bluuebunny/crossref_metadata_2025 huggingface dataset:
Gastropod radulae and the assessment of form in evolutionary paleontology
Authors: Hickman Carole S.
Published: July 2016

Evolutionary paleontologists seek explanations of form and pattern in a diversity of contexts. A review of this diversity is presented to underscore the difficulty of producing a general model of the factors controlling the morphology of real organisms. A model specific to the marine prosobranch gastropod radula identifies seven factors contributing to form and pattern: (1) phylogenetic, (2) mechanical, (3) ecological, (4) programmatic, (5) maturational, (6) degenerative, and (7) constructional. Aspects of radular morphology attributable to each factor are illustrated in scanning electron micrographs of radulae of marine prosobranch gastropods. Applications of and extrapolations from the radular model are made to promote recognition of a broader range of interacting factors in other systems. Some aspects of form and pattern convey important information about non-evolutionary processes and phenomena and are best examined outside of the evolutionary framework.
Bathymetric anomalies in the Neogene fossil record: The role of diving marine birds
Authors: Lindberg David R. , Kellogg Michael G.
Published: July 2016

Associations of bathymetrically anomalous fossils, a phenomenon particularly common in shallow-water, marine, Neogene faunas of the northeastern Pacific, have yet to be explained adequately. No known physical transport processes can selectively move species upslope from deep water into diverse nearshore, shallow-water habitats. Previously proposed biological explanations are based on undocumented phenomena. We document bathymetric anomalies in Recent mollusc accumulations on Southeast Farallon Island, California, that are created by the activities of diving marine birds. Application of these observations to patterns in the Neogene fossil record is direct, involving the same genera and often the same species. We argue that the process has occurred since at least the Miocene when diving marine birds radiated rapidly, probably in response to trophic resources created by intensive upwelling (Lipps and Mitchell 1976) and that they could have transported specimens upslope in sufficient quantity to have contributed anomalous species to the Neogene fossil record. Specific examples from the literature are discussed.
The improbability of animal phyla with few species
Authors: Strathmann Richard R. , Slatkin Montgomery
Published: July 2016

At present there are many animal phyla that contain only a few species. The existence of these small phyla can be used to test assumptions about speciation and extinction in multicellular animals.

We first model the number of species in a monophyletic clade with a birth and death process that assumes rates of speciation and extinction are constant. If no new phyla have evolved since the Cambrian and speciation and extinction rates for minor phyla are similar to or greater than those estimated from fossils, then our model shows that the probabilities of minor phyla surviving to the present are small. Random variation in extinction and speciation rates does not improve the chances for persistence. If speciation rates exceed extinction rates at the initial radiation of the clade, but before the clade becomes large, speciation rates come to equal extinction rates and both are low, persistence from before the Ordovician up to the present becomes likely. These models show that if speciation and extinction rates are independent of the number of species in a clade, then conditions before the Ordovician strongly influence today's distribution of species among taxa.

We also discuss a model in which speciation and extinction rates depend on the number of species in a clade. This alternative model can account for the persistence of phyla with few species to the present and predicts a short duration for phyla that did not exceed a threshold number of species.
On the early origins of major biologic groups
Authors: Raup David M.
Published: July 2016

The commonly observed fact that the principal biologic groups appear early in an adaptive radiation is shown to be an artifact of the geometry of the evolutionary tree. For marine invertebrates of the Phanerozoic, total diversity and species turnover rates combine to make early origins of major groups inevitable. This is demonstrated through an analysis of the theoretical probability distribution of divergence time (time to most recent common ancestor) in comparison with observed distributions. Whereas the fact of early origins does not require ecological explanations, the interpretation of the morphologic distinctness of major groups when they first appear remains an ecological problem.
Evolutionary tempo in Jurassic and Cretaceous ammonites
Authors: Ward Peter D. , Signor Philip W.
Published: July 2016

Stage-level range data for 983 Jurassic and Cretaceous ammonite genera, distributed within 83 families, were analyzed by assigning absolute ages to stages or substages. We found ammonite genera to have a mean generic range of 7.3 Myr/ammonite genus. Using a similar methodology, mean generic range per family was also computed. The distribution of long-ranging genera (arbitrarily chosen as those ammonite genera ranging for 12 Myr or more) among families was found to be nonrandom. Instead, long-ranging genera were found to be concentrated in a few families, resulting in significant heterogeneity in the distribution of generic longevities within families (taxotely sensu Raup and Marshall 1980). Although some of the long-ranging genera were found to be morphologically simpler than shorter-ranging genera, others were equally or even more complex, indicating that longevity among ammonite genera is not merely a taxonomic artifact, dependent on degree of differentiable conch morphology. Those Cretaceous families composed of a large number of long-ranging genera were also among the leaders in mean species longevity per family, based on species-level range data for Cretaceous ammonites of the Great Valley Sequence of California. Many of the long-ranging genera and species possess a similar morphologic attribute: siphuncular tubes (connecting rings) of small diameter but high wall thickness.
Precambrian-Cambrian Boundary: the spike is driven and the monolith crumbles
Authors: Sepkoski J. John , Knoll Andrew H.
Published: July 2016

