Median Dorsal Plate Research Articles

Nahecaris stuertzi, a phyllocarid crustacean from the Lower Devonian Hunsrück Slate

Restudy ofNahecaris based on new stereo radiographs and re-examination of actual specimens has added the following new information.Nahecaris ranges in shape from long and slender to short and stout, the carapace from deep and rounded to long and narrow. Carapace ornament consists either of thin ridges parallel to the ventral border or inclined rows of irregular lines or cusps, but these characters cannot be consistently correlated with other variables to permit definition of separate species. The carapace is fused to the head, and possibly to the anterior end to the thorax in the area in front of the median dorsal plate. A post-mandibular pyritised band may represent the carapace adductor muscle, which inserts on the mesolateral carinae. The mandible resembles that ofCeratiocaris, while the maxillae are like those of extant leptostracans. The 8 thoracopods are biramous: they have an exopod with finger-like lobes and a stenopodous endopod. Posteriorly the endopods reduce markedly, forming a conical, anteriorly open feeding basket. The predominance of dorso-ventrally flattened specimens, and the wide ventral gape of the carapace valves suggests those valves were spread apart in life.Nahecaris may have pressed itself to the substrate by flexing the carapace rim outward and incurving the ventral region of the carapace. The stout second antennae may have stabilised and propelled the animal over the substrate.Nahecaris is thought to have been benthic and to have fed, likeCeratiocaris, by lifting the substrate into suspension, as well as by picking up large fragments directly with the mandibles.

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.

A possible occurrence of Remigolepis in the topmost Old Red Sandstone of Berwickshire

Synopsis The occurrence of an incomplete fish plate of very large size in a loose block of Upper Old Red Sandstone at Pease Bay, near Cockburnspath, Berwickshire, is recorded. The bone is described, and identified as the anterior median dorsal plate of an antiarch (Placodermi), ?Remigolepis sp., a genus hitherto known only from beds of debatable age (Famennian or Lower Carboniferous) in East Greenland and Australia. The British discovery may allow more accurate correlation with these beds, which bear very important vertebrate faunas including the earliest known land vertebrates, of which the ichthyostegid amphibians from East Greenland are best known. The actual age of these strata, however, remains to be evaluated. The possibility of amphibians occurring in association with ?Remigolepis sp. in Berwickshire is considered extremely remote.