Persistent Oceanic Anoxia, Mass (?) Extinction Separate Cambrian, Ordovician Explosions

Persistent oceanic anoxia and elevated extinction rates separate the Cambrian and Ordovician radiations

Authors:

Saltzman et al

Abstract:

Recurrent mass extinction events (at "biomere"—a biostratigraphic unit—boundaries) characterize the middle Cambrian to Early Ordovician (Tremadocian) time interval that is between the major Cambrian and Ordovician radiations of animal life. A role for anoxia in maintaining elevated extinction rates in the late Cambrian has been proposed based on coincidence of an extinction with positive excursions in δ13Ccarb and δ34SCAS (CAS—carbonate-associated sulfate). Here we examine an Early Ordovician extinction event at the base of the North American Stairsian Stage (upper Tremadocian), and demonstrate concurrent onset of positive excursions in δ13C and δ34S inferred to reflect enhanced organic matter burial under anoxic waters. Sea-level rise may have brought anoxic waters onto the shelf to initiate extinctions. The evidence for δ13C excursions and elevated extinction rates appears to wane in the Tremadocian, consistent with progressive oxygenation of the oceans reaching a threshold that helped facilitate initial stages of the Great Ordovician Biodiversification Event.

Aegirocassis benmoulae: a 2+ Meter (!!!) Long, Filter Feeding Anomalocaridid From Tremadocian Ordovician Morocco

Anomalocaridid trunk limb homology revealed by a giant filter-feeder with paired flaps

Authors:

Van Roy et al

Abstract:

Exceptionally preserved fossils from the Palaeozoic era provide crucial insights into arthropod evolution, with recent discoveries bringing phylogeny and character homology into sharp focus. Integral to such studies are anomalocaridids, a clade of stem arthropods whose remarkable morphology illuminates early arthropod relationships and Cambrian ecology. Although recent work has focused on the anomalocaridid head, the nature of their trunk has been debated widely. Here we describe new anomalocaridid specimens from the Early Ordovician Fezouata Biota of Morocco, which not only show well-preserved head appendages providing key ecological data, but also elucidate the nature of anomalocaridid trunk flaps, resolving their homology with arthropod trunk limbs. The new material shows that each trunk segment bears a separate dorsal and ventral pair of flaps, with a series of setal blades attached at the base of the dorsal flaps. Comparisons with other stem lineage arthropods indicate that anomalocaridid ventral flaps are homologous with lobopodous walking limbs and the endopod of the euarthropod biramous limb, whereas the dorsal flaps and associated setal blades are homologous with the flaps of gilled lobopodians (for example, Kerygmachela kierkegaardi, Pambdelurion whittingtoni) and exites of the ‘Cambrian biramous limb’ . This evidence shows that anomalocaridids represent a stage before the fusion of exite and endopod into the ‘Cambrian biramous limb’, confirming their basal placement in the euarthropod stem, rather than in the arthropod crown or with cycloneuralian worms. Unlike other anomalocaridids, the Fezouata taxon combines head appendages convergently adapted for filter-feeding with an unprecedented body length exceeding 2 m, indicating a new direction in the feeding ecology of the clade. The evolution of giant filter-feeding anomalocaridids may reflect the establishment of highly developed planktic ecosystems during the Great Ordovician Biodiversification Event.

Trace Fossils of Brachiopods Found in Tremadocian Ordovician Argentina

First record of Podichnus in orthide brachiopods from the Lower Ordovician (Tremadocian) of NW Argentina and its relation to the early use of an ethological strategy

Authors:

Santos et al

Abstract:

Well preserved etchings from the pedicle attachment of brachiopods occur on orthide brachiopod shells from Lower Ordovician (Tremadocian) of NW Argentina. Under the name Podichnus conicus isp. n. this trace fossil is interpreted as a fixichnia produced by the attachment of brachiopod pedicles upon hard substrates. The trace is characterized by an irregular and asymmetrical cluster of circular pits. Each individual pit has a typical conical shape and presents the external mean diameter/length ratio close to 1. The occurrence of P. conicus isp. n. in the Tremadocian is the oldest record of this ichnogenus, and points to orthide brachiopods as Podichnus producers for the first time. The presence of this structure provides new information regarding colonization of biogenic hard substrates during the Early Ordovician and an adaptive strategy of its use by orthide brachiopods.