The Precambrian-Cambrian Boundary traditionally has been viewed as a fundamental discontinuity in the historical record of life. Almost from the birth of stratigraphic geology, it was recognized that the richly fossiliferous strata of Cambrian and younger periods were underlain by sedimentary rocks largely devoid of macroscopic fossils. Darwin (1872) was well aware of this dichotomy and of the “sudden manner in which several groups of species first appear” in the rock record. In the sixth edition of theOrigin of Species(1872), he wrote, “The case [for this sudden appearance] at present must remain inexplicable … and may be truly urged as a valid argument against the views [on evolution] here entertained.” Careful biostratigraphic work by Walcott and others in the closing decades of the nineteenth century accentuated rather than dispelled the problem. Diverse Early Cambrian invertebrate assemblages were documented from North America and Scandinavia, but repeated searches for comparable fossils in Precambrian strata produced only a few, enigmatic remains (see review by Yochelson [1979]).
Rancho La Brea: status and future
Authors: Akersten William A. , Shaw Christopher A. , Jefferson George T. , Page George C.
Published: July 2016

Rancho La Brea is unquestionably the most famous fossil deposit in North America, and its fauna occupies a key position in vertebrate paleontology as the basis for the Rancholabrean Land Mammal Age. The 23 acres now known as Hancock Park yielded the bulk of fossil specimens from this locality. A minimum of 518 taxa have been identified to date including 79 algae (all but one of which are diatoms), 72 vascular plants, 9 nonmarine mollusks (13 taxa of marine mollusks were carried to the site by early Indians), 135 arthropods, 27 lower vertebrates, 138 birds, and 58 mammals. A detailed list of the biota, too lengthy for inclusion here, is available from the authors on request.
Character diversification and patterns of evolution in early vascular plants
Authors: Knoll Andrew H. , Niklas Karl J. , Gensel Patricia G. , Tiffney Bruce H.
Published: July 2016

Available data on the stratigraphic ranges of latest Silurian and Devonian vascular plant macro-fossils (sporophytes) and spores provide insights into the tempo and mode of early tracheophyte evolution. Patterns of diversification, origination, and extinction conform in general to the predictions of Sepkoski's kinetic model of diversification. Rates of generic origination and extinction vary not only through time but also between organ systems for a single time interval. This fact, coupled with data on longevity and turnover and comparative morphological observations, can be used to document mosaic evolution in early vascular plant history. Mosaic evolution is an important theme in plant evolution; indeed, what we recognize as macroevolutionary events often correlate with brief periods of pronounced mosaicism. Such evolutionary patterns reflect the developmental biology of tracheophytes in which individual organs often have life spans that are considerably shorter than the life of the whole plant. Under these conditions, individual organs or organ systems can respond to different sets of evolutionary pressures.

The major period of early vascular plant diversification occurred during the late Early and early Middle Devonian Period, 30 Myr or more after the origin of the group. Such lags in diversification are not uncommon in the fossil record. Sometimes they reflect extrinsic controls on diversification, but in other cases they appear to be a consequence of intrinsic rates of origination and extinction.
Generation times and the Quaternary evolution of reef-building corals
Authors: Potts D. C.
Published: July 2016

Faunal stasis among Indo-Pacific reef-building corals is explained as the result of chronic environmental disruptions preventing evolutionary processes from approaching completion since the Late Pliocene. The model assumes shallow reefal habitats (<20 m) on the continental shelves are major sites of scleractinian evolution and explores ecological and evolutionary consequences of high-frequency sea-level fluctuations and their associated transgression-regression cycles. Because single generations, dominated by a few large clonal genotypes, may persist indefinitely, local populations may not have experienced enough generations to approach evolutionary equilibrium with their environments during the estimated average duration (≈3200 yr) of existence of shallow habitats. Persistent consequences of chronic evolutionary disturbance may be the extensive intraspecific variation so characteristic of the dominant genera of shallow Indo-Pacific corals and the apparent paucity of recently evolved endemic species. The same disturbances may have accelerated speciation rates among reefal organisms with much shorter generation times.
The cost of phenotypic evolution
Authors: Charlesworth Brian
Published: July 2016

