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PubMed2002

Geologic emissions of methane to the atmosphere.

The atmospheric methane budget is commonly defined assuming that major sources derive from the biosphere (wetlands, rice paddies, animals, termites) and that fossil, radiocarbon-free CH4 emission is due to and mediated by anthropogenic activity (natural gas production and distribution, and coal mining). However, the amount of radiocarbon-free CH4 in the atmosphere, estimated at approximately 20% of atmospheric CH4, is higher than the estimates from statistical data of CH4 emission from fossil fuel related anthropogenic sources. This work documents that significant amounts of "old" methane, produced within the Earth crust, can be released naturally into the atmosphere through gas permeable faults and fractured rocks. Major geologic emissions of methane are related to hydrocarbon production in sedimentary basins (biogenic and thermogenic methane) and, subordinately, to inorganic reactions (Fischer-Tropsch type) in geothermal systems. Geologic CH4 emissions include diffuse fluxes over wide areas, or microseepage, on the order of 10(0)-10(2) mg m(-2) day(-1), and localised flows and gas vents, on the order of 10(2) t y(-1), both on land and on the seafloor. Mud volcanoes producing flows of up to 10(3) t y(-1) represent the largest visible expression of geologic methane emission. Several studies have indicated that methanotrophic consumption in soil may be insufficient to consume all leaking geologic CH4 and positive fluxes into the atmosphere can take place in dry or seasonally cold environments. Unsaturated soils have generally been considered a major sink for atmospheric methane, and never a continuous, intermittent, or localised source to the atmosphere. Although geologic CH4 sources need to be quantified more accurately, a preliminary global estimate indicates that there are likely more than enough sources to provide the amount of methane required to account for the suspected missing source of fossil CH4.

Air Pollutants↗

PubMed2002

The Scaphopoda.

The Scaphopoda are marine infaunal carnivores that feed on foraminiferans and other microorganisms selected and manipulated by their unique feeding tentacles or captacula. Their tusk-like shell is open at both ends; the burrowing foot and captacula protrude anteriorly, while respiratory currents pass through the posterior opening. Although the scaphopods comprise one of the smallest molluskan classes in terms of species diversity, they have a worldwide distribution ranging from intertidal to depths in excess of 6000 m. Despite detailed monographic work from the late nineteenth and early twentieth centuries, however, the biology of scaphopods is today among the least understood of mollusks. To some extent, this is related to a significant departure from more typical molluscan organization inferred from studies of the larger classes. For example, the mantle cavity is elongate and restricted in space, and several associated organs are lost, including the osphradium and ctenidia; the latter is associated with the loss of auricles from the scaphopod heart. There is also little record of ontogeny in the adult shell, as the older portion is periodically decollated to maintain the posterior aperture for passage of inhalant and exhalant currents. This and other constraints on scaphopod diversification are reflected in the range limits of shell shape within the class. In contrast, the "Dentalium" egg has been a model system for experimental embryology, particularly in studies of cell lineage and early morphogenesis. Later larval development and most organogenesis is nonetheless poorly known, although recent studies have considerably enhanced our knowledge in this area. Global biogeographic patterns of scaphopod diversity have been studied only preliminarily. A general decrease in diversity with depth is broken by small diversity peaks at bathyal depths. There is also evidence for a marked latitudinal diversity gradient in the world's oceans, near equatorial in the Pacific but at approximately 20 degrees N in the Atlantic. Scaphopods have a wide diversity of ecto- and endo-symbiotic associations with other organisms, including commensal bacteria and ciliates, mutualistic anemones and corals, and parasitic algae and platyhelminths. Other documented associations include predation by naticid gastropods and ratfish, and the inhabitation of empty scaphopod shells by a variety of sipunculans and hermit crabs. Phylogeny within the class is still unresolved, although significant progress has been made recently in documenting morphological variation among families, genera, and species for application in a cladistic context. The evolutionary relationships of Scaphopoda within Mollusca are farther from resolution, with every conchiferan class a proposed sister group among modern analyses. Molecular avenues should provide significant progress in scaphopod phylogenetics, as indicated by recent comparative sequence analysis and gene expression patterns. Scaphopods comprise the most recent class of mollusks to appear in the fossil record, dating at least from the Mississippian Carboniferous (362.5 My). They appear from first-order occurrence data to have a substantially documented fossil record, with several sharp extinction and radiation events evident during their comparatively short geologic history. The unresolved position of the Scaphopoda within molluscan phylogeny, and their relatively limited range of morphological disparity but substantial derivation from other conchiferans, renders the scaphopods an enigmatic group. They represent a significant evolutionary avenue within the Mollusca, but one that is poorly examined. Nonetheless, comprehensive phylogenetic analysis and documentation of diversity within this small, monophyletic, class-level taxon, well-preserved in the fossil record, is imminently tractable, and presents a singular opportunity in the study of molluscan evolution.

