When less means more: evolutionary and developmental hypotheses in rodent molars.

Area of Science:

• Evolutionary biology
• Developmental biology
• Paleontology

Background:

• Rodent evolution shows a reduction in tooth number, characterized by a diastema and loss of canines and premolars.
• The inhibitory cascade (IC) model explains molar development but doesn't account for premolar influence.
• The role of the premolar (p4) in shaping molar proportions during rodent evolution is not well understood.

Purpose of the Study:

• To investigate whether the presence or absence of the premolar (p4) influences molar proportions in rodents.
• To analyze the impact of p4 on molar size variation across extinct and extant rodent families.
• To refine evolutionary models of tooth development by incorporating premolar effects.

Main Methods:

• Analysis of a large dataset encompassing extinct and extant rodent families over 40 million years.
• Comparative analysis of molar areas and proportions in relation to premolar presence/absence.
• Application and evaluation of the inhibitory cascade (IC) model in light of premolar data.

Main Results:

• Rodents exhibit diverse molar proportions, with the IC model explaining approximately 80% of taxa.
• Molar proportions are significantly influenced by the presence or absence of the p4.
• The first (m1) and third (m3) molars show the most variability, especially when p4 is absent, with m1 potentially comprising half the total molar area.

Conclusions:

• The p4 plays a crucial role in constraining molar proportions during rodent evolution.
• Loss of the p4 likely released developmental constraints on m1, leading to increased size variability and influencing the entire molar row.
• Understanding premolar evolution is key to fully explaining patterns of molar reduction and diversification in rodents.