Middle Cambrian fossil Pikaia and the evolution of chordate swimming
Date
2012-06-13
Authors
Lacalli, Thurston
Journal Title
Journal ISSN
Volume Title
Publisher
BioMed Central
Abstract
Conway Morris and Caron (2012) have recently published an account of virtually all the available information on Pikaia
gracilens, a well-known Cambrian fossil and supposed basal chordate, and propose on this basis some new ideas about
Pikaia’s anatomy and evolutionary significance. Chief among its chordate-like features are the putative myomeres, a
regular series of vertical bands that extends the length of the body. These differ from the myomeres of living chordates
in that boundaries between them (the myosepta) are gently curved, with minimal overlap, whereas amphioxus and
vertebrates have strongly overlapping V- and W-shaped myomeres. The implication, on biomechanical grounds, is that
myomeres in Pikaia exerted much less tension on the myosepta, so the animal would have been incapable of
swimming as rapidly as living chordates operating in the fast-twitch mode used for escape and attack. Pikaia either
lacked the fast-twitch fibers necessary for such speeds, having instead only slow-twitch fibers, or it had an ancestral
fiber type with functional capabilities more like modern slow fibers than fast ones. The first option is supported by the
sequence of development in zebrafish, where both myoseptum formation and fast fiber deployment show a
dependence on slow fibers, which develop first. For Pikaia, the absence of fast fibers has both behavioral and
anatomical implications, which are discussed. Among the latter is the possibility that a notochord may not have been
needed as a primary stiffening device if other structures (for example, the dorsal organ) could perform that role.
Description
BioMed Central
Keywords
Citation
Lacalli: The Middle Cambrian fossil Pikaia and the evolution of chordate swimming. EvoDevo 2012 3:12