Animals↗

PubMed1996

Global increases in allergic respiratory disease: the possible role of diesel exhaust particles.

OBJECTIVE: Reading this article will enable the readers to recognize and evaluate i e potential relationship between allergic respiratory disease and polyaromatic hydrocarbons as air pollutants from industrial and automotive fuel sources. In this article we review the long-term trends in the prevalence of allergic airway diseases (rhinitis and asthma). We then examine the epidemiologic and other research data relating to the role that hydrocarbon fuel emissions may have had on allergic respiratory disease. DESIGN: Published literature on the relationship between specific air pollutants and trends in allergic respiratory disease were reviewed. Reports of research on pollutant effects on allergic antibody (IgE) were also studied. In both cases, the Melvyl-Medline database since 1975 was used for literature searches. Older references were identified from the bibliographies of relevant articles and books and with the help of the rare books collection at UCLA's Louis M. Darling Biomedical library. RESULTS: Examination of the historical record indicates that allergic rhinitis and allergic asthma have significantly increased in prevalence over the past two centuries. Although the reasons for this increase are not fully elucidated, epidemiologic data suggest that certain pollutants such as those produced from the burning of fossil fuels may have played an important role in the prevalence changes. Also important are studies showing that diesel exhaust, a prototypical fossil fuel, is able to enhance in vitro and in vivo IgE production. CONCLUSION: Increased levels of the compounds resulting from fossil fuel combustion may be partly responsible for the increased prevalence of allergic respiratory disease. If the nature of these compounds and the mechanisms by which they exacerbate allergic disease can be identified, steps can be taken to reduce the production or the impact of these allergy producing compounds.

Humans↗

PubMed2005

Isotopic constraints on the fate of petroleum residues sequestered in salt marsh sediments.

To provide a new perspective on the fate of petroleum in the marine environment, we utilized variations in the natural abundance of radiocarbon (14C) to detect and quantify petroleum residues that have persisted in Wild Harbor sediments, West Falmouth, MA, for more than 30 years. The 5730-yr half-life of 14C makes this isotope ideal for the detection of fossil-fuel-derived contaminants (14C free) within different fractions of natural organic matter (modern 14C content) in environmental matrixes. Samples of both contaminated and uncontaminated sediments were sequentially treated, first by solvent extraction, followed by saponification, and then acid hydrolysis. Radiocarbon analysis of the sediment residues and select extracts was performed to probe for the presence of fossil fuel contaminants and/or their metabolites in different pools of sedimentary organic matter. Our results indicate that the majority of fossil carbon is solvent-extractable and has not been incorporated in the insoluble organic matter in sediment. Unextracted sediments contaminated with petroleum contain significantly less 14C than extracted sediments, and isotope mass balance calculations suggest that up to approximately 9% of the total organic carbon (TOC) in the petroleum contaminated sediment horizons is derived from solvent-extractable petroleum. These estimates are similar to values calculated when the total quantities of oil (measured by gas chromatography with flame ionization detector (GC-FID)) are compared to TOC content (determined by elemental analysis). These results pave the way for applications of this isotopic approach to more complex environmental systems where the fate of contaminants is less certain.

