The fossil record is sparse, which is no surprise for soft-bodied animals, but surprisingly the hard stylets are not even found. A fossil (Archisymplectes) which may represent a Nemertean was found in Mazon Creek (Illinois), and is dated to the Pennsylvanian (325-299 MYA). The Nemerteans are believed to have been present over 500 MYA in the Cambrian or Pre-Cambrian even though fossil evidence doesn’t imply this.
(not actual picture of Nemertea fossil)
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