correlated to similar modes of life (e.g., symmetrical cornutes and primitive mitrocystitids). Hierarchical clustering allowed the identification of three main morphological sets (subdivided into 11 subsets) within the global stylophoran morphospace. These morphological sets are used to analyze the spatiotemporal variations of disparity. The initial radiation of stylophorans is characterized by a low diversity and a rapid increase in disparity (Middle Cambrian–Tremadocian). The subsequent diversification involved filling and little expansion of morphospace (Arenig–Middle Ordovician). Finally, both stylophoran diversity and disparity decreased relatively steadily from the Late Ordovician to the Middle Devonian, with the exception of a second (lower) peak in the Early Devonian. Such a pattern is comparable to that of other Paleozoic marine invertebrates such as blastozoans and orthid brachiopods. During the Lower to Middle Ordovician, the most dramatic diversification of stylophorans took place with a paleogeographic dispersion essentially limited to the periphery of Gondwana. In the Late Ordovician, stylophorans steadily extended toward lower paleolatitudes, and new environmental conditions, where some of them radiated, and finally survived the end-Ordovician mass extinction (e.g., anomalocystitids). This pattern of paleobiogeographic dispersion is comparable to that of other examples of Paleozoic groups of marine invertebrates, such as bivalve mollusks.