Accidents↗

PubMed1992

Evolution and environment in the Hominoidea.

Between 10 and 20 million years ago, a variety of hominoid primates lived in Africa, Europe and Asia. The question of which of these, if any, lie closest to the ancestries of humans and modern apes remains a lively source of debate. Recent fossil discoveries, though, shed light on the environments in which the various groups of hominoid emerged and, it is hoped, on their evolution. But the lack of a hominid fossil record before about 5 million years ago--and any fossil record for the African apes--is still a frustrating barrier.

Animals↗

PubMed2002

Use of Chlorella vulgaris for CO(2) mitigation in a photobioreactor.

Carbon dioxide (CO(2)) is a colorless gas that exists at a concentration of approximately 330 ppm in the atmosphere and is released in great quantities when fossil fuels are burned. The current flux of carbon out of fossil fuels is about 600 times greater than that into fossil fuels. With increased concerns about global warming and greenhouse gas emissions, there have been several approaches proposed for managing the levels of CO(2) emitted into the atmosphere. One of the most understudied methods for CO(2) mitigation is the use of biological processes in engineered systems such as photobioreactors. This research project describes the effectiveness of Chlorella vulgaris, used in a photobioreactor with a very short gas residence time, in sequestering CO(2) from an elevated CO(2) airstream. We evaluated a flow-through photobioreactor's operational parameters, as well as the growth characteristics of the C. vulgaris inoculum when exposed to an airstream with over 1850 ppm CO(2). When using dry weight, chlorophyll, and direct microscopic measurements, it was apparent that the photobioreactor's algal inoculum responded well to the elevated CO(2) levels and there was no build-up of CO(2) or carbonic acid in the photobioreactor. The photobioreactor, with a gas residence time of approximately 2 s, was able to remove up to 74% of the CO(2) in the airstream to ambient levels. This corresponded to a 63.9-g/m(3)/h bulk removal for the experimental photobioreactor. Consequently, this photobioreactor shows that biological processes may have some promise for treating point source emissions of CO(2) and deserve further study.

Bioreactors↗

PubMed2002

The evolution and development of cranial form in Homosapiens.

Despite much data, there is no unanimity over how to define Homo sapiens in the fossil record. Here, we examine cranial variation among Pleistocene and recent human fossils by using a model of cranial growth to identify unique derived features (autapomorphies) that reliably distinguish fossils attributed to "anatomically modern" H. sapiens (AMHS) from those attributed to various taxa of "archaic" Homo spp. (AH) and to test hypotheses about the changes in cranial development that underlie the origin of modern human cranial form. In terms of pattern, AMHS crania are uniquely characterized by two general structural autapomorphies: facial retraction and neurocranial globularity. Morphometric analysis of the ontogeny of these autapomorphies indicates that the developmental changes that led to modern human cranial form derive from a combination of shifts in cranial base angle, cranial fossae length and width, and facial length. These morphological changes, some of which may have occurred because of relative size increases in the temporal and possibly the frontal lobes, occur early in ontogeny, and their effects on facial retraction and neurocranial globularity discriminate AMHS from AH crania. The existence of these autapomorphies supports the hypothesis that AMHS is a distinct species from taxa of "archaic" Homo (e.g., Homo neanderthalensis).

Animals↗

PubMed2004

Estimating metazoan divergence times with a molecular clock.

Accurately dating when the first bilaterally symmetrical animals arose is crucial to our understanding of early animal evolution. The earliest unequivocally bilaterian fossils are approximately 555 million years old. In contrast, molecular-clock analyses calibrated by using the fossil record of vertebrates estimate that vertebrates split from dipterans (Drosophila) approximately 900 million years ago (Ma). Nonetheless, comparative genomic analyses suggest that a significant rate difference exists between vertebrates and dipterans, because the percentage difference between the genomes of mosquito and fly is greater than between fish and mouse, even though the vertebrate divergence is almost twice that of the dipteran. Here we show that the dipteran rate of molecular evolution is similar to other invertebrate taxa (echinoderms and bivalve molluscs) but not to vertebrates, which significantly decreased their rate of molecular evolution with respect to invertebrates. Using a data set consisting of the concatenation of seven different amino acid sequences from 23 ingroup taxa (giving a total of 11 different invertebrate calibration points scattered throughout the bilaterian tree and across the Phanerozoic), we estimate that the last common ancestor of bilaterians arose somewhere between 573 and 656 Ma, depending on the value assigned to the parameter scaling molecular substitution rate heterogeneity. These results are in accord with the known fossil record and support the view that the Cambrian explosion reflects, in part, the diversification of bilaterian phyla.

