A general model of taxonomic diversity, incorporating diversity-dependent rates of origination and extinction, is constructed to examine the dynamic responses of diversity to perturbation. The model predicts that the trajectories of diversification increase and decrease are substantially different. The trajectories of diversity during disequilibrium conditions are displayed in phase diagrams to permit a simple graphical analysis of stability. A positive displacement of diversity from equilibrium results in a rapid decline in diversity and may involve an initial overshoot of the equilibrium condition. A negative displacement of equal magnitude results in a gradual increase in diversity. The model is expressed as a nonlinear difference equation to incorporate intrinsically a delay time due to the characteristic noninstantaneous response of origination and extinction. The model initially assumes a parabolic curve expressing total taxon origination rate as a function of diversity. A second model, constructed assuming a sigmoidal total taxon origination rate derived from considerations of allopatric speciation, enhances the asymmetry of the diversity response. The delayed recovery of the Triassic fauna is shown to be characteristic of return to equilibrium from an undersaturated condition, whereas the more rapid “catastrophic” decline in the Late Permian fauna is shown to be characteristic of return to equilibrium from the oversaturated condition. It is proposed, although not assumed, that perturbation may include a degree of selectivity related to the dispersal abilities of organisms, thereby enhancing the observed asymmetry.

Arcoid bivalves occupy an intermediate position, in terms both of morphology and of adaptive range between the Pterioida and the Veneroida. The range and limits of arcoid adaptations are related to the growth patterns of their shells. Both the arcoid hinge and ligament grow by the serial repetition of simple structures, in contrast with the development of more specialized, complex structures in other groups. These simple growth patterns place significant mechanical constraints on the range of possible shell forms. Most arcoids live in moderately unstable environments, where they are liable to be excavated or detached from their substrates. Many employ recovery strategies, being adapted to regain their life positions. However, a variety of specialized forms, convergent on other groups of bivalves, have become adapted to avoid being dislodged in the first place. Thus, intrinsic growth patterns and substrate relationships have been the major factors in the evolution of the Arcoida.

Morphological evolution in the Late Cretaceous (Maastrichtian) Contusotruncana lineage of planktonic foraminifera was studied at DSDP Sites 525 (South Atlantic) and 384 (North Atlantic). A multivariable approach was used to separate aspects of form controlled by geographical variation (size, spiral roundness of the test, percentage of kummerform specimens) from those due to changes that occurred simultaneously in geographically distant populations of the lineage (shell conicity, number of chambers in the last whorl).

A gradual increase in mean shell conicity was observed over the last 3 million years of the Cretaceous. It arose from the combination of a rapid development of highly conical shells after 68.5 Ma and a long-term trend of progressive disappearance of the ancestral morphotype. Therefore, despite the gradual change in “mean form,” the morphological evolution in the Contusotruncana lineage differs from the classical image of phyletic gradualism. The gradual increase in mean shell conicity in the lineage was accompanied by a remarkable decrease in its absolute abundance (shell accumulation rate), suggesting that the changes in shell morphology might not have been neutral with respect to natural selection. Apparently, gradual change in “mean form” of fossil lineages does not require an equally gradual development of morphological novelties. It may be caused by natural selection operating on a constant range of variation in populations living in environments without geographical barriers.

Dickinsonia is reconstructed as a benthic polypoid of generally cnidarian design. The oral surface was without tentacles but contained a median oral slit that probably led through a pharynx into an enteron, which was divided into digestive diverticulae by radiating mesenteries; feeding may have been via ciliary tracts. The mesenteries and body wall contained a stiff form of mesogloea. There seems to be no need to postulate a novel constructional grade for this organism, bringing the concept of the Vendozoa into question.

Four independent lines of evidence, (1) the quality of specimen preservation, (2) taxonomic collection curves, (3) molecular divergence estimates, and (4) ghost lineage analysis of a genus-level cladogram, point to echinoids having a much poorer fossil record in the Triassic than in the Lower Jurassic. Furthermore, preservational differences between Triassic and Lower Jurassic echinoids have remained a consistent feature over 160 years of discovery. Differences exist in how effectively paleontologists have collected the fauna from available outcrops in the Triassic and Lower Jurassic. Collection curves suggest that rocks have been more efficiently searched for their fossils in Europe than elsewhere in the world, and that Lower Jurassic faunas are better sampled from available outcrop than Triassic faunas. The discovery of Triassic taxa has quickened in pace over the past 4 decades (though largely driven by a single Lagerstätte—the St. Cassian beds) while discoveries of new taxa from the Lower Jurassic have slowed. Molecular analysis of extant families and ghost lineage analysis of Triassic and Lower Jurassic genera both point to poorer sampling of Triassic faunas. This difference in the quality of the fossil record may be partially explained by differences in rock outcrop area, as marine sedimentary rocks are much less common in the Triassic than in the Lower Jurassic. However, improving biomechanical design of the echinoid test over this critical time interval was probably as important, and better explains observed preservational trends. Changes in the quality of the echinoid fossil record were thus driven as much by intrinsic biological factors as by sampling patterns.

