The evolutionary dynamics of the early Palaeozoic marine biodiversity accumulation

The early Palaeozoic Era records the initial biodiversification of the Phanerozoic. The increase in biodiversity involved drastic changes in taxon longevity, and in rates of origination and extinction. Here, we calculate these variables in unprecedented temporal resolution. We find that highly volatile origination and extinction rates are associated with short genus longevities during the Cambrian Period. During the Ordovician and Silurian periods, evolutionary rates were less volatile and genera persisted for increasingly longer intervals. The 90%-genus life expectancy doubled from 5 Myr in the late Cambrian to more than 10 Myr in the Ordovician-Silurian periods. Intervals with widespread ecosystem disruption are associated with short genus longevities during the Cambrian and with exceptionally high longevities during the Ordovician and Silurian periods. The post-Cambrian increase in persistence of genera, therefore, indicates an elevated ability of the changing early Palaeozoic marine ecosystems to sustainably maintain existing genera. This is evidence of a new level of ecosystem resilience which evolved during the Ordovician Period.Peer reviewe

Middle Ordovician carbonate facies development, conodont biostratigraphy and faunal diversity patterns at the Lynna River, northwestern Russia

The Ordovician Period has emerged as a highly dynamic time in Earth history. Comprehensive work on chrono, chemo-and biostratigraphy has resulted in an overall wellconstrained systemic framework, but several local successions around the globe still await detailed analysis in many respects. Herein we perform a highresolution analysis of abiotic and biotic signals in the Lynna River section, a key locality in northwestern Russia. As this section has been pivotal in documenting the temporal evolution of the Great Ordovician Biodiversification Event on Baltica, the macroscopic and microscopic characteristics of the local succession reveal important paleoenvironmental information that ties into the global development during the Middle Ordovician. The results add particularly to the understanding of the characteristics and largescale sedimentary ‘behavior’ of the Baltoscandian paleobasin. Microfacies vary consistently with the macroscopic appearance of the rocks, with intervals characterized by competent limestone being associated with coarser carbonate textures and intervals dominated by marly beds associated with finer textures. Along with carbonate textures, fossil grain assemblages vary in a rhythmic (~cyclic) manner. The local rocks are commonly partly dolomitized, with the proportion of dolomitization increasing upsection. Regional comparisons suggest that the changes in overall macro and microfacies were strongly related to variations in sea level. New highresolution conodont biostratigraphic data largely confirm previous regional correlations based on lithostratigraphy and trilobite faunas, and enable more robust correlations worldwide

The Late Ordovician extinction conundrum

The Late Ordovician mass extinction (LOME) has long been known for its association with the Hirnantian glaciations. Two extinction pulses seem to reflect global cooling and warming, respectively. The effects climate change had on Ordovician life are well documented through palaeontological evidence, several geochemical proxies and further simulated in modelling scenarios. Preceding the Hirnantian extinction interval was a phase of prolific faunal migrations in notably the later parts of the Katian. Well-documented evidence shows that low-latitude faunas dispersed to high latitudes, and taxa that had previously been endemic to particularly Baltica and South China, suddenly began to appear in Laurentia. These events, referred to as the Boda warming event and the Richmondian invasion, have been suggested to reflect a biotic response to warmer climate indicating that the onset of the subsequent Hirnantian icehouse marked a considerable environmental shift from the latest Katian warming phase. Whereas a lot of focus has been on untangling the selective effects of the two Hirnantian extinction pulses on different clades, less focus has been on what led to the transition from the Middle Ordovician biodiversity rise to the dispersal phase seen during the Katian. It appears that most clades did not â during any point of time in the Ordovician â surpass the richness levels they had achieved by the earliest Katian. Rather, a plateau was established when all metazoan clades are compiled together. This could suggest that extinction rates began to increase relative to originations. Either a threshold was reached by the early Katian prohibiting ecosystems to expand, or an extinction pulse occurred that decimated overall biodiversity accumulation. Two lines of evidence suggest the latter to be the case. Firstly, several new clades became hugely diverse by the Katian. These include bryozoans, crinoids, rugose and tabulate corals, as well as molluscs such as bivalves and gastropods, and show that obviously increased ecosystem complexity with, for instance, more epifauna and tiering occurred. Secondly, large richness datasets compiled from all metazoan clades differ from biodiversity curves based on individual clades in that they show a drop in richness already by the earliest Katian (as opposed to the classic two-pulsed Hirnantian scenario). This discrepancy has long been an overlooked conundrum that arguably has been ascribed to the larger datasets having been temporally less well-resolved. However, these new multiclade Ordovician biodiversity curves are â in some cases â resolved down to the scale of millennia, and they still depict this large fall in richness levels during the early Katian. As this pattern is observed across vastly dispersed regions, it is arguably a true signal depicting a global extinction pulse that precedes the two classic Hirnantian extinction waves by several million years. If such a three-phased âextended-LOMEâ interval is to be corroborated, it would imply a con­siderably different extinction scenario from the classic Hirnantian one. For instance, most of the Katian would then have to be viewed as being part of a protracted survival phase. In such a scenario, the global dispersal of faunas during the Boda and Richmnondian events could reflect faunas adapting to changing climatic and environmental conditions during not just the classic Hirnantian icehouseâgreenhouse scenario but as part of a prolonged phase culminating with the Hirnantian extinctions. It would further entail a new perspective on the possible mechanisms driving this long extinction phase. Several extinction determinants, both intrinsic and extrinsic may be culprits, but it is hard to overlook the apparent correlation between the start of this âextended-LOMEâ phase and some of the largest volcanic eruptions known in Earthâs history. Future better temporally resolved research into both the interplay between life and the environment during the earlier parts of the Late Ordovician hopefully will shed more light on this fascinating mass extinction event

