Data from: Histology and affinity of anaspids, and the early evolution of the vertebrate dermal skeleton

The assembly of the gnathostome bodyplan constitutes a formative episode
in vertebrate evolutionary history, an interval in which the mineralized
skeleton and its canonical suite of cell and tissue types originated.
Fossil jawless fishes, assigned to the gnathostome stem-lineage, provide
an unparalleled insight into the origin and evolution of the skeleton,
hindered only by uncertainty over the phylogenetic position and
evolutionary significance of key clades. Chief among these are the jawless
anaspids, whose skeletal composition, a rich source of phylogenetic
information, is poorly characterized. Here we survey the histology of
representatives spanning anaspid diversity and infer their generalized
skeletal architecture. The anaspid dermal skeleton is composed of
odontodes comprising spheritic dentine and enameloid, overlying a basal
layer of acellular parallel fibre bone containing an extensive shallow
canal network. A recoded and revised phylogenetic analysis using equal and
implied weights parsimony resolves anaspids as monophyletic, nested among
stem-gnathostomes. Our results suggest the anaspid dermal skeleton is a
degenerate derivative of a histologically more complex ancestral
vertebrate skeleton, rather than reflecting primitive simplicity.
Hypotheses that anaspids are ancestral skeletonizing lampreys, or a
derived lineage of jawless vertebrates with paired fins, are rejected.