Scratch-digging sauropods, revisited

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.

Keywords:

• sauropod
• claws
• digging
• nesting
• behaviour
• reproduction

Acknowledgements

For permissions to reproduce videos: thanks to Grace Smith-Bishop (Gopherus); Timothy and Joan Frank (wombat digging); and Dave Stegenga, Honolulu Zoo, www.honoluluzoo.org (Geochelone nigra digging). For access to specimens: thanks to Jack Horner (MOR), Jay Rotella (MSU), Mark Goodwin and the UCMP staff, and Carol L. Spencer (MCZ). Thanks to Matt Bonnan, Frankie Jackson and Dave Varricchio for general discussion and reviews; Carrie Ancell, Bob Harmon, Jamie Jette, Linda Roberts, and MOR field crews for collection and preparation of specimens. Additional thanks to Bobby Boessenecker, Liz Freedman, Mike Knell, Ash Poust, John Scannella, Melissa Svee, and Holly Woodward for reading through the manuscript and helpful discussion.