James R. Thomka

The Utility of Isolated Crinoid Ossicles and Fragmentary Crinoid Remains in Taphonomic and Paleoenvironmental Analysis: An Example from the Upper Pennsylvanian of Oklahoma, United States

Abstract The crinoid fossil record is dominated by isolated ossicles, pluricolumnals, arm segments, and other fragmentary remains resulting from postmortem skeletal disarticulation; however, few studies to date have focused on dissociated crinoid elements in taphonomic and/or paleoenvironmental analysis. A diverse, abundant, and taphonomically variable crinoid fauna recovered from a thin mudstone interval within the Upper Pennsylvanian Barnsdall Formation in northeastern Oklahoma presents a unique opportunity to test the value of incomplete crinoid remains in reconstructing paleoenvironmental conditions and understanding taphonomic patterns. Isolated radial plates were identified to the most precise level possible, commonly genus or species, and used to calculate the minimum number of completely disarticulated individuals; this value was then compared to the number of articulated specimens representing that same taxon to determine the proportion of individuals with cups that have undergone disarticulation into separate ossicles. Cladid taxa are shown to be particularly prone to total disarticulation, with disparid microcrinoids and, somewhat surprisingly, flexibles demonstrating more resistance to disarticulation. Genus-level taphonomic trends among cladid taxa indicate that genera with large but thin cup plates, short anal sacs, and arms capable of adopting a trauma posture are less prone to total disarticulation. Analysis of fragmentary crinoid material recovered from disaggregation of bulk mudstone slabs reveals that thin horizons containing abundant articulated crinoid crowns are enriched in skeletal material and encrusted ossicles relative to thicker subjacent and superjacent intervals, providing further evidence that such horizons represent periods of sediment starvation on the distal shelf that were episodically punctuated by storm events.

Genus-level taphonomic variation within cladid crinoids from the Upper Pennsylvanian Barnsdall Formation, northeastern Oklahoma

Abstract Previous comparative taphonomic studies have convincingly demonstrated that the taphonomic state of crinoid fossils is controlled largely by paleoenvironmental processes and constructional morphology. While taphonomic variability among depositional facies has a long history of investigation, the degree to which preservational heterogeneity is controlled by crinoid morphology has only recently been addressed and only at relatively coarse levels. Most studies to date have focused on taphonomic variability at the subclass or ordinal level, with little documentation of lower level taphonomic variation within a single crinoid subclass. A remarkably diverse, abundant, and well-preserved crinoid fauna, recovered from a single mudstone interval within the Upper Pennsylvanian (Missourian) Barnsdall Formation in northeastern Oklahoma, midcontinent North America, provides a unique opportunity to examine taphonomic trends among poteriocrine cladid crinoids at refined taxonomic levels. Genus-level variations in specimen completeness, axis of compaction, arm position, and features attributed to decay and scavenging are observed within this assemblage and taphonomic trends related primarily to the size of individuals are detected. These results indicate that taphonomic variability extends at least to genus level within the subclass Cladida. Understanding this variability is important in interpreting the genesis and nature of crinoid-bearing units, as minor variations in morphology, ethology, and scavenger preferences impart unexpected biostratinomic heterogeneity to Copan crinoid fauna that would otherwise be difficult to explain. Taphonomic variability at low taxonomic levels and the influence of preferential scavenging should be accounted for in future crinoid taphonomic grade studies, particularly in Pennsylvanian and younger deposits, in order to avoid taphonomic assumptions that may be overly broad.

Taphonomy of diploporite (Echinodermata) holdfasts from a Silurian hardground, southeastern Indiana, United States: palaeoecologic and stratigraphic significance

A microbioherm-bearing hardground within the middle Silurian (Wenlock) Massie Formation near Napoleon, southeastern Indiana, United States is encrusted by the attachment structures of numerous pelmatozoan echinoderms. Among the most common of these holdfasts are multi-plated discoidal structures representing the thecal attachments of diploporite ‘cystoids’. This large population of holdfasts permits the first detailed taphonomic and palaeoecologic study of hardground diploporite attachments, allowing for increased morphological understanding of these rarely studied structures and facilitating identification of holdfasts in deposits where they might have been overlooked or misidentified. The biostratinomic sequence commences with detachment of thecae, followed by weathering of isolated discoidal holdfasts to bring out radiating canal structures and plate sutures, eventually leading to removal of the interior floor to expose the underlying substrate. Continued exposure can result in separation of component holdfast plates, though cementation to the substrate prevents scattering of plates. Diagenetic precipitation of pyrite occurred after burial; the large size of crystals suggests late diagenesis, perhaps seeded by early diagenetic pyrite crystallites produced by decay of ligamentary tissue. Extrinsic taphonomic factors include overgrowth of holdfasts by laminar stenolaemate bryozoans and other echinoderm attachment structures. Diploporite holdfasts are not bored and are absent on microbioherms. Taphonomic data indicate the time-averaged nature of this hardground and its diploporite assemblage and permit prediction of similar occurrences at major flooding surfaces.

