The Embryology and Evolutionary Biology of The Tetrapod Limb Musculature

Tetrapods came out of water in great part thanks to the evolution of fins into limbs. This transition greatly modified the skeletal structure of fins but also its muscular anatomy. Land vertebrates exhibit a wide range of morphologies and lifestyles, enabled in many cases by their different limb morphologies. Different as they might be, a wing of a bird and a flipper of a whale are composed of the same bones, and the same muscles, but while we have a more achieved understanding of the homologies of the skeletal elements, the evolutionary identities and relationships between muscles of the limb remain more obscure. Adult anatomy might indicate some homology hypotheses; however, the development of muscles has for long time been ignored and the clues it might provide to understand how same muscles have evolved to allow a bird to fly or a lizard to climb need to be considered. The first chapter of this thesis will follow the embryonic development of the forelimb musculature in a variety of amniotes representing six different kinds of both reptiles and mammals. Among the findings presented in this chapter, perhaps the most surprising is the conserved early patterns of muscle mass cleavage observed in all species studied, which allows to track down one-to-one homologies for all the forelimb muscles across amniotes. Many of these newly identified homologies challenge the previous understanding of the identity and evolution of particular muscles and provide new understanding of how muscles develop and evolve. The second chapter will follow the development of the leg musculature. As observed for the forearm, leg muscles develop following an initially stereotyped pattern of divisions, which allow to identify the identity of muscle groups that have been previously overlooked, highlighting the high degree of transformation that limb muscles have undergone during evolution in the case of birds and mammals. Based on the findings of the previous chapter, it appeared natural to compare the development and homologies of tetrapod forelimb musculature in general. However, the third chapter of this thesis shows how different lissamphibian limb musculature development is compared not only to amniotes, but to vertebrates in general. The particularities of lissamphibian development make it challenging to use the same previous method for establishing homologies while also highlight the extraordinary developmental evolutionary tale of the fin to limb transition. Having studied the developmental patterns of muscle formation in the limbs and fins of different vertebrate clades, the fourth chapter investigates what happens in the development of limb reduced and limbless reptiles. Squamates have evolved limb reduced and limbless forms multiple times. Investigating the development of the apparently limbless skink Brachymeles lukbani it was possible to identify the persistence of rudimentary limb from embryonic time into adulthood and understand the relationship between persistent early limb development and the presence of particular muscles in limbless and limb reduced species. Finally, Chapter 5 presents an account of the embryonic development of the Musk turtle, Sternotherus odoratus. This small turtle originated in North America was employed in the first chapter as the representative of turtles used to study muscle development. Its small size made it ideal for studying the development of muscles, but also allowed for following the development of other anatomical structures. A full account of its development, from the earliest moments until hatching is provided