This study focuses on the relationships of limb bone morphologies to body size, locomotor behavior, and phytogeny among small to medium Recent carnivores, and applies the information to hyaenodontids from the Eocene to early Oligocene of North America to better understand locomotor adaptations among the group. Limb bone structure are analyzed using linear regression analyses of limb bone dimensions (bone length, shaft sectional properties, and articular dimensions) and morphometric analyses of distal humeral and femoral articulations.

The Recent sample includes 47 placental and marsupial carnivore species, which were categorized into the locomotor groups (arboreal, scansorial, terrestrial with generalized, semi-fossorial, and semi-aquatic subgroups, and cursorial). Among the groups, cursors are the easiest to characterize by some features, which relate to the restriction to parasagittal movement and lightening of bones (e.g., long limbs, large medullary cavities, large articular volume relative to surface area, distal humeri with large trochlea, deep trochlear groove, and medially positioned capitulum, and mediolaterally narrow and symmetrical distal femora with deep and narrow patellar groove). In arborealists, joint structures indicate greater mobility (e.g., large articular surface area relative to volume, distal humeri with laterally expanded capitulum and wide trochlear groove, and wide distal femora and patellar groove), and shafts are more rigid, presumably to resist the greater loadings by musculature. Non-cursorial terrestrialists have shorter limbs, except in larger terrestrial generalists, and low shaft rigidities, with the exception of the humerus in large semi-fossorialists. Semi-fossorial terrestrialists have articulations which seem suited for transmitting heavy loads at the expense of mobility (e.g., large contact surface for ulna and relatively large femoral condyles). Scansors are intermediate between other groups in their limb bone structure. Distal humeral and femoral articular morphology may be more affected by phytogeny than by behavior, especially in scansors and terrestrialists.

All limb dimensions examined here correlated well with body mass and indicated low errors for the body mass estimates. Though limb bone dimensions do not scale strongly allometrically, body proportions change slightly in relation to size:​ larger animals have stronger humeri and tibiae relative to femora, larger distal humeral articulation relative to humeral head, and larger femoral head relative to distal femoral articulation, which may relate to changes in limb posture and joint mobilities.

Hyaenodontids show similarity to marsupials in some characters (e.g., distal humeral shape, and greater shaft rigidities relative to articular dimensions), presumably suggesting retention of primitive traits. Relatively stronger femora and anteroposteriorly elongated tibial shaft are unique in hyaenodontids compared with Recent carnivores.

North American hyaenodontids were non-cursorial terrestrial animals in general, but there were some variations. Wasatchian hyaenodontids, such as Prototomus, had less clear locomotor specializations. Incipient running adaptations (e.g., the relative humeral dimensions) existed in Arfia and in Pyrocyon;​ however, Pyrocyon has distal humeri similar to that of arboreal camivorans. Distal femora of Prolimnocyon resemble those of semifossorial camivorans, but Prolimnocyon was adapted for tree-climbing based on humeral articular structures. For Bridgerian proviverrine Sinopa and Tritemnodon, scansorial caniform-like locomotion was suggested based on distal humeral and femoral articular shapes. The limb bone dimensions of Sinopa also have unspecialized non-cursorial terrestrial characters. In the Bridgerian and Uintan limnocyonines (Thinocyon, Limnocyon, and Machaeroides), the strong humeral shaft relative to bone length and distal femoral articular morphology suggests digging specializations. Distal humeral shape of Hyaenodon horridus from the Orellan indicates more cursorial adaptations, and that of H. crucians suggests felid-like scansorial locomotion;​ however, they were less cursorial than modem canids based on robust and short limbs.

Estimated body masss (kg) for hyaenodontids are as follows:​ Arfia shoshonienesis, 5.4;​ A. opisthotoma, 9.5;​ Prototomus spp.,< 6.0;​ Pyrocyon dioctetus, 2.6;​ Sinopa spp., 1.3- 14;​ Tritemnodon spp., 7.6- 13;​ Prolimnocyon atavus, 1.6;​ Thinocyon velox, 0.7 -1.5;​ T. medius, 1.2 - 2.5 (dimorphic);​ Machaeroides eothen, 12;​ Limnocyon verus, 7.8;​ L. potens, 16;​ Hyaenodon crucians, 9.1 - 25;​ H. horridus, 25 - 43 (dimorphic). Body size range of hyaenodontids gradually expanded from middle Wasatchian to middle Bridgerian and shifted strongly upwards after Bridgerian.