Evolution of the earliest graptolites
and other hemichordates.
Between Narva and the Pakri Islands, in the klint and in the valleys cutting into the bedrock, graptolite
argillite and phosphorite rich in U are exposed or spread beneath the Quaternary cover.
The base of the Upper Ordovician Series has been formally designated at the base of the Nemagraptus gracilis graptolite
Biozone (Bergstrom et al.
Herein we provide new information on the chronostratigraphy of the "Siluro-Devonian" succession in the subsurface of West Texas based on conodonts from a core from the Pegasus field in Midland County, Texas, and a reevaluation of graptolites
originally described by Decker (1942, 1952) from wells in adjoining Crane County.
In contrast, fossils are extremely rare in the more distal basin sediments; the few present are typically a graptolite
Thus, the base of the Sixth Stage of the Ordovician System correlates with the lower to middle part of the Keila Stage in Baltoscandia, and is in close proximity to the foliaceous/clingani graptolite
Knowlton and Jackson (1994) have argued that--far more often than generally acknowledged--the apparent plasticity of coral reef cnidarians reflects genetic differentiation, Indeed, the literature provides numerous hints of genetic controls on size and form: genetically based, species-level differences in colony form within the Montastraea annularis complex (Weil and Knowlton, 1994); differences in regeneration among clones of the reef coral Stylophora pistillata (Rinkevich, 2000); age effects on the survival of stony corals (Hughes and Connell, 1987); graptolites
with highly determinate patterns of colony size (Mitchell, 1986); and botryllid tunicates that exhibit zooid and colony senescence (Rinkevich et al., 1992).
fossils are preserved in the Bear River, Feltzen, and Lumsden Dam formations (White 2010a).
A number of graptolite
genuses were found by Almera (1891), Schriel (1929), and Riba (1949), who attributed the black series to the Silurian (Llandovery, Wenlock and Ludlow).
The boundary between the Hunneberg and Billingen "groups" (later referred to as stages, substages or series) was placed between the Megistaspis (Paramegistaspis) planilimbata and Megalaspides dalecarlicus trilobite biozones when established (Tjernvik 1956), coinciding with the lower boundary of the Phyllograptus densus graptolite
The upper parts of the Bear River formation in the northwestern area and Feltzen formation in the southeastern areas locally contains the Early Ordovician graptolite
Rhabdinopora flabelliformis, suggesting that the underlying Acacia Brook, Cunard, Bloomfield, and Moshers Island formations are Middle to Late Cambrian, and that a significant unconformity exists between the Halifax Group and the overlying Silurian White Rock Formation.
Other acid-resistant microfossils encountered during this study include acritarchs, foraminiferans, melanoscleritoids, chitinous hydroids, and graptolite
siculae, but these were not studied quantitatively.