Thyroid and endostyle development in cyclostomes provides new insights into the evolutionary history of vertebrates.

Based on the direct development of the thyroid gland both in hagfish and gnathostomes, and the shared expression profile of thyroid-related transcription factors in the cyclostomes, we challenge the plesiomorphic status of the lamprey endostyle and propose an alternative hypothesis where the lamprey endostyle could be obtained secondarily in crown lampreys.