More than 300 coots (Fulica americana) became frozen in Spring Lake, Tazewell County, Illinois, on December 1, 1985. This catastrophic event permitted 8 weeks of taphonomic observations, which showed that ice forms a stable substrate which permits terrestrial taphonomic processes to be imprinted on lacustrine deposits. Bird and mammal scavengers attacked coot carcasses in different manners, resulting in distinct disarticulation sequences. Bird scavengers preferentially fed on the head, neck, and breast-wing complex, causing early disarticulation of bones in these areas, late loss of hindlimb joints, and minimal bone damage. In contrast, mammal scavengers concentrated their attention on the hindlimb and tail region, resulting in bone breakage and early disarticulation of these body parts, but late disarticulation of the breast-wing complex. These data demonstrate that scavenger-specific feeding behaviors significantly influence disarticulation patterns early in assemblage formation, while anatomy may exert increasingly greater influence on disarticulation patterns as carasses become less attractive to scavengers. Finally, because taphonomic processes change in intensity and type through time, bone frequency and modification patterns will vary according to the time at which the patterns are arrested by burial. Thus, bone frequency and modification patterns should provide an index to the relative importance of specific biotic agents and of anatomy in fossil disarticulation patterns as well as an estimate of the time between death and burial.

Shells of 375 sexed specimens of Nautilus cf. N. pompilius trapped at 300–700 ft (90–215 m) depth off Mutremdiu Point and Ngemelis Island, Palau, exhibit distinctive dimorphism in shell proportion and overall size. Of 221 mature animals, males have larger shells (mean diameter = 209 mm), a broader aperture (mean maximum width = 99 mm; mean width at apertural salient = 92 mm) and greater weight (mean shell + body weight = 1426 gm). Female shells are smaller (mean diameter = 198 mm), narrower (mean maximum width = 91 mm; mean apertural salient width = 81 mm) and the overall weight is less (mean = 1157 gm). Dimorphic differences are not apparent in young shells (less than 180 mm diameter), but develop during the final ½ to ¼ whorl. This is accompanied by development of the spadix in males, indicating that shell dimorphism reflects sexual maturity. Of 375 animals trapped, 28% were female; no depth segregation was apparent. Traditional identification of larger fossil dimorphic cephalopods as females and smaller forms as males is the reverse of observed dimorphism in Nautilus and should be discontinued in favor of macroconch and microconch designations.

Coiled cephalopods constitute a major part of the Paleozoic nekton. They emerged in the Early Ordovician but nearly vanished in the Silurian. The Emsian appearance of ammonoids started a story of evolutionary success of coiled cephalopods, which lasted until the end-Permian extinction event. This story is investigated by using a taxonomic database of 1346 species of 253 genera of coiled nautiloids and 1114 genera of ammonoids. The per capita sampling diversities, the Van Valen metrics of origination and extinction, and the probabilities of origination and extinction were calculated at stage intervals. The outcome of these estimations largely reflects the known biotic events of the Paleozoic. The polyphyletic, iterative appearance of coiled cephalopods within this time frame is interpreted to be a process of adaptation to shell-crushing predatory pressure. The evolution of the diversity of coiled nautiloids and ammonoids is strongly correlated within the time intervals. Once established, assemblages of coiled cephalopods are related to changes in sea level. The general trends of decreasing mean (or background) origination and extinction rates during the Paleozoic are interpreted to reflect a successive stabilization of the coiled cephalopod assemblages. Different reproduction strategies in ammonoids and nautiloids apparently resulted in different modes of competition and morphological trends. Significant morphological trends toward a stronger ornamentation and a centrally positioned siphuncle characterize the evolution of Paleozoic nautiloids.

