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. 2012 Jul 6;3(1):12.
doi: 10.1186/2041-9139-3-12.

The Middle Cambrian Fossil Pikaia and the Evolution of Chordate Swimming

Free PMC article

The Middle Cambrian Fossil Pikaia and the Evolution of Chordate Swimming

Thurston Lacalli. Evodevo. .
Free PMC article


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.


Figure 1
Figure 1
Pikaia gracilens, as reconstructed by Conway Morris and Caron[1].The head bears a pair of tentacles, probably sensory in nature, and paired rows of ventrolateral projections that may be gills. Not shown: the expanded anterior (pharyngeal) region of the digestive tract, and the dorsal shield-like structure, the anterior dorsal unit, that lies above it. The boxed detail shows the main axial features: the dorsal organ (do), and the putative notochord (not) and digestive tract (dt). The size range among specimens is 1.5 to 6 cm, which makes this animal very close in size to the adult stage of modern lancelets (amphioxus)
Figure 2
Figure 2
The beneficial consequences of having chevron- or V-shaped myomeres, illustrated using amphioxus. (A) The V shape guarantees that the force acting perpendicular to the myoseptum (vector P) is less than the force of contraction (vector F) by an amount that increases with increasing angle (Θ) to the vertical. The degree of incline shown is typical for amphioxus larvae, and increases with increasing age. (B) Somite overlap in young adult amphioxus, modified from [11]. The central components of the locomotory system are the notochord (not, shown in violet) and the nerve cord (green). These are bound to the myomeres (pink) by sheaths of basal lamina (blue). The V-shaped myomeres are positioned so that the tip of the caudal-most in any section is adjacent to the notochord, while the extended tails of progressively more anterior myomeres (shown in progressively lighter shades, compare with A) are ranged above and below. Because every point along the notochord has essentially the same complement of septa, this arrangement ensures that the force of contraction experienced by the notochord is distributed evenly along its anteroposterior axis, rather than being borne at specific sites

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