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Fossil Record (fossil + record)
Selected AbstractsEXTINCTION DURING EVOLUTIONARY RADIATIONS: RECONCILING THE FOSSIL RECORD WITH MOLECULAR PHYLOGENIESEVOLUTION, Issue 12 2009Tiago B. Quental Recent application of time-varying birth,death models to molecular phylogenies suggests that a decreasing diversification rate can only be observed if there was a decreasing speciation rate coupled with extremely low or no extinction. However, from a paleontological perspective, zero extinction rates during evolutionary radiations seem unlikely. Here, with a more comprehensive set of computer simulations, we show that substantial extinction can occur without erasing the signal of decreasing diversification rate in a molecular phylogeny. We also find, in agreement with the previous work, that a decrease in diversification rate cannot be observed in a molecular phylogeny with an increasing extinction rate alone. Further, we find that the ability to observe decreasing diversification rates in molecular phylogenies is controlled (in part) by the ratio of the initial speciation rate (Lambda) to the extinction rate (Mu) at equilibrium (the LiMe ratio), and not by their absolute values. Here we show in principle, how estimates of initial speciation rates may be calculated using both the fossil record and the shape of lineage through time plots derived from molecular phylogenies. This is important because the fossil record provides more reliable estimates of equilibrium extinction rates than initial speciation rates. [source] A New Mantispid-Like Species of Rhachiberothinae (Neuroptera: Berothidae) from Baltic Amber, with a Critical Review of the Fossil Record of the SubfamilyACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 4 2010Vladiir N. MAKARKIN Abstract: Whalfera wiszniewskii sp. nov. is described from the Late Eocene Baltic amber. The genus Whalfera is considered as the only fossil genus confidently assigned to the Rhachiberothinae. Others previously placed in this subfamily belong to Paraberothinae (except perhaps for Oisea). The Late Eocene/present Rhachiberothinae and the Cretaceous Paraberothinae are considered to be the subfamilies of Berothidae. [source] A New Species Group in the Genus Tanytarsus van der Wulp (Diptera: Chironomidae) Based on a Fossil Record from Baltic AmberACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 4 2010Wojciech GI Abstract: A new fossil chlronomid, Tanytarsus serafini, found in Baltic amber is described and illustrated based on adult males. The new species and similar extant species of the genus Tanytarsus van der Wulp are compared. Due to several distinct characters of wing, legs and hypopygium, a new species group for Tanytarsus serafini is proposed, and its diagnostic features are evaluated. [source] The first fossil Bothriocerinae from Eocene Baltic amber with notes on recent taxa (Hemiptera, Fulgoromorpha, Cixiidae)MITTEILUNGEN AUS DEM MUSEUM FUER NATURKUNDE IN BERLIN-DEUTSCHE ENTOMOLOGISCHE ZEITSCHRIFT, Issue 2 2002Jacek Szwedo Abstract The first fossil representative of Bothriocerinae (Hemiptera: Fulgoromorpha: Cixiidae) from Eocene Baltic amber is described. This small group is currently distributed only in the New World. The fossil presents the evidence of a wider distribution of this group in the past. Bothriobaltia pietrzeniukae gen. and sp. n. is described and illustrated. Fossil record of the family, paleogeography of the Eocene, distribution pattern, phylogeny and ecology of the group in view of fossil record are discussed. [source] Patterns of isozyme variation as indicators of biogeographic history in Pilgerodendron uviferum (D. Don) FlorínDIVERSITY AND DISTRIBUTIONS, Issue 2 2002A. C. Premoli Abstract. The effects of Pleistocene glaciations on the genetic characteristics of the most austral conifer in the world, Pilgerodendron uviferum, were analysed with specific reference to the hypothesis that the species persisted locally in ice-free areas in temperate South America. It was expected that genetic variation would decrease with latitude, given that ice fields were larger in southern Patagonia and thus refugia were probably located towards the northern distributional limit of the species as suggested by the fossil record. In addition, an increase in among-population genetic divergence was expected with increasing distance to putative glacial refugia. We examined the relationship between location and within-population variability indices of 20 Pilgerodendron populations derived from isozyme analyses. We analysed possible refugia hypotheses by the distribution of allele frequencies using multivariate discriminant analysis. The degree of genetic differentiation with geographical distance between all population pairs was investigated by Mantel tests. Results indicated that Pilgerodendron populations are highly monomorphic, probably reflecting past population bottlenecks and reduced gene flow. Southernmost populations tend to be the least genetically variable and were therefore probably more affected by glacial activity than northern ones. Populations located outside ice limits seem to have been isolated during the glacial period. The presence of centres of genetic diversity, together with the lack of a significant correlation between genetic and geographical distances and the absence of geographical patterns of allelic frequencies at most analysed alleles, may indicate that Pilgerodendron did not advance southward after the last glaciation from a unique northern refugium, but spread from several surviving populations in ice-free areas in Patagonia instead. [source] Taxonomic diversity gradients through geological timeDIVERSITY AND DISTRIBUTIONS, Issue 4 2001J. Alistair Crame Abstract., There is evidence from the fossil record to suggest that latitudinal gradients in taxonomic diversity may be time-invariant features, although almost certainly not on the same scale as that seen at the present day. It is now apparent that both latitudinal and longitudinal gradients increased dramatically in strength through the Cenozoic era (i.e. the last 65 my) to become more pronounced today than at any time in the geological past. Present-day taxonomic diversity gradients, in both the marine and terrestrial