Bruhathkayosaurus (; meaning "huge-bodied lizard") is an extinct 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 titanosaurian sauropod. Length estimates by researchers exceed those of the titanosaur Argentinosaurus, as longer than and weighing over 80 tonnes. A 2023 estimate placed Bruhathkayosaurus as potentially weighing approximately . If the upper estimates of the 2023 records are accurate, Bruhathkayosaurus may have rivalled the blue whale as one of the largest animals to ever exist. However, all of the estimates are based on the dimensions of the fossils described in Yadagiri and Ayyasami (1987), and in 2017, it was reported that the holotype fossils had disintegrated and no longer exist.
Discovery and naming
The holotype of Bruhathkayosaurus, GSI PAL/SR/20, was discovered around 1978 near the southern tip of India, specifically in the Tiruchirappalli district of Tamil Nadu, northeast of Kallamedu village. It was recovered from rocks of the Kallamedu Formation, which are dated to the Maastrichtian stage of the Late Cretaceous, about 70 million years ago. The fossilized remains include hip bones (the ilium and ischium), partial leg bones (femur and tibia), a forearm (radius) and a tail bone (part of a vertebra, specifically a platycoelous caudal centrum). The remains were originally classified as belonging to a carnosaurian theropod by Yadagiri and Ayyasami in 1987 (not 1989, as some sources indicate). 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 carnosaur (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. However, a 2022 review by Pal and Ayyasami suggested that the skeleton was real and that the genus is likely valid. Additional previously unseen photographs were provided of the tibia bone at the excavation site and in a plaster jacket. Pal and Ayyasami also reinforced the taxon's position within Titanosauria.
The only known remains of Bruhathkayosaurus have been lost so the validity of the genus and any size estimates are questionable.
Size estimates
According to the published description, the shin bone (tibia) of Bruhathkayosaurus was long. This is 29 percent larger than the tibia of Argentinosaurus, which is only long. The fragmentary femur was similarly huge; across the distal end, it measured , 33% larger than the femur of Antarctosaurus giganteus, which measures . The ilium measured in length.