Placoderm Fishes Research Articles

The Sinolepidae, a family of antiarchs (placoderm fishes) from the Devonian of South China and eastern Australia

Two new antiarchs are described, from the Late Devonian Hunter Siltstone near Grenfell in south-eastern Australia (Grenfellaspis branagani n.gen., n.sp.), and from the Early Middle Devonian Dayaoshan Group in Guangxi, south-eastern China (Dayaoshania youngi n.gen., n.sp.). New material is described of Xichonolepis qujingensis P'an & Wang, 1978 from the Middle Devonian of Yunnan, and new interpretations are presented for Sinolepis Liu & P'an, 1958 from the Late Devonian of Jiangsu. All four genera are placed in the family Sinolepidae Liu & P' an, of which the most obvious defining character is the much reduced ventral laminae of the anterior and posterior ventrolateral plates of the trunk armour, and the presumed absence of a median ventral plate. Emended diagnoses are presented for the family Sinolepidae and the genera Xichonolepis and Sinolepis. It is suggested that Grenfellaspis and Sinolepis are immediately related, and the biostratigraphic, biogeographic, and palaeogeographic implications of this relationship are discussed. The vertebrate fauna from the Hunter Siltstone is regarded as the youngest nonmarine vertebrate horizon known from the Devonian of south-eastern Australia. A close palaeogeographic connection between south-eastern Australia and South and North China is indicated for the latest Devonian and earliest Carboniferous (late Famennian-early Tournaisian), which contrasts with the distinctive Devonian vertebrate faunas from the two regions in earlier strata. Other Devonian fossil groups showing a similar biogeographic pattern are considered in the context of competing hypotheses concerning the palaeogeographic relationships of Gondwana and Asia during the Middle Palaeozoic. 319 320 Records of the Australian Museum (1992) Vo1.44 RITCHIE, A., S. WANG, G.C. YOUNG & G. ZHANG, 1992. The Sinolepidae, a family of antiarchs (placoderm fishes) from the Devonian of South China and eastern Australia. Records of the Australian Museum 44(3): 319-370.

A new antiarch (placoderm fish) from the Early Devonian of South China

A new bothriolepidoid antiarch of small size, Luquanolepis pileos gen. et sp. nov. is described from the Early Devonian of Luquan County, Yunnan Province. Only remains of the trunk armour are known. It is characterised by the long dorsal wall, with a strong posterior process formed by the posterior median dorsal and posterior dorsolateral plates, the posterior median dorsal longer and broader than the anterior median dorsal, a strongly developed posterior internal tranverse crista anteriorly placed at about the middle of the length of the dorsal trunk armour, and an independent posterior lateral plate. It is one of the few antiarchs known from the Early Devonian which has a developed brachial process and segmented pectoral fin. The small unpaired semilunar plate, subanal lamina and other features indicate bothriolepid affinities, and the separate posterior lateral suggests it is a stem taxon within the bothriolepidoids, a group to which 13 genera are assigned. Distribution of 19 characters amongst these taxa is summarised in a data matrix used in a computer-assisted cladistic analysis of the group to produce over 100 equally parsimonious trees. By eliminating poorly known taxa and characters from the analysis and inferring some missing character states 14 equally parsimonious trees were produced for 10 bothriolepidoid taxa. Luquanolepis is regarded as the most primitive member within the clade, consistent with the fact that it is the oldest bothriolepidoid genus so far discovered.

Contribution to the correlation of the Emsian (Lower Devonian) on the basis of placoderm fishes

Contribution to the correlation of the Emsian (Lower Devonian) on the basis of placoderm fishes

Early Devonian conodonts from the Kuan Tung Formation, Thailand: systematics and biogeographic considerations

A small conodont assemblage, obtained from a limestone marking the boundary between the middle and upper units of the Kuan Tung Formation, Satun Province, South Thailand, contains inter alia Pandorinellina steinhornensis steinhornensis, Polygnathus labiosus mawsonae n.subsp., and Pseudooneotodus kuangtungensis n.sp., indicating an Emsian age (perbonus-serotinus Zones). Studies on ontogenetic variation in Pa elements of Polygnathus labiosus mawsonae n.subsp. show that smaller Pa elements have fewer transverse ridges on the posterior third of the platform, more distinct marginal denticulation on the platform and a proportionately smaller basal cavity. The basal cavity lips may break off in mature specimens; this gives an appearance of a small basal cavity. The distribution of P. labiosus, P. pseudoserotinus, P. perbonus and Pseudooneotodus kuantungensis n.sp., in the Shan-Thai and South China Terranes, together with supporting evidence from thelodontid and placoderm fishes, suggests that these terranes were in close proximity to the eastern margin of Gondwana at the end of the Early Devonian.

The relationships of placoderm fishes

The relationships of placoderm fishes

Observations on Ctenurella (Ptyctodontida) and the classification of placoderm fishes

The structure of the hyoid arch of the ptyctodont Ctenurella is described and discussed with reference to theories of jaw suspension in placoderm fishes. It is concluded that primitive placoderms had a modified hyoid arch but that the hyomandibular took no direct role in supporting the jaws. The relationships of ptyctodonts are discussed and it is concluded that they are placoderm fishes. Several different classifications of placoderm fishes are evaluated and are shown to be weakly based, chiefly because of lack of precise knowledge of character distribution. An attempt is made to produce a classification by using a simple cladistic computer analysis. The result highlights homoplasy in character distribution amongst traditionally recognized placoderm groups.