realms, are underpinned by the tropical radiations of a comparatively small number of species-rich clades. Quite why these particular taxa proliferated through the Cenozoic is uncertain, but it could be that at least part of the explanation involves the phenomenon of evolutionary escalation. This is, in essence, a theory of biological diversification through evolutionary feedback mechanisms between predators and prey; first one develops an adaptive advantage, and then the other. However, there may also have been some form of extrinsic control on the process of tropical diversification, and this was most likely centred on the phenomenon of global climate change. This is especially so over the last 15 my Various Late Cenozoic (Neogene) vicariant events effectively partitioned the tropics into a series of high diversity centres, or foci. It has been suggested that, in the largest of these in the marine realm (the Indo-West Pacific or IWP centre), a critical patterns of islands acted as a template for rapid speciation during glacioeustatic sea level cycles. The same process occurred in the Atlantic, Caribbean and East Pacific (ACEP) centre, though on a lesser scale. Tropical terrestrial diversity may also have been promoted by rapid range expansions and contractions in concert with glacial cycles (a modified refugium hypothesis). We are beginning to appreciate that an integrated sequence of Neogene tectonic and climatic events greatly influenced the formation of contemporary taxonomic diversity patterns. [source] The origin of higher taxa: macroevolutionary processes, and the case of the mammalsACTA ZOOLOGICA, Issue 1 2007T. S. Kemp Abstract The origin of a new higher taxon is characterized by a long-term phylogenetic trend, involving evolutionary changes in a large number of characters. At this phylogenetic level, the conflict between internal integration of the phenotype and its evolvability can be resolved by the correlated progression model, in which many disparate traits evolve by a sequence of small increments in loose correlation with one another, rather than by the modularity model. The trend leading to the new higher taxon implies the existence of a long ridge in an adaptive landscape. An evolutionary lineage tracking it requires adaptive changes in broad biological characteristics, involving many traits. Species selection is a possible additional driver of the trend. These conclusions are tested against the synapsid fossil record of the origin of mammals. The reconstructed sequence of acquisition of mammalian traits supports the correlated progression model. The adaptive ridge involved is postulated to have been a sequence of overlapping niches requiring increasing ability to remain active in daily and seasonally fluctuating environments by means of increasing internal regulation. An inferred speciation bias in favour of relatively small, relatively more progressive carnivores indicates that species selection was also involved in driving the trend. Palaeoenvironmental evidence indicates that ecological opportunity probably played a role at certain points along the lineage. [source] The Great American Biotic Interchange revisitedECOGRAPHY, Issue 2 2010Sarah Cody The "Great American Biotic Interchange" (GABI) is regarded as a defining event in the biogeography of the Americas. It is hypothesized to have occurred when the Isthmus of Panama closed ca three million years ago (Ma), ending the isolation of South America and permitting the mixing of its biota with that of North America. This view of the GABI is based largely upon the animal fossil record, but recent molecular biogeographic studies of plants that show repeated instances of long-distance dispersal over major oceanic barriers suggest that perhaps the land bridge provided by the isthmus may have been less necessary for plant migration. Here we show that plants have significantly earlier divergence time estimates than animals for historical migration events across the Isthmus of Panama region. This difference in timing indicates that plants had a greater propensity for dispersal over the isthmus before its closure compared with animals. The GABI was therefore asynchronous for plants and animals, which has fundamental implications for the historical assembly of tropical biomes in the most species-rich forests on the planet. [source] The profound influence of the Late Pliocene Panamanian uplift on the exchange, diversification, and distribution of New World birdsECOGRAPHY, Issue 2 2010Brian Tilston Smith Separated throughout most of the Cenozoic era, North and South America were joined during the mid-Pliocene when the uplift of Panama formed a land bridge between these two continents. The fossil record indicates that this connection allowed an unprecedented degree of inter-continental exchange to occur between unique, previously isolated biotic assemblages, a phenomenon now recognized as the "Great American Biotic Interchange". However, a relatively poor avian fossil record has prevented our understanding the role of the land bridge in shaping New World avian communities. To address the question of avian participation in the GABI, we compiled 64 avian phylogenetic studies and applied a relaxed molecular clock to estimate the timing of trans-isthmus diversification events. Here, we show that a significant pulse of avian interchange occurred in concert with the isthmus uplift. The avian exchange was temporally consistent with the well understood mammalian interchange, despite the presumed greater vagility of birds. Birds inhabiting a variety of habitats and elevational zones responded to the newly available corridor. Within the tropics, exchange was equal in both directions although between extratropical and tropical regions it was not. Avian lineages with Nearctic origins have repeatedly invaded the tropics and radiated throughout South America; whereas, lineages with South American tropical origins remain largely restricted to the confines of the Neotropical region. This previously unrecognized pattern of asymmetric niche conservatism may represent an important and underappreciated contributor to the latitude diversity gradient. [source] Conserving biodiversity under climate change: the rear edge mattersECOLOGY LETTERS, Issue 5 