Diploporite (Echinodermata, Blastozoa) Thecal Attachment Structures from the Silurian of Southeastern Indiana

Abstract Taxonomic descriptions of diploporites from the middle Silurian of eastern Laurentia have focused nearly entirely on thecal plating, with minimal description or figuring of attachment structures. A recently discovered hardground surface within the Wenlock-age (Sheinwoodian) Massie Formation that is encrusted by numerous well-preserved pelmatozoan holdfasts, including structures identifiable as diploporite thecal attachments, provides an opportunity to document the morphology of these rarely described structures. Moderately thin-walled structures with a parabolic, depressed central area composed of seven distinct marginal plates with prominent pores appear to represent thecal attachments of the broad-based trematocystinid holocystitid Paulicystis. Thick-walled, steep-sided structures composed of five to seven fused or partially fused plates with less prominent pores, a deep stelar depression, and radiating canals appear to represent thecal attachments of undetermined, possibly holocystinid or pentacystinid holocystitids. All diploporite holdfasts are cemented to fine-grained, well-sorted skeletal substrates on elevated crests of the hardground surface, but are not found on microbioherms.

Anatomy of a compound sequence boundary: a basal Silurian karstic unconformity in the Cincinnati Arch region

The Ordovician–Silurian contact in parts of the Cincinnati Arch region is an irregularly pitted erosion surface reflecting karstic weathering during a lower-mid Silurian (Aeronian) lowstand, superimposed on the Hirnantian–Rhuddanian Cherokee Unconformity. This irregular unconformity is overlain by a unit termed the “golden Brassfield”, which grades laterally into strata identified as the “red Brassfield”, a probable equivalent of the Oldham Limestone to the east–southeast. These units are Aeronian or early Telychian, considerably younger than the Rhuddanian–early Aeronian Brassfield Formation sensu stricto. Preservation of erosional topography, resulting from rapid flooding and burial of karstic surfaces, is typical of unconformities representing moderate durations.

Sequence boundaries and chronostratigraphic gaps in the Llandovery of Ohio and Kentucky: The record of early Silurian paleoceanographic events in east-central North America

New and published data are integrated herein to resolve the age and stratigraphic relationships for problematic strata of the Aeronian and Telychian (Llandovery; Silurian) in Ohio and Kentucky (USA). At least two major depositional sequences were traced along the eastern flank of the Cincinnati Arch; these are separated by a regionally angular unconformity with complex topography. Underlying units are progressively truncated to the northwest while overlying strata change facies, condense, and onlap in the same direction. The basal unit of the upper sequence is the Waco Member of the Alger Shale Formation in Kentucky and southern Ohio and the Dayton Formation in western Ohio. A persistent, positive carbonate carbon isotope (δ13Ccarb) excursion associated with the mid-Telychian Valgu Event is recognized in the upper subunit of the Waco Member; the absence of a comparable signal in the Dayton Formation corroborates interpretations that it is significantly younger. The correlations proposed here can be used to understand the nuanced depositional history and chronostratigraphic completeness of the lower Silurian in eastern North America. This framework can be used to characterize sea-level history and local conditions that prevailed during global paleoenvironmental events.

First report of Silurian crinoid columnals with tetralobate and hexalobate lumen structures

Two specimens of crinoid columnals with previously undocumented lumen morphologies are described from Silurian (Wenlock) strata of the Cincinnati Arch region, eastern-midcontinent North America. An attachment structure with a prominent tetralobate lumen, the first tetralobate lumen in a columnal reported from the Silurian, was recovered from a microbioherm in the Massie Formation of southeastern Indiana. An isolated columnal ossicle with a large hexalobate lumen, the first hexalobate lumen-bearing columnal documented, was recovered from a skeletal dolowackestone in the Lilley Formation of southwestern Ohio. These discoveries significantly increase the disparity of the otherwise morphologically conservative Silurian crinoid-columnal fauna.

