Shared patterns of segment size development in trilobites and vertebrates

Area of Science:

• Developmental biology
• Evolutionary biology
• Paleontology

Background:

• Animal body segment size is crucial for form and function, impacting evolutionary trajectories.
• The inhibitory cascade (IC) model describes a vertebrate developmental bias influencing serially homologous structures.
• This model is considered the default for vertebrate segment development, with significant evolutionary consequences.

Purpose of the Study:

• To investigate if the inhibitory cascade (IC) or a similar model controls segment size development in trilobites.
• To determine the prevalence and evolutionary impact of the IC model in arthropods.

Main Methods:

• Analysis of segment size patterning across 128 trilobite species.
• Examination of ontogenetic growth patterns in three trilobite species.
• Comparative analysis with stem and modern arthropods.

Main Results:

• Trilobites exhibit prominent linear relative segment size patterning in adult forms.
• Strict regulation of segment patterning is observed in developing pygidial segments.
• The IC model appears to be a common developmental mode across arthropods.

Conclusions:

• The inhibitory cascade (IC) model is a widespread developmental mechanism in arthropods, similar to vertebrates.
• This developmental bias has likely influenced morphological evolution across arthropod lineages.
• The IC model provides a unifying framework for understanding segment development and evolution in diverse animal groups.