bluuebunny/crossref_metadata_2025 huggingface dataset:
Evolution of taxonomic diversity gradients in the marine realm: evidence from the composition of Recent bivalve faunas
Authors: Crame J. Alistair
Published: July 2006

A major new inventory of living marine Bivalvia (Mollusca) is based on 29 regional faunas. These again pick out strong latitudinal and longitudinal gradients in taxonomic diversity, but there are indications that the patterns are not so regular as previously thought. There are signs of asymmetry between the Northern and Southern Hemisphere latitudinal gradients, with the former tending to be more regular than the latter. Northern gradients are also characterized by a marked inflection at approximately 30°N, and the three Australian provinces seem to form a distinct “hot-spot” in the Southern Hemisphere. The larger of the two tropical high-diversity foci (the southern China-Indonesia-NE Australia one) appears to be much more nearly arcuate in plan view than oval and is closely associated with the world's richest development of coral reefs.

A taxonomic and stratigraphic analysis reveals that the steepest latitudinal gradients are associated with the youngest bivalve clades. The most striking pattern is that shown by the heteroconchs, an essentially infaunal taxon that radiated extensively throughout the Cenozoic era. Steep gradients are also characteristic of the relatively young anomalodesmatan and arcoid clades and, somewhat surprisingly, the predominantly epifaunal pteriomorphs. Although the latter taxon falls within an older (i.e., “late Paleozoic-Jurassic”) group of clades, it is apparent that certain elements within it (and in particular the Pectinidae) radiated extensively in the latest Mesozoic-Cenozoic. A small but significant component of the later stages of the adaptive radiation of the Bivalvia comprised epifaunal taxa.

The presence of the steepest latitudinal gradients in the youngest clades provides further evidence that the Tropics have served as a major center of evolutionary innovation. Even though some sort of retraction mechanism cannot be completely ruled out, these gradients are most likely the product of primary radiations. Clade history can be an important determinant of contemporary large-scale biodiversity patterns. The markedly lower diversity of some bivalve clades, such as the heteroconchs, in the high-latitude and polar regions may simply reflect the fact that they are not yet fully established there. In a way that is reminiscent of the onshore-offshore radiation of certain benthic marine invertebrate taxa, it may take periods of tens or even hundreds of millions of years for bivalve clades to disseminate fully across the earth's surface.

The persistent spread of taxa from low- to high-latitude regions should perhaps come as no great surprise, as the tropical ocean is very much older than either of the polar ones. The late Cretaceous-Cenozoic evolutionary radiation of the Bivalvia was accompanied by a marked deterioration in global climates, and many new groups have yet to be fully assimilated into cool- and cold-water benthic ecosystems.
Escalation in Late Cretaceous-early Paleocene oysters (Gryphaeidae) from the Atlantic Coastal Plain
Authors: Dietl Gregory P. , Alexander Richard R. , Bien Walter F.
Published: July 2006

More than 1600 valves of Late Cretaceous and early Paleocene Northern Atlantic Coastal Plain gryphaeid oysters (Exogyrinae and Pycnodonteinae) were examined for breakage-induced shell repair and morphologic variability to evaluate the hypothesis of escalation. The Exogyrinae show disproportionately higher average repair frequency (0.41) relative to the ecologically and functionally similar unornamented pycnodonts (0.19). An increase in repair frequency (independent evidence of the action of a selective agent, e.g., predation) through the stratigraphic interval supports escalation. Variation in repair frequencies may reflect differences in oyster morphology and in the strength and diversity of shell crushers across an onshore-offshore gradient. Escalation of antipredatory adaptation characterized the evolutionary response of gryphaeid oysters to their durophagous predators. Adaptation generally occurred by the enhancement of existing traits in both oyster lineages. Characters that confer a selective advantage against predators are not all expressed or improved concurrently in both oyster lineages. Morphologic adaptations to minimize shell breakage include the development of expansive, broad commissural shelves, thickened valves, and surface ornamentation (Exogyrinae). Surface ornament in the Exogyrinae gradually increased with time. For some characters, such as thickness, conflicting functional demands (e.g., valve stabilization) may have limited adaptation to predators.
Suture and location of the coiling axis in gastropod shells
Authors: Van Osselaer Christian , Grosjean Philippe
Published: July 2006

The general allometric equations for the logarithmic helicospiral can fit many extraconical shapes, but the isometric conditions traditionally used limit study only to conical growth. We present evidence to show that in real gastropod shells, the logarithmic helicospiral equations fit the suture. Poor location of the coiling axis and / or an inappropriate pole for the logarithmic helicospiral has often led to the rejection of this model. The differences between the errors associated with measurement or previously available models are discussed. Two methods, based on suture trace measurements, are proposed to locate the coiling axis both in apical and lateral views. The first is a graphical method based on an elementary property of the logarithmic spiral. The second is a computational method based on iterative reprojections of the suture. It is shown that the protoconch and the teleoconch must be treated separately. The precision of the new methods (especially the computing method) enables deviations from logarithmic helicospiral trajectory to be identified and differentiated from irregularities of the shell and sequential growth phases. Application of these methods may be useful not only for other gastropod morphological features, but also for other taxa such as brachiopods and other mollusks.
The evolution of pelvic aspiration in archosaurs
Authors: Carrier David R. , Farmer Colleen G.
Published: July 2006