Formulae are developed for the fraction of the population that must be eliminated by selection in each generation in order to account for a given rate of evolution in a metrical trait. A combination of directional and stabilizing selection is assumed. The effects of competition based on phenotypic value are also considered. The formulae are applied to data on evolutionary rates and their variances. It is concluded that most selective elimination is due to stabilizing selection, and that even very rapid evolutionary change in a single character usually involves low levels of additional elimination. It is suggested that long-sustained evolutionary trends are unlikely to be caused either by gradual change of the optimum under stabilizing selection or by the effects of competitive selection in favor of extreme individuals. It is possible that genetic correlations between traits under selection may limit the response of a single trait to directional selection and hence produce very gradual change.
Testing for variation in taxonomic extinction probabilities: a suggested methodology and some results
Authors: Conroy Michael J. , Nichols James D.
Published: July 2016

Several important questions in evolutionary biology and paleobiology involve sources of variation in extinction rates. In all cases of which we are aware, extinction rates have been estimated from data in which the probability that an observation (e.g., a fossil taxon) will occur is related both to extinction rates and to what we term encounter probabilities. Any statistical method for analyzing fossil data should at a minimum permit separate inferences on these two components. We develop a method for estimating taxonomic extinction rates from stratigraphic range data and for testing hypotheses about variability in these rates. We use this method to estimate extinction rates and to test the hypothesis of constant extinction rates for several sets of stratigraphic range data. The results of our tests support the hypothesis that extinction rates varied over the geologic time periods examined. We also present a test that can be used to identify periods of high or low extinction probabilities and provide an example using Phanerozoic invertebrate data. Extinction rates should be analyzed using stochastic models, in which it is recognized that stratigraphic samples are random variates and that sampling is imperfect.
Disarticulation patterns of some modern East African mammals
Authors: Hill Andrew , Behrensmeyer Anna K.
Published: July 2016

The sequences of skeletal disarticulation in a broad range of African mammals in a tropical savanna environment suggest that in general the process is very consistent. There is some variation among species, but this cannot yet be convincingly related to higher taxonomic categories or to body size. Disarticulation begins shortly after death, proceeds more or less continuously, and is almost complete after 5 yr. Because of the overall predictability of disarticulation, the stage of disarticulation reached by comparable fossil skeletons provides useful information about their taphonomic history prior to burial.
Does archaeogastropod respiration fail in turbid water?
Authors: Gilinsky Norman L.
Published: July 2016

C. M. Yonge's idea that differences in habitat distribution and species diversity between archaeogastropods and caenogastropods are caused by differences in their respective abilities to obtain oxygen in turbid water is tested experimentally for the first time. Of 11 species of marine prosobranchs studied—six archaeogastropods and five caenogastropods—three archaeogastropods and two caenogastropods show significant reductions of respiration in response to turbidity, as shown using one-way analysis of covariance (ANCOVA). Two-way nested ANCOVA, which goes beyond one-way ANCOVA by incorporating all data into a single analysis, was also unable to detect any significant difference between archaeogastropods and caenogastropods in their respiratory responses to turbidity. Though the number of species studied was limited, the data and statistical tests suggest that there may be no biologically important systematic difference between archaeogastropods and caenogastropods in their respiratory responses to turbid conditions. Results of this study cast doubt on the validity of Yonge's hypothesis and suggest the need for new research into the controls of gastropod distribution and species diversity.
Heterochrony and the achievement of the multibrachiate grade in camerate crinoids
Authors: Broadhead Thomas W.
Published: July 2016

The multibrachiate grade of organization in camerate crinoids was principally attained through pinnule differentiation: the accelerated development of a proximal pinnule that ultimately became a pinnule-bearing arm. Although pinnule differentiation is not the only means of achieving a multibrachiate organization in modern crinoids, it was the primary mode in producing both dichotomous and exotomous branching patterns in multibrachiate camerates. Whereas most of the proximal branchings, commonly fixed in the cup of camerates, were due to pinnule differentiaton, the first dichotomy and distal dichotomies above the cup apparently developed from direct modification of a brachial plate at the growing tip of the immature crinoid arm.
Aerosol suspension feeding and current velocities: distributional controls for late Osagean crinoids
Authors: Kammer Thomas W. , Ausich William I.
Published: July 2016

Distributional patterns of late Osagean (Mississippian) crinoids from the east-central United States are examined using multivariate analysis of crinoid species diversity and species abundance data. We confirm previous hypotheses that three well-defined crinoid associations existed during the late Osagean. These associations were dominated, respectively, by 1) monobathrid camerates preserved in carbonate packstones; 2) poteriocrine inadunates in higher-energy siltstones and sandstones; and 3) disparid inadunates, cyathocrine inadunates, and flexibles in mudstones where neither monobathrids nor poteriocrines dominated. In conjunction with petrologic data on the enclosing sediments, the analyses suggest that these associations occurred along a spectrum of increasing current velocity at the seafloor. Camerates, poteriocrine inadunates, and flexibles are interpreted to display preferences for specific environmental conditions, whereas disparid and cyathocrine inadunates are inferred to be environmental generalists.

