A survey was undertaken of the developmental types of the neogastropod families Volutidae, Fasciolariidae, Nassariidae, Mitridae, Buccinidae and Olividae in the Palaeocene and Eocene series of the Gulf of Mexico. Although the families were dominated by planktotrophic development in the Paleocene, most show a general increase in numbers of species with nonplanktotrophic development through the Eocene. Present day environmental correlations with larval development do not adequately explain the observed shift in developmental types in the Early Tertiary. It is proposed that higher speciation rates for nonplanktotrophs and a unidirectional trend in developmental change may account for the progressive increase of nonplanktotrophic development.

Traditional wisdom of the evolution of lungs in fishes is that lungs arose when gill ventilation was hindered by an aquatic habitat that was low in oxygen. This scenario has been buttressed primarily by a proposed correlation between extant air-breathing fishes and hypoxic habitats, as well as by the fact that early vertebrate fossils were found in sediments believed to indicate a semi-arid environment. There are problems with this scenario, yet it retains a dominant influence on how the evolution of aerial respiration is viewed. This paper presents a new hypothesis for lung evolution that is more consistent with the fossil record and physiology of extant animals than the traditional scenario; I propose that lungs evolved to supply the heart with oxygen. The primitive vertebrate heart was spongy in architecture and devoid of coronary support, obtaining oxygen from luminal blood. By supplying oxygen to this tissue, lungs may have been important in ancient fishes for sustaining activity, regardless of environment. Furthermore, this function for lungs may have influenced cardiovascular adaptations of tetrapods because their divided cardiovascular system isolates the right side of the heart from pulmonary oxygen. I propose that three innovations compensate for this isolation: In extant amphibians oxygen-rich blood from cutaneous and buccal respiration enters the right side of the heart; in chelonians and lepidosaurs the intracardiac shunt washes oxygen-rich blood into the right side of the heart; in mammals, birds, and perhaps in crocodilians, support of the heart by coronary vasculature eliminates this problem.

Analysis of several thousand mammalian fossils from late Pleistocene sediments in California provide data on sampling in mammalian paleoecology. Recovery of bones and teeth from the screenwashed bulk sediment sample residue is considered nearly total. Neither surface collecting alone, nor small bulk samples provide satisfactory quantitative data on original community structure or postmortem alterations in community organization. Minimum sample size for the analysis of diversity is discussed. Diversity and size-trophic ratios of the total identifiable mammalian component of this fauna (N = 1222) are similar to those expected in living communities, and therefore suggest adequate sampling, and minimally biased samples.

Approximately half of the Caribbean Oligocene reef coral fauna became locally extinct during the Early Miocene; roughly two thirds of the genera driven to local extinction still survive in the Indo-Pacific. Coral genera with lecithotrophic larvae (brooders) preferentially survived, over those with planktotrophic larvae (broadcasters). Among 37 genera for which we inferred reproductive mode, 73% of brooding genera survived the Oligocene/Miocene extinction events, while only 29% of the broadcasting genera survived. The proportion of brooders to broadcasters also increased markedly. During the late Oligocene, 47% of Caribbean reef coral genera were broadcasters, but in the middle Miocene, only 32% of the genera were broadcasters.

Survivorship in Puerto Rican reefs was correlated with tolerance of cold and turbid conditions. Genera tolerant of both cold water and turbidity had much higher survival rates than those tolerant of turbidity alone. Only 25% of the genera that could tolerate neither cold water nor turbidity survived. Most of the eurytopic genera were brooders, while most of the stenotypic genera were broadcasters.

We present two hypotheses that may account for the preferential survivorship of brooders: the recruitment hypothesis, and the dispersal hypothesis. The recruitment hypothesis holds that brooders survive preferentially because lecithotrophic larvae have higher recruitment success than do planktotrophic larvae in marginal habitats, such as upwelling zones. This is supported by the correlation of brooding and eurytopy. The dispersal hypothesis suggests that brooders survive preferentially because lecithotrophic larvae, which typically inherit zooxanthellae from the egg, have a longer larval lifespan and, hence, a wider potential dispersal range, than planktotrophic larvae, which typically capture zooxanthellae from the water column. Biogeographic range data, however, do not support this second hypothesis: modern Indo-Pacific brooding and broadcasting genera have nearly identical ranges, and many brooding species have narrower longitudinal ranges than do broadcasting species. Preferential survivorship of brooding corals contrasts sharply with survivorship patterns among molluscs during extinction events; among molluscs, broadcasters are favored over brooders.

