Tetrapods are organisms which evolved from a fish to have four limbs and fingers. These animals include frogs, cats and even humans. After analyzing over 100 skull bones of extinct and non-extinct tetrapods, the researchers found that tetrapods have fewer skull bones than fish – both extinct and living. A a new study suggests that the limited number of cranial bones may have hindered evolution over millions of years.

Researchers from the University of Bristol, Pompeu Fabra University in Barcelona and University College London analyzed fossilized skulls of animals from the transitional period between aquatic and terrestrial environments. According to a press releasetheir findings show that tetrapods had a more complex relationship between their cranial bones than fish.

Although it might seem that this difference in bone connection would lead to evolutionary changes among land animals, these changes actually hindered the evolution of the tetrapod skull.

“Tetrapod skulls generally have fewer cranial bones than their fish ancestors, but simply counting the number of bones misses some important data,” said study lead author James Rawson in a press release. “We used a technique called network analysis, where the arrangement of the skull bones – which bones connect to which – is recorded in addition to the number of bones.”

Network analysis helps to provide graphs and numerical data. This data then helps determine how subjects in the dataset relate to each other.

‚ÄúTraditionally, anatomical research has been primarily descriptive or qualitative. “Network analysis provides a robust mathematical framework for quantifying the anatomical relationships between bones: a type of data often overlooked in most studies of morphological evolution,” co-author Borja Esteve-Altava, an expert in the technique, said in a press release.

To their surprise, the researchers found that the evolutionary process slowed down with fewer bones in the skull.

“It may seem strange, but having fewer bones means that each of those bones has to connect with more of its neighbors, resulting in a more complex arrangement,” Rawson said in a press release. “Modern frogs and salamanders had the most complex skulls of any animal we studied.”

The authors go on to say that early tetrapod skulls were consolidated into a single unit, while fish skulls were made of several sections.

According to a press release, the researchers also found that the origin of tetrapods coincided with a decline in various cranial bone arrangements.

“We were surprised to find that these changes in the skull appear to have limited the evolution of tetrapods, rather than promoting radiation to new habitats on land.” We think neck evolution, extinction events, or a bottleneck in skull development may be responsible,” co-author Emily Rayfield said in a press release.

The researchers observed a similar change in bone diversity in tetrapods that occurred much earlier than skull bones.

“We also see a similar decline in structural variability for limb bones in early tetrapods, but the decline in limbs occurs 10 million years earlier.” A variety of factors appear to have influenced skull and limb evolution in early tetrapods, and we have so much more to learn about this crucial moment in our own evolutionary history,” Rawson said in a press release.

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