Using tangential thin sections, we examined variation in porosity and water vapor conductance across two eggs of Troodon formosus, a small (∼50 kg) theropod dinosaur from the North American Upper Cretaceous, testing two hypotheses of egg incubation: (1) full burial within sediments or vegetation and (2) partial burial with exposed upper egg portions. We divided and sampled the eggs in five zones, 1 through 5 from blunt top to more pointed bottom. A geometric model composed of a hemisphere, cone, and paraboloid was used to estimate total and zonal volumes and surface areas. The 138 × 67 mm idealized Troodon egg has a volume, surface area, and mass of 296.4 cm3, 239.23 cm2, and 314.2 g, respectively. Zonal surface areas and volumes highlight the strongly asymmetric and elongate form of the Troodon egg. Geometric modeling provides better estimates of volume and surface area where egg shape diverges markedly from that of a typical bird egg. Porosity varies significantly across both Troodon eggs, with zones 2 and 3 having the largest pores and a majority (70–78%) of total conductance, whereas zone 5 has very low conductance. Total water vapor conductance in the two eggs are 31.85 and 40.62 mg H2O day− Torr−, values 76% and 97% of those predicted for an avian egg of similar size. Low total conductance compares favorably to values in extant birds and non-avian reptiles that incubate in open nests, arguing against full burial incubation. Together with nesting site evidence, low conductance values favor partial burial and incubation by a Troodon adult. Asymmetric egg shape concentrates volume, surface area, and conductance near or at the point of subaerial exposure. Among non-avian dinosaurs, the eggs of Troodon and troodontids are most similar to those of modern birds in having an asymmetric shape, low porosity, no ornamentation, and three structural eggshell layers.

Relative abundance data are of primary importance in paleoecology, but it is not always obvious how they should be interpreted. Because relative abundance is expressed as a proportion of the total sample, change in the abundance of one group necessarily changes the relative abundance of all groups in the sample. There are two possible interpretations for a trend in the relative abundance of a taxon: an “active” scenario in which the trend reflects change in the population density of the group itself, or a “passive” scenario in which the change is driven by population changes in other taxa. To discriminate between these scenarios it is necessary to collect absolute abundance data (abundance expressed as a function of sample area or volume).

We examine both absolute and relative abundance trends through a major paleoecological transition: the shift from trilobite-dominated to brachiopod-dominated paleocommunities in shallow marine carbonates spanning the Lower/Middle Ordovician boundary in western Utah and eastern Nevada. We sampled 61 carbonate mudstone and wackestone beds from the upper Ibex Series (Lower Ordovician) and lower Whiterock Series (Middle Ordovician) at three sections that span the boundary. All samples come from the shallow subtidal Bathyurid trilobite biofacies. Samples were broken into small pieces, and all skeletal fragments >2 mm were identified to the finest possible taxonomic level. Consistent with previous work on this interval, the relative abundance of trilobites declines sharply across the boundary, while the relative abundance of brachiopods increases. Absolute abundance data indicate that the decline in trilobite abundance is genuine and not an artifact of normalization. The trend is not easily explained by sampling bias, facies distribution, taphonomic regime, or sedimentation style.

The dramatic shift in abundance contrasts with relatively minor changes in relative genus richness across the boundary. This is partly ascribable to differences in the relative abundance structure of trilobite faunas. Though comparable numbers of trilobite and brachiopod genera occur above and below the boundary, the trilobite fauna from the upper Ibex Series has lower evenness then the lower Whiterock Series fauna. Hence sampled trilobite richness is high in the lower Whiterock despite the small number of specimens. This highlights the importance of collecting abundance data. Although these data suggest that in at least some cases richness and abundance patterns are not closely coupled, more robust richness data are necessary to confirm this conclusion.

Sublethal injuries recorded in the shells of five Upper Mississippian ammonoid species from the Imo Formation of northwest Arkansas are manifested as repaired shell breaks, which can be categorized as minor, moderate, massive, deep-acute, or as perforations. Overall, 15% of the ammonoids exhibit some form of repaired break. The injuries are distributed as follows: Anthracoceras discus 9%; Fayettevillea bransoni 13%; Fayettevillea friscoense 21%; Rhadinites miseri 24%; Richardsonites mapesi 38%. These figures are substantially lower than in living Nautilus, in which more than one-half of adult specimens exhibit repaired shell breaks. The different frequencies of injuries may reflect species-selective predation, differential abilities to sustain and to repair shell damage, or they may be due to interspecific differences in physical parameters such as shell thickness and body chamber length. The most likely possible perpetrators of the injuries include sharks, other fishes, and cephalopods.

