Inner ear sensory system changes as extinct crocodylomorphs transitioned from land to water

Area of Science:

• Paleontology
• Evolutionary Biology
• Comparative Anatomy

Background:

• Major evolutionary transitions involve significant changes in body plans and habitats.
• The transition of land mammals to cetaceans (whales) is a well-studied example of secondary aquatic adaptation.
• Reptile transitions to marine environments during the Mesozoic Era are less understood.

Purpose of the Study:

• To investigate the inner ear vestibular system changes in extinct crocodile relatives (thalattosuchians) during their transition to pelagic life.
• To compare the evolutionary pathways of thalattosuchians and cetaceans becoming secondarily aquatic.

Main Methods:

• Computed tomography (CT) scans were used to analyze the inner ear vestibular system morphology.
• Morphological data of the vestibular system were correlated with inferred habitat (terrestrial vs. pelagic).

Main Results:

• Pelagic thalattosuchians displayed a more compact vestibular labyrinth with wider semicircular canals and an enlarged vestibule.
• These inner ear modifications in thalattosuchians resemble those found in other marine reptiles and cetaceans.
• Thalattosuchian inner ear changes occurred after postcranial skeletal adaptations for swimming, following a prolonged semiaquatic phase.
• Cetaceans, in contrast, modified their inner ears early in their transition to water, without an extended semiaquatic period.

Conclusions:

• Thalattosuchians and cetaceans represent distinct evolutionary routes and paces for becoming secondarily aquatic.
• Inner ear morphology serves as a key indicator of habitat adaptation during major evolutionary transitions.