Article: Devonian sharks from south-eastern Australia and Antarctica
Devonian shark remains from the Aztec Siltstone in south Victoria Land, Antarctica, are described as Antarctilamna prisca gen. et sp. nov., Xenacanthus sp., and Mcmurdodus? cf. featherensis White. Similar spines, teeth, and associated endocranial and jaw remains from the Bunga Beds on the far south coast of New South Wales are also referred to A. prisca. In this genus the teeth are diplodont, the fin-spines ctenacanthiform, and the braincase had a long otic region, prominent subocular shelves, and probably a persistent lateral occipital fissure. The double mandibular joint on the palatoquadrate resembles that of acanthodians. The other taxa are known only from isolated teeth. Both are freshwater occurrences of late Givetian to early Frasnian age. In a new hypothesis of interrelationships for early elasmobranchs, diplodont teeth are regarded as a synapomorphy of Antarctilamna and Xenacanthus, with the fin-spines of the former interpreted as a primitive feature. A ctenacanth origin for euselachians is rejected, and hybodonts and euselachians are considered to be closely related. A single dorsal fin and spine are interpreted as the primitive elasmobranch condition, shared with holocephalans, and the absence of spines in some Palaeozoic sharks is regarded as secondary.