The evolution of higher taxa among early Paleozoic gastropods is similar to that among early metazoans as a whole, as higher taxa diversified rapidly and early. There are two issues pertinent to this pattern. First, were greater morphologic changes concentrated in the early phases of evolution? Second, does the pattern better fit models of increasing phylogenetic constraints or increasing ecologic restrictions? This paper presents a phylogeny-based method designed to test whether amounts of morphologic evolution decreased over time. It also explores whether the data better fits models of increasing phylogenetic (i.e., developmental or genetic) constraint or increasing ecologic restriction. Two metrics of morphologic separation (i.e., the morphologic difference between sister-species) are used: (1) Euclidean distance in morphospace and (2) transition magnitude. The latter metric is calculated by a multivariate analysis of sister-species contrasts, which determines both types and magnitudes of morphologic transitions. The advantage of using transition magnitudes is that it balances the effects of transitions that either affect more morphometric characters or occur more frequently. Both metrics indicate that larger morphologic separations between sister-species were concentrated early in gastropod evolution. Among gastropods, gross shell morphology often reflects basic trophic strategy and function whereas basic internal anatomy does not. Transition magnitudes can be broken down into transitions associated with differences in basic trophic strategies and shell functional biology (“external”), and those associated with differences in basic internal anatomy (“internal”). Internal transition magnitudes show a highly significant decrease over time (p < 10–04) whereas external transition magnitudes show a much less significant decrease over time (p < 0.10) and no significant decrease after the earliest Ordovician (p ≅ 0.50). The results therefore suggest that increasing phylogenetic constraints played a greater role in the early evolution of gastropods than did increasing ecologic ones.

Several metrics, including average difference among species, range of occupied morphological space, and number of character-state combinations, are used to investigate morphological diversification in Paleozoic crinoids. Despite several phases of taxonomic diversification, the maximal level of disparity reached in the Ordovician remained essentially unsurpassed. Although new regions in morphological space were occupied after the Devonian, these were not as extensive as those that had been evacuated prior to the Carboniferous. This discordance between extensive total morphological change and limited net change further supports previous arguments for the importance of morphological constraints in crinoid evolution. Major changes in the occupation of morphological space correspond with changes in taxonomic diversity within certain higher taxa. The extent to which advanced cladids (Poteriocrinina) appear to expand into new morphological space is exaggerated by the large number of very similar species in this group. If fewer species are sampled, by considering only those forms that differ from each other by at least some prescribed amount, poteriocrines appear to be less extreme morphologically. In contrast, other groups that seem to occupy unique regions in morphological space continue to do so even if fewer of them are sampled. Major crinoid clades—Camerata and Cladida+Flexibilia—do not show the same evolutionary pattern as Crinoidea, but instead exhibit a more gradual diversification of morphology. This observation provides additional support for the existence of qualitative differences among taxa of different rank.

Documenting past environmental disturbances will provide a very incomplete explanation of extinctions until more data on intrinsic (e.g., phylogenetic) responses to disturbances are collected. Taxonomic selectivity can be used to infer phylogenetic inheritance of extinction-biasing traits. Selectivity patterns among higher taxa, such as between mammals and bivalves, are well documented. Selectivity patterns among lower taxa (genus, species) have great potential for understanding the dynamics underlying higher taxic turnover. Two echinoid data sets, of fossil and living taxa, indicate that species extinctions do not occur randomly within genera. Reverse rarefaction estimates of past species extinction rates assume random species extinction within higher taxa, so these widely cited extinction estimates may be inaccurate. Revised estimates based on a simulated curve imply that past species extinctions rates may be 6%–15% lower than previously cited. Possible causes for the observed selectivity patterns are discussed. These include nonrandom phylogenetic nesting of species with traits often cited as enhancing extinction vulnerability, into certain taxa. Such traits include low abundance, large body size, narrow niche breadth, and many others. Phylogenetic nesting of extinction-biasing traits at many taxonomic levels does not predict that a dichotomy of mass-background selectivity based on a few traits will occur. Instead, it predicts patterns of selectivity at many taxonomic levels, and at many spatio-temporal scales.

Documentation of morphologic variation within and among fossil species (and larger clades) provides fundamental data needed for studies of evolution, paleoecology, and the systematic foundation required for most fields of paleobiology. In paleontological (and, frequently, biological) studies, morphologic variation is used as a general proxy for genetic variation. Although the occurrence of ecophenotypic variation is well appreciated in these studies, it is only with the use of colonial (clonal) organisms that the scope and significance of phenotypic variation can be evaluated directly. Systematic evaluation of intracolonial morphologic variation (transects through growth series) can yield insights about ecophenotypic variation in bryozoans and suggest the most appropriate methods for data collection in paleobiologic and taxonomic studies.

In this study, morphological conservatism is documented within local segments of bryozoan colonies; each zooid is generally more similar to adjacent zooids than to distant zooids within the same colony. One region of a colony, therefore, can be more similar to a region of a different colony than to a distant region of its own colony. Variation within one colony does not, however, represent the total variation among a group of specimens, indicating a colonial level of morphologic control (genetic or macroenvironmental) over morphogenesis. Directional morphogenetic gradients (associated with successive ontogenetic histories) are not recognized in these specimens, but fluctuating trends within colonies (some cyclic), were observed and are indicative of changing microenvironmental influence during skeletal formation. In order to best document morphologic variation within a population, for any type of paleobiological study, individual measurements should be widely distributed over large colony fragments and (or) a minimal number of measurements collected from each of a large number of smaller fragments.