The different environmental distributions of the major crinoid groups are interpreted to be a function of the mode or modes of aerosol filtration feeding used by each group. This inference is possible through functional morphologic and morphometric studies of crinoid arms, because the skeletal elements of the arms, which are commonly preserved, are directly involved in feeding.
Asymmetrical patterns of origination and extinction in higher taxa.
Authors: Gilinsky Norman L. , Bambach Richard K.
Published: July 2016

Using Sepkoski's compendium of fossil marine families (1982a, and updates), we have analyzed the changing pace of familial origination and extinction within 55 extinct and 44 extant higher taxa of marine organisms. Eight different metrics were calculated, and least-squares regression analysis was used to identify within-taxon trends in the data. All metrics and analyses gave essentially the same results. Origination metrics decline significantly with time during the histories of higher taxa, while extinction metrics increase significantly. The number of statistically significant declines of origination metric, however, substantially and invariably exceeds the number of statistically significant increases of extinction metric for each pair of corresponding metrics analyzed. It follows, therefore, that temporal trends in the pace of origination and extinction within higher taxa are highly asymmetrical.

Further analysis shows that truncating data from temporal endpoints has little effect upon the intensity of origination trends, implying that declining pace of origination is a sustained property of the long term histories of taxa. Such truncation, however, reduces the intensity of extinction trends to statistical insignificance and confirms Van Valen's (1985a) suggestion that extinction behaves largely as a stationary process. If the histories of higher taxa are characterized by substantial declines in the pace of origination while the pace of extinction remains largely stationary, it follows that declining pace of origination is an important controlling factor in long term taxic evolution.
On the statistical detection of cycles in extinctions in the marine fossil record
Authors: Quinn James F.
Published: July 2016

Periodicity has recently been reported in the extinction rates of fossil marine families since the Permian. The analysis used appears particularly sensitive to parameter estimation techniques, particularly in the definition of mass extinctions. It also fails to incorporate autocorrelation in the fossil record into its null hypothesis and rests on an inappropriatea posterioricomparison to the null hypothesis. An alternative analysis, examining the time-lags between periods of high extinction rates, produces no evidence of a cycle.
Macroevolutionary implications of mass extinction — evidence from an Upper Cambrian stage boundary
Authors: Westrop Stephen R.
Published: July 2016

Survival of North American shelf trilobite families during a mass extinction across the upper boundary of the Upper Cambrian Sunwaptan Stage (=“Ptychaspid Biomere”) cannot be predicted from patterns of turnover among their component species: on the shelf, stratigraphic ranges of species belonging to surviving families do not differ significantly from those of eliminated families. Thus, the sorting of families during the extinctions cannot be explained by simple upward causation from the individual level. However, families that ranged from shelf to slope environments before the extinctions (most of which are pandemic) fared significantly better than those confined to shelf settings (which tend to be endemic to North America), indicating that family survival was influenced by geographic and environmental distribution, a property emergent above the individual level. One factor implicated in clade survival during normal background times, high species richness, did not influence the outcome of mass extinction. The results affirm the importance of a hierarchical approach to the interpretation of macroevolutionary patterns and provide some support to the suggestion that sorting processes operating during mass extinctions differ from those of background times.
How good was the fossil record? Clues from the Californian Pleistocene
Authors: Valentine James W.
Published: July 2016

The living members of 113 families of bivalves and gastropods of the Californian Province include 698 species living at shelf depths, of which 538 or 77% are known as Pleistocene fossils from the same region; another 113 fossil species are extralimital, and 98 are extinct. Living species not found as fossils are chiefly rare today, and/or minute, fragile, and/or from deeper shelf habitats. Sampling of the Pleistocene record has been biased towards shallow-water assemblages. Fragile and minute forms are probably underrepresented in the record. Rare forms, however, are still appearing as new studies are conducted, and many rare species are yet to be discovered. At least 85% of durably skeletonized living species may have been captured in the record. It is probable that most durably skeletonized invertebrate species were represented in lithostratigraphic units throughout the Phanerozoic, but that this record is lost owing to erosion, burial, and destruction of skeletons in situ. The bulk of the marine invertebrate fossil record does not represent a series of unusual skeletal accumulations, but rather the preserved remnants of an excellent original record formed through ordinary though episodic processes.
Growth in encrusting cheilostome bryozoans: I. Evolutionary trends
Authors: Lidgard Scott , Jackson Jeremy B. C.
Published: July 2016

