A recently published review article (Donoghue, Forey, and Aldridge, 2000,
Conodont affinity and chordate phylogeny. Biological Reviews, Cambridge
Philosophical Society 75:191-252) supports the assignment of protoconodonts
to the phylum Chaetognatha. Protoconodonts first appear in the late
Precambrian. The relationships of the phylum Chaetognatha to other
bilaterian animals remain rather uncertain. It is a modern phylum in the
Precambrian, however, assuming that you do not follow the terminology of
the following article in the same volume and number (Budd and Jensen). In
that case, modern phyla would be thought to appear throughout the fossil
record because of redefinition. I do not think such crown-based
terminology is advisable due to its instability and difficulty of
application.
David C.
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