Movements of the pelvic girdle have recently been found to contribute to inspiratory airflow in both crocodilians and birds. Although the mechanisms are quite different in birds and crocodilians, participation of the pelvic girdle in the production of inspiration is rare among vertebrates. This raises the possibility that the pelvic musculoskeletal system may have played a role in the ventilation of basal archosaurs. Judging from the mechanism of pelvic aspiration in crocodilians and the structure of gastralia in basal archosaurs, we suggest that an ischiotruncus muscle pulled the medial aspect of the gastralia caudally, and thereby helped to produce inspiration by increasing the volume of the abdominal cavity. From this basal mechanism, several archosaur lineages appear to have evolved specialized gastralia, pelvic kinesis, and/or pelvic mobility. Kinetic pubes appear to have evolved independently in at least two clades of Crocodylomorpha. This convergence suggests that a diaphragmatic muscle may be basal for Crocodylomorpha. The pelvis of pterosaurs was long, open ventrally, and had prepubic elements that resembled the pubic bones of Recent crocodilians. These characters suggest convergence on the pelvic aspiratory systems of both birds and crocodilians. The derived configuration of the pubis, ischium and gastralia of non-avian theropods appears to have enhanced the basal gastral breathing mechanism. Changes in structure of the pelvic musculoskeletal system that were present in both dromaeosaurs and basal birds may have set the stage for a gradual reduction in the importance of gastral breathing and for the evolution of the pelvic aspiration system of Recent birds. Lastly, the structure of the pelvis of some ornithischians appears to have been permissive of pubic and ischial kinesis. Large platelike prepubic processes evolved three times in Ornithischia. These plates are suggested to have been instrumental in an active expansion of the lateral abdominal wall to produce inspiratory flow. Thus, many of the unique features found in the pelvic girdles of various archosaur groups may be related to the function of lung ventilation rather than to locomotion.
Understanding the dynamics of trends within evolving lineages
Authors: Alroy John
Published: July 2006

The study of evolutionary trends is one of the oldest and most intriguing topics in evolutionary biology and paleobiology (McNamara 1990). Workers since Cuvier, Lyell, and Owen have wanted to know if the fossil record demonstrates “progression” within temporal sequences of related organisms. Regardless of whether changes in the average values of morphological characters are progressive in any meaningful sense, these changes are still of great interest. In practice, questions about trends are most commonly framed by paleontologists in terms of “complexity” (however defined) or body size (McShea 1998a).
Trends, tools, and terminology
Authors: McShea Daniel W.
Published: July 2006

It has been said that most scientists would rather use another scientist's toothbrush than his terminology. In this issue's Matters of the Record, John Alroy suggests certain conceptual and methodological changes for the study of mechanisms, or dynamics, of large-scale trends, and also a revision of current trend-mechanism terminology. On the conceptual and methodological matters, Alroy's suggestions are good ones (with one important exception), and here I applaud them and explain further why they ought to be taken seriously. Predictably, however, I do not endorse (some of) his terminological revisions.
Calcified metazoans in thrombolite-stromatolite reefs of the terminal Proterozoic Nama Group, Namibia
Authors: Grotzinger John P. , Watters Wesley A. , Knoll Andrew H.
Published: July 2006

Reefs containing abundant calcified metazoans occur at several stratigraphic levels within carbonate platforms of the terminal Proterozoic Nama Group, central and southern Namibia. The reef-bearing strata span an interval ranging from approximately 550 Ma to 543 Ma. The reefs are composed of thrombolites (clotted internal texture) and stromatolites (laminated internal texture) that form laterally continuous biostromes, isolated patch reefs, and isolated pinnacle reefs ranging in scale from a meter to several kilometers in width. Stromatolite-dominated reefs occur in depositionally updip positions within carbonate ramps, whereas thrombolite-dominated reefs occur broadly across the ramp profile and are well developed as pinnacle reefs in downdip positions.

The three-dimensional morphology of reef-associated fossils was reconstructed by computer, based on digitized images of sections taken at 25-micron intervals through 15 fossil specimens and additionally supported by observations of over 90 sets of serial sections. Most variation observed in outcrop can be accounted for by a single species of cm-scale, lightly calcified goblet-shaped fossils herein described asNamacalathus hermanastesgen. et sp. nov. These fossils are characterized by a hollow stem open at both ends attached to a broadly spheroidal cup marked by a circular opening with a downturned lip and six (or seven) side holes interpreted as diagenetic features of underlying biological structure. The goblets lived atop the rough topography created by ecologically complex microbial-algal carpets; they appear to have been sessile benthos attached either to the biohermal substrate or to soft-bodied macrobenthos such as seaweeds that grew on the reef surface. The phylogenetic affinities ofNamacalathusare uncertain, although preserved morphology is consistent with a cnidarian-like bodyplan. In general aspect, these fossils resemble some of the unmineralized, radially symmetric taxa found in contemporaneous sandstones and shales, but do not appear to be closely related to the well-skeletonized bilaterian animals that radiated in younger oceans. Nama reefs demonstrate that biohermal associations of invertebrates and thrombolite-forming microorganisms antedate the Cambrian Period.
Bangiomorpha pubescensn. gen., n. sp.: implications for the evolution of sex, multicellularity, and the Mesoproterozoic/Neoproterozoic radiation of eukaryotes
Authors: Butterfield Nicholas J.
Published: July 2006

