South American Giant Carcharodontosaurid — Meraxes

Meraxes gigas (圖片來源：Carlos Papolio，採用 CC BY-SA 4.0 授權)

Discovery site of Meraxes gigas

Time period when Meraxes gigas lived（白稜製作）

When people think of carnivorous theropod dinosaurs, the image that often comes to mind is a creature with an enormous skull, powerful hind limbs, and a pair of seemingly useless tiny forelimbs. This body plan appears repeatedly across several theropod groups, including the Tyrannosauridae, Abelisauridae, and Carcharodontosauridae. Among them, carcharodontosaurids remain one of the less thoroughly understood lineages. Although the past three decades have yielded an increasing number of fossil discoveries from this family, relatively few specimens preserve complete anatomical elements such as the skull, forelimbs, and hind limbs, limiting detailed study of their morphology.

In 2022, the discovery of the giant species Meraxes gigas significantly expanded our understanding of this group. The fossils were unearthed in the Huincul Formation of northern Patagonia, Argentina, a geological unit dating to the Late Cretaceous. The genus name Meraxes is derived from the dragon ridden by Rhaenys Targaryen in A Song of Ice and Fire. The holotype specimen, MMCh-PV 65, is estimated to represent a mature individual between 39 and 53 years old. The fossil preserves a nearly complete skull (although the lower jaw is absent), the pectoral girdle, pelvic girdle, portions of the forelimbs and hind limbs, fragments of cervical and dorsal vertebrae, a complete sacrum, and several anterior and middle caudal vertebrae.

Meraxes gigas (圖片來源：Ansh Saxena，採用 CC BY-SA 4.0 授權)

Several distinctive anatomical features can be observed on the skull of this specimen. Two small openings occur along the anterior margin of the antorbital fenestra. At the junction between the postorbital and squamosal bones, a low and rounded lateral projection forms a noticeable corner. The quadratojugal bone presents a deep and rounded surface facing the infratemporal fenestra. Numerous dermal bones are present on the skull, arranged in a pattern similar to that of Acrocanthosaurus, another member of Carcharodontosauridae. The nasal region is covered with folds and ridges, while the area immediately surrounding the external nostrils is comparatively smooth, a condition also observed in other carcharodontosaurids. A robust brow ridge extends laterally from the postorbital bone. The posterior margin of the quadratojugal bears several elongated spines, and the supraoccipital exhibits a pronounced posterior projection. The total skull length of Meraxes is estimated to be approximately 127 cm. Because a complete maxilla of the related giant Giganotosaurus has never been found, researchers used the skull proportions of Meraxes to infer that the skull of Giganotosaurus may have reached roughly 162 cm in length.

The vertebral column also provides important insights. The sacral vertebrae display co-ossification, meaning that individual sacral elements fused together during bone formation. Similar conditions occur in Acrocanthosaurus and Giganotosaurus, though the fusion appears more pronounced in Meraxes. The anterior caudal vertebrae possess hyposphene–hypantrum articulations, a specialized interlocking structure that strengthens the spine.

The forelimb elements show a distinctive configuration. The scapula is long and slightly curved, while the humerus is robust and the ulna short and thick. Only metacarpals II and III are preserved, accompanied by nine phalanges. In contrast, the hind limbs are remarkably well preserved. Metatarsal III is the longest, whereas metatarsal I is the shortest. The overall proportions of the hind limbs closely resemble those of Acrocanthosaurus and Concavenator.

Phylogenetic analyses based on fossil morphology place Meraxes within an early branch of the Giganotosaurini lineage. It represents one of the large South American carcharodontosaurids from the middle to late Cretaceous, living during roughly the same period as Taurovenator. The South American record of carcharodontosaurids includes genera such as Tyrannotitan, Giganotosaurus, Meraxes, Taurovenator, and Mapusaurus. These taxa appear primarily from the Albian to Turonian stages of the Cretaceous. By the late Turonian, the entire family had disappeared, suggesting that carcharodontosaurids achieved relatively high species diversity shortly before their extinction.

The study of the Meraxes fossils also revealed that its forelimbs were extremely reduced, measuring only about half the length of the femur. This observation prompted researchers to investigate why theropods such as Meraxes evolved such short forelimbs. Their analyses indicate that forelimb reduction evolved independently within four separate theropod lineages: Abelisauridae, Carcharodontosauridae, Tyrannosauridae, and Alvarezsauridae. Furthermore, the reduction tends to be more pronounced in later members of each lineage. For example, although Acrocanthosaurus is similar in body size to Meraxes, the more derived Meraxes possesses markedly shorter forelimbs. These findings suggest that forelimb reduction is not directly linked to overall body size, nor does it correlate strictly with the amount of muscle attachment or functional use of the forelimb.

Various hypotheses have been proposed to explain the reduction of forelimbs in groups such as tyrannosaurids, abelisaurids, and carcharodontosaurids. Some researchers have suggested that smaller arms may have offered advantages during mating behavior or predatory strategies. Others argue that the reduction may have arisen indirectly through selection acting on other anatomical traits. Using phylogenetic regression analysis, the present study supports the latter interpretation, proposing that shortened forelimbs are associated with the evolution of extremely large skulls and therefore represent a correlated evolutionary outcome rather than a direct adaptation.

Author: Shui Ye-You

Second author: Bai Leng

Reference:

Canale, J. I. et al. (2022). New giant carnivorous dinosaur reveals convergent evolutionary trends in theropod arm reduction. Current Biology.