Afromimus

Afromimus (meaning "Africa mimic") is a genus of theropod dinosaur from the Early Cretaceous Elrhaz Formation of Niger. It contains a single species, A. tenerensis, named in 2017 by Paul Sereno from parts of the right leg, vertebrae, and ribs found in the Ténéré Desert. It was originally classified as an ornithomimosaurian, but subsequently it was argued to be an abelisauroid.
Discovery and naming.
The only known specimen of "Afromimus" is a partial skeleton consisting of seven (originating from the middle of the tail and estimated to be the 15th, 16th, 18th, 20th, 22nd, 24th, and 27th tail vertebrae), bones from the right hindlimb (the , , , and ), and part of a rib, all found within a radius of . It was discovered in 1997 at the Gadoufaoua locality in the Elrhaz Formation, in the Ténéré Desert of Niger. The specimen is catalogued as MNBH GAD112 in the Musée National Boubou Hama, the national museum of Niger in the city of Niamey. Since the specimen was exposed to the elements when it was found, it may have been more complete when it was still buried. The hindlimb elements were discovered in place, although they were weathered; the tail vertebrae were out of place, and the last vertebra in the sequence was found several meters away from the rest of the specimen.
In 2017, Paul Sereno published a description of MNBH GAD112 as the holotype specimen of a new genus and species, "Afromimus tenerensis". The genus name combines the Latin prefix "Afro-" ("Africa") and the Greek word "mimos" ("mimic"), in reference to the interpretation of the specimen as a member of the Ornithomimosauria. The species name combines the name of the Ténéré Desert and the Latin suffix "-ensis" ("from"). Subsequently, in 2019, Mauricio Cerroni and colleagues published a reassessment of "Afromimus" as a member of the Abelisauroidea.
Description.
"Afromimus" was 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.
Despite various characteristics shared with abelisauroids, Sereno assigned "Afromimus" to the Ornithomimosauria in 2017. To support this position, he stated that the "peanut-shaped" centra and the rimmed depressions on the prezygapophyses in the tail vertebrae have been reported in "Sinornithomimus", "Gallimimus", and other ornithomimosaurs ("Deinocheirus" also has broad centra, although it lacks the prezygapophyseal depressions), while the shape of the centra contrasts with the subcircular centra in "Majungasaurus", "Masiakasaurus", and alvarezsauroids.
He also drew attention to characteristics of the foot claws, namely the flat bottom face with a V-shaped platform and the recessed flexor attachment, which are shared with ornithomimosaurs. While he noted that the first two traits are shared with abelisauroids, as well as the additional sheath attachment groove near the top of the claw, he noted that the latter condition was not consistent among abelisauroids: the grooves in abelisauroids usually reach the tip of the claw, while they dissipate beforehand in "Afromimus", and an unnamed noasaurid also from the Elrhaz Formation. Sereno also noted that the expansion of the fibula's bottom end resembles "Masiakasaurus", but not the unnamed noasaurid.
Within the Ornithomimosauria, Sereno did not provide a concrete position for "Afromimus" other than noting it was a basal (early-diverging) member. He noted that the prezygapophyses were relatively short compared to "Harpymimus" and "Shenzhousaurus", and that the ascending process of the astragalus is much less extensive in both "Afromimus" and "Nqwebasaurus" compared to most other ornithomimosaurs. Sereno concluded that the affinities of "Afromimus" "will likely be tested most effectively by discovery of new material".
In their 2019 paper, Cerroni and colleagues systematically analyzed the traits that Sereno noted as being shared between "Afromimus" and ornithomimosaurs, and concluded that many of the same traits can also be found among abelisauroids (see §Description for further discussion of the analysis). In particular, "Afromimus" shares with abelisauroids:
To rigorously test the affinities of "Afromimus", they added it to three phylogenetic datasets: two generally encompassing theropods, from Carrano "et al." (2012) and Choiniere "et al." (2012); and one focusing specifically on the Ceratosauria, from Brissón Egli "et al." (2016). The most parsimonious phylogenetic trees for the first two analyses found "Afromimus" to be a member of the Ceratosauria, grouping with "Masiakasaurus" and "Majungasaurus" in the former and with "Masiakasaurus" in the latter. Forcing "Afromimus" to be an ornithomimosaur resulted in a less parsimonious tree for the latter analysis. Finally, adding "Afromimus" to the ceratosaur-centric matrix resulted in it being found as part of the Noasauridae in a polytomy (unresolved group). In this analysis, the inclusion of "Afromimus" in the Ceratosauria is supported by the prominent iliofibular tubercle; its inclusion in the Noasauridae is supported by the flattened bottom end of the tibia and the fusion of the astragalar ascending process to the fibula. The phylogenetic tree from the analysis is partially replicated below.
Given these results, Cerroni and colleagues referred "Afromimus" to the Abelisauroidea, with a likely position in the Noasauridae. However, they maintained it as a valid taxon because of the large posterior process of the chevron and the extent to which the tibial crest of the fibula covers the fibular fossa.
Evolution of ornithomimosaurs.
In 2014, Romain Allain and colleagues suggested that the close relation of the African "Nqwebasaurus" to Eurasian basal ornithomimosaurs indicates that the group was widespread before the breakup of Pangaea. In his description of "Afromimus", Sereno suggested that southern ornithomimosaurs such as "Afromimus" and "Nqwebasaurus", which he labelled as the only unambiguous ornithomimosaurs from the southern hemisphere, were an early phase in the group's evolution. However, the removal of "Afromimus" from the Ornithomimosauria by Cerroni and colleagues weakens this hypothesis, as do several analyses excluding "Nqwebasaurus" from the Ornithomimosauria; they considered the presence of ornithomimosaurs in the southern hemisphere to be equivocal.
Paleoecology.
No other fossil vertebrates were found with the remains of "Afromimus" at the Gadoufaoua locality. However, other dinosaurs are known from Gaoufaoua: the theropods "Suchomimus"/"Cristatusaurus", "Kryptops", and "Eocarcharia"; the sauropod "Nigersaurus"; and the ornithopods "Ouranosaurus", "Lurdusaurus", and "Elrhazosaurus". Other fauna include crocodilians, such as "Sarcosuchus", "Stolokrosuchus", "Araripesuchus", and "Anatosuchus"; turtles of the genera "Teneremys", "Platycheloides", "Taquetochelys", and "Araripemys"; and fish, including the coelacanth "Mawsonia", the lungfish "Ceratodus", and the ray-finned fish "Lepidotes" and "Pliodetes".