Redescription of the gnathostome fish fauna from the mid-Palaeozoic Silverband Formation, the Grampians, Victoria

Gnathostome vertebrate remains from fine-grained sandstones of the Silverband Formation in the Grampians, Victoria include dissociated fin spines, scales and teeth. These elements arc assigned herein to the acanthodians Sinacanthus? micracanthus (fin spines) and Radioporacanthodes sp. cf. R. qujingensis (scales and tooth whorls). This fauna indicates a Late Silurian (?late Ludlow) age for the vertebrate-beating Stratum. Under current systematic groupings, the two gnathostome taxa from the Silverband Formation belong to two different families, the Sinacanthidae and the Poracanthodidae. However. the preserved association could indicate that the three element types derived from the same biological species. The possibility that the Sinacanthidae is a sister group to the Climatiidae and the Poracanthodidae is raised by this scenario. The Sinacanthidae is tentatively reassigned to the Acanthodii, as it is considered to lack diagnostic chondrichthyan characters.

A redescription of Atopacanthus dentatus Hussakof and Bryant, 1918 (Acanthodii, Ischnacanthidae)

The holotype specimen of the ischnacanthid acanthodian Alopacanthus dentatus comprises most of the posterior half of a dentigerous jaw bone, rather than a fragment from the middle of the jaw as was previously believed. A new diagnosis and revised description for the taxon are based on the holotype from the Rhinestreet Shale (Frasnian) and other specimens front the North Evans and Genundewa limestones (early Frasnian) of the Genesee Formation (late Givetian-early Frasnian); all of: these dentigerous jaw bones were collected near Hamburg, New York state, U.S.A. An emended diagnosis for Atopacanthus clarifies the differences between it and other ischnacanthid genera.

Diplacanthid acanthodians from the Aztec Siltstone (late Middle Devonian) of southern Victoria Land, Antartica

One articulated and several partial, semi-articulated specimens of acanthodians were collected in 1970 from the freshwater deposits of the Aztec Siltstone (Middle Devonian; Givetian), Portal Mountain, southern Victoria Land, Antarctica, during a Victoria University of Wellington Antarctic Expedition. The Portal Mountain fish fauna, preserved in a finely laminated, non-calcareous siltstone, includes acanthodians, palaeoniscoids, and bothriolepid placoderms. The articulated acanthodian specimens are the most complete fossil fish remains documented so far from the Aztec assemblage, which is the most diverse fossil vertebrate fauna known from Antarctica. They are described as a new taxon, Milesacanthus antarctica gen. et sp. nov., which is assigned to the family Diplacanthidae. Its fin spines show some similarities to spine fragments named Byssacanthoides debenhami from glacial moraine at Granite Harbour, Antarctica, and much larger spines named Antarctonchus glacialis from outcrops of the Aztec Siltstone in the Boomerang Range, southern Victoria Land. Both of these are reviewed, and retained as form taxa for isolated spines. Various isolated remains of fin spines and scales are described from Portal Mountain and Mount Crean (Lashly Range), and referred to Milesacanthus antarctica gen. et sp. nov. The histology of spines and scales is documented for the first time, and compared with acanthodian material from the Devonian of Australia and Europe. Distinctive fin spines from Mount Crean are provisionally assigned to Culmacanthus antarctica Young, 1989b. Several features on the most complete of the new fish specimens - in particular, the apparent lack of an enlarged cheek plate - suggest a revision of the diagnosis for the Diplacanthidae.

A new Late Devonian acanthodian fish from the Hunter Formation near Grenfell, New South Wales

The ischnacanthid acanthodian Grenfellacanthus zerinae gen. et sp. nov. is described on the basis of two large jaw bones from the Late Devonian (late Famennian) Hunter Formation, near Grenfell, N.S.W. The new species is the youngest known ischnacanthid, and the largest ischnacanthid from Gondwana. As for many ischnacanthids, the structure of the jaws and teeth indicate that Grenfellacanthus was probably an ambush predator.

