The origin of conodonts and of vertebrate mineralized skeletons

Conodonts are an extinct group of jawless vertebrates whose toothlike
elements are the earliest instance of a mineralized skeleton in
the vertebrate lineage, inspiring the ‘inside-out’ hypothesis that
teeth evolved independently of the vertebrate dermal skeleton and
before the origin of jaws. However, these propositions have been
based on evidence from derived euconodonts. Here we test hypotheses
of a paraconodont ancestry of euconodonts using synchrotron
radiation X-ray tomographic microscopy to characterize
and compare the microstructure of morphologically similar euconodont
and paraconodont elements. Paraconodonts exhibit a
range of grades of structural differentiation, including tissues
and a pattern of growth common to euconodont basal bodies.
The different grades of structural differentiation exhibited by
paraconodonts demonstrate the stepwise acquisition of euconodont
characters, resolving debate over the relationship between
these two groups. By implication, the putative homology of euconodont
crown tissue and vertebrate enamel must be rejected as
these tissues have evolved independently and convergently. Thus,
the precise ontogenetic, structural and topological similarities
between conodont elements and vertebrate odontodes appear to
be a remarkable instance of convergence. The last common ancestor
of conodonts and jawed vertebrates probably lacked mineralized
skeletal tissues. The hypothesis that teeth evolved before jaws and
the inside-out hypothesis of dental evolution must be rejected; teeth
seem to have evolved through the extension of odontogenic competence
from the external dermis to internal epithelium soon after
the origin of jaws.