Pikaia gracilens is an extinct basal chordate from the Middle Cambrian (~505 million years ago) that grew to approximately 50 mm in length.
First discovered by Charles Walcott in the Burgess Shale of Canada, Pikaia has long occupied an important position in discussions surrounding early vertebrate evolution. Once regarded as the oldest known chordate and the direct ancestor of all vertebrates, it is now more commonly interpreted as part of a stem lineage closely related to the ancestors of modern vertebrates.
Pikaia's most defining feature is the putative notochord that runs along the head-tail axis of its body. The notochord is a rod-like flexible organ that, among other roles, acts as an anchor for muscles within an organism which in turn facilitates movement. Over many generations the notochord was to become encased within the calcified bones of a vertebral column – the defining feature of all vertebrates.
Another interesting feature of Pikaia are the distinctive ‘S’ shaped muscle arrays, or myomeres, that run along the length of the body almost perpendicular to the notochord (Conway Morris & Caron, 2012). The structure of these tissues is suggestive of the more complex muscle arrays of later vertebrates, strengthening the likelihood of an evolutionary connection.
Of further interest are the signs of cephalization that Pikaia displays - it has a well-defined head with sensory tentacles in close proximity to a possible pharyngeal gill array.
The evolution of an internal skeletal framework from the ancestral notochord eventually provided vertebrates with greater structural support and muscular leverage, facilitating the later colonisation of terrestrial environments. Arthropods appear to have reached land earlier, possibly during the Silurian period, but vertebrate skeletal mechanics ultimately supported the evolution of larger-bodied terrestrial organisms with increasingly complex locomotor and sensory systems.
With thanks to Dr Jean-Bernard Caron for feedback with anatomical detail.
