The variable phylogenetic distribution of Wormian bones within mammals

Wormian bones, also known as intrasutural bones, are irregularly shaped bones that variably develop from separate ossification centers between sutures. Cranial bones derive embryologically from neural crest cells (anterior skull) or mesoderm (posterior skull), divided by the coronal suture. However, the interparietal bone, posterior to the parietal and superior to the occipital, derives from neural crest cells, which differ from its neighboring bones.

There is no concrete explanation for how and when intrasutural bones form, although both genetic and environmental factors have been proposed. Brain development can affect signaling of skull formation and suture timing. Parts of the embryonic brain are associated with specific parts of the skull formation; the presence of an intrasutural bone could reflect development of that associated brain region. Additionally, mechanical stress on the skull may lead to the formation of intrasutural bones to reduce strain. Restriction of bone growth, such as coronal suture synostosis, has been associated with surrounding sutures remaining open, with an increased incidence of intrasutural bones. Intrasutural bones have been highly correlated with central nervous abnormalities in humans such as osteogenesis imperfecta, rickets, and pyknodysostosis. The incidence and location of intrasutural bones vary among species. In order to comprehensively document this variability in the occurrence of different types of intrasutural bones within Mammalia, we conducted a comprehensive literature review.

We searched electronic databases to retrieve species that satisfied our criteria for Wormian bones. Synonyms for the main search term ‘Wormian bone’ included supernumerary, intrasutural, and fontanelle bone. Searches of Wormian bones may have contained specific identifiers based on suture location. References were screened for photos and illustrations.

Entries for each Wormian bone include the species classification, location of bone, and an illustration (if provided). Based on the location of the bone, the species were sorted into an intrasutural bone category: bregma, coronal, frontal-maxilla, interparietal (1, 2, 3, and 4 elements), lambdoid, metopic, nasal, orbital, pterion, and sagittal. A cladogram was created based on existing trees was created to map the relationship between the species and occurrences of Wormian bones.

Intrasutural bones are identified within most mammal groups in various locations. The bregmatic bone has a variable occurrence throughout Mammalia. When a supernumerary bone has been identified in the frontal-maxilla or orbital region, there is not an instance of a bregmatic bone. Instances of a supernumerary bone towards the front of the skull (metopic, nasal, and coronal) are rarely identified together in the same species. Supernumerary bones at the pterion are mostly recorded in primates. The bregmatic and interparietal bones are less variable among species compared to smaller, anterior intrasutural bones.

The pattern of Wormian bones anterior to the coronal suture may fluctuate more than those posterior, possibly due to greater variability in neural crest-derived bones development vs. mesoderm-derived bones. Alternatively, the more variable bones among mammals could result from greater variation of the associated brain region itself between species. Future work seeks to more extensively survey individual species to assess how variable each Wormian bone is within a species.