John E. Repetski

A Fish from the Upper Cambrian of North America

Phosphatic dermal fragments of Anatolepis, interpreted as a heterostracan fish (class Agnatha), have been discovered in the Deadwood Formation of Late Cambrian age in northeastern Wyoming. This discovery extends back the age of the earliest known vertebrate fossils by approximately 40 million years. Other occurrences of Anatolepis in North America, Greenland, and Spitzbergen show that these fish had a widespread geographic distribution in Late Cambrian and Early Ordovician marine environments.

Fossilized embryos are widespread but the record is temporally and taxonomically biased

SUMMARY We report new discoveries of embryos and egg capsules from the Lower Cambrian of Siberia, Middle Cambrian of Australia and Lower Ordovician of North America. Together with existing records, embryos have now been recorded from four of the seven continents. However, the new discoveries highlight secular and systematic biases in the fossil record of embryonic stages. The temporal window within which the embryos and egg capsules are found is of relatively short duration; it ends in the Early Ordovician and is roughly coincident with that of typical “Orsten”‐type faunas. The reduced occurrence of such fossils has been attributed to reducing levels of phosphate in marine waters during the early Paleozoic, but may also be owing to the increasing depth of sediment mixing by infaunal metazoans. Furthermore, most records younger than the earliest Cambrian are of a single kind—large eggs and embryos of the priapulid‐like scalidophoran Markuelia. We explore alternative explanations for the low taxonomic diversity of embryos recovered thus far, including sampling, size, anatomy, ecology, and environment, concluding that the preponderance of Markuelia embryos is due to its precocious development of cuticle at an embryonic stage, predisposing it to preservation through action as a substrate on which microbially mediated precipitation of authigenic calcium phosphate may occur. The fossil record of embryos may be limited to a late Neoproterozoic to early Ordovician snapshot that is subject to dramatic systematic bias. Together, these biases must be considered seriously in attempts to use the fossil record to arbitrate between hypotheses of developmental and life history evolution implicated in the origin of metazoan clades.

Did intense volcanism trigger the first Late Ordovician icehouse

Oxygen isotopes measured on Late Ordovician conodonts from Minnesota and Kentucky (United States) were studied to reconstruct the paleotemperature history during late Sandbian to Katian (Mohawkian–Cincinnatian) time. This time interval was characterized by intense volcanism, as shown by the prominent Deicke, Millbrig, and other K-bentonite beds. A prominent carbon isotope excursion (Guttenberg δ 13 C excursion, GICE) postdates the Millbrig volcanic eruptions, and has been interpreted to reflect a drawdown of atmospheric carbon dioxide and climatic cooling. The oxygen isotope record in conodont apatite contradicts this earlier interpretation. An increase in δ 18 O of 1.5‰ (Vienna standard mean ocean water) just above the Deicke K-bentonite suggests an abrupt and short-lived cooling that possibly initiated a first short-term glacial episode well before the major Hirnantian glaciation. The decrease in δ 18 O immediately after the mega-eruptions indicates warming before the GICE, and no cooling is shown in the GICE interval. The coincidence of the Deicke mega-eruption with a cooling event suggests that this major volcanic event had a profound effect on Late Ordovician (late Mohawkian) climate.

Lithostratigraphic, conodont, and other faunal links between lower Paleozoic strata in northern and central Alaska and northeastern Russia

Lower Paleozoic platform carbonate strata in northern Alaska (parts of the Arctic Alaska, York, and Seward terranes; herein called the North Alaska carbonate platform) and central Alaska (Farewell terrane) share distinctive lithologic and faunal features, and may have formed on a single continental fragment situated between Siberia and Laurentia. Sedimentary successions in northern and central Alaska overlie Late Proterozoic metamorphosed basement; contain Late Proterozoic ooid-rich dolostones, Middle Cambrian outer shelf deposits, and Ordovician, Silurian, and Devonian shallow-water platform facies, and include fossils of both Siberian and Laurentian biotic provinces. The presence in the Alaskan terranes of Siberian forms not seen in wellstudied cratonal margin sequences of western Laurentia implies that the Alaskan rocks were not attached to Laurentia during the early Paleozoic. The Siberian cratonal succession includes Archean basement, Ordovician shallow-water siliciclastic rocks, and Upper Silurian-Devonian evaporites, none of which have counterparts in the Alaskan successions, and contains only a few of the Laurentian conodonts that occur in Alaska. Thus we conclude that the lower Paleozoic platform successions of northern and central Alaska were not part of the Siberian craton during their deposition, but may have formed on a crustal fragment rifted away from Siberia during the Late Proterozoic. The Alaskan strata have more similarities to coeval rocks in some peri-Siberian terranes of northeastern Russia (Kotelny, Chukotka, and Omulevka). Lithologic ties between northern Alaska, the Farewell terrane, and the peri-Siberian terranes diminish after the Middle Devonian, but Siberian affinities in northern and central Alaskan biotas persist into the late Paleozoic.

