Gavin C. Young

Two well-preserved articulated tristichopterids are known from the Canowindra fossil fish locality (Frasnian) in central New South Wales: Mandageria Johanson & Ahlberg, 1997 Johanson, Z. and Ahlberg, P. E. 1997. A new tristichopterid (Osteolepiformes: Sarcopterygii) from the Mandagery Sandstone (Late Devonian, Famennian) near Canowindra, NSW. Transactions of the Royal Society of Edinburgh: Earth Sciences, 88: 39–68. [Crossref] [Google Scholar] and Cabonnichthys Ahlberg & Johanson, 1997 Johanson, Z. and Ahlberg, P. E. 1997. A new tristichopterid (Osteolepiformes: Sarcopterygii) from the Mandagery Sandstone (Late Devonian, Famennian) near Canowindra, NSW. Transactions of the Royal Society of Edinburgh: Earth Sciences, 88: 39–68. [Crossref] [Google Scholar] . Two other large tristichopterids are Notorhizodon Young et al., 1992 Young, G. C. , Long, J. A. and Ritchie, A. 1992. Crossopterygian fishes from the Devonian of Antarctica: systematics, relationships and biogeographic significance. Records of the Australian Museum, Supplement, 14: 1–77. [Crossref] [Google Scholar] from the Aztec Siltstone fauna of Antarctica (Givetian), and the Famennian Eusthenodon Jarvik, 1952 Jarvik, E. 1952. On the fish-like tail in the ichthyostegid stegocephalians with descriptions of a new stegocephalian and a new crossopterygian from the Upper Devonian of East Greenland. Meddelelser om Gr⊘nland, 114: 1–90. [Google Scholar] , first described from East Greenland and recently identified in eastern Australia. Relationships of the East Gondwana tristichopterids were previously assessed as Notorhizodon [Platycephalichthys [Cabonnichthys [Mandageria [Eusthenodon]]]], with no endemic Gondwanan clade, and more basal tristichopterids (Tristichopterus, Eusthenopteron, Jarvikina) being Northern Hemisphere forms. This suggested that the group may have originated in Laurussia, with advanced tristichopterids later reaching Gondwana (Middle–Late Devonian). Reconsideration of evidence from structure of the palate and scale morphology suggests an alternative phylogeny, with an endemic Gondwanan clade (subfamily Mandageriidae nov.) comprising at least three genera: Cabonnichthys and Mandageria from Canowindra, an undescribed tristichopterid from Eden, NSW, and possibly Notorhizodon from Antarctica. Distinctive isolated scales of ‘mandageriid’ type are illustrated from the Antarctic Aztec Siltstone assemblage and contrasted with typical tristichopterid scales, including new examples illustrated from the Givetian Bunga beds, NSW. Biogeographic implications include a possible Gondwanan origin for tristichopterids.

Exceptional braincase preservation in some Devonian placoderm fishes permits interpretation of muscles and cranial nerves controlling eye movement. Placoderms are the only jawed vertebrates with anterior/posterior obliques as in the jawless lamprey, but with the same function as the superior/inferior obliques of other gnathostomes. Evidence of up to seven extraocular muscles suggests that this may be the primitive number for jawed vertebrates. Two muscles innervated by cranial nerve 6 suggest homologies with lampreys and tetrapods. If the extra muscle acquired by gnathostomes was the internal rectus, Devonian fossils show that it had a similar insertion above and behind the eyestalk in both placoderms and basal osteichthyans.

A diverse fossil vertebrate record in the Devonian of coastal southeastern Australia includes at least 30 genera and species representing all three major groups of extant jawed vertebrates (bony and cartilaginous fishes, and tetrapods), and both extinct groups (placoderm and acanthodian fishes). A bone recorded by W. B. Clarke in 1860 from Twofold Bay is the first published record of a Devonian vertebrate from the Southern Hemisphere. Abundant plant remains at some localities include large impressions of arborescent lycopsids, indicating one of the earliest forest environments for the Gondwana supercontinent. An early terrestrial invertebrate fauna is evidenced by fossil millipede remains. A review of the evidence for age control using paleontology and isotopic dating suggests that the lower part of the Devonian sequence (Bunga beds, Boyd Volcanic Complex) may be somewhat older (Emsian – Eifelian) than recently proposed; the tetrapod trackways at Genoa River in Victoria (lower part of the Combyingbar Formation) may correlate with the lowermost Twofold Bay Formation of the Merrimbula Group, which is overlain by the marine Bellbird Creek Formation, a manifestation of the global late Frasnian marine transgression and extinction event. Abundant placoderm remains at all higher fossil fish horizons in the overlying Worange Point Formation indicate a latest Devonian rather than Carboniferous age for the upper Merrimbula Group.