Homology of fin lepidotrichia in osteichthyan fishes

Lepidotrichia are dermal elements located at the distal margin of osteichthyan fins. In sarcopterygians and actinopterygians, the term has been used to denote the most distal bony hemisegments and also the more proximal, scale-covered segments which overlie endochondral bones of the fin. In certain sarcopterygian fishes, including the Rhizodontida, these more proximal, basal segments are very long, extending at least half the length of the fin. The basal segments have a subcircular cross section, rather than the crescentic cross section of the distal lepidotrichial hemisegments, which lack a scale cover and comprise short, generally regular, elements. In rhizodonts and other sarcopterygians, e.g. Eusthenopteron, the basal elements are the first to appear during fin development, followed by the endochondral bones and then the distal lepidotrichia. This sequence contradicts the 'clock-face model' of fin development proposed by Thorogood in which the formation of endochondral bones is followed by development of lepidotrichia. However, if elongate basal 'lepidotrichia' are not homologous with more distal, jointed lepidotrichia and if the latter form within a distal fin-fold and the former outside this fold, then Thorogood's 'clock-face' model remains valid. This interpretation might indicate that the fin-fold has been lost in early digited stem-tetrapods such as Acanthostega and Ichthyostega and elongate basal elements, but not true lepidotrichia, occur in the caudal fins of these taxa.

X-ray microtomography of 410 million-year-old optic capsules from placoderm fishes

The structure of two small ossified optic capsules from mid-Palaeozoic placoderm fishes has been revealed in fine detail, by the use of Xray microtomography analysis and 3D visualisation software. These two specimens are 410 million-year-old; they were collected from an Early Devonian (Lochkovian) limestone in central New South Wales, and are the oldest known optic capsules from jawed fishes. The capsules show attachment areas for seven extrinsic eye muscles, rather than the six until recently deemed universal for gnathostomes. The analysis also revealed structures within the ossified cartilage which covered the medial surface of the eyeball, including nerve tracts, vascular canals, and possibly a choroid rete mirabile. (c) 2005 Elsevier Ltd. All rights reserved.

Effects of harvesting callianassid (ghost) shrimps on subtropical benthic communities

The effects of harvesting of callianassid shrimp (Trypaea australiensis) on the abundance and composition of macrobenthic assemblages in unvegetated sediments of a subtropical coastal embayment in Queensland, Australia were examined using a combination of sampling and manipulative experiments. First, the abundance and composition of the benthic infauna in an area regularly used for the collection of shrimp for bait by recreational anglers was compared with multiple reference areas. Second, a BACI design, with multiple reference areas, was used to examine the short-term effects of harvesting on the benthic assemblages from an intensive commercialised fishing competition. Third, a large-scale, controlled manipulative experiment, where shrimp were harvested from 10,000 m(2) plots at intensities commensurate with those from recreational and commercial operators, was done to determine the impacts on different components of the infaunal assemblage. Only a few benthic taxa showed significant declines in abundance in response to the removal of ghost shrimp from the unvegetated sediments. There was evidence, however, of more subtle effects with changes in the degree of spatial variation (patchiness) of several taxa as a result of harvesting.. Groups such as capitellid polychaetes, gammarid amphipods and some bivalves were significantly more patchy in their distribution in areas subjected to harvesting than reference areas, at a scale of tens of metres. This scale corresponds to the patterns of movement and activity of recreational harvesters working in these areas. In contrast, patchiness in the abundance of ghost shrimp decreased significantly under harvesting at scales of hundreds of metres, in response to harvesters focussing their efforts on areas with greater numbers of burrow entrances, leading to a more even distribution of the animals. Controlled experimental harvesting caused declines in the abundance of soldier crabs (Mictyris longicarpus), polychaetes and amphipods and an increase in the spatial patchiness of polychaetes. Populations of ghost shrimp were, however, resilient to harvesting over extended periods of time. In conclusion, harvesting of ghost shrimp for bait by recreational and commercial fishers causes significant but localised impacts on a limited range of benthic fauna in unvegetated sediments, including changes in the degree of spatial patchiness in their distribution. (c) 2005 Elsevier B.V. All rights reserved.

