Pikaia

Australia: The Land Where Time Began

Pikaia

Pikaia was found in the the Burgess Shale in British Columbia, Canada, of Middle Cambrian age. Several features of Pikaia give indications of its affinity to chordates, though it has the superficial appearance of a worm. It appears to have cartilage rods supporting what is believed to be a tail fin and appears to have a notochord. In its body form and its overall anatomy it seems similar to lancelets of the present, though at the time of writing it has not been studied in detail, resulting its evolutionary position remaining enigmatic.

The discovery of this animal demonstrates that animals similar to Pikaia had evolved by at least 525 Ma. Though it could be an early, possibly basal, deuterostome (an organism with 2 openings, that includes all chordates, as well as 3 other phyla) it was previously classified as a cephalochordate. Pikaia is another fossil that has been referred to as "a squashed slug" kind that has been reported to have a notochord.

Long reports that after examining all the main fossils of this animal Philippe Janvier of Paris has said he cannot confirm that any of them had any chordate features, suggesting Pikaia could have been closely related to Yunnanozoon.

Pikaia gracilens (Walcott) is a chordate from the Cambrian but no detailed description of it has been available prior to the study by Morris & Caron (Morris & Caron, 2012)³. According to the authors³ they studied 114 specimens that were available, concluding that it was indeed a chordate, and went on to explore its relationships to chordates, extinct and extant, as well as other deuterostomes. Pikaia has a fusiform body that is laterally compressed and has about 100 myomeres. The bilobed small head has 2 narrow tentacles and there is no evidence of eyes, and with the exception of a thin dorsal fin, with no fin rays, and a series of appendages that are bilaterally arranged with possible pharyngeal pores at the anterior end, and there are no other external features. As well as the musculature the internal anatomy includes an alimentary canal that has a prominent lenticular unit that is present almost always preserved in positive relief. The mouth is implied to be terminal as the cavity is interpreted as pharyngeal. The anus appears to be terminal, though the posterior extension of the gut is unclear. A reflectively preserved unit is the most prominent structure, that they suggest is possibly hollow, that has been termed the dorsal organ. It was formerly interpreted as a notochord, they suggest it is less likely to be a notochord because of its position and size. They suggest it could have formed a storage organ, though its original function is uncertain. There is a narrow stand of tissue, interpreted a being the nerve chord and notochord, that is ventral to the dorsal organ. The authors³ also suggest there is evidence of a vascular system, that includes a ventral blood vessel.

The authors³ say the presence of sigmoidal myomeres is largely the basis for the position of Pikaia in the chordates, and in many other respects the more tentative identification of a notochord, Pikaia differs from what is expected for the nature of primitive chordates, as occurs in such animals as amphioxus and the fossil record of the Cambrian, such as Cathaymyrus, Haikouichthys, Metaspriggina, Myllokunmingia, and Zhongxiniscus. The authors³ say they prefer a scenario regarding Pikaia as the most stemward of the chordates, linked to the yunnanozoans, that are controversial, though there is the possibility that Pikaia is convergent on the chordates. There are implications for evolution of notochord, gills and myomeres. The wealth of Pikaia material indicates that the consistency of preservational details allowed a reconstruction of the anatomy that is reliable, and does not significantly erode phylogenetically relevant characters, though by definition there must be some sort of taphonomic imprint.