A major increase in upwelling at the Oligocene/Miocene boundary was probably responsible for this extinction/geographic restriction event. Preferential survival of brooding and mixed mode coral genera appears to be a product of their being better able to recruit and survive in marginal conditions such as upwelling zones.

We review and synthesize multiple biotic and abiotic proxies for marine nutrient and food availability, primary productivity, and food quality (stoichiometry) and propose what their relationships may have been to macroevolutionary processes, especially speciation. This review confirms earlier suggestions that there has been an overall increase in marine primary productivity over the Phanerozoic, but indicates that the increase has been irregular and that present levels may not be the peak. We integrate these indicators into a new estimate of relative primary productivity in the global ocean through the Phanerozoic. We then combine multiple, frequently conflicting ecological-evolutionary hypotheses into a general model for how primary production may affect speciation over geological time scales. This model, an elaboration and extension of the “speciation cycle” previously proposed by Grant and Grant, attempts to explain why an increase in food supply sometimes is associated with decreased diversity, and at other times with increased diversification. We propose some simple tests for the application of this model to the fossil record.

Geographic range size is one of the few traits that promoted survivorship during both mass and background extinctions, but the exact reason (or reasons) why a large geographic range confers extinction resistance remains unclear. Proposed explanations have focused on the roles of dispersal ability, climate tolerance, global abundance, and widespread ranges in predicting taxon longevities. This study uses biogeographic data for late Paleozoic brachiopod genera to test the relative contribution of these traits to genus longevities, using simple but accurate proxy measurements. The results demonstrate a strong positive relationship between genus longevity and geographic range size, which is robust to several potential errors. Further, latitudinal range, which predominantly reflects climate tolerance, was no more important than longitudinal range, which predominantly reflects dispersal ability, in predicting genus longevities. Rather, longevities were an outcome of the total number of occurrences, which estimates global abundance, and the advantages of widespread distribution, regardless of which particular traits were responsible for generating the total geographic range. The advantages of a large geographic range were apparent during both background and mass extinctions of late Paleozoic time. Although not statistically significant, there was a tendency for the greatest selectivity to occur in intervals with the lowest extinction rates. The correlation of genus longevity and geographic range size had a profound consequence for the secular pattern of global brachiopod diversity: because the diversity of genera with small geographic ranges was more volatile owing to their correspondingly shorter longevities, global diversity and mean geographic range size paralleled each other almost exactly. Given that the correlation between taxon longevity and geographic range size has also been demonstrated for other taxonomic groups and at other time intervals, these results suggest that global diversity curves compiled from taxonomic databases dominantly reflect changes in the diversity of genera with small geographic ranges.

Systematics and paleontology have had a long conceptual relationship, united by the common goal of reconstructing the history of life. Yet, with few exceptions, paleontologists have had little input into formulating systematic theory and methodology. The reasons for this apparently relate to two conceptual-philosophical traditions of post-Darwinian paleontology: (1) the widespread adoption of a species concept in which taxa are viewed as nondiscrete, arbitrarily designated segments of evolutionary continua, and (2) the belief that phylogenetic reconstruction is primarily an empirical matter of tracing evolutionary change through the stratigraphic record.

Available systematic evidence supports the hypothesis that species are real, discrete units in space and time and that, unless they are postulated to be directly ancestral to another species, they can be defined by the possession of one or more evolutionary novelties (derived characters). Species beginnings are delineated by speciation (vicariance) events and their terminations by subsequent speciation events or by extinctions.

Natural groups are composed of taxa that have shared a common genealogical history. Cladistic analysis is a method to construct and test hypotheses of monophyly and thereby define natural groups. Cladistic hypotheses are necessary to investigate many of the major questions within contemporary paleobiology. Virtually no studies of evolutionary rates, patterns of taxonomic diversity, modes of taxic evolution, and patterns of morphological diversification can be undertaken without reference to cladistic hypotheses about the composition of natural groups.

Because paleobiology is historical in its content, paleontologists are greatly limited in their ability to use paleontological data to investigate questions about the evolutionary process. According to current evolutionary theory, the concepts of adaptation and natural selection relate to genetic and ecological processes that take place within local populations (microevolution). If so, then data relevant to examining these phenomena are likely to be lacking in paleontological samples. Consequently, explanations of paleontological pattern that include process-related concepts such as adaptation and natural selection are axiomatic in their logical structure and thus cannot be falsified or critically evaluated by that paleontological pattern.