Multicellular filaments from the ca. 1200-Ma Hunting Formation (Somerset Island, arctic Canada) are identified as bangiacean red algae on the basis of diagnostic cell-division patterns. As the oldest taxonomically resolved eukaryote on recordBangiomorpha pubescensn. gen. n. sp. provides a key datum point for constraining protistan phylogeny. Combined with an increasingly resolved record of other Proterozoic eukaryotes, these fossils mark the onset of a major protistan radiation near the Mesoproterozoic/Neoproterozoic boundary.

Differential spore/gamete formation showsBangiomorpha pubescensto have been sexually reproducing, the oldest reported occurrence in the fossil record. Sex was critical for the subsequent success of eukaryotes, not so much for the advantages of genetic recombination, but because it allowed for complex multicellularity. The selective advantages of complex multicellularity are considered sufficient for it to have arisen immediately following the appearance of sexual reproduction. As such, the most reliable proxy for the first appearance of sex will be the first stratigraphic occurrence of complex multicellularity.

Bangiomorpha pubescensis the first occurrence of complex multicellularity in the fossil record. A differentiated basal holdfast structure allowed for positive substrate attachment and thus the selective advantages of vertical orientation; i.e., an early example of ecological tiering. More generally, eukaryotic multicellularity is the innovation that established organismal morphology as a significant factor in the evolutionary process. As complex eukaryotes modified, and created entirely novel, environments, their inherent capacity for reciprocalmorphologicaladaptation, gave rise to the “biological environment” of directional evolution and “progress.” The evolution of sex, as a proximal cause of complex multicellularity, may thus account for the Mesoproterozoic/Neoproterozoic radiation of eukaryotes.
Morphometric analysis of Rhynia and Asteroxylon: testing functional aspects of early land plant evolution
Authors: Roth-Nebelsick A. , Grimm G. , Mosbrugger V. , Hass H. , Kerp H.
Published: July 2006

New morphometric data gathered from cross-sections of two Lower Devonian land plants (Rhynia gwynne-vaughanii and Asteroxylon mackiei) are interpreted in terms of the evolution of the function of vascular bundles in early land plants. The following conclusions can be drawn from these new data: (1) The ratio of the cross-sectional area of the xylem (representing the conducting volume supplying the axis with water) to the xylem perimeter (representing the “contact area” between xylem and parenchyma through which water leaves the xylem and enters the parenchyma) is not constant for Rhynia axes, almost constant for Asteroxylon axes, and different between Rhynia and Asteroxylon. Thus, Bowers hypothesis that the ratio of cross-sectional area of the xylem to xylem perimeter is constant during ontogenetic development is true for Asteroxylon. That this ratio is constant during phylogeny, however, is not supported by our data. (2) The ratio between cross-sectional area of xylem to parenchyma is higher in Asteroxylon than in Rhynia. (3) As predicted by previous computer simulations, the ratio of the xylem perimeter to the axis perimeter plays a major role in determining water transport performance of the transpiring axis. This ratio is constant within ontogeny but is different in Asteroxylon and Rhynia. In Asteroxylon axes, this ratio is about twice as large as in Rhynia axes. (4) Contrary to the expectations, the distance between the outermost layer of the xylem and the transpiring surface, which represents the low-conductivity pathway through the parenchyma, appears not to be a limiting factor for the water transport in axes of Rhynia and Asteroxylon. (5) From the analysis of the geometric parameters, it is evident that Rhynia and Asteroxylon with their distinct stelar geometries represent two different constructional types for which no transitional stages are known.
Delayed herbivory and the assembly of marine benthic ecosystems
Authors: Vermeij Geerat J. , Lindberg David R.
Published: July 2006

Phylogenetic analysis of the metazoan evolutionary tree as a whole, and of trees of component major clades, indicates that marine herbivores, defined here as macrophagous consumers of living multicellular attached marine plants, always occupy terminal positions at several scales of analysis. Nearly all living benthic marine herbivores are derived from microphages, detritivores, or predators, and most have post-Paleozoic origins. The derived nature of herbivory in the sea parallels the evolutionary situation among land animals. Pre-Mesozoic marine benthic ecosystems, characterized by relatively low rates of flow of energy and nutrients, may have relied even more heavily on decomposers for the transfer of carbon from primary producers to animals than do living marine ecosystems in the photic zone.
Likelihood tests of hypothesized durations: determining and accommodating biasing factors
Authors: Wagner Peter J.
Published: July 2006

Paleobiologists frequently hypothesize that a taxon's duration (i.e., the true span from origination to extinction) exceeds its stratigraphic range (i.e., the span from first appearance to last appearance in the fossil record). One can test hypothesized duration by assessing the plausibility of the implicitly hypothesized gaps between origination and first appearance and / or between last appearance and extinction. Several tests assess the probability of not finding a taxon over some stratigraphic gap. Because the likelihood of a hypothesis given data reflects the probability of the data given that hypothesis, these probabilities also give the likelihood of a hypothesized duration. However, many probability / likelihood tests require simplifying assumptions about unknown sampling parameters such as the consistency of sampling over time, sampling intensities for unknown ancestors, and actual sampling intensities themselves.

