M. Paul Smith

Nonbiomineralized carapaces in Cambrian seafloor landscapes (Sirius Passet, Greenland): Opening a new window into early Phanerozoic benthic ecology

Widespread microbial mats and the absence of significant bioturbation resulted in a poorly developed mixed layer and extensive cohesive substrates during the Ediacaran-Cambrian transition. Large nonbiomineralized arthropod carapaces overprinted with trails, interconnected burrow systems, narrow-caliber structures with dendritic terminations, and annulated burrows are abundant in the Early Cambrian Sirius Passet Lagerstatte (Greenland). Taphonomic controls were partially responsible for the pronounced association of these structures and carapaces, but ecologic conditions are envisioned as playing a significant role. Although some trace fossils (e.g., annulated structures) may be related to scavenging and/or deposit feeding, morphologic evidence suggests that this is not always the case. Interconnected burrow systems provide evidence for re-use of structures, suggesting grazing on or farming of bacteria. Sulfur bacteria were probably abundant in Cambrian sediments, requiring a source of sulfur or sulfide and limited quantities of oxygen. Anoxic sediment provided a source of hydrogen sulfide, whereas burrow outlets connected to the oxygenated sediment-water interface. Carcasses and molds of nonbiomineralized organisms may have acted as attractors, supporting a community of small invertebrates. Carapaces created a sharp boundary, further decreasing oxygen diffusion into the sediment. This steep oxygen gradient at the organic surface of the carapace promoted the growth of sulfur bacteria that could have served as the primary food for small macrofauna and meiofauna.

Oxygen isotope variability in conodonts: implications for reconstructing Palaeozoic palaeoclimates and palaeoceanography

Conodonts have the potential to elucidate the intricacies of Palaeozoic climates, especially if δ18O values of single apatitic tooth-like ‘elements’ can be used to map evolving sea surface temperatures and differentiate oceanic water masses. Their ecological distribution as pelagic and nektobenthic organisms, high-resolution biostratigraphy, and abundance in Cambrian–Triassic rocks qualifies them as potentially robust climate archives. Previous ion microprobe conodont δ18O studies have proceeded directly to palaeotemperature interpretation without appreciation of inter- and intra-element variability or post-mortem artefacts. Here, ion microprobe analyses of Ordovician and Silurian conodonts establishes that: intra-element crown tissue δ18O typically varies by ≤1‰ (53% of conodonts analysed), is normally ≤2‰ (92% of analyses), and rarely varies by 2–4‰; δ18O can vary across elements, suggesting a microstructural and/or diagenetic control; δ18O can vary between species representatives by c. 3‰; δ18O of pelagic and nektobenthic taxa can be offset by 2–3‰; elements processed with formic acid have highly variable δ18O; and thermal alteration does affect δ18O. Conodont ion microprobe δ18O values are comparable with those of bulk methods, but utilization of material with no consideration of geological context or processing history may introduce significant artefacts. A protocol for future conodont oxygen isotope ion microprobe studies is proposed. Supplementary material: Full results of oxygen isotope analyses reported in this paper are available at www.geolsoc.org.uk/SUP18516.

Cambro‐Ordovician stratigraphy of Bjørnøya and North Greenland: constraints on tectonic models for the Arctic Caledonides and the Tertiary opening of the Greenland Sea

The southern tip of Bjørnøya is composed of poorly known sedimentary rocks of Precambrian and Lower Palaeozoic age. Biostratigraphic and sedimentological investigations have demonstrated that the sediments of the Younger Dolomite are of Early, but not earliest, Ordovician age and that those of the conformably overlying Tetradium Limestone are of Black Riveran (Mid‐Ordovician) age. A significant hiatus lies between the base of the Younger Dolomite and the underlying Vendian glacial diamictites of the Sørhamna Formation. Comparison of this distinctive stratigraphic context with other sequences in Arctic Laurentia shows that a similar hiatus is present only in eastern North Greenland, where the Early Ordovician Wandel Valley Formation rests unconformably on a variety of Proterozoic units. Consideration of other geological characters, including the presence of west‐directed Caledonian thrusting and the location of the platform‐trough boundary in the Franklinian Basin, indicates that Bjørnøya was an integral part of the Franklinian Basin during Early Palaeozoic deposition and was attached to eastern North Greenland until early Tertiary rifting. A corollary of this interpretation of the Phanerozoic location of Bjørnøya indicates that any Caledonian strike‐slip, orogen‐parallel, displacements must have been located outboard of the combined Bjørnøya–Greenland craton. Furthermore, it provides strong constraints on interpretations of the geometry of Greenland Sea opening during early Tertiary time, and of the Vestbakken Volcanic Province and pull‐apart basin with its bounding faults, the Hornsund and Senja fracture zones.