The origin of conodonts and of vertebrate mineralized skeletons

Conodonts are an extinct group of jawless vertebrates whose tooth-like elements are the earliest instance of a mineralized skeleton in the vertebrate lineage, inspiring the ‘inside-out’ hypothesis that teeth evolved independently of the vertebrate dermal skeleton and before the origin of jaws. However, these propositions have been based on evidence from derived euconodonts. Here we test hypotheses of a paraconodont ancestry of euconodonts using synchrotron radiation X-ray tomographic microscopy to characterize and compare the microstructure of morphologically similar euconodont and paraconodont elements. Paraconodonts exhibit a range of grades of structural differentiation, including tissues and a pattern of growth common to euconodont basal bodies. The different grades of structural differentiation exhibited by paraconodonts demonstrate the stepwise acquisition of euconodont characters, resolving debate over the relationship between these two groups. By implication, the putative homology of euconodont crown tissue and vertebrate enamel must be rejected as these tissues have evolved independently and convergently. Thus, the precise ontogenetic, structural and topological similarities between conodont elements and vertebrate odontodes appear to be a remarkable instance of convergence. The last common ancestor of conodonts and jawed vertebrates probably lacked mineralized skeletal tissues. The hypothesis that teeth evolved before jaws and the inside-out hypothesis of dental evolution must be rejected; teeth seem to have evolved through the extension of odontogenic competence from the external dermis to internal epithelium soon after the origin of jaws.

A new Late Cambrian pentastomid and a review of the relationships of this parasitic group

Pentastomida, tongue worms, are a taxon of about 130 species of parasites, living exclusively in the respiratory tracts of vertebrates. Three-dimensionally preserved Upper Cambrian larvae already demonstrate a high degree of adaptation to parasitism, striking morphological conservatism, and a high diversification by the Late Cambrian, thereby suggesting a likewise diversified host group. Not least due to their highly modified morphology, the systematic affinities of pentastomids remain controversial. The two major alternatives place the group as either close to branchiuran crustaceans or as stem-lineage derivatives of the Euarthropoda. To this set of Cambrian fossil representatives of the pentastomids we can add a new form from Lower Ordovician boundary beds from Sweden, most likely reworked from Upper Cambrian horizons. Based on this new species, named Aengapentastomum andresi gen. et sp. nov., and the available information about fossil and Recent pentastomids, we review the diverging ideas on the systematic position of this fully parasitic taxon.

First record of Early Cambrian shelly microfossils from west Africa

Two incomplete specimens of Aldanella attleborensis were recovered from calcareous dolomite of the Walidiala Group, Taoudeni Basin, northern Guinea. This is the first record of Cambrian shelly microfossils from west Africa and the first record of Aldanella from a western Gondwanan continent. These fossils, the regional stratigraphy, and radiometric dating place the dolomite, previously considered to be Late Proterozoic in age, in the Early Cambrian. Proterozoic and lower Paleozoic strata in the Taoudeni Basin are generally unfossiliferous. Hence, A. attleborensis and associated microfossils are biostratigraphic markers of geologic importance in west Africa. The Lower Cambrian dolomite near Walidiala is separated from Precambrian tillite by a previously unrecognized disconformity within the Walidiala Group that represents a hiatus of uncertain duration.

