bluuebunny/crossref_metadata_2025 huggingface dataset:
Early ontogeny ofEutrephocerascompared to RecentNautilusand Mesozoic ammonites: evidence from shell morphology and light stable isotopes
Authors: Landman Neil H. , Rye Danny M. , Shelton Kevin L.
Published: July 2016

Observations on the morphology of the early whorls ofEutrephoceras dekayi(Morton), a widespread Cretaceous nautilid, are supplemented with oxygen and carbon isotopic analyses (δ18O and δ13C) of the early septa of five well-preserved specimens to help identify the point of hatching on the shell. Septa 4 and 5 are more closely spaced than preceding septa and probably correspond in time of formation with a constriction or first broken aperture on the outer shell one-third whorl forward of the fourth septum. In modernNautilus, morphologic, isotopic, and observational data suggest that similar features mark hatching. Between the fourth and fifth septa inE. dekayi, δ18O values show a shift of variable magnitude from heavy to lighter values followed by a return to heavier values over the next one to three septa. This isotopic shift is compatible with a hatching interpretation and may be explained as the result of kinetic and equilibrium effects on emergence from an egg capsule.

Eutrephoceras dekayihatched at about 9 mm in diameter, one-third the hatching size of modernNautilus.LikeNautilus, E. dekayiprobably produced few young, all of which were active swimmers at hatching. In contrast, Mesozoic ammonoids produced numerous offspring ranging from 0.5 to 1.5 mm in diameter which may have spent some time in the plankton. These differences in life history may correlate with differences in ecologic specialization, environmental tolerance, and habitat between ammonoids and nautilids and may have contributed to their disparate rates of evolution during the Mesozoic.
Small mammal bone accumulations produced by mammalian carnivores
Authors: Andrews Peter , Evans E. M. Nesbit
Published: July 2016

Small mammal bone assemblages from the scats of three families of mammalian carnivores are described here as a means of assessing paleontological bone assemblages. These are the Viverridae, Canidae, and Mustelidae. Marking behavior by the carnivores or their use of latrine areas can produce concentrations of bone comparable with those produced from owl pellets, but the nature of the bone assemblages is very different. Skeletal element preservation has a characteristic pattern related to the relative strength of the individual elements, so that the strongest elements are the ones that best survive the physical abrasion of the predator's teeth. Bone breakage is greatest in canids and least in some viverrids. Digestion of the bone occurs to a certain extent with all carnivores, with moderate rounding but little corrosion of either bones or teeth in viverrid-derived bone assemblages, very great rounding and severe corrosion of bone and tooth enamel (but little corrosion of tooth dentine) in canids, and moderate rounding and slight to moderate corrosion of bone and tooth dentine (but less of enamel) in mustelid-derived bone assemblages. Tooth marks are generally rare except in canid-derived bone assemblages. These patterns may enable the identification of predators responsible for the accumulation of fossil predator assemblages, and a consideration of the hunting and other behavior of that type of predator then permit the identification of possible biases in the fossil fauna. In the viverrids, for instance, the mongoose selected for large prey size and against cranial elements, while the genet took all prey sizes available to it and all body parts. Larger canids such as foxes and coyotes took both large and small species, but proportionally more of the most common prey species were present. Skulls were underrepresented. The small canid, the bat-eared fox, selected for the smallest prey species available to it but took all body parts. Mustelids took all sizes of prey, although not necessarily in proportion to their abundance in the habitat, and large species were underrepresented. If the predator type is known, therefore, for a fossil assemblage, these biases can be taken into account in assessing the paleoecological significance of the fauna. An example from the Olduvai Pleistocene deposits is given.
Evidence for punctuated gradualism in the Late NeogeneGloborotalia tumidalineage of planktonic foraminifera
Authors: Malmgren Björn A. , Berggren W. A. , Lohmann G. P.
Published: July 2016

The warm-water planktonic foraminiferalGloborotalia tumidalineage has been studied in a 10-Myr-long stratigraphic sequence (Late Miocene through Recent) from the Indian Ocean to determine long-term evolutionary patterns through the lineage's history, and particularly to study in great detail the evolutionary transition fromG. plesiotumidatoG. tumidaacross the Miocene/Pliocene boundary. Sampling resolution was very good, between 5 × 103and 15 × 103yr across the Miocene/Pliocene boundary and about 2 × 105yr otherwise. The test shape was analyzed in edge view, permitting determinations of variation in inflation and elongation of the test. Shape was analyzed quantitatively using eigenshape analysis. This method represents the greatest proportion of variation observed among a collection of shapes by the least number of different shapes. The Late Miocene (10.4-5.6 Myr B.P.) populations exhibited only minor fluctuations in shape that did not result in any net phyletic change. This period of stasis was followed by an 0.6-Myr-long period (between 5.6 and 5.0 Myr B.P.) of gradual transformation of the Late Miocene morphotype(G. plesiotumida)into the Early Pliocene morphotype(G. tumida).The populations were again more or less in stasis in the Pliocene and Pleistocene (5.0 Myr to the present day), so that no major modifications of the newly evolved Early Pliocene morphotype occurred during these 5 Myr. Thus it would appear that theG. tumidalineage, while remaining in relative stasis over a considerable part of its total duration underwent periodic, relatively rapid, morphologic change that did not lead to lineage branching. This pattern does not conform to the gradualistic model of evolution, because that would assume gradual changes throughout the history of the lineage. It also does not conform to the punctuational model, because (1) there was no speciation (lineage branching) in this lineage and (2) the transition was not rapid enough (<1% of the descendant species' duration according to definition). For this evolutionary modality we propose the term “punctuated gradualism” and suggest that this may be a common norm for evolution—at least within the planktonic foraminifera.
Divergences and phyletic transformations in the history of the Globorotalia inflata lineage
Authors: Scott George H.
Published: July 2016

