Nov 27, 2008 - from the Gneudna Formation, Carnarvon Basin, Western Australia, ... terms and conditions of use: http://www.tandfonline.com/page/terms-and-.
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Late Devonian thelodonts (Agnatha) from the Gneudna Formation, Carnarvon Basin, Western Australia a
Susan Turner & Roger S. Dring a
b
Hancock Museum, Newcastle upon Tyne, NE2 4PT, England
b
Department of Geology, University of Western Australia, Nedlands, Australia, 6009 Available online: 27 Nov 2008
To cite this article: Susan Turner & Roger S. Dring (1981): Late Devonian thelodonts (Agnatha) from the Gneudna Formation, Carnarvon Basin, Western Australia, Alcheringa: An Australasian Journal of Palaeontology, 5:1, 39-48 To link to this article: http://dx.doi.org/10.1080/03115518108565432
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Late Devonian thelodonts (Agnatha) from the Gneudna Formation, Carnarvon Basin, Western Australia SUSAN T U R N E R A N D R O G E R S. D R I N G
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TURNER, S. & DRING, R. S., 1981:3:2. Late Devonian thelodonts from the Gneudna Formation, Carnarvon Basin, Western Australia. AIcheringa 5, 39-48. ISSN 0311-
5518. Dermal denticles of two thelodont agnathans, Australolepis seddoni gen. et sp. nov., and a possible nikoliviid gen. et sp. indet, are described from the Gneudna Formation, a marine sequence of interbedded limestone and shale exposed on the eastern edge of the Carnarvon Basin, Western Australia. The age and significance of these new forms are discussed. The Gneudna Formation is Late Devonian, probably early Frasnian, which, if confirmed, makes these the youngest thelodonts known to date. Susan Turner, Hancock Museum, Newcastle upon Tyne, NE2 4PT, England; Roger S. Dring, Department of Geology, University of Western Australia, Nedlands, Australia 6009; received 5 March 1980.
MICROSCOPIC fish fragments were collected by R. S. Dring in the course o f research on the palaeobiology of the Gneudna Formation, a richly fossiliferous Devonian marine unit exposed in the Carnarvon Basin. Many of the microvertebrate fragments proved to be dermal denticles o f thelodonts, small scales averaging 1 m m 3, with crown, neck, and base with pulp opening. The terminology o f thelodont scales is described by Gross (1967), Turner (1976) and KaratajuteTalimaa (1978). Few scales (5-30 denticles in 3 kg o f limestone) are found in any one residue. The scales are not greatly water-worn, etched or broken, and thus it seems reasonable to assume that they are in situ in the Gneudna Formation and not derived either from a contemporaneous freshwater environment or reworked from an older rock. Two thelodont scales similar to those described in this paper have previously been isolated f r o m the type locality o f the Gneudna Formation by Seddon (1969, pl.2, figs 2,6), but he was unaware of their affinities, interpreting them as fish teeth. Fortuitously, Seddon appears to have figured one scale f r o m each o f the 'scale species' described in this paper. Other figured specimens (Seddon, 1969, pl.2, figs 1;3) are probably acanthodian scales which commonly occur with thelodont scales in the Gneudna Formation and elsewhere. 0311/5518/81/010039-10 $3.00 ©AAP
One such scale is figured here (Fig. 6D). The only previous report o f thelodonts from Western Australia was made by Gross (1971) who described Turinia australiensis, o f presumed Early Devonian (Dittonian) age, from Wilson Cliffs No. 1 Well in the Southern Canning Basin. The present paper aims to document the two Gneudna forms, which m a y be the youngest thelodonts known to date. Fossils described are housed in the Department o f Geology, University o f Western Australia (UWA).
Geological setting The Carnarvon Basin is an elongate meridional crustal downwarp with an area of about 235,000 km 2, of which slightly more than half is offshore (Playford et al., 1975). The sedimentary, predominantly marine, contents o f the Carnarvon Basin range in age from Silurian to Quaternary (Fig. 1). Devonian outcrops are confined to the eastern margin o f the Carnarvon Basin, where they directly overlie the Precambrian basement, but deep drillholes to the west and north o f the outcrop area indicate that Devonian sediments are present in the subsurface throughout most o f the northern half of the basin. The Devonian sequence consists o f mainly Late Devonian conformable shallow marine shelf sediments laid down in a slowly sinking basin that was probably open to the north. These rocks comprise
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40
SUSAN TURNER AND ROGER S. DRING
ALCHERINGA
M~
[] Homestead ® Well z~ Dam Track River or Creek Fault
'---]Mainly Tertiary ~
Mainly Permian
~
Munabia Ss.& V
~
Gneudna Form~
['~
Precambrian I
Fig. 1. Location and geologicalmap of exposures of the Gneudna Formation, Carnarvon Basin, Western Australia.
Specimens described in this paper are from the type section of the Gneudna Formation, 5 km south of Gneudna Well (indicated by a bar).
locally derived terrigenous clastics and shelf carbonate, and have a total thickness o f about 1,400 m. In ascending order, the
following formations are recognized: Nannyarra Greywacke (about 80 m thick), a clastic wedge o f conglomeratic and
DEVONIAN THELODONTS
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ALCHERINGA
feldspathic sandstone of inferred Middle Devonian or Frasnian age; Gneudna Formation (about 450 m), a richly fossiliferous, probably early Frasnian marine sequence of interbedded limestone, dolostone, shale and transitional lithologies, with some fine sandstone towards the top; Munabia Sandstone (about 550 m), a fine- to medium-grained shelf sandstone of Late Devonian (?Frasnian) age; and Willaraddie Formation (about 300 m), a conglomeratic pebbly sandstone and siltstone unit of Late Devonian (possibly Famennian) age, that is probably marine. Disconformably overlying the Devonian sequence in the outcrop area are further calcareous/clastic marine deposits of Early Carboniferous age. The Gneudna Formation occurs as narrow linear outcrops (Fig. 1) in the form of parallel sets of limestone beds, typically about 25 cm thick and running for several kilometres along strike. These beds commonly have a relief of 5-25 cm and are separated from each other by intervals of soil and limestone rubble typically 10 m wide. The regional dip of the beds is variable, but usually falls within the range 30-45°W. Both the lower and the upper boundaries are indistinct and the unit grades into the Nannyarra Greywacke below and the the Munabia Sandstone above by progressive changes in lithological character. With the possible exception of palynomorphs, Gneudna fossil assemblages tend to be rich in numbers but low in diversity. Most organisms seem to have been hardy forms capable of tolerating the fluctuating environmental conditions and corresponding range of sediment types that characterize the unit. Common fossil associations are crinoid-brachiopod, ostracode-crinoid-brachiopod, ostracode-eridostracan-mollusc-alga, and stromatoporoidcoral. Fish fragments are found sporadically throughout the section but are rare in limemudstone facies and in stromatoporoidcoral biotas. They are most common in the limestone (packstone), and are generally rare in the shale which is not exposed but common in cores. Other microvertebrate fragments recognized by Dring (in prep.) are indeterminate acanthodians, palaeoniscids, dipnoans and placoderms (?ar-
41
throdires). A few macroscopic remains of dipnoans and placoderms have also been found.
MUNABIA SANDSTONE 530~%~J
vv~ 320 --X-
Bed 62
290
z
Bed 4 5
260
Bed 38 Bed 3 6 230
Q