Histological structure of the cancellous bone layer in Bothriolepis canadensis (Antiarchi, Placodermi)

The Placodermi are extinct basal gnathostomes which had extensive dermal and perichondral bone, but which lacked the endochondral bone which characterizes the more derived bony fishes. Thin sections of bone from a specimen of the antiarch placoderm Bothriolepis canadensis, from the Escuminac Formation ( Frasnian, Upper Devonian), Quebec, Canada, reveal that part of the cancellous layer in its dermal and endoskeletal bone formed from perichondral bone trabeculae growing around cartilage spheres. The resultant structure mimics that of osteichthyan endochondral bone. The layout and dimensions of this polygonal mosaic patterning of the bone trabeculae and flattened cartilage spheres resemble those of the prismatic layers of calcified cartilage in chondrichthyans. If the lack of endoskeletal bone in chondrichthyans is a derived character, then the structure identified in B. canadensis could represent a 'template' for the formation of prismatic calcified cartilage in the absence of bone.

Gyracanthides hawkinsi sp nov (Acanthodii, Gyracanthidae) from the Lower Carboniferous of Queensland, Australia, with a review of gyracanthid taxa

A new species of Gyracanthides from the mid-Visean Ducabrook Formation of Middle Paddock site, near Springsure in the Drummond Basin, central Queensland, is based on isolated three-dimensionally preserved elements. The specimens comprise paired and unpaired spines and pectoral girdle elements, procoracoids and scapulocoracoids, and include growth series. The morphology, especially of the shoulder girdle bones and the form and tubercular ornamentation of the paired fin spines, is used to distinguish the new taxon. These characters also help differentiate the numerous described gyracanthid species. Aspects of palaeobiology including possible sexual dimorphism are explored. A hypothetical reconstruction of the fish is based on our interpretation of the articulation of isolated elements combined with examination of wear patterns on fin spines. Gyracanthides hawkinsi sp. nov. is compared with other Australian taxa as well as with gyracanthids from North America, Europe, Russia, Iran, Africa and Antarctica, some of which are tentatively reassigned here to the Gondwanan genus Gyracanthides.

Effect of soil type on burrowing behavior and cocoon formation in the green-striped burrowing frog, Cyclorana alboguttata

This study examined the effect of soil type on burrowing behaviour and cocoon formation during aestivation in the green-striped burrowing frog, Cyclorana alboguttata (Gunther, 1867). Given a choice, frogs always chose to burrow in wet sand in preference to wet clay. Frogs buried themselves faster and dug deeper burrows in sandy soil. However, under my laboratory conditions, there was little difference in the pattern of soil drying between the two soil types. Frogs in both sand and clay soil experienced hydrating conditions for the first 3amonths and dehydrating conditions for the last 3amonths of the 6-month aestivation period, and cocoons were not formed until after 3amonths of aestivation. After 6amonths, there were more layers in the cocoons of frogs aestivating in sand than those aestivating in clay. Frogs were able to absorb water from sandy soil with water potentials greater than -400akPa, but lost water when placed on sand with a water potential of -1000akPa.

Gnathostome microremains from the lower Devonian Jawf Formation, Saudi Arabia

Calcareotis horizons in the Qasr and Hammamiyat members (Lower Devonian, ?Pragian and lower Emsian) of file Jawf Formation, northwestern Saudi Arabia, yielded a rich assemblage of microremains from acanthodian, placoderm. chondrichthyan, and sarcopterygian vertebrates. The most abundant elements are scales from acanthodians Nostolepis spp., Milesacanthus ancestralis n. sp., Canadatepis? sp., and Gomphonchus? fromensis. scales and dermal bone fragments from acanthothoracid and ?rhenanid placoderms, and teeth from onychodontids. Rarer occurrences include ?chondrichthyan scales of several different morphotypes, and petalichthid and ?ptyctodontid placoderm elements. The Qasr Member assemblage shows a close resemblance to slightly older faunas front the Lochkovian of Brittany and Spain. The Hammamiyat Member microvertebrate fauna shows closest affinity with that of the stratigraphically lower Qasr Member, with similarities also to coeval faunas from southeastern Australia, late Emsian/Eifelian faunas from west-central Europe, and the Givetian Aztec Siltstone fauna from Antarctica.