One post is about Pikaia, a very important fossil from the Burgess Shale:
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.