Meeting Abstract

56.1  Sunday, Jan. 5 13:30  The cranial anatomy of living jawless fishes and the interrelationships of early vertebrates MIYASHITA, T.*; PALMER, A. R.; University of Alberta; University of Alberta tetsuto@ualberta.ca

No other structure better characterizes vertebrates than the head, with its massive skull, highly specialized muscles, and intricate innervations by cranial nerves. Did the first vertebrate possess all of those traits, or did early vertebrates acquire them in steps? Answers may lie in the basal grade of jawless vertebrates. Based on dissections, a histological analysis, and micro-CT imaging of cartilages, muscles, and nerves in hagfish and lampreys, we demonstrate that the biomechanics of the jawless vertebrate head differs from that of jawed vertebrates in two distinct ways: linear muscle antagonism and elastic recoiling of the skeleton. Homologues in the cranial musculature of hagfish and lampreys are established at more than one level of organization. That is, a group of muscles may withstand a test of homology, but individual muscles may or may not be compared. To highlight examples, we emphasize the muscle groups controlled by the trigeminal and facial nerves. After incorporating the nested patterns of homologues in coding of the characters, the cranial musculature neither supports nor rejects the hypothesis that hagfish and lampreys are each other’s sister group. The solution to this problem lies in identifying possible correlates of the cranial muscles in fossil jawless vertebrate lineages. A phylogenetic bracket approach potentially changes the basal vertebrate phylogeny from the modern view of successive acquisition of gnathostome-like characteristics to a highly divergent, mosaic pattern of character evolution among the lineages.