Meeting Abstract
The agnathan/gnathostome (jawless vertebrate/jawed vertebrate) transition represents one of the most significant faunal turnovers in the earth’s history. From the Silurian period (444 million years ago) through the Devonian period (359 mya) vertebrate faunas transitioned from diverse groups of agnathans to faunas dominated almost entirely by gnathostomes. Despite the significance of this transition, the circumstances that drove gnathostome dominance are unclear. Competition between gnathostomes and agnathans is the most commonly proposed hypothesis; however, this hypothesis has remained largely untested. When two groups are in competition, there should be at least some overlap in niche. Greater overlap is associated with greater competition. Body form can act as a proxy of niche, where differences in body form can correspond with differences in habitat use. Gnathostomes with similar body form to agnathans would be consistent with hypotheses of competition, while taxa that have different body forms would not be consistent with a hypothesis of competition. Our dataset consists of distance based measurements for numerous complete or nearly complete body fossils of early vertebrates. In a preliminary principal coordinate analysis we looked for similarities and differences in body form between Silurian and Devonian agnathan and gnathostome taxa. Gnathostome and agnathans body forms show considerable separation by fin position or presence and shape of the head and armor; however, antiarch placoderms (gnathostomes) overlap with osteostracans (agnathans) along these morphospace axes. Future analyses will investigate the amount of morphological separation necessary to reject competition between these taxa.
