bluuebunny/crossref_metadata_2025 huggingface dataset:
Origin of the Late Ordovician Lepidocyclus brachiopod fauna in North America and its biogeographic significance
Authors: Jin Jisuo
Published: January 2018

Three rhynchonellid brachiopod genera, Hiscobeccus, Lepidocyclus, and Hypsiptycha, are the most diagnostic elements of the Lepidocyclus fauna of North America in Late Ordovician time. These are characterized by relatively large, strongly biconvex to globular shells with coarse imbricating growth lamellae and, internally, with septiform cardinal processes in brachial valves. Among the three genera, Hiscobeccus appears the earliest, now known from rocks of late Trentonian-Edenian age in the Canadian Rocky Mountains and Mackenzie Mountains. Morphologically, Hiscobeccus is distinguished from the other two genera by its open delthyrium in the pedicle valve. Early forms of Hiscobeccus show close morphological similarity to Rhynchotrema in their non-globular biconvex shells covered by strong growth lamellae only in the anterior portions. It has been suggested that Hiscobeccus evolved from the Rhynchotrema wisconsinense stock through increase in shell size, globosity, and strength of growth lamellae. Earliest species of Rhynchotrema has been documented convincingly from rocks of early Trentonian age, and the derivation of Hiscobeccus most likely took place during the mid-Trentonian. Lepidocyclus and Hypsiptycha evolved from either Rhynchotrema or Hiscobeccus by developing a pair of deltidial plates covering the delthyrium.

Rhynchotrema and other rhynchonellids that evolved before mid-Trentonian time are common to the North American (Laurentian) and the Siberia-Kazakhstan paleocontinents. In contrast, Hiscobeccus, Lepidocyclus, and Hypsiptycha that evolved after the mid-Trentonian are virtually restricted to Laurentia. Therefore, Rhynchotrema marked the last successful intercontinental migration of rhynchonellids during their Llandeilian-Caradocian cosmopolitanism. The pronounced provincialism of the North American Lepidocyclus fauna may have been caused by a number of factors. Facies control is not likely the explanation because these rhynchonellids occur in nearly all the inland and marginal platform seas of Laurentia and commonly are found together in the same types of rocks. Plate tectonics and sea-level changes are considered major causes. The Ordovician rhynchonellids lived in shallow marine (intertidal-subtidal) environments and were incapable of crossing vast, deep oceanic barriers because of their sedentary mode of life and short-lived motile larval stages. The widening of the ocean between North America and Siberia, coupled with high sea-level stand, may have created a sufficiently wide oceanic barrier to interrupt faunal mixing between the two paleocontinents by late Trentonian time. Moreover, the rise in sea level would have resulted in the disappearance of island faunas, which could have served as stepping stones for intercontinental migration of shallow-water benthic faunas during low sea-level stand.
Late Pennsylvanian and Permian Turrilepadida (Machaeridia) from the western and south-central United States
Authors: Kaasa Maurice E.
Published: January 2018

The Machaeridia is an enigmatic marine fossil group which has been variously allied with the cirripedians, molluscs, annelids, and echinoderms. Although machaeridians have previously been unreported from rocks younger than earliest Pennsylvanian (late Namurian), members of the machaeridian order Turrilepadida are now found to be locally abundant well into the Permian. New occurrences of turrilepadidan sclerites are reported from the Late Pennsylvanian of Texas and the Early Permian of Texas and Nevada.

Turrilepadidan sclerites are common in marine shales of Missourian and Virgilian age in north-central Texas. Well preserved turrilepadidan sclerites have been found in the Wolf Mountain Shale (Missourian) and in the Colony Creek, Finis, and Necessity Shales (Virgilian) of north-central Texas. A few sclerites have a high number of closely spaced rugae and are similar to those of Turrilepas, but the most common types of sclerites have relatively few, widely spaced rugae, and many of these show radial ornament like that found on sclerites assigned to the genus Clarkeolepis. Although most forms would be placed in the family Turrilepadidae, a partial specimen from the Finis Shale appears to be a lepidocoleidid sclerite.

Silicified sclerites of Permian age have been found in two marine carbonate units: the upper part of the Bird Spring Formation (late Wolfcampian or early Leonardian) of southern Nevada and in the Cathedral Mountain Formation (Leonardian) of West Texas. Although their details are generally poorly preserved, these sclerites appear to be representative of members of the family Turrilepadidae with a different taxon found at each of the two localities.

