Afromimus was initially described as "medium-sized" for an ornithomimosaur. Judging by the fusion in the vertebrae and between the fibula and tarsus of the holotype specimen of
Afromimus, the individual had reached adult size before it died. The tibia has a length of . Sereno noted that the largest specimen of the ornithomimosaur
Sinornithomimus (a subadult at least seven years old) had a tibia of a similar length, and concluded that
Afromimus had a somewhat smaller adult size, making it half the size of
Gallimimus, twice the size of
Shenzhousaurus, and three times the size of
Nqwebasaurus.
Tail vertebrae Sereno identified several distinctive characteristics in the tail vertebrae of
Afromimus. The 16th tail vertebra had depressions along the top and bottom surfaces of the broad , which Sereno described as "peanut-shaped". Such a structure would have stabilized the tail from flexing to the side. The depressions on the bottom surfaces are less evident towards the rear of the tail. This trait is shared with
Gallimimus and an unnamed
ornithomimid from the
Dinosaur Park Formation, and contrasts with the less-broad centra of the abelisauroids
Majungasaurus and
Masiakasaurus; however, among the Abelisauroidea, the
noasaurid Elaphrosaurus also has similar vertebrae. Additionally, the prezygapophyses and postzygapophyses ( at the front and rear of the vertebrae that interlock with adjacent vertebrae) are distinctive. The sides of the prezygapophyses have a rough texture, which Sereno considered to be an
autapomorphy, or distinguishing trait; however, these rough surfaces are also seen in the abelisauroids
Majungasaurus,
Ceratosaurus, and
Ekrixinatosaurus, and Cerroni and colleagues suggested that this trait may be more widespread as indications of muscle or ligament attachments. The sides of the postzygapophyses are rough as well. The facets where the prezygapophyses interlock are also distinctive — a rounded corner on the outer surface of the postzygapophysis fit into an oval facet on the inner surface of the prezygapophysis, and the tip of the postzygapophysis fit into a rimmed depression on the prezygapophysis which was situated closer to the centrum. The purpose of these structures was likely to stiffen the tail. Sereno noted that the latter trait is shared with
Sinornithomimus and more advanced ornithomimids, but the same is also present in
Majungasaurus,
Masiakasaurus, and several Indian abelisauroids. Several processes on the tail vertebrae of
Afromimus are present as low crests or ridges, including the and , although the transverse processes become smooth by the 27th tail vertebra. Below the transverse processes, the 16th tail vertebra also has an accessory ridge, accompanied by a depression below it; the depression also becomes smooth by the 27th tail vertebra. On the bottom of the vertebra, there is a pair of prominent, roughened ridges where the s attach. The pedicels, or surfaces where the chevrons articulate with their corresponding vertebrae, are fused into a continuous, crescent-shaped surface in
Afromimus. Sereno considered as an autapomorphy the fact that the fused surface was half as wide front-to-back than it was side-to-side, although
Ceratosaurus,
Majungasaurus, and
Masiakasaurus also have the same condition. Despite this, Cerroni and colleagues noted a potential distinguishing trait in that the rearward-projecting (posterior) process of the chevron is much larger than the forward-projecting (anterior) process, which is not seen in any ornithomimosaurs or abelisauroids.
Hindlimbs At the top end of the tibia, Sereno noted an unusual elliptical attachment scar bearing a series of roughened ridges, located below the expanded
lateral condyle on the outer surface of the bone. Most ornithomimosaurs lack this scar, but it is present in the abelisauroids
Masiakasaurus,
Ekrixinatosaurus,
Skorpiovenator, and
Carnotaurus, as well as to a lesser extent in
Velocisaurus and
Ceratosaurus. Just to the front of the scar is the fibular crest, which is shorter and narrower than those of
Sinornithomimus and
Gallimimus and also becomes deeper at its bottom end. This crest stretches to the top of the bone, which is typical of abelisauroids but is unusual for ornithomimosaurs and other
tetanuran theropods, where it is separated by a notch from the top. Like the two ornithomimosaurs, however, the bottom edge of the lateral condyle is clearly demarcated from the shaft of the bone. The shaft of the tibia is compressed at the front and rear, and bows outwards, unlike the relatively straight shafts of the ornithomimosaurs
Sinornithomimus,
Gallimimus, and
Beishanlong, and the noasaurid
Elaphrosaurus, but like the noasaurids
Masiakasaurus and
Velocisaurus. The bottom end of the tibia expands outwards to form the lateral malleolus, which is one-third the width of the bottom end and is partially fused to the (one of the tarsal bones). This partial fusion is unknown in ornithomimosaurs, but it is present in
Ceratosaurus, noasaurids, and the
Alvarezsauroidea. The actual articulating surface of the astragalus on the tibia is roughened and bears a raised lip, the medial buttress, at its inner rim, which is unknown in ornithomimosaurs but is present in
Ceratosaurus and various abelisauroids. The inner edge of the astragalus is offset from the buttress, which is similar to
Nqwebasaurus. Compared to ornithomimosaurs, the articulating surface for an ascending process of the astragalus on the tibia is much smaller and lower. Accordingly,
Afromimus had a small, subrectangular ascending process on the astragalus like those of abelisauroids, but unlike
Sinornithomimus,
Gallimimus, and
Harpymimus. The (another tarsal bone) is also more exposed in
Afromimus. Unlike
Sinornithomimus and
Gallimimus, the fibular crest on the tibia does not join with the iliofibular tubercle or anterior trochanter (the attachment of the iliofibularis muscle) on front of the fibula, because the anterior tubercle is situated further towards the rear by about . The tubercle is roughened and better developed than in ornithomimosaurs, although a well-developed tubercle is typical of
Ceratosaurus and various abelisauroids. On the inner side of the fibula, there is a deep that extends smoothly onto the shaft in the form of a trough running in front of the anterior tubercle. The fossa is covered partially by the flared tibial crest, and it is also demarcated above by a crest angled upwards at the rear. There is also a roughened attachment surface below the tibial crest (considered by Sereno to be an autapomorphy). These characteristics are similar to
Ceratosaurus,
Masiakasaurus,
Skorpiovenator,
Arcovenator, and
Deltadromeus. Some tetanurans, like
Tyrannosaurus,
Neovenator, and
Beishanlong, also have the deep fossa, but lack the surrounding crests. Nevertheless, Cerroni and colleagues noted that the tibial crest covers the fibial fossa to a much greater extent than
Masiakasaurus, which is an autapomorphy. The bottom end of the fibula in
Afromimus is relatively expanded for an ornithomimosaur, but it resembles
Masiakasaurus,
Skorpiovenator, and
Xenotarsosaurus in this respect. The foot claws of
Afromimus show characteristics shared between abelisauroids and ornithomimosaurs. The bottom surfaces of the claws are relatively flat, which is the case for most ornithomimosaurs (except for
Beishanlong and
Deinocheirus) and noasaurids. There are two grooves for the attachment of a
keratin sheath on the side of the claw, one near the top and one near the bottom, which is common to some noasaurids; however, many ornithomimosaurs only have the groove on the bottom. On the bottom of the claw, there is a V-shaped platform for the attachment of the sheath, with a deep pit in between. Sereno inferred that the sheath would have been a subtriangular "hoof" in life. The inner edge of the platform is a sharp ridge while the outer edge is rounded. Some noasaurids have a similar platform. The attachment for the flexor muscle at the rear of the claw is also recessed, which is seen in ornithomimosaurs. ==Classification==