Placoderms

From a set of new characters, the phylogeny of placoderms taken as a whole has been investigated using the computer programs PAUP and Mac Clade. The result corroborate the hypothesis of an inclusion of the Phylolepida within the Arthrodira and the basal position of the Acanthothoraci among Placodermata.

Almost all gnathostomes or jawed vertebrates (including osteichthyans, chondrichthyans, ‘acanthodians’ and most placoderms) possess paired pectoral and pelvic fins. To date, it has generally been believed that antiarch placoderms (extinct armoured jawed fishes from the Silurian–Devonian periods) lacked pelvic fins. The putative absence of pelvic fins is a key character bearing on the monophyly or paraphyly of placoderms. It also has far-reaching implications for studying the sequence of origin of pelvic girdles versus that of movable jaws in the course of vertebrate evolution. Parayunnanolepis xitunensis represents the only example of a primitive antiarch with extensive post-thoracic preservation, and its original description has been cited as confirming the primitive lack of pelvic fins in early antiarchs. Here, we present a revised description of Parayunnanolepis and offer the first unambiguous evidence for the presence of pelvic girdles in antiarchs. As antiarchs are placed at the base of the gnathostome radiation in several recent studies, our finding shows that all jawed vertebrates (including antiarch placoderms) primitively possess both pectoral and pelvic fins and that the pelvic fins did not arise within gnathostomes at a point subsequent to the origin of jaws.

Phyllolepid placoderms were heretofore known almost entirely from Greenland, western Europe, Australia, and Antarctica. Now, a suite of 31 specimens are here recognized from the United States. The locality yielding these fossils is Red Hill, a roadcut exposing late Famennian redbeds (Duncannon Member, Catskill Formation, Uppermost Devonian), east of Renovo, Clinton County, north-central Pennsylvania. One partially articulated individual, plus several isolated plates, represent Phyllolepis rossimontina sp. nov., described herein.
Additional dissociated plates may represent variants of this new species, or alternatively may belong to the previously recognized species Phyllolepis concentrica, P. nielseni, P. undulata, or P. woodwardi. A single plate described as P. delicatula over a century ago from slightly older strata farther east in Pennsylvania is morphologically distinct from any of the Red Hill specimens.