The paleoecology of turrilepadidans is not well known, but each of these occurrences is from a locality that represented an area with a muddy bottom, a slow rate of deposition, and an unusually diverse fauna. The fauna at each horizon is dominated by a large number of brachiopod and mollusc species, many of which were conspicuously encrusted or bored during life or afterwards. Although a few of the sclerites show postmortem encrustations on their inner or outer surfaces, the overall lack of borings or encrustations suggests that the turrilepadidans were mobile, possibly infaunal organisms.
The flat clam controversy: Where did they come from? Where did they go?
Authors: Kauffman Erle G. , Tomás Villamil , Harries Peter J. , Meyer Christian A. , Sageman Bradley B.
Published: January 2018

Thin-shelled, weakly articulated, epifaunal byssate to free-living flat clams characterize Devonian-Cretaceous dark, organic-rich shale facies. Convergent evolution on the flat-clam morph occurs among several families of Bivalvia. Flat clams regularly occurred in dense populations, dominating low diversity, inequitable communities for which they were commonly the pioneer species and formed shell islands for the colonization of smaller, firm-substrate dependent invertebrates. The unique flat-clam dominated ecosystem, as well as the widespread environments to which they were adapted, disappeared at the end of the Cretaceous. Although locally, low-oxygen benthic muds exist through the Cenozoic to Recent, they do not support large populations of flat clams, whose ecological niche was never filled after the Cretaceous. Flat clams underwent an exponential increase in size and apparent growth rates through the mid- and Late Cretaceous, reaching over 3 m in diameter (Inoceramidae). Most of this increase in shell size also involved expansion of mantle tissue and probably gills, relative to the size of the visceral mass. This suggests specific adaptations to colonization of oxygen-poor benthic habitats with hydrogensulfide enriched substrates, which broadly characterized Paleozoic and Mesozoic epicontinental seas, during greenhouse intervals. Such conditions were particularly well-developed during Cretaceous Oceanic Anoxic Events, at which time flat clams thrived.

Functional morphology, geochemistry, and facies associations suggest that many flat clams were chemosymbiotic and/or had greatly expanded oxygen-absorption surfaces. These adaptations allowed them to be opportunistic in chemically stressed benthic environments; growth rates also seem to be enhanced in these environments, a characteristic of living chemosymbiotic species. Many lines of evidence prove that flat clams lived in these environments and do not represent pseudoplanktic rain. A combination of factors seems to have been responsible for their disappearance near the end of the Cretaceous; i.e., mass extinction, loss of primary habitats, and major radiation among bottom-feeding bivalve predators.
Biostratigraphy and biogeography of Frasnian, Upper Devonian conodonts
Authors: Klapper Gilbert
Published: January 2018

Analysis of Frasnian conodont sequences in the Montagne Noire, southern France, results in the recognition of a conventional zonation consisting of thirteen zones. Graphic correlation of eight Montagne Noire sections with a total of 11 sections in the Alberta Rockies, the Hay River-Trout River areas of the southern Northwest Territories, Canada, the Midcontinent and New York sequences in the United States, and the Canning Basin, Western Australia divides the Frasnian into 34 composite standard units. Most of these sections are dominated by conodonts of the outer-shelf to basinal Palmatolepis biofacies, but those of Hay River-Trout River are occupied by the quite contrasting inner-shelf Polygnathus biofacies. Correlation of the latter with the Montagne Noire zonation is effected only through graphic correlation. The taxonomy of many of the critical species involved in the correlations is based on shape analysis and multielement taxonomy of Palmatolepis.

Prevailing opinion is that the Frasnian was a time of cosmopolitanism as exemplified both by benthic organisms, especially corals and brachiopods, as well as conodonts. This contrasts with the Early and Middle Devonian which was a time of significant endemism in benthic fossils and in conodonts. The high point in conodont endemism according to the zone-by-zone analysis of the Early and Middle Devonian (Klapper and Johnson, 1980) was the early Eifelian when there were about 70% endemic species. Although endemism did not reach this high in the Frasnian, analysis of a limited number of intensively collected and studied sections in the Montagne Noire, U.S. and Canada, Western Australia, and the Russian Platform demonstrates levels of endemism ranging from about 30 to 45% in three composites of two zones each. Endemic species occur in both the Palmatolepis and Polygnathus biofacies. Using the Probabilistic Index of Similarity of Raup and Crick (1979), the null hypothesis of randomness is rejected for some paired comparisons between localities representing different biofacies, as would be expected, but it is also rejected between some localities representing the same biofacies on different continents. Although the explanation of significantly dissimilar faunas can be accepted at 95% confidence levels in only a limited number of instances, there are dissimilarities of somewhat lesser magnitude among various paired comparisons. This seems to indicate that the Frasnian was not a time of conodont cosmopolitanism even within the Palmatolepis biofacies.
The development of Early Paleozoic shell concentrations: evidence from the Cambrian and Ordovician of the Great Basin
Authors: Li Xing , Droser Mary
Published: January 2018

Shell concentrations have constituted an important and conspicuous part of the stratigraphic record since the Early Cambrian. The paleontological and stratigraphic significance of shell beds is well understood, primarily from Mesozoic and Cenozoic examples. Lower Paleozoic fossil concentrations, however, have not received much attention. The Cambrian and Ordovician evolutionary radiations were two of the most significant events in the history of life and established the Cambrian and Paleozoic faunas respectively. In order to determine the effect of these radiations on the development of fossil accumulations, a systematic study of early Paleozoic shell beds was conducted in the Great Basin areas of California, Nevada, and Utah.

