Just above the medial condyle is a low yet noticeable ridge which juts away from the rest of the bone, towards the midline of the animal's body. This ridge is known as a mesiodistal crest. The area immediately above the condyles possesses a shallow yet wide lowered area known as an extensor groove. Overall the femur is almost identical to that of other abelisaurids. The proximal (upper or innermost) part of the tibia possesses a myriad of complex features. A large and hatchet-shaped structure known as a cnemial crest points forwards at the proximal portion of the tibia. The tip of the cnemial crest is hooked due to the presence of a downward pointing spur, known as a ventral process. Although Coria (2001) considered a hooked cnemial crest to be unique to "Quilmesaurus", Valieri "et al." (2007) noted that this structure was also possessed by "Aucasaurus" and "Majungasaurus", as well as the ambiguous abelisaurid "Genusaurus". The distal part of the tibia possesses its own projections for connecting to ankle bones, known as malleoli. This part has the form of an asymmetrical triangle when seen from the front, with the massive lateral malleolus projecting further distally than the smaller medial malleolus. This combination of distal tibia features was also once presumed to have been unique to "Quilmesaurus". However, Valieri "et al." (2007) note that the distal tibia of "Rajasaurus" was very similar to that of "Quilmesaurus". In 2016, "Quilmesaurus" was estimated to have measured in length. This would have made it among the smallest derived abelisaurids, although its legs were proportionally robust like those of "Pycnonemosaurus", one of the largest members of the family. When originally described, Coria could not find a more precise placement for "Quilmesaurus" than Theropoda. The presence of a notch in the distal articular surface of the tibia was cited by him as evidence of a possible relationship with basal Tetanurae, which would be surprising as "Quilmesaurus" lived during a time when South American theropod assemblages were dominated by abelisaurids and carcharodontosaurs. Other theropod material has been recovered from within these same strata and has in 2005 also provisionally been referred to the Tetanurae.