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

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.

Relative taxonomic and ecologic stability in Devonian marine faunas of New York State: a test of coordinated stasis

The concept of coordinated stasis, manifest as a pattern of long intervals of concurrent taxonomic and ecologic persistence separated by comparatively abrupt periods of biotic change, has been challenged in recent studies that claim a lack of prolonged persistence of taxa and associations. A key problem has been the difficulty of distinguishing faunal change owing to localized, short-term environmental fluctuation or patchiness from that indicating regionally pervasive, long-term evolutionary or ecological change. Here, we use an extensive database from the Middle Devonian Hamilton Group of the Appalachian Basin to test for taxonomic and ecologic persistence within this ecological-evolutionary subunit, a succession of purported relative stability. Replicate samples collected from many localities and stratigraphic horizons over a wide geographic area allow us to address the effects of small-scale environmental variation and localized faunal patchiness while exploring basin-scale variation in faunal composition within and between the formations of the Hamilton Group.Observed stratigraphic distributions of fossils are consistent with a scenario in which all taxa are present from bottom to top of the Hamilton Group, and absences result only from sampling failure. Although small-scale variation in faunal composition indeed does occur, there is no more variation among formations than occurs within them. Assemblages from different formations, whether they are defined by taxonomic or ecologic composition, are statistically indistinguishable according to several independent metrics, including ANOSIM and a maximum likelihood estimation that evaluates stratigraphic turnover using Bayesian “Information Criterion.” Simulated data sets indicate that test results are most consistent with species-level extinction of 2.6% per Myr within the Hamilton Group, far lower than the Givetian rate of 11.5% per Myr generic extinction derived from a global database. Such faunal persistence over the ~5.5 Myr encompassed by this unit is consistent with the pattern of coordinated stasis. Earlier studies showing greater amounts of temporal turnover in Hamilton Group faunas are likely influenced by their smaller geographic scale of analysis, suggesting that regional studies done elsewhere may yield similar results.