Redescription and neotype designation of the Middle Devonian microconchid (Tentaculita) species ‘Spirorbis’AngulatusHall, 1861

Abstract

The Middle Devonian (Givetian) microconchid tubeworm species known asSpirorbis angulatusHall, 1861 is redescribed here on the basis of a new collection from the Traverse Group of Michigan and Hamilton Group of Ohio and New York, U.S.A. Its general characteristics and ornamentation indicate it belongs to the genusPalaeoconchusVinn, 2006. Because the original specimens on which the species was erected are lost, a neotype is here designated for a specimen from the same area (Erie County, New York) and stratigraphic interval (Hamilton Group, Givetian) as the original specimens described by Hall (1861). The new specimens ofPalaeoconchus angulatus(Hall) show that the species is characterized by a spectrum of morphological variability, possibly ranging from a nearly smooth tube, though weakly ornamented, to one distinctly tuberculated on which thicker nodes and thin spines may occur together. Assemblages of microconchids from particular stratigraphic divisions are characterized by a dominance of small (up to 1.5 mm in diameter, probably juvenile) individuals, while larger individuals (>2 mm) are rare. This suggests that the populations may have experienced occasional mass mortalities. Although substrate (in this case brachiopod shells and rugosan coral thecae) overturning and an increase in sedimentation rate are possible factors, an episodic anoxia/dysoxia of the bottom waters seems likely as it is known that the host dark shales record such occasional oxygen deficiency.