Chordates form a very diverse phylum with species living all over the planet (1). The extant animals in the phylum include the vertebrates—a familiar group that includes fish, amphibians, reptiles, mammals, and birds—plus less well-known creatures including hagfish (which, together, with vertebrates make up the group Craniata), lancelets, and tunicates(1). A crucial defining feature of chordates is a long, cartilage-like(2) structure called the notochord (1,2), which runs along the central axis of the embryo of all chordates (2) and is important in embryonic development (1,2). While in the more modern chordates, the notochord turns into bone before birth(2), in some ancient vertebrates, such as lampreys and sturgeons, the notochord remains in the body for all of the animals’ lives(2); in the even older chordates such as tunicates and lancelets, which do not have backbones, the notochord remains for part or all of the animals’ lives as well, providing the structural support needed for them to swim(1,2). These creatures, particularly lancelets, are probably related to the oldest chordate fossils ever found, which date back to the Early Cambrian period, some 525 million years ago(1).
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