In order to minimize taphonomic variations in original chemical and physical conditions, shell beds were compared from strata deposited in similar depositional environments from similar tectonic settings. Preliminary analysis of the shell beds from relatively pure carbonate facies and mixed carbonate and siliciclastic facies shows: 1) that shell concentrations became a significant stratigraphic feature in the later Early Cambrian; 2) the thickness and lateral extent of the shell beds increase from Early Cambrian to Middle Ordovician; 3) the abundance and internal complexity of the shell beds increase from Early Cambrian to Middle Ordovician; and 4) the Cambrian and Early Ordovician shell beds are primarily, if not exclusively, dominated by trilobites whereas the Middle Ordovician shell beds are dominated by brachiopods and ostracodes.

These data show a temporal trend in the development of the early Paleozoic shell beds. The nature of the Cambrian and Ordovician shell beds differs qualitatively and quantitatively. There is an increase in physical scale, abundance, and internal complexity through time. The thickness and abundance of the trilobite beds increase through the Cambrian. Interestingly, although trilobites were still diverse and abundant, they did not commonly generate thick trilobite beds after the Late Cambrian. The early Middle Ordovician is a critical time in the development of early Paleozoic shell beds. A variety of monotaxic and polytaxic shell beds, including 6m thick composite beds, first appeared at this time. While the brachiopods and ostracodes generate laterally extensive, commonly monotaxic, shell beds, the gastropods and bryozoans only formed lenticular concentrations.
Comparative ecology of bryozoan radiations: origin of novelties in cyclostomes and cheilostomes
Authors: Lidgard Scott , Taylor Paul D. , Jablonski David
Published: January 2018

The cyclostome and cheilostome bryozoan clades diversified at different times and consequently in different ecological contexts. Cyclostomes began their rebound from a Permo-Triassic bottleneck in the early Jurassic, prior to increases in bioturbation, durgophagous predation, and other ecological changes commonly termed the Mesozoic Marine Revolution. Cheilostomes did not appear until the latest Jurassic and rapid diversification began only in the mid-Cretaceous, when the Mesozoic Revolution was well under way. We compared the radiations of these clades to test for similarities reflecting general patterns in the origin of within-clade novelties, and for differences that might be due to ecological context or clade attributes. We examined the timing and environment of first occurrence of eight morphological novelties that evolved convergently in both clades (ovicells, cylindrical arborescent colony, multilayered encrusting growth [frontal budding s.l.], bilaminate erect colony, fenestrate colony, articulated colony, calcified operculae, avicularia), as well as novelties unique to cheilostomes (cribrimorph and ascophoran grades, and lunulitiform colonies) or to cyclostomes (apertures arranged in fascicles). First occurrences were traced from the literature, BMNH collections, and unpublished data provided by Prof. Dr. E. Voigt; environments were inferred within an onshore-offshore gradient using the protocol of Jablonski and Bottjer (1990).

As seen for other invertebrate groups, within-clade novelties were not concentrated in onshore settings for either bryozoan clade, in contrast to the origination patterns at the ordinal level (including the Cheilostomata). Differences in sequence or environment of first occurrence were not obviously related to the Mesozoic Revolution, or to the distinction between zooid- and colony-level characters. Novelties showed a weak tendency to originate in similar sequence and in similar environments for both clades, but these numbers are small. Differences in sequence may partly reflect clade-specific constraints: for example, opercula are considered a prerequisite for the development of avicularia, so that cyclostomes needed to evolve that precursor before avicularia could appear, whereas opercula are primitive (though uncalcified) in cheilostomes and thus did not constitute such a constraint. If avicularia are omitted, the rank-order correlation of novelties becomes marginally significant (p = 0.05, Spearman rank test). The most striking difference is in the temporal pattern of novelty acquisition. In cyclostomes, these are rather evenly spread over 100 Ma, whereas in cheilostomes six of the novelties appeared in the Late Albian-Early Cenomanian during a period of rapid diversification. Despite a slow start, the cheilostome radiation enters an explosive phase that may characterize successful establishment of clades founded late in the Phanerozoic.
Levels of selection: an analysis of the forces driving diversification in the turritellid gastropods
Authors: Lieberman Bruce S. , Allmon Warren D. , Eldredge Niles
Published: January 2018

This analysis examines the evolution of the greater diversity of non-planktonic species relative to planktonic species in the turritellid gastropods. Two mechanisms for generating diversity gradients in larval types have been proposed in the literature. The first, species selection or the effect hypothesis, focused on the population biology of larval types. The second proposed that factors in development were responsible. Turritellids have been cited as a classic example of species selection. In order to test the validity of these mechanisms for the evolution of the turritellids two phylogenetic models are formulated. One is constructed to fit the predicted evolutionary results if species selection or the effect hypothesis are the operative processes driving diversification. The second model is constructed as if developmental constraints prohibiting the origination of planktonic species in non-planktonic lineages were responsible for the relative increase in non-planktonic species.

