Middle Cambrian Pikaia enters the LRT

Pikaia gracilens (Fig. 1) is an extinct, primitive chordate animal known from the Middle Cambrian Burgess Shale of British Columbia. Mallatt and Holland 2013 mistakenly located the anus at the very end of the hagfish-like body, opposite the end with the sensory tentacles.

The Royal Ontario Museum website reports,
“Pikaia is considered to represent a primitive chordate (Conway Morris, 1979; Conway Morris et al., 1982) possibly close to craniates (Janvier, 1998); a stem-chordate (Smith et al., 2001); or a cephalochordate (Shu et al., 1999). Its exact position within the chordates is still uncertain and this animal awaits a full redescription.”

Figure 1. Pikaia diagram and in situ specimens from Mallatt and Holland 2013. Here a post-anal tail is identified (red arrow), distinct from the diagram (red x).

Here
in the large reptile tree (LRT, 1820+ taxa; subset Fig. 2) Pikaia nests with the lancelet, Branchiostoma, basal to the hagfish, Myxine. Lancelets are chordate ancestors to hemichordates and echinoderms, as briefly described here and here. Pikaia is closer to hagfish, Myxine, and the rest of the craniates, vertebrates and gnathostomes.

Mallatt and Holland 2013 wrote:
“We deduce that Pikaia, not amphioxus, is specialized. We performed a cladistic analysis that showed the character interpretations of CMC are consistent with their wide‐ranging evolutionary scenario, but that these interpretations leave unresolved the position of Pikaia within chordates.”

The LRT sees things differently.
Pikaia is a suitable transitional taxon between the nematode Enoplus (a taxon omitted or ignored by Mallatt and Holland and all other prior workers) and higher chordates. The term ‘specialized’ can only be applied to amphioxus (= lancelets, = Branchiostoma) which has the specialized atrium not found in other chordates, but retained by all hemichordates and echinoderms. In addition, lancelets have a specialized semi-sessile adulthood with their tail buried in the substrate while filter feeding. No other chordates are semi-sessile, but tunicates and crinoids are sessile and filter feed.

The ROM web page on Pikaia reports,
“Walcott placed Pikaia in a now defunct group called the Gephyrea with other vermiform fossils such as Banffia, Ottoia and Oesia. Pikaia was later considered to be a primitive chordate (Conway Morris, 1979; Conway Morris et al., 1982), an interpretation which has since been followed to some degree in most discussions about early chordate evolution (e.g., Janvier, 1998). Pikaia played a major part in Gould’s interpretations of the Burgess Shale fossils in Wonderful Life (Gould, 1989; see also Briggs and Fortey, 2005). A full redescription of this animal is currently under way (Conway Morris and Caron, in prep.).”

“Pikaia is relatively rare, known from more than 60 specimens, all from the Walcott Quarry where it represents 0.03% of the specimens counted in the community (Caron and Jackson, 2008). Maximum size: 55 mm. The eel-like morphology and musculature of the animal suggest that it was likely free-swimming, although it probably spent time on the sea floor. The tentacles may have had a sensory function, and the presence of mud in its gut suggests that Pikaia was potentially a deposit feeder.”

Future reversals
Several times in vertebrate history eel-like forms re-appeared. Think of the moray eel, Gymnothorax, for instance. This morphology is always in the gene pool, going back to Pikaia.


References
Conway Morris S 1979. The Burgess Shale (Middle Cambrian) fauna. Annual Review of Ecology and Systematics, 10(1): 327-349.
Conway Morris SHB, et al. (4 co-authors) 1982. Atlas of the Burgess Shale. Palaeontological Association, 31 p. + 23 pl.
Mallatt J and Holland N 2013. Pikaia gracilens Walcott: Stem Chordate, or Already Specialized in the Cambrian? Journal of Experimental Zoology Molecular and Developmental Evolution 320(4):247–271.
Walcott CD 1911. Cambrian geology and paleontology. Middle Cambrian annelids. Smithsonian Miscellaneous Collections 57(5):109–144.

ROM web page devoted to Pikaia

wiki/Pikaia

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