Animals↗

PubMed2003

Genera of the human lineage.

Human fossils dated between 3.5 and nearly 7 million years old discovered during the last 8 years have been assigned to as many as four new genera of the family Hominidae: Ardipithecus, Orrorin, Kenyanthropus, and Sahelanthropus. These specimens are described as having morphological traits that justify placing them in the family Hominidae while creating a new genus for the classification of each. The discovery of these fossils pushed backward by >2 million years the date of the oldest hominids known. Only two or three hominid genera, Australopithecus, Paranthropus, and Homo, had been previously accepted, with Paranthropus considered a subgenus of Australopithecus by some authors. Two questions arise from the classification of the newly discovered fossils: (i) Should each one of these specimens be placed in the family Hominidae? (ii) Are these specimens sufficiently distinct to justify the creation of four new genera? The answers depend, in turn, on the concepts of what is a hominid and how the genus category is defined. These specimens seem to possess a sufficient number of morphological traits to be placed in the Hominidae. However, the nature of the morphological evidence and the adaptation-rooted concept of what a genus is do not justify the establishment of four new genera. We propose a classification that includes four well defined genera: Praeanthropus, Ardipithecus, Australopithecus, and Homo, plus one tentative incertae sedis genus: Sahelanthropus.

Animals↗

PubMed2000

A formicine in New Jersey cretaceous amber (Hymenoptera: formicidae) and early evolution of the ants.

A worker ant preserved with microscopic detail has been discovered in Turonian-aged New Jersey amber [ca. 92 mega-annum (Ma)]. The apex of the gaster has an acidopore and, thus, allows definitive assignment of the fossil to the large extant subfamily Formicinae, members of which use a defensive spray of formic acid. This specimen is the only Cretaceous record of the subfamily, and only two other fossil ants are known from the Cretaceous that unequivocally belong to an extant subfamily (Brownimecia and Canapone of the Ponerinae, in New Jersey and Canadian amber, respectively). In lieu of a cladogram of formicine genera, generalized morphology of this fossil suggests a basal position in the subfamily. Formicinae and Ponerinae in the mid Cretaceous indicate divergence of basal lineages of ants near the Albian (ca. 105-110 Ma) when they presumably diverged from the Sphecomyrminae. Sphecomyrmines are the plesiomorphic sister group to all other ants, or they are a paraphyletic stem group ancestral to all other ants-they apparently became extinct in the Late Cretaceous. Ant abundance in major deposits of Cretaceous and Tertiary insects indicates that they did not become common and presumably dominant in terrestrial ecosystems until the Eocene (ca. 45 Ma). It is at this time that modern genera that form very large colonies (at least 10,000 individuals) first appear. During the Cretaceous, eusocial termites, bees, and vespid wasps also first appear-they show a similar pattern of diversification and proliferation in the Tertiary. The Cretaceous ants have further implications for interpreting distributions of modern ants.

Animals↗

PubMed1974

Nonasymptotic species richness models and the insects of british trees.