Growth of the colony is a basic element of morphological evolution and life history in cheilostome bryozoans. Here we consider the occurrence of different modes of growth in encrusting cheilostomes through geologic time and in well-studied living associations. We assess patterns of zooid formation by direct examination of skeletal characters in species from nearly all diverse fossil assemblages reported from North America and quantify within-assemblage diversities and abundance rankings for fossil encrusting species with different modes of growth. These data document macroevolutionary trends showing a transition from dominance of an apparently primitive mode of budding in the Early Cretaceous to derived modes through the Tertiary. The trends are characterized by their long duration and apparent convergence among systematic subgroups within the Cheilostomata. We then consider the validity of our observations as adaptive trends. Patterns of ecological dominance among living and fossil species indicate that different patterns of zooid formation are important determinants of success of colonies as reflected by their abundance, competitive ability, survivorship, and recovery from injury or predation. The consistency of the long-term trends and evidence for the existence of ecological mechanisms in fossil assemblages suggest a major evolutionary role for biotic interactions.
The ichthyosaurian tailbend: a verification problem facilitated by computed tomography
Authors: McGowan C.
Published: July 2016

The tailbend, a downward flexure of the vertebral column, is a prominent feature of post-Triassic ichthyosaurs. However, due to the vagaries of preservation and of preparation, its presence cannot be verified in all skeletons. Computed tomography offers a solution to the problem by identifying the wedge-shaped centra that contribute to the tailbend. Leptopterygius tenuirostris appears to have a tailbend, albeit slightly downturned. There is also evidence that its close relative Eurhinosaurus longirostris had a tailbend, too.
From regional to total geographic ranges: testing the relationship in Recent bivalves
Authors: Jablonski David , Valentine James W.
Published: July 2016

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.
The continuum in soft-bodied biotas from transitional environments: a quantitative comparison of Triassic and Carboniferous Konservat-Lagerstätten
Authors: Briggs Derek E. G. , Gall Jean-Claude
Published: July 2016

The exceptionally preserved Triassic (Anisian) Grès à Voltzia fauna from the Vosges of northeastern France is compared with four major Carboniferous Konservat-Lagerstätten. The problem of comparing faunas with different types of preservation and degrees of taxonomic determination is addressed with a newly devised similarity coefficient. This coefficient quantifies and combines data from different levels in the taxonomic hierarchy and allows comparisons between biotas in a consistent direction regardless of relative diversity. The rank order of similarity between the four Carboniferous Konservat-Lagerstätten and the Grès à Voltzia is as follows: 1. Mazon Creek (Westphalian D) of Illinois; 2. Glencartholm (Visean) of southern Scotland; 3. Bear Gulch (Namurian) of Montana; and 4. Blanzy-Montceau (Stephanian) of France. These occurrences represent conditions transitional between nearshore fully marine and fresh water. The Grès à Voltzia fauna is significantly closer to the Mazon Creek fauna than to the others; the taxonomic overlap with Blanzy-Montceau and Bear Gulch is limited.

Stratigraphic age has an insignificant influence on the result; indeed, the fauna closest in age to the Grès à Voltzia, that of Blanzy-Montceau, is least similar. Taphonomic factors are important in determining the range of organisms preserved. The Glencartholm fauna is represented only by forms with either mineralized or robust chitinous skeletons, implying a greater degree of decay prior to the onset of diagenetic mineralization than in the other Konservat-Lagerstätten. Environment, however, is the major control on similarity. The Mazon Creek biota, like that of Grès à Voltzia, represents settings transitional between terrestrial and marine-influenced delta. Groups common to both Konservat-Lagerstätten include medusae, brachiopods, polychaetes, bivalve and gastropod molluscs, limulids, scorpions, spiders, branchiopods, ostracodes, malacostracans, cycloids, euthycarcinoid and myriapod uniramians, insects, fish, and tetrapods. There is a striking continuity between the faunas of Carboniferous and Triassic transitional sedimentary environments. Groups that were adapted to fluctuating conditions (e.g., shifting salinity) show strong congruence at the family and lower levels and were little affected by Permian extinctions. The major taxonomic contrasts are in the eumalacostracans and insects: many of the groups represented in the Grès à Voltzia appeared in the Permian and radiated across the Permo-Triassic boundary as Paleozoic forms became extinct.
Organic preservation of non-mineralizing organisms and the taphonomy of the Burgess Shale
Authors: Butterfield Nicholas J.
Published: July 2016