To the student of evolution, Malmgren and Kennett's (1981) detailed study of the Globorotalia inflata lineage demonstrates the continuity of the stratigraphic record and the abundance of planktic microfossils in deep-sea sediments. Their work recalls the prediction (Prothero and Lazarus 1980) that planktic microfossils might serve as the laboratory animals of evolutionary studies and indicates the potential value of fossils in determining ancestry. However, for theorists to judge the evolutionary import of Malmgren and Kennett's high-resolution morphometric study, broader consideration of lineage relationships is needed. They portray the G. inflata lineage as a set of five successional taxa in simple ancestor-descendant relationship. Trends, established morphometrically and claimed to be “one of the clearest examples of gradualism in any group of fossils” (Malmgren and Kennett 1981, p. 236), are shown to progressively transform a compressed shell with peripheral keel (G. conoidea) into a globose shell (G. inflata) without a keel. While the succession of taxa given by Malmgren and Kennett has been supported in other studies of western South Pacific sequences (e.g., McInnes 1965; Walters 1965; Kennett 1966; 1973; Jenkins 1971; Scott 1979; Hornibrook 1982), it is not established that the order indicates simple phyletic transformation without branching, as Malmgren and Kennett imply. Crucial evidence concerns the history and status of keeled populations which resemble the ancestral G. conoidea but occur in the Pliocene, associated with advanced members of the G. inflata lineage. These populations are not mentioned by Malmgren and Kennett, yet, if they are also descendants of G. conoidea, their occurrence strongly suggests that lineage divergence occurred. I wish to review evidence in support of this view.
Mechanical and photosynthetic constraints on the evolution of plant shape
Authors: Niklas Karl J. , Kerchner Vincent
Published: July 2016

A computer model is presented which is capable of calculating both the photosynthetic efficiency (I) of any specified plant shape and the stress related to the total moment arm (M) imposed on vertical branching patterns. Computer simulations indicate that a flattened plant thallus and an erect branching growth habit are two plant shapes capable of optimizing photosynthetic efficiency during indeterminate growth. These two morphologies have geometric analogues in the dorsiventral thalli of some bryophytes and in the vertical axes of mosses and tracheophytes, respectively.

Extension of the model to complex, three-dimensional branching patterns indicates thatIandI/Mare maximized when branching is overtopped (treelike, with lateral branches on a main axis) and when lateral branching systems are planated (frondlike). Geometric alterations of branching patterns that result in optimization ofIandI/Mcan be simulated by computer and are shown to be similar to morphologic alterations attending the early evolution of vascular land plants. It is suggested that a number of major evolutionary trends seen in Upper Silurian to Upper Devonian times can be expressed in terms of optimizing the display of photosynthetic tissues (I) or the balance between photosynthetic efficiency and incurred moment arms (I/M).
Allometry and heterochrony in an Eocene echinoid lineage: morphological change as a by-product of size selection
Authors: McKinney Michael L.
Published: July 2016

Three closely related species of the irregular echinoid Oligopygus succeed one another in the late Eocene deposits of Florida. This apparent lineage, where each species is similar but larger than the preceding species, offers an excellent opportunity to examine relationships between heterochrony and environmental change. Bivariate and multivariate techniques illustrate patterns of simple and complex allometry and indicate that simple size increase related to changes in the timing of maturation account for most of the morphological differences among species. Those differences that are not the by-products of size increase are due to heterochronic changes dissociated from the otherwise global (i.e., whole organism) hypermorphosis and were probably needed to increase the relative food intake of the larger forms. Unlike the vast majority of allometric studies, which show constant growth ratios, the ontogenies analyzed here often exhibit curvilinear trajectories. Thus, heterochronic events are seen as systematic alterations of already changing growth ratios. It is suggested that these heterochronic changes provided quick, efficient responses to selective pressures on a plurality of characters and behaviors. As the Florida carbonate environment became more stable, responses associated with K-type life history strategies were favored, resulting in “size selection.” Thus, the morphological differentia among these species may not have resulted from selection directed at those traits, but were either simple sequelae of size increase or of changes which occurred to permit the size increase.
Radiations and extinctions in relation to environmental change in the marine Lower Jurassic of northwest Europe
Authors: Hallam A.
Published: July 2016