This paper examines the effects of sampling parameters on probability / likelihood tests and presents methods for testing hypotheses about these unknowns while testing hypotheses about true durations. Two data sets are used here as examples. One analysis tests the origination times among Paleozoic gastropods implied by phylogenetic inferences. The other analysis tests the extinction times among Maastrichtian ammonites implied by different numbers of extinction events. In both cases, hypotheses positing many gaps in the fossil record become more likely after accommodating uncertainty about sampling. However, the increased likelihoods are insufficient to prevent these hypotheses from being rejected in favor of hypotheses positing fewer gaps. In both cases, the conclusions are identical to those derived by simple methods using simple models for unknown sampling parameters. Although numerous factors can exaggerate the implausibility of gaps, making these factors parts of testable hypotheses is possible. Thus, excessive assumptions about sampling parameters need not hinder empirical testing of hypothesized durations.
Forelimb posture in neoceratopsian dinosaurs: implications for gait and locomotion
Authors: Paul Gregory S. , Christiansen Per
Published: July 2006

Ceratopsid dinosaurs traditionally have been restored with sprawling forelimbs and were considered unable to run at high speeds. An alternative view restores the ceratopsids as rhinoceros-like with parasagittal forelimb kinematics and the ability to run faster than extant elephants. Several anatomical difficulties concerning the mounting of ceratopsid skeletons with nearly parasagittal forelimbs stem not from the forelimb itself, but from errors in rib and vertebral articulation. Matching a skeletal restoration to a probable ceratopsid trackway shows that the hands were placed directly beneath the glenoids, and that manual impressions were directed laterally, not medially as in sprawling reptiles. Pedal impressions in trackways are medial to the manual impressions, owing to the slightly averted elbow and to the asymmetrical distal femoral condyles, which directed the crus slightly medially. The limbs of ceratopsians of all sizes display substantial joint flexure, strongly indicating that the elephantine forelimb posture that has sometimes been suggested as the alternative to a sprawling posture is erroneous. The articular surfaces of uncrushed ceratopsian scapulocoracoids and forelimb joints confirm that the forelimb operated in a near-parasagittal plane with the elbows only slightly averted. The maximal running speed of even the largest ceratopsids is inferred to have significantly exceeded that of elephants and was probably broadly similar to that of rhinos.
Homoplasy and the evolution of dinosaur locomotion
Authors: Carrano Matthew T.
Published: July 2006

In this paper, I survey hindlimb and pelvic anatomy across non-avian dinosaurs and analyze these within a cladistic framework to quantify patterns of change within the locomotor apparatus. Specifically, I attempt to identify where homoplasy constitutes parallelism and may thereby be used to infer similar selective pressures on hindlimb function. Traditional methods of discrete character optimization are used along with two methods for evaluating changes in continuous characters in a phylogenetic context (squared-change parsimony and clade rank correlation). Resultant patterns are evaluated in light of the biomechanics of locomotion and the relationship between form and function in extant terrestrial vertebrates.

Although non-avian dinosaurian locomotor morphology is strikingly uniform, these analyses reveal the repeated derivations of several morphological features that have potential relevance for hindlimb locomotor function. Anterior and posterior iliac expansion, a medially oriented femoral head, and an elevated femoral lesser trochanter each evolved independently multiple times within Dinosauria. These changes probably reflect enlargement of several hindlimb muscles as well as a general switch in their predominant function from abduction-adduction (characteristic of “sprawling” limb postures) to protraction-retraction (characteristic of parasagittal, or “erect,” limb postures). Several “avian” characteristics are shared with more basal theropods, and many were acquired convergently in other dinosaurian lineages. The evolution of the avian hindlimb therefore represents a cumulative acquisition of characters, many of which were quite far removed in time and function from the origin of flight.
Two genomic paths to the evolution of complexity in bodyplans
Authors: Valentine James W.
Published: July 2006

