Pikaia gracilens is an extinct basal chordate from the Middle Cambrian period (~505 Mya). It grew up to 50 mm in length and it represents an important stepping stone in the evolution of modern vertebrates.
First discovered by Charles Walcott in the famous Burgess Shale there has been much debate about Pikaia's taxonomic classification – in particular its relationship to the vertebrate clade. Once thought to be the oldest known chordate and the ancestor to modernvertebrates - Pikaia is now considered to belong to a stem-group of chordates. Even if Pikaia is not the direct ancestor of modern vertebrates a creature with many of the same anatomical features almost certainly was.
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. This is another important stage of evolutionary development that can be associated with Pikaia gracilens.
The internalised vertebrate skeleton that evolved from the notochord eventually facilitated a transition from water onto land. Vertebrates weren’t the first group to do this as there are indications that arthropods were already moving onto land as early as the Silurian period. The internal bone and muscle configurations of vertebrate morphology provides far greater leverage efficiencies than the limited jointed limbs of arthropods. These efficiencies enabled larger, denser brained creatures to begin radiating across the Earth.
Vauxia gracilens is an extinct sponge-like organism with a distinctive branching morphology. Vauxia lived on the Cambrian seabed in a sessile manner occupying the same environmental niche as modern sponges. Although it lacks the siliceous spicules of modern sponges it is thought to have filtered nutrients in much the same way - by passing water through fibrous spongin walls.
With thanks to Dr Jean-Bernard Caron for feedback with anatomical detail.