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M J Benton

Publications and source records attributed to M J Benton.

At least 19 recordsLinked to original sources
PubMed2004

Ecosystem remodelling among vertebrates at the Permian-Triassic boundary in Russia.

The mass extinction at the Permian-Triassic boundary, 251 million years (Myr) ago, is accepted as the most profound loss of life on record. Global data compilations indicate a loss of 50% of families or more, both in the sea and on land, and these figures scale to a loss of 80-96% of species, based on rarefaction analyses. This level of loss is confirmed by local and regional-scale studies of marine sections, but the terrestrial record has been harder to analyse in such close detail. Here we document the nature of the event in Russia in a comprehensive survey of 675 specimens of amphibians and reptiles from 289 localities spanning 13 successive geological time zones in the South Urals basin. These changes in diversity and turnover cannot be explained simply by sampling effects. There was a profound loss of genera and families, and simplification of ecosystems, with the loss of small fish-eaters and insect-eaters, medium and large herbivores and large carnivores. Faunal dynamics also changed, from high rates of turnover through the Late Permian period to greater stability at low diversity through the Early Triassic period. Even after 15 Myr of ecosystem rebuilding, some guilds were apparently still absent-small fish-eaters, small insect-eaters, large herbivores and top carnivores.

Amphibians↗
PubMed2001

Finding the tree of life: matching phylogenetic trees to the fossil record through the 20th century.

Phylogenies, or evolutionary trees, are fundamental to biology. Systematists have laboured since the time of Darwin to discover the tree of life. Recent developments in systematics, such as cladistics and molecular sequencing, have led practitioners to believe that their phylogenies are more testable now than equivalent efforts from the 1960s or earlier. Whole trees, and nodes within trees, may be assessed for their robustness. However, these quantitative approaches cannot be used to demonstrate that one tree is more likely to be correct than another. Congruence assessments may help. Comparison of a sample of 1000 published trees with an essentially independent standard (dates of origin of groups in geological time) shows that the order of branching has improved slightly, but the disparity between estimated times of origination from phylogeny and stratigraphy has, if anything, become worse. Controlled comparisons of phylogenies of four major groups (Agnatha, Sarcopterygii, Sauria and Mammalia) do not show uniform improvement, or decline, of fit to stratigraphy through the twentieth century. Nor do morphological or molecular trees differ uniformly in their performance.

Animals↗
PubMed2000

Quality of the fossil record through time.

Does the fossil record present a true picture of the history of life, or should it be viewed with caution? Raup argued that plots of the diversification of life were an illustration of bias: the older the rocks, the less we know. The debate was partially resolved by the observation that different data sets gave similar patterns of rising diversity through time. Here we show that new assessment methods, in which the order of fossils in the rocks (stratigraphy) is compared with the order inherent in evolutionary trees (phylogeny), provide a more convincing analytical tool: stratigraphy and phylogeny offer independent data on history. Assessments of congruence between stratigraphy and phylogeny for a sample of 1,000 published phylogenies show no evidence of diminution of quality backwards in time. Ancient rocks clearly preserve less information, on average, than more recent rocks. However, if scaled to the stratigraphic level of the stage and the taxonomic level of the family, the past 540 million years of the fossil record provide uniformly good documentation of the life of the past.

Animals↗
PubMed2000

Stems, nodes, crown clades, and rank-free lists: is Linnaeus dead?

Recent radical proposals to overhaul the methods of biological classification are reviewed. The proposals of phylogenetic nomenclature are to translate cladistic phylogenies directly into classifications, and to define taxon names in terms of clades. The method has a number of radical consequences for biologists: taxon names must depend rigidly on the particular cladogram favoured at the moment, familiar names may be reassigned to unfamiliar groupings, Linnaean category terms (e.g. phylum, order, family) are abandoned, and the Linnaean binomen (e.g. Homo sapiens) is abandoned. The tenets of phylogenetic nomenclature have gained strong support among some vocal theoreticians, and rigid principles for legislative control of clade names and definitions have been outlined in the PhyloCode. The consequences of this semantic maelstrom have not been worked out. In pratice, phylogenetic nomenclature will bc disastrous, promoting confusion and instability, and it should be abandoned. It is based on a fundamental misunderstanding of the difference between a phylogeny (which is real) and a classification (which is utilitarian). Under the new view, classifications are identical to phlylogenies, and so the proponents of phylogenetic nomenclature will end up abandoning classifications altogether.