2005Arndt Hampe Abstract Modern climate change is producing poleward range shifts of numerous taxa, communities and ecosystems worldwide. The response of species to changing environments is likely to be determined largely by population responses at range margins. In contrast to the expanding edge, the low-latitude limit (rear edge) of species ranges remains understudied, and the critical importance of rear edge populations as long-term stores of species' genetic diversity and foci of speciation has been little acknowledged. We review recent findings from the fossil record, phylogeography and ecology to illustrate that rear edge populations are often disproportionately important for the survival and evolution of biota. Their ecological features, dynamics and conservation requirements differ from those of populations in other parts of the range, and some commonly recommended conservation practices might therefore be of little use or even counterproductive for rear edge populations. [source] Invasion success: does size really matter?ECOLOGY LETTERS, Issue 2 2002A. W. Miller The recent paper by Roy et al. (2001) presents a compelling relationship between range limit shifts, climatic fluctuations, and body size for marine bivalves in the fossil record. However, their extension of body size as a correlate for contemporary marine bivalve introductions is problematic and requires further scrutiny. Unlike their analysis of the fossil assemblage, the approach used for contemporary invasions does not adequately control for dispersal mechanism (vector) or source region. First, their analysis included mariculture species, intentionally introduced because of their large size, creating a vector-specific bias. Second, successful invaders from multiple source regions (Northern Hemisphere) were compared with potential invaders from a single source region (north-eastern Pacific), leaving both source and vector as uncontrolled variables. We present an analysis of body size for bivalve introductions from a single vector and source region, indicating no correlation between body size and invasion success when eliminating intentional introduction, source region and transport vector as confounding factors. [source] EXTINCTION DURING EVOLUTIONARY RADIATIONS: RECONCILING THE FOSSIL RECORD WITH MOLECULAR PHYLOGENIESEVOLUTION, Issue 12 2009Tiago B. Quental Recent application of time-varying birth,death models to molecular phylogenies suggests that a decreasing diversification rate can only be observed if there was a decreasing speciation rate coupled with extremely low or no extinction. However, from a paleontological perspective, zero extinction rates during evolutionary radiations seem unlikely. Here, with a more comprehensive set of computer simulations, we show that substantial extinction can occur without erasing the signal of decreasing diversification rate in a molecular phylogeny. We also find, in agreement with the previous work, that a decrease in diversification rate cannot be observed in a molecular phylogeny with an increasing extinction rate alone. Further, we find that the ability to observe decreasing diversification rates in molecular phylogenies is controlled (in part) by the ratio of the initial speciation rate (Lambda) to the extinction rate (Mu) at equilibrium (the LiMe ratio), and not by their absolute values. Here we show in principle, how estimates of initial speciation rates may be calculated using both the fossil record and the shape of lineage through time plots derived from molecular phylogenies. This is important because the fossil record provides more reliable estimates of equilibrium extinction rates than initial speciation rates. [source] BIOTIC INTERACTIONS AND MACROEVOLUTION: EXTENSIONS AND MISMATCHES ACROSS SCALES AND LEVELSEVOLUTION, Issue 4 2008David Jablonski Clade dynamics in the fossil record broadly fit expectations from the operation of competition, predation, and mutualism, but data from both modern and ancient systems suggest mismatches across scales and levels. Indirect effects, as when antagonistic or mutualistic interactions restrict geographic range and thereby elevate extinction risk, are probably widespread and may flow in both directions, as when species- or organismic-level factors increase extinction risk or speciation probabilities. Apparent contradictions across scales and levels have been neglected, including (1) the individualistic geographic shifts of species on centennial and millennial timescales versus evidence for fine-tuned coevolutionary relationships; (2) the extensive and dynamic networks of interactions faced by most species versus the evolution of costly enemy-specific defenses and finely attuned mutualisms; and (3) the macroevolutionary lags often seen between the origin and the diversification of a clade or an evolutionary novelty versus the rapid microevolution of advantageous phenotypes and the invasibility of most communities. Resolution of these and other cross-level tensions presumably hinges on how organismic interactions impinge on genetic population structures, geographic ranges, and the persistence of incipient species, but generalizations are not yet possible. Paleontological and neontological data are both incomplete and so the most powerful response to these problems will require novel integrative approaches. Promising research areas include more realistic approaches to modeling and empirical analysis of large-scale diversity dynamics of ostensibly competing clades; spatial and phylogenetic dissections of clades involved in escalatory dynamics (where prey respond evolutionarily to a broad and shifting array of enemies); analyses of the short- versus long-term consequences of mutualistic symbioses; and fuller use of abundant natural experiments on the evolutionary impacts of ecosystem engineers. [source] DOMESTICATION OF MAIZE, SORGHUM, AND SUGARCANE DID NOT DRIVE THE DIVERGENCE OF THEIR SMUT PATHOGENSEVOLUTION, Issue 2 2007Andrew B. Munkacsi We investigated two alternative hypotheses for the origin of crop pathogen species: that human-mediated agricultural practices drove the divergence of many crop plant pathogen species or that coevolutionary processes in natural populations of the crops' ancestors drove divergence of pathogen