The phenomenon of selective biotic extinction is commonly explained as the result of the extinction of specialized organisms and the survival of generalized organisms during a time of environmental stress. Using 48 orders of aquatic free-living arthropods, we test the hypothesis that generalized organisms tend to have longer geologic ranges than specialized organisms. The degree of specialization, as expressed through morphological complexity, is measured by the diversity of limb pairs on the arthropod body. The resulting “tagmosis value” reflects the functional differentiation of limb pairs to perform specific tasks and thus may estimate ecological specialization. No statistically significant correlation exists between an order's typical degree of morphological specialization and its geologic range. Degree of specialization is not an important determinant of evolutionary persistence at this taxonomic level. Evolutionary longevity may depend on the adaptive type (e.g., trophic role, relationship to substrate, etc.) represented by the taxon and on the assortment of these adaptive types.

The occupation of new environments by evolutionary lineages is frequently associated with morphological changes. This covariation of ecotype and phenotype is expected due to the process of natural selection, whereby environmental pressures lead to the proliferation of morphological variants that are a better fit for the prevailing abiotic conditions. One primary mechanism by which phenotypic variants are known to arise is through changes in the timing or duration of organismal development resulting in alterations to adult morphology, a process known as heterochrony. While numerous studies have demonstrated heterochronic trends in association with environmental gradients, few have done so within a phylogenetic context. Understanding species interrelationships is necessary to determine whether morphological change is due to heterochronic processes; however, research is hampered by the lack of a quantitative metric with which to assess the degree of heterochronic traits expressed within and among species. Here I present a new metric for quantifying heterochronic change, expressed as a heterochronic weighting, and apply it to xiphosuran chelicerates within a phylogenetic context to reveal concerted independent heterochronic trends. These trends correlate with shifts in environmental occupation from marine to nonmarine habitats, resulting in a macroevolutionary ratchet. Critically, the distribution of heterochronic weightings among species shows evidence of being influenced by both historical, phylogenetic processes and external ecological pressures. Heterochronic weighting proves to be an effective method to quantify heterochronic trends within a phylogenetic framework and is readily applicable to any group of organisms that have well-defined morphological characteristics, ontogenetic information, and resolved internal relationships.

Interpretations of morphologic radiations and macroevolutionary patterns are dependent on a priori choices of taxonomic and geographic scales of study. The results of disparity analysis at varying taxonomic (species and genus) and geographic (regional, biofacies, and community) scales are examined in a study of Ordovician though Early Silurian crinoids. Using discrete morphologic characters, we examined the disparity of 421 crinoids from 65 Laurentian biofacies. Crinoid disparity differs when analyzed at the regional and biofacies levels. Regardless of fluctuations in regional crinoid disparity, average within-biofacies disparity was static throughout the Ordovician, deviating only during the Silurian because of the proliferation of the morphologically aberrant myelodactylid crinoids. The choice of taxonomic level does not have an effect at the biofacies level. However, at the regional level, the two taxonomic scales (genus and species) can produce different results because of variation in the number of species per genus through time and the amount of morphologic variation within individual genera. Weighting disparity by abundance provides a metric combining morphology and community structure. Average weighted disparity at the community level showed patterns similar to that of the biofacies-level disparity curve, but this metric has a greater degree of variation between biofacies. Biofacies with a low ratio of weighted to unweighted disparity display the distinctive community structure (based on aerosol filtration theory) that is often reported in crinoid assemblages.

A number of facts indicate that the diversity of chthamaloid barnacles is much lower now than it has been in the past. Among these are the relict, often disjunct geographic distributions of distinctive chthamaloid higher taxa and their representation by only a few living species. The living chthamaloid barnacles also have a disjunct bathymetric distribution: as we will document here in detail, they are concentrated in the intertidal zone and in deep water. In his rejoinder to our paper, Paine makes no cogent argument against our interpretations of this powerful evidence.

Our case that the balanoids have competitively displaced the more primitive chthamaloids is based on several facts: (1) the Chthamaloidea have dwindled during the Cenozoic Era, so that most have relict or refugial distributions, whereas the Balanoidea have undergone a remarkable adaptive radiation; (2) balanoid species are known to defeat chthamaloid species in competition for space; (3) the competitive success of the balanoids can be attributed to an inherent biological feature—a tubiferous skeleton, which fosters rapid growth; (4) the peak diversity of living balanoids coincides precisely with the bathymetric gap in the chthamaloids’ distribution.