Animals↗
PubMed1999

Evolutionary patterns from mass originations and mass extinctions.

The Fossil Record 2 database gives a stratigraphic range of most known animal and plant families. We have used it to plot the number of families extant through time and argue for an exponential fit, rather than a logistic one, on the basis of power spectra of the residuals from the exponential. The times of origins and extinctions, when plotted for all families of marine and terrestrial organisms over the last 600 Myr, reveal different origination and extinction peaks. This suggests that patterns of biological evolution are driven by its own internal dynamics as well as responding to upsets from external causes. Spectral analysis shows that the residuals from the exponential model of the marine system are more consistent with 1/f noise suggesting that self-organized criticality phenomena may be involved.

Animals↗
PubMed1999

Early origins of modern birds and mammals: molecules vs. morphology.

Recent claims from molecular evidence that modern orders of birds and mammals arose in the Early Cretaceous, over 100 million years (Myr) ago, are contrary to palaeontological evidence. The oldest fossils generally fall in the time range from 70-50 Myr ago, with no earlier finds. If the molecular results are correct, then the first half of the fossil record of modern birds and mammals is missing. Suggestions that this early history was played out in unexplored parts of the world, or that the early progenitors were obscure forms, are unlikely. Intense collecting over hundreds of years has failed to identify these missing fossils. Control experiments, in the form of numerous Cretaceous-age fossil localities which yield excellently preserved lizards, salamanders, birds, and mammals, fail to show the modern forms. The most likely explanation is that they simply did not exist, and that the molecular clock runs fast during major radiations.

Animals↗
PubMed1998

Molecular and morphological phylogenies of mammals: congruence with stratigraphic data.

Tests of a sample of 206 cladograms of mammals show that morphological data seem to predict phylogenies that match the known fossil record better than molecular trees. Three metrics that assess the rank order of branching points, the stratigraphic consistency of those nodes, and the ratio of ghost range to known range show a considerable diversity of values. Some published trees show excellent matching with fossil-record data; others show almost no correspondence whatsoever. Morphological trees are nearly twice as good as molecular trees in terms of matching of the rank orders of nodes and oldest fossils, while morphological trees are 10% better than molecular in terms of stratigraphic consistency of the nodes. The ratios of ghost range to known range are lower for molecular trees. Among the molecular trees, those based on gene data are considerably better than those based on protein sequences, at least in terms of the rank order of nodes and the stratigraphic consistency of nodes. Protein trees, however, were best of all in terms of minimizing the proportion of ghost range. These findings probably indicate real phenomena, but the match of molecular trees to the expectations of stratigraphy may improve as the study of molecular phylogeny matures.

Animals↗
PubMed1995

Diversification and extinction in the history of life.

Analysis of the fossil record of microbes, algae, fungi, protists, plants, and animals shows that the diversity of both marine and continental life increased exponentially since the end of the Precambrian. This diversification was interrupted by mass extinctions, the largest of which occurred in the Early Cambrian, Late Ordovician, Late Devonian, Late Permian, Early Triassic, Late Triassic, and end-Cretaceous. Most of these extinctions were experienced by both marine and continental organisms. As for the periodicity of mass extinctions, no support was found: Seven mass extinction peaks in the last 250 million years are spaced 20 to 60 million years apart.

Animals↗
PubMed1994

Evaluation of growth and energy storage as biological markers of DDT exposure in sailfin mollies.

Direct and indirect measures of growth and energy storage were evaluated as indicators of subchronic 1,1,1-trichloro-2-(o-chlorophenyl)-2-(p-chlorophenyl)ethane (o,p'-DDT) exposure in juvenile sailfin mollies (Poecilia latipinna). Three-day-old mollies were exposed to 0, 1, 10, 25, 50, 75, and 100 micrograms/liter o,p'-DDT for 21 days. Tissue residues, percentage weight gain, RNA and DNA content, RNA:DNA ratio, percentage total lipid, percentage triglyceride, percentage total protein, and triglyceride:total lipid ratio were measured following exposure. Mortality was concentration and time dependent, with 100% mortality at 75 and 100 micrograms/liter. Among controls and remaining treatments, tissue residues (0.50 to 363 ng/mg dry wt), percentage weight gain (116 to 596%), percentage total lipid (2.84 to 4.33%), and percentage triglyceride (1.01 to 3.22%) were significantly different. Tissue residues were positively correlated with concentration, while percentage weight gain, percentage lipid, percentage triglyceride, and triglyceride:total lipid ratio were negatively correlated with concentration. Direct measures are likely to remain the method of choice for evaluating effects of toxicants on growth in laboratory exposures because of their relative simplicity and reliability. However, indirect measures of energy storage, such as triglyceride:total lipid ratio, rather than direct measures of various lipid fractions may be more reliable indicators of the energetic cost of toxicant stress.