Nonasymptotic models of species diversity are those that do not consider negative feedback between number of species in a biota and the net rate of species addition. These models propose species richness differences to be primarily the product of geologic age differences among biotas. Nonasymptotic explanations are traditional for various diversity difference spectra, including latitudinal diversity gradients and the greater species richness of ancient compared to young lakes. I review new evidence that renders these nonasymptotic explanations doubtful.The only uncontested evidence in favor of nonasymptotic species accumulation is Southwood's correlation between the number of insect species associated with British tree taxa and the number of Quaternary fossil records of these taxa. I show that nonasymptotic explanations of species richness variation are unacceptable for this system also. The variation in insect species richness among these taxa is well accounted for by a species-area relationship. The species richness asymptote is attained within a few hundred years, and the influence of cumulative host taxon abundance (number of fossil records) upon insect species richness is insignificant after the statistical influence of present-day host plant range is removed. I conclude Southwood's correlation to lack causality; it probably derives from a correlation between the number of fossil records of the host taxa and their present range.

Journal Article↗

PubMed1989

Bilaterians of the Precambrian-Cambrian transition and the annelid-arthropod relationship.

The Late Proterozoic fossil record contains the remains of animals that may represent a grade of organization not found among living metazoans. These forms were segmented and large enough to require a hemocoel, yet evidently were not capable of forming penetrating burrows, which are essentially absent from contemporaneous sediments containing locally common but chiefly horizontal trace fossils. As has been noted, there is no evidence that Late Proterozoic invertebrates possessed a coelom suited for peristaltic burrowing. Therefore, the annelidan body plan had probably not appeared. It is not implausible, however, that coelomic spaces in the form of ducts or organ sacs were present in Late Proterozoic segmented forms. Uniramians, some of which employ the hemocoel hydrostatically in lobopodal locomotion, may be allied to segmented hemocoelic forms not unlike sprigginids. Coelomic spaces may have been exploited in some protoarthropod lineages to enhance pedal-wave locomotion, but probably there are no eucoelomic forms in arthropodan ancestry. Annelids may represent an early divergent branch of seriated worms, perhaps rather nemertine-like at first, that developed eucoelomic compartments only in Cambrian time. The extinct grade is most likely to have arisen from flatworm-like ancestors. Of all the proposed phylogenies examined, only that of Manton closely anticipates these interpretations of the early metazoan fossil record.

Journal Article↗

PubMed1995

The challenge of paleoecological stasis: reassessing sources of evolutionary stability.

The paleontological record of the lower and middle Paleozoic Appalachian foreland basin demonstrates an unprecedented level of ecological and morphological stability on geological time scales. Some 70-80% of fossil morphospecies within assemblages persist in similar relative abundances in coordinated packages lasting as long as 7 million years despite evidence for environmental change and biotic disturbances. These intervals of stability are separated by much shorter periods of ecological and evolutionary change. This pattern appears widespread in the fossil record. Existing concepts of the evolutionary process are unable to explain this uniquely paleontological observation of faunawide coordinated stasis. A principle of evolutionary stability that arises from the ecosystem is explored here. We propose that hierarchical ecosystem theory, when extended to geological time scales, can explain long-term paleoecological stability as the result of ecosystem organization in response to high-frequency disturbance. The accompanying stability of fossil morphologies results from "ecological locking," in which selection is seen as a high-rate response of populations that is hierarchically constrained by lower-rate ecological processes. When disturbance exceeds the capacity of the system, ecological crashes remove these higher-level constraints, and evolution is free to proceed at high rates of directional selection during the organization of a new stable ecological hierarchy.

Journal Article↗

PubMed2003

Number of ancestral human species: a molecular perspective.