Morphologically complex metazoans appear abruptly during the Cambrian explosion. Suggested measures of metazoan complexity include number of cell morphotypes and aspects of the genome such as the amount of DNA, the number of genes, and the information content of the genome or egg. Estimates of gene numbers are now available for metazoan species belonging to five different phyla or subphyla. There is little correlation between gene number and morphological complexity in the invertebrates: relatively complex forms can have fewer genes than relatively simple forms. Presumably, the more complex forms use more gene-expression events during development, implying that, on average,cis-regulatory elements of more complex invertebrates are richer in binding sites than are those of simpler forms. Vertebrates have many more genes than invertebrates and therefore have more total gene-expression events during development, although they may have, on average, fewer expression events per gene than the invertebrates. There are thus two genomic pathways in the evolution of metazoan complexity: one involves increasing the number of genes, the other involves increasing the number ofcis-regulatory binding sites. Both modes were associated with the origin of bodyplans that first appear as fossils during the Cambrian explosion.
Theoretical Morphology: State of the Art - Theoretical Morphology: The Concept and Its Applications. George R. McGhee Jr. Columbia University Press, New York. 1999. 316 pages. Cloth $60.00, paper $26.50.
Authors: Eble Gunther J.
Published: July 2006

The morphological diversity of life has captivated systematists in the construction of classifications, embryologists in the study of development, and evolutionists in the formulation of theories of organic change. In a century marked by the advances of molecular biology, has the discipline of morphology produced anything … new? Yes. The solidification of paleontology and systematics and the emergence of macroevolution as a legitimate field owe much to an increased rigor in the analysis of morphological data. At the same time, the discipline of morphology has achieved an unprecedented sophistication through another development, the very expression of its maturity: theoretical morphology. Theoretical morphology forms the subject of McGhee's landmark book, an elegant combination of compendium and manifesto. Its richness and scope provide an opportunity for a critical appraisal of the discipline of morphology, particularly quantitative and developmental morphology.
Are homalozoans echinoderms? An answer from the extraxial-axial theory
Authors: David Bruno , Lefebvre Bertrand , Mooi Rich , Parsley Ronald
Published: July 2006

Homalozoans include four classes of non-pentamerous Paleozoic echinoderms: Homostelea (cinctans), Ctenocystoidea (ctenoid-bearing homalozoans), Homoiostelea (solutes), and Stylophora (cornutes and mitrates). Their atypical morphologies have historically made it difficult to relate them to other classes. Therefore, their systematic positions have been represented by two hypotheses (H): as stem taxa to echinoderms (H1) or as stem taxa to chordates (H2). These conclusions rest on previous inability to recognize synapomorphies with more crownward echinoderms, resulting in a forcing of the homalozoans down the phylogenetic tree that is more artifactual than evolutionary. The Extraxial-Axial Theory (EAT) identifies body-wall homologies, common ontogenetic patterns, and major events in bodyplan evolution. Therefore, the EAT can identify synapomorphies among even the most disparate of echinoderms. Application of the EAT undermines both H1 and H2 and strongly suggests that the bizarre asymmetry of homalozoans is a derived characteristic, and not indicative of plesiomorphic morphology for either chordates or echinoderms. Each of the four homalozoan clades and their major features are reexamined using the EAT. New findings are presented concerning homologies of thecal body wall, but we focus on stems, arms, and brachioles, which are recognized as very distinct products of independent evolutionary events. The results support a new interpretation (H3) of homalozoans as a polyphyletic assemblage that can be parsed out into other, clearly echinoderm clades. The Homoiostelea and Homostelea share the blastozoan synapomorphy of a brachiole. The enigmatic Ctenocystoidea also seem to have brachioles. The Stylophora have an arm as in crinoids. H3 is also more congruent with the known fossil record. Although they are stratigraphically early echinoderms, homalozoans are not indicative of the plesiomorphic morphology of the phylum.
A theoretical morphologic analysis of convergently evolved erect helical colony form in the Bryozoa
Authors: McGhee George R. , McKinney Frank K.
Published: July 2006

Exploration of the theoretical morphospace of erect helical colony form in Bryozoa, created by McKinney and Raup (1982), reveals that only a small volume of the three-dimensional space of hypothetical form is occupied by actual colonies of the Paleozoic fenestrates (Class Stenolaemata) Archimedes and Helicopora, helical species of the cheilostome (Class Gymnolaemata) Bugula, and the cyclostome (Class Stenolaemata) Crisidmonea archimediformis. Actual helical-colony bryozoans are not found in regions of the morphospace characterized by colony geometries that possess the largest surface areas of filtration sheet. Examination of computer-simulated colonies in the theoretical morphospace reveals that, although possessing high surface areas, colonies in the empty region of high-surface-area morphospace possess other aspects of geometry that are unrealistic as filter-feeding geometries: the filtration-sheet whorls are held at small acute angles to the central colony axis and are deeply nested within one another, both of which are disadvantageous conditions for the system of filter feeding used by the extant cheilostome Bugula, and presumably by extinct helical-colony bryozoans as well.