A diversity and turnover analysis has been undertaken for a number of invertebrate groups in the Liassic of northwest Europe. There is a more or less steady rise in diversity from the early Hettangian through to the Pliensbachian, followed by a marked decline into the early Toarcian, after which it tends once more to increase. Ammonites stand out from the other invertebrates as having had an exceptionally high rate of turnover, with very short species durations.

Increase of neritic habitat area due to rise of sea level, and recolonization following the end-Triassic mass extinction event appear to be the promoters of diversity increase or radiation. Severe reductions of neritic habitat area with associated environmental deterioration, related either to episodic marine regressions or spreads of anoxic bottom waters, and bound up respectively with sea-level fall and rise, are seen as the prime factors responsible for increase of extinction rate. While the environmentally sensitive ammonites were affected by even minor regressions, the other, more eurytopic groups were evidently more resistant to these. The only event that warrants the term mass extinction, affecting nearly all the benthos and nekton but not the plankton, correlates precisely with the early Toarcian anoxic event. Several episodes can be recognized of migrations of organisms into Europe following extinctions.
Extinctions in a model taxonomic hierarchy
Authors: Valentine James W. , Walker Timothy D.
Published: July 2016

A computer model of background and mass extinctions in a taxonomic hierarchy has been used to study the effects of different extinction patterns in a search for clues as to the causes of actual extinction events. Model taxa at four levels were built up from speciation events in adaptive space according to rules of origination which seem plausible biologically. The frequency distribution of species among the three higher taxonomic levels in the model is similar to that in living marine taxa which have good fossil records. Three mass extinction patterns were imposed on the model after species diversity had attained equilibrium (i.e., when speciation = background extinction): random; bloc (contiguous niches were cleared); and clade (all members of selected higher taxa were removed). Effects on the taxonomic profile varied with pattern. Four of the five historical mass extinctions resemble the effects of the random pattern. End-Permian families were harder hit than those in the random model, but this may be a result of an extremely high species extinction level. It is concluded that the effect of extinctions on the taxonomic hierarchy provides a tool to help in understanding extinction causes.
Plant taphonomy of late Holocene deposits in Trinity (Clair Engle) Lake, northern California
Authors: Spicer Robert A. , Wolfe Jack A.
Published: July 2016

Six drainage basins at the margins of Trinity Lake were analyzed to determine the relations of source vegetation (largely coniferous forest) to plant debris in deltaic deposits that represent high-energy depositional environments. Thirty-one samples containing an estimated 1,043,000 identifiable plant fragments were subjected to a multivariate statistical (correspondence) analysis; in the resulting ordination, samples from the more mesic side of the lake clustered separately from samples from the drier side of the lake. The distribution of a species in the source vegetation is generally paralleled by the distribution of the same species in the samples, and all major forest trees are present in the samples. However, relative abundances in the source vegetation have no direct relation to relative abundances in the samples. Plants growing far away from depositional sites are poorly represented, even if these plants produce organs suited to long-distance transport; this occurs by dilution by material derived from plants in proximity to depositional sites and suggests that megafossil assemblages that contain putative mixtures of high-altitude “temperate” and low-altitude “thermophilic” taxa represent true biotic associations. The information gained from the Trinity analysis is compared to information gained from analysis of plant debris in low-energy depositional environments; each environment contains different kinds of information that is significant in paleoecological reconstructions of regional vegetation. Whereas high-energy environments contain the best taxonomic representation of source vegetation, low-energy environments retain information on spatial distributions of taxa within source vegetation. High-energy environments typically also contain the best representation of different organs of a given taxon.
Plate translocation in spatangoid echinoids: its morphological, functional and phylogenetic significance
Authors: McNamara Kenneth J.
Published: July 2016

Analysis of relative changes in plate position during ontogeny was made on a number of Cenozoic spatangoid echinoids: species ofBreynia, Lovenia, ProtenasterandEchinocardium. Contrary to the generally held view that adjacent ambulacral and interambulacral columns in echinoids always remain in a fixed relative position during ontogeny, many spatangoids show great fluidity in plate growth, with adjacent columns ‘sliding past’ one another during ontogeny. Furthermore, in many genera plates from one column may undergo strong lateral growth and bisect pairs of plates in adjacent rows. This phenomenon of relative plate movement, both meridional and equatorial, is herein termed ‘plate translocation’. It is considered to occur by localized resorption and redeposition in a narrow zone along adjacent plate boundaries. Plate translocation has been of considerable phylogenetic significance to spatangoids and, combined with an increase in differential allometries between plates, has been one of the most important factors in the evolution of the Spatangoida. Furthermore, the initiation of plate translocation in the apical system in certain Jurassic echinoids may have been the trigger for the migration of the periproct out of the apical system, and a major factor in the evolution of irregular echinoids. Heterochrony in plate growth has been critical in controlling the course of evolution in many lineages of spatangoid echinoids.
Random walk and the existence of evolutionary rates
Authors: Bookstein Fred L.
Published: July 2016