In order to test these two proposed mechanisms a phylogenetic analysis of the turritellids using molecular sequence data was performed to determine the evolution of larval types in this clade. This is compared to the phylogenetic patterns predicted by the two models. The model asserting the control of species selection or the effect hypothesis is rejected. It is predicted that species with non-planktonic larvae will increase at the expense of planktonic species because planktonic organisms are developmentally more open to change, while developmental change is buffered in the non-planktonics. Once a lineage has become non-planktonic, there is little chance for it to undergo subsequent changes in larval style through phylogenetic time. The process producing this pattern is termed the cell-lineage effect, and it is a mechanism governing changes in the timing of germ line sequestration in organisms. Thus, the fossil and recent turritellid biota will show a bias towards non-planktonic forms due to biases and constraints in development.
The origin of mammals: chronology and paleobiogeography
Authors: Lucas Spencer G. , Hunt Adrian P.
Published: January 2018

A comprehensive review of the stratigraphic and geographic distribution of the advanced, non-mammalian cynodonts (traversodontids, tritylodontids and tritheledontids) and the Late Triassic-Early Jurassic mammals indicates that: (1) traversodontids were a wholly Triassic group that disappeared during the Rhaetian; during the late Carnian-early Norian they were rare but widespread components of Pangaean land-vertebrate faunas; (2) tritylodontids first appeared in Europe during the Rhaetian, were a cosmopolitan group by the Sinemurian/Pliensbachian and disappeared during the Bathonian; (3) tritheledontids ranged in age from late Carnian to Sinemurian/Pliensbachian and were mostly a New World group; and (4) the oldest mammal is of late Carnian age from West Texas, but there is at least a 10-million-year gap between it and the next oldest mammals from the late Norian of Europe.

Advanced cynodont and mammalian distributions of the Late Triassic-Early Jurassic do not suggest Late Triassic paleoprovinciality, but they do support the notion of a cosmopolitan vertebrate fauna during the Sinemurian/Pliensbachian. Proponents of a tritylodontid ancestry of mammals must explain away a 15-million-year-long absence of tritylodontids from the Late Triassic fossil record. In contrast, proponents of a tritheledontid ancestry of mammals need offer no such explanation since tritheledontids and mammals appeared simultaneously during the late Carnian.
Patterns of faunal turnover and diversity in the Wyoming-Montana Paleogene in relation to regional and global events
Authors: Maas M.C. , Gingerich P.D. , Gunnell G. , Krause D.W.
Published: January 2018

The Bighorn Basin of Wyoming and its northerly extension, the Crazy Mountains Basin of Montana, contain a continuous record of the middle Paleocene through early Eocene mammalian biota. This interval, spanning some eight million years, is subdivided into 18 faunal zones. Durations of the zones, ranging from 0.3 to 0.75 million years, are extrapolated from intrabasinal depositional rates and correlated with the early Tertiary paleomagnetic reversal record. This stratigraphic resolution allows us to document in detail regional diversity and turnover.

In the Bighorn and Crazy Mountains basins, standing generic richness (estimated number of genera at the midpoint of an interval) decreased slightly between the Torrejonian and Tiffanian (middle and late Paleocene), increased from the latest Tiffanian and Clarkforkian (latest Paleocene) to the early Wasatchian (early Eocene), and declined during the later Wasatchian.

Changes in diversity reflect first and last appearances (including originations, extinctions, and dispersal). Because first and last appearance rates are in part dependent on diversity and the duration of each interval, we plot generic turnover as rate quotients, ratios of observed to expected first or last appearances. Expected turnover rates are calculated from multiple regression of turnover on duration and generic richness. Turnover rates are evaluated in relation to the quality of the fossil record for each interval. Rate quotients show that the slight decline in generic richness in the Bighorn and Crazy Mountains basins between the Torrejonian and Tiffanian resulted from a higher than expected disappearance rate at the end of the Torrejonian. Lower than expected disappearance rates and higher than expected appearance rates in the early Clarkforkian and, particularly, the earliest Wasatchian contributed to the increase in richness during this period. The middle through late Wasatchian decline in generic richness resulted from surprisingly low appearance and high disappearance rates.

There was little change in taxonomic composition of faunas between the middle and late Paleocene; faunas from this interval were dominated by archaic groups (e.g., “condylarths”, multituberculates, plesiadapiforms). In contrast, faunal composition changed markedly during the latest Paleocene and the early Wasatchian, with the introduction of major modern groups (e.g., rodents, artiodactyls, perissodactyls, primates) and the decline in diversity of the archaic forms that dominated earlier Paleocene faunas. There is strong evidence that this faunal change was influenced by global climatic factors: climatic warming, beginning in the latest Paleocene, opened new routes for intercontinental dispersal, and had a major effect on worldwide patterns of faunal composition and diversity, including those of the Bighorn and Crazy Mountains basins.
Patterns of loop development in post-Paleozoic terebratulid brachiopods and their evolutionary significance
Authors: Mackinnon David I.
Published: January 2018

Studies of loop ultrastructure reveal much about both the processes and patterns of loop growth in brachiopods. Such studies also provide useful insight into evolutionary relationships between various main loop-bearing brachiopod taxa. At least five different loop types, two short-looped and three long-looped, are recognisable in Cenozoic Terebratulida.