species. We distinguished between these two hypotheses by constructing a robust multigene phylogeny and estimating the dates of divergence among four, monophyletic species of smut fungi (Ustilago maydis, U. scitaminea, Sporisorium reilianum, S. sorghi) known to specifically infect maize, sorghum, sugarcane, and their wild ancestors. Without a fossil record for smut fungi, we calibrated the pathogen species' divergence times to their plant host divergence times. Specifically, a calibration date of 10,000 years was employed to test the hypothesis that the fungal species originated at the time of domestication of their current hosts and a calibration date of 50 million years was employed to test the hypothesis that the fungal species originated on wild ancestors of their domesticated hosts. Substitution rates at five protein coding genes were calculated and rates obtained for the 10,000 year calibration date were orders of magnitude faster than those commonly reported for eukaryotes, thus rejecting the hypothesis that these smut pathogen species diverged at the time of domestication. In contrast, substitution rates obtained for the 50 million year calibration were comparable to eukaryotic substitution rates. We used the 50 million year calibration to estimate divergence times of taxa in two datasets, one comprised solely the focal species and one comprised the focal species and additional related taxa. Both datasets indicate that all taxa diverged millions of years ago, strongly supporting the hypothesis that smut species diverged before the time of domestication and modern agriculture. Thus, smut species diverged in the ecological context of natural host plant and fungal populations. [source] METAMORPHOSIS AND NEOTENY: ALTERNATIVE PATHWAYS IN AN EXTINCT AMPHIBIAN CLADEEVOLUTION, Issue 7 2006Rainer R. Schoch Abstract The Branchiosauridae was a clade of small amphibians from the Permo-Carboniferous with an overall salamander-like appearance. The clade is distinguished by an extraordinary fossil record that comprises hundreds of well-preserved specimens, representing a wide range of ontogenetic stages. Branchiosaurids had external gills and weakly ossified skeletons, and due to this larval appearance their status as neotenic (perennibranchiate) froms has long been accepted. Despite their extensive fossil record large specimens with an adult morphology appeared to be lacking altogether, but recently two adult specimens were identified in a rich sample of Apateon gracilis collected in the 19th century from a locality near Dresden, Saxony. These specimens are unique among branchiosaurids in showing a high level of ossification, including bones that have never been reported in a branchiosaur. These highlight the successive formation of features believed to indicate terrestrial locomotion, as well as feeding on larger prey items. Moreover, these transformations occurred in a small time window (whereas the degree of size increase is used as a proxy of time) and the degree of concentration of developmental events in branchiosaurids is unique among tetrapods outside the lissamphibians. These specimens are compared with large adults of the neotenic branchiosaurid Apateon caducus from the Saar-Nahe Basin, which despite their largetr body size lack the features found in the adult. A. gracilis specimens. These specimens give new insight into patterns of metamorphosis (morphological transformation) in branchiosaurids that are believed to be correlated to a change of habitat, and clearly show that different life-history pathways comparable to those of modern salamanders were already estabilshed in this Paleozoic clade. [source] Fossilized embryos are widespread but the record is temporally and taxonomically biasedEVOLUTION AND DEVELOPMENT, Issue 2 2006Philip C. J. Donoghue SUMMARY We report new discoveries of embryos and egg capsules from the Lower Cambrian of Siberia, Middle Cambrian of Australia and Lower Ordovician of North America. Together with existing records, embryos have now been recorded from four of the seven continents. However, the new discoveries highlight secular and systematic biases in the fossil record of embryonic stages. The temporal window within which the embryos and egg capsules are found is of relatively short duration; it ends in the Early Ordovician and is roughly coincident with that of typical "Orsten"-type faunas. The reduced occurrence of such fossils has been attributed to reducing levels of phosphate in marine waters during the early Paleozoic, but may also be owing to the increasing depth of sediment mixing by infaunal metazoans. Furthermore, most records younger than the earliest Cambrian are of a single kind,large eggs and embryos of the priapulid-like scalidophoran Markuelia. We explore alternative explanations for the low taxonomic diversity of embryos recovered thus far, including sampling, size, anatomy, ecology, and environment, concluding that the preponderance of Markuelia embryos is due to its precocious development of cuticle at an embryonic stage, predisposing it to preservation through action as a substrate on which microbially mediated precipitation of authigenic calcium phosphate may occur. The fossil record of embryos may be limited to a late Neoproterozoic to early Ordovician snapshot that is subject to dramatic systematic bias. Together, these biases must be considered seriously in attempts to use the fossil record to arbitrate between hypotheses of developmental and life history evolution implicated in the origin of metazoan clades. [source] Terminal addition, the Cambrian radiation and the Phanerozoic evolution of bilaterian formEVOLUTION AND DEVELOPMENT, Issue 6 2005David K. Jacobs Summary We examine terminal addition, the process of addition of serial elements in a posterior subterminal growth zone during animal development, across modern taxa and fossil material. We argue that terminal addition was the basal condition in Bilateria, and that modification of terminal addition was an important component of the rapid Cambrian evolution of novel bilaterian morphology. We categorize the often-convergent modifications of terminal addition from the presumed ancestral condition. Our focus on terminal addition and its modification highlights trends in the history of