Before one can study evolutionary rates one must reject the null model ofsymmetric random walk. for which the requisite quantity does not exist. As random walks reliably simulate all the features we find so compelling in the fossil record—jumps, trends, and irregular cycles—rejection of this irritating hypothesis is much more difficult than one might hope. This paper reviews principal theorems from the mathematical literature of random walk and shows how they may be applied to empirical data by scaling net changes according to the square root of elapsed time. The notorious pair of “opposite” findings, equilibrium and anagenesis, may be construed as deviations from random walk in opposite directions. Malmgren's data onGloborotalia tumida, previously interpreted as an example of punctuated anagenesis, are consistent with a random walk showing neither punctuation nor anagenesis, but instead varying in speed over four subsequences.
On the estimation of taxonomic longevity from Lyellian curves
Authors: Levinton Jeffrey S. , Farris James S.
Published: July 2016

Stanley's (1979) method of estimating taxonomic longevity from Lyellian curves differs from Kurtén's (1960) because of different underlying assumptions. Kurtén's model supposes that taxon richness and longevity are stochastically invariate over the period used for the Lyellian estimate. Both Kurtén and Stanley seek to calculate the average longevity of a taxon, when the temporal stratigraphic ranges are known after and before a given time datum. Under the exponential decay model used by Kurtén, mean taxonomic longevity is indeed approximately 2.89 half-lives, as he maintained. Stanley's estimate is based on the unrealistic assumptions of uniform longevity and evenly-spaced origins of fossil taxa. This leads to an estimate of longevity as exactly two half-lives.
Biogeographic and evolutionary patterns of continental margin benthic foraminifera
Authors: Buzas Martin A. , Culver Stephen J.
Published: July 2016

Several very large, taxonomically standardized data sets have been compiled and utilized to investigate biogeographic and evolutionary patterns of continental margin benthic foraminifera. Mean partial species durations for 87 frequently occurring and 180 rarely occurring species on the Atlantic continental margin of North America are the same, namely 21 m.y. The global fossil record of these species indicates no center or centers of origin and indicates very rapid dispersal. The Miocene had the greatest number of first occurrences with 46%, followed by the Pleistocene, Pliocene and Oligocene with approximately 13% each. The remaining 14% first occur in the Eocene, Paleocene, and Cretaceous. Species with a wide geographic distribution often exhibit longer species durations than those with narrow geographic ranges. The vast majority of endemic species (150 of 175) occur rarely and have no fossil record.
Larval ecology, life history strategies, and patterns of extinction and survivorship among Ordovician trilobites
Authors: Chatterton Brian D. E. , Speyer Stephen E.
Published: July 2016

Differences in the larval ecology of Ordovician trilobites directly influenced the outcome of the Ashgill extinction (latest Ordovician) and indirectly governed the pattern of evolution in post-Ordovician trilobites. Larval ecology also affected survivorship patterns within the Ordovician, particularly between the Llandeilo and the Caradoc stages. All taxa with pelagic adults became extinct by the end of the Ordovician. Similarly, trilobites with entirely planktonic larvae had all but disappeared by the end of the Ordovician. Although suffering a significant loss in diversity, taxa with benthic larvae provided the ancestral stock for the majority of post-Ordovician lineages. Trilobites with a two-stage protaspid period (single planktonic followed by benthic larvae) suffered least during the Ashgill extinction, giving rise to approximately 23% of the new genera appearing in the early Silurian.

Patterns of extinction/survivorship among trilobite taxa with different developmental strategies indicate that the so-called Ashgill extinction was most likely the result of a composite phenomenon, including environmental perturbations, ecosystem breakdown and biogeographic restriction, and was not the consequence of a single catastrophic event (e.g., bolide collision). Indeed, our results are consistent with extinction models which invoke periodic global cooling and sea level regression associated with glaciation. Taxa with planktonic larvae first suffered a marked decline in diversity between the Llandeilo and the Caradoc (end of Middle Ordovician), coincident with the onset of the Ordovician-Silurian glaciation. During this time interval, the paleogeographic ranges of most surviving genera with solely planktonic larvae were severely constricted to lower latitude paleobiogeographic provinces. During the Ashgill (Rawtheyan to Hirnantian), climatic and sea level fluctuations were most extreme because of continued and more extensive glaciation. At this time, when extinctions were particularly severe, there was also a significant effect upon the survivorship of taxa with benthic larvae.