Species arise and establish themselves over the geologic time scale. This process is manifested as a change in the relative frequency of occurrences of a given species in the global pool of species. Our main goal here is to model this rise and the eventual decline of microfossil species using a mixed-effects model where groups each have a characteristic occurrence trajectory (main effects) and each species belonging to those groups is allowed to deviate from the given group trajectory (random effects). Our model can be described as a “hat” with logistic forms in the periods of increase and decline. Using the estimated timings of rises and falls, we find that the lengths of the periods of rise are about as long as the lengths of the periods when species are above 50% of their estimated maximal occurrences. These latter periods are here termed periods of dominance, which are in turn about the same length as the species' periods of fall. The peak rates of the rises of microfossils are in general faster than their peak rates of falls. These quantified observations may have broad macroevolutionary and macroecological implications. Further, we hypothesize that species that have experienced and survived high levels of environmental volatility (specifically, periods of greater than average variation in temperature and productivity) during their formative periods should have longer periods of dominance. This is because subsequent environmental variations should not drive them to decline with ease. We find that higher estimated environmental volatility early in the life of a species positively correlates with lengths of periods of dominance, given that a species survives the initial stress of the environmental fluctuations. However, we find no evidence that the steepness of the rise of a species is affected by environmental volatility in the early phases of its life.

Ecological ordination can reveal gradients in the species composition of fossil assemblages that can be correlated with paleoenvironmental gradients. Ordinations of simulated data sets suggest that nonmetric multidimensional scaling (NMDS) generally produces less distorted results than detrended correspondence analysis (DCA). We ordinated 113 brachiopod-dominated samples from the Frasnian (Late Devonian) Brallier, Scherr, and lower Foreknobs Formations of southwest Virginia, which represent a range of siliciclastic marine paleoenvironments. A clear environmental signal in the ordination results was obscured by (apparently) opportunistic species that occurred at high abundance in multiple environments; samples dominated by these species aggregated in ordination space regardless of paleoenvironmental provenance. After the opportunist-dominated samples were removed, NMDS revealed a gradient in species composition that was highly correlated with substrate (grain size); a second, orthogonal gradient likely reflects variation in disturbance intensity or frequency within grain-size regimes. Additional environmental or ecological factors, such as oxygenation, may also be related to the gradients. These two gradients, plus the environmental factors that controlled the occurrence of opportunistic species, explain much of the variation in assemblage composition in the fauna. In general, the composition of fossil assemblages is probably influenced by multiple paleoecological and paleoenvironmental factors, but many of these can be decomposed and analyzed.

Forty-six specimens of Nautilus pompilius Linnaeus were captured in depths varying between 100 and 500 m outside of the fringing reef near Suva, Fiji Islands. Thirty-eight of the specimens were male. Air weight per individual varied between 347 and 630 g. Sexual dimorphism in size is indicated, since mature shell modifications (approximated septa, blackened aperture) were present in two females weighing about 350 g (soft parts plus shell) and one weighing slightly over 400 g; the smallest male showing mature shell modifications weighed 496 g. All newly captured specimens were heavier than seawater, with mean weight in seawater of 1.87 g determined for twenty-five specimens. Total volumes of cameral liquid ranged between 13.5 and 0 ml. Thirteen of twenty-five sampled specimens showed less than 1.0 ml of cameral liquid from all chambers. Average cameral liquid osmolarity was lower than that observed in sampled populations of N. macromphalus from New Caledonia and N. pompilius from the Philippine Islands. Maximum swimming rates were 0.25 m/sec. N. pompilius exhibits two common color polymorphs.

The middle (Wuliuan Stage) Cambrian Burgess Shale is famous for its exceptional preservation of diverse and abundant soft-bodied animals through the “thick” Stephen Formation. However, with the exception of the Walcott Quarry (Fossil Ridge) and the stratigraphically older Tulip Beds (Mount Stephen), which are both in Yoho National Park (British Columbia), quantitative assessments of the Burgess Shale have remained limited. Here we first provide a detailed quantitative overview of the diversity and structure of the Marble Canyon Burgess Shale locality based on 16,438 specimens. Located 40 km southeast of the Walcott Quarry in Kootenay National Park (British Columbia), Marble Canyon represents the youngest site of the “thick” Stephen Formation. We then combine paleoecological data sets from Marble Canyon, Walcott Quarry, Tulip Beds, and Raymond Quarry, which lies approximately 20 m directly above the Walcott Quarry, to yield a combined species abundance data set of 77,179 specimens encompassing 234 species-level taxa. Marble Canyon shows significant temporal changes in both taxonomic and ecological groups, suggesting periods of stasis followed by rapid turnover patterns at local and short temporal scales. At wider geographic and temporal scales, the different Burgess Shale sites occupy distinct areas in multivariate space. Overall, this suggests that the Burgess Shale paleocommunity is far patchier than previously thought and varies at both local and regional scales through the “thick” Stephen Formation. This underscores that our understanding of Cambrian diversity and ecological networks, particularly in early animal ecosystems, remains limited and highly dependent on new discoveries.