animal evolution evident in the fossil record. These trends appear to be the product of departure from the initial terminal addition state, as is evident in evolutionary patterns within-fossil groups such as trilobites, but is also more generally related to shifts in types of morphologic change through the early Phanerozoic. Our argument is contingent on dates of metazoan divergence that are roughly convergent with the first appearance of metazoan fossils in the latest Proterozoic and Cambrian, as well as on an inference of homology of terminal addition across bilaterian Metazoa. [source] Conflicting phylogenetic signals at the base of the metazoan treeEVOLUTION AND DEVELOPMENT, Issue 4 2003Antonis Rokas Summary A phylogenetic framework is essential for under-standing the origin and evolution of metazoan development. Despite a number of recent molecular studies and a rich fossil record of sponges and cnidarians, the evolutionary relationships of the early branching metazoan groups to each other and to a putative outgroup, the choanoflagellates, remain uncertain. This situation may be the result of the limited amount of phylogenetic information found in single genes and the small number of relevant taxa surveyed. To alleviate the effect of these analytical factors in the phylogenetic recons-truction of early branching metazoan lineages, we cloned multiple protein-coding genes from two choanoflagellates and diverse sponges, cnidarians, and a ctenophore. Comparisons of sequences for ,-tubulin, ,-tubulin, elongation factor 2, HSP90, and HSP70 robustly support the hypothesis that choanoflagellates are closely affiliated with animals. However, analyses of single and concatenated amino acid sequences fail to resolve the relationships either between early branching metazoan groups or between Metazoa and choano-flagellates. We demonstrate that variable rates of evolution among lineages, sensitivity of the analyses to taxon selection, and conflicts in the phylogenetic signal contained in different amino acid sequences obscure the phylogenetic associations among the early branching Metazoa. These factors raise concerns about the ability to resolve the phylogenetic history of animals with molecular sequences. A consensus view of animal evolution may require investigations of genome-scale characters. [source] Modularity, evolvability, and adaptive radiations: a comparison of the hemi- and holometabolous insectsEVOLUTION AND DEVELOPMENT, Issue 2 2001Andrew S. Yang SUMMARY Despite recent attention given to the concept of modularity and its potential contribution to the evolvability of organisms, there has been little mention of how such a contribution may affect rates of diversification or how this would be assessed. A first key prediction is that lineages with relatively greater degrees of modularity in given traits should exhibit higher rates of diversification. Four general conditions for testing this prediction of the modular evolvability hypothesis are outlined here. The potential role of modularity as a deterministic factor in adaptive radiations is best examined by looking at historic patterns of diversification rather than just levels of extant diversity, the focus of most analyses of key innovations. Recent developmental evidence supports the notion that phenotypes of juvenile and adult stages of insects with "complete" metamorphosis (Holometabola) are distinct developmental and evolvable modules compared to the highly correlated life stages of insects with "incomplete" metamorphosis (Hemimetabola). Family-level rates of diversification for these two groups were calculated from the fossil record. The Holometabola was found to have a significantly and characteristically higher rate of diversification compared to the less modular Hemimetabola, consistent with the idea that intrinsic differences in modularity can influence the long-term evolvability of organisms. The modular evolvability hypothesis also makes a second key prediction: that characters in more modular clades will exhibit greater levels of variation due to their independence. This provides an independent, phenotypically based test of the hypothesis. We discuss here how this second prediction may be tested in the case of the Hemi- and Holometabola. [source] Human evolution at the Matuyama-Brunhes boundaryEVOLUTIONARY ANTHROPOLOGY, Issue 1 2004Article first published online: 12 FEB 200, Giorgio Manzi Abstract The cranial morphology of fossil hominids between the end of the Early Pleistocene and the beginning of the Middle Pleistocene provides crucial evidence to understand the distribution in time and space of the genus Homo. This evidence is critical for evaluating the competing models regarding diversity within our genus. The debate focuses on two alternative hypotheses, one basically anagenetic and the other cladogenetic. The first suggests that morphological change is so diffused, slow, and steady that it is meaningless to apply species names to segments of a single lineage. The second is that the morphological variation observed in the fossil record can best be described as a number of distinct species that are not connected in a linear ancestor-descendant sequence. Today much more fossil evidence is available than was in the past to test these alternative hypotheses, as well as intermediate variants. Special attention must be paid to Africa because this is the most probable continental homeland for both the origin of the genus Homo (around 2.5,2 Ma),1 as well as the site, two million or so years later, of the emergence of the species H. sapiens.2 However, the African fossil record is very poorly represented between 1 Ma and 600 ka. Europe furnishes recent discoveries in this time range around the Matuyama-Brunhes chron boundary (780,000 years ago), a period for which, at present, we have no noteworthy fossil evidence in Africa or the Levant. Two penecontemporaneous sources of European fossil evidence, the Ceprano calvaria (Italy)3 and the TD6 fossil assemblage of Atapuerca (Spain)4 are thus of great interest for testing hypotheses about human evolution in the fundamental time span bracketed between the late Early and the Middle Pleistocene. This paper is based on a phenetic approach to cranial variation aimed at reviewing the