Meraxes

Meraxes Temporal range: Late Cretaceous, (late Cenomanian), ~
Reconstructed skull
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Clade:	Saurischia
Clade:	Theropoda
Family:	†Carcharodontosauridae
Tribe:	†Giganotosaurini
Genus:	†Meraxes Canale et al., 2022
Type species
†Meraxes gigas Canale et al., 2022

Meraxes is a genus of carcharodontosaurid theropod dinosaur from the Late Cretaceous Huincul Formation of Argentine Patagonia. The genus contains a single species, Meraxes gigas.

Discovery and naming

The holotype of Meraxes, previously called the "Campanas carcharodontosaurid", MMCh-PV 65, was discovered in 2012. Known bones include 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 has the most complete carcharodontosaurid skeleton known from the Southern Hemisphere.

Meraxes gigas was described in 2022 by Canale et al. based on these remains. The generic name, "Meraxes", honors a dragon from the George R.R. Martin fantasy novel series, A Song of Ice and Fire. The specific name, "gigas", is derived from a Greek word meaning "giant", in reference to its large size.

Description

Size compared to a human

Meraxes was a very large theropod, weighing approximately 4.26 metric tons (4.70 short tons). Its skull alone is 1.27 m (4.2 ft) long, rivalling that of Acrocanthosaurus which has a skull length of 1.23 m (4.0 ft). The shapes and proportions of various bones (i.e. skull, scapula, metacarpals, ischial shaft, foot, etc.) indicate that Meraxes and Acrocanthosaurus had similar proportions and body size. It also possessed reduced forelimbs, an instance of convergent evolution that occurred independently in four different lineages: Carcharodontosauridae, Abelisauridae, Tyrannosauridae, and Alvarezsauridae.

Life restoration

Osteohistological analysis of the holotype suggests the individual could have been up to 53 years old when it died, having reached skeletal maturity approximately 4 years prior to its death, making it the longest-lived non-avian theropod currently known. Meraxes was also 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.

Classification

Meraxes represents the earliest diverging member of the tribe Giganotosaurini within the Carcharodontosauridae. The cladogram below displays the results of the phylogenetic analysis by Canale et al.

Carcharodontosauridae	Neovenator       Concavenator     Eocarcharia     Lajasvenator     Lusovenator       Acrocanthosaurus       Shaochilong   Carcharodontosaurinae   Carcharodontosaurus spp.   Giganotosaurini   Meraxes       Tyrannotitan       Giganotosaurus     Mapusaurus

Paleoenvironment

Several dinosaurs from the Huincul Formation (Meraxes in dark blue, left)

The fossil remains of Meraxes were recovered from the Huincul Formation. A substantial number of taxa are known to have inhabited this paleoenvironment. The theropods of the area are represented by the paravian Overoraptor, the elaphrosaurine Huinculsaurus, the abelisaurs Skorpiovenator, Tralkasaurus, and Ilokelesia, the fellow giant carcharodontosaurid Mapusaurus, and the megaraptoran Aoniraptor. However, Meraxes was found in an older rock layer than Mapusaurus, so they probably did not coexist. The herbivores of the area are represented by the rebbachisaurid sauropods Cathartesaura and Limaysaurus, the titanosaurs Argentinosaurus and Choconsaurus, and indeterminate iguanodonts.