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.
Highlights
Pterosaurs were a group of flying archosaurian reptiles that evolved during the Triassic, making them the earliest vertebrates capable of powered flight by over 50 million years
Pneumaticity is most often studied on a presence or absence basis (e.g. [9,15,18]), but can be quantified through a relative pneumaticity index [12,19], or by looking at the volume of pneumatic cavities. This air space proportion (ASP), is defined as the proportion of bone cross section occupied by air space compared to the total cross-sectional area [10]
While WP1 and WP2 appear to be similar in their ASP value and distribution, the single WP3 in our dataset (UP WP3) has a lower central shaft ASP value than any other bone studied, 15% lower than the proximal head, and 10% lower than the lowest ASP recorded in any other bone
Summary
Pterosaurs were a group of flying archosaurian reptiles that evolved during the Triassic, making them the earliest vertebrates capable of powered flight by over 50 million years. The bird-like respiratory system present in pterosaurs [6] comprised of lungs and an extensive air sac system with diverticula that pneumatized the postcranial skeleton. [9,15,18]), but can be quantified through a relative pneumaticity index [12,19], or by looking at the volume of pneumatic cavities This air space proportion (ASP), is defined as the proportion of bone cross section occupied by air space compared to the total cross-sectional area [10]. ASP is equal to K2 for a given circular long bone shaft of constant cortical thickness, which allows the conversion of published K values to ASP values
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