Early-to-Middle Pleistocene trajectories of human evolution. The focus of the paper is on neither the origin nor the end of the story of the genus Homo, but rather its chronological and phylogenetic core. Elucidation of the evolutionary events that happened around 780 ka during the transition from the Early to Middle Pleistocene is one of the new frontiers for human paleontology, and is critical for understanding the processes that ultimately led to the origin of H. sapiens. [source] Evolution and development of the primate limb skeletonEVOLUTIONARY ANTHROPOLOGY, Issue 3 2002Chi-Hua Chiu Abstract The order Primates is composed of many closely related lineages, each having a relatively well established phylogeny supported by both the fossil record and molecular data.1 Primate evolution is characterized by a series of adaptive radiations beginning early in the Cenozoic era. Studies of these radiations have uncovered two major trends. One is that substantial amounts of morphological diversity have been produced over short periods of evolutionary time.2 The other is that consistent and repeated patterns (variational tendencies3) are detected. Taxa within clades, such as the strepsirrhines of Madagascar and the platyrrhines of the Neotropics, have diversified in body size, substrate preference, and diet.2, 4,6 The diversification of adaptive strategies within such clades is accompanied by repeated patterns of change in cheiridial proportions7, 8 (Fig. 1) and tooth-cusp morphology.9 There are obvious adaptive, natural-selection based explanations for these patterns. The hands and feet are in direct contact with a substrate, so their form would be expected to reflect substrate preference, whereas tooth shape is related directly to the functional demands of masticating foods having different mechanical properties. What remains unclear, however, is the role of developmental and genetic processes that underlie the evolutionary diversity of the primate body plan. Are variational tendencies a signature of constraints in developmental pathways? What is the genetic basis for similar morphological transformations among closely related species? These are a sampling of the types of questions we believe can be addressed by future research integrating evidence from paleontology, comparative morphology, and developmental genetics. [source] Fossil fruits of Salsola L. s.l. and Halanthium K.Koch (Chenopodiaceae) from Lower Pleistocene lacustrine sediments in Armenia,FEDDES REPERTORIUM, Issue 3-4 2008Janna Akopian Dr. For the first time imprints of fossil fruits of Salsola L. s.l. and Halanthium K.Koch (Chenopodiaceae, Salsoloideae,) are reported from the territory of Armenia, the latter being the first fossil record of the Southwest Asian genus worldwide. They were collected from fluvio-lacustrine diatomite layers of the Lower Pleistocene (Sisian series) in the upper-middle Vorotan river valley recently dated at 1.4,0.935 my (Early Pleistocene). The fossils are described, figured and compared with recent relatives. Due to poor preservation, the identity of the Salsola fruit cannot be specified but the similar genus Kochia can be excluded. The Halanthium fruit seems to differ from those of extant species. Both are also discussed in the context of the very poor fossil Salsoloideae record and of the actual and lower Pleistocene vegetation and environment. The findings of truly semidesert species underline the presence of open landscapes and extremely semiarid to arid conditions. However, stratigraphically more refined analyses are needed to draw conclusions about the former vegetation and the duration of arid periods because most macrofossils reported from the same strata indicate semiarid and even humid environments. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) Fossile Früchte von Salsola L. und Halanthium K.Koch (Chenopodiaceae) aus unterpleistozänen lakustrischen Sedimenten in Armenien Erstmals wird über Funde fossiler Früchte der Gattungen Salsola L. s.l. und Halanthium K.Koch (Chenopodiaceae, Salsoloideae) aus Armenien berichtet. Für die letztere Gattung handelt es sich weltweit um einen Erstfund. Die nur mäßig guten Abdrucke stammen aus fluvio-lakustrischen Diatomiten der Sisian Serie vom oberen Vorotan-Tal, die nach neuesten Datierungen 1,4,0,935 Millionen Jahre alt sind und damit in das Unterpleistozän gehören. Die Fossilien werden beschrieben, abgebildet und mit den nächsten rezenten Verwandten verglichen. Während bei der Salsola -Frucht wegen Beschädigungen vor der Einbettung keine Artbestimmung möglich ist, die Zugehörigkeit zu Kochia aber ausgeschlossen werden kann, stimmt die besser erhaltene Halanthium- Frucht mit keiner rezenten Art überein. Beide Arten werden im Zusammenhang mit dem bisher äußerst spärlichen Fossilbefund und im Hinblick auf ihren möglichen ökologischen und vegetationskundlichen Indikatorwert diskutiert. Die Nachweise dieser Halbwüstenpflanzen sprechen für eine waldfreie Umgebung und ein extremsemiarides oder arides Klima zur Zeit der Einbettung. Die gleichen Schichten enthalten aber vor allem Makrofossilien mesophytischer Gehölze, die an ein semiarides bis humides Klima gebunden sind. Weitergehende Rückschlüsse sind erst dann möglich, wenn eine feinstratigraphische Analyse vorliegt, durch die größere klimatische Fluktuationen nachgewiesen werden könnten. [source] Undirected motility of filamentous cyanobacteria produces reticulate matsGEOBIOLOGY, Issue 3 2010R. N. SHEPARD The roles of biology in the morphogenesis of microbial mats and stromatolites remain enigmatic due to the vast array of physical and chemical influences on morphology. However, certain microbial behaviors produce complex morphological features that can be directly attributed to motility patterns. Specifically, laboratory experiments with a strain of the cyanobacteria Pseudanabaena demonstrate that distinctive morphologies arise from the undirected gliding and colliding of filaments. When filamentous cells collide, they align and clump, producing intersecting ridges surrounding areas with low cell density, i.e. reticulate structures. Cell motility is essential for the development of reticulates and associated structures: filaments organize into reticulates faster than cell division and growth, and conditions