Analysis of molluscan collections from a 3+ m.y. interval around the Cretaceous–Tertiary (K–T) interval in east Texas suggests that molluscs suffered an extinction at or near the K–T boundary, followed by a prolonged period of stress which lasted through the P0 and P1a planktic foraminiferal zones. The stressed period was characterized by low species richness, low abundances of individuals, high species turnover and a dominance of deposit feeders. Species richness and the relative abundance of deposit feeders generally track the 13C depletion curve suggesting that the stress was caused by a lack of primary production. A stable, relatively diverse, suspension feeding molluscan community was reestablished less than two million years after the K–T boundary. The total number of species within the habitat did not recover to Cretaceous levels within the study interval.

Key morphological traits reveal changes in functional morphospace occupation of reef fish assemblages over time. We used measurements of key functional attributes (i.e., lower jaw length and orbit diameter) of 208 fossil fish species from five geological periods to create bivariate plots of functional morphological traits through time. These plots were used to examine possible function and ecological characteristics of fossil reef fish assemblages throughout the Mesozoic and Cenozoic. A previously unknown trend of increasing orbit diameter over time became apparent. The Teleostei are the principal drivers of this change. The Eocene appears to mark a dramatic increase in two previously rare feeding modes in fishes: nocturnal feeding and high-precision benthic feeding. Interestingly, members of the Pycnodontiformes had relatively large eyes since the Triassic and appear to be the ecological precursors of their later teleost counterparts and may have been among the earliest nocturnal feeding fishes. Our results highlight potential changes in the roles of fishes on coral reefs through time.

The Late Devonian extinction event was not geologically “instantaneous,” in that extinctions during the epoch are not concentrated into a single sharp pulse at the end of the Frasnian. Extinction rates are elevated for a period of at least 2 to 4 m.y. during the middle and late phases of the Frasnian, with maximum rates occurring generally 2 m.y. before the terminal Frasnian. Neither was the Late Devonian biotic crisis a “gradual” event. In the analysis of the evolution of ecosystems, it is misleading to consider the pattern of extinction rates alone. Frasnian marine ecosystems flourished during the same time interval characterized by elevated extinction rates because origination rates of new species are higher, per time interval, than corresponding extinction rates. This pattern of relative origination/extinction rates abruptly reversed during the latest Frasnian—precipitating a rapid loss of species diversity. Within limits of current stratigraphic correlation, the ecosystem collapse appears to have occurred simultaneously in such widespread geographic regions as New York State (U.S.A.) and the southern Urals (U.S.S.R.).

In viewing the Late Devonian event from an ecological perspective, the most important question is not “What triggered the elevated extinction rates?”, but rather “What was the inhibiting factor that caused the cessation of new species originations?”

Extinctions at the top of the Sunwaptan Stage (=“Ptychaspid Biomere”) near the Cambrian-Ordovician boundary eliminated about half of North American trilobite families. The families that extend from the shelf into the upper slope show significantly higher survival than those confined to the shelf. Biofacies and lithofacies distribution patterns indicate that the extinctions cannot be attributed to a shelfwide physical environmental perturbation, such as a fall in water temperature or the spread of anoxic waters. We develop a simple biogeographic model which suggests that diversity of a faunal province is influenced profoundly by changes in the number of component biofacies. This model is tested with an analysis of biofacies distribution patterns across the upper boundary of the Sunwaptan Stage. The extinctions correspond closely to lithofacies shifts in the outer shelf that indicate the initiation of major paleogeographic changes, possibly in response to a sea-level rise. The effects of these changes cascade across the entire shelf by the shoreward migration of off-shelf and shelf-margin taxa. Biofacies become reduced in number through telescoping and their environmental ranges expand during the extinction interval, suggesting an increase in the proportion of eurytopic taxa. Selective survival of wide-ranging eurytopes may have influenced the dynamics of faunal replacement by lowering speciation rates of shelf taxa. Consequently, the proportion of shelf endemics will decline and biofacies will be dominated by immigrant taxa. There are sufficient similarities in extinction patterns across the upper boundary of the Sunwaptan Stage and those at other Upper Cambrian stage boundaries to suggest that the biogeographic model developed here may have broader application.