Loop development in short-looped forms is relatively unspectacular and of limited value for evolutionary studies. It consists of the growth of either an undulating transverse ribbon extending between two short crura (in the Family Terebratulidae) or, where crural processes are united (as in most Cancellothyrididae), a ring. In all short-looped forms the loop is derived from, and hence supported solely by, the crura throughout ontogeny.

The early stages of loop development in the great majority of post-Paleozoic long-looped brachiopods are similar. A septal pillar, located centrally in the dorsal valve floor, appears generally very early in ontogeny. Rudimentary descending branch elements are generated both from the crura and the flanks of the septal pillar. Rudimentary ascending elements develop at either the ventral or posteroventral edge of the septal pillar and consist firstly of an inverted or steeply inclined and continuously expanding cone or hood which, through resorption of its posteriorly located apex, becomes transformed into a ring. Further growth sees firstly the fusion of the anterior extremeties of adjacent ascending and descending loop elements followed by their progressive anterolateral separation from the septal pillar.

Contrasting modes of growth characterise the later stages of loop development in the three main groups of long-looped brachiopods. In Dallinidae, ascending and descending elements detach from the septum to give rise to a long, reflected loop supported only by the crura. In Terebratellidae, a pair of narrow, laterally directed but anteriorly migrating struts anchor the the descending branches to the median septum. These struts, in structural continuity with the median septum, are built up entirely from secondary layer fibres. In Laqueidae, in addition to a pair of septal struts, a pair of anterolaterally migrating vertical bands spanning the gap between adjacent ascending and descending branches provide further support. Unlike the septal struts but like the remaining ascending and descending loop elements, the vertical bands are constructed from both primary and secondary shell layers. Growth line discordancies at the junctions between the vertical bands and contiguous ascending and descending elements indicate the structural integrity of the vertical bands.
Two new carnivorous cynodonts from the Ischigualasto Formation (Upper Triassic) of Argentina
Authors: Martínez Ricardo N. , Forster Catherine A. , May Cathleen L.
Published: January 2018

Two skulls representing new forms of carnivorous cynodonts were discovered recently in the Upper Triassic (Carnian) Ischigualasto Formation of northwestern Argentina. These specimens are the first complete carnivorous cynodonts described from the Late Triassic of Argentina, and provide critical evidence for the evolution of cranial form between well known Middle Triassic cynodonts, such as Probainognathus, and the first mammals in the latest Triassic.

The first skull (PVSJ 422) is proportionately long, with a posterior skull width only 35 per cent of basal skull length (9.5 cm.). Other distinctive features of the skull include the form of the postcanine crowns, which have two cusps posterior to the principal cusp and fine serations on their leading edges, and which overlap each other in an imbricate pattern. Postcanines 5-7 are subequal in size, overlap the dentary laterally, and are substantially larger than postcanines 2-4. The dental formula in the upper jaw is I4-C1-PC7, with the small first postcanine separated fore and aft by small diastemas. As in other advanced eucynodonts, no pineal foramen is present. Some postcranial elements, as yet unstudied, were also found with this specimen.

The second skull (PVSJ 411) has broader proportions, the width between quadrates almost half that of basal skull length (7.5 cm.). These proportions, in combination with deep, curved zygomatic arches, resemble other Middle Triassic cynodonts such as Probelesodon. Distinctive features of this skull include bicusped postcanines in the upper jaw similar to those of Probelesodon, and a mandibular symphysis that extends back to the level of the fourth postcanine. The dental formula in the upper jaw is I4-C1-PC6. As in the first skull, a pineal foramen is absent.

Several advanced cranial features, such as a fused mandibular symphysis, occur in both skulls and strongly suggest that these new forms belong within the Eucynodontia (Hopson, 1991). Other advanced features, such as the absence of a pineal foramen, further suggest that these new forms are closer to mammals than are traversodontid cynodonts. In contrast, other characters appear to be curiosly primitive, such as a short hard palate in the first skull. These new cynodonts thus may introduce previously unseen features and character combinations to the study of cynodont cranial evolution.
The geological history of polyploidy in woody angiosperms
Authors: Masterson Jane
Published: January 2018

Polyploidy, the condition where an organism has more than two complete sets of chromosomes, is a common and important evolutionary phenomenon in flowering plants (angiosperms). Estimates of the percentage of angiosperms (approximately 250-300,000 living species) derived from polyploids range from 47-90%. The historical development of polyploidy is currently very poorly understood and whether polyploidy is a major causal factor in angiosperm diversification is widely debated.

