Petrosal morphology and cochlear function in Mesozoic stem therians

Area of Science:

• Paleontology
• Comparative Anatomy
• Evolutionary Biology

Background:

• Understanding the evolution of the mammalian inner ear is crucial for reconstructing the sensory capabilities of early mammals.
• Plesiomorphic crown mammalian petrosal specimens offer unique insights into the transitional anatomy of early mammals.
• Micro-computed tomography (micro-CT) enables detailed examination of fossilized osteological structures.

Purpose of the Study:

• To describe the bony anatomy of the inner ear in three early mammalian petrosal specimens: Priacodon fruitaensis and two Höövör petrosals.
• To investigate the osteological correlates of neurovascular features, particularly along the abneural wall of the cochlear canal.
• To clarify hypotheses regarding the mosaic evolution of the cochlear endocast in early mammals.

Main Methods:

• Micro-CT imaging of three fossil petrosal specimens (one triconodont, two stem therians) and extant mammalian taxa.
• Detailed anatomical description of the bony labyrinth and associated structures.
• Comparative analysis of fossil and extant mammalian inner ear anatomy.

Main Results:

• The membranous cochlear duct adhered abneurally to a secondary bony lamina before the evolution of a primary bony lamina or tractus foraminosus.
• Fossil stem therian inner ears show localized enlargement of the intrapetrosal venous plexus, interpreted as the vein of the cochlear aqueduct.
• These early specimens appear to have limited high-frequency hearing capabilities.

Conclusions:

• The development of specialized venous drainage in the cochlear endocast predates the evolution of high-frequency hearing in mammals.
• The stria vascularis and high-frequency hearing capacity likely evolved independently or later in mammalian evolution.
• Early mammalian inner ear evolution involved a mosaic of primitive and derived features.