An asterolepidoid antiarch (placoderm fish) from the Early Devonian of the Georgina Basin, central Australia

A small collection of well-preserved antiarch remains from the basal calcareous unit of the Cravens Peak Beds is described and figured. This represents the earliest record of antiarchs from the Australian continent. An Emsian age, based on correlations with the Mulga Downs Group in the Darling Basin, makes this the earliest recorded antiarch occurrence outside of the south China province. All specimens are of small size and may be juvenile. Some resemblances to Pterichthyodes are noted, but the material is insufficient for formal taxonomic treatment. The structure of the asterolepidoid pectoral fin is discussed, and it is concluded that this is a shared primitive feature of euantiarchs generally. It is suggested that the primary distribution of asterolepidoid antiarchs did not include south China, and some implications of a vicariance explanation for the origin of bothriolepidoids in south China, and of asterolepidoids elsewhere, are briefly considered.

New Early Devonian brachythoracids (placoderm fishes) from the Taemas - Wee Jasper region of New South Wales

Two new brachythoracids are described from the Taemas Limestone. Arenipiscis westolli gen. et sp. nov. possesses a skull-roof with deeply embayed posterior and elongate anterolateral margins, a large fused rostropineal plate, and orbits anteriorly placed and deeply notching the skull. The visceral skull-roof surface is developed with strong relief. The trunk-shield is poorly known, but the median dorsal plate probably had a prominent carinal process. Errolosteus goodradigbeensis gen. et sp. nov. is known only by an incomplete skull-roof, and anterior lateral and posterior ventrolateral plates. It has a highly distinctive ridged ornament. The affinities of these forms are considered in a discussion of brachythoracid interrelationships, and it is concluded that they are interchangeable on a cladogram with Buchanosteus. Other brachythoracids from the fauna, recently described by White, are considered, and it is suggested that Goodradigbeeon is phyletically primitive and Taemasosteus phyletically advanced relative to Buchanosteus. Schemes of brachythoracid interrelationships recently proposed by Denison and by Miles & Dennis are compared, and a new scheme is proposed.

Stratigraphic occurrence of some placoderm fishes in the Middle and Late Devonian

Stratigraphic occurrence of some placoderm fishes in the Middle and Late Devonian

The Holonematidae (placoderm fishes), a review based on new specimens of Holonema from the Upper Devonian of Western Australia

A new species of the placoderm fish Holonema is described from the Frasnian of Western Australia as H. westolli . A detailed account of the head and trunk shields is given; the palatoquadrate, opercular cartilage and gnathals are also described and receive special attention in view of the interest that attaches to these structures. Restorations of the armour are given in connexion with a discussion of the possible mode of life, and its convergent resemblance to antiarchs is noted. In a section on comparative morphology, some specimens of Arctolepis decipiens from Spitsbergen are partly redescribed; the variation in the dermal bones of the snout in Holonema westolli is analysed with respect to their possible ontogeny; and the structure of the nasal capsules and nasal openings in arthrodires is discussed, with the result that the usual elasmobranch model used in their interpretation is discarded; three possible interpretations of the snout in Holonema are proposed and one is tentatively accepted after a discussion of the available evidence, although it implies that some of the normal morphological relationships in the arthrodire snout have been seriously modified. The comparative morphology of the lateral-lines of the cheek and flank is considered, as well as of the jaw apparatus and gill cover. This last discussion is summarized in a general account of jaw suspension in placoderms. In a review of the family Holonematidae, the type and all referred genera ( Holomena, Deirosteus, Rhenonema, Gyroplacosteus, Megaloplax, Deveonema ) are tentatively defined, although the available information is not fully adequate for this purpose, and all named species and recent records are considered. The poorly known genera Aspidichthys, Glyptaspis, Groenlandaspis, Grazosteus, Tropidosteus and Arctonema , are excluded from the family. New material is described from Germany, Belgium and Scotland as Holonema spp. Throughout this review Holonema westolli is used as a standard of comparison rather than the poorly known type species H. rugosum . Finally the relationships and classification of the holonematids are considered. It is concluded that they are arthrodires at the advanced coccosteomorph level of organization, phylogenetically equally distinct from all other brachythoracids, and it is recommended that the Brachythoraci should be divided into the collateral taxa Holonematei and Eubrachythoracei, the former with the single family Holonematidae.

A New Acanthodian Fish from the Upper Mississippian of Indiana

This organ genus pertains to the fin and free pectoral spines of acanthodian placoderm fishes which are widely distributed in late Paleozoic rocks. Specimens of the best known species, G. formosus Agassiz of the Pennsylvanian of Great Britain, may be as much as 40 centimeters in length, indicating fishes of very respectable size, and the specimen dealt with here was probably originally much larger but has lost part of both extremities.