THE EVOLUTIONARY HISTORY OF INTEGUMENTARY SENSE ORGANS IN CROCODYLOMORPHS

A suite of five primary senses (sight, hearing, smell, touch and taste) enables vertebrates to function in their habitats. Additionally many vertebrates have evolved novel sensory mechanisms that convey improved hunting ability, indirectly leading to enhanced relative fitness. Examples include infrared detection in snakes, and echolocation in bats. Extant crocodilians possess small domed structures called “Integumentary Sense Organs” (ISOs) that are hypothesized to be mechanosensors sensitive to water-borne vibrations caused by potential prey. They are restricted to the head in alligatorids, but are also present on postcranial regions in other crocodilians. The distribution of ISOs on the faces of extant crocodilians corresponds well with the distributions of foramina within their bones of the jaws. The foramina permit innervation and supply of blood to ISOs, and may be used as indicators of the presence of ISOs in now extinct crocodylomorphs.

Crocodilian ISOs were thought to have first evolved during the Jurassic period, and only have been present in genera adopting a semi-aquatic lifestyle. This study indicates that ISOs evolved much earlier, during the Triassic period (~200 million years ago). In fact, homologous foramina are present in the skulls and mandibles of the earliest crocodylomorphs (Triassic), and in some related basal archosaurs. The most ancient crocodylomorphs lived in terrestrial habitats, compared to the fully or semi-aquatic lifestyles of the later crocodilians. This is reflected in the differences in distribution and density of foramina along the jaws of these animals. Several phases in crocodylomorph evolution are identified based on foramina distributions, leading up to the modern condition. It is possible that the function of these ISOs has changed since the Triassic period, perhaps reflecting changes in ecology.