Roger K. Pabian

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.

Growth stages of the middle and late Pennsylvanian trilobite Ditomopyge scitula from the North American midcontinent

Larval forms of the middle and late Pennsylvanian trilobite Ditomopyge scitula (Meek and Worthen, 1865) have been collected from seven localities in the Desmoinesian of Oklahoma, Missourian of Oklahoma and Texas, and Virgilian of Nebraska and Texas. These larval trilobites have been interpreted as degree 1 through degree 8 meraspides. Although these larval forms are extremely rare, an essentially complete growth sequence was reconstructed and an ontogeny synthesized. During growth, D. scitula absorbed the brim and posterior spines into the pygidium and developed first the lateral then the median preoccipital glabellar lobes; the anterior cephalic margin is more prominent in juvenile stages, becoming narrower in late mersapide molts and finally overlapped by the glabella in early holaspide stages.