Even though actual bryozoans are found only in the low to intermediate surface-area regions of the theoretical morphospace, surface area of filtration sheet is a major determinant of form in these helical colonies, as is evidenced by a negative correlation in values of the parameters BWANG and ELEV exhibited by the colony data. Minimum values of BWANG are even further constrained by the apparent need of the Archimedes colonies to maintain filtration-sheet branching densities within the range of 20 to 50.
Origination and extinction components of taxonomic diversity: Paleozoic and post-Paleozoic dynamics
Authors: Foote Mike
Published: July 2006

Changes in genus diversity within higher taxa of marine animals on the temporal scale of a few million years are more strongly correlated with changes in extinction rate than with changes in origination rate during the Paleozoic. After the Paleozoic the relative roles of origination and extinction in diversity dynamics are reversed. Metazoa as well as individual higher taxa shift from one mode of diversity dynamics to the other. The magnitude of taxonomic rates, the relative variance of origination and extinction rates, and the presence or absence of a long-term secular increase in diversity all fail to account for the shift in importance of origination and extinction in diversity changes. Origination and extinction rates both tend to be diversity-dependent, but different modes of diversity-dependence may contribute to the change in diversity dynamics from the Paleozoic to the post-Paleozoic. During the Paleozoic, there is a weak tendency for extinction rates to be more diversity-dependent than origination rates, whereas after the Paleozoic the two rates are about equally diversity-dependent on average.
Testing for a mass extinction without selecting taxa
Authors: Solow Andrew R. , Smith Woollcott K.
Published: July 2006

Statistical inference about mass extinction events is commonly based on the pattern of fossil finds among a group of taxa. An important issue for existing methods is the selection of taxa for inclusion in the analysis. A common approach is to select taxa on the basis of the stratigraphic height of their uppermost finds. This approach creates a bias in favor of detecting a mass extinction event. This paper describes and illustrates an approach that avoids this problem.
Relationships between leaf morphology and climate, Bolivia: implications for estimating paleoclimate from fossil floras
Authors: Gregory-Wodzicki Kathryn M.
Published: July 2006

Fossil floras are an important source of quantitative terrestrial paleoclimate data. Many paleoclimate estimates are based on relationships observed in modern vegetation between leaf morphology and climate, such as the increase in the percentage of entire-margined species with increasing temperature and the increase in leaf size with increasing precipitation. An important question is whether these observed relationships are universal or regional; for example, recent studies suggest that significant differences exist between floras from three domains: the Northern Hemisphere, New Zealand/Australia, and subalpine zones. Also, debate exists over which statistical models of modern data sets, univariate or multivariate, provide the most accurate estimates of paleoclimate. In this study, 12 foliage samples from living Bolivian forests are compared with data sets from different regions. Models based on data sets from North America and Japan, namely the Climate-Leaf Analysis Multivariate Program (CLAMP) data set of J. A. Wolfe, and from east Asia produce reasonably accurate estimates of temperature and precipitation, suggesting that the climate-leaf morphology relationships for Bolivian vegetation do not differ significantly from those for Northern Hemisphere vegetation. The mean leaf size for a given mean annual precipitation is smaller than for a data set from the Western Hemisphere and Africa, but this difference is most likely due to different sampling methods. As for estimating climate from fossil floras, these results, along with the analysis of four other regional data sets, imply that the most accurate climate estimates will be produced by the predictor data set with the most similar climate-leaf morphology relationships. Unfortunately, our present lack of understanding of why climate-morphology relationships vary between the North America/Japan, New Zealand/Australia, and subalpine domains makes it difficult to identify data sets similar to paleofloras. Until we learn more, it is probably best to compare fossil floras to predictor data sets from the same domain. The performance of the various statistical methods depends on the nature of the predictor data set. Multiple regression analysis tends to produce the most accurate estimates for small data sets with a narrow range of environmental variation that have similar relationships to the flora, and linear regression or canonical correspondence analysis for the larger and more varied CLAMP data set. If a similar predictor data set is not available, then nearest-neighbor analysis can still produce accurate paleoclimate estimates.
Sexual dimorphism and paleoecology inTeleoceras, a North American Miocene rhinoceros
Authors: Mead Alfred J.
Published: July 2006

Sexual dimorphism is documented in 35 articulated adult skeletons, 24 females, and 11 males, of the Miocene rhinocerosTeleoceras majorfrom Ashfall Fossil Beds, Nebraska. Morphometric analysis of 51 cranial, mandibular, forelimb, and hindlimb characters reveals larger male mean values in 50 of the 51 measurements, of which 23 are significantly different (p≤ 0.01). The most clearly dimorphic feature is the i2 diameter. The dimorphism evident in additional mandibular and cranial characters is conservative when compared with the dimorphism present in the fore- and hindlimbs. Non-overlapping male and female ranges are recorded for humerus length, radius length, radius proximal width, and femur length, with corresponding dimorphism ratios (DR = male ÷ female) of 1.11, 1.12, 1.11, and 1.10. Maximum male longbone lengths exceed minimum female lengths by an average of 24% (20–29%). Developmental maturity is apparently asynchronous inT. major, with fusion of longbone epiphyses delayed a minimum of two relative adult age classes in males. Significant sexual dimorphism is evident in the radius (DR = 1.34) and femur (DR = 1.19) cross-sectional areas. Estimates of body mass suggest a DR value between 1.13 and 1.23. The cranial, mandibular, and body-size dimorphism inT. majorapproaches that seen in the extant rhinocerosesCeratotherium simumandRhinoceros unicornis.However, the apparent herd structure and breeding-age sex ratio for the Ashfall herd suggests a behavioral ecology forT. majordifferent from that of extant rhinoceroses.Teleoceraswas likely a herding polygynous species ecologically more similar to extantHippopotamus amphibiusof Africa.
New methods for quantifying macroevolutionary patterns and processes
Authors: Alroy John
Published: July 2006

