Comparative analysis of the vertebral pneumatization in pterosaurs (Reptilia: Pterosauria) and extant birds (Avialae: Neornithes)

Abstract

Birds and pterosaurs have pneumatic bones, a feature likely related to their flight capabilities but whose evolution and origin is still poorly understood. Pneumatic foramina are present on the external surface of the bone and are reliable indicators of post-cranial skeletal pneumatization present in Pterosauria, Eusauropoda, and Neotheropoda. Here, we carried out a qualitative analysis of the position, size and number of pneumatic foramina of the cervical and thoracic/dorsal vertebrae of pterosaurs and birds, as they have the potential to challenge hypotheses about the emergence and evolution of the respiratory trait in these groups. We also discussed differences between pneumatic and vascular foramina for identification purposes. Besides phylogenetic representativeness, the pterosaur taxonomic sampling considered the preservation of specimens and, for birds, their life habit, as this relates to the level of pneumatization. Pneumatic foramina on the lateral faces of the centrum of the mid-cervical vertebrae of pterosaurs and birds differ in position and size, and those adjacent to the neural canal additionally differ in number. The avian posterior cervical vertebrae show a higher number of pneumatic foramina in comparison to their mid-cervicals, while the opposite is true for pterosaurs, suggesting differences in the cervical air sac of these clades. Pneumatic foramina were found at the base of the transverse processes of the notarial vertebrae of birds, while they were absent from some of the pterosaurs analyzed here, revealing the presence of a pneumatic hiatus in the vertebral column that might be explained due to the distance of this structure to the cervical air sac. These findings indicate that, although the overall skeletal pneumatization of pterosaurs and birds present deep homologies, some pneumatic features occurred convergently because variation in the number of pneumatic foramina along the vertebral column is related to the position of the air sacs in pterosaurs and birds and/or the habit of each species. There is an evident reduction of the pneumatic foramina in birds that have aquatic foraging and an increase in the ones which perform static soaring. Although we did not find any external anatomical difference between pneumatic and vascular foramina, we observed that vascular foramina occur at specific sites and thus identification on the basis of location is reliable.

Assessing ontogenetic maturity in extinct saurian reptiles

“…Müller, 1908;Wedel, 2009). The degree of pneumatization could reasonably be used to assess maturity in extinct groups which possess these features (namely, pterosaurs, Biological Reviews 96 (2021) 470-525 © 2020 Cambridge Philosophical Society neotheropods, and eusauropods; Wedel, 2009;Buchmann, Avilla & Rodrigues, 2019) because skeletal pneumatization changes during ontogeny in living dinosaurs. Field observation of skull pneumatization in living passerine birds by wetting the skull and pulling back feathers is a common method to determine the age of an individual in ornithology (e.g.…”

Quantitative assessment of the vertebral pneumaticity in an anhanguerid pterosaur using micro-CT scanning

“…In this case, the ASP values of SNSB/BSPG 1991 I 27 and SMNK 3985 would be similar. Such high degree of pneumatisation had been already suggested for late pterodactyloids 17 , 18 , 22 , 46 , 47 .…”

“…The mid-cervical vertebrae of SNSB/BSPG 1991 I 27 have no pneumatic foramina adjacent to the neural canal, only on the lateral surface of the centra 29 , differing from some other anhanguerians in which foramina are present in both regions 17 , 25 , 38 . Our analysis of the sections of SNSB/BSPG 1991 I 27 indicates that internal pneumatic cavities close to the sides of the centrum spread out dorsally and increase in the area of the pedicle of the neural arches, forming large air spaces above the neural canal and establishing a highly pneumatised region independent of the presence of pneumatic foramina in that region (Fig.…”