Our results indicate that the proportion of the life cycle spent in the water column, dependent upon a planktonic food source, was a critical factor in the survivorship of trilobite taxa during Middle to Late Ordovician time. Consequently, differences in life history patterns predisposed individual taxa to survival or extinction.
Kinematics of accretionary shell growth, with examples from brachiopods and molluscs
Authors: Ackerly Spafford C.
Published: July 2016

A moving reference frame is introduced for the analysis of accretionary shell growth. Simple principles of motion and stepwise growth define the model. At each growth step, the aperture migrates from its present position to a new position, according to locally defined rules. The aperture becomes the focus of the analysis, mathematically and conceptually, in conformity with biological reality. Kinematic principles provide the analytical framework for describing the aperture's trajectory (kinematics is the study of motion). The aperture “translates,” “rotates,” and “dilates.” The model offers exceptional flexibility in the analysis of accretionary growth forms and is particularly well-suited to analysis of conical and loosely coiled shell geometries. Computer simulations illustrate the principles of a moving reference model. The inverse problem of finding the aperture motions from actual shell data is rigorously specified, for both planispiralled and helicospiralled shell forms.
Ontogenetic regulatory mechanisms, heterochrony, and eccentricity in dendrasterid sand dollars
Authors: Beadle Steven C.
Published: July 2016

Posterior eccentricity has evolved independently in three lineages of northern Pacific sand dollars; it is best known in the genusDendraster. In these forms, the anterior areas of the aboral surface are much more highly developed than the posterior areas; consequently, the apical system is located posteriorly, rather than centrally. This morphology is linked to an unusual and highly successful mode of suspension-feeding. The evolution of eccentricity appears to be related to regulatory mechanisms found in many non-eccentric sand dollars, such asEchinarachnius parma. During the ontogeny of this form, the growth rates of the ambulacra and interambulacra are correlated with their position along the longitudinal axis. Early in ontogeny, the anterior areas develop at a faster rate than the posterior areas; later in ontogeny, this relationship is completely reversed. Normally, these two phases of unequal growth counterbalance each other, and the mature test appears symmetrical. However, if the balance between the two phases were upset, eccentricity would naturally ensue. In fact, aberrant Recent and fossilEchinarachniuswith the predicted anteriorly-and posteriorly-eccentric morphologies actually do exist. Posterior eccentricity is apparently produced by retention of the earlier unequal growth pattern, which favors anterior development. This represents trait neoteny; however, since the retained trait is a regulatory mechanism that controls growth rates over the entire aboral surface, the morphological effects are particularly profound. Thus, the seemingly bizarre morphology ofDendrastercan be derived by a change in the timing of an existing regulatory mechanism. This may help to explain the sudden appearance ofDendrasterin the fossil record and the absence of transitional forms. The unusual suspension-feeding behavior ofDendrastermay have been derived from a righting response that is common among other sand dollars.
New evidence for the life habit of graptoloids from physical modelling
Authors: Rigby Susan , Rickards Barrie
Published: July 2016

Physical models of graptolites have been constructed for a range of morphologies, with emphasis on planar, multiramous forms. The models are life size and have the density of a living graptolite, based on the now-established collagenous nature of the periderm and unavoidable assumptions about the amount of extrathecal tissue present in the living colony. These models have been used to test the two main hypotheses of graptolite life habit developed by Bulman, Rickards, Kirk, and others. Testing of graptoloid models in water suggests that many rhabdosome shapes were designed for passive rotation within the water column. This is caused in the models by a variety of modifications, including changes in thecal and stipe orientation, alterations of colony shape and the addition of vanes and hooks. Rotation would only have been useful when the rhabdosome was in directional motion and the frequency of such modifications seems anomalous if no such movement occurred. Thus movement by some means is required, either passively, by changes in buoyancy, or by automobility. Spiralling action would increase the harvesting path of an individual living on a planar, multiramous colony, making this a theoretically advantageous mode of life for the morphology. It would prevent the individual zooids of scandent biserial and uniserial colonies from feeding from the same narrow band of water.
Milankovitch cycles and their effects on species in ecological and evolutionary time
Authors: Bennett K. D.
Published: July 2016