We argue that the radiation of balanoids during the past 40 Myr or so has caused a decline in chthamaloid diversity by reducing and destabilizing low intertidal and shallow subtidal populations, thus elevating average rate of extinction and depressing average rate of speciation. This model allows for exceptions to the rule of balanoid competitive dominance, but contrary to Paine's claim, the exceptions that exist are either trivial or support our view.

Paine offers no alternative to our competition hypothesis. He suggests that an increase in predation has caused a decline in chthamaloid body size, but neither a Cenozoic trend in body size nor one in predation pressure has empirical support; furthermore, the body-size hypothesis has no direct bearing on the case we have presented.

The hypothesis of limiting similarity, which postulates that morphologically and/or ecologically similar species will differ enough in shape, size, or other variables to minimize competition, has been controversial among ecologists and paleoecologists. Many studies have reported the occurrence of limiting similarity in modern environments or in time-averaged fossil deposits; however, empirical high-resolution time series demonstrating limiting similarity over longer time scales are lacking. We have integrated radiocarbon-calibrated amino acid dating techniques, stable isotope estimates, and morphometric data to test the hypothesis of limiting similarity in late Quaternary land snails from the Canary Islands over a period of 42,500 years. We tested for both ecological character displacement (two closely related species will differ in size in order to minimize competition in sympatry and these differences will be minimized in allopatry) and communitywide character displacement (overdispersion of body size among competitors in a guild). Multiple proxies of body size consistently show that two endemic congeneric pulmonate gastropod species (Theba geminata and T. arinagae) maintained a difference in size from ~42,500 B.P. through the last occurrence of T. arinagae 14,900 B.P., with a concomitant trend of a decreasing body size. Theba geminata body size did not converge on that of T. arinagae and variation in T. geminata body size did not increase significantly following the extinction of T. arinagae; therefore, ecological character displacement and release did not occur. Community-wide character displacement was found in only one time bin over the last 42,500 years. These results suggest that limiting similarity is a transient ecological phenomenon rather than a long-term evolutionary process. This study not only demonstrates the problems inherent in biological “snapshot” studies and geological studies of time-averaged deposits to test limiting similarity adequately, but it also presents a more adequate research protocol to test the importance of interspecific competition in the history of life.

The use of clade-shape statistics, particularly center of gravity, on paraphyletic groups has been questioned since the introduction of these descriptive statistics. In addition, previous studies have found that early-arising groups of organisms are bottom-heavy in center of gravity relative to later-arising groups, leading to macroevolutionary hypotheses about the nature of diversification. Paraphyletic groups have been asserted to be inherently bottom-heavy because of preconceived ideas as to how paraphyletic group formation affects center of gravity. In turn, early-arising groups of organisms have been asserted to be bottom-heavy because they also tend to be paraphyletic, thus calling into question the biological significance of the bottom-heaviness of early-arising groups.

In this study, both computer simulations and the evolutionary history of families of mammals show that paraphyletic groups are inherently top-heavy. Simulated monophyletic clades and monophyletic families of mammals have centers of gravity near their expected positions at the midpoint of clade duration, while paraphyletic groups are significantly top-heavy.

Early-arising families of mammals are found to be inherently top-heavy, rather than bottom-heavy as found by earlier studies. This new result is likely to be due to the use of updated stratigraphic range data and different taxonomic assignments rather than to bias due to inclusion of paraphyletic groups or bias due to unequal time-unit lengths in the Cenozoic time scale.

Differences in zooid skeletal ontogeny are strongly linked to the distribution of bryozoan species, even along such large-scale environmental gradients as depth and latitude. Three growth patterns broadly characterize zooid formation for encrusting cheilostome bryozoans: zooidal and intrazooidal budding, which facilitate growth in two dimensions at colony margins; and frontal budding, which also permits upward growth in the third dimension. Analyses of skeletal growth patterns of encrusting species present in 230 Recent assemblages from the North Atlantic, Mediterranean, North Sea, and Gulf of Mexico show that species with zooidal budding dominate overwhelmingly at lower latitudes and in shallower water, as assessed by percentages of species within assemblages. In contrast, patterns within moderately diverse fossil assemblages from North America indicate that within the geographic and environmental limits sampled, species with intrazooidal budding once dominated and probably originated in shallow, warm water environments. Intrazooidal budding now occurs with greater relative frequency among species from deeper and higher latitude assemblages. Patterns for frontal budding suggest a slight decrease with increasing depth, but no consistent relationship with latitude. These paleoenvironmental trends occur withn a putatively monophyletic clade and are markedly similar to onshore/offshore trends recognized in other groups of marine benthos, based on patterns of occurrence of supra-specific taxa and novel morphotypes. Results presented here are based on relative dominance of species within assemblages and thus provide a novel comparative test of these previously reported trends.