The role of preservation on the quantification of morphology and patterns of disparity within Paleozoic echinoderms

Abstract. The loss of information resulting from taphonomic degradation could represent a significant bias in the study of morphological diversity. This potential bias is even more concerning given the uneven effect of taphonomy across taxonomic groups, depositional facies, and stratigraphic successions and in response to secular changes through the Phanerozoic. The effect of taphonomic degradation is examined using character-based morphological data sets describing disparity in Paleozoic crinoids and blastozoans. Characters were sequentially excluded from the analyses following progressive taphonomic loss to determine how morphologic metrics, such as the relative distribution of taxa in morphospace and partial disparity, changed with increasing taphonomic alteration. Blastozoans showed very little change in these metrics with decreasing preservational quality, which is a result of characters that create distance in morphospace being recognizable in isolated plates. The opposite result is present in crinoids as the characters that are important in structuring the morphospace require intact modules (i.e., the calyx) to accurately assess. Temporal and stratigraphic trends produced encouraging results in that patterns could be largely recovered even with exaggerated taphonomic biases. However, certain parts of a stratigraphic sequence should be avoided and morphological outliers could potentially play a larger role through time, though both of these biases can be easily identified and avoided. The methods presented in this study provide a way to assess potential taphonomic biases in character-based studies of morphological diversity.

SIDERITE CONCRETIONS IN THE COPAN CRINOID LAGERSTÄTTE (UPPER PENNSYLVANIAN, OKLAHOMA): IMPLICATIONS FOR INTERPRETING TAPHONOMIC AND DEPOSITIONAL PROCESSES IN MUDSTONE SUCCESSIONS

ABSTRACT A thin interval of bioturbated, fossiliferous mudstone within the middle portion of the Upper Pennsylvanian Barnsdall Formation crops out near Copan, northeastern Oklahoma. Representing slow background sedimentation in an oxygenated distal shelf setting, this exposure has yielded an exceptionally diverse and well-preserved crinoid fauna consisting of over 1200 articulated or partially articulated specimens. The interval is also characterized by abundant siderite concretions, present as four morphologies. Large concretions without a distinct skeletal nucleus are located in thin horizons containing abundant articulated crinoids; these indicate sediment starvation on the shelf. Also in these thin, crinoid-bearing units are extremely localized siderite concretions precipitated around sites where soft tissues would have been volumetrically more abundant within large macrofossils, notably tegmina and proximal arms of articulated crinoids, indicating extremely rapid burial events that episodically punctuated the sediment-starved conditions. Small concretions, generally <60 mm in diameter and commonly precipitated around endobenthic-fossil nuclei, are located in thicker units with few articulated crinoids. These indicate higher sedimentation rates and thicker individual burial events. Sideritized large-diameter burrows with sharply defined walls indicate firmer substrates created by minor erosive events. These are located in the thicker units alongside the smaller concretions and indicate higher-energy storm events than those represented in the thinner, crinoid-bearing units. Collectively, the evidence provided by siderite concretion morphologies and their relationships to macrofossil preservation demonstrate that the Copan crinoid Lagerstätte formed through periods of sediment starvation occasionally interrupted by distal storm events. These periods alternated with intervals of increased sedimentation and more violent storms.

A noteworthy accumulation of disparid crinoids from the type Cincinnatian (Upper Ordovician) of southwestern Ohio, USA: implications for the palaeoecology and taphonomy of crinoid “logjam” assemblages

Crinoids are a common and well-studied faunal component of the Upper Ordovician (Katian; Edenian) Kope Formation in the greater Cincinnati Arch region, USA. However, a relatively fresh outcrop exposing the Southgate and McMicken members of the Kope Formation at Cleves, Hamilton County, southwestern Ohio, has yielded a crinoid specimen worthy of description and comment. The specimen is a “logjam” of numerous articulated columns of Iocrinus subcrassus displaying parallel alignment, reflecting the influence of storm-generated currents. Iocrinus is not typically found in such a state; the genera Ectenocrinus and Cincinnaticrinus are generally associated with “logjams” in the type Cincinnatian, making this an unusual occurrence. At least one of the columns has the coiled dististele of another, smaller I. subcrassus tightly wrapped around it. Although I. subcrassus is known to employ a coiled dististele as an attachment strategy, ramose bryozoans are generally utilised as substrates and tight coiling around larger I. subcrassus columns has not previously been reported. Preservation of coiled dististeles, in general, is a feature not previously documented in Cincinnatian crinoid “logjams.” This specimen illustrates that unusual, noteworthy and/or rare material representing relatively common organisms continue to be discovered even within extensively studied units in the type Cincinnatian.