Geographic range appears to be an important aspect of the biology of species, but ranges cannot be unambiguously determined from the fossil record: ancient species can rarely be traced to the full extent of their original geographic ranges, and some ancient provinces are far better characterized than others. Here we test the degree to which paleogeographic range data for species within ancient provinces can be used to estimate the relative magnitudes of total geographic ranges, focusing on the 212 living bivalve species of the Oregonian Province, a north-south (N-S) province of the northeastern Pacific shelf.

We break the total range of each species into its within-province and extraprovincial components. Nonparametric rank correlation tests and simple linear regressions yield highly significant correlations, both between within-province and total ranges (which are not independent variables) and between within-province and extraprovincial ranges. The strongest correlations are obtained when north-ranging species are excluded. These north-ranging species are thermally tolerant and have access to broad east-west (E-W) provinces, thus weakening the relation between their within-province and total ranges. On a coarser scale, we also found that the extraprovincial ranges of species with small within-province ranges (<650 km, ca. 25% of focal province) are significantly less than those for species with large within-province ranges (> 1950 km, ca. 75% of focal province). The significant correlations of within-province range with both total and extraprovincial ranges hold for species with within-province ranges >650 km, so caution is needed regarding species that extend into a focal province only a short distance from its edges. We conclude that within-province geographic range can generally be used to rank species by total geographic range and thus is an adequate proxy in most comparative studies of the paleobiologic consequences of range magnitudes.

We analyze relationships among a range of ecological and biological traits—geographic range size, body size, life mode, larval type, and feeding type—in order to identify those traits that are associated significantly with species duration in New Zealand Cenozoic marine molluscs, during a time of background extinction. Using log-linear modeling, we find that bivalves have only a small number of simple, two-way associations between the studied traits and duration. In contrast, gastropods display more complex interactions involving three-way associations between traits, a pattern that suggests greater macroecological complexity of gastropods. This is not an artifact caused by the larger number of gastropods than bivalves in our data set. We used stratified randomized resampling of families to test for associations between traits that might result from shared inheritance rather than ecological trait interactions; we found no evidence of phylogenetic effects in any associations examined. The relationships revealed by our study should serve to constrain the range of possible biological mechanisms that underlie these relationships. As previously observed, two-way associations are present between large geographic range and increased duration, and between large geographic range and large body size, in both bivalves and gastropods. In gastropods, planktotrophic larval type is associated with large range size through a three-way interaction that also involves duration; there is no direct association of larval type and geographic range. Gastropods also display two-way associations between duration and life mode, and duration and feeding type. We note that in gastropods, an infaunal life mode is associated with large range size, whereas in bivalves infaunality is associated with reduced range size.

Mammalian species often exhibit clinal geographic variation in body size: individuals tend to be larger in areas with lower mean annual temperature. Climatic change involving increasing or decreasing mean annual temperature may cause clines to shift geographically, resulting in a phenotypic shift at all affected locales within a species' range. I assess the potential of shifting geographic clines to produce morphological trends in the fossil record. Five extant North American mammalian species (Didelphis virginiana, Mephitis mephitis, Odocoileus virginianus, Scalopus aquaticus, and Sciurus carolinensis) are examined to quantify size change along latitudinal clines and to estimate the geographic range and temperature difference commonly associated with a given difference in body size. Relative to body size, the observed size range of skeletal characters within each of these five species is comparable to that seen in a much larger sample of North American mammals. Thus patterns of variation documented for the five species may be used to assess the likelihood of dine translocation as an explanation of size change in the mammalian fossil record. As a case study, I examine three lineages from the Early Eocene of the Bighorn Basin, Wyoming. I determine that size change in these chronoclines represents evolutionary change and is not merely the result of shifting geographic clines.

Open-ended tubes were found extending inward from the inner surfaces of brachial valves of three different species of Emsian spiriferid brachiopods. It is believed that these tubes were formed by the outer epithelium of the brachiopods, precipitating shell material around a small commensal filter feeding organism living in the outer mantle cavity. The position of the open ends of these tubes, the business end of the filter feeding organism, in every case discovered is between the bases of the two spiralia. This lends support to the suggestion of Ager (1961) and Wallace and Ager (1966) that spiriferid brachiopods had median inhalant and lateral exhalant feeding currents. These structures are named Burrinjuckia spiriferidophilia n.gen. n.sp. They are trace fossils of indeterminate taxonomic position.

Cretaceous floras in Alaska, when compared to those at mid-latitudes, generally indicate later appearances in Alaska of major clades and major leaf morphologies. Compared to mid-latitude floras, Alaskan Late Cretaceous floras contain few major clades. The Alaskan clades diversified but at a low taxonomic level. Migrational pathways into high latitudes were probably along streams. Similar patterns characterized the Alaskan Tertiary, although some southward migrations of lineages occurred during the Neogene.

