Morphological Analysis of the Blastoid Taxa Metablastus Wortheni and Tricoelocrinus Woodmani with Emphasis on Taxonomic Implications

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Metablastus wortheni and Tricoelocrinus woodmani are relatively uncommon blastoids from North America stretching from the early to the middle Mississippian age. These taxa, along with other blastoids, serve as important indicators of the ancient benthic marine community and can facilitate modern hypotheses relating to paleo environment and evolution. Central to these inferences, however, is species recognition. Historically, these taxa were both assigned to the genus Pentremites (currently recognized as genus Metablastus). These taxa exhibit distinct morphological variation in their developmental growth progressions, yet, this variation appears to relate to overall size rather than taxonomic identity. This becomes problematic at smaller sizes, as the two taxa are notoriously difficult to separate, with samples and identifications relying on the basis of where each occurs in the fossil record. Those found in earlier Warsaw Formation have usually been assigned to M. wortheni while those found in Salem Formation have been assigned as to T. woodmani. Morphologically, M. woodmani juveniles and adults have a narrow elongate theca, typical of many troosticrinids, while Tricoelocrinus maintains a similar profile to the typical Metablastus as juveniles, but member species diverge as adults where they reach appreciably larger sizes than M. woodmani. The objective of this study was to assess the hypothesis that T. woodmani exhibits shared ontogenic morphological characteristics with M. wortheni and only diverges morphologically from this species in larger size categories as a result of allometry. A representative sample of M. wortheni and T. woodmani specimens comprised of a complete growth series for both taxa were analyzed using geometric morphometric techniques as well as linear measurements from both longitudinal and summit orientations. General linear models indicate strong allometric relationships between shape and overall size (estimated by length of theca). Significant differences between species were found to become evident as theca size exceeded 20 mm, whereby T. woodmani consistently developed significant differences in morphology of longitudinal and summit profiles compared with M. wortheni of similar size and T. woodmani under 20 mm in theca length. We suggest this shared morphology at smaller size classes preceding shape divergence in T. woodmani is a result of heterochrony among sister taxa and that these two taxa represent a chronospecies rather than two distinct genera that have converged on an identical juvenile morphology. [ABSTRACT FROM AUTHOR]

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