Many questions have emerged recently regarding the importance and methodology of analysis of adaptation. Divergent views reflect both problems of definition and more substantive issues of interpretation. Defining the state of adaptation in terms of its contribution to current fitness, rather than origin by natural selection, is essential if natural selection is to be considered an explanation of adaptation. The context dependency and relativity of fitness apply also to adaptation. Design criteria are essential components of adaptation, but only to the extent that they are subsumed as elements of the causal interactions determining relative reproductive potential. The local, relational, contingent character of adaptation supports only limited reference to improvement. Most long-term patterns of change can be better described as diffusion within a structured design-space than as progressive improvement of design. The analysis of adaptation is part of a broader inquiry into the processes and constraints that control form and the history of changing form. It offers one perspective on how organisms operate on ecological time scales and how their configurations might be maintained or transformed over evolutionary time. Hypotheses concerning adaptation are sometimes tested by reference to predictions concerning the central tendency or trend of some aspect of an anatomical-behavioral system. These can be interpreted with minimal reference to assumptions of optimality if the analysis is viewed in terms of Bayesian inference. However, an alternative and frequently preferable approach to testing relies on limit-oriented predictions. Analysis of adaptation can be visualized as inferring the pattern and nature of interactions comprising the causal plexus that determines fitness. A comprehensive understanding of form and form-change requires that this be integrated with the perspective offered by studies of development, genetics, phylogenetic history, and external perturbations acting on the system.

Cave bears, an extinct subgenus (Spelearctos) of Ursus, were versatile enough to inhabit large areas of the northern hemisphere during the middle and late Pleistocene, yet they had evolved a specialized dentition that emphasized grinding functions, implying a heavy dietary reliance on tough, fibrous foods (i.e., plants). Isotope studies have yielded conflicting results on cave bear diet, however, often without consideration of the provenance of the samples or the possible contradictions that taphonomic and morphologic evidence might pose to dietary interpretations. It is likely that cave bear habits varied somewhat in response to environmental circumstance, and the limits on their abilities to do so remain unknown. If the larger goal of paleontological inquiry is to reconstruct the adaptations of cave bear species, then variation and commonalities among populations must be tracked closely, and the disparate lines of evidence currently available examined together on a case by case basis. Clearly, no single analytical technique can achieve this. By way of example we present the results of a cross-disciplinary collaboration that combines osteometric, isotopic, and taphonomic approaches to studying the paleoecology of a bear assemblage from Yarimburgaz Cave in northwest Turkey. Reference information on the linkages between diet, hibernation, and population structure in modern bears provides test implications for the investigation. Osteometric techniques demonstrate the presence of two coextant middle Pleistocene bear species in the sample–Ursus (Spelearctos) deningeri, a form of cave bear, and U. arctos or brown bear–the former abundant in the sample, the latter rare. An attritional mortality pattern for the bears and the condition of their bones show that most or all of the animals died in the cave from nonviolent causes in the context of hibernation. The study also elucidates several characteristics of the cave bear population in this region. Osteometric techniques show that the adult sex ratio of the cave bears is only slightly skewed toward females. This pattern lies near one extreme of the full range of possible outcomes in modern bear species and can only reflect a strong dietary dependence on seasonally available plants and invertebrates, showing that hibernation was a crucial overwintering strategy for both sexes; the results specifically contradict the possibility of regular, heavy emphasis on large game (hunted or scavenged) as a winter food source. The nature of wear and breakage to the adult cave bear teeth indicates that food frequently was obtained from cryptic sources, requiring digging and prying, and that extensive mastication was necessary, leading to complete obliteration of some cheek tooth crowns in old individuals. The patterns of tooth damage during life corroborate the dietary implications of the adult sex ratio and also argue for a diet rich in tough, abrasive materials such as nuts, tubers, and associated grit. The carbon and oxygen isotopic compositions of cave and brown bear tooth enamel from the site are virtually identical, and there is no evidence of a strong marine signal in either species, despite the cave's proximity to a modern estuary of the Sea of Marmara; nitrogen isotope ratios could not be examined because of poor protein preservation. The isotope results suggest that both bear species were highly omnivorous in the region during the middle Pleistocene and obtained nearly all of their food from terrestrial and fresh-water habitats. Bone pathologies, usually originating from trauma, occur in some of the adult bears, testifying to long lifespans of some individuals in this fossil population. The Yarimburgaz cave bears also exhibit great size dimorphism between the sexes, based on weight-bearing carpal bone dimensions, with adult males attaining roughly twice the body mass of adult females.

