PRELIMINARY REVISION OF HETERASTER (ECHINOIDEA) FROM THE LOWER CRETACEOUS OF TEXAS

Currently, six species of the abundant echinoid genus Heteraster are recognized from the Lower Cretaceous Fredericksburg Group of Texas. However, confusion riddles the literature when distinguishing amongst these species. The apparent similarities amongst these species have led most workers to utilize Heteraster texanus as a “catchall” assignment (Cooke, 1955). Most of this apparent confusion can be attributed to vague species descriptions and poor distinguishing characters. For example, Adkins (1928) differentiates H. adkinsi by citing a “narrower ambulacrum” which is a relative character subject to variation.

In order to determine how many of the species designations are valid, specimens of Heteraster were collected from three different stratigraphic horizons within the Fredericksburg group. These horizons were chosen because the type specimens of many of the Heteraster species were collected there. The present study quantitatively examines 102 specimens of Heteraster for ten morphological characters historically used to differentiate the species. T tests did not show any significant (alpha greater than 0.05) difference in any specimen when compared to every other specimen. Principle Component Analysis (PCA) has shown that there is no distinction between any of the measured specimens. Loadings seem to indicate that position of a specimen on the PCA plot is controlled by test length.

These results indicate that H. adkinsi, H. bohmi, and H. mexicanus should be synonymized with H. texanus. Several other species (H. bosei and H. electus) could not be tested since the type specimens were reported from strata not tested in this study, but are probably also synonyms. Several explanations may explain why this confusion has existed: 1. some of the named species are probably juveniles of H. texanus; 2. ecophenotypic variation may exist as some variability was evident in the data analyzed. Juveniles of H. texanus could superficially be distinct from the adult counterparts, perhaps explaining much of the confusion.

A clearer understanding of the taxonomic relationships between species of Heteraster in the Lower Cretaceous of Texas will yield better biostratigraphic, evolutionary, and paleoecological data.