The acanthodians were a common and widespread group of fishes throughout the world during the mid-Palaeozoic. In this study, a full-scale systematic analysis of Silurian to Middle Devonian acanthodian taxa of eastern Australia was undertaken, incorporating a review and updating of historical records and descriptions of taxa.
Phylogenetic relationships within the group and with other early gnathostomes are uncertain. The structure, function and modes of growth of acanthodian scales were described and investigated, and these features were used for comparisons between different taxa within the group, and between acanthodians and other early vertebrates. Histological and morphological characters of the scales were incorporated in a cladistic analysis of genera erected for articulated fish. This analysis did not support the traditional ordinal level groups, the Climatiida, Ischnacanthida and Acanthodida. Therefore, the highest taxonomic level used in the study was the family.
Rarely were acanthodians preserved as articulated fossils. The only examples known from the Silurian-Middle Devonian of Australia are one specimen of the putative acanthodian Yealepis douglasi Burrow & Young 1999 from the Ludlow of Victoria, five specimens of an indeterminate ?ischnacanthidid from the late Givetian of New South Wales and a rich assemblage from the Givetian lacustrine shales of Mt Howitt, Victoria. The latter fauna (originally dated as Late Devonian) includes six incomplete specimens of the culmacanthidid Culmacanthus stewarti Long 1983 and about 60 specimens of acanthodidid Howittacanthus kentoni Long 1986. In contrast, disassociated remains of the group are ubiquitous in microvertebrate faunas from the mid-Palaeozoic of eastern Australia. Although scales of other fish groups are sometimes more common in particular facies, acanthodian elements are found in all depositional environments, from deep shelf marine to transitional to freshwater. Most of the taxa, particularly those only preserved as isolated scales, had not been systematically described by other workers. This thesis incorporates descriptions of new taxa, and revision and updating of other taxa. Several overseas studies have produced biostratigraphic charts based on acanthodian scales. A similar biostratigraphic overview was undertaken based on a systematic analysis of the Early Devonian acanthodians of eastern Australia, permitting comparisons with acanthodian faunas of other regions.
Acanthodian scales and fin spines are the most common elements in the few vertebrate faunas that are known from the Silurian of Australia. Diversity and geographic distribution of the acanthodian faunas peaked during the Early Devonian. This study has been hampered by the low numbers of scales in many samples, and by uncertainty over their dating (particularly for the faunas from non-limestone deposits). Nevertheless, the work shows that changes in the marine assemblages are broadly correlated with the pattern of marine transgressions and regressions. Composition of the acanthodian faunas, and their abundance relative to other vertebrates in the assemblages, depend on the depositional environment. This correlation is best illustrated in Lower Devonian deposits, in which acanthodians are the most prolific and diverse. In transitional and marginal marine deposits, thelodonts are dominant, and acanthodians a minor element of the fauna. In off-shore assemblages, acanthodians and placoderms are dominant, and thelodonts are rare or absent.
Vertebrate faunas are poor in the earliest Devonian deposits, but become more common by the late Lochkovian, with near-shore assemblages characterized by Trundlelepis cervicostulata and "Nostolepis" guangxiensis, and deeper shelf assemblages by a new genus, possibly assignable to the Ischnacanthidae. The vertebrate record is sparse through the middle Pragian, though "N." guangxiensis is present low in the Coopers Creek Limestone (upper sulcatus-pireneae zones), being replaced by Nostolepoides platymarginata, Gomphonchus? bogongensis, and Radioporacanthodes sp. aff. R. (Gomphonchus) liujingensis by the kindlei Zone. Microvertebrate assemblages are more common by the late Pragian (pireneae Zone), with Radioporacanthodes sp. aff. R. liujingensis in deeper shelf deposits, and N. platymarginata and G.? bogongensis dominating near-shore assemblages. The earliest Emsian (dehiscens Zone) is marked by the incoming Cheiracanthoides wangi. Middle Emsian (perbonus-serotinus zones) assemblages are characterized by two new species, possibly assignable to Gomphonchus. The Middle Devonian cosmopolitan association of Cheiracanthoides comptus and "Acanthoides" dublinensis, which characterizes early Middle Devonian faunas from North America, Europe and China, appears first in the latest Emsian at the serotinus-patulus zone boundary.
As well as showing the value of acanthodians in biostratigraphy and as indicators of environmental settings, their use in biogeography was demonstrated. Although many of the acanthodian taxa are endemic, several are also found in other regions. The Silurian to earliest Devonian faunas of eastern Australia are most closely related to coeval Chinese assemblages. Several latest Silurian-earliest Devonian taxa are also recorded from the circum-Arctic region. The late Lochkovian to early Emsian assemblages, particularly from south-eastern Australia, have many taxa in common with Chinese faunas. The mid-Emsian taxa show highest endemicity; and the latest Emsian-Eifelian assemblages have the most cosmopolitan aspect. Acanthodian faunas become rarer and depauperate in the Middle Devonian, particularly in the south-eastern comer, and are mostly in poorly dated, ?fluviatile/marginal marine deposits.
This study of acanthodian faunas has encompassed a full scale systematic review of the group in this region, an appraisal of phylogenetic relationships within the group and with other early vertebrates, their palaeoecology, and their use in biostratigraphy and biogeography.