The first land animals had fewer skull bones than fish •

Tetrapods are vertebrates (animals with backbones) that have four limbs or leg-like structures. They include all amphibians, reptiles, birds, and mammals and are believed to have evolved from finned fish way back in the Devonian period, about 400 million years ago. The first tetrapods were aquatic and adapted to life in the water, but over time new species evolved that became more terrestrial and less dependent on aquatic environments for nutrition, reproduction, and survival.

In a study published in the journal science progressand researchers from University of BristolPompeu Fabra University in Barcelona and University College London have investigated the skulls of more than 100 living and fossil tetrapods to understand how the arrangements of the skull bones have changed during the evolution of this important group. Scientists found that the skulls of tetrapods had far fewer bones than the skulls of fish, both extinct and living, but that these bones had more complex connections between them than those of fish skulls.

“The skulls of tetrapods generally have fewer skull bones than their fish ancestors, but just counting the number of bones lacks some important data,” said study lead author James Rawson. “We used a technique called network analysis, in which the arrangement of the bones of the skull – to which the bones are connected – is recorded, as well as the number of bones.”

Traditionally, anatomy research has mostly been descriptive or qualitative. The network analysis provides a sound mathematical framework for estimating anatomical relationships between bones: a type of data that is often overlooked in most studies of morphological evolution,” explained study co-author Dr. Borja Estiv-Altava.

One might expect that having fewer bones would make the organization of the skull simpler, but that’s not what the researchers found. They concluded that for tetrapods, having fewer skull bones makes the organization of their skulls more complex.

“It may seem strange, but having fewer bones means that each of these bones has to connect to a greater number of its neighbors, resulting in a more complex arrangement,” Rawson said. “Modern frogs and salamanders have the most complex skulls of all the animals we have studied.” The skulls of the first tetrapods became more coherent into a single unit, while their fish ancestors had skulls made up of several distinct sections.

By looking at a variety of skull bone arrangements over time, the authors also discovered that the origin of tetrapods coincided with a decrease in this diversity. “We were surprised that these changes to the skull seemed to limit the evolution of tetrapods, rather than promoting radiation into new habitats on Earth,” said the study’s senior author. Emily Rayfield. “We think that neck evolution, extinction events, or asphyxiation in skull growth may be responsible.”

“We also see a similar decrease in the skeletal variation of limb bones in early tetrapods, but the decline in limbs occurred 10 million years ago,” Rawson said. “Various factors appear to influence the development of the skull and limbs in early tetrapods, and we have a lot to learn about this crucial time in our evolutionary history.”

The study authors conclude that the cranial development of tetrapods is characterized by increased complexity in terms of bone arrangements but decreasing diversity. They say that rather than encouraging the diversity of life on Earth, these changes in skull anatomy actually limited the evolution of tetrapods for millions of years.

Image Credit: Science Photo Library / Alamy Stock Photo ID (2G70HK2) Original Artist: Mark Garlick

by Alison BosmanAnd the crew clerk