RELATIONSHIPS BETWEEN TAPHONOMY AND ONTOGENY IN THE ADVANCED CLADID CRINOID APOGRAPHIOCRINUS TYPICALIS FROM THE UPPER PENNSYLVANIAN OF OKLAHOMA

The final taphonomic state of crinoid remains is controlled by a large number of complexly related variables that can be biological, physical, and chemical in nature and can influence crinoid preservation throughout the post-mortem history of the fossil or assemblage under study. Among the most poorly understood of such taphonomic influences is ontogeny, in that variations in morphology and ethology (or other variables) may result in differences in preservation between juveniles and adults belonging to a single taxon. The advanced (poteriocrine) cladid crinoid Apographiocrinus typicalis is the most abundant articulated taxon within the diverse crinoid fauna recovered from the Upper Pennsylvanian Barnsdall Formation near Copan, northeastern Oklahoma. The abundance of A. typicalis from this locality (>300 specimens) permitted construction of a detailed growth series and allowed the taphonomic effects of growth within a single assemblage to be tested. Data collected included specimen completeness, axis of compaction, arm configuration, and radial width (used as the size metric). Small individuals are nearly entirely compressed laterally (perpendicular to the long axis of the crown) and have complete or considerable arms attached to the cup; this is herein termed the erisoid condition. In contrast, large individuals are rarely compressed laterally, instead being compressed obliquely or parallel to the long axis of the crown, and predominantly lack arms or display only proximal brachials; this is herein termed the apographioid condition. The co-occurring advanced cladid Erisocrinus typus did not display a similar pattern, but rather exemplified the erisoid condition throughout ontogeny. Study of specimens preserved in matrix indicate that this reflects an increase in the strength of cup plate sutures and, hence, greater rigidity of the cup with growth. Adult A. typicalis responded to compression through rotation and “shearing off” of arms whereas juveniles responded by flattening laterally, crushing the cup but retaining the arms. The transition from erisoid to apographioid condition throughout growth of this taxon has important implications for taphonomic interpretations of cladid-dominated crinoid assemblages and highlights the role of ontogenetic stage in crinoid preservation.