A DEVELOPMENTAL MODEL OF SEGMENT SIZE PATTERNING SHARED AMONG TRILOBITES AND VERTEBRATES

The relative size of body segments is one of the major determinants of the shape and functionality of an animal’s body. The inhibitory cascade is a developmental rule that predicts that sizes of serial structures like teeth, vertebrae, limbs and digits in vertebrates follow a linear pattern in size difference. Establishing the ubiquity of this rule across the two main segmented clades – vertebrates and arthropods – could reveal a universal design principle in animals. We propose that body segments in trilobites, the most diverse extinct arthropod lineage, also developed in accordance with the inhibitory cascade, and that it is the default and ancestral state of segment size generation. To test this, we examined holaspid (adult) segment size patterning in 128 trilobite species and during post-embryonic ontogenetic growth in three trilobite species. We show that linear patterning is prominent throughout trilobite species in the holaspid form and that the inhibitory cascade has near complete control over newly developing segments in the pygidium. In the species examined, the inhibitory cascade pattern is consistently strongly expressed initially in development, but can either be maintained throughout development, or modified by differential growth rates to produce alternative adult body plans. Additionally, the frontal appendages from some stem arthropods such as radiodontids also show strong signals of the inhibitory cascade, as do the abdomens of two modern insect species during ontogeny. Such a developmental rule has likely influenced (and constrained) the evolution of body size and shape in the largest group of animals on Earth. We further highlight the power of integrating evo-devo and the fossil record, while encouraging new thought into how ancestral segments developed and evolved.