Published on May 31st, 2020 | by Vishruth Venkat


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.

Professor Mike Coates and graduate Student Benjamin Otoo are part of the Coates lab in the Darwinian Sciences cluster at the University of Chicago. The lab focuses on the biology and diversity of early tetrapods, as well as the evolutionary relationships of early ray-finned fishes, fossil jawless vertebrates and the intersection of palaeontology and evolutionary-developmental biology. 
Before joining the Coates lab, ben studied with Professor Jenny Clack at the University of Cambridge where he began work on the TW:eed project (the Tetrapod World: early evolution and diversification), studying the diversity of tetrapods from the Early Carboniferous. His current work continues this theme, focusing on understanding the ecological and evolutionary relationships of early tetrapods, particularly the enigmatic fossil 
Ben was recently awarded the Field Museum’s Armour Graduate Fellowship for the year 2019-2020, awarded to one student who works in close collaboration with the Field Museum at Chicago. 
Mike Coates is Professor of Organismal Biology and Anatomy and Chair of the Committee on Evolutionary Biology at the University of Chicago. He began his career at the University of Newcastle, where he and Professor Clack were lab mates. This research collaboration has persisted ever since, leading to the many seminal papers the two published together on early tetrapod evolution.
Arguably the most famous work from the team was the description of Acanthostega, an early tetrapod from the Late Devonian of Greenland and the first tetrapod with fully formed digits. Pictured here, the specimen nicknamed ‘Grace’. 
Profesor Clack was instrumental in bringing the Acanthostega material to be worked on and studied in the UK from Denmark. Much of this material needed preparing out of the rock. 
At their basement prep lab in Cambridge, Professor Clack and her team were able to work on these fossils, carefully removing sediments and revealing unprecedented anatomical details. 
Stunning specimens like Boris (pictured above, now prepped) yielded significant details about the anatomy of Acanthostega. This specimen was nicknamed after Russian President Boris Yeltsin. 
As a result of Professor Clack and Professor Coates’ work, Acanthostega is arguably the best known early tetrapod. Here is Professor Coates’ reconstruction of Acanthostega, showing the entirety of the axial skeleton, including the skull and vertebral column, as well as both shoulder and pelvic girdles and limbs. 
The reconstruction of limb elements from Acanthostega revealed that it had eight fully formed digits- indicating that the ancestral condition in tetrapods may have been to have more than five digits- as opposed to all the major lineages of tetrapods alive today, which have five or fewer digits.
Reconstruction of the forelimb and shoulder girldle, indicating the the characteristic eight fully formed digits.
Cladogram showing Acanthostega’s relationships to other early tetrapods. From Daeschler et al. 2006.

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