Episode 110: The Fin-Limb Transition and Early Tetrapod Biodiversity
One of the great themes in palaeobiology is the water-land transition, or how and when the ancestors of today’s four-legged terrestrial animals moved to land. Lines of questioning have included understanding the anatomy and biomechanics of the axial skeleton- head and vertebrae (focusing on biting and swallowing) and the appendicular skeleton (focusing on how the earliest tetrapods walked or swam). Our picture of this story has drastically changed in the last three decades, as new fossils have filled in crucial gaps in the tetrapod evolutionary tree. This changing picture really came to the fore with the work of the late Professor Jenny clack, who’s work at Cambridge in early tetrapods from Greenland and elsewhere brought the water land question back in fashion.
We speak with one of Professor Clack’s close collaborators – Professor Mike Coates, who tells us about their description of the tetrapod fossil that started it all, Acanthostega gunnarii. Acanthostega is the first tetrapod int he fossil record with fully formed digits, making the complete shift away from the fin rays of fishes. Work on Acanthostega, and other early tetrapods like Ichthyostega and Tiktaalik (Ep 100) have shown us that most of the adaptations to land- like limbs and digits- actually evolved in the water in mostly aquatic organisms. Professor Coates further highlights the history of the question itself, dating back to Richard Owen, and how new fossils and data are drastically changing our understanding of the story.
Another aspect of the transition we’re only beginning to understand is the palaeobiology and diversity of early tetrapods- what sort of environments they lived in, what their population structure was like, and how these tetrapods interacted with other animals in the local food web. Throwng light on some of these questions is fourth year Chicago graduate student Benjamin Otoo, who in his master’s work with Professor Clack studied the diversity and distribution of tetrapods from a historically enigmatic geological period called Romer’s gap. Ben’s work, continuing in Chicago, is trying to understand the ecological composition of tetrapod and fish communities from Romer’s gap, and trying to understand how the diversity of these fascinating animals changed through time.