Morphological disparity and evolutionary transformations in the primate hyoid apparatus.

Area of Science:

• Evolutionary Biology
• Comparative Anatomy
• Primate Morphology

Background:

• The hyoid apparatus is crucial for mammalian swallowing, respiration, and vocalization.
• Placental mammals typically possess a rod-shaped basihyal with an anterior cornu, unlike anthropoid primates with broad, suspended basihyals.
• The evolutionary trajectory of hyoid morphology in primates and related mammals remains largely uncharacterized.

Purpose of the Study:

• To investigate the evolutionary history of hyoid apparatus morphology within primates and their euarchontan outgroups.
• To identify patterns of hyoid evolution, particularly focusing on basihyal shape and anterior cornu reduction.
• To explore potential drivers, such as vocalization, behind observed hyoid morphological changes.

Main Methods:

• Phylogenetic comparative methods were employed to analyze hyoid evolution across primate and euarchontan lineages.
• Linear morphometrics were used to quantify and compare hyoid apparatus features, including basihyal shape and anterior cornu presence.
• Sexual dimorphism in hyoid morphology was assessed in Cynocephalus volans.

Main Results:

• Ancestral mammals and most non-anthropoid primates exhibit a rod-shaped basihyal with a fully ossified anterior cornu.
• Haplorhine primates show a reduced anterior cornu, while anthropoids display increased basihyal aspect ratio and volume.
• Lorisoid strepsirrhines convergently evolved broad basihyals and reduced anterior cornua, suggesting independent evolutionary pathways.

Conclusions:

• The ancestral mammalian hyoid morphology, characterized by a rod-shaped basihyal and anterior cornu, is conserved in many non-primate euarchontans and basal primates.
• The reduction of the anterior cornu and broadening of the basihyal represent significant evolutionary trends within primates, particularly in haplorhines and anthropoids.
• Vocalization adaptations alone are insufficient to explain the iterative reduction of anterior cornua across primate evolution, indicating other functional pressures may be involved.