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Conodonts from the Prout Dolomite of north-central Ohio and Givetian (upper Middle Devonian) correlation problems
- Dale R. Sparling
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- Journal:
- Journal of Paleontology / Volume 73 / Issue 5 / September 1999
- Published online by Cambridge University Press:
- 14 July 2015, pp. 892-907
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The Prout Dolomite of north-central Ohio lies disconformably above the lowest Givetian (upper Middle Devonian) Plum Brook Shale and below the lowest Famennian (upper Upper Devonian) Ohio Shale. A sample from its base yielded over 4,000 diverse conodont specimens. Included is Polygnathus ansatus Ziegler and Klapper, 1976, the index species for the Middle varcus Subzone, which is not reported from strata of this age in Ontario and Indiana, a fact that long caused their miscorrelation with the Lower varcus Subzone. Also present is P. rhenanus Klapper, Philip, and Jackson, 1970, considered to be also indicative of the Middle varcus Subzone in North America, and P. ovatinodosus Ziegler and Klapper, 1976. Lowest occurrences of the latter are in the middle part of this subzone; its presence indicates correlation with the lower Tully Limestone of New York, the basal unit of the Taghanic Series. The Prout and equivalent strata in the region therefore represent a long unrecognized continuous time-rock unit created by Johnson's (1970) Taghanic onlap. The collection includes a new species of Ancyrolepis, A. huntleyi; a new species of Polygnathus is left in open nomenclature, as are nine specimens assigned to Tortodus but of otherwise uncertain taxonomic status.
Givetian conodont correlation between North America and the Global Stratotype Section and Point established by the Subcommission on Devonian Stratigraphy (SDS) in Morocco is extremely problematical because of relatively erratic distribution (probably owing to limited ecologic adaptability) of P. ansatus and P. hemiansatus Bultynck, considered herein to be an early morphotype of P. ansatus. The base of the Givetian Stage has been defined by the SDS as coinciding with the lowest occurrence of P. hemiansatus. The only possible evidence for the SDS's hemiansatus Zone in North America involves reported occurrence of that morphotype in the uppermost Arkona Shale of Ontario, a position above the top of the Plum Brook Shale, which has been considered to be of Givetian age for decades. Also it appears that the interval between the Eifelian (lower Middle Devonian) kockelianus Zone and the hemiansatus Zone at the SDS's global-stratotype section in Morocco is of questionable age and probably too thin to represent continuous sedimentation. Adoption of a widely recognized faunal break at the base of strata deposited during the If T-R cycle of North America and Europe as the base of the Givetian could provide a sound alternative.
Conodonts from the Middle Devonian Plum Brook Shale of north-central Ohio
- Dale R. Sparling
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- Journal:
- Journal of Paleontology / Volume 69 / Issue 6 / November 1995
- Published online by Cambridge University Press:
- 14 July 2015, pp. 1123-1139
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The Plum Brook Shale of north-central Ohio lies disconformably between the Eifelian (lower Middle Devonian) Delaware Limestone and the middle Givetian (upper Middle Devonian) Prout Dolomite. Its diverse conodont fauna indicates that it represents a transgression into the area in earliest Givetian time accompanied by the products of a notable adaptive radiation within two lineages of Icriodus. One of these (I. expansus Group) includes I. expansus, I. arkonensis and I. brevis, and complete intergradation of forms suggests that the group at this level involves conspecific ecotypes. A more extensive radiation involved descendants of I. regularicrescens (I. regularicrescens Group), including three new species, I. eriensis, I. janeae and I. ohioensis. Other representatives of this lineage occur here as well as in strata of comparable age in the type region of the Givet Limestone as described by Bultynck (1987). A third lineage is represented by I. excavatus in both areas, but evidence of radiation within this branch is limited to the Givet Limestone. Remarkable diversity also occurs among forms considered herein to involve Polygnathus pseudofoliatus and its descendants. The existence of intermediate forms and shared characteristics between and among the various species, including some that first appeared well before the end of the Eifelian, implies that they may be conspecific ecotypic variants at this level. New morphotypes within this group are treated informally. The greatest overall diversity occurs just above the basal Plum Brook, in strata interpreted to reflect inner-sublittoral conditions. The entire Plum Brook belongs to the upper ensensis Zone. A considerable part of the type Givetian placed in the Lower varcus Subzone by Bultynck (1987) lies below the lowest occurrence of its index species (P. timorensis); it contains some of the species found in the Plum Brook, and much of this section may also belong to the ensensis Zone.
On the age of the Hungry Hollow Formation
- Dale R. Sparling
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- Journal:
- Journal of Paleontology / Volume 66 / Issue 2 / March 1992
- Published online by Cambridge University Press:
- 14 July 2015, p. 339
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In the paper by J. Keith Rigby (1991) in the January issue of the Journal, the Hungry Hollow Formation of southwestern Ontario is correlated with the basal Ludlowvillian Centerfield Limestone of New York. The new species of heteractinid sponge described in the paper was thus considered to be older than the designated type species of his new genus Gondekia, which is from the middle Ludlowvillian Wanakah Shale. The correlation was referred to as that of Rickard (1984) and was consistent with others going back several decades. It was also included in subsequent correlations based on conodont biostratigraphy (Johnson et al., 1985; Sparling, 1985) that are otherwise largely at odds with Rickard's conclusions. However, Brett and Baird (1985) provided convincing evidence that the Centerfield is a regressive unit between shales representing deeper water sedimentation, whereas the Hungry Hollow appears to be the basal unit of a transgressive sequence of strata and thus unlikely to be of the same age. Evidence for this was supplied by Landing and Brett (1987), who described a previously unrecognized disconformity beneath the Hungry Hollow and noted its regional significance. They also reported finding a single specimen of the conodont Polygnathus timorensis in the Hungry Hollow. This is the guide fossil for the Lower varcus Subzone, to which the Centerfield has been assigned, so placement of both units in at least the same conodont subzone was justified at that time.