Many questions about the base chromosome number of angiosperms (important for establishing the extent of polyploidy) and the origin of polyploidy can be addressed using fossil plants. One of the physical effects of polyploidy is a general increase in cell volume; the size of guard cells (leaf epidermal cells that control the opening and closing of the gas exchange pores [stomata]) has been used successfully to determine the relative number of chromosomes present or ploidy level in living Carva (hickories), Betula (birch), Magnolia, and Pvrus (pear). At least two previous studies have used guard cells to estimate ploidy level in fossil taxa (Metasequoia, Sequoia, and Paleorubiaceophyllum).

The base haploid chromosome number in angiosperms generally has been assumed to be between n=7-9, but this is apparently based either on the dubious assumption that the most common condition is likely to be primitive (cf. egg laying in mammals) or the inference that because this number is found in many different clades it is likely to be basic in each of these groups. The Gnetales (the living gymnosperms most closely related to angiosperms) have a haploid basic number of 7, but Donoghue and Doyle (1989) find that it is almost equally parsimonious to assume the base number is n=12-14 because of uncertainties over the chromosome number of certain critical extinct outgroups. My guard cell measurements of three angiosperm families (Platanaceae, Magnoliaceae and Lauraceae) provide additional empirical support for the n=7-9 hypothesis and suggest that extinct diploid members existed in the mid-Cretaceous and early Cenozoic floras. These results also falsify the hypothesis that polyploidization triggered the angiosperm radiation in the Cretaceous in these families. Other hypotheses regarding the historical timing of major increases in polyploidy are being tested. These include climatic deterioration and associated modification and mixing of plant distributions during the Eocene and Pleistocene. It may be that polyploids survive major disturbances at a higher rate than diploids because they often have broader ecological tolerances and wider geographic ranges. Thus the accumulation of polyploids in plant families may not occur because they are speciation prone as usually inferred but because they are extinction resistant. My preliminary results in Platanaceae support an increased level of polyploidy after major disturbances.
Paleobotany of the Paleocene St. Stephens site, Berkeley County, South Carolina
Authors: Melchior Robert
Published: January 2018

Plant fossils including silicified wood, amber and a diverse suite of palynomorphs were recovered from beds of the Rhems and Williamsburg Formations of the Black Mingo group. The Rhems beds are Danian in age and the Williamsburg is Thanetian. Both formations constitute prograding delta sequences. The palynomorph assemblage was recovered from the Rhems Formation and consists of 112 species of pollen and spores of angiosperms, gymnosperms and cryptogams as well as 32 species of fungal spores. Silicified wood represents 10 taxa and the amber is suggestive of two but precise stratigraphic position of these latter fossils is uncertain.

An analysis of the ranges of palynomorph taxa within the Rhems Formation yields three assemblages. An assemblage concentrated in the lower, pro-delta beds of the unit is felt to represent a coastal community, a group concentrated in the upper delta-front and topset beds is representative of an inland assemblage and a third group ranges throughout the section. The coastal assemblage is dominated by palms with gymnosperms also being abundant. The inland assemblage consists primarily of a diverse array of dicots associated with the Normapolles group. The long ranging assemblage is associated with both the above groups in about equal numbers. A habitat mosaic consisting of xeric to mesic and hydric elements within the coastal and inland community zones is inferred. Analysis using the modern affinities of the palynomorphs suggests a subtropical, summer dry climate near its meteorological boundary with the tropical zone such as that found in parts of coastal southeast Asia today.

Wood taxa from the site include specimens assignable to Podocarpoxylon, Myrica, Fagaceaoxylon, Magnoliaeoxylon, Ulminium, Liquidambar, Nyssoxylon, the Olacaceae, Apocynaceae and possibly the Icacinaceae. Calculated “V” values and anatomical details of these woods correlate well with the habitat mosaic and paleoclimatic implications suggested by the palynomorphs.

Infra-red spectra of the ambers suggest affinities of these fossils with the Caesalpiniaceae and the Araucariaceae. Pollen that might be attributable to these groups includes Margocolporites, Rhoipites, and Classopollis (Pagiophyllum).

The fungal palynomorphs are both abundant and diverse in some samples from the site. These fossils include both hyphae and conidia in place in the silicified wood and dispersed spores.
Advances in deepsea paleoecology: introduction to the symposium
Authors: Miller W.
Published: January 2018

This symposium will cover new developments in the ecology and paleoecology of deepsea organisms, based largely on advances in the study of protists, metazoan associations, and biogenic sedimentary structures. Bringing together ecologists and paleoecologists will establish a bridge between workers who have devoted their attention exclusively to the modern biota and those who have only considered fossil evidence from pelagic, turbidite, and related deep basinal rock sequences. Contributors will emphasize patterns and processes from historical or developmental perspectives as well as from experimental or actualistic perspectives, and will attempt to identify the promising directions for future research.

