1,721,020 research outputs found
Quantitative analysis of substrate preference in Carboniferous stem group echinoids
No full textA wealth of knowledge exists concerning echinoid substrate preference in recent and post-Paleozoic environments, however, relatively little is known of the environmental distribution and paleoecology of echinoids during the Paleozoic. The Paleozoic echinoids encompass the vast majority of the echinoid stem group, and were generally rare, but often present in Carboniferous communities across a range of paleoenvironments. We analyzed substrate preference in stem group echinoids during the Carboniferous Period, utilizing a dataset of echinoid occurrences compiled from museum collections and the literature. We focused on preference for substrate lithology (carbonate vs. siliciclastic) and grain size (fine-grained vs. coarse-grained). Using three different quantitative metrics, we analyzed substrate preferences in five families of echinoids, the Palaechinidae, the Archaeocidaridae, the Proterocidaridae, the Lepidesthidae, and the Lepidocentridae. Broadly, we found that most families showed a preference for carbonate environments, however relative affinities varied amongst families. The palaechinids showed a strong relative affinity for carbonate environments and were relatively more tolerant of coarse-grained environments than other echinoids. The Archaeocidaridae showed the widest environmental tolerance of any clade, with a high tolerance for siliciclastic and coarse-grained substrates. The Proterocidaridae and Lepidesthidae tended to have a relative affinity for fine-grained substrates. These differential tolerances and affinities for particular environments may have controlled the varying macroevolutionary histories of these clades in the Late Paleozoic
Biotic impacts of temperature before, during, and after the end-Permian extinction: A multi-metric and multi-scale approach to modeling extinction and recovery dynamics
No full textExtinction and delayed recovery during the end-Permian extinction and Early Triassic has been linked to environmental instability brought on by volcanic outgassing and greenhouse conditions, but the relative importance of the myriad of environmental stressors at this time on recovery dynamics is not well understood. Previous workers have documented both overall delayed biotic recovery for the entirety of the Early Triassic, but also incipient recoveries that appear to occur relatively early after the initial extinction event. Here, we explore the patterns of extinction and recovery using several metrics of ecological complexity in marine benthic communities using a global dataset, and compare several multiple regression models to determine which set of abiotic factors best predicts extinction and recovery dynamics. We additionally test the importance of temporal scale of analysis in interpretations of recovery dynamics and modeling results, by including analyses at the epoch, stage, and substage scales bracketing the interval of extinction and recovery. We find differences in mode of recovery between the ecological metrics analyzed, with some metrics exhibiting an Early Triassic recovery lag, while others recover continuously or immediately following the initial extinction event. We also find evidence of a global ‘Dienerian minimum’, with overall levels of community complexity significantly lower than earlier Griesbachian communities, suggesting a synchronous disturbance to the progression of recovery at this time. The regression model with δ18Oapatite mean values as the response variable is most often found to be the best-fit model across all time scales analyzed, though proxies of rock record fidelity and paleontological sampling effort become more important in finer timescale analyses, likely due to dwindling sample numbers. Out of the models tested, these results suggest that global ocean temperatures best predict patterns of extinction and recovery across several ecological metrics, and that thermal episodes during the initial extinction event and subsequently in the Early Triassic recovery period significantly suppressed benthic marine community health
A diverse assemblage of Permian echinoids (Echinodermata, Echinoidea) and implications for character evolution in early crown group echinoids