Organic preservation of non-mineralizing animals constitutes an important part of the paleontological record, yet the processes involved have not been investigated in detail. Organic-walled fossils are generally explicable as a coincidence of original, relatively recalcitrant, extra-cellular materials and more or less anti-biotic depositional circumstances. One of the most pervasive natural inhibitors of biodegradation results from substrate and enzyme adsorption onto, and within, clay minerals; such interactions are likely responsible for many of the organic-walled fossils preserved in clastic sediments. Close examination of the fossilLagerstätteof the Burgess Shale (Middle Cambrian, British Columbia) reveals that most of its so-called soft-bodied fossils are composed of primary (although kerogenized) organic carbon. Their preservation can be attributed to pervasive clay-organic interactions as the organisms were transported in a moving sediment cloud and buried with all cavities and spaces permeated with fine grained clays. The organic-walled Burgess Shale fossils were studied both in petrographic thin section and isolated from the rock matrix, following careful acid maceration. Isotopic analysis of bulk organic and carbonate carbon yielded values consistent with a normal marine paleoenvironment. Anatomical and histological consideration of the enigmatic Burgess wormAmiskwiasuggest that it may in fact be a chaetognath, while the putative chordatePikaiaappears not to be related to modern cephalochordates.
Age and origin of subalpine forest zone
Authors: Axelrod Daniel I.
Published: July 2016

Genera of montane conifers first appeared in the fossil record in large numbers in the Middle Eocene (47-45 Ma) as widespread volcanism in the Cordilleran region provided scattered, cold upland tracts suited for them. Higher subalpine environments, generally above 2,000 m, probably were confined then to upper slopes of scattered strato-volcanos and basement highs covered with thick pyroclastic deposits. As relief increased in the middle Tertiary, and especially during the Pliocene-Quaternary, the area suited for a subalpine forest zone increased significantly, enabling montane conifers to spread into that higher, colder environment and form a new major forest zone. During glaciations the subalpine zone was much larger and probably from 900-1,000 m lower than at present. Spreading post-glacial warmth and drought account for its retreat to higher levels and for the discontinuous distribution of the forest and its species today. Recency of the subalpine adaptive zone is implied also by the close relations of its varieties and subspecies of herbaceous perennials, a few annuals, and pines to those in vegetation zones below it.
Landmark-based morphometrics of spiral accretionary growth
Authors: Johnston Mark R. , Tabachnick R. Elena , Bookstein Fred L.
Published: July 2016

Many organisms continue to grow their skeletons throughout ontogeny. In the shells of molluscs, protists, and brachiopods and in bovid horns, accretionary spiral growth provides a detailed and continuous growth history. Although a shell may be described as a single static form, the overall morphology is a summation of the ongoing accretionary process. For this reason, an explicitly ontogenetic characterization of form provides insight into the final form achieved. Analysis of landmark transformations offers direct access to major components of morphological variation, both among adult individuals and through an individual's ontogeny. Parameters of preconceived, abstract geometric models can also be used to characterize morphological variation, but there is no guarantee that these parameters will coincide with the major features of shape variation.

In order to locate landmarks at equivalent ontogenetic stages, features that indicate ontogenetic stage of coiled forms must be identified (e.g., growth increments, age, size, whorls). The gastropod Epitonium (Nitidiscala) tinctum exhibits prominent varices that provide landmark locations throughout ontogeny. Recent specimens of this species were obtained from three localities in Baja, Mexico. The morphological variation among individuals, treated as whole shells and within individual ontogenies, was analyzed using shape coordinates of landmark configurations. Deformation of shape is expressed in the uniform and nonuniform shape subspaces. The empirical components of shape variation found are similar to those generated by two parameters of an equiangular spiral: θ, the angle between consecutive varices, and W, the whorl expansion rate. The distribution of individuals is examined within morphospaces constructed from these shape features.

Three scales of analysis are necessary to characterize adequately the shape variation within and among specimens. The smallest scale is equivalent to increment-by-increment changes in θ and W. The middle scale comprises variation equivalent to whorls resulting from systematic changes in θ and W during an individual's ontogeny. Finally, there is the overall ontogenetic trajectory. Mean shape must be a function of initial shape and ontogenetic trajectory in shape. Mean forms that are found to have similar shapes at the same arbitrary growth increment may achieve that shape in different ways.
A model of onshore-offshore change in faunal diversity
Authors: Sepkoski J. John
Published: July 2016

