Earlier we looked at
PILs, parallel interphalangeal lines (Peters 2000, 2010), which appear in a wide variety of tetrapod extremities, typically, but not always, aligning phalanges in parallel sets. Note the PILs of the plantigrade pes of Kryptobaatar (from Kielan-Jaworowska and Gambaryan 1994; Fig. 1 left) become better aligned when the pes is elevated to the digitigrade configuration (Fig. 1 right), as illustrated by Kielan-Jaworowska and Gambaryan in their 1994 paper (Fig. 1 middle).
Living taxa that are both digitigrade and arboreal
include the big cats. The claws are not so sharp and not so curved, but they are long and their keratin sheath may have been much sharper and more curved.
Note the very short astragalus
and the calcaneum positioned at an odd lateral angle. These unique traits suggested to early researchers that Kryptobataar could have rotated its ankle to a great degree, ideal for establishing a good grip from any angle, even inverted, on a tree limb.
Kielan-Jaworowska Z and Gambaryan PP 1994. Postcranial anatomy and habits of Asian multituberbulate mammals. Fossils & Strata 36:1-92.
Peters D 2000. Description and Interpretation of Interphalangeal Lines in Tetrapods. Ichnos 7:11-41.
Peters D 2010. In defence of parallel interphalangeal lines. Historical Biology iFirst article, 2010, 1–6 DOI: 10.1080/08912961003663500
Wible JR Rougier GW 2000. Cranial anatomy of Kryptobaatar dashzevegi (Mammalia, Multituberculata), and its bearing on the evolution of mammalian characters. Bulletin of the American Museum of Natural History 247:1–120. doi:10.1206/0003-0090(2000)247<0001:CAOKDM>2.0.CO;2.