Earliest land animals had fewer skull bones than fish, restricting their evolution

The skulls of thepods had less bones than living fish, limiting their evolution for millions of years.

Researchers from the University of Bristol, Barcelona's Universitat Pompeu Fabra and University College London found that fish had more complex connections between their skull bones than tetrapods. The evolution of the tetrapods was restricted by the changes to skull anatomy.

The first land animals with limbs and digits were the tembopods.

The research, published this week in Science Advances, quantified the organization of skull bones in more than 100 living and fossil animals.

According to James Rawson of Bristol's School of Earth Sciences, turtles have fewer skull bones than fish, but counting the number of bones misses some important data. We used a technique called network analysis to record the arrangement of skull bones.

Dr. Borja Esteve-Altava is an expert in this technique. Network analysis provides a mathematical framework to quantify the relationship between bones.

The organization of the skulls of fish and tetrapods were found to be more complex.

Mr. Rawson said, "having fewer bones means each of those bones must connect with more of its neighbors, resulting in a more complex arrangement." The skulls of modern frog and salamanders were complex. The skulls of the earliest tetrapods were consolidated into a single unit.

The authors looked at the variety of skull bone arrangements over time and found that there was a drop in the number of skull bone arrangements. The senior author of the study, Professor Emily Rayfield, said they were surprised that the changes to the skull seemed to limit the evolution of thepods. The evolution of a neck, extinction events or a bottleneck in skull development may be to blame.

The drop in the limbs happens 10 million years earlier than the drop in the limbs. It seems that different factors were affecting skull and limb evolution, and we have so much more to learn.

In Science Advances, James Rawson, Dr. Borja Esteve-Altava, Dr. Laura Porro, Dr. Hugo Dutel, and Professor Emily Rayfield wrote about the early evolution of the brain.

More information: James Rawson et al, Early tetrapod cranial evolution is characterised by increased complexity, constraint and an offset from fin-limb evolution, Science Advances (2022). DOI: 10.1126/sciadv.adc8875. www.science.org/doi/10.1126/sciadv.adc8875 Journal information: Science Advances