Cascading trend of Early Paleozoic marine radiations paused by Late Ordovician extinctions

The greatest relative changes in marine biodiversity accumulation occurred during the Early Paleozoic. The precision of temporal constraints on these changes is crude, hampering our understanding of their timing, duration, and links to causal mechanisms. We match fossil occurrence data to their lithostratigraphical ranges in the Paleobiology Database and correlate this inferred taxon range to a constructed set of biostratigraphically defined high-resolution time slices. In addition, we apply capture-recapture modeling approaches to calculate a biodiversity curve that also considers taphonomy and sampling biases with four times better resolution of previous estimates. Our method reveals a stepwise biodiversity increase with distinct Cambrian and Ordovician radiation events that are clearly separated by a 50-million-year-long period of slow biodiversity accumulation. The Ordovician Radiation is confined to a 15-million-year phase after which the Late Ordovician extinctions lowered generic richness and further delayed a biodiversity rebound by at least 35 million years. Based on a first-differences approach on potential abiotic drivers controlling richness, we find an overall correlation with oxygen levels, with temperature also exhibiting a coordinated trend once equatorial sea surface temperatures fell to present-day levels during the Middle Ordovician Darriwilian Age. Contrary to the traditional view of the Late Ordovician extinctions, our study suggests a protracted crisis interval linked to intense volcanism during the middle Late Ordovician Katian Age. As richness levels did not return to prior levels during the Silurian-a time of continental amalgamation-we further argue that plate tectonics exerted an overarching control on biodiversity accumulation.Peer reviewe

Late Ordovician and early Silurian virgianid and stricklandioid brachiopods from North Greenland: implications for a warm‐water faunal province

An unusually rich and diverse suite of virgianid brachiopods, hitherto poorly known, is systematically described here for the first time from the Ordovician–Silurian boundary interval (late Katian – Aeronian) of North Greenland. The Late Ordovician virgianids comprise typical taxa of the warm‐water Tcherskidium fauna (e.g. Tcherskidium tenuicostatum, Proconchidium schleyi, Holorhynchus giganteus and Deloprosopus dawesi sp. nov.). Among the early Silurian taxa, Virgiana hursti sp. nov. occurs as abundant shell beds, similar to other congeneric species in Laurentia, but has somewhat larger internal skeletal structures, albeit not as extravagantly developed as in the late Katian virgianids; Borealoides balderi gen. et sp. nov. shows extreme thickening of the shell wall and internal structures, approaching the extravagant calcification of Katian virgianids. The highly distinctive mid‐Aeronian stricklandioid brachiopod genus, Kulumbella, characterized by a shell with criss‐cross (divaricate) ribbing, also occurs in North Greenland, represented by K. heimdali sp. nov., which has the largest and most strongly biconvex shells for the genus. Palaeogeographically, the Late Ordovician virgianid fauna of Laurentia was highly distinct, confined to the low–mid tropical latitudes north of the palaeoequator. In comparison, the early Silurian (Rhuddanian) Virgiana and some related taxa in Laurentia spanned the tropics of both hemispheres, forming extensive shell beds in carbonate basins, although Borealis and Borealoides gen. nov. remained confined largely to the northern hemisphere, suggesting a certain level of provincialism extending into the earliest Silurian. A palaeoecological preference for warm‐water carbonate settings would explain the unusual abundance and richness of the virgianid faunas in North Greenland

Initial results from the Caltech/DRSI balloon-borne isotope experiment

The Caltech/DSRI balloonborne High Energy Isotope Spectrometer Telescope (HEIST) was flown successfully from Palestine, Texas on 14 May, 1984. The experiment was designed to measure cosmic ray isotopic abundances from neon through iron, with incident particle energies from approx. 1.5 to 2.2 GeV/nucleon depending on the element. During approximately 38 hours at float altitude, 100,000 events were recorded with Z or = 6 and incident energies approx. 1.5 GeV/nucleon. We present results from the ongoing data analysis associated with both the preflight Bevalac calibration and the flight data

A Measurement of the Isotopic Composition of Cosmic Ray Iron

We present a new measurement of the isotopic composition of cosmic ray iron in the energy interval ~1550-2200 MeV /nucleon. The data were collected during the May 1984 flight of a balloon-borne spectrometer and show an average mass resolution of ~0.7 amu. The instrument employed the Cerenkov-Energy technique for mass determination. The observed ^(54)Fe/^(56)Fe ratio, 0.14 +0.18/-0.11 at the top of the atmosphere, is consistent with a solar system composition at the cosmic ray source. We also place an upper limit on the ^(58)Fe/^(56)Fe ratio at the top of the atmosphere of ≤.07. Both are consistent with previous measurements at lower energies

A measurement of cosmic ray deuterium from 0.5–2.9 GeV/nucleon

The rare isotopes ^(2)H and ^(3)He in cosmic rays are believed to originate mainly from the interaction of high energy protons and helium with the galactic interstellar medium. The unique propagation history of these rare isotopes provides important constraints on galactic cosmic ray source spectra and on models for their propagation within the Galaxy. Hydrogen and helium isotopes were measured with the balloon-borne experiment, IMAX, which flew from Lynn Lake, Manitoba in 1992. The energy spectrum of deuterium between 0.5 and 3.2 GeV/nucleon measured by the IMAX experiment as well as previously published results of ^(3)He from the same instrument will be compared with predictions of cosmic ray galactic propagation models. The observed composition of the light isotopes is found to be generally consistent with the predictions of the standard Leaky Box Model derived to fit observations of heavier nucle