Specifically, attention will be focused on benthic organisms and associations, and on biogenic structures, because these entities make up ecologic patterns that are most accessible to study by both biologists and paleontologists. On the biologic side, contributors will review the significance of new discoveries and theories, present summaries of path breaking work on foraminiferids and xenophyophores, and characterize modern organism-sediment interactions and trace associations. Paleontologists will emphasize processes requiring large areas of seafloor and long time spans in their review of the origins of deepsea communities and ichnofacies, large-scale dispersal patterns, long-term ecosystem dynamics, and taphonomy and time-averaging of deepsea fossil deposits. This is the first attempt to bring ecologic and paleoecologic efforts into closer coordination in order to identify the most significant themes in the historical biology of deepwater marine organisms.

Some examples of research issues amenable to this kind of coordinated approach include: 1) origination of deepsea inhabitants from shallow marine sources vs. elaboration and diversification of taxa already established in deepwater; 2) importance of small-scale, frequent disturbance vs. large-scale, rare events (turbidity currents, anoxia episodes) in shaping the deepsea biota; 3) taxonomic replacements and invasions at varied spatiotemporal scales; 4) biotic interactions vs. environmental pacing as mechanisms producing community structure; 5) evolutionary importance of allochthonous vs. autochthonous food supplies; 6) relationships between trace fossil assemblages and the development of deepsea ecosystems; 7) the nature and history of linkages between deepsea ecologic systems, and terrestrial and shallow marine systems; and 8) preservation potential of the dominant benthic organisms, and their symbionts or traces, and its implications in paleoecologic reconstructions. In general, combining biologic and paleontologic information should result in more realistic and useful interpretations of ancient biotas (rather than simply the description of more unique fossil finds), as well as an increased awareness of the long-term development of the modern biota (rather than attention directed exclusively to living organisms, short-term processes, and small-scale patterns).
Ostracode paleoecology of the Bangor-Pennington transition (Chesterian, Mississippian) in northeastern Alabama
Authors: Olson William , Dewey Chris
Published: January 2018

The Pennington Formation (Chesterian, Mississippian) of northeastern Alabama is a regressive, distal lobe of the Pennington/Lee clastic wedge, located on the western flank of the Appalachians in the Tennessee structural salient. The clastic units of the Pennington include sandstone, siltstone, mudstone, clay shale and coal and are genetically related to the effects of progradation. The presence of limestones and dolostones within the southwestwardly prograding and thinning Pennington is due to the lateral equivalency and interfingering relationships of the Pennington with the upper part of the adjacent Bangor Limestone shelf.

A highly diverse ostracode fauna as been collected from outcrops along the Pennington-Bangor transition in northeastern Alabama. The most common assemblage of ostracodes is dominated by kirkbyaceans and includes species of Amphissites, Polytylites, Kirkbyella, Kirkbya and Reviya. Bairdiaceans are noteably absent from this assemblage. The Kirkbyacean Assemblage is only found in middle to outer shelf palaeoenvironments where clastic input is high, but where salinity conditions are thought to have been typically marine. In middle to outer shelf limestone-producing palaeoenvironments, a Bairdiacean Assemblage is dominant, although kirkbyaceans are still present.

A Sansabellid Assemblage, consisting of kloedenellaceans such as Sansabella, Geisina, Geffenina, Nufferella and Glyptopleura appears to be produced under rapidly changing nearshore, inner shelf conditions, where terrigenous input was often high and palaeosalinities may have been variable or less than normal marine. The Sansabellid Assemblage contains occasional kirkbyaceans, however bairdiaceans are noteably absent. Intermediate assemblages contain elements of the three main assemblages and reflect the transition from the inner to outer shelf.

In summary, the ostracode assemblages of the Pennington-Bangor transition in northeast Alabama provide indicators of palaeoenvironmental change during the southwestward progradation of the Pennington deltaic wedge onto the Bangor Limestone shelf. The prime factors resulting in palaeoecological response appear to have been variations in terrigenous input and palaeosalinity.
Morphology and movement of the presacral vertebral column in Phenacodus vortmani and Phenacodus primaevus
Authors: Otts Charlotte
Published: January 2018

Phenacodus vortmani and P. primaevus, members of the mammalian radiation (Tiffanian - Bridgerian) in western North America, were digitigrade five-toed ungulates who were good runners compared to their contemporaries. P. vortmani was smaller and more slender (presacral vertebral column = 463 mm) while P. primaevus was larger and more robust (presacral vertebral column = 814 mm).

The presacral vertebral column in these phenacodontids consists of 7 cervical, 15 thoracic, and 6 lumbar vertebrae, with the sectional length proportions being very similar in P. vortmani and P. primaevus.