Review of other Arctic paleontological data from Ellesmere Island, previously used to suggest that the Arctic was a major center of origin during the Late Cretaceous, indicates that the ages of supposedly substantiating dinoflagellate floras were misinterpreted. When the dinoflagellate data are interpreted according to standard methodology, first occurrences of genera and species groups on Ellesmere are, like the Alaskan occurrences, later than first occurrences at middle latitudes.

Distinguishing the differential roles of hardpart-input rates and burial rates in the formation of shell beds is important in paleobiologic and sedimentologic studies, because high shelliness can reflect either high population density of shell producers or lack of sediment. The modeling in this paper shows that differences in the relative importance of burial rates and hardpart-input rates lead to distinct patterns with respect to the degree of shelliness and taphonomic alteration in shell beds. Our approach substantially complements other models because it allows computation of both shelliness and assemblage-level alteration. To estimate shelliness, we dissected hardpart-input rates into dead-shell production and shell destruction rates. To estimate assemblage-level alteration, we computed an alteration rate that describes how rapidly shells accrue postmortem damage. Under decreasing burial rates but constant hardpart-input rates, a positive correlation between alteration and shelliness is expected (Kidwell's R-sediment model). In contrast, under decreased destruction rates and/or increased dead-shell production rates and constant burial rates (Kidwell's R-hardpart model), a negative correlation between shelliness and alteration is expected. The contrasting predictions thus provide a theoretical basis for distinguishing whether high shell density in shell beds reflects passive shell accumulation due to a lack of sediment dilution or whether it instead reflects high shell input from a life assemblage. This approach should be applicable for any fossil assemblages that vary in shell density and assemblage-level alteration. An example from the Lower Jurassic of Morocco, which has shell-rich samples less altered than shell-poor samples, suggests that the higher shelliness correlates with higher community-level abundance and lower proportion of juveniles of the main shell producer, supporting the driving role of hardpart-input rates in the origin of the shell-rich samples in this case. This is of significance in paleoecologic analyses because variations in shelliness can directly reflect fluctuations in population density of shell producers.

Incremental stages of major evolutionary transitions within a single animal lineage are rarely observed in the fossil record. However, the extraordinarily complete sequence of well preserved material spanning the 27-Myr existence of the marine squamate subfamily Mosasaurinae provides a unique exception. By comparison with extant and extinct analogs, the tail morphology of four mosasaurine genera is examined, revealing a pattern of evolution that begins with the generalized varanoid anatomy and culminates in a high-aspect-ratio fin, similar to that of sharks. However, unlike the epicercal caudal fluke of selachians in which the tail bends dorsocaudally, derived mosasaurs develop a hypocercal tail with a ventrocaudal bend. Progressive caudal regionalization, reduced intervertebral mobility, increased tail depth due to a marked downturn of the posterior caudal segment, and the development of finlike paired appendages reveal a pattern of adaptation toward an optimized marine existence. This change in morphology reflects a transition from anguilliform or sub-carangiform locomotion to carangiform locomotion, and indicates a progressive shift from nearshore dwellers to pelagic cruisers—a change in foraging habitat independently corroborated by paleobiogeographic, stable isotope, osteohistological, and paleopathological data. Evolutionary patterns similar to those observed in mosasaurine mosasaurs are seen in other secondarily aquatically adapted amniotes, notably metriorhynchid crocodyliforms, cetaceans, and ichthyosaurs, and may be explained by developmental modularity governing the observed phenotypic expression.

Spectral algorithms have been shown to work well in a wide range of situations that involve the task of ordering. When applied to the localities of a set of European Neogene land mammal taxa, spectral ordering relies almost entirely on the most common genera, depends on connectivity more than on length of taxon lists, and is robust to noise from rarer and less connected taxa. The spectral coefficients for localities are highly correlated with known geochronological ages. Although elementary compared with more sophisticated biochronological tools, spectral ordering allows a fast and standardized way to generate biochronological ordering of localities when other information than faunal lists is lacking. Compared with the conventional mammal Neogene (MN) units, spectral ordering of localities appears to lack distinct temporal boundaries in taxon content and render a much lower count of Lazarus events. If, as seems to be the case, biochronology depends mainly on the most common taxa and if evolutionary change is also most clearly reflected in them, then the main evolutionary patterns should be detectable at a modest level of sampling.