Onshore-offshore patterns of faunal change occurred at many taxonomic scales during the Paleozoic Era, ranging from replacement of the Cambrian evolutionary fauna by the Paleozoic fauna to the environmental expansion of many orders and classes. A simple mathematical model is constructed to investigate such change. The environmental gradient across the marine shelf-slope is treated as a linear array of discrete habitats, each of which holds a set number of species, as observed in the fossil record. During any interval of time, some portion of the species in each habitat becomes extinct by background processes, with rates of extinction varying among both clades and habitats, as also observed in the record. After extinction, species are replaced from within the habitat and from immediately adjacent habitats, with proportions dependent on surviving species. This model leads to the prediction that extinction-resistant clades will always diversify at the expense of extinction-prone clades. But if extinction intensity is highest in nearshore habitats, extinction-resistant clades will expand preferentially in the onshore direction, build up diversity there, and then diversify outward toward the offshore. Thus, onshore-offshore patterns of diversification may be the expectation for faunal change quite independently of whether or not clades originate onshore. When the model is parameterized for Paleozoic trilobites and brachiopods, numerical solutions exhibit both a pattern of faunal change and a time span for diversification similar to that seen in the fossil record. They also generate structure similar to that seen in global diversification, including logistic patterns of growth, declining origination but constant extinction within clades through time, and declining overall extinction across clades through time.
The function of astrorhizae in stromatoporoids: quantitative tests
Authors: LaBarbera Michael , Boyajian George E.
Published: July 2016

Using the diameters of the channels at branch points, we quantitatively test three alternative models of the function of astrorhizae in stromatoporoids. The distribution of diameters at branch points is significantly different from the distribution that would be predicted from models of either a diffusive function or a bulk-flow system in which resistance to flow was constant at all levels of the branching hierarchy. The distribution of channel diameters is virtually identical to that predicted by a model (Murray's law) that simultaneously minimizes resistance to flow and some volume-related cost function. Astrorhizae thus carried a bulk flow of fluid and can be inferred to have been lined with cellular elements; the exchange sites associated with the fluid-transport system were distributed throughout the soft tissues of the stromatoporoid animal. The most parsimonious hypothesis of function, that the fluid-transport system was associated with suspension feeding, implies strong similarities between the structure of the stromatoporoid animal and living sponges.
Punctuated anagenesis and the importance of stratigraphy to paleobiology
Authors: MacLeod Norman
Published: July 2016

The depositional history of Upper Miocene through Recent sediments from DSDP Site 214 (Ninetyeast Ridge, Indian Ocean) is reexamined. Samples of the Globorotalia tumida planktic foraminiferal lineage, originally obtained from these sediments by Malmgren et al. (1983), serve as the empirical basis for the recognition of punctuated anagenesis as a distinct mode of phenotypic evolution and have been the subject of numerous additional investigations. However, conclusions reached by previous authors depend strictly on the validity of the original chronostratigraphic interpretation of these sediments. Graphic correlation analysis of first- and last-appearance datum levels for a total of 41 planktic foraminiferal, radiolarian, and calcareous nannoplankton taxa provides evidence for a more complex depositional history at this deep-sea site than originally believed. Based on a conservative model of variation in the pattern of sediment accumulation rates, the lowermost portion of the studied section (6.5-4.3 Ma) represents an interval of temporally condensed sediment accumulation (1.88 cm/1,000 yr) followed by an interval (4.3-2.8 Ma) of temporally expanded sediment accumulation (3.97 cm/1,000 yr). This interval, in turn, is followed by a depositional hiatus or an extremely condensed interval, at least 800,000 yr in duration, which is followed by another relatively condensed (1.36 cm/1,000 yr) interval from 2.0 Ma-Recent. Although this chronostratigraphic reinterpretation deviates substantially from the original, which recognized Site 214 as being both temporally continuous and exhibiting a constant sediment accumulation rate from the Upper Miocene through the Upper Pliocene, it is more consistent with expectations based on Neogene eustatic sea-level fluctuations and global surveys of Neogene hiatus distributions. Age assignments for samples of the Gr. tumida lineage based on the revised chronostratigraphic model reverse some findings of previous investigators with respect to the distinctiveness of phenotypic evolutionary rates characterizing the transition from Gr. plesiotumida to Gr. tumida. Finally, a brief survey of similar marine invertebrate lineage studies shows that changes in the rate of phenotypic evolution often appear to coincide with major physical changes in the paleoceanographic environment. Such correspondences may be due, at least in part, to the effect of these environmental changes on sediment accumulation rates. Paleobiologists who seek to understand patterns of phenotypic change over time must remove the effects of variations in sediment accumulation rates from their data before evolutionary hypothesis testing and remain aware of the limitations imposed on their interpretations by the uncertain nature of chronostratigraphic inference.
Calibrating the paleothermometer: climate, communities, and the evolution of size
Authors: Dayan Tamar , Simberloff Daniel , Tchernov Eitan , Yom-Tov Yoram
Published: July 2016

