The urohyal: Development and homology within osteichthyans

Area of Science:

• Comparative anatomy
• Osteichthyan evolution
• Phylogenetics

Background:

• The urohyal bone's formation and presence vary significantly across osteichthyan fishes.
• Previous studies have described urohyal structures in sarcopterygians, Polypterus, Amia, Lepisosteus, and teleosts, but homology has been debated.

Purpose of the Study:

• To investigate the homology and evolutionary origin of the urohyal and parurohyal structures in osteichthyans.
• To clarify the developmental and anatomical differences in urohyal formation across various fish groups.
• To establish the parurohyal as a synapomorphy of siluroids.

Main Methods:

• Comparative anatomical analysis of urohyal and parurohyal structures across diverse osteichthyan taxa.
• Examination of ontogenetic development and ossification patterns in key species.
• Phylogenetic analysis to infer homology and evolutionary relationships.

Main Results:

• The teleostean urohyal, formed from the sternohyoideus muscle tendon, is nonhomologous to the sarcopterygian cartilaginous urohyal.
• The parurohyal, a compound structure in siluroids, is homologous to the teleostean urohyal.
• The parurohyal is identified as a synapomorphy of siluroids, exhibiting unique articulation and vascularization features.

Conclusions:

• The urohyal and parurohyal represent distinct evolutionary trajectories within osteichthyans.
• The parurohyal's unique characteristics confirm its status as a defining feature of the siluroid clade.
• Understanding these structures provides insights into hyoid arch evolution and fish phylogeny.