Laurentian origin for the North Slope of Alaska: Implications for the tectonic evolution of the Arctic

The composite Arctic Alaska-Chukotka terrane plays a central role in tectonic reconstructions of the Arctic. An exotic, non-Laurentian origin of Arctic Alaska-Chukotka has been proposed based on paleobiogeographic faunal affi nities and various geochronological constraints from the southwestern portions of the terrane. Here, we report early Paleozoic trilobite and conodont taxa that support a Laurentian origin for the North Slope subterrane of Arctic Alaska, as well as new Neoproterozoic-Cambrian detrital zircon geochronological data, which are both consistent with a Laurentian origin and profoundly different from those derived from similar-aged strata in the southwestern subterranes of Arctic Alaska-Chukotka. The North Slope subterrane is accordingly interpreted as allochthonous with respect to northwestern Laurentia, but it most likely originated farther east along the Canadian Arctic or Atlantic margins. These data demonstrate that construction of the com- posite Arctic Alaska-Chukotka terrane resulted from juxtaposition of the exotic southwestern fragments of the terrane against the northern margin of Laurentia during protracted Devonian(?)-Carboniferous tectonism.

New paleontologic evidence constraining the age and paleotectonic setting of the Talladega slate belt, southern Appalachians

The Talladega slate belt in the southern Appalachian orogen of Alabama and Georgia is a thick sequence of lower greenschist-facies metaclastic, metacarbonate, and metavolcanic rocks thrust above miogeoclinal rocks of the foreland and overthrust by higher grade metamorphic rocks of the eastern Blue Ridge terrane(s). The age assignments and tectonic affinities of this sequence have been highly controversial. Recent fossil discoveries in key stratigraphic units, however, combined with confirmed fossil occurrences in the Jemison Chert, have established a firm correlation with the Appalachian foreland, thus stratigraphically linking the Talladega belt with Laurentia. The lower predominantly clastic sequence, tectonically bounded at its base by the frontal Blue Ridge thrust system, grades upward into a 3.5-km-thick marble sequence. The basal carbonate unit (Jumbo Dolomite) contains Early Cambrian archaeocyathids, and these fossils, in addition to the stratigraphic position and carbonate lithofacies, establish correlation of this unit with the Lower Cambrian Shady Dolomite. The uppermost unit in the marble sequence (Gantts Quarry Formation) contains Early Ordovician (middle to late Canadian; = early to middle Arenigian) conodonts that confirm correlation of this unit with the Newala Limestone and Kingsport and Mascot Formations of the Appalachian foreland. The carbonate platform sequence is unconformably overlain by a thick clastic sequence that accumulated in a deep successor basin; the Lay Dam Formation is a marine fanlike deposit at the base of this sequence. Conodont molds from the top of the Lay Dam and fossils from the stratigraphically higher Jemison Chert indicate a Silurian to Early Devonian age for the Lay Dam Formation. Paleontologic data and the stratigraphic and structural setting indicate that the Talladega slate belt is the most distally preserved and relatively complete fragment of the Appalachian miogeocline; thus, the tectonic evolution of the Talladega belt is crucial to understanding the western margin of Iapetus. Linkage of the Talladega slate belt rocks with those of the western Blue Ridge to the northeast suggests that the latter once contained a thick Cambrian to Devonian cover sequence which subsequently has been mostly removed.

Calibration of a conodont apatite-based Ordovician 87Sr/86Sr curve to biostratigraphy and geochronology: Implications for stratigraphic resolution

The Ordovician 87 Sr/ 86 Sr isotope seawater curve is well established and shows a decreasing trend until the mid-Katian. However, uncertainties in calibration of this curve to biostratigraphy and geochronology have made it diffi cult to determine how the rates of 87 Sr/ 86 Sr decrease may have varied, which has implications for both the stratigraphic resolution possible using Sr isotope stratigraphy and efforts to model the effects of Ordovician geologic events. We measured 87 Sr/ 86 Sr in conodont apatite in North American Ordovician sections that are well studied for conodont biostratigraphy, primarily in Nevada, Oklahoma, the Appalachian region, and Ohio Valley. Our results indicate that conodont apatite may provide an accurate medium for Sr isotope stratigraphy and strengthen previous reports that point toward a signifi cant increase in the rate of fall in seawater 87 Sr/ 86 Sr during the Middle Ordovician Darriwilian Stage. Our 87 Sr/ 86 Sr results suggest that Sr isotope stratigraphy will be most useful as a high-resolution tool for global correlation in the mid-Darriwilian to mid-Sandbian, when the maximum rate of fall in Sr/