Animals↗
PubMed1994

A genetic and morphometric comparison of Helisoma trivolvis and Gambusia holbrooki from clean and contaminated habitats.

Genetic and morphometric data from freshwater snail (Helisoma trivolvis) and mosquitofish (Gambusia holbrooki) populations from a relatively clean and a severely contaminated habitat were compared. Within the clean habitat, snail genetic patterns may have been more influenced than those of mosquitofish by site-specific selection because of the lesser likelihood of gene flow among snail subpopulations. Distinct genetic patterns within the contaminated habitat, combined with data from other published work, suggest that selection for tolerant genotypes may have occurred in both species. Body size in both species was associated with glucosephosphate isomerase allozyme genotype. In snails, apparent selection for a particular allele in the contaminated habitat may be related to its contaminant tolerance and body-size plasticity. In mosquitofish, a particular genotype associated with small body size appears to have been favored in the contaminated environment.

Alleles↗
PubMed1990

Phylogeny of the major tetrapod groups: morphological data and divergence dates.

The phylogeny of the major groups of tetrapods (amphibians, reptiles, birds, and mammals) has until recently been poorly understood. Cladistic analyses of morphological data are producing new hypotheses concerning the relationships of the major groups, with a focus on the identification of monophyletic groups. Molecular phylogenies support some of these views and dispute others. Geological dates of the major evolutionary branching points are recalculated on the basis of the cladograms and new fossil finds.

Amphibians↗
PubMed1989

Mass extinctions among tetrapods and the quality of the fossil record.

The fossil record of tetrapods is very patchy because of the problems of preservation, in terrestrial sediments in particular, and because vertebrates are rarely very abundant. However, the fossil record of tetrapods has the advantages that it is easier to establish a phylogenetic taxonomy than for many invertebrate groups, and there is the potential for more detailed ecological analyses. The relative incompleteness of a fossil record may be assessed readily, and this can be used to test whether drops in overall diversity are related to mass extinctions or to gaps in our knowledge. Absolute incompleteness cannot be assessed directly, but a historical approach may offer clues to future improvements in our knowledge. One of the key problems facing palaeobiologists is paraphyly, the fact that many higher taxa in common use do not contain all of the descendants of the common ancestor. This may be overcome by cladistic analysis and the identification of monophyletic groups. The diversity of tetrapods increased from the Devonian to the Permian, remained roughly constant during the Mesozoic, and then began to increase in the late Cretaceous, and continued to do so during the Tertiary. The rapid radiation of 'modern' tetrapod groups--frogs, salamanders, lizards, snakes, turtles, crocodilians, birds and mammals--was hardly affected by the celebrated end-Cretaceous extinction event. Major mass extinctions among tetrapods took place in the early Permian, late Permian, early Triassic, late Triassic, late Cretaceous, early Oligocene and late Miocene. Many of these events appear to coincide with the major mass extinctions among marine invertebrates, but the tetrapod record is largely equivocal with regard to the theory of periodicity of mass extinctions.

Animals↗
PubMed1987

Selective destruction of leucocytes by freezing as a potential means of modulating tissue immunogenicity: membrane integrity of lymphocytes and macrophages.

