The holotype of Meraxes, MMCh-PV 65, was discovered in 2012. Known from a nearly complete skull, pectoral and pelvic elements, partial forelimbs, complete hindlimbs, fragmentary ribs and cervical and dorsal vertebrae, a sacrum, and several complete caudal vertebrae, it represents the most complete carcharodontosaurid skeleton known from the Southern Hemisphere.[1] The specimen was referred to as the "Campanas carcharodontosaurid" before its description as a new species in 2022.[2]
Meraxes is one of the largest theropods, weighing approximately 4.26 metric tons (4.70 short tons).[1] Henderson (2023) listed a body length estimate of 9–10 metres (30–33 ft), referencing Canale et al. (2022), but also estimated a body length of 10.2–11.6 metres (33–38 ft) using the pelvic area.[3] Its skull is 1.27 m (4.2 ft) long, similar to that of Acrocanthosaurus, which has a skull length of 1.23–1.29 m (4.0–4.2 ft).[1][4] The shapes and proportions of various bones, including the skull, scapula, metacarpals, ischial shaft, and foot, indicate that Meraxes and Acrocanthosaurus had similar proportions and body size. Meraxes possessed reduced forelimbs, an instance of convergent evolution that occurred independently in four different lineages: Carcharodontosauridae, Abelisauridae, Tyrannosauridae, and Alvarezsauridae. Additionally, the second toes possess an enlarged claw, almost twice as long as the claw on the fourth toe.[1]
Osteohistological analysis of the holotype suggests the individual could have been between 39 and 53 years old when it died, having reached skeletal maturity approximately 4 years prior to its death (between 35 and 49 years old), making it the longest-lived non-avian theropod currently known. Meraxes was determined to have grown to large size by extending its growth period (hypermorphosis), rather than increasing its relative growth rate (acceleration) through development as in Tyrannosaurus, to which it was compared.[1][2]
Classification
Canale et al. (2022) recovered Meraxes as the earliest diverging member of the tribe Giganotosaurini within the Carcharodontosauridae. The results of their phylogenetic analyses are displayed in the cladogram below:[1]
Cau (2024) also recovered similar relationships for Meraxes, but not in a clade with Tyrannotitan, Giganotosaurus, and Mapusaurus, as suggested by Canale et al. (2022).[5]
^Cerroni, M.A.; Motta, M.J.; Agnolín, F.L.; Aranciaga Rolando, A.M.; Brissón Egli, F.; Novas, F.E. (2020). "A new abelisaurid from the Huincul Formation (Cenomanian-Turonian; Upper Cretaceous) of Río Negro province, Argentina". Journal of South American Earth Sciences. 98: 102445. Bibcode:2020JSAES..9802445C. doi:10.1016/j.jsames.2019.102445. S2CID213781725.
^Rolando, Alexis M. Aranciaga; Motta, Matías J.; Agnolín, Federico L.; Tsuihiji, Takanobu; Miner, Santiago; Brissón-Egli, Federico; Novas, Fernando E. (9 October 2024). "A new carcharodontosaurid specimen sheds light on the anatomy of South American giant predatory dinosaurs". The Science of Nature. 111 (6): 56. doi:10.1007/s00114-024-01942-4. ISSN1432-1904.