Direct extrapolation of these results to non-colonial organisms is not appropriate at this time. However, additional, related studies with bryozoans and other colonial organisms (e.g., corals, graptolites), should provide a greater, general appreciation of relationships between morphology and genetics.

Interior chamber walls of ammonites range from smoothly undulating surfaces in some taxa to complex surfaces, corrugated on many scales, in others. The ammonite suture, which is the expression of the intersection of these walls on the exterior of the shell, has been used to assess anatomical complexity. We used the fractal dimension to measure sutural complexity and to investigate complexity over evolutionary time and showed that the range of variation in sutural complexity increased through time. In this paper we extend our analyses and consider two new parameters that measure the range of scales over which fractal geometry is a satisfactory metric of a suture. We use a principal components analysis of these parameters and the fractal dimension to establish a two-dimensional morphospace in which the shapes of sutures can be plotted and in which variations and evolution of suture morphology can be investigated. Our results show that morphospace coordinates of ammonitic sutures correspond to visually perceptible differences in suture shape. However, three main classes of sutures (goniatitic, ceratitic, and ammonitic) are not unambiguously discriminated in this morphospace. Interestingly, ammonitic sutures occupy a smaller morphospace than other suture types (roughly one-half of the morphospace of goniatitic and ceratitic sutures combined), and the space they occupied did not change dimensions from the Jurassic to the late Cretaceous.

We also compare two methods commonly used to measure the fractal dimension of linear features: the Box method and the Richardson (or divider) method. Both methods yield comparable results for ammonitic sutures but the Richardson method yields more precise results for less complex sutures.

The most widely cited explanation for the functional enigma of sutural complexity in ammonoids, the Buckland hypothesis, has related septal folding and fluting to buttressing, providing increased shell strength against implosion, along with increased efficiency and decreased weight in shell and septum construction. In Paleozoic ammonoids, sutures ranged from simple to extremely complex. Comparison of shell and septum thickness (in polished sections) with sutural complexity in 49 Paleozoic ammonoid genera (Middle Devonian–Upper Permian) indicates that no significant reduction in either septum thickness or shell thickness accompanied a one-hundred-fold increase in sutural complexity. These preliminary results fail to support the Buckland hypothesis, suggest there may have been alternative incentives for increasing sutural complexity, and add support to views that septal fluting may have been related to buoyancy control.

Theoretical and experimental biomechanical approaches are used to test the effect regular archaeocyathan central cavity diameter has on the generation of passive flow through the skeleton. These results are then used to predict a correspondence between gross morphology and paleoenvironmental occurrence. Previous work has demonstrated that regular archaeocyathan morphology generates passive flow, via Bernoulli and viscous entrainment effects, through its porous walls for suspension feeding, a phenomenon that occurs in modern sponges. Efficacy of entrainment depends upon the area of the excurrent pore (i.e., central cavity) over which the ambient flow is moving. Consequently, archaeocyaths should have maximized their central cavity diameters.

Five-centimeter-long, conical and cylindrical acrylic pipes with varying end diameters were tested in a flume to document the relative effects of Bernoulli and viscous entrainment. Each pipe was oriented perpendicular to the flow direction in a uniform flow field, and fluorescein dye was injected at the pipe's mid-length for flow visualization. Models with different-sized apertures consistently exhibit dye movement to the larger opening and greater dye entrainment speeds than models with identically sized apertures, thereby suggesting that viscous entrainment effects are significant and operating in concert with Bernoulli effects. To test for similar effects in archaeocyaths, four brass models were constructed with varying central cavity diameters. Both volume flux and excurrent flow speed of the exiting water increased as the central cavity diameter increased. An analysis of the morphologies that occur in nature confirm these results. Regular archaeocyaths most commonly have central cavity diameters close to their outer wall diameter, thereby maximizing the excurrent pore area.

These results have implications for archaeocyathan paleoecology. Environments with low-magnitude currents should support individuals with larger central cavity diameters than higher energy settings. Data on the occurrence of morphotypes within bioherms of varying flow energies from South Australia support this prediction.

The unique manner in which molars from members of the family Elephantidae erupt in the jaw and wear obliquely and sequentially has profound effects upon dental function and phylogenetic change within the group. Three-dimensional modeling using a “molar matrix” of elephantid dentition, and application of such models to systematic and functional studies, allows a more refined description of dental morphology. A method of examining variation within elephantid teeth is presented based on successive staging of worn molars. Results indicate that individual plates exhibit increasingly derived features with wear (relative to the systematic analysis used here), while successively worn plates exhibit successively more plesiomorphic features apically and posteriorly. Further, results indicate that the patterns developed by wear on the surface of elephantid molars are conserved throughout life despite their unique successive replacement pattern. The cheek teeth in a molar series act as a single, continuous masticating unit, here termed a “cheek tooth battery.” Overall, the tools developed here, wear staging and molar matrices, allow for a more refined understanding of morphological variation within and between elephantids, with application to more conservative elephantoid taxa.