Bruhathkayosaurus (; meaning "huge-bodied lizard") is a genus of sauropod dinosaur found in the Kallamedu Formation of India. The fragmentary remains were originally described as a theropod, but it was later determined to be a titanosaur sauropod. Length estimates by researchers exceed those of the titanosaur Argentinosaurus, If the upper estimates of the 2023 records are accurate, Bruhathkayosaurus may have rivaled the blue whale as one of the largest animals to ever exist. The generic name chosen, "Bruhathkayosaurus", is derived from a combination of the Sanskrit word Bruhathkaya (bṛhat , 'huge, heavy' and kāya, काय 'body'), and the Greek sauros (lizard). The specific epithet, "matleyi", honours British palaeontologist Charles Alfred Matley, who discovered many fossils in India.
The monsoon season, combined with the sands and clays of the Kallamedu Formation, creates water-saturated fossils which are very friable. During the dry season, expansion during the day and contraction during the night can cause fossils to split apart. This results in poorly preserved bones that can be impossible to extract without damage. In 2017, Galton and Ayyasami reported that the Bruhathkayosaurus fossils started to disintegrate inside their field jackets before reaching the Geological Survey of India (GSI) and no longer exist.
Classification
Bruhathkayosaurus was originally classified as a carnosaurian theropod (like Allosaurus), of an uncertain position (incertae sedis). However, Chatterjee (1995) re-examined the remains and demonstrated that Bruhathkayosaurus is actually a titanosaur sauropod. Some later studies listed Bruhathkayosaurus as an indeterminate sauropod or as a nomen dubium.
The original publication described little in the way of diagnostic characteristics and was only supported by a few line drawings and photographs of the fossils as they lay in the ground. This led to online speculation by researchers that the bones might actually have been petrified wood, akin to the way the original discoverers of Sauroposeidon initially believed their find to be fossilized tree trunks. A 2022 review by Pal and Ayyasami presented additional previously unseen photographs of the tibia bone at the excavation site and in a plaster jacket, and reinforced the taxon's position within Titanosauria. However, as Dhananjay M. Mohabey and colleagues noted in 2024, "this new information has not resolved the identity of the element nor the validity of the taxon."
Size estimates
According to the published description, the shin bone (tibia) of Bruhathkayosaurus was long. In a May 2008 article for the blog Sauropod Vertebra Picture of the Week, paleontologist Matt Wedel used a comparison with Argentinosaurus and calculated the weight of Bruhathkayosaurus at up to . In 2019, Gregory S. Paul suggested that the supposed tibia was probably a degraded femur, in which case its length was slightly greater than that of Dreadnoughtus (1.91 meters) and Futalognkosaurus (1.98 meters). Its ilium is similar in length to that of Dreadnoughtus whereas the width of the distal femur appears to slightly exceed that of Patagotitan. Paul estimated its mass at around , much lower than any previous estimation. In 2020, Molina-Perez and Larramendi suggested that the long tibia is probably a fibula, and estimated the size of the animal at and .
By comparison, the titanosaur Argentinosaurus is estimated to have reached in length, and to have weighed 65–100 tonnes. These sauropods are known only from partial or fragmentary remains, so the size estimates are uncertain. Length is calculated by comparing existing bones to the bones of similar dinosaurs, which are known from more complete skeletons and scaling them up isometrically. However, such extrapolation can never be more than an educated guess and the length of the tail, in particular, is often hard to judge. Determining mass is even more difficult because little evidence of soft tissues survives in the fossil record. In addition, isometric scaling is based on the assumption that body proportions remain the same, which is not necessarily the case. In particular, the proportions of the titanosaurs are not well known, due to a limited number of relatively complete specimens. with estimates of up to .
Another poorly known sauropod that shares similar size estimates to Bruhathkayosaurus is Maraapunisaurus fragillimus, which is based on a now-missing dorsal vertebra. In 2006, Kenneth Carpenter used Diplodocus as a guide and estimated Maraapunisaurus to be in length and weigh only about . In 2018, however, Carpenter estimated Maraapunisaurus to be in length based upon comparisons with rebbachisaurids. In 2019, Paul gave a higher estimation of and a weight of . Nevertheless, Paul stressed that the idea of these 'super-sauropods' or 'land-whales' aren't as far-fetched as one would think given the long period of sauropod existence as well as their very efficient body-plan (i.e. small heads, large body, efficient ventilated air-sacs, strong and flexible muscle and bone structure, and large source of static food), indicating that the existence of 150+ ton sauropods such as Bruhathkayosaurus and Maraapunisaurus are in the realm of possibility.