The role of antagonistic organismal interactions in the production of long-term macroevolutionary trends has been debated for decades. Some evidence seems to suggest that temporal trends in predation frequency share a common causative mechanism with genus-level diversity, whereas studies on the role of parasites in “shaping” the evolutionary process are rare indeed. Digenean trematodes (Phylum Platyhelminthes) infest molluscs in at least one stage of their complex life cycle. Trematodes leave characteristic oval-shaped pits with raised rims on the interior of their bivalve hosts, and these pits are preserved in the fossil record. Here we survey 11,785 valves from the Pleistocene–Holocene deposits of the Po Plain and from nearby modern coastal environments on the northeast Adriatic coast of Italy. Of these, 205 valves exhibited trematode-induced pits. Trematodes were selective parasites in terms of host taxonomy and host body size. Infestation was restricted to lower shoreface/transition-to-platform paleoenvironments. During the Holocene, individuals from the transgressive systems tract were significantly more likely to be infested than those from highstand systems tracts. Temporal trends in infestation frequency cannot be explained as an ecological/evolutionary phenomenon (e.g., the hypothesis of escalation); instead the trend seems controlled by environmental variation induced by glacio-eustatic sea-level changes and inadequate sampling. Because this interaction appears to be ephemeral, both temporally and spatially, it is not likely that any selective pressure would be continuous over geologic time in this region. Furthermore, these results support the hypothesis that antagonistic interactions are lower in the northern Adriatic Sea in comparison to other midlatitude shallow marine settings.

Fish eagles (genus Haliaeetus) potentially have contributed the bones of their prey to many late Cenozoic concentrations of vertebrate microfossils. To evaluate possible biases in the taxa and skeletal elements preserved in assemblages accumulated by fish eagles, which could affect paleoecological and evolutionary interpretations derived from Cenozoic microfaunal localities, a sample of 1883 bones accumulated by modern African fish eagles (H. vocifer) was collected from a roost near Lake Turkana, Kenya. Characteristics of the bone assemblage include (1) taxonomic composition dominated by fish; (2) diverse but small-sized and ecologically restricted animals including inshore fish, aquatic birds, very small nocturnal mammals (which may have been contributed by other raptors), and small reptiles (possible predators on the roost); (3) poor survivorship for specific categories of skeletal elements; (4) considerable bone breakage for all but the smallest animals; and (5) a distinctive pattern of postcranial fragmentation for bird and mammal elements. The fish eagle element profile has some similarities to those of other diurnal raptors, but its cumulative signature can be distinguished from those of other bone accumulators and includes characteristics that would also be manifest in fossil assemblages. Examination of fossil microfaunas from the Plio-Pleistocene localities of Kanapoi (Kenya), Inolelo (Tanzania), and Olduvai Gorge (Tanzania) shows that the patterns of element preservation for Kanapoi and Inolelo do not match those of the Lake Turkana fish eagle roost. Nonetheless, some aspects of the Olduvai Gorge FLKN assemblage show taxonomic and skeletal similarites. In light of the selective choices of prey by fish eagles, the fauna recovered from Olduvai Gorge FLKN might not accurately reflect faunal diversity or patterns of abundance at this locality during the Pleistocene. Because fish eagles of the genus Haliaeetus are found on three continents, have a fossil record extending to the Miocene, and deposit bones in a habitat conducive to preservation, the taxa preserved at many other late Cenozoic microfaunal sites also might be biased. Therefore, taphonomic profiles of late Cenozoic microfaunal assemblages should be compared against the taphonomic profiles typical of fish eagle roosts before faunal characterizations of sites are applied in analyses of paleoecological evolution.

