Is My Skeleton Fat? Asked the Quetzalcoatlus Pterosaur & Elizabeth Martin Answered...

A novel method of estimating pterosaur skeletal mass using computed tomography scans

Authors:

Martin et al

Abstract:

Body mass is an important, basic parameter of life. It is central to understanding many aspects of an animal's ecology and behavior and is potentially the most important factor affecting locomotor performance (Biewener, 1989). Body mass is particularly significant for volant animals, because it affects the speed required for take off and landing, flight speed, and maneuverability in flight and influences the upper body size limit of flight (Alexander, 1998). Accordingly, accurate mass estimates are key to understanding the behavior and capability of extinct animals.

Estimates of body mass have been obtained for a number of extinct volant animals, including birds (e.g., Hone et al., 2008) and pterosaurs (e.g., Bramwell and Whitfield, 1974; Brower and Veinus, 1981; Witton, 2008; Henderson, 2010). Because pterosaurs were both the first vertebrates to achieve powered flight and the largest animals ever to do so, accurate mass estimates are crucial to our understanding of pterosaurian biomechanics and the upper size limits on their flight. The two main approaches that have previously been taken to calculate pterosaur body mass rely either on estimates of total body volume and density distributions (Bramwell and Whitfield, 1974; Brower and Veinus, 1981; Henderson, 2010) or the relationship between skeletal mass and total mass (Witton, 2008), using data from modern birds (Prange et al., 1979).

The results vary greatly from method to method—for example, the mass of the 10.5-m wingspan Quetzalcoatlus northropi has been estimated at 75 kg (Brower and Veinus, 1981), 259 kg (Witton, 2008), and 544 kg (Henderson, 2010), although this larger estimate is thought to be based on an anatomically incorrect reconstruction (Witton and Habib, 2010). This range is so wide that it spans from an animal with an improbably low implied average body density of 0.15 g/cm3 (Witton, 2008) to a heavy animal, incapable of flight (Henderson, 2010). Even Witton's (2008) estimate of 259 kg would render the animal flightless, according to some analyses (e.g., Chatterjee and Templin, 2004), but not others (e.g., Witton and Habib, 2010).

Witton (2008) applied the skeletal mass approach to pterosaurs, estimating skeletal mass from bone volume calculated using simple geometric shapes—a pragmatic method, and the only feasible approach in the absence of comprehensive three-dimensional (3D) morphological information. To estimate cortical thickness, Witton (2008) applied a regression analysis of thickness-to-diameter data for a range of pterosaur bones. Then, to estimate total body mass, the relationship found by Prange et al. (1979) between skeletal mass and total mass in birds was applied, allowing determination of the body mass of various pterosaur species over a wide range of animal sizes. This relationship was deemed to be appropriate because it applies to both birds and terrestrial mammals, which are distant in phylogenetic terms, and it was therefore thought to likely be accurate for pterosaurs as well.

Here, we present a novel method that uses computed tomography (CT) scans to estimate the volume, and hence the mass, of pterosaur wing bones and thus provides a more robust foundation for the methodology adopted by Witton (2008). The use of CT scans to analyze fossils is a relatively new process, but representatives of a number of different groups, including arthropods (e.g., Penney et al., 2007), mammals (e.g., Ni et al., 2012), and dinosaurs (e.g., Martinez et al., 2011), have already been studied. However, few CT studies have been performed on pterosaurs, focusing mainly on the skull (e.g., Witmer et al., 2003) or axial skeleton (Claessens et al., 2009). Only one study has reported CT scans for any part of the pterosaurian appendicular skeleton—a single humerus of Bennettazhia (Habib, 2008). To demonstrate the method, we estimate the mass of three first wing phalanges from different ornithocheirid pterosaur individuals, using CT scans that are new to the scientific literature.