The hind limb of Captorhinus aguti and the step cycle of basal amniotes
The hind limb of Captorhinus aguti and the step cycle of basal amniotes (in Studies on the evolution of vertebrates; papers in honour of Robert Lynn Carroll--Etudes sur l'evolution des vertebres; articles en l'honneur de Robert Lynn Carroll, Hans-Dieter Sues (editor), Alison M. Murray (editor) and Jason S. Anderson (editor))
Canadian Journal of Earth Sciences = Revue Canadienne des Sciences de la Terre (April 2003) 40 (4): 515-526
Exquisitely preserved limb elements of Captorhinus aguti from the Lower Permian locality of Richards Spur, Oklahoma, form an excellent basis for analysing the step cycle of the hind limb of basal amniotes. An articulated composite hind limb skeleton is moved through the step cycle. These movements are constrained by the intrinsic skeletal anatomy and data from fossilized footprints attributed to Palaeozoic amniotes. Two well-defined hinge joints are evident: a distal tarsometatarsal joint, and a mesotarsal joint between the calcaneum-astragalus-centrale unit and distal tarsals. A strict homology between the latter and mesotarsal joint in lepidosaurs is uncertain. The conspicuous size disparity between the distal end of the tibia and the much larger tibial facet of the astragalus, a widespread feature of archaic amniotes, permits considerable movement between the distal end of the tibia and astragalus during the stance phase. Differential dorsoflexion, concentrated at the preaxial side of the mesotarsal joint, allows the pes to remain stationary (rather than slide laterally) on the substrate as the femur is retracted and the tarsus shifts from an anterior to lateral orientation. The distal end of the femur describes an arc of only approximately 60 degrees during the stance phase, but could rotate about 60 degrees about its long axis, a figure comparable to that recorded for living lizards.