This paper documents a series of methodological innovations that are relevant to macroevolutionary studies. The new methods are applied to updated faunal and body mass data sets for North American fossil mammals, documenting several key trends across the late Cretaceous and Cenozoic. The methods are (1) A maximum likelihood formulation of appearance event ordination. The reformulated criterion involves generating a maximally likely hypothesized relative ordering of first and last appearances (i.e., an age range chart). The criterion takes faunal occurrences, stratigraphic relationships, and the sampling probability of individual genera and species into account. (2) A nonparametric temporal interpolation method called “shrink-wrapping” that makes it possible to employ the greatest possible number of tie points without violating monotonicity or allowing abrupt changes in slopes. The new calibration method is used in computing provisional definitions of boundaries among North American land mammal ages. (3) Additional methods for randomized subsampling of faunal lists, one weighting the number of lists that have been drawn by the sum of the square of the number of occurrences in each list, and one further modifying this approach to account for long-term changes in average local species richness. (4) Foote's new equations for instantaneous speciation and extinction rates. The equations are rederived and used to generate time series, confirm that logistic dynamics result from the diversity dependence of speciation but not extinction, and define the median duration of species (i.e., 2.6 m.y. for Eocene-Pleistocene mammals). (5) A method employing the G likelihood ratio statistic that is used to quantify the volatility of changes in the relative proportion of species falling in each of several major taxonomic groups. (6) Univariate measures of body mass distributions based on ordinary moment statistics (mean, standard deviation, skewness, kurtosis). These measures are favored over the method of cenogram analysis. Data are presented showing that even diverse individual fossil collections merely yield a noisy version of the same pattern seen in the overall continental data set. Peaks in speciation rates, extinction rates, proportional volatility, and shifts in body mass distributions occur at different times, suggesting that environmental perturbations do not have simple effects on the biota.
Adductors, abductors, and the evolution of archosaur locomotion
Authors: Hutchinson John R. , Gatesy Stephen M.
Published: July 2006

Living crocodilians (Crocodylia) and birds (Neornithes) differ in many aspects of hindlimb anatomy and locomotor function. How did this disparity evolve? We integrate information from fossils with functional descriptions of locomotion in living crocodilians and birds, using a phylogenetic perspective. We then outline the major changes in three-dimensional control of the hip joint along the line from the ancestral archosaur to birds. Our analysis reveals that most aspects of hip morphology and function inAlligatorare ancestral for Archosauria. Femoral protractors and retractors are located cranial and caudal to the hip, respectively. Similarly, femoral adductors and abductors are located ventral and dorsal to the hip. Transformations of this ancestral pattern on the line to birds involved modifications in osteology, myology, and neural control. In some cases, homologous muscles changed function by acquiring new activity patterns. In others, activity was conserved, but origins and/or insertions were altered. Fossil theropods document the stepwise evolution of a novel mechanism of limb adduction/abduction involving long-axis rotation of the femur. This mechanism accounts for the conspicuous absence of significant musculature ventral and dorsal to the hip joint in extant birds.
Snake origins and the need for scientific agreement on vernacular names
Authors: Lee Michael S. Y.
Published: July 2006

When transitional fossils blur previously clear boundaries between major groups, drawing that line can be problematic. While the need to rigorously define formal taxon names is widely acknowledged (e.g., de Queiroz and Gauthier 1992; Cantino et al. 1997), the need for similar precision in the scientific use of vernacular terms is not widely appreciated. The lack of accepted usages for common names has engendered endless arguments about when a fish becomes (also?) a tetrapod (e.g., Clack 1997), when a dinosaur becomes (also?) a bird (e.g., Padian and Chiappe 1998), and when a mammal-like reptile becomes (also?) a mammal (e.g., Rowe and Gauthier 1992). Recent descriptions of fossil snakes with well-developed hindlimbs have raised similar questions about where to draw the line between lizards and snakes and initiated a lively debate over the origin of snakes. However, lack of a precise definition of the vernacular term “snake,” as well as lack of a consensus on what constitutes a higher taxon's “origins,” has seriously hindered discussion of “snake origins.” Here, precise definitions of both terms are proposed and justified and their paleobiological implications discussed. The origin of higher taxa remains one of the most intriguing macroevolutionary problems, but scientists risk arguing at cross-purposes unless they agree on the exact boundaries of vernacular groups, and the exact meaning of the term “origin.”
A fresh look at sideritic “coprolites”
Authors: Seilacher Adolf , Marshall Cynthia , Skinner H. Catherine W. , Tsuihiji Takanobu
Published: July 2006