Morphologies and relationships of the vertebral structure in the cervical section allow both dorsoventral and mediolateral movements in both animals. Structures in the cranial thoracic section allow some mediolateral movement but restrict dorsoventral motion, adding stability and allowing forward transfer of momentum generated in the hind limb and caudal vertebral column. Structures in the caudal thoracic and especially the lumbar sections allow greater dorsoventral movements which enabled both phenacodontids to increase stride length beyond that accomplished by the limbs alone. In the smaller P. primaevus. the orientation of the zygapophyses emphasizes dorsoventral movements and restricts mediolateral movements compared to that in P. vortmani. providing the larger animal with greater efficiency in the flexion and extension of the posterior vertebral column during running.
The meaning of fossils, 1992
Authors: Patterson Colin
Published: January 2018

Martin Rudwick's 1972 book “The Meaning of Fossils” was structured around five events in the history of paleontology, set in 1565, 1666, 1796, 1829, and 1857. Each event was tied to developments in paleontology, geology and biology in the surrounding decades. The central conceptual issues in those five episodes—the origin of fossils, geochronology, extinction, gradualism or catastrophism, and the origin of species—are all still with us. Rudwick's history ended with the 1870s, and the gap between then and now is almost as great as that between his two most widely separated episodes. My own experience in paleontology spans one-third of that gap; I shall talk about the meaning of fossils today, in the context of recent developments in paleontology (mostly of vertebrates) and biology, and how those affect our understanding of fossils. My main headings are: techniques and methods (of preparation and interpretation of fossils), the completeness of the fossil record (how we assess that, and how our assessment is changed by new finds and by new interpretations), the rise of molecular biology and its impact on paleontology, and some aspects of the relation between evolutionary theory and paleontology.
Preservation of Mollusca in Copano Bay, Texas. The long-term record
Authors: Powell Eric N.
Published: January 2018

Downcore changes in carbonate content can be produced by varying rates of taphonomy, physical and biological reworking or changes in rates of carbonate addition. Only the latter can be used to investigate changes in carbonate productivity in the living community. Only the latter indicate unequivocal changes in community structure. The stratigraphic, taphonomic and biologic records from two cores in Copano Bay, Texas were analyzed to determine (1) whether variations in shell content with depth were caused by variations in carbonate preservation, carbonate production or sedimentation rate and (2) the extent to which characteristics of fossil assemblages such as species composition, numerical abundance, biomass, and trophic and habitat structure, identified similar or different trends. Below the top few cm of the sedimentary column, variations in carbonate content with depth could be attributed to variations in carbonate production. Most biological attributes varied similarly with depth and, hence, time on both long (≥ 100 yr) and short (~ 10 yr) temporal scales. These variations could not be explained by any taphonomic process, sediment reworking and burial, or sedimentation rate. Despite a vigorous taphonomic milieu, explaining the shell content of Copano Bay sediments requires the preservation of nearly all carbonate produced. Doing so requires the preservation of most of the relatively large-shelled biota (large species and adults) which retain important evidence of changes in the community's history in this area. The results reemphasize the importance of large individuals and biomass in paleontologic reconstruction and suggest that changes in community productivity, which in paleontologic usage, must be carbonate productivity, are preserved in the fossil record in some assemblages. Taphofacies analyses might be used to identify such assemblages.
Large-body impact: the least unlikely cause of pulsed extinction
Authors: Raup David M.
Published: January 2018

In the past year, a strong consensus in the geological community has developed in favor of comet or asteroid impact as the ultimate cause of the K-T mass extinction, although many paleontologists remain doubtful. The discovery of tektites and craters with argon-argon ages matching the K-T boundary has finally removed the “smoking gun” problem. It is important, therefore, to evaluate large-body impact as a possible cause of other Phanerozoic extinctions, and to do so as carefully as possible before enthusiasm for the K-T success overwhelms objectivity in this research area.

Approximately 60% of all species extinctions in the Phanerozoic occurred in “pulsed” extinctions, defined as ecologically and geographically pervasive episodes in which the number of species going extinct in a geologically short interval far exceeds any reasonable estimate based on chance coincidence of independent events. In addition to the well-known mass extinctions, pulsed extinctions often mark system, series, and stage boundaries, and probably some zonal boundaries.

To be plausible, any proposed cause of pulsed extinctions must be (1) geographically pervasive (regional or global), (2) effective in diverse habitats, and (3) relatively quick-acting (to inhibit survival of species by migration or adaptation). The stresses causing the extinctions must be sufficiently severe and rare to be beyond the reach of natural selection, so that species do not have prior opportunity to evolve defenses.

These requirements severely limit the possibilities to phenomena that occur on time scales of one million to tens of millions of years, far longer than the tens or hundreds of years available for study by traditional actualistic approaches. In view of the Phanerozoic record, it is not surprising that the earth has not experienced a pulsed extinction in historic times (excluding human influences). This suggests that extinction is one case where the present is a not the key to the past, and a full exploration of the problem will require a substantial re-ordering of thinking.

Furthermore, because pulsed extinctions involve geographically widespread species (as well as restricted taxa), and because extinction of widespread species may be qualitatively different from that of local endemics, the common extrapolation from studies of extinction in local populations is risky.

Of the many phenomena suggested as causes of pulsed extinction, large-body impact is the one that most nearly satisfies the requirements, and is therefore the least unlikely of the candidates. Temporal distribution is especially critical. Currently accepted flux estimates for comet and asteroid impacts are in the appropriate frequency range to explain the extinction record, but testing the hypothesis will depend on better radiometric dating of the 100+ craters and other confirmed impact events.