Tetrapods are organisms that evolved from fish to have four limbs and digits. These animals include frogs, cats and even humans. After analyzing over 100 skull bones in extinct and non-extinct tetrapods, researchers found that tetrapods have fewer skull bones than fish — both extinct and living. A new study suggests that the limited number of skull bones could have prevented evolution for millions of years.
Researchers from the University of Bristol, Barcelona's Universitat Pompeu Fabra and University College London analyzed fossilized skulls of animals from the transitional period between aquatic and terrestrial environments. According to a press release, their findings indicate that tetrapods had a more complex connection between their skull bones than fish.
While it may seem like 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 skull bones than their fish ancestors, but simply counting the number of bones misses some important data," says lead study author James Rawson in a press release. "We used a technique called network analysis, where the arrangement of skull bones — which bones connect to which — is recorded in addition to bone number."
Network analysis helps provide graph and numerical data. This data then helps pinpoint how the subjects in the data set relate to each other.
"Traditionally, anatomy research has been mostly descriptive or qualitative. Network analysis provides a sound mathematical framework to quantify anatomical relations among bones: a kind of data often overlooked in most studies on morphological evolution," says co-author Borja Esteve-Altava, an expert in this technique, in a press release.
To their surprise, the researchers found that the evolution process was slowed with fewer skull bones.
"It might seem strange, but having fewer bones means each of those bones must connect with more of its neighbors, resulting in a more complex arrangement," says Rawson in a press release. "Modern frogs and salamanders had the most complex skulls of all the animals we studied."
The authors went on to say that early tetrapod skulls consolidated into a single piece, whereas fish skulls were made of several sections.
According to a press release, researchers also found that tetrapods' origins coincide with a drop in various skull bone arrangements.
"We were surprised to find these changes to the skull seemed to limit tetrapod evolution rather than promoting radiation to new habitats on land. We think that the evolution of a neck, extinction events or a bottleneck in skull development may be responsible," says co-author Emily Rayfield in a press release.
Researchers noticed a similar change in bone variety in tetrapods that occurred much earlier than the skull bones.
"We also see a similar drop in structural variability for the limb bones in early tetrapods, but the drop in the limbs happens 10 million years earlier. It seems that different factors were affecting skull and limb evolution in early tetrapods, and we have so much more to learn about this crucial time in our own evolutionary history," says Rawson in a press release.