The postorbital palatoquadrate articulation in elasmobranchs

Area of Science:

• Paleontology
• Ichthyology
• Evolutionary Biology

Background:

• Modern hexanchiforms are the only extant elasmobranchs with a postorbital articulation.
• This articulation is considered a secondary connection, not a basal trait.
• Paleozoic sharks possessed a postorbital articulation, but it differed morphologically and topographically from the hexanchiform condition.

Purpose of the Study:

• To re-evaluate the evolution of the postorbital articulation in sharks.
• To propose new terminology for different types of shark jaw articulations.
• To understand the primitive jaw suspension pattern in chondrichthyans.

Main Methods:

• Comparative morphological analysis of fossil and extant shark specimens.
• Cladistic analyses (morphological and molecular) to infer evolutionary relationships.
• Review of existing literature on shark anatomy and paleontology.

Main Results:

• The term amphistylic is proposed for the hexanchiform postorbital articulation.
• The term archaeostylic is proposed for the Paleozoic shark postorbital articulation.
• The autodiastylic pattern is likely not primitive for chondrichthyans; holocephalans may have secondarily lost a postorbital articulation.

Conclusions:

• The postorbital articulation in modern hexanchiforms is apomorphic (derived).
• The archaeostylic condition may represent a basal chondrichthyan synapomorphy.
• Jaw suspension patterns in cartilaginous fishes have evolved multiple times independently.