CC211: Invertebrate-vertebrate transition

Claim CC211:

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Response:

1. There are Cambrian fossils transitional between vertebrate and invertebrate:
   1. Pikaia, an early invertebrate chordate. It was at first interpreted as a segmented worm until a reanalysis showed it had a notochord.
   2. Yunnanozoon, an early chordate.
   3. Haikouella, a chordate similar to Yunnanozoon, but with additional traits, such as a heart and a relatively larger brain (Chen et al. 1999).
   4. Conodont animals had bony teeth, but the rest of their body was soft. They also had a notochord (Briggs et al. 1983; Sansom et al. 1992).
   5. Cathaymyrus diadexus, the oldest known chordate (535 million years old; Shu et al. 1996).
   6. Myllokunmingia and Haikouichthys, two early vertebrates that still lack a clear head and bony skeletons and teeth. They differ from earlier invertebrate chordates in having a zigzag arrangement of segmented muscles, and their gill arrangement is more complex than a simple slit (Monastersky 1999).
   
   
2. There are living invertebrate chordates (Branchiostoma [Amphioxus], urochordates [tunicates]) and living basal near-vertebrates (hagfish, lampreys) that show plausible intermediate forms.

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