The central goal of evolutionary morphology is the explanation of form, behavior, and adaptation in the context of phylogenetic constraints and possibilities. The most direct way to do this is to make comparative studies within well-defined, monophyletic lineages. My long-term goal is to better understand the origin and subsequent radiation of bony fishes, one of the key events in vertebrate history. To do this, my laboratory is involved in a variety of intensive studies of extant lineages of phylogenetically primitive fishes (e.g., elasmobranchs, holocephalans, coelacanths, lungfishes, polypterids, sturgeons, paddlefishes, and bowfins) combined with studies of fossil representatives. Within each lineage, we employ synthetic approaches in order to develop a more complete evolutionary picture. For example, recent studies of paddlefishes have combined analyses of developmental series, phylogenetic reconstruction, comparison of fossil and recent taxa, and functional studies of living animals combining video and advanced physiological methods. Typically, we focus on cranial anatomy, usually the feeding and sensory systems, because these systems are of great functional and phylogenetic significance and often yield evolutionary hypotheses that stimulate the next round of research.
William E. Bemis
William E. Bemis
Evolutionary Morphology of
Fishes and Amphibians
Fishes and Amphibians
My main ties with the Program in Neuroscience and Behavior stem from the wealth of behavioral and neuroanatomical information that still needs to be collected for these lineages.
For example, our research has shown that jawed vertebrates have independently evolved several different muscular systems to open the lower jaw. In some cases, muscles that normally act in jaw closing have been converted during evolution to act in jaw opening. This finding implies a corresponding phylogenetic change in the generation of muscle activity patterns, which is relevant to understanding the evolution of motor patterning. A second point is the role which electroreception and other lateralis senses have played in the early evolution of fishes. Paddlefishes are ideal for such studies because they possess the largest number of electroreceptive ampullary organs of any vertebrate. An ongoing study of the development of their electroreception system documents the developmental similarities and differences among all of the acousticolateralis senses (hearing, lateral line sense, and electroreception)
Like other vertebrates, the receptors and nerves of the acousticolateralis senses develop from a series of placodes on the side of the head, but the receptors of the electrosensory system are the last of the three sensory systems to differentiate. This work also suggests that there may be major differences in the developmental potential of pre- and post-otic placodes. A third area where new neuroanatomical insight is being generated from the study of phylogenetically primitive fishes concerns the innervation of the basicranial muscle in the coelacanth, Latimeria chalumnae. This study is based on a serially sectioned embryo, and it shows that the basicranial muscle is innervated by the VI nerve (n. abducens), not the Xth nerve as previously reported. This means that the basicranial muscle must develop from the same pre-otic somite as the lateral rectus muscle, a finding that is significant from the standpoint of understanding the segmental organization of the vertebrate head. It also suggests that the basicranial muscle may be the homologue of the retractor bulbi muscle in amphibians. Such studies can contribute much to the analysis of large-scale evolutionary patterns of fishes and amphibians and they form a natural avenue for the next many years of research.
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