| Summary: | Armadillos are fossorial mammals found primarily in South America. Extant genera vary in size, behavior, and habitat. Armadillos use their forelimbs to break the soil when digging; hindlimb use varies. Some use hindlimbs purely for bracing; others actively excavate with their hindlimbs. Use of the hindlimbs in excavation is related to burrow creation. Three armadillo species were examined in the present study: Dasypus novemcinctus, Tolypeutes matacus, and Chaetophractus vellerosus. Dramatic differences were present in both the osteology and myology of the hindlimb. Identification of muscles was a non-trivial task. Among the difficulties was identification of divisions of the gluteus superficialis. This muscle traditionally is divided into the gluteus maximus and femorococcygeus. In armadillos the natural division appears to be tripartite. The hindlimb of Tolypeutes is dramatically different from that of Dasypus. Chaetophractus has a more intermediate morphology, but generally resembles Dasypus. Many anatomical differences appear related to burrowing. Tolypeutes is the least fossorial, but most cursorial of extant armadillos. Both Dasypus and Chaetophractus are accomplished burrowers, and share several features which appear to be specializations for using the hindlimb to excavate soil. The tibia and fibula are more robust in Dasypus than in Tolypeutes. A well developed flange, site of origin for the peronei, is present on the proximal fibula in Dasypus. The tibia-fibula of Chaetophractus is intermediate. The muscles of Dasypus, particularly the gluteals and hamstrings, are better developed than those of Chaetophractus, which in turn, has better developed muscles than Tolypeutes. Two muscles, the caudofemoralis and crurococcygeus, are present in Dasypus, but absent in the other two armadillos. The sartorius and tenuissimus are present in Chaetophractus and Dasypus, but absent in Tolypeutes. Intrinsic pedal musculature is better differentiated and developed in Dasypus and Chaetophractus. Fossoriality has yet to be directly measured in armadillos. Behavioral descriptions have suggested Chaetophractus is more fossorial than Dasypus. Hindlimb anatomy suggests Dasypus is the better burrower. Extending the hindlimb analysis to a controversial fossil armadillo, †Peltephilus, suggests it was not a burrower.
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