Computed tomography visualization of Wormian bones in the pen-tailed tree shrew (Ptilocercus lowii)

Wormian bones are variably ossifying bones within the sutures of skulls. While widespread among mammals, they do not occur in every specimen of species that are known to form Wormian bones. While the exact reason(s) for why these bones form is unknown, it has been hypothesized that they may form due to prolonged cranial stress. Wormian bones in primates have been studied for over a century, but to our knowledge, they have not yet been reported from Ptilocercus lowii (Pen-tailed tree shrew), a member of Scandentia, a mammalian order sometimes recovered as an outgroup to Primatomorpha, the group that includes primates. We focused on two main questions: 1) How does the location and morphology of Wormian bones vary within P. lowii? and 2) Are the locations of the Wormian bones in P. lowii the same as in primates?

To examine tree shrew skulls for Wormian bones, we sourced computed tomography (CT) scans from Morphosource. The Ptilocercus lowii skulls are female specimens from different nature reserves in Malaysia. These scans were first viewed on surface meshes to search for Wormian bones. The CT scans with potential Wormian bones were then uploaded to Avizo 2020.1. Once in Avizo, modifications were made to the scan to better locate the Wormian bone. Avizo’s transformation function was used to align the scan into anatomical position. The gamma filter was then applied to show only the whitest pixels. This technique ensured that only bone was visible and no debris from the scanning process was picked up in the image. Each slice of the CT scan was viewed in search of the potential Wormian bone before segmenting it manually using the Avizo paintbrush. The bone was then examined by all authors, and once it was agreed that it was indeed a Wormian bone, portions of the adjacent bones were segmented manually.

Both tree shrew specimens have Wormian bones, but they are not in the same location in the skull. Specimen USNM 481107 has a Wormian bone in the lambda (where the sagittal and lambdoidal sutures meet). This Wormian bone can also be classified as an interparietal. USNM 481103 has a Wormian bone in the suture between the frontal and nasal bones and lacks a bone in the lambda. Interestingly, both Wormian bones overlapped surrounding skull bones along parts of their sutures, a morphology not readily apparent from the external views alone. Wormian bones occur in similar locations in primates. These findings show that the locations of Wormian bones are variable within P. lowii; however, with only two specimens, it is difficult to draw firm conclusions about what this variation means. These data also show us that the locations are consistent among primates and at least one scandentian taxon, suggesting the possibility of a homologous developmental origin for these bones.

We plan to expand our sample to determine whether additional specimens will show consistent Wormian bone locations within P. lowii and to further compare the varying locations of Wormian bones in P. lowii to those of other mammals.