Severe extinction and rapid recovery of mammals across the Cretaceous-Palaeogene boundary, and the effects of rarity on patterns of extinction and recovery.

The end-Cretaceous mass extinction ranks among the most severe extinctions of all time; however, patterns of extinction and recovery remain incompletely understood. In particular, it is unclear how severe the extinction was, how rapid the recovery was and how sampling biases might affect our understanding of these processes. To better understand terrestrial extinction and recovery and how sampling influences these patterns, we collected data on the occurrence and abundance of fossil mammals to examine mammalian diversity across the K-Pg boundary in North America. Our data show that the extinction was more severe and the recovery more rapid than previously thought. Extinction rates are markedly higher than previously estimated: of 59 species, four survived (93% species extinction, 86% of genera). Survival is correlated with geographic range size and abundance, with widespread, common species tending to survive. This creates a sampling artefact in which rare species are both more vulnerable to extinction and less likely to be recovered, such that the fossil record is inherently biased towards the survivors. The recovery was remarkably rapid. Within 300 000 years, local diversity recovered and regional diversity rose to twice Cretaceous levels, driven by increased endemicity; morphological disparity increased above levels observed in the Cretaceous. The speed of the recovery tends to be obscured by sampling effects; faunas show increased endemicity, such that a rapid, regional increase in diversity and disparity is not seen in geographically restricted studies. Sampling biases that operate against rare taxa appear to obscure the severity of extinction and the pace of recovery across the K-Pg boundary, and similar biases may operate during other extinction events.

The disparity of priapulid, archaeopriapulid and palaeoscolecid worms in the light of new data.

Priapulids and their extinct relatives, the archaeopriapulids and palaeoscolecids, are vermiform, carnivorous ecdysozoans with an armoured, extensible proboscis. These worms were an important component of marine communities during the Palaeozoic, but were especially abundant and diverse in the Cambrian. Today, they comprise just seven genera in four families. Priapulids were among the first groups used to test hypotheses concerning the morphological disparity of Cambrian fossils relative to the extant fauna. A previous study sampled at the generic level, concluding that Cambrian genera embodied marginally less morphological diversity than their extant counterparts. Here, we sample predominantly at the species level and include numerous fossils and some extant forms described in the last fifteen years. Empirical morphospaces for priapulids, archaeopriapulids and palaeoscolecids are relatively insensitive to changes in the taxon or character sample: their overall form has altered little, despite the markedly improved sampling. Cambrian and post-Cambrian genera occupy adjacent rather than broadly overlapping regions of these spaces, and Cambrian species still show lower morphological disparity than their post-Cambrian counterparts. Crucially, the significance of this difference has increased with improved taxon sampling over research time. In contrast with empirical morphospaces, the phylogeny of priapulids, archaeopriapulids and palaeoscolecids derived from morphological characters is extremely sensitive to details of taxon sampling and the manner in which characters are weighted. However, the extant Priapulidae and Halicryptidae invariably resolve as sister families, with this entire clade subsequently being sister group to the Maccabeidae. In our most inclusive trees, the extant Tubiluchidae are separated from these other living taxa by a number of small, intervening fossil clades.

Morphological disparity in the cambrian.

An analysis of the range of morphology among arthropods demonstrates that disparity among living arthropods is similar to that in Cambrian arthropods. The range of morphological design resulting from the Cambrian "explosion" has been overestimated, reflecting a tendency to separate as "problematic" taxa that cannot be accommodated in the classification on the basis of the living biota. Problematic taxa are largely an artifact of an inadequate taxonomy. Special evolutionary processes may not be necessary to explain the early radiation of the metazoans.

Fossil ghost ranges are most common in some of the oldest and some of the youngest strata.