The Quaternary ice ages were paced by astronomical cycles with periodicities of 20–100 k.y. (Milankovitch cycles). These cycles have been present throughout earth history. The Quaternary fossil record, marine and terrestrial, near to and remote from centers of glaciation, shows that communities of plants and animals are temporary, lasting only a few thousand years at the most. Response of populations to the climatic changes of Quaternary Milankovitch cycles can be taken as typical of the way populations have behaved throughout earth history. Milankovitch cycles thus force an instability of climate and other aspects of the biotic and abiotic environment on time scales much less than typical species durations (1–30 m.y.). Any microevolutionary change that accumulates on a time scale of thousands of years is likely to be lost as communities are reorganized following climatic changes. A four-tier hierarchy of time scales for evolutionary processes can be constructed as follows: ecological time (thousands of years), Milankovitch cycles (20–100 k.y.), geological time (millions of years), mass extinctions (approximately 26 m.y.). “Ecological time” and “geological time” are defined temporally as the intervals between events of the second and fourth tiers, respectively. Gould's (1985) “paradox of the first tier” can be resolved, at least in part, through the undoing of Darwinian natural selection at the first tier by Milankovitch cycles at the second tier.
Rates of extinction in marine invertebrates: further comparison between background and mass extinctions
Authors: Thackeray J. Francis
Published: July 2016

Prominent extinction “events” have been recognized from statistical analyses of marine invertebrate genera represented in Mesozoic and Cenozoic assemblages, contrasting with relatively low “background” extinction intensities measured in terms of a “percentage extinction” index. On a logarithmic scale, the slope of the relationship between time and extinction intensity for background extinctions is shown to be parallel to the slope obtained for most extinction events, characterized by intensities 100.35 above prevailing background levels. Although extinction intensities are variable, this study suggests that the magnitude of the factor(s) primarily associated with most mass extinctions in a 260-m.y. period (N = 9) need not necessarily have been very different from one event to another, an exception being the mass extinction at the end of the Cretaceous.
Miniaturization and heterochrony inHomoeorhynchia meridionalisandH. cynocephala(Brachiopoda, Rhynchonellidae) from the Jurassic of the Iberian Range, Spain
Authors: Laurin Bernard , García-Joral Fernando
Published: July 2016

The miniaturization of shells, often observed in Mesozoic brachiopods, is analyzed in two species of the well-known genusHomoeorhynchia.We investigated shell development in the context of recent works that consider growth as a function of size, shape, and age. The miniaturization results from heterochronic disturbances during development. Although these shells exhibit a large morphological plasticity, architectural constraints limit their morphogenesis, so that small adult specimens are not exactly reduced replicas of large ones. Adult morphology depends largely on the pattern of rib formation, with miniaturization arising by two different processes affecting this pattern: predisplacement of the appearance of ribs inH. meridionalisand acceleration inH. cynocephala. The first case seems to be a reversible process without evolutionary consequences, but the second occurs within an evolutionary trend. In both cases, the miniaturization seems to be related to environmental influences. Study of the growth pathway of these species allows recognition of basic characters in the morphological variation of populations.
Life-history and the evolution of ontogeny in the ostracode genus Cyprideis
Authors: Schweitzer Peter N. , Lohmann G. P.
Published: July 2016

A large body of paleontological literature concerns the importance of ontogeny as a source of morphological variation for evolution; morphologies that appear during one stage of an organism's development are made available for use in another simply by modifying the developmental program. Paleontologists need to know why this occurs, so they can study the process of evolution in extinct animals and so they can discuss the fossil record in terms that are applicable to modern forms. If most cases of heterochrony can be attributed to life-history evolution then the fossil record provides evidence of the nature of selection (in particular the age-specific mortality) that extinct animals experienced. The hypothesis of interest here is that species in which maturity is accelerated will also show generalized morphology and small size, while those with delayed maturity will have more specialized morphology and large size.

Four species of the ostracode genus Cyprideis were studied to determine whether differences in age at maturity are correlated with heterochrony in the expected manner. For each species the changes in size and shape through geological time were evaluated in the statistical context of modern geographic and seasonal variation. Living populations were sampled regularly to detect differences in seasonality and to estimate the duration of development.

Evolution of ontogeny is apparent at the level of species in this group, but it is not simply related to differences in life-history. In comparisons among species, we find evidence of heterochrony where there is no difference in the age at maturity, and a difference in age at maturity where there is no heterochrony. Similarly, three of the four species show the expected positive correlation between size and age at maturity, yet the fourth species is relatively large and matures rapidly. Cyprideis does not support the generalization that life-history evolution causes heterochrony, and casts doubt on the inference of life-history evolution from heterochrony where the data are drawn exclusively from extinct forms.
Addendum to: Analysis of clade shape using queueing theory and the fast Fourier transform
Authors: Gilinsky Norman L. , Good I. J.
Published: July 2016

This short contribution clarifies similarities and differences between the findings of Gilinsky and Bambach (1986) and Gilinsky and Good (1989) regarding the shapes of clades. The methods developed in the latter contribution are preferred.
Historical tests of the absolute completeness of the fossil record of tetrapods
Authors: Maxwell W. Desmond , Benton Michael J.
Published: July 2016

