Soft-bodied worms from the Burgess Shale fossil beds in Yoho National Park, British Columbia, Canada have been known for over 100 years. They are known by the name Spartobranchus tenuis and are considered one of the most abundant species in the Walcott Quarry community. However, only recently has a detailed examination of their characteristics been made, leading to the conclusion that they are ancient examples of acorn worms. One member of the research team was Christopher Cameron from the University of Montreal, who studies modern-day acorn worms. Identifying the fossils was not problematic: “Spartobranchus is clearly an acorn worm,” he said in an interview. “It’s almost like someone took a picture of a modern-day animal – it’s absolutely astonishing.” These animals are not true worms but belong to a very different phylum: the Hemichordates. The name means “half chordate” indicating that hemichordates share some of the characters associated with chordates. This is relevant to some of the discussion below. Hemichordates comprise three classes: the Enteropneusta or acorn worms; the Pterobranchia – minute colonial tube-dwelling organisms; and the Graptolithina or graptolites. Pterobranchs were already known to be part of the Cambrian Explosion, so the inclusion of acorn worms in this flowering of animals is a significant addition to knowledge. According to Henry Gee in Nature:
“Before this report, the earliest-known fossil enteropneusts were Triassic, between 250 million years and 200 million years old. That Caron et al. extend the fossil record of enteropneusts back to the Cambrian period makes their findings notable, even with no other contribution.”
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