Carbonate isotopes reveal a link between past ocean temperatures and mass extinction.

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Ontogenetic stages of trilobites have traditionally been recognized on the basis of the\ud development of exoskeletal segmentation. The established protaspid, meraspid, and holaspid phases\ud relate specifically to the development of articulated joints between exoskeletal elements. Transitions\ud between these phases were marked by the first and last appearances of new trunk segment\ud articulations. Here we propose an additional and complementary ontogenetic scheme based on the\ud generation of new trunk segments. It includes an anamorphic phase during which new trunk segments\ud appeared, and an epimorphic phase during which the number of segments in the trunk remained\ud constant. In some trilobites an ontogenetic boundary can also be recognized at the first\ud appearance of morphologically distinct posterior trunk segments. Comparison of the phase boundaries\ud of these different aspects of segment ontogeny highlights rich variation in the segmentation\ud process among Trilobita. Cases in which the onset of the holaspid phase preceded onset of the\ud epimorphic phase are here termed protarthrous, synchronous onset of both phases is termed synarthromeric,\ud and onset of the epimorphic phase before onset of the holaspid phase is termed protomeric.\ud Although these conditions varied among close relatives and perhaps even intraspecifically\ud in some cases, particular conditions may have been prevalent within some clades.\ud Trilobites displayed hemianamorphic development that was accomplished over an extended\ud series of juvenile and mature free-living instars. Although developmental schedules varied markedly\ud among species, morphological transitions during trilobite development were generally regular,\ud limited in scope, and extended over a large number of instars when compared with those\ud of many living arthropods. Hemianamorphic, direct development with modest change between\ud instars is also seen among basal members of the Crustacea, basal myriapods, pycnogonids, and\ud in some fossil chelicerates. This mode may represent the ancestral condition of euarthropod development

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The depositional age and stratigraphic correlations of metamorphosed and variably deformed rocks of Mount Everest are poorly known because of limited recovery of diagnostic fossils. Detailed study of Cambrian and Ordovician strata from along the length of the Himalaya has produced a coherent stratigraphy that stretches from northern India to Tibet. Our work also demonstrates that the North Col Formation rocks (= Everest series), between the Qomolangma and Lhotse detachments of the South Tibetan detachment system, still locally preserve sedimentary textures and primary stratigraphy that match those within Cambrian strata ~1100 km to the west in northern India. This demonstrates a coherency of depositional systems and stratigraphic architecture for Cambrian deposits along much of the Himalaya Tethyan margin. It also allows, for the fi rst time, identifi cation of precise depositional ages of several units in the Everest region, in particular, the Yellow Band carbonate and directly underlying siliciclastic strata, which are both shown to be late Middle Cambrian in age. Detrital zircon data presented herein for a sample from these siliciclastic strata contain a similar age spectrum to those from Middle Cambrian strata in northern India, as well as grains as young as ca. 526 Ma, both of which support the depositional age and continuity of depositional systems along the length of the Himalaya. Highly fractured rocks of the Ordovician lower Chiatsun Group in the hanging wall of the South Tibetan detachment system in Nyalam, 75 km to the west of Everest, correlate with Ordovician strata of the Mount Qomolangma Formation on Mount Everest. Our correlations indicate that the base of the summit pyramid of Everest, the foot of the "Third Step," is composed of a 60-m-thick, white-weathering thrombolite bed. The top of this ancient microbial deposit crops out only 70 m below the summit of Mount Everest.

The good fossil record of trilobite exoskeletal anatomy and ontogeny, coupled with information on their nonbiomineralized tissues, permits analysis of how the trilobite body was organized and developed, and the various evolutionary modifications of such patterning within the group. In several respects trilobite development and form appears comparable with that which may have characterized the ancestor of most or all euarthropods, giving studies of trilobite body organization special relevance in the light of recent advances in the understanding of arthropod evolution and development. The Cambrian diversification of trilobites displayed modifications in the patterning of the trunk region comparable with those seen among the closest relatives of Trilobita. In contrast, the Ordovician diversification of trilobites, although contributing greatly to the overall diversity within the clade, did so within a narrower range of trunk conditions. Trilobite evolution is consistent with an increased premium on effective enrollment and protective strategies, and with an evolutionary trade-off between the flexibility to vary the number of trunk segments and the ability to regionalize portions of the trunk.

Trilobites offer the opportunity to explore postembryonic development within the fossil record of arthropod evolution. In contrast to most trilobites, the Silurian proetid Aulacopleura konincki from the Czech Republic exhibits marked variation in the mature number of thoracic segments, with five morphs with 18-22 thoracic segments. The combination of abundant articulated specimens available from a narrow stratigraphic interval and segmental intraspecific variation makes this trilobite singularly useful for studying postembryonic growth and segmentation. Trunk segmentation followed a hemianamorphic pattern, as seen in other arthropods and as characteristic of the Trilobita; during a first anamorphic phase, segments were accreted, while in the subsequent epimorphic phase, segmentation did not proceed further despite continued growth. Size increment during the anamorphic phase was targeted and followed Dyar's rule, a geometric progression typical of many arthropods. We consider alternative hypotheses for the control of the switch from anamorphic to epimorphic phases of development. Our analysis favors a scenario in which the mature number of thoracic segments was determined quite early in development rather than at a late stage in association with a critical size threshold. This study demonstrates that hypotheses concerning developmental pattern and control can be tested in organisms belonging to an extinct clade.