Meeting Abstract
The ontogeny of lampreys holds a special place in the historical development of ideas about the early evolution of vertebrates. Ammocoete larvae of living lampreys (cephalochordate-like, sand-burrowing filter feeders) have served as a model for primitive vertebrates, whereas the eel-like, typically blood-sucking adults are considered specific to that lineage. This recapitulatory view of lamprey ontogeny has long predicted the last common ancestor of all living vertebrates to be an ammocoete-like filter feeder. If the Ammocoete Model is correct, a filter-feeding larval stage should have existed in lamprey stem taxa. We test this prediction using newly discovered specimens of Priscomyzon riniensis from the Devonian Witpoort Formation of South Africa. Seven specimens form an ontogenetic series from a 15 mm-long individual (slightly larger than living lamprey hatchlings) to an adult. None of these specimens has any skeletal correlates of filter feeding. Instead, traits associated with the predatory life mode of living adult lampreys are present in all of specimens, including: prominent eyes; oral sucker; keratinous teeth; tectal cartilages; short branchial region; and pericardial closure of branchial basket. Therefore, Priscomyzon likely had no ammocoate stage. To test whether the absence of the ammocoete stage is (a) a tip state unique to Priscomyzon or (b) a general condition of the lamprey stem, we compared three other Paleozoic stem lampreys (Hardistiella, Mayomyzon, and Pipiscius) with the Priscomyzon series. The smallest specimens of each taxon closely resemble the late larva to post-metamorphic juvenile stages in Priscomyzon, which implies that the ammocoete larval stage of living lampreys represents a secondarily evolved condition, convergent with cephalochordates due to feeding habits.