Six compilations of fossil tetrapod families, spanning 100 years, each contain a broadly similar diversity pattern since the Upper Devonian. Comparison of four recent data bases, one of which is derived from a strict cladistic treatment, reveals widespread taxonomic and stratigraphic inaccuracies in three earlier data bases. Improvement of our interpretation of the tetrapod fossil record will come through continued taxonomic and stratigraphic revision as well as discovery of new fossils.
Sutural pattern and shell stress inBaculiteswith implications for other cephalopod shell morphologies
Authors: Jacobs David K.
Published: July 2016

InBaculites, a straight shelled ammonite, the constructional limits on shell shape resulting from the limited strength of nacre in tension are circumvented by a system of vaults in the phragmocone. Vaults bridge between regions of the phragmocone supported by the complex ammonite septal suture, and maintain the shell wall in compression when hydrostatic load induces bending moments. To determine how these vaults interact in the phragmocone to resist hydrostatic loading, measurements were made on a suite ofBaculitesspecimens. InBaculitesthere is a statistically significant inverse relationship between circumferential curvature (radius of curvature) of the vaulted shell surface and the size of vaults spanning between sutural elements supporting the phragmocone. The inverse relationship between radius of curvature and the sizes of spans in this system of vaults results in the generation of comparable reactive forces at the ends of the vault spans where adjacent vaults interact. The equivalence of these reactive forces prevents the lateral displacement of the vault ends. Consequently, compressive stresses from adjacent vaults are superimposed on, and reduce, the tensional stress component of bending. Limiting tensile stress is of utmost importance in a lightweight shell composed of a brittle material such as nacre, which is strong in compression but weak in tension.

Baculitesis particularly appropriate for this study because its straight shell is curved only in the circumferential direction, thus simplifying the problem. However, sutural patterns in coiled ammonites appear to be similarly constrained to produce vaults in the phragmocone which vary inversely in curvature and span size.
On the origin of ammonite sutures
Authors: García-Ruiz Juan M. , Checa Antonio , Rivas Pascual
Published: July 2016

The origin of ammonite sutures is one of the most interesting open problems in paleontology. Present theories of suture formation deal with function rather than morphogenesis. Here we present a study showing that sutures are scale-invariant. In each of ten different ammonite genera, the ontogenetic sequence of the suture can be characterized by the same fractal dimension. We propose a morphogenetic theory for ammonite sutures that is based on the similarity between sutures and the Saffman-Taylor instability in both fractal characters and physical mechanism.
Evolution of dental traits since latest Pleistocene in meadow voles (Microtus pennsylvanicus) from Virginia
Authors: Barnosky Anthony D.
Published: July 2016

Digitizing the third upper molars ofMicrotus pennsylvanicusreveals evolutionary change in some traits but stability in others during the last 30,000 years. Fossils from Strait Canyon, Virginia (radiocarbon dated between 29,870 + 1,800/-1,400 and 17,880 ± 150 yr B.P.) were compared with modern samples from the margin of the species' conterminous geographic range: Virginia, Colorado, Alaska, and Hudson Bay. Since the late Pleistocene a modification from narrow to wide teeth took place in the eastern populations but not in the western ones, and populations in Virginia apparently evolved wider confluence between triangles 1 and 2. Traits that remained stable through time in the Virginia area, but not necessarily elsewhere, include the numerical shape factors of the occlusal surface and the posterior loop. Potentially widespread stability is evident in the perimeter of the posterior loop relative to total tooth perimeter. Tooth length forms a geographic cline, in which both the fossil and modern populations from Virginia occupy the same position. A peripheral population from southern Colorado shows the most derived dental morphology. These morphological relationships suggest that mosaic evolution operates in arvicoline lineages and provide testable paradigms for future studies.
Intracolony variation in skeletal growth rates in Paleozoic ramose trepostome bryozoans
Authors: Key Marcus M.
Published: July 2016

All erect, branching (ramose) organisms adhere to the same fundamental geometric growth law: the rate of distal growth decreases away from the branch axis. Regardless of the phylogenetic history of an organism, the formation of cylindrical branches requires adherence to this law. In colonial ramose organisms such as trees, corals, and bryozoans, this law poses a problem. How do colonies coordinate the growth rates of the individual modules to produce an integrated branch? This question is addressed in the context of three Ordovician and three Devonian species of ramose trepostome bryozoans. Using remnant growing tips in the endozone as isochronous surfaces, relative rates of skeletal secretion among zooids were measured. Measurements of skeletal and void spaces across a colony branch enabled calculation of the volume of skeletal material secreted by zooids between successive remnant growing tips. Results indicate that rate of skeletal secretion systematically decreases from the branch axis outward into the exozone. This suggests that zooid morphogenesis is controlled to a certain degree by the colony. Colonial control over zooidal growth rates in turn regulates the shape of the colony.
A kill curve for Phanerozoic marine species
Authors: Raup David M.
Published: July 2016