Sideritic “coprolites” from the late Miocene of southwest Washington, the Upper Cretaceous of Saskatchewan and Madagascar, and the Permian of China have often been claimed to be pseudofossils. They are here interpreted as intestinal casts (cololites) prefossilized by bacterial activity and later transformed into siderite with no traces of original food particles left. All occurrences are found within fluvial overbank deposits that carry no other vertebrate remains. Their absence could be due to aquifer roll-fronts that destroyed phosphatic bones and teeth but favored siderite precipitation.
Evolution of hindlimb posture in nonmammalian therapsids: biomechanical tests of paleontological hypotheses
Authors: Blob Richard W.
Published: July 2006

Analyses of limb joint morphology in nonmammalian therapsid “mammal-like reptiles” have suggested that among many lineages, individual animals were capable of shifting between sprawling and upright hindlimb postures, much like modern crocodilians. The ability to use multiple limb postures thus might have been ancestral to the generally more upright posture that evolved during the transition from “mammal-like reptiles” to mammals. Here I derive a biomechanical model to test this hypothesis through calculations of expected posture-related changes in femoral stress for therapsid taxa using different limb postures. The model incorporates morphological data from fossil specimens and experimental data from force platform experiments on iguanas and alligators.

Experimental data suggest that the evolutionary transition from sprawling to nonsprawling posture was accompanied by a change in the predominant loading regime of the limb bones, from torsion to bending. Changes in the cross-sectional morphology of the hindlimb bones between sphenacodontid “pelycosaurs” and gorgonopsid therapsids are consistent with the hypothesis that bending loads increased in importance early in therapsid evolution; thus, bending stresses are an appropriate model for the maximal loads experienced by the limb bones of theriodont therapsids. Results from the model used to estimate stresses in these taxa do not refute the use of both sprawling and more upright stance among basal theriodont therapsids. Thus, the hypothesis that the use of multiple postures was ancestral to the more upright posture typical of most mammals is biomechanically plausible. Model calculations also indicate that the axial rotation of the femur typical in sprawling locomotion can reduce peak bending stresses. Therefore, as experimental data from alligators and iguanas suggest, the evolution of nonsprawling limb posture and kinematics in therapsids might have been accompanied by increased limb bone bending stress.
Comparative osteohistology of some embryonic and perinatal archosaurs: developmental and behavioral implications for dinosaurs
Authors: Horner John R. , Padian Kevin , de Ricqlès Armand
Published: July 2006

Histologic studies of embryonic and perinatal longbones of living birds, non-avian dinosaurs, and other reptiles show a strong phylogenetic signal in the distribution of tissues and patterns of vascularization in both the shafts and the bone ends. The embryonic bones of basal archosaurs and other reptiles have thin-walled cortices and large marrow cavities that are sometimes subdivided by erosion rooms in early stages of growth. The cortices of basal reptiles are poorly vascularized, and osteocyte lacunae are common but randomly organized. Additionally, there is no evidence of fibrolamellar tissue organization around the vascular spaces. Compared with turtles, basal archosaurs show an increase in vascularization, better organized osteocytes, and some fibrolamellar tissue organization. In dinosaurs, including birds, vascularization is greater than in basal archosaurs, as is cortical thickness, and the osteocyte lacunae are more abundant and less randomly organized. Fibrolamellar tissues are evident around vascular canals and form organized primary osteons in older perinates and juveniles.

Metaphyseal (“epiphyseal”) morphology varies with the acquisition of new features in derived groups. The cartilage cone, persistent through the Reptilia (crown-group reptiles, including birds), is completely calcified in ornithischian dinosaurs before it is eroded by marrow processes; cartilage canals, absent in basal archosaurs, are present in Dinosauria; a thickened calcified hypertrophy zone in Dinosauria indicates an acceleration of longitudinal bone growth.

Variations in this set of histological synapomorphies overlap between birds and non-avian dinosaurs. In birds, these variations are strongly correlated with life-history strategies. This overlap, plus independent evidence from nesting sites, reinforces the hypothesis that variations in bone growth strategies in Mesozoic dinosaurs reflect different life-history strategies, including nesting behavior of neonates and parental care.
Theoretical morphology of the Archosaur (Reptilia: Diapsida) pelvic girdle
Authors: Rasskin-Gutman Diego , Buscalioni Angela D.
Published: July 2006

Theoretical models of skeletal structures provide suitable frameworks to assess macroevolutionary patterns of form change. We discuss three theoretical approaches to account for morphological patterns of the pelvic girdle in archosaurs. Every approach targets a different level of organization within the concept of morphospace. First, we build a morphocline by applying a mathematical transformation to the outline of the hip of the theropod dinosaurDeinonychus antirrhopus, in order to look at theoretical paths of evolutionary change based on changes of proportion. Second, we analyze the variability of a sample of 86 hips within a theoretical construction that incorporates information about the spatial orientation of the three paired bones that build this skeletal compound. Finally, we look at boundary patterns within these hips as a basis for generating a formalism based on graph theory. Insights about the evolution and development of the archosaur triradiate pelvis and its morphological trends are suggested in the light of each theoretical approach, with a special focus on the convergent evolution of a retroverted pubis in ornithischians and birds.