Studies in the past 20 years have often interpreted size fluctuations in fossil mammal remains as a response to climatic change, in accord with Bergmann's rule. However, such paleoecological inference requires careful consideration of changes in community composition that could cause ecological character displacement or release. Recent size gradients of mammals should be screened for the possibility of character displacement if fossil size gradients are to be used as a “paleothermometer” for past climate. The use of teeth in paleontological studies as a measure of body size presents a further complication; for several carnivore guilds, it appears that competitive pressures act most strongly on tooth size. Teeth may therefore inaccurately estimate body size, even if body size accurately indicates climatic conditions. In the fossil record different species exhibit different size patterns under the same conditions of climatic change, and the same species may show diametrically opposite size fluctuations under similar conditions of climatic change, in different regions.
Faunal interchange and Miocene terrestrial vertebrates of southern Asia
Authors: Barry John C. , Morgan Michele E. , Winkler Alisa J. , Flynn Lawrence J. , Lindsay Everett H. , Jacobs Louis L. , Pilbeam David
Published: July 2016

Problems of stratigraphic completeness and poor temporal resolution make analysis of faunal change in terrestrial sequences difficult. The fluvial Neogene Siwalik formations of India and Pakistan are an exception. They contain a long vertebrate record and have good chronostratigraphic control, making it possible to assess the influence of biotic interchange on Siwalik fossil communities. In Pakistan, the interval between 18 and 7 Ma has been most intensively studied and changes in diversity and relative abundance of ruminant artiodactyls and muroid rodents are documented with temporal resolution of 200,000 years. Within this interval, diversity varies considerably, including an abrupt rise in species number between 15 and 13 Ma, followed by a decline in ruminant diversity after 12 Ma and a decline in muroid diversity in two steps at 13 and 10 Ma. Significant changes in relative abundance of taxa include an increase in bovids between 16.5 and 15 Ma, a decrease in tragulids after 9 Ma, and a very abrupt increase in murids at 12 Ma. Megacricetodontine rodents also decrease significantly at 12 Ma, and smaller declines are recorded among myocricetodontine and copemyine rodents after 16 Ma. An increase of dendromurine rodents at 15.5 Ma is also observed. There is also a trend of progressive size increase among giraffoids and bovids throughout the sequence.

We have also investigated relationships between biotic interchange and diversity, body size, and relative abundance, concluding that (1) the rapid increase in ruminant and muroid diversity was largely due to immigration, whereas in situ speciation had only a secondary role; (2) during intervals of increasing diversity, resident lineages did not have higher than average rates of in situ speciation; (3) during intervals with rising diversity, greater extinction did not accompany increased immigration; (4) during intervals with falling diversity, there may have been greater extinction in recently invading lineages; and (5) change in diversity was independent of changes in relative abundance and body size.
Iterative evolution of hypercarnivory in canids (Mammalia: Carnivora): evolutionary interactions among sympatric predators
Authors: Van Valkenburgh Blaire
Published: July 2016

Convergent evolution of hypercarnivorous adaptations in canids has occurred a number of times in the last 40 m.y. among distantly related taxa. The adaptations include an increase in carnassial blade length, reduction or loss of post-carnassial molars, and transformation of the talonid of the lower first molar from a basinlike depression into a trenchant, bladelike cusp. Although the diversity of these specialized canids is typically low in past and present communities, it was unusually high during the Late Oligocene of North America and the Pleistocene of South America. These two comparable events provide an opportunity for exploring possible causes of the evolution of hypercarnivory in canids. Plots of generic diversity against time for North American predators reveal a roughly inverse relationship between the number of hypercarnivorous canid taxa and the numbers of other hypercarnivores, such as creodonts, nimravids, mustelids, and amphicyonids. Similarly, the radiation of hypercarnivorous canids in South America occurred at a time of relatively low diversity of other hypercarnivores. Analysis of trophic diversity within the North American carnivore paleoguild before, during, and after the Late Oligocene reveals considerable taxonomic turnover among carnivores because of immigration and speciation. Late Oligocene hypercarnivorous canids appear to have been replaced first by amphicyonids and large mustelids, and then by felids.

Despite the repeated tendency of canids to evolve adaptations for hypercarnivory, a canid has yet to appear that is completely catlike, that is, without any post-carnassial molars. This possible constraint on morphological evolution in canids is argued to have resulted, paradoxically, in increased flexibility over evolutionary time and a great potential for rapid diversification and clade survivorship. Finally, it is suggested that the iterative pattern of specialization of the lower molars for meat-slicing that is seen in all families of carnivores, past and present, is probably a result of intraspecific competition for food, perhaps among littermates. This intraspecific selective force is countered by competition among species, since there are limits on the number of sympatric hypercarnivorous species within a single community.