Susan Turner

The oldest articulated chondrichthyan from the Early Devonian period

Chondrichthyans (including living sharks, skates, rays and chimaeras) have a fossil record of scales and dermal denticles perhaps dating back to the Late Ordovician period, about 455 million years ago. Their fossil tooth record extends to the earliest Devonian period, almost 418 million years ago, whereas the oldest known articulated shark remains date from the Early Devonian period, about 394 million years ago. Here we report the discovery of an articulated shark that is almost 409 million years old from the Early Devonian (early Emsian) period of New Brunswick, Canada. The specimen, identified as Doliodus problematicus (Woodward), sheds light on the earliest chondrichthyans and their interrelationships with basal jawed vertebrates. This species has been truly problematic. Previously known only from isolated teeth, it has been identified as an acanthodian and a chondrichthyan. This specimen is the oldest shark showing the tooth families in situ, and preserves one of the oldest chondrichthyan braincases. More notably, it shows the presence of paired pectoral fin-spines, previously unknown in cartilaginous fishes.

False teeth: conodont-vertebrate phylogenetic relationships revisited

ABSTRACT An evidence-based reassessment of the phylogenetic relationships of conodonts shows that they are not “stem” gnathostomes, nor vertebrates, and not even craniates. A significant group of conodont workers have proposed or accepted a craniate designation for the conodont animal, an interpretation that is increasingly becoming established as accepted “fact”. Against this prevailing trend, our conclusion is based on a revised analysis of traditional morphological features of both discrete conodont elements and apparatuses, histological investigation and a revised cladistic analysis modifying that used in the keystone publication promoted as proof of the hypothesis that conodonts are vertebrates. Our study suggests that conodonts possibly were not even chordates but demonstration of this is beyond the scope of this paper. To summarize, in conodonts there is low cephalization; presence of simple V-shaped trunk musculature and unique large-crystal albid material in the elements; lack of a dermal skeleton including characteristic vertebrate hard tissues of bone, dentine and enamel; lack of odontodes with bone of attachment and a unique pulp system; lack of segmentally-arranged paraxial elements and dermal elements in median fins, all of which supports neither a vertebrate nor a craniate relationship for conodonts.

Siluro-Devonian thelodonts from the Welsh Borderland

Silurian, Downtonian and Dittonian strata from the Welsh Borderland contain a succession of thelodonts (Agnatha). Isolated scales have been identified and a new species, Apalolepis toombsi, is described. Four scale assemblages are recognized: Turinia pagei in Dittonian; Goniporus/Katoporus in U. Red Downton Group; Acanthodians (thelodonts absent) in Holdgate Sandstones Group and Thelodus parvidens in the Silurian—Lower Red Downton Group. The Thelodus parvidens assemblage occurs above and below the Ludlow Bone Bed, which, on the basis of the thelodonts, does not correspond to a major break in the succession. The incoming of the Turinia pagei assemblage at the beginning of the Dittonian marks a principal faunal change, and this can be correlated with the N. German Beyrichienkalk erratics, and the Polish-Lithuanian and Spitsbergen sequences. Comparison of the Welsh Borderland thelodont succession with that of Scotland, Europe, Russia and Canada suggests the existence of distinct faunal provinces, influenced by palaeogeography.

Australian Jurassic sedimentary and fossil successions: current work and future prospects for marine and non-marine correlation

Strata of Jurassic age occur extensively across onshore Australia, but they are predominantly of non-marine origin. Marine Jurassic strata have only limited onshore exposure in northwestern and central-western Australia, with thick marine sequences lying offshore on the North West Shelf. The richest petroleum province in Australia is located at the shelfs southern end, where the Dingo Claystone represents an important source rock for oil and gas. By and large, non-marine deposits, including economic coals, are distributed in the eastern states. Jurassic stage boundaries, in the main, are poorly constrained with respect to the Australian sedimentary succession. New work on microfossils, plants, fish, and zircon dating is providing a basis for improved correlation across Australian basins, with overseas successions, and recent international IUGS geologic timescales.

The last dicynodont: an Australian Cretaceous relict

Some long–forgotten fossil evidence reveals that a dicynodont (mammal-like reptile of the infraorder Dicynodontia) inhabited Australia as recently as the Early Cretaceous, ca. 110 Myr after the supposed extinction of dicynodonts in the Late Triassic. This remarkably late occurrence more than doubles the known duration of dicynodont history (from ca. 63 Myr to ca. 170 Myr) and betrays the profound impact of geographical isolation on Australian terrestrial faunas through the Mesozoic. Australias late–surviving dicynodont may be envisaged as a counterpart of the ceratopians (horned dinosaurs) in Cretaceous tetrapod faunas of Asia and North America.

A review of placoderm scales, and their significance in placoderm phylogeny

An historical review of the literature relating to placoderm scales preserved in association with articulated dermal plates, or as isolated units in microvertebrate assemblages, is followed by a discussion of their relevance in phylogenetic analyses of the Placodermi. The dentinous tissue forming the tubercles of Early Devonian acanthothoracid scales and dermal bone is similar to that of the dermal bone ornament of some osteostracans, and denticles of the vertebrate Skiichthys from the Ordovician Harding Sandstone. This similarity supports the proposition that the gnathostomes are the sister-group of the Osteostraci, with the Placodermi branching earliest within the gnathostomes, and the Acanthothoraci branching earliest within the Placodermi. The meso-semidentine in acanthothoracid tubercles, rather than semidentine (sensu stricto), is most likely to be synapomorphic for the Placodermi.