No full textAbstractThe Permian is regarded as one of the most crucial intervals during echinoid evolution because crown group echinoids are first widely known from the Permian. New faunas provide important information regarding the diversity of echinoids during this significant interval as well as the morphological characterization of the earliest crown group and latest stem group echinoids. A new fauna from the Capitanian Lamar Member of the Bell Canyon Formation in the Guadalupe Mountains of West Texas comprises at least three new taxa, includingEotiaris guadalupensisThompson n. sp. an indeterminate archaeocidarid, andPronechinus? sp. All specimens represented are silicified and known from disarticulated or semiarticulated interambulacral and ambulacral plates and spines. This assemblage is one of the most diverse echinoid assemblages known from the Permian and, as such, informs the paleoecological setting in which the earliest crown group echinoids lived. This new fauna indicates that crown group echinoids occupied the same environments as stem group echinoids of the Archaeocidaridae and Proterocidaridae. Furthermore, the echinoids described herein begin to elucidate the order of character transitions that likely took place between stem group and crown group echinoids. At least one of the morphological innovations once thought to be characteristic of early crown group echinoids, crenulate tubercles, was in fact widespread in a number of stem group taxa from the Permian as well. Crenulate tubercles are reported from two taxa, and putative cidaroid style U-shaped teeth are present in the fauna. The presence of crenulate tubercles in the archaeocidarid indicates that crenulate tubercles were present in stem group echinoids, and thus the evolution of this character likely preceded the evolution of many of the synapomorphies that define the echinoid crown group.</jats:p
Reorganization of sea urchin gene regulatory networks at least 268 million years ago as revealed by oldest fossil cidaroid echinoid
Full text linkEchinoids, or sea urchins, are rare in the Palaeozoic fossil record, and thus the details regarding the early diversification of crown group echinoids are unclear. Here we report on the earliest probable crown group echinoid from the fossil record, recovered from Permian (Roadian-Capitanian) rocks of west Texas, which has important implications for the timing of the divergence of crown group echinoids. The presence of apophyses and rigidly sutured interambulacral areas with two columns of plates indicates this species is a cidaroid echinoid. The species, Eotiaris guadalupensis, n. sp. is therefore the earliest stem group cidaroid. The occurrence of this species in Roadian strata pushes back the divergence of cidaroids and euechinoids, the clades that comprise all living echinoids, to at least 268.8 Ma, ten million years older than the previously oldest known cidaroid. Furthermore, the genomic regulation of development in echinoids is amongst the best known, and this new species informs the timing of large-scale reorganization in echinoid gene regulatory networks that occurred at the cidaroid-euechinoid divergence, indicating that these changes took place by the Roadian stage of the Permian
The Ordovician diversification of sea urchins: systematics of the Bothriocidaroida (Echinodermata: Echinoidea)
No full textThe echinoids of the order Bothriocidaroida represent the initial burst of sea urchin diversification. They were the first echinoids to achieve widespread biogeographical dispersal and achieved high levels of species richness compared to other clades of stem group echinoids. Following long-standing controversy regarding their phylogenetic affinities within the phylum Echinodermata, bothriocidaroids are now regarded as echinoids. The species- and genus-level phylogenetic relationships of the bothriocidaroids, are, however, less clearly known. We herein compile a database of bothriocidaroid occurrences, undertake detailed phylogenetic analyses of all named species within the Bothriocidaroida, and provide updated diagnoses for all species. Our phylogenetic analysis includes species belonging to three genera: Bothriocidaris, Unibothriocidaris and Neobothriocidaris. Furthermore, we describe a new species, Neobothriocidaris pentlandensis sp. nov., from the Silurian of Scotland and Sweden, using micro-computed tomography and scanning electron microscopy. This species has implications for understanding the diversity dynamics of bothriocidaroids before and after the Late Ordovician mass extinction and the palaeobiogeographical distribution of bothriocidaroids in the Silurian. Using Bayesian and parsimony-based phylogenetics, we demonstrate that Neobothriocidaris is a distinct clade, and depending upon the analytical technique, Unibothrioidaris and Bothriocidaris are also resolved as monophyletic. Unibothriocidaris appears to have become extinct in the Sandbian, while Neobothriocidaris and Bothriocidaris survived the Late Ordovician extinction events and lasted at least until the Ludlow. Furthermore, we performed statistical time-calibration of our phylogenetic trees and show that bothriocidaroids originated in the Dapingian or Darriwilian, diversifying in the Darriwilian to Sandbian and reaching peak diversity in the Sandbian and Katian. This Sandbian and Katian peak is similar to that observed in other echinoderm clades, as well as other animal groups during the Ordovician radiation, and may be linked to heightened sea level and expansive tropical shelves present in the Upper Ordovician, and to new inhabitable environments linked with Ordovician oxygenation. http://zoobank.org/urn:lsid:zoobank.org:pub:B55D0B22-D4AB-474A-80B9-D19BB43107F0.</p
