Skull Bones Restricted the Evolution of Earth’s Earliest Land Animals



According to recent studies, tetrapods' evolution was constrained for millions of years because their skulls had fewer bones than extinct and current fish.

Tetrapods have more intricate connections between their skull bones than fish, according to research on fossilized animal skulls during the transition from an aquatic to a terrestrial environment. These alterations to skull structure didn't help to promote the diversification of terrestrial life; instead, they hindered the development of tetrapod skulls.

Tetrapods were the first land animals with limbs and digits and diverged from fish about 400 million years ago. From amphibians to humans, they are the ancestors of all living things.

To better understand how skulls altered as tetrapods evolved, the study, which was released on September 9 in the journal Science Advances, quantified the arrangement of skull bones in over 100 extant and extinct creatures. Researchers from the University of Bristol, Barcelona's Universitat Pompeu Fabra, and University College London carried out the study.

In average, tetrapod skulls have fewer skull bones than their fish forebears, but measuring the number of bones ignores some crucial information, according to lead scientist James Rawson of Bristol's School of Earth Sciences. In addition to bone number, we recorded the arrangement of the skull's bones, including which bones connect to which. This method is known as network analysis.

Expert in this method and author Dr. Borja Esteve-Altava said: "Traditionally, anatomy research has been primarily descriptive or qualitative. Anatomical relationships between bones can be quantified using network analysis, a method that is frequently ignored in research on morphological evolution.

Although it may seem paradoxical at first, the researchers found that tetrapods' skulls were more intricately organized despite having fewer skull bones than fish.

The fact that there are fewer bones means that each one must connect with more of its neighbors, creating a more intricate arrangement, Mr. Rawson continued. Of all the creatures we examined, modern frogs and salamanders had the most complicated skulls. The earliest tetrapods' skulls also developed into a more cohesive whole, as opposed to their fish forebears' multiple discrete pieces.

The researchers also discovered that the emergence of tetrapods is associated with a decline in the variety of skull bone configurations by examining the variation of skull bone arrangements across time.

"We were startled to find that these alterations to the skull seemed to limit tetrapod evolution, rather than promoting radiation to new habitats on land," said Professor Emily Rayfield, the study's principal author. We speculate that this might be caused by the evolution of a neck, extinction events, or a skull development bottleneck.

"We also observe a comparable loss in structural variety for the limb bones in early tetrapods, although the drop in the limbs occurs 10 million years earlier," Mr. Rawson said in his conclusion. We still have a lot to learn about this critical period in the evolution of tetrapods, including how various circumstances may have affected the development of their skulls and limbs.

By UNIVERSITY OF BRISTOL 

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