A fossil fish assemblage associated with marine invertebrates from the Coonardoo Sandstone (Wallingalair Group) at Boor Hill (eastern limb of Tullamore Syncline) contains phyllolepid and bothriolepid placoderms of probable Late Devonian age. An angular unconformity with the overlying Hervey Group indicates erosion and folding during the Middle – Late Devonian, and evidently younger than the main Tabberabberan orogenic event. Invertebrate remains demonstrate a Late Devonian marine interval, not previously recognised as far west as the Tullamore Syncline, and assumed to represent the global maximum sea-level in the late Frasnian immediately preceding the Frasnian – Famennian extinction event. A phyllolepid placoderm plate from a sedimentary interbed of the Dulladerry Volcanics in the Hervey Syncline compares with abundant phyllolepid material from the Merriganowry Shale Member of the Dulladerry Volcanics near Cowra, and similar occurrences in the Comerong Volcanics and Boyd Volcanic Complex in southeastern New South Wales. Biostratigraphic data suggest a late Middle Devonian (Givetian) age for the Merriganowry Shale Member of the Dulladerry Volcanics, which appears conformable beneath the Upper Devonian Hervey Group.

Phyllolepid placoderm remains from the Aztec Siltstone fish fauna are described as Austrophyllolepis quiltyi sp. nov., Austrophyllolepis cf. A. youngi, Placolepis tingeyi sp. nov., and phyllolepid indet. The new Antarctic species of two genera previously only known from Australia reinforce evidence from other fish taxa of close biogeographic affinity, as part of the eastern margin of Palaeozoic Gondwana. At least three genera and four species gives the Aztec Siltstone fish fauna the most diverse phyllolepid assemblage known, and probably the oldest documented so far (?late Middle Devonian). Specimens of Austrophyllolepis from the type locality at Mount Howitt, Victoria, are refigured for comparison with the Antarctic species. The order Phyllolepida is a key group for understanding Devonian vertebrate biogeography and palaeogeography, with a unique disjunct distribution in both time and space between the Southern and Northern hemispheres. Phyllolepids document one of the major dispersal events in early vertebrate history, which approximately coincided with the Frasnian–Famennian boundary mass extinction during the Late Devonian.

Phyllolepid placoderms from the Devonian Boyd Volcanic Complex on the south coast of New South Wales are described as Cobandrahlepis petyrwardi, gen. et sp. nov., and Yurammia browni, gen. et sp. nov. Both are more primitive than other described phyllolepids in retaining a posterior dorsolateral plate in the trunk armor. Cobandrahlepis resembles Placolepis Ritchie, 1984, in the shape of the nuchal plate in the skull, and Austrophyllolepis Long, 1984, in the more lateral position of the sensory groove on the paranuchal plate. Yurammia, gen. nov., differs from all other phyllolepids in lacking ridged ornament on the dermal bones. These new taxa add to the diversity of early members of the order Phyllolepida from East Gondwana, before the appearance of the genus Phyllolepis in the Northern Hemisphere during the last stage of the Late Devonian (Famennian). Phyllolepid placoderms, only recorded from non-marine deposits, have a unique disjunct distribution in both time and space between the Southern an...

A second species of the placoderm genus Placolepis (Pl. harajica sp. nov.), based on a single articulated specimen from Givetian–Frasnian strata in the MacDonnell Ranges, demonstrates the occurrence of this taxon across the Australian craton. Placolepis (order Phyllolepida) is endemic to east Gondwana, and other phyllolepids are widespread in the Givetian and younger of Gondwana (Australia, Antarctica, Turkey, Venezuela), but do not occur until Late Devonian (Famennian) time in the Northern Hemisphere (Europe, Russia, Greenland, North America). The disjunct space–time distribution of the Phyllolepida is inconsistent with palaeomagnetic evidence indicating a wide equatorial ocean between Gondwana and Laurussia in Late Devonian time. This new species provides additional evidence supporting a Gondwana origin for the group, and later access to northern landmasses resulting from closure of the ocean between Gondwana and Laurussia and continental connection at or near the Frasnian–Famennian boundary.

Cavanosteus, a new genus of homostiid arthrodire, with C. australis (McCoy) as type species, is erected for new material from the Early Devonian (Emsian) of the Burrinjuck area, NSW. This new material compares closely with the holotype from Buchan, Victoria, identified as the central plates from the skull; it includes lower jawbones (infragnathals) lacking denticulation also closely similar to those of Homostius from the Middle Devonian of the Baltic Province and Scotland. The only bone from the dermal trunk armour referred to the new genus is an anterior dorsolateral plate. Relationships of other large Early Devonian arthrodires assigned to the family Homostiidae are discussed. Cavanosteus gen. nov. is probably the closest relative to the type genus known from the Early Devonian.