Paraxenisaurus (, meaning "strange lizard") is an extinct genus of ornithomimosaurian theropod from the Late Cretaceous Cerro del Pueblo Formation of Coahuila in Mexico. The genus contains a single species, P. normalensis, which is known from a few bones of tail, hips, hands, and feet. The specific epithet was given in honor of the Benemérita Normal School of Coahuila, a teacher training institution, where the fossils were reposited. It is a member of the family Deinocheiridae and is the only member of that clade known from Laramidia.
Discovery
During the 1990s, ornithomimosaur fossils were discovered at three sites in the Cerro del Pueblo Formation of Coahuila state. The known specimens of Paraxenisaurus were collected from three different localities near the towns of Parras de la Fuente and General Cepeda. Two decades later, these remains were identified as belonging to a distinct North American ornithomimosaur taxon. In 2020, they were named and described by Mexican paleontologists Claudia Inés Serrano-Brañas, Belinda Espinosa-Chávez, Sarah Augusta Maccracken, Cirene Gutiérrez-Blando, Claudio de León-Dávila and José Flores Ventura. The holotype specimen of Paraxenisaurus was given the designation BENC 2/2-001 and was stored at the Normal School of Coahuila. It consists of a manual phalanx, an astragalus, a calcaneum, and several bones of the feet including a partial metatarsal, several pedal phalanges, and unguals. Four additional specimens were referred to the genus and consist of various partial bones of the hands and feet in addition to a partial femur, and several caudal vertebrae. Each named specimen consists of several bones that were disarticulated when they were found. However, the authors of the description stated that all the bones of each specimen were found in association with one another and were almost certainly from the same animal.
Description
The known remains of Paraxenisaurus are highly incomplete, and it is not known how ontogenetically old the known specimens were when they died and became fossilized, so making an accurate estimate of their full size in life is very difficult. The authors who described the fossils estimated that Paraxenisaurus was about 5.7 metres (19 ft) long and weighed about 600 kilograms (1,300 lb). They suggested this based on the size estimates for the similarly-sized and more completely known genus Beishanlong, which is believed to be a close relative. The holotype specimen, BENC 2/2-001, is relatively fragmentary, but it preserves enough detail to identify Paraxenisaurus as a genus of deinocheirid. Serrano-Brañas and colleagues diagnosed the genus by the following autapomorphies: a deep, concave, elliptical surface on the articular surface of the claw on the first finger, vertically-oriented articulations between the prezygapophyses on the anterior caudal vertebrae, ventromedially-oriented prezygapophyses on the caudal vertebrae, a large flared condyle of the second metatarsal, and a rounded foramen on the medial surface of the toe claws. Paraxenisaurus also lacks the arctometatarsalian condition seen in many other ornithomimosaurs. They also noticed several apomorphic features which are not unique to Paraxenisaurus itself. These synapomorphies include: laterally-compressed hand claws with a deep sulcus along the flexor tubercle, a proximally-expanded articular surface of the third metacarpal, the presence of a first pedal digit, a semi-ginglymoid distal articular surface of the third metatarsal, and broad, strongly curved toe claws with enlarged and vertical proximodorsal processes.
Several other specimens were found at nearby localities. These include BENC 1/2-0054 (a few metacarpal and phalanx elements), BENC 1/2-0091 (part of a femur, several toe fragments, and vertebrae), BENC 1/2-0092 (caudal vertebrae), BENC 30/2-001 (two pedal unguals). The quality of their preservation varies considerably between the specimens. While these elements were described in detail by Serrano-Brañas and colleagues, they were not used to diagnose the genus. Paraxenisaurus had distinct morphology from many other ornithomimosaurs in several regards. Their caudal vertebrae were highly elongated in comparison with most other ornithomimosaurs, with only Gallimimus and Beishanlong exhibiting a similar condition. The anterior portion of the tail of Paraxenisaurus was also likely quite stiff and rod-like in comparison with related taxa and it likely had very little dorsoventral mobility based on the articulation of the zygapophyses. The metatarsals of Paraxenisaurus are also distinct from most of its relatives due to the full articulation of the third metatarsal with the tibia. This is the plesiomorphic condition for coelurosaurs, but most ornithomimosaurs possess an arctometatarsus. The only ones which do not, besides Paraxenisaurus, are Deinocheirus, Nqwebasaurus, and Beishanlong. Paraxenisaurus also retains the first pedal digit, which is also seen in Nqwebasaurus, Garudimimus, and Beishanlong. Serrano-Brañas and colleagues also noted that Paraxenisaurus has numerous morphological similarities to an unnamed taxon from the Dinosaur Park Formation, known only as TMP 1967.19.14.
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