Pikaiidae
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Pikaia Temporal range: Middle Cambrian |
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| Life reconstruction of Pikaia gracilens | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Family: | †Pikaiidae |
| Genus: |
†Pikaia Walcott, 1911 |
| Species: | †P. gracilens |
| Binomial name | |
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Pikaia gracilens Walcott, 1911 |
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Pikaia gracilens is an extinct cephalochordate animal known from the Middle Cambrian Burgess Shale of British Columbia. Sixteen specimens are known from the Greater Phyllopod bed, where they comprised 0.03% of the community. It resembled the lancelet and perhaps swam much like an eel.
Pikaia is a primitive chordate that lacked a well-defined head and averaged about 1 1⁄2 inches (3.8 cm) in length. Because it was once thought to be closely related to the ancestor of all vertebrates; as such, it receives otherwise-inordinate attention among the multitude of animal fossils found in the famous Burgess Shale in the mountains of British Columbia, Canada. Pikaia had a pair of large, antenna-like tentacles on its head, and a series of short appendages, which may be linked to gill slits, on either side of its head. In these ways, it differs from the living lancelet. The "tentacles" on its head may be comparable to those in the present-day hagfish, a jawless chordate.
Although primitive, Pikaia shows the essential prerequisites for vertebrates. When alive, Pikaia was a compressed, leaf-shaped animal with an expanded tail fin; the flattened body is divided into pairs of segmented muscle blocks, seen as faint vertical lines. The muscles lie on either side of a flexible structure resembling a rod that runs from the tip of the head to the tip of the tail. It likely swam by throwing its body into a series of S-shaped, zigzag curves, similar to the movement of eels; fish inherited the same swimming movement, but they generally have stiffer backbones. These adaptations may have allowed Pikaia to filter particles from the water as it swam along.Pikaia was probably a slow swimmer, since it lacked the fast-twitch fibers that are associated with rapid swimming in modern chordates.
Conway Morris and Caron (2012) published an exhaustive description based on all 114 of the known fossil specimens; they discovered new and unexpected characteristics that they recognized as primitive features of the first chordate animals. On the basis of these findings, they constructed a new scenario for chordate evolution. Subsequently, Mallatt and Holland reconsidered Conway Morris and Caron's description, and concluded that many of the newly recognized characters are unique, already-divergent specializations that would not be helpful for establishing Pikaia as a basal chordate.
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