Middle Palaeozoic elasmobranch remains from Australia

ABSTRACT Shark teeth have been found in Upper Devonian rocks from the Broken River embayment and Burdekin Star shelf of north Queensland and in Lower Carboniferous rocks from Broken River and from near Gresford, New South Wales. Harpago ferox gen. et sp. nov. is a new tooth type which occurs in the Upper Bundock Creek Formation (possible Late Devonian), Queensland, and in the Early Carboniferous Bingleburra Formation, New South Wales. Other Late Devonian forms found in Queensland include Protacrodus spp. and Phoebodus cf. P. politus Newberry, 1889, known previously from North America, Europe and India. From one Early Carboniferous site in Queensland came a tooth of Xenacanthus sp. and one of Cladodus thomasi sp. nov., a form previously described from the Laurel Formation of Western Australia where it is associated with bradyodont teeth referred here to Helodus.

A REDESCRIPTION AND REINTERPRETATION OF GYRACANTHIDES MURRAYI WOODWARD 1906 (ACANTHODII, GYRACANTHIDAE) FROM THE LOWER CARBONIFEROUS OF THE MANSFIELD BASIN, VICTORIA, AUSTRALIA

Abstract The articulated acanthodian Gyracanthides murrayi Woodward from the Lower Carboniferous of Mansfield, Victoria, Australia, is redescribed from original and newly aquired material. It includes the only known head region from a member of the Gyracanthidae. A new reconstruction and interpretation of the pectoral area are proposed, incorporating the elements from the original description as well as a new bone interpreted as a procoracoid attached to a prepectoral spine. The posterior free pectoral spine described by Woodward is reinterpreted as a scapulocoracoid. A subsequent reconstruction of the pectoral girdle which synonymised the anterior and posterior prepectoral spines is rejected. A clear difference in size and shape of the pectoral and pelvic fin spines is established. The morphology of the tubercular ornament of the paired fin spines is used to distinguish species of Gyracanthides. Gyracanthides murrayi had a covering of dermal polyodontodia with spinose crowns and a concave base with a central vascular canal opening. The systematic position of the Gyracanthidae remains unclear, with the family retained within the Climatiiformes only on the basis of the broad-based, fin spines with nodose ornament.

Shark teeth from the Early-Middle Devonian Cravens Peak Beds, Georgina Basin, Queensland

The oldest shark teeth so far recorded from Australia are described from the Early-Middle Devonian of western Queensland. The teeth show characters in common with those of the Jurassic-Recent hexanchoid sharks, and the possible relationship to these so-called ‘living fossils’ is discussed. The teeth are ascribed to a new species, Mcmurdodus whitei, of a genus found previously in the Middle-Late Devonian of Antarctica. Scales and prismatic cartilage found in the Cravens Peak Beds probably belong to this shark.

Global Ordovician vertebrate biogeography

Abstract Cambrian–Ordovician vertebrate and supposed vertebrate occurrences have been repeatedly claimed during recent decades, with confirmed taxa bearing mineralized tissues with a vertebrate histomorphology still relatively rare. The only biogeographic province that we can presently recognize is the Gondwana Endemic Assemblage (GEA) with possible Late Cambrian fragmentary remains from Australia but more definite Early Ordovician (Arenigian) to early Late Ordovician (Caradocian) arandaspids (i.e., Sacabambaspis, Arandaspis) and other taxa known from South America and Australia. Certain chondrichthyans (‘sharks’, at first without teeth and which might not constitute a monophyletic group) might have originated in East Gondwana province and then are found in the Late Ordovician and Early Silurian of Mongolia, Tarim, and South China. The GEA fauna proper disappears by middle–late Caradocian and vertebrates do not reappear in Gondwana until mid Late Silurian. Late Ordovician (−455 Myr or earlier) vertebrates are also known with certainty from Laurentia, viz., North America (pteraspidomorphs Astraspis, Eriptychius, and various gnathostome-like taxa including chondrichthyan-, placoderm- and acanthodian-like remains), and Siberia (astraspid-like microremains with an unusual histology, which might correspond to a new group of lower vertebrates) as well as scales from putative loganiid and thelodontidid thelodonts from North America and Russia (Timan–Pechora, the Severnaya Zemlya archipelago and Siberia). This is defined as the Laurentia–Baltica–Siberia Assemblage (LBSA). We also mention one enigmatic reference to a Late Ordovician anaspid in South Africa. There is no clear association of taxa between the GEA and LBSA despite a small overlap in time. Various recent palaeogeographic models published for the Ordovician are critically analyzed and considered within four groups: the archetypal palaeogeographic reconstructions, two alternative solutions, and a compact version. Habitats of vertebrates in mostly BA1 (marine intertidal) to BA3 (shallow subtidal) environments, and their dispersal capabilities are evaluated with regard to those models. The main feature of Ordovician vertebrate biogeography is endemism. Furthermore, the present lack of complete descriptions of most taxa, which are often represented only by isolated microremains, and the need for a thorough phylogenetic analysis preclude any phylogenetic palaeobiogeographic study. In such a framework, we also evaluate possible links between external, physical factors and the Ordovician radiation of vertebrates. The Late Proterozoic deposition of oceanic phosphate and the Early Cambrian increase in oxygen on Earth might have been the spur for vertebrate evolution before the phase when hard tissues appeared. The sharp decline of the marine strontium isotope ratio during the Middle to Late Ordovician transition, interpreted as having been controlled primarily by continental collisional tectonics and its associated erosion and weathering, has been proposed as the consequence of a possible mantle superplume event which could have caused the prominent Caradocian transgressive phase. This might have been a factor in the changeover from a Gondwanan to a Laurentian focus for vertebrates.