Biologists routinely compare inferences about the order of evolutionary branching (phylogeny) with the order in which groups appear in the fossil record (stratigraphy). Where they conflict, ghost ranges are inferred: intervals of geological time where a fossil lineage should exist, but for which there is no direct evidence. The presence of very numerous and/or extensive ghost ranges is often believed to imply spurious phylogenies or a misleadingly patchy fossil record, or both. It has usually been assumed that the frequency of ghost ranges should increase with the age of rocks. Previous studies measuring ghost ranges for whole trees in just a small number of temporal bins have found no significant increase with antiquity. This study uses a much higher resolution approach to investigate the gappiness implied by 1,000 animal and plant cladograms over 77 series and stages of the Phanerozoic. It demonstrates that ghost ranges are indeed relatively common in some of the oldest strata. Surprisingly, however, ghost ranges are also relatively common in some of the youngest, fossil-rich rocks. This pattern results from the interplay between several complex factors and is not a simple function of the completeness of the fossil record. The Early Palaeozoic record is likely to be less organismically and stratigraphically complete, and its fossils -- many of which are invertebrates-may be more difficult to analyse cladistically. The Late Cenozoic is subject to the pull of the Recent, but this accounts only partially for the increased gappiness in the younger strata.

The shape of life: how much is written in stone?

Considering the enormous diversity of living organisms, representing mostly untapped resources for studying ecological, ontogenetic and phylogenetic patterns and processes, why should evolutionary biologists concern themselves with the remains of animals and plants that died out tens or even hundreds of millions of years ago? The reason is that important new insights into some of the most vexing evolutionary questions are being revealed at the interfaces of palaeontology, developmental biology and molecular biology. Attempts to synthesise information from these disciplines, however, often encounter their greatest hurdles in considerations of the radiation of the Metazoa. Ongoing challenges relate to the origins of body plans, the relationships of the metazoan phyla and the timing of major evolutionary radiations. Palaeontology not only has its own unique contributions to the study of evolutionary processes, but provides a lynchpin for many of the emerging techniques.

Contractile activity and oxidative metabolism in single adult myocytes during normoxia and hypoxia.

Contractile activity and redox metabolism were measured by optical techniques in single myocytes isolated from adult rats. Observations were made during periods of perfusion with normoxic and hypoxic solutions. Contractile activity was determined by quantitation of light transmitted through myocytes, and the state of redox metabolism was determined by measurement of flavoprotein fluorescence. Contractile activity was found to decrease gradually and in proportion to the duration of hypoxia, and this decline followed a similar but more abrupt decline in flavoprotein fluorescence which indicated reduction of the electron transport system. This observation suggests the decline in contractile activity was due to a decrease in the oxidative formation of ATP, and also indicates the response of isolated myocytes to hypoxia is dissimilar to the response of in vivo myocytes to conditions of ischaemia.

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.

Poly(A) tail-dependent exonuclease AtRrp41p from Arabidopsis thaliana rescues 5.8 S rRNA processing and mRNA decay defects of the yeast ski6 mutant and is found in an exosome-sized complex in plant and yeast cells.

Eukaryotic 3'-->5' exonucleolytic activities are essential for a wide variety of reactions of RNA maturation and metabolism, including processing of rRNA, small nuclear RNA, and small nucleolar RNA, and mRNA decay. Two related but distinct forms of a complex containing 10 3'-->5' exonucleases, the exosome, are found in yeast nucleus and cytoplasm, respectively, and related complexes exist in human cells. Here we report on the characterization of the AtRrp41p, an Arabidopsis thaliana homolog of the Saccharomyces cerevisiae exosome subunit Rrp41p (Ski6p). Purified recombinant AtRrp41p displays a processive phosphorolytic exonuclease activity and requires a single-stranded poly(A) tail on a substrate RNA as a "loading pad." The expression of the Arabidopsis RRP41 cDNA in yeast rescues the 5.8 S rRNA processing and 3'-->5' mRNA degradation defects of the yeast ski6-100 mutant. However, neither of these defects can explain the conditional lethal phenotype of the ski6-100 strain. Importantly, AtRrp41p shares additional function(s) with the yeast Rrp41p which are essential for cell viability because it also rescues the rrp41 (ski6) null mutant. AtRrp41p is found predominantly in a high molecular mass complex in Arabidopsis and in yeast cells, and it interacts in vitro with the yeast Rrp44p and Rrp4p exosome subunits, suggesting that it can participate in evolutionarily conserved interactions that could be essential for the integrity of the exosome complex.