From Evolution: The Whole Story, Thames & Hudson, Steve Parker, p.111, some data points:
- acorn worms go back all the way to the middle Cambrian, 505 million years ago
- as a hemichordate, the acorn worm is a Deuterostome
- broadly speaking:
- protostomes: invertebrates; all living invertebrates belong to Protostomia
- deuterostomes: vertebrates; some invertebrates (echinoderms: starfish and sea urchins) belong to Deuterostomia; deuterstomes are all chordates + the invertebrate echinoderms; also include lancelets and tunicates
- among extant species, there are a gazillion protostomes compare the paltry number of deuterostomes
- so acorn worms, extending back to the middle Cambrian, are deuterostomes, perhaps starting the long lineage all the way to humans
Internet links:
- most important, "Life Before Dinosaurs"
- google echinoderms
- google starfish
- google acorn worms
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