that inhibit motility also inhibit reticulate formation. Cell density of the inoculum affects the frequency of cell,cell collisions, and thus the time required for biofilm organization into reticulate structures. This also affects the specific geometry of the reticulates. These patterns are propagated into larger structures as cyanobacterial cell numbers increase and cells remain motile. Thus, cell motility is important for templating and maintaining the morphology of these microbial communities, demonstrating a direct link between a microbial behavior and a community morphology. Reticulate geometries have been identified in natural microbial mats as well as in the fossil record, and these structures can be attributed to the motility of filamentous bacteria. [source] Garnet-filled trails associated with carbonaceous matter mimicking microbial filaments in Archean basaltGEOBIOLOGY, Issue 4 2009K. LEPOT The study of the earliest traces of life on Earth can be complicated by abiotically formed biomorphs. We report here the finding of clustered micrometer-sized filaments of iron- and calcium-rich garnets associated with carbonaceous matter in an agate amygdale from a 2.7-billion-year-old basalt of the Maddina Formation, Western Australia. The distribution of carbonaceous matter and the mineral phases composing the filaments were analyzed using a combination of confocal laser scanning microscopy, laser-Raman micro-spectroscopy, focused ion beam sectioning and transmission electron microscopy. The results allow consideration of possible biogenic and abiotic processes that produced the filamentous structures. The filaments have a range of sizes, morphologies and distributions similar to those of certain modern iron-mineralized filamentous bacteria and some ancient filamentous structures interpreted as microfossils. They also share a high morphological similarity with tubular structures produced by microbial boring activity. However, the microstructures and the distribution of carbonaceous matter are more suggestive of an abiotic origin for the filaments. They are characteristic features of trails produced by the displacement of inclusions associated with local dissolution of their silica matrix. Organic compounds found in kerogen or bitumen inclusions may have contributed significantly to the dissolution of the quartz (or silica gel) matrix driving filamentous growth. Discriminating the products of such abiotic organic-mediated processes from filamentous microfossils or microbial borings is important to the interpretation of the scarce Precambrian fossil record and requires investigation down to the nanoscale. [source] Experimental silicification of the extremophilic Archaea Pyrococcus abyssi and Methanocaldococcus jannaschii: applications in the search for evidence of life in early Earth and extraterrestrial rocksGEOBIOLOGY, Issue 4 2009F. ORANGE Hydrothermal activity was common on the early Earth and associated micro-organisms would most likely have included thermophilic to hyperthermophilic species. 3.5,3.3 billion-year-old, hydrothermally influenced rocks contain silicified microbial mats and colonies that must have been bathed in warm to hot hydrothermal emanations. Could they represent thermophilic or hyperthermophilic micro-organisms and if so, how were they preserved? We present the results of an experiment to silicify anaerobic, hyperthermophilic micro-organisms from the Archaea Domain Pyrococcus abyssi and Methanocaldococcus jannaschii, that could have lived on the early Earth. The micro-organisms were placed in a silica-saturated medium for periods up to 1 year. Pyrococcus abyssi cells were fossilized but the M. jannaschii cells lysed naturally after the exponential growth phase, apart from a few cells and cell remains, and were not silicified although their extracellular polymeric substances were. In this first simulated fossilization of archaeal strains, our results suggest that differences between species have a strong influence on the potential for different micro-organisms to be preserved by fossilization and that those found in the fossil record represent probably only a part of the original diversity. Our results have important consequences for biosignatures in hydrothermal or hydrothermally influenced deposits on Earth, as well as on early Mars, as environmental conditions were similar on the young terrestrial planets and traces of early Martian life may have been similarly preserved as silicified microfossils. [source] Earliest fossil record of bacterial,cyanobacterial mat consortia: the early Silurian Passage Creek biota (440 Ma, Virginia, USA)GEOBIOLOGY, Issue 2 2008A. M. F. TOMESCU ABSTRACT Cyanobacteria in terrestrial and aquatic habitats are frequently associated with heterotrophic bacteria, and such associations are most often metabolically interactive. Functionally, the members of such bacterial,cyanobacterial consortia benefit from diverse metabolic capabilities of their associates, thus exceeding the sum of their parts. Such associations may have been just as ubiquitous in the past, but the fossil record has not produced any direct evidence for such associations to date. In this paper, we document fossil bacteria associated with a macrophytic cyanobacterial mat in the early Silurian (Llandovery) Massanutten Sandstone of Virginia, USA. Both the bacterial and the cyanobacterial cells are preserved by mineral replacement (pyrite subsequently replaced by iron oxyhydroxides) within an amorphous carbonaceous matrix which represents the common exopolysaccharide investment of the cyanobacterial colony. The bacteria are rod-shaped, over 370 nm long and 100 nm in diameter, and occur both as isolated cells and as short filaments. This occurrence represents the oldest fossil evidence for bacterial,cyanobacterial associations, documenting that such consortia were present 440 Ma ago, and revealing the potential for them to be recognized deeper in the fossil record. [source] An actualistic perspective into Archean worlds , (cyano-)bacterially induced sedimentary structures in the siliciclastic Nhlazatse Section, 2.9 Ga Pongola Supergroup, South AfricaGEOBIOLOGY, Issue 1 2008N. NOFFKE ABSTRACT Extensive microbial mats colonize sandy tidal flats that form along the coasts of