A record of late Paleozoic foraminiferal diversity, origination and extinction frequencies, and provincialism at million-year temporal resolution and species-level taxonomic resolution has been achieved by analyzing composite standard databases. Foraminiferal species diversity increased throughout Mississippian and Pennsylvanian time leading up to its peak at the Pennsylvanian/Permian boundary. Foraminifers then experienced a steep decline in diversity during the Early Permian. Frequencies of origination and extinction broadly tracked changes in global diversity. From Late Mississippian time onward, patterns in total foraminiferal diversity were dominated by fusulinoideans. There is no clear relationship between rates of foraminiferal evolution and the alternating glacial and nonglacial intervals that characterized the late Paleozoic ice age. Rather, high rates of origination and extinction might reflect instability of neritic environments as a consequence of high-frequency, high-amplitude base-level fluctuations (cyclothemic deposition). Further, the advent of algal symbiosis in fusulinoideans was a physiologic innovation that promoted diversification as these symbiont-bearing taxa experimented with morphologic adaptations for partitioning the low-nutrient environments to which they were specialized. Growth to large size and delayed maturation in fusulinoideans might have been enabled by the late Paleozoic hyperoxic atmosphere and the widespread development of oligotrophic, carbonate platform and shelf environments. The late Paleozoic history of foraminiferal diversification was influenced also by closure of the Rheic Ocean beginning in Late Mississippian time. Foraminiferal associations on opposite sides of Pangea exhibited relatively high similarity prior to the closure, but then similarity decreased steadily after destruction of the subequatorial marine corridor. Arctic-Eurasian and North American associations were nearly isolated from one another throughout the main burst of fusulinoidean diversification, so that parallel lineages developed independently in the two regions, resulting in many instances of convergence.

Two communities dominated by deposit-feeding bivalves are recognized in Quisset Harbor, Massachusetts. Interference with life position and sediment preconditioning restrict the distribution of some species. The Eel Grass Community, dominated by Nucula proxima, comprises primarily non-siphonate forms which live in fecal-pellet rich, high-water-content, oxygenated sediment. The sediment texture results from active burrowing. This soft, mobile sediment readily clogs the siphons of siphonate bivalves. The Channel Community, dominated by Yoldia limatula, comprises primarily siphonate forms which live in partly compacted, low-water-content muds. Each community is characterized by abundant surface, shallow, and deep feeding species. Competition for food has generated this distinct stratification in feeding depth of the dominant species with each community.

Similar competitive relationships, affecting both species occurrence and trophic stratification, are apparent in three Silurian age deposit-feeding bivalve communities from the McAdam Brook Formation of Nova Scotia. A community dominated by four non-siphonate species inhabited soupy, high-water-content muds. A community dominated by three siphonate species lived in cohesive muds. A mixed community, dominated by four species, two non-siphonate and two siphonate species (the most abundant of which are present, but not as the most abundant forms, in the other communities), characterized the mid-way point of the ecotone between the other two communities.

These deposit-feeding bivalve dominated communities have similar structural patterns although separated by 400 million years and despite environmental differences and evolutionary changes. Both response to physical factors, such as sediment character, and interspecific competition impose and maintain this type of community structure.

We present a morphometric analysis of water transport cells within a physiologically explicit three-dimensional space. Previous work has shown that cell length, diameter, and pit resistance govern the hydraulic resistance of individual conducting cells; thus, we use these three parameters as axes for our morphospace. We compare living and extinct plants within this space to investigate how patterns of plant conductivity have changed over evolutionary time. Extinct coniferophytes fall within the range of living conifers, despite differences in tracheid-level anatomy. Living cycads, Ginkgo biloba, the Miocene fossil Ginkgo beckii, and extinct cycadeoids overlap with both conifers and vesselless angiosperms. Three Paleozoic seed plants, however, occur in a portion of the morphospace that no living seed plant occupies. Lyginopteris, Callistophyton, and, especially, Medullosa evolved tracheids with high conductivities similar to those of some vessel-bearing angiosperms. Such fossils indicate that extinct seed plants evolved a structural and functional diversity of xylem architectures broader, in some ways, than the range observable in living seed plants.