Cyclostomes, the living jawless vertebrates including hagfishes and
lampreys, represent the most basal lineage of vertebrates. Although the
monophyly of cyclostomes has been supported by recent molecular
analyses, the phenotypic traits of hagfishes, especially the lack of
some vertebrate-defining features and the reported endodermal origin of
the adenohypophysis, have been interpreted as hagfishes exhibiting a
more ancestral state than those of all other vertebrates. Furthermore,
the adult anatomy of hagfishes cannot be compared easily with that of
lampreys. Here we describe the craniofacial development of a series of
staged hagfish embryos, which shows that their adenohypophysis arises
ectodermally, consistent with the molecular phylogenetic data. This
finding also allowed us to identify a pan-cyclostome pattern, one not
shared by jawed vertebrates. Comparative analyses indicated that many of
the hagfish-specific traits can be explained by changes secondarily
introduced into the hagfish lineage. We also propose a possibility that
the pan-cyclostome pattern may reflect the ancestral programme for the
craniofacial development of all living vertebrates.
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