Development of the bony skull in common sole: brief survey of morpho-functional aspects of ossification sequence

The postembryonic development of the bony cephalic skeleton in the common sole Solea solea,
observed from hatching to the juvenile stage or postmetamorphic larva, appears to follow a
similar chronological order to that observed in other Pleuronectiformes and Perciformes and
the sequence in bone formation is a response to functional demands. At hatching, S. solea has
no bony structure. On day 4, only the outlines of maxillaries and opercular bones are visible.
On day 6, a thin parasphenoid appears between the orbits and isolates the braincase from the
buccal cavity making food ingestion possible without any impact on the brain. On day 8, the
dentaries form and two small preopercular bones appear on each side of the head. On day 9,
at weaning from the yolk sac, branchial arches support the gill filaments (used for respiration
and trapping phytoplankton which pass through the open mouth). On day 10, the premaxil-
laries develop in front of the maxillaries. The superimposing of the maxillaries and the
premaxillaries is a typical feature of species possessing an acanthopterygian protractile mouth
at the adult stage. On day 12, the frontals develop above the orbits and the set of opercular
bones is complete. On day 18, the migration of the left eye begins. On day 20, the left eye has
moved to the median crest of the head. On day 23, both eyes are located on the same side. On
day 26, the braincase is formed by a basioccipital, exoccipitals, pterotics, sphenotics and a
supraoccipital. On day 50, new structures have appeared, others have developed and have
undergone an extensive remodeling due to metamorphosis.