A biased fossil record can preserve reliable phylogenetic signal
No full textThe fossil record is notoriously imperfect and biased in representation, hindering our ability to place fossil specimens into an evolutionary context. For groups with fossil records mostly consisting of disarticulated parts (e.g., vertebrates, echinoderms, plants), the limited morphological information preserved sparks concerns about whether fossils retain reliable evidence of phylogenetic relationships and lends uncertainty to analyses of diversification, paleobiogeography, and biostratigraphy in Earth's history. To address whether a fragmentary past can be trusted, we need to assess whether incompleteness affects the quality of phylogenetic information contained in fossil data. Herein, we characterize skeletal incompleteness bias in a large dataset (6585 specimens; 14,417 skeletal elements) of fossil squamates (lizards, snakes, amphisbaenians, and mosasaurs). We show that jaws + palatal bones, vertebrae, and ribs appear more frequently in the fossil record than other parts of the skeleton. This incomplete anatomical representation in the fossil record is biased against regions of the skeleton that contain the majority of morphological phylogenetic characters used to assess squamate evolutionary relationships. Despite this bias, parsimony- and model-based comparative analyses indicate that the most frequently occurring parts of the skeleton in the fossil record retain similar levels of phylogenetic signal as parts of the skeleton that are rarer. These results demonstrate that the biased squamate fossil record contains reliable phylogenetic information and support our ability to place incomplete fossils in the tree of life.</p
Lower Pennsylvanian (Bashkirian) echinoids from the Marble Falls Formation, San Saba, Texas, USA
No full textA new species of the genus Archaeocidaris is described from the Marble Falls Formation of San Saba County, Texas, USA. This species, Archaeocidaris marmorcataractensis new species is the fourth species of Archaeocidaris described from relatively complete test material from the Pennsylvanian of North America. The test, spines and elements of the Aristotle's lantern are preserved
Evolutionary models in the Early Triassic marine realm
No full textThe relative influences of extrinsic compared to intrinsic drivers of evolutionary change have long been theorized and debated in the fossil record. Ecological recoveries from mass extinction events present records in which to examine these contrasts. Competition in a low diversity world, reproductive strategy, reconstruction of trophic systems and ecological specialization represent possible intrinsic controls on diversification. Feedback between diversity and abundance shifts of marine organisms with biogeochemical cycling and environmental conditions act as extrinsic controls on recovery process and rate. Disentangling these evolutionary pressures is a major challenge for understanding evolutionary recovery from mass extinction.The end-Permian mass extinction (251.88 Ma) represents the largest mass extinction in Earth history and led to an extended recovery interval which lasted the duration of the Early Triassic (~ 4.8 Myr) and beyond. Recent analyses suggest that the survivors of the mass extinction were biased toward organisms with higher metabolic rates that were more resilient against the volatile environmental changes that pervaded the Early Triassic including extreme temperature events, low pH, and low oxygen conditions. We use the Early Triassic recovery of gastropods, echinoids, and ammonoids to examine the processes of taxonomic and ecological evolution in response to, or in spite of, extrinsic environmental perturbations.The case studies of benthic gastropods and echinoids, when compared to pelagic ammonoids reflect similarities and differences in recovery following the end-Permian mass extinction. Gastropods and echinoids exhibit evidence of strong extrinsic environmental limitations which implicate the availability of refugia as a control on recovery. Low initial taxonomic diversity of survivors may have also limited the evolutionary recovery of both clades. Abundant and diverse microgastropod assemblages are interpreted as an adaptation to extreme environmental conditions. The morphological diversity of disarticulated echinoid spines and plates described in the southwestern United States, and examination of phylogenetic ghost lineages hints at a significant “hidden diversity” of Early Triassic echinoids. Ammonoids experienced taxonomic resets but are shown to be resilient to repeated environmental perturbations in the Boreal Ocean over the duration of the Early Triassic. Ammonoids may have adapted to persistent latitudinal temperature gradients and oxygen minimum zones that developed in the Early Triassic ocean basins
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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