A new study led by Zurich University researchers sheds light on the origin of a puzzling skull bone, known as the interparietal, in humans and all other mammals, and discovers that it consists not of two, as previously believed, but of four elements.

The mammalian skull, including that of humans, is composed of about 20 bones. Fish, reptile and bird skulls, however, have considerably more. When mammals evolved 320 million years ago, the skull’s structure became simplified during its development and the number of skull bones decreased. Some bones were lost in the lineage leading to mammals in the course of evolution, especially a number of skull roof bones.

The interparietal bone, also referred to as ‘os interparietale,’ is a median triangular bone lying at the junction of the parietal and occipital bones of the mammalian skull.

The bone has particularly puzzled researchers: on the one hand, it seems to have survived in carnivores, ungulates and humans (it is frequently found in contemporary indigenous peoples of the southern Andes as well as in those of mummies of the Inca civilization as a separate bone in the skull); on the other hand, it is not found in all mammals.

By studying fossils and embryos of over 300 species of vertebrates, the researchers have now been able to identify the bone in all of them. They used non-invasive micro-CT imaging to analyze rare embryos of different species from museum collections.

“The interparietal was clearly discernible in specimens from the embryonic period as the skull bones were fused less strongly here,” said Marcelo Sánchez, a professor of paleontology at the University of Zurich and co-author of the paper published in the Proceedings of the National Academy of Sciences.

At the same time, he sees the fact that the bone is only clearly and easily discernible in the embryonic period as the reason why previous researchers failed to recognize it: “It would seem that many anatomists have overlooked the presence of the interparietal in numerous mammalian lineages as the bone becomes fused to other skull bones during growth and is unrecognizable in adult individuals.”

Another result that also refutes previous assumptions concerns the origin of the bone.

“Whilst it was previously assumed that the mammalian interparietal was composed of two elements, we discovered that it develops from four elements: a medial and a lateral pair,” explained lead author Dr Daisuke Koyabu, a post-doctoral student at the University of Zurich.

The tabular bones of our ancestors and fish correspond to the lateral interparietal bones, which were overlooked until now. According to the new results, however, they have survived in mammalian lineages after all.

“The evidence of the continuation of fish bones in mammals provides new insights into the origins of our own anatomy,” Prof Sánchez concluded.

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Bibliographic information: Koyabu et al. Paleontological and developmental evidence resolve the homology and dual embryonic origin of a mammalian skull bone, the interparietal. PNAS, August 14, 2012; doi: 10.1073/pnas.1208693109