It is now known, when a tissue allograft is transplanted, that antigen recognition alone is not sufficient for lymphocyte activation in the host. "Passenger" leucocytes (antigen-presenting cells) present in the donor tissue are now recognized as a major immunogenic stimulus. Removal of these contaminating leucocytes, using a variety of procedures, has enabled the immunogenicity of allografts to be reduced, thus enhancing the survival of tissue allografts. This initial study explores the possibility of using a cryobiological approach to modulating the immunogenicity of tissues by virtue of the well-recognized differential susceptibility of different cell types to freezing injury. The investigation was prompted by demonstrations that pancreatic islets can secrete insulin in response to a graded glucose challenge after cryopreservation using relatively fast cooling rates which would be expected to be suboptimal for leucocyte survival. Batches of rat peripheral blood lymphocytes, or peritoneal exudate cells (macrophages) were cooled at 0.3, 1, 5, 20, 75, or 200 degrees C/min using three different cryopreservation protocols reported to yield viable pancreatic islets. Cell survival was evaluated in terms of the numbers of cells recovered after freezing as well as a fluorometric viability assay which assessed the membrane integrity of cells. Optimum survival of both lymphocytes and macrophages after freezing and thawing was found at cooling rates in the range of 0.3 to 5 degrees C/min. A significant number (10-40%) of these lymphoid cells survived freezing at 20 degrees C/min and only after cooling at rates greater than 75 degrees C/min was survival reduced to a negligible level.

Analysis of Variance↗
PubMed1987

Interaction of cooling rate, warming rate, and extent of permeation of cryoprotectant in determining survival of isolated rat islets of Langerhans during cryopreservation.

Cryopreservation of islets of Langerhans offers a number of important benefits for attempts to cure diabetes by transplantation. In the published literature, a variety of cooling rates, ranging from 0.25 to 75 degrees C/min, in conjunction with warming rates of 4-200 degrees C/min have been proposed to give optimal preservation of islets. In view of the general importance of rates of temperature change in determining survival and because of the possibility of modulating tissue immunogenicity by freezing and thawing, we have studied the interaction of cooling rate and warming rate for isolated rat islets that had been either fully or partially equilibrated with 2 M dimethyl sulfoxide (DMSO). Batches of islets were stored at -196 degrees C after cooling at 0.3, 3.0, 10, 30, 60, 150, or greater than 1000 degrees C/min and then warmed at either 10 or 50 degrees C/min. Survival was assessed by measuring the secretion of insulin during static incubation in alternating nonstimulatory and stimulatory media. Cooling rates extending over three orders of magnitude proved not to be a major determinant of survival when the islets were equilibrated with 2 M DMSO: greater than 50% survival was achieved at all cooling rates studied when the warming rate was at 50 degrees C/min. Peak survival (83%) was attained at a cooling rate of 0.3 degrees C/min, but only slightly lower recoveries were obtained at 60 and greater than 1000 degrees C/min. However, in islets only partially equilibrated with cryoprotectants, functional recovery was highly dependent on the cooling and warming rates, with peak survivals after slow cooling and rapid warming. Full permeation of the tissue with cryoprotectant offered maximal recovery of function.

Animals↗
PubMed1987

Selective killing of leucocytes by freezing: potential for reducing the immunogenicity of pancreatic islets.

Recent developments in transplantation immunobiology, concerning the mechanism of tissue rejection, clearly indicate that antigen recognition alone is not sufficient for lymphocyte activation. "Passenger" leucocytes (antigen presenting cells) carried in the donor tissue are now recognized as the major immunogenic stimulus, such that removal of these contaminating leucocytes, using a variety of procedures, has enabled the immunogenicity of allografts to be reduced and the survival of the graft to be significantly extended. Remarkable advances have been made in recent years in preventing rejection of islet allografts, and even xenografts, in experimental animals by using procedures which do not involve continuous immunosuppressive therapy. Cryopreservation offers not only the means by which donor tissue can be stored effectively during such procedures but also the possibility that, under appropriate conditions, the freezing and thawing process itself could modulate tissue immunogenicity by allowing the selective killing of immunocompetent leucocytes whilst preserving the function of parenchymal cells in the graft. In this preliminary study we have characterized the survival of leucocytes and islets from the same species (rat) after cryopreservation by the same technique using dimethyl sulphoxide (Me2SO) as the cryoprotectant. Optimum survival of rat lymphocytes and macrophages was found at cooling rates in the range of 0.3-5 degrees C/min, after cooling at rates greater than 75 degrees C/min, survival was reduced to a negligible level. On the other hand, recovery of islets was 73 +/- 9% at 75 degrees C/min, indicating that depletion of lymphoid cells, with satisfactory preservation of endocrine cells, should be obtainable at this cooling rate.

Animals↗