Despite the remarkable developments in molecular biology over the past three decades, anthropological genetics has had only limited impact on systematics in human evolution. Genetics offers the opportunity to objectively test taxonomies based on morphology and may be used to supplement conventional approaches to hominid systematics. Our analyses, examining chromosomes and 46 estimates of genetic distance, indicate there may have been only around 4 species on the direct line to modern humans and 5 species in total. This contrasts with current taxonomies recognising up to 23 species. The genetic proximity of humans and chimpanzees has been used to suggest these species are congeneric. Our analysis of genetic distances between them is consistent with this proposal. It is time that chimpanzees, living humans and all fossil humans be classified in Homo. The creation of new genera can no longer be a solution to the complexities of fossil morphologies. Published genetic distances between common chimpanzees and bonobos, along with evidence for interbreeding, suggest they should be assigned to a single species. The short distance between humans and chimpanzees also places a strict limit on the number of possible evolutionary 'side branches' that might be recognised on the human lineage. All fossil taxa were genetically very close to each other and likely to have been below congeneric genetic distances seen for many mammals. Our estimates of genetic divergence suggest that periods of around 2 million years are required to produce sufficient genetic distance to represent speciation. Therefore, Neanderthals and so-called H. erectus were genetically so close to contemporary H. sapiens they were unlikely to have been separate species. Thus, it is likely there was only one species of human (H. sapiens) for most of the last 2 million years. We estimate the divergence time of H. sapiens from 16 genetic distances to be around 1.7 Ma which is consistent with evidence for the earliest migration out of Africa. These findings call into question the mitochondrial "African Eve" hypothesis based on a far more recent origin for H. sapiens and show that humans did not go through a bottleneck in their recent evolutionary history. Given the large offset in evolutionary rates of molecules and morphology seen in human evolution, Homo species are likely to be characterised by high levels of morphological variation and low levels of genetic variability. Thus, molecular data suggest the limits for intraspecific morphological variation used by many palaeoanthropologists have been set too low. The role of phenotypic plasticity has been greatly underestimated in human evolution. We call into question the use of mtDNA for studies of human evolution. This DNA is under strong selection, which violates the assumption of selective neutrality. This issue should be addressed by geneticists, including a reassessment of its use for molecular clocks. There is a need for greater cooperation between palaeoanthropologists and anthropological geneticists to better understand human evolution and to bring palaeoanthropology into the mainstream of evolutionary biology.

Animals↗

PubMed2005

Mitogenomic perspectives on the origin and phylogeny of living amphibians.

Establishing the relationships among modern amphibians (lissamphibians) and their ancient relatives is necessary for our understanding of early tetrapod evolution. However, the phylogeny is still intractable because of the highly specialized anatomy and poor fossil record of lissamphibians. Paleobiologists are still not sure whether lissamphibians are monophyletic or polyphyletic, and which ancient group (temnospondyls or lepospondyls) is most closely related to them. In an attempt to address these problems, eight mitochondrial genomes of living amphibians were determined and compared with previously published amphibian sequences. A comprehensive molecular phylogenetic analysis of nucleotide sequences yields a highly resolved tree congruent with the traditional hypotheses (Batrachia). By using a molecular clock-independent approach for inferring dating information from molecular phylogenies, we present here the first molecular timescale for lissamphibian evolution, which suggests that lissamphibians first emerged about 330 million years ago. By observing the fit between molecular and fossil times, we suggest that the temnospondyl-origin hypothesis for lissamphibians is more credible than other hypotheses. Moreover, under this timescale, the potential geographic origins of the main living amphibian groups are discussed: (i) advanced frogs (neobatrachians) may possess an Africa-India origin; (ii) salamanders may have originated in east Asia; (iii) the tropic forest of the Triassic Pangaea may be the place of origin for the ancient caecilians. An accurate phylogeny with divergence times can be also helpful to direct the search for "missing" fossils, and can benefit comparative studies of amphibian evolution.

Amphibians↗

PubMed2000

Timing transantarctic disjunctions in the Atherospermataceae (Laurales): evidence from coding and noncoding chloroplast sequences.