today's Earth. The microbenthos (mainly cyanobacteria) respond to the prevailing physical sediment dynamics by biostabilization, baffling and trapping, as well as binding. This biotic,physical interaction gives rise to characteristic microbially induced sedimentary structures (MISS) that differ greatly from both purely physical structures and from stromatolites. Actualistic studies of the MISS on modern tidal flats have been shown to be the key for understanding equivalent fossil structures that occur in tidal and shelf sandstones of all Earth ages. However, until now the fossil record of Archean MISS has been poor, and relatively few specimens have been found. This paper describes a study location that displays a unique assemblage with a multitude of exceptionally preserved MISS in the 2.9-Ga-old Pongola Supergroup, South Africa. The ,Nhlazatse Section' includes structures such as ,erosional remnants and pockets', ,multidirected ripple marks', ,polygonal oscillation cracks', and ,gas domes'. Optical and geochemical analyses support the biogenicity of microscopic textures such as filamentous laminae or ,orientated grains'. Textures resembling filaments are lined by iron oxide and hydroxides, as well as clay minerals. They contain organic matter, whose isotope composition is consistent with carbon of biological origin. The ancient tidal flats of the Nhlazatse Section record four microbial mat facies that occur in modern tidal settings as well. We distinguish endobenthic and epibenthic microbial mats, including planar, tufted, and spongy subtypes. Each microbial mat facies is characterized by a distinct set of MISS, and relates to a typical tidal zone. The microbial mat structures are preserved in situ, and are consistent with similar features constructed today by benthic cyanobacteria. However, other mat-constructing microorganisms also could have formed the structures in the Archean tidal flats. [source] The fate of the homoctenids (Tentaculitoidea) during the Frasnian,Famennian mass extinction (Late Devonian)GEOBIOLOGY, Issue 3 2006DAVID BONDArticle first published online: 18 AUG 200 ABSTRACT The homoctenids (Tentaculitoidea) are small, conical-shelled marine animals that are among the most abundant and widespread of all Late Devonian fossils. They were a principal casualty of the Frasnian,Famennian (F-F, Late Devonian) mass extinction, and thus provide an insight into the extinction dynamics. Despite their abundance during the Late Devonian, they have been largely neglected by extinction studies. A number of Frasnian,Famennian boundary sections have been studied, in Poland, Germany, France, and the USA. These sections have yielded homoctenids, which allow precise recognition of the timing of the mass extinction. It is clear that the homoctenids almost disappear from the fossil record during the latest Frasnian ,Upper Kellwasser Event'. The coincident extinction of this pelagic group, and the widespread development of intense marine anoxia within the water column, provides a causal link between anoxia and the F-F extinction. Most notable is the sudden demise of a group, which had been present in rock-forming densities, during this anoxic event. One new species, belonging to Homoctenus is described, but is not formally named here. [source] The genetic response to Snowball Earth: role of HSP90 in the Cambrian explosionGEOBIOLOGY, Issue 1 2006M. E. BAKER ABSTRACT The events that shaped the Cambrian explosion from 545 to 530 Ma, when multicellular animals suddenly appeared in the fossil record, are not fully understood. It is likely that the evolution of new transcription factors and other signal transduction proteins that regulated developmental networks was important in the emergence of diverse animal phyla seen in the Cambrian. I propose that one or both extensive glaciations that ended about 670 and 635 Ma were important in the evolution of signal transduction proteins in small animals in the Neoproterozoic/Proterozoic. These glaciations have been called Snowball Earth. One consequence of these glaciations is that they increased the expression of genetic diversity in animals due to the effect of extreme climatic stress on heat-shock protein 90 (HSP90). Climatic stress diverted HSP90 from chaperoning the folding and proper intracellular localization of many signal transduction proteins that regulate development in animals. As a result, pre-existing mutant signal transduction proteins and developmental pathways were expressed in animals. Selectively advantageous mutations were fixed in stem group animals and later were a source for the expansion of animal phyla during the Cambrian. [source] How good is the fossil record of arthropods?GEOLOGICAL JOURNAL, Issue 3-4 2001An assessment using the stratigraphic congruence of cladograms Abstract A total of 179 published cladograms of arthropods was tested for their congruence with the palaeontological record. Congruence for data sets including 510 tetrapod and 157 fish trees was also calculated for comparison. Results provide support for the hypothesis that the fossil record of arthropods is less complete (at levels of stratigraphic and taxonomic resolution pertinent to most macroevolutionary studies) than the records of several other taxa. Differences in tree size, shape, balance and the distribution of first stratigraphic appearances (potential sources of bias) were controlled for by various randomization procedures. Most measures of congruence for arthropods were statistically poorer than those for other groups, even when the sample was divided into broad temporal bins. Many of the most robust and widely reproduced sister groupings of arthropods are attended by ghost ranges of many tens or hundreds of millions of years. Fossils of a number of presently very diverse and abundant arthropod groups are conspicuously absent from the record, despite many spectacular examples of the detailed preservation of others. There is probably no single reason for the apparently poor record of arthropods. Low preservation potential, small size of individuals, small numbers of individuals, and restricted palaeobiogeography almost certainly play a role in particular cases. Copyright © 2001 John Wiley & Sons, Ltd. 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