REVISED PHYLOGENETIC HYPOTHESIS OF BLASTOIDEA (ECHINODERMATA)

Phylogenetic inference of extinct forms relies heavily on external character data. Consequently, missing character data is a significant issue when utilizing fossil taxa. Including taxa with sparse character data can result in instability within tree topology or incorrect placement of taxa within the consensus tree. Conversely, some taxa with a large number of missing characters can still influence tree topology in positive ways because they may contain unique combinations of character states that group taxa. Many studies have examined the relationships among previously ascribed blastoid groupings by examining thecal morphology but often include a limited amount of taxa and character data.

This study of blastoid phylogeny incorporates approximately 40 ingroup taxa and 143 informative character states. All previously utilized thecal characters were reevaluated and revised to better characterize external morphology. New character data was included that discriminated specific aspects of morphology (e.g., ambulacra and anal area plating). We inspected changes in tree stability by including and excluding taxa and characters with a high degree of missing data, to identify areas of varying stability. In addition, a new suite of character data has been generated that describes the internal anatomy more fully, especially the respiratory structures (hydrospires) by generating 3-D anatomical models from serial sections of thecae. The majority of species, for which have internal anatomical models fit into a large grouping of spiraculates. We incorporated the hydrospire data into a subsampled dataset of the spiraculate grouping to further examine tree stability at a finer scale.

Major evolutionary relationships remain stable with the inclusion or exclusion of partial data; this includes a polyphyletic spiraculate grouping within the larger fissiculate clade. Minor relationships either collapsed or were more fully resolved. This improved phylogenetic hypothesis provides a sufficient framework to begin testing evolutionary questions on clade evolution.