Eocene animal trace fossils in 1.7-billion-year-old metaquartzites.

The Paleoproterozoic (1.7 Ga [billion years ago]) metasedimentary rocks of the Mount Barren Group in southwestern Australia contain burrows indistinguishable from ichnogenera Thalassinoides, Ophiomorpha, Teichichnus, and Taenidium, known from firmgrounds and softgrounds. The metamorphic fabric in the host rock is largely retained, and because the most resilient rocks in the sequence, the metaquartzites, are too hard for animal burrowing, the trace fossils have been interpreted as predating the last metamorphic event in the region. Since this event is dated at 1.2 Ga, this would bestow advanced animals an anomalously early age. We have studied the field relationships, petrographic fabric, and geochronology of the rocks and demonstrate that the burrowing took place during an Eocene transgression over a weathered regolith. At this time, the metaquartzites of the inundated surface had been weathered to friable sandstones or loose sands (arenized), allowing for animal burrowing. Subsequent to this event, there was a resilicification of the quartzites, filling the pore space with syntaxial quartz cement forming silcretes. Where the sand grains had not been dislocated during weathering, the metamorphic fabric was seemingly restored, and the rocks again assumed the appearance of hard metaquartzites impenetrable to animal burrowing.

Ancestral state reconstruction of ontogeny supports a bilaterian affinity for Dickinsonia.

Despite numerous attempts, classification of the Precambrian fossil Dickinsonia has eluded scientific consensus. This is largely because Dickinsonia and its relatives are structurally simple, lacking morphological synapomorphies to clarify their relationship to modern taxa. However, there is increasing precedence for using ontogeny to constrain enigmatic fossils, and growth of the type species Dickinsonia costata is well understood. This study formalizes the connection between ontogeny in Dickinsonia-which grows by the addition of metameric units onto one end of its primary axis-with terminal addition, defined as growth and patterning from a posterior, subtermial growth zone. We employ ancestral state reconstruction and stochastic character mapping to conclude that terminal addition is a synapomorphy of bilaterian animals. Thus, terminal addition allies Dickinsonia with the bilaterians, providing evidence that large stem- or crown-group bilaterians made up a significant proportion of the Precambrian biota. This study also illustrates the potential for combining developmental and phylogenetic data in constraining the placement of ancient problematic fossil taxa on the evolutionary tree.

Pulse of atmospheric oxygen during the late Cambrian.

A rise in atmospheric O(2) has been linked to the Cambrian explosion of life. For the plankton and animal radiation that began some 40 million yr later and continued through much of the Ordovician (Great Ordovician Biodiversification Event), the search for an environmental trigger(s) has remained elusive. Here we present a carbon and sulfur isotope mass balance model for the latest Cambrian time interval spanning the globally recognized Steptoean Positive Carbon Isotope Excursion (SPICE) that indicates a major increase in atmospheric O(2). We estimate that this organic carbon and pyrite burial event added approximately 19 × 10(18) moles of O(2) to the atmosphere (i.e., equal to change from an initial starting point for O(2) between 10-18% to a peak of 20-28% O(2)) beginning at approximately 500 million years. We further report on new paired carbon isotope results from carbonate and organic matter through the SPICE in North America, Australia, and China that reveal an approximately 2‰ increase in biological fractionation, also consistent with a major increase in atmospheric O(2). The SPICE is followed by an increase in plankton diversity that may relate to changes in macro- and micronutrient abundances in increasingly oxic marine environments, representing a critical initial step in the trophic chain. Ecologically diverse plankton groups could provide new food sources for an animal biota expanding into progressively more ventilated marine habitats during the Ordovician, ultimately establishing complex ecosystems that are a hallmark of the Great Ordovician Biodiversification Event.

The NASA Astrobiology Roadmap.

The NASA Astrobiology Roadmap provides guidance for research and technology development across the NASA enterprises that encompass the space, Earth, and biological sciences. The ongoing development of astrobiology roadmaps embodies the contributions of diverse scientists and technologists from government, universities, and private institutions. The Roadmap addresses three basic questions: how does life begin and evolve, does life exist elsewhere in the universe, and what is the future of life on Earth and beyond? Seven Science Goals outline the following key domains of investigation: understanding the nature and distribution of habitable environments in the universe, exploring for habitable environments and life in our own Solar System, understanding the emergence of life, determining how early life on Earth interacted and evolved with its changing environment, understanding the evolutionary mechanisms and environmental limits of life, determining the principles that will shape life in the future, and recognizing signatures of life on other worlds and on early Earth. For each of these goals, Science Objectives outline more specific high priority efforts for the next three to five years. These eighteen objectives are being integrated with NASA strategic planning.

Articulated Palaeozoic fossil with 17 plates greatly expands disparity of early chitons.

Modern chitons (Mollusca: Polyplacophora) possess a highly conserved skeleton of eight shell plates (valves) surrounded by spicules or scales, and fossil evidence suggests that the chiton skeleton has changed little since the first appearance of the class in the Late Cambrian period (about 500 million years before present, Myr bp). However, the Palaeozoic problematic taxon Multiplacophora, in spite of having a more complex skeleton, shares several derived characters with chitons. The enigmatic status of the Multiplacophora is due in part to the fact that its members had an exoskeleton of numerous calcium carbonate valves that usually separated after death. A new articulated specimen from the Carboniferous period (about 335 Myr bp) of Indiana reveals that multiplacophorans had a dorsal protective surface composed of head and tail valves, left and right columns of overlapping valves (five on each side), and a central zone of five smaller valves, all surrounded by an annulus of large spines. Here we describe and name the articulated specimen and present evidence that multiplacophorans were chitons. Thus the highly conserved body plan of living chitons belies the broad disparity of this clade during the Palaeozoic era.