The rarefaction graphs and trophic nuclei of dead molluscan shell cumulates and their associated live molluscan fauna from seven ‘ecological’ habitats in the Essex Chenier Plain facies are examined and compared. The results show that while the trophic nuclei of the live and dead faunas tend to be dissimilar, the rarefaction graphs indicate that the dead fauna will be more diverse than the live, though changes in the diversity of the live fauna tend to be mirrored in the dead fauna.

We outline a plausible evolutionary sequence that led from prokaryotes to the origin of the first nucleated cell. The nucleus is postulated to evolve after the archaebacterium and eubacterium merged to form the symbiotic ancestor of amitochondriate protists. Descendants of these amitochondriate cells (archaeprotists) today thrive in organic-rich anoxic habitats where they are amenable to study. Eukaryosis, the origin of nucleated cells, occurred by the middle Proterozoic Eon prior to the deposition in sediments of well-preserved microfossils such as Vandalosphaeridium and the spiny spheres in the Doushantou cherts of China.

Significant warming of Earth's climate in the near term seems increasingly likely. If significant enough, this climatic regime could, in the long term, come to resemble previous greenhouse intervals in earth history. Consequently, analysis of the fossil record during periods of extreme warmth may provide important lessons for species biology, including biogeography, in a much warmer world. To explore this issue, we analyzed the biogeographic response of 63 molluscan species to the long-term global warmth in the Late Cretaceous Western Interior Seaway (WIS) of North America, using Geographic Information Systems (GIS) to quantitatively measure changes in range size and distribution throughout this interval. We specifically considered the role that geographic range size played in mediating extinction resistance and invasion potential of these WIS species. We found no relationship between geographic range size and survivorship. However, endemic species with small range sizes were more likely to become invasive. Finally, mollusks did not experience a poleward shift in range out of the tropics during this warm regime. To the extent that these patterns are representative, and the WIS and taxa considered constitute a reasonable ancient analogue to a warmer future world, these results suggest that some biogeographic “rules” may not prevail under greenhouse conditions of long-term, equable warmth. They also suggest that other factors beyond geographic range size, including distinctive niche characteristics, may play quite important roles in species survival and invasion potential. This potentially complicates predictions regarding the future responses of extant species to long-term warming.

The covariation patterns of the postcranial skeleton in eight species of moas (Aves, Dinornithidae) from the Pleistocene of New Zealand are described using multiple factor analysis. Rotation schemes include an orthogonal Varimax and oblique (direct quartimin) simple structure solutions.

Four major patterns of covariation are resolved: a length factor, primarily that of leg length; a width factor, including pelvis width and long bone widths; a sternal breadth factor; and a sternal length-posterior pelvis length factor. The first two patterns represent a functional separation between body size and those aspects of the skeleton, scaling allometrically, adapted to support body weight. The sternal breadth factor may indicate that features contributing to trunk support are independent from those width measurements of the pelvis and hindlimb bones. The sternal length-posterior pelvis length factor reveals a pattern of covariation that is somewhat independent of other patterns (long bone lengths) determining variability in body size.

Wolpoff's (1984) recent discussion of evolutionary rates in Homo erectus deserves careful study. Whether Homo erectus or other hominid species exhibit gradual, continuous change in key characters or whether instead there is evidence for morphological stasis in the fossil record is an important question. By allocating all Homo erectus specimens to three groups of early, intermediate, and later geological age and by comparing group means for 13 measurements, Wolpoff attempts to show that gradualism is the rule for this mid-Pleistocene taxon. This method is straightforward, but it is crucial that the samples be composed in a manner which is biologically reasonable. I argue here that Wolpoff has not done this. While there may be legitimate doubt concerning sorting of the fossils, especially where specimens are incomplete, several of the individuals said to be representative of Homo erectus are simply inappropriate for use in this analysis. Wolpoff insists that he has employed a “conservative” definition of the species, but instead he has measured everything in sight. This approach to the record does influence his results.

Consideration of the physics of sinking of hollow, rigid bodies leads to equations of motion for sinking cephalopod shells. We have derived equations of motion for three post-mortem sinking situations: sinking with a fixed amount of water in the phragmocone; rapid phragmocone filling (no siphuncular tube); and slow phragmocone filling (siphuncular tube intact). In all three cases sinking speed can be closely approximated by the terminal velocity calculated from the total weight, buoyancy, and drag parameters of the shell.