Previous studies of the small Southern Hemisphere family Atherospermataceae have drawn contradictory conclusions regarding the number of transantarctic disjunctions and role of transoceanic dispersal in its evolution. Clarification of intergeneric relationships is critical to resolving (1) whether the two Chilean species, Laurelia sempervirens and Laureliopsis philippiana, are related to different Austral-Pacific species, implying two transantarctic disjunctions as suggested by morphology; (2) where the group is likely to have originated; and (3) whether observed disjunctions reflect the breakup of Gondwana. We analyzed chloroplast DNA sequences from six regions (the rbcL gene, the rpl16 intron, and the trnL-trnF, trnT-trnL, psbA-trnH, and atpB-rbcL spacer regions; for all six regions, 4,372 bp) for all genera and most species of Atherospermataceae, using parsimony and maximum likelihood (ML). The family's sister group, the Chilean endemic Gomortega nitida (Gomortegaceae), was used to root the tree. Parsimony and ML yielded identical single best trees that contain three well-supported clades (> or = 75% bootstrap): Daphnandra and Doryphora from south-eastern Australia; Atherosperma and Nemuaron from Australia-Tasmania and New Caledonia, respectively; and Laurelia novac-zelandiac and Laureliopsis philippiana from New Zealand and Chile, respectively. The second Chilean species, Laurelia sempervirens, is sister to this last clade. Likelihood ratio testing did not reject the molecular clock assumption for the rbcL data, which can therefore be used for divergence time estimates. The atherosperm fossil record, which goes back to the Upper Cretaceous, includes pollen, wood, and leaf fossils from Europe, Africa, South America, Antarctica, New Zealand, and Tasmania. Calibration of rbcL substitution rates with the fossils suggests an initial diversification of the family at 100-140 million years ago (MYA), probably in West Gondwana, early entry into Antarctica, and long-distance dispersal to New Zealand and New Caledonia at 50-30 MYA by the ancestors of L. novae-zelandiae and Nemuaron.

Antarctic Regions↗

PubMed2000

Leaves of Salicaceae (Salix and Populus) from Oligocene Sediments Near Tepexi de Rodríguez, Puebla, Mexico.

Eleven new salicaceous taxa based on fossil leaves, seven Salix and four Populus species, are described from the Oligocene Los Ahuehuetes locality, Pie de Vaca Formation, near Tepexi de Rodríguez in Puebla, Mexico. They are characterized by the presence of typical salicoid teeth, camptodromous secondary venation, and elliptic or lanceolate shape. Morphological differences between these new fossil plants and those reported previously from the United States and elsewhere indicate that diversification among these taxa occurred early during their differentiation. Furthermore, the limited morphological similarities of the fossil and extant plants growing naturally in Mexico also indicate a high rate of evolution. Documenting the expansion of high-latitude North American taxa into lower latitudes at the time of deposition of the Pie de Vaca Formation confirms that at least some of the typical boreal taxa were part of dry tropical communities in low-latitude North America and underscores the need to reevaluate the origin of this southern vegetation.

Journal Article↗

PubMed2002

The evolutionary origin and diversification of feathers.

Progress on the evolutionary origin and diversification of feathers has been hampered by conceptual problems and by the lack of plesiomorphic feather fossils. Recently, both of these limitations have been overcome by the proposal of the developmental theory of the origin of feathers, and the discovery of primitive feather fossils on nonavian theropod dinosaurs. The conceptual problems of previous theories of the origin of feathers are reviewed, and the alternative developmental theory is presented and discussed. The developmental theory proposes that feathers evolved through a series of evolutionary novelties in developmental mechanisms of the follicle and feather germ. The discovery of primitive and derived fossil feathers on a diversity of coelurosaurian theropod dinosaurs documents that feathers evolved and diversified in nonavian theropods before the origin of birds and before the origin of flight. The morphologies of these primitive feathers are congruent with the predictions of the developmental theory. Alternatives to the theropod origin of feathers are critique and rejected. Hypotheses for the initial function of feathers are reviewed. The aerodynamic theory of feather origins is falsified, but many other functions remain developmentally and phylogenetically plausible. Whatever their function, feathers evolved by selection for a follicle that would grow an emergent tubular appendage. Feathers are inherently tubular structures. The homology of feathers and scales is weakly supported. Feathers are composed of a suite of evolutionary novelties that evolved by the duplication, hierarchical organization, interaction, dissociation, and differentiation of morphological modules. The unique capacity for modular subdivision of the tubular feather follicle and germ has fostered the evolution of numerous innovations that characterize feathers. The evolution of feather keratin and the molecular basis of feather development are also discussed.

Adaptation, Physiological↗