Testing the patterning cascade model of cusp development in Macaca fascicularis mandibular molars

Area of Science:

• Primate paleontology
• Developmental biology
• Geometric morphometrics

Background:

• Molar cusp number is crucial for primate systematics and inferring phylogenetic relationships.
• Developmental mechanisms, such as the patterning cascade model (PCM), may explain molar cusp formation.
• The PCM suggests early cusp development dictates later cusp presence and positioning.

Purpose of the Study:

• To test if accessory cusp variation in Macaca fascicularis mandibular second molars (M2s) aligns with PCM predictions.
• To investigate the correlation between enamel-dentine junction (EDJ) shape and accessory cusp presence.

Main Methods:

• Micro-computed tomography (micro-CT) imaging of 13 Macaca fascicularis M2s.
• Geometric morphometrics applied to EDJ shape variation.
• Analysis of accessory cusp presence in relation to molar size and cusp height.

Main Results:

• Accessory cusp patterning in macaque M2s is largely consistent with the PCM.
• Molars with accessory cusps were larger and had shorter relative cusp heights.
• Peripheral cusp formation correlated with more centrally positioned primary cusps.

Conclusions:

• The PCM provides a suitable framework for understanding cusp variation in Macaca fascicularis molars.
• The PCM does not fully explain all instances of accessory cusp expression in the sample.