Pikaia

Pikaia gracilens is an extinct, primitive chordate marine animal known from the Middle Cambrian Burgess Shale of British Columbia. Described in 1911 by Charles Doolittle Walcott as an annelid, and in 1979 by Harry B. Whittington and Simon Conway Morris as a chordate, it became "the most famous early chordate fossil", or "famously known as the earliest described Cambrian chordate". It is estimated to have lived during the latter period of the Cambrian explosion. Since its initial discovery, more than a hundred specimens have been recovered. The body structure resembles that of the lancelet and it swam perhaps much like an eel. A notochord and myomeres (segmented blocks of skeletal muscles) span the entire length of the body, and are considered the defining signatures of chordate characters. Its primitive nature is indicated by the body covering, a cuticle, which is characteristic of invertebrates and some protochordates. A reinterpretation in 2024 found evidence of the gut canal, dorsal nerve cord and myomeres, and suggested that the taxon was previously interpreted upside down. The exact phylogenetic position is unclear, though recent studies suggest that it is likely a stem-chordate with crown group traits. Previously proposed affinities include those of cephalochordata, craniata, or a stem-chordate not closely related to any extant lineage. Popularly but incorrectly attributed as an ancestor of all vertebrates, or the oldest fish, or the oldest ancestor of humans, it is generally viewed as a basal chordate alongside other Cambrian chordates; it is a close relative of vertebrate ancestors but it is not an ancestor itself.

Discovery

The fossils of Pikaia gracilens was discovered by Charles Walcott from the Burgess Shale member of the Stephen Formation in British Columbia, and described it in 1911. He named it after Pika Peak, a mountain in Alberta, Canada. Based on the obvious and regular segmentation of the body, as is the feature of annelids, Walcott classified it as a polychaete worm and created a new family Pikaidae for it. (Princeton palaeontologist Benjamin Franklin Howell changed the name of the family to Pikaiidae in 1962.) Walcott was aware of the limitation of his classification, as he noted: "I am unable to place it within any of the families of the Polychaeta, owing to the absence of parapodia [paired protrusions on the sides of polychaete worms] on the body segments back of the fifth."

University of Cambridge palaeontologist Harry B. Whittington and his student Simon Conway Morris re-examined the Burgess Shale fauna and noted the anatomical details of Pikaia for the first time. The fossil specimens bears features of notochord and muscle blocks that are fundamental structures of chordates, and not of annelids. In 1977, Conway Morris presented a paper that indicated the possible chordate position, without further explanation. He and Whittington were convinced that the animal was obviously a chordate, as they wrote in Scientific American in 1979:Finally, we find among the Burgess Shale fauna one of the earliest-known invertebrate representatives of our own conspicuous corner of the animal kingdom: the chordate phylum... The chordates are represented in the Burgess Shale by the genus Pikaia and the single species P. gracilens.Conway Morris formally placed P. gracilens among the chordates in a paper in the Annual Review of Ecology and Systematics that same year. However, he provided no structural analyses such as using microscopes to confirm the chordate features. The comparative description only earned a "putative" chordate status. The fossil's chordate nature was received sceptically for several decades. Only in 2012, when detailed analysis was reported by Conway Morris and Jean-Bernard Caron, that the chordate position became generally accepted. The fossils are found only in a restricted series of horizons in the strata exposed on Fossil Ridge, close to the Yoho National Park. From the same location, other fish-like animal fossils named Metaspriggina were discovered in 1993. Conway Morris identified the animals as another Cambrian chordate. The fossil specimens are preserved in the Smithsonian Institution and the Royal Ontario Museum.

Description Pikaia has a lancelet-like body, tapering at both ends, laterally flat and lacked a well-defined head. It measures an average of about 1+1⁄2 inches (3.8 cm) in length. Walcott recorded the longest individuals as 5 cm (2.0 in) in length. Pikaia has a pair of large, antenna-like tentacles on its head that resembles those of invertebrates such as snails. The attachment of the tentacles makes a two-lobed structure of the head. The tentacles may be comparable to those in the present-day hagfish, a jawless chordate. It has a small circular mouth that could be used to eat small food particles in a single bite. There is a series of short appendages on either side of the underside of the head just after the mouth, and their exact nature or function is unknown. The pharynx is associated with six pairs of slits with tiny filaments that could be used for respiratory apparatus. In these ways, it differs from the modern lancelets, which have distinct pharyngeal gill slits on either sides of the pharynx and are used for filter feeding. A major primitive structure of Pikaia is a cuticle as its body covering. Cuticle is a hard protein layer predominantly found in invertebrates such as arthropods, molluscs, echinoderms and nematodes. Unlike a typical cuticle, the cuticle of Pikaia does not have hard extracellular (exoskeleton) protection, and the entire body is essentially soft-bodied. 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. Pikaia was an active and free swimmer. 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.

Reinterpretations

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