Pikaia: One of the Earliest Chordates

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Pikaiais one of the most celebrated of the Burgess Shale fossils due to its status as one of the earliest chordate (or close to chordate) animals. Originally described as an annelid, it was then reinterpreted as a (stem) cephalochordate by Simon Conway Morris when interest in the Burgess Shale rebounded in the 1970s. Alternative, not highly-supported, interpretations include that it’s more basal than cephalochordates (Simonetta et al., 1999), a protostome (Butterfield, 1990), or a hagfish (Janvier, 1997). Take a look for yourself with the fossils above, from Conway Morris & Caron (2012), by far the most comprehensive review of Pikaia. Read it for any information you want (PDF available on request); I didn’t make this post a full profile of Pikaia for fear of inadvertently plagiarising it! Once you’re done considering, read on.

Pikaia‘s general appearance resembles that of modern-day lancelets, as drawn above (Conway Morris & Caron, 2012), a resemblance reinforced by its small size and fins. Pikaia was ~5cm long and had a collagenous body wall, preserved in the Burgess Shale fossils as a silvery film. A dorsal thread can be seen running along the body, flanked by putative V-shaped muscle blocks; this is interpreted as a notochord or even a combination nerve- and notochord. This notochord and the muscles are key to Pikaia‘s positioning as an early chordate. A pharynx (i.e. a mouth) has been suggested at the anterior end of the animal, based on concentrations of sediment at the interior of the animals there (Shu et al., 1996); if accepted, this further solidifies a chordate interpretation. No eyes have been discovered in any specimen.

Two features separate it from known modern chordates, though. It has a strange organ running down the dorsal trunk, shaped like a sausage and with a possible stabilisatory function (Conway Morris & Caron, 2012). Chordates use the notochord for this, but Pikaia‘s notochord may not have been up to the task yet. It also has a so-called dorsal unit, a sort of head shield. It also has a couple of tentacles on its head, although these can be interpreted as gills. These features are probably apomorphic for Pikaia – after all, just because it’s in the “stem group”, it doesn’t mean it can’t have its own evolutionary quirks (this is a point that needs to be stressed to any beginners in phylogenetics!).

In summary, Pikaia is considered to be a stem-group chordate, probably a derived cephalochordate, based on the presence of a notochord, mytomes (muscles) flanking it, gill slits, and the pharynx. Note: unlike what is written in most popular science articles, Pikaia is not a direct vertebrate ancestor.

It was most probably a suspension or deposit feeder swimming like a blind eel near the benthos, although a burrowing lifestyle can’t be discounted (Holland & Chen, 2001).

If you’re interested in other Cambrian chordates and chordate-likes, check out Yunnanozoon, Cathaymyrus, Haikouichthys, Metaspriggina, and mylokunmingiids.

References:

Butterfield NJ. 1990. Organic preservation of non-mineralizing organisms and the taphonomy of the Burgess Shale. Paleobiology 16, 272-286.

Conway Morris S & Caron J-B. 2012. Pikaia gracilens Walcott, a stem-group chordate from the Middle Cambrian of British Columbia. Biological Reviews 87, 480-512.

Holland ND & Chen JY. 2001. Origin and early evolution of the vertebrates: new insights from advances in molecular biology, anatomy, and palaeontology. Bioessays 23, 142-151.

Janvier P. 1997. The vertebrates before the Silurian. Geobios 30, 931-950.

Shu D-G, Conway Morris S & Zhang X-L. 1996. A Pikaia-like chordate from the Lower Cambrian of China. Nature 384, 157-158.

Simonetta AM, Pucci A & Dzik J. 1999. Hypotheses on the origin and early evolution of chordates in the light of recent zoological and palaeontological evidence. Italian Journal of Zoology 66, 99-119.