The spatial and temporal diversification of Early Palaeozoic vertebrates

Abstract Recent discoveries have dramatically altered traditional views of the stratigraphic distribution and phylogeny of Early Palaeozoic vertebrates and permit a reappraisal of biogeographic patterns and processes over the first 120 million years of vertebrate evolution. Stratigraphic calibration of the phylogenetic trees indicates that most of the pre-Silurian record can be inferred only through ghost ranges. Assessment of the available data suggests that this is due to a shift in ecological niches after the latest Ordovician extinction event and a broadening of geographical range following the amalgamation of Euramerica during the early Silurian. Two major patterns are apparent in the biogeographic data. Firstly, the majority of jawless fishes with dermoskeletal, plated ‘armour’ were highly endemic during Cambrian-Ordovician time, with arandaspids restricted to Gondwana, galeaspids to China, and anatolepids, astraspids and, possibly, heterostracans confined to Laurentia. These Laurentian groups began to disperse to other continental blocks as the ‘Old Red Sandstone continent’ amalgamated through a series of tectonic collisions. The second major pattern, in contrast, encompasses a number of microsquamous and naked, jawed and jawless primitive vertebrates such as conodonts, thelodonts, placoderms, chondrichthyans and acanthodians, which dispersed rapidly and crossed oceanic barriers to attain cosmopolitan distributions, although many have Laurentian origins. A clear difference in dispersal potential exists between these two types of fishes. Overall, the development of biogeographic patterns in Early Palaeozoic vertebrates involved a complex interaction of dispersal, vicariance and tectonic convergence.

Paleogeography of the Great American Carbonate Bank of Laurentia in the Earliest Ordovician (Early Tremadocian): The Stonehenge Transgression

This chapter describes and presents a newly compiled map illustrating the paleogeography of Laurentia during the earliest Ordovician, a time when the great American carbonate bank was at one of its greatest extents and a period for which the most is understood. The map depicts the known or postulated extent of the inner detrital belt, the great American carbonate bank and the more problematic (commonly structurally relocated) outer detrital belt. The period on which the map is based and discussed in the accompanying text is based on the Early Ordovician (early Ibexian) (early Tremadocian) Stonehenge transgression.

The harding sandstone revisited -a new look at some old bones

Abstract Although the Harding Sandstone (basal Franklinian, Caradoc, Ordovician) of Canon City, Fremont County, Colorado, U.S.A., has been known for over one hundred years, recent fieldwork in the type area and the preparation of new material has yielded a substantial amount of new data on the vertebrate fauna and palaeoecology of the unit. The preliminary results of this work include a revision of the diversity of fish present during the Lower Caradoc, with major implications for the timing of the first diversification of the fishes.

The affinity of Anatolepis Bockelie & Fortey

Abstract Anatolepis was initially described as the dermal armour of a heterostracan agnathan, an interpretation which has subsequently engendered considerable debate. Criticism of a vertebrate affinity has centred on the ornament of the sclerites, which has been said to fall outside the known range of vertebrate morphology, and the histology of the hard tissues which has been considered as either incompatible with a vertebrate affinity or inadequately known for positive interpretation. New work on the morphology and histology of large collections of Anatolepis ranging in age from Late Cambrian to Early-Middle Ordovician has demonstrated that the taxon is unequivocally vertebrate and represents the oldest known agnathan.

Sequence Stratigraphy of the Scottish Laurentian Margin and Recognition of the Sauk Megasequence

The Cambrian–Ordovician succession of northwestern Scotland represents a fragment of a once-continuous southeastern Laurentian margin stretching between western Newfoundland and eastern Greenland. The subdivisions of the Sauk megasequence are recognized within the Scottish succession for the first time. The Sauk I supersequence corresponds to deposition of the Ardvreck Group, which is composed of a succession of arkoses, quartz arenites, and siltstones. The overlying Sauk II and III supersequences are represented by carbonates of the Durness Group. Within the Durness Group, several smaller sequences can be identified (interpreted as third order). The Sauk II supersequence can be divided into two sequences of 83- and 75-m (272- and 246-ft) thickness. These may correlate with Cambrian grand cycles recognized in other parts of Laurentia. The Sauk III supersequence contains four smaller sequences, some of which correlate with sequences observed in western Newfoundland. The top of the Sauk megasequence is not observed because of truncation of the available section by a thrust fault.

The Sirius Passet Lagerstätte of North Greenland—A geochemical window on early Cambrian low-oxygen environments and ecosystems

Abstract The early Cambrian Sirius Passet fauna of northernmost Greenland (Cambrian Series 2, Stage 3) contains exceptionally preserved soft tissues that provide an important window to early animal evolution, while the surrounding sediment holds critical data on the palaeodepositional water‐column chemistry. The present study combines palaeontological data with a multiproxy geochemical approach based on samples collected in situ at high stratigraphic resolution from Sirius Passet. After careful consideration of chemical alterations during burial, our results demonstrate that fossil preservation and biodiversity show significant correlation with iron enrichments (FeHR/FeT), trace metal behaviour (V/Al), and changes in nitrogen cycling (δ15N). These data, together with Mo/Al and the preservation of organic carbon (TOC), are consistent with a water column that was transiently low in oxygen concentration, or even intermittently anoxic. When compared with the biogeochemical characteristics of modern oxygen minimum zones (OMZs), geochemical and palaeontological data collectively suggest that oxygen concentrations as low as 0.2–0.4 ml/L restricted bioturbation but not the development of a largely nektobenthic community of predators and scavengers. We envisage for the Sirius Passet biota a depositional setting where anoxic water column conditions developed and passed over the depositional site, possibly in association with sea‐level change, and where this early Cambrian biota was established in conditions with very low oxygen.