Hadronomas and the case of the missing fifth digit: a study of fifth digit proportions in extant and extinct kangaroos

Sthenurines (Sthenurinae) contains Pliocene–Pleistocene species that possess monodactyl feet, the fifth (V) digit present in extant kangaroos being lost. Miocene sthenurines retain the digit V, and its loss in geologically younger taxa has hitherto been a mystery. An investigation of metatarsal (Mt) proportions in macropodoids (Macropodoidea) shows that robusticity of Mt five (V) in extant species is inversely correlated with the length of Mt four (IV)—longer-footed species having more slender digit Vs. This also correlates with mode of locomotion, as species that are specifically adapted to higher speed endurance hopping have longer feet. However, the late Miocene sthenurine Hadronomas puckridgi differs from extant kangaroos in having a long digit four (IV) but retaining a robust digit V. The Mt-V is also highly resistant to bending, and comprised a significant percentage of cortical bone. We propose that the pedal morphology of H. puckridgi represents an intermediate stage in sthenurine locomotion, during which bipedal striding had been adopted but the astragalus was not yet fully modified to resist medial shifting of the weight. The robust digit V was, therefore, important for lateral support of the body weight on the long hind leg. George Watts [[e-mail]], Bristol Palaeobiology Group, School of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK; Centre for Integrative Anatomy, Department of Cell and Developmental Biology, University College London, Gower Street, London, WC1E 6BT, UK; Suresh Singh [[e-mail]], Bristol Palaeobiology Group, School of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK; School of Environment, Earth & Ecosystem Sciences, The Open University, Milton Keynes, MK7 6AA, UK; Emily J. Rayfield [[e-mail]], Bristol Palaeobiology Group, School of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK; Christine M. Janis [[e-mail]], Bristol Palaeobiology Group, School of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK; Department of Ecology, Evolution and Organismal Biology, Brown University, Providence, RI 02912, USA.