Anne Weil

Delayed biological recovery from extinctions throughout the fossil record.

How quickly does biodiversity rebound after extinctions? Palaeobiologists have examined the temporal, taxonomic and geographic patterns of recovery following individual mass extinctions in detail, but have not analysed recoveries from extinctions throughout the fossil record as a whole. Here, we measure how fast biodiversity rebounds after extinctions in general, rather than after individual mass extinctions, by calculating the cross-correlation between extinction and origination rates across the entire Phanerozoic marine fossil record. Our results show that extinction rates are not significantly correlated with contemporaneous origination rates, but instead are correlated with origination rates roughly 10 million years later. This lagged correlation persists when we remove the ‘Big Five’ major mass extinctions, indicating that recovery times following mass extinctions and background extinctions are similar. Our results suggest that there are intrinsic limits to how quickly global biodiversity can recover after extinction events, regardless of their magnitude. They also imply that todays anthropogenic extinctions will diminish biodiversity for millions of years to come.

No fractals in fossil extinction statistics

Statistical analyses of the fossil record seek to discover the mechanisms controlling biotic diversity throughout the Earths history. Solé et al. reported that many extinction time series are statistically self-similar, with 1/f power spectra, suggesting that extinctions are driven by self-organized criticality or by other scale-free internal dynamics of the biosphere. Here we show that the apparent self-similarity and 1/f scaling reported by Solé et al. are artefacts of their interpolation methods. Extinction records that are not interpolated show no evidence of fractal scaling.

Correlations in fossil extinction and origination rates through geological time

Recent analyses have suggested that extinction and origination rates exhibit long-range correlations, implying that the fossil record may be controlled by self–organized criticality or other scale–free internal dynamics of the biosphere. Here we directly test for correlations in the fossil record by calculating the autocorrelation of the extinction and origination rates through time. Our results show that extinction rates are uncorrelated beyond the average duration of a stratigraphic interval. Thus, they lack the longrange correlations predicted by the self–organized criticality hypothesis. In contrast, origination rates show strong autocorrelations due to long–term trends. After detrending, origination rates generally show weak positive correlations at lags of 5–10 million years (Myr) and weak negative correlations at lags of 10–30 Myr, consistent with aperiodic oscillations around their long–term trends. We hypothesize that origination rates are more correlated than extinction rates because originations of new taxa create new ecological niches and new evolutionary pathways for reaching them, thus creating conditions that favour further diversification.

AN EXCEPTIONAL SPECIMEN OF CIMEXOMYS JUDITHAE (MAMMALIA: MULTITUBERCULATA) FROM THE CAMPANIAN TWO MEDICINE FORMATION OF MONTANA, AND THE PHYLOGENETIC STATUS OF CIMEXOMYS

Abstract We here describe a near-complete dentition, partial rostrum, and associated lumbar vertebrae of the multituberculate mammal Cimexomys judithae, discovered on a dinosaur nest horizon at Egg Mountain, in Montanas Campanian Two Medicine Formation. This specimen confirms associations of isolated teeth made by previous authors and provides a wealth of phylogenetically useful information. A phylogenetic analysis including this specimen indicates that Cimexomys is diagnosed on the basis of primitive and widely distributed character states, is probably not monophyletic, and that C. judithae is more derived than other species of Cimexomys. Species of Cimexomys are too primitive to be included in the suborders Taeniolabidoidea or Ptilodontoidea, and are thus useful as outgroups in future analyses, but they must be scored separately to insure monophyly.

Biodiversity: Fossils make waves

A 62-million-year cycle in biodiversity emerges from scrutiny of a marine-fossil database, but its causes remain mysterious. Thus, this discovery is likely to provoke a flurry of theoretical speculation.The 62-million-year questionWith the posthumously published A Compendium of Fossil Marine Animal Genera, Jack Sepkoski initiated a new wave in palaeontology: the exploration of major patterns in the history of life as recorded by compilations of taxonomic data. This database, which records first and last stratigraphic appearances of over 36,000 marine genera, has been reanalysed in the light of the latest stratigraphic time scales, and a previously unrecognized 62-million-year cycle in the diversity of fossil genera emerges. As yet there is no explanation, but various models involving climate, environment, geological and astrophysical factors should provide testable predictions to help solve the mystery.

Evolutionary biology: Teeth as tools

What determines the shapes of mammalian teeth? When tools are designed to cut to the meat of the question, form follows function rather than developmental or evolutionary constraints.

Mammalian palaeobiology: living large in the Cretaceous.

Discoveries of large, carnivorous mammals from the Cretaceous challenge the long-held view that primitive mammals were small and uninteresting. Have palaeontologists been asking the wrong questions?Mammal bites dinosaurWe tend to think of the the mammals that lived in the shadow of the dinosaurs as scuttling around trying not to get eaten or trodden on. They were small, the size of a modern shrew or rat, feeding on insects, and active during evening hours. That picture is changing. The extinct mammal Repenomamus robustus, discovered five years ago, broke the mould: it was much bigger, the size of a Virginia opossum. And now another member of the genus Repenomamus has been found that is half as big again, the size of a badger or jackal. Whats more, a specimen of R. robustus found preserved with its last meal inside its — stomach the bones of young dinosaurs. These carnivorous mammals were beginning to come out of the shadows.

Mammalian evolution: Relationships to chew over

Did advanced mammals evolve on the southern continents and then move north? Not according to a new study, which concludes that such mammals evolved in both the south and the north.

Mammalian evolution: A beast of the southern wild

A newly discovered skull from the Cretaceous period belongs to a mammal that was big, strange and fast-moving. The fossil solves a long-standing mystery, and helps to resolve a controversy about mammalian evolution. See Article p.512 The gondwanatheres were mammals that lived the southern continents alongside the dinosaurs during the Late Cretaceous and early Paleocene. Known only from a few teeth and some jaw fragments, their appearance and evolutionary relationships remained obscure. The entire skull of a bizarre and badger-sized fossil mammal from the Cretaceous of Madagascar changes all that. Although almost certainly highly derived — as one would expect from a member of the unique endemic island fauna of Madagascar at that time — Vintana is clearly a gondwanathere. The anatomy of the herbivorous, large-eyed and agile creature shows that gondwanatheres were related to the better-known multituberculates, a long-lived and successful group of (now also extinct) rodent-like mammals.