Experiments on modern Nautilus shells yield sinking velocities in agreement with calculated values. The experiments also show that orientation of a sinking Nautilus shell varies as the phragmocone fills with water. With small negative buoyancy the shell sinks with its plane of symmetry upright, but as it fills, it begins to rock from side to side and leans over and sinks with its plane of symmetry horizontal when the camerae are about 55% full.

The maximum sinking speed of upright adult Nautilus shells is approximately 30 cm/sec, which appears to be too small for embedding in the bottom upon impact. The maximum depth to which Nautilus sinks in the upright position ranges from about 7 m for rapidly filling shells to as much as 600 m for slowly filling shells. In the latter case, the shell will continue to fill after coming to rest on the bottom, and the stability of the vertical orientation will be removed within 1 or 2 days. Thus, primary vertical preservation of cephalopod shells indicates water depths less than about 10 m.

The linear survivorship curve of Van Valen's Law can be generated from stochastic loss of lineages, given a decreasing sample size within an adaptive zone. No special biological explanation is required or called for in order to explain “linearity” in taxonomic survivorship curves.

Changes in morphological variability within fossil populations of planktonic protists provide insight into the processes responsible for morphological change. Evolution in the radiolarian genera Theocorythium and Lamprocyclas is documented biometrically in Pliocene and Pleistocene core material from the equatorial Pacific and Indian Oceans. The patterns of morphological change in Theocorythium within a single Pacific core could be interpreted as indicating the in situ evolution of T. trachelium trachelium from T. vetulum via intermediate forms during the Pleistocene. However, consideration of biogeographic data shows that this is not the case and supports instead two alternate interpretations: (1) immigration coupled with extensive hybridization and introgression, or (2) multiple polymorphs of a single species changing in frequency along a cline. Analyses of evolutionary change in Theocorythium and the closely related genus Lamprocyclas during the Pliocene suggest that hybridization may be a significant factor in the production of new species. Evolutionary models and phylogenetic analyses should consider that barriers to gene flow may vary significantly with time and permit the exchange of genetic information between populations previously reproductively isolated.

An unusual isolated specimen representing an immature rhabdosome of Dicellograptus sp. from the Middle Ordovician Athens Shale of Alabama has a flattened sphere attached to the distal end of the nema. The flattened sphere can be interpreted as a float that probably conferred buoyancy on the rhabdosome. Such floats, which became detached in mature rhabdosomes, may be common in the fossil record. The presence of nematophorous siculae in all planktonic graptolites suggests that for immature rhabdosomes a passive, buoyant mode of life, attained by means of a float, might have been widespread taxonomically. This mode of life is compatible with the theory of passive response for the mode of life of mature rhabdosomes, but it is difficult to reconcile with the theory of automobility.

Fish scales accumulating in marine laminated sediments can provide a record of population variability of small pelagic fishes. Although some studies have noted signs of scale degradation that could affect estimates of population variability, there are presently no well-developed means to evaluate degradation. We developed several indices as indicators of fish scale preservation in two box-cores that we collected off Pisco (14°S), one at 301 m near the center of the oxygen minimum zone (OMZ), and the other at 201 m near the upper limit of the OMZ. These indices include (1) an index of fish scale integrity (estimate of scale wholeness relative to fragmentation), (2) the fungi-free area of fish scales and vertebrae, (3) the ratio of fish scales to vertebrae (as well as fish scales to vertebrae and bones), and (4) the ratio of whole scales to fragments. We address whether lower numbers of anchovy scales occurring in association with reduced total organic carbon fluxes and higher bottom-water oxygen concentrations are due entirely to lower abundances of anchovy or whether differential preservation of the fish scales in the sediments plays an important role in reduced scale abundances. Comparison of temporal sequences between the two cores provides the means to assess whether there are differences in the preservation of fish scales. The combined indices indicate that the lower numbers of fish scales in the earliest period have been affected by degradation, and to a greater degree in the box-core from 201 meters, which can be subject to higher oxygen concentrations. On the other hand, decadal-scale variations in fish scale abundance within the period of better preservation are unlikely to be caused by degradation. We discuss the utility and drawbacks of different indices of preservation for reconstructing past changes in fish population sizes with fluxes of fish debris and also briefly discuss the utility of these indices to other paleobiological systems.