A STUDY OF THE ONTOGENY OF THE MISSISSIPPIAN BLASTOID TRICOELOCRINUS AND IT'S RELATIONSHIP TO METABLASTUS

Tricoelocrinus is an uncommon blastoid found in mid-continent Mississippian (Meramecian) age formations. Members of the genus grow to relatively large sizes and have one of the most dramatically varied ontogenies recorded in the blastoid record. Early workers assigned species based on the size and shape of the theca. Some species may actually be artificial constructs, based on ontogenetic stages. Furthermore, the genus Tricoelocrinus appears to be derived from the blastoid genus Metablastus, through the process of heterochrony.

In order to investigate species relationships among members of the genus Tricoelocrinus, as well as the relationship between Tricoelocrinus and Metablastus, an analysis of the ontogeny of both groups was undertaken. The ontogeny of these groups has been little studied, and never quantified using modern morphometric methods. A growth series of at least 15 specimens of the rare blastoid Tricoelocrinus woodmani was used. Specimens ranged in size from 9 mm to 63 mm. The specimens were digitized, and analyzed using the morphometric technique Procrustes Method. Procrustes Method is a least squares method that attempts to resolve objects of similar shape regardless of size by translating, scaling, and rotating the objects to achieve the best fit. The generalized Procrustes Method of superimposition generates an ‘average' shape from all the input shapes and then offers residuals between the ‘average' shape and the original input shapes. This method is well suited for analyzing the growth trajectories among blastoid species.

The definition of the parameters of this ontogeny may allow the synonymy of some Tricoelocrinus species. Furthermore, the analysis has also provided additional evidence suggesting that Tricoelocrinus was derived from the blastoid genus Metablastus, through heterochronic process of peramorphosis.