FOWLER, D.W., & HALL, L.E. (2011) Scratch-digging sauropods, revisited, Historical Biology doi:10.1080/08912963.2010.504852 [link to paper]

ABSTRACT

Unguals of sauropod dinosaurs are notable for their unusual shape and orientation, and differ from those of graviportal mammals to which they are often compared. Early in their evolution, sauropod manual unguals underwent severe reduction, being lost on all digits except digit (D)-I. In contrast, the pedal unguals become hypertrophied and laterally compressed with an unusual angled orientation. Recent workers dismissed an early suggestion of a scratch-digging function, supporting instead substrate-gripping during locomotion, although neither hypothesis has been adequately tested, and rejection of the scratch-digging hypothesis was based on inadequate comparison. Here, we show that sauropod nesting traces are morphologically consistent with being excavated by scratch-digging, and that this behaviour is also observed in extant tortoises, which possess flattened unguals with a functionally analogous orientation. In this initial qualitative study, we surveyed claw morphology and use in a range of tortoises, based on previously published accounts and observation of specimens and video footage. Our findings suggest that there is a good case for re-examining the scratch-digging hypothesis for sauropods, although further work is required. The influence of reproductive behaviours on morphology is discussed, including the suggestion that the single manus claw of sauropods may have functioned for mate-gripping during copulation.