A kill curve for Phanerozoic species is developed from an analysis of the stratigraphic ranges of 17,621 genera, as compiled by Sepkoski. The kill curve shows that a typical species' risk of extinction varies greatly, with most time intervals being characterized by very low risk. The mean extinction rate of 0.25/m.y. is thus a mixture of long periods of negligible extinction and occasional pulses of much higher rate. Because the kill curve is merely a description of the fossil record, it does not speak directly to the causes of extinction. The kill curve may be useful, however, to limit choices of extinction mechanisms.
Taxon age and selectivity of extinction
Authors: Boyajian George E.
Published: July 2016

Taxon-age distributions were compiled for families of marine animals surviving or becoming extinct in each stage of the Phanerozoic. I demonstrate, through the use of a modified bootstrap analysis, that there is no difference between the longevity of families becoming extinct during times of background extinction and times of mass extinction. In both mass and background extinction intervals the mean age of families that become extinct is 2 standard deviations below the geometric mean taxon age of families available for extinction. Young families are more susceptible to extinction, perhaps as the result of lower species richness or of occupying a smaller geographic range. There is no tendency during mass extinctions toward loss of families with different taxon ages other than those that become extinct during background times. Thus, in terms of family survival, mass extinction appears to be an exaggeration of processes of background extinction.
Incumbent replacement: evidence for long-term evolutionary progress
Authors: Rosenzweig Michael L. , McCord Robert D.
Published: July 2016

Evolutionary progress is a trend that relaxes trade-off rules. It begins with the evolution of a key adaptation. It continues with the spread of the key adaptation as the clade that contains it replaces some older clade that lacks it. Key adaptations are those that allow for improvement in at least one organismal function at a reduced fitness cost in other functions.

Replacement almost certainly involves more than pure chance. It may not often involve competitive extinction. Instead, species from the new clade produce new species to replace already extinct species from the old clade. The key adaptation gives them a higher competitive speciation rate than old-clade sources of replacement. The process, termedincumbent replacement,proceeds at a rate limited by extinction rate. Thus, replacement often seems linked to mass extinction events.

The incumbent-replacement hypothesis explains what we know about the replacement of straight-neck turtles (Amphichelydia) by those that can flex their necks and protect their heads in their shells. This replacement occurred four or five times in different biotic provinces. It happened as long ago as the Cretaceous in Eurasia, and as recently as the Pleistocene in mainland Australia. It was accomplished in Gondwanaland by turtles flexing their necks sideways (Pleurodira), and in the north by those flexing their necks into an S-curve (Cryptodira). As is typical of replacements, amphichelydian replacement took millions of years to accomplish wherever it occurred, and much of it in North America took place in a burst associated with and immediately subsequent to a mass extinction.
Biogeography of early Mesozoic continental tetrapods: patterns and implications
Authors: Shubin Neil H. , Sues Hans-Dieter
Published: July 2016

The stratigraphic framework for Triassic and Early Jurassic continental strata has greatly changed in recent years. These revised correlations necessitate a review of traditional views of early Mesozoic continental faunal succession and biogeography. We have examined the relationship between tetrapod distribution and paleogeographic context during the Triassic and Early Jurassic on the basis of a data base comprising updated faunal lists for major early Mesozoic assemblages of continental tetrapods. Analysis of these data supports the hypothesis that there were few barriers to biotic interchange among continental tetrapods throughout the Triassic and Early Jurassic. Early Mesozoic tetrapod assemblages are dominated by widely distributed, often cosmopolitan families. Late Triassic patterns of latitudinal variation among tetrapod assemblages appear to be correlated to those seen among terrestrial plants and contrast with the extremely uniform distribution of Early Jurassic continental biotas.
Enrichment and stability in the Pliocene mammalian fauna of North China
Authors: Flynn Lawrence J. , Tedford Richard H. , Zhanxiang Qiu
Published: July 2016

The Late Neogene vertebrate fossil record from Yushe Basin presents multiple, superposed assemblages from a single area, spanning roughly the interval of 6–2 Ma. Both large and small mammals show peak species richness in the middle Pliocene but indicate relative faunal stability throughout the Pliocene. Large mammals show turnover, especially extinction, around 5 and 2.5 Ma. Small mammals indicate change (over half of the species and several genera), as well as turnover at the species level, between 4 and 3.4 Ma. The loosely controlled dating of these events does not disprove hypothetical correlation with events in North America and with global climatic shifts. Elements that lack Yushe antecedents, some being long-distance dispersers, appear throughout the section, but with little effect on the resident assemblage. First records of well-documented immigrants (from North America, Europe, Africa, southern Asia, or high latitudes) generally do not coincide with ecomorph extinctions. Early Pliocene exchange between Asia and North America appears to have been balanced in both directions and involved a small proportion of the fauna. Immigration probably was opportunistic and contributed to faunal enrichment. We interpret the Yushe Pliocene mammalian assemblages as representing a fauna that was stable from ca. 5 to 2.5 Ma and changed mainly by additions and congeneric species substitutions.