Examining competition during the agnathan/gnathostome transition using distance-based morphometrics

The rise of jawed vertebrates (gnathostomes) and extinction of nearly all jawless vertebrates (agnathans) is one of the most important transitions in vertebrate evolution, but the causes are poorly understood. Competition between agnathans and gnathostomes during the Devonian period is the most commonly hypothesized cause; however, no formal attempts to test this hypothesis have been made. Generally, competition between species increases as morphological similarity increases; therefore, this study uses the largest-to-date morphometric comparison of Silurian and Devonian agnathan and gnathostome groups to determine which groups were most and least likely to have competed. Five agnathan groups (Anaspida, Heterostraci, Osteostraci, Thelodonti, and Furcacaudiformes) were compared with five gnathostome groups (Acanthodii, Actinopterygii, Chondrichthyes, Placodermi, Sarcopterygii) including taxa from most major orders. Morphological dissimilarity was measured by Gower’s dissimilarity coefficient, and the differences between agnathan and gnathostome body forms across early vertebrate morphospace were compared using principal coordinate analysis. Our results indicate competition between some agnathans and gnathostomes is plausible, but not all agnathan groups were similar to gnathostomes. Furcacaudiformes (fork-tailed thelodonts) are distinct from other early vertebrate groups and the least likely to have competed with other groups.

Methods
Supplementary table T3: 29 measurements of body form and the size, shape, and position of eyes and fins taken from fossil specimens of Silurian and Devonian vertebrates using calipers and, in rare cases for large specimens, tailor tape. Categorized by Class. Proportions of each measurement relative to standard length (tip of the rostrum to the caudal peduncle) calculated for each specimen. For taxa with multiple specimens, averages were taken. length or leading edge of structures that were absent were recorded as "0", distances (from the rostrum) and missing measurements (from incomplete specimens) were recorded as "-" or not applicable. proportions of 120 taxa used in associated article for this data were z-transformed.

Supplementary file containing additional supplementary tables and figures, with results, are included; as is the r-script and necessary files for calculating Gower's dissimilarity matrix and Principal Coordinate Analysis, as well as correlation coefficients and the coefficient of determination for each variable on the first four axes.