Air Space Proportion in Pterosaur Limb Bones Using Computed Tomography and Its Implications for Previous Estimates of Pneumaticity

Abstract

Air Space Proportion (ASP) is a measure of how much air is present within a bone, which allows for a quantifiable comparison of pneumaticity between specimens and species. Measured from zero to one, higher ASP means more air and less bone. Conventionally, it is estimated from measurements of the internal and external bone diameter, or by analyzing cross-sections. To date, the only pterosaur ASP study has been carried out by visual inspection of sectioned bones within matrix. Here, computed tomography (CT) scans are used to calculate ASP in a small sample of pterosaur wing bones (mainly phalanges) and to assess how the values change throughout the bone. These results show higher ASPs than previous pterosaur pneumaticity studies, and more significantly, higher ASP values in the heads of wing bones than the shaft. This suggests that pneumaticity has been underestimated previously in pterosaurs, birds, and other archosaurs when shaft cross-sections are used to estimate ASP. Furthermore, ASP in pterosaurs is higher than those found in birds and most sauropod dinosaurs, giving them among the highest ASP values of animals studied so far, supporting the view that pterosaurs were some of the most pneumatized animals to have lived. The high degree of pneumaticity found in pterosaurs is proposed to be a response to the wing bone bending stiffness requirements of flight rather than a means to reduce mass, as is often suggested. Mass reduction may be a secondary result of pneumaticity that subsequently aids flight.

The first pterosaur pelvic material from the Dinosaur Park Formation (Campanian) and implications for azhdarchid locomotion

“…6). These ratios are low compared with other estimates (mean 77%) of pterosaur pneumaticity based on long bones (Martin and Palmer 2014) and other skeletal elements (mean = 60.5%; Elgin and Hone 2013), but do not include the large unenclosed dorsal pneumatic spaces of UALVP 56200. CT scans reveal four main internal chambers, in addition to the large, broken dorsal pneumatic chamber (Supplementary Material 2).…”

Histology and pneumaticity of Aoniraptor libertatem (Dinosauria, Theropoda), an enigmatic mid‐sized megaraptoran from Patagonia

“…However, postcranial pneumaticity is not unique to birds, being observed in non‐avian dinosaurs (Britt, 1993; Wedel, 2003; O´Connor, 2006, 2007; O’Connor and Claessens, 2006; Sereno et al ., 2008; Makovicky and Zanno, 2011; Benson et al ., 2012; Xu et al ., 2014; Watanabe et al ., 2015; Lambertz et al ., 2018) as well as pterosaurs (Britt, 1993; Butler et al ., 2009; Claessens et al ., 2009; Martin and Palmer, 2014). The presence of this condition in non‐volant taxa indicates that the interaction between postcranial pneumaticity and flight capability was secondarily acquired and suggests that the main function of postcranial pneumatization is not flight.…”

Exploring the Relationship between Skeletal Mass and Total Body Mass in Birds

“…Pterosaurs are significantly morphologically different than birds, having a single extended digit forming the main wing spar, a flexible membrane forming the wing surface, a filamentous integumentary covering of pycnofibres rather than feathers, and many more [ 53 ]. Furthermore, it appears that pterosaurs (at least the larger species) were even more highly pneumatized than birds [ 54 ], making them substantially different. These morphological differences combined with the results here indicate that phylogeny plays an important role in this relationship and suggests that this relationship should not be used to determine pterosaur body mass, and alternative methods should be sought.…”