A 67-million-year-old Tyrannosaurus rex fossil known as "Gus" has shattered the world record for a dinosaur sold at auction, fetching a staggering $US50.1 million ($A71.7 million) from an anonymous telephone bidder — far exceeding pre-sale expectations and cementing its place in natural history auction history.

The sale, conducted live and online, lasted just 10 minutes and saw Gus attract bids from seven competing buyers before the mystery winner — who has chosen to remain unidentified — secured the lot. Ahead of the auction, the specimen had been expected to bring in somewhere between $US20 million and $US30 million ($A28–$A42 million).

Why 'Gus' is considered an extraordinary T-rex specimen

Discovered in 2021 on a South Dakota ranch, Gus stands approximately 3.8 metres tall and stretches 11.5 metres in length, making it one of the largest known T-rex specimens in existence. The fossil is mounted upright, tail extended and right foot slightly raised — a pose that captures the predator mid-stride.

At around 61% complete, Gus includes what experts describe as an exceptionally well-preserved skull featuring a jaw lined with powerful teeth, two largely intact feet, and a number of rarely recovered bones — among them a furcula, or wishbone. The fossil was named in tribute to Gary Licking, the ranch's owner, who passed away during the roughly five-year process of excavating, restoring and mounting the skeleton.

"Gus is not only an exceptional find, but a specimen that's been excavated, documented, prepared, and cared for with real excellence," said Sotheby's vice chair Cassandra Hatton following the sale. "The market responds when great specimens are taken care of in the right way."

At one point during bidding, auctioneer Phyllis Kao urged participants to go higher with a nod to the lot itself: "Try a bigger bite — it's a T-rex, after all."

A new benchmark in dinosaur auction records

The sale price comfortably surpasses the previous record set in 2024, when a nearly complete stegosaurus skeleton sold by the same auction house for just under $US45 million ($A64 million). Before that, the benchmark had been held by a T-rex nicknamed "Stan," which changed hands in 2020 for close to $US32 million ($A45 million). Stan is now on display at the Natural History Museum Abu Dhabi, dramatically posed in combat with another fossilised T-rex over triceratops remains.

Earlier in auction history, a T-rex named "Sue" — the first dinosaur ever sold at auction, also through the same house in 1997 — became a centrepiece exhibit at the Field Museum in Chicago. The stegosaurus that held the most recent record before Gus is currently on long-term loan to the American Museum of Natural History in Manhattan.

Scientists urge new owner to place 'Gus' on public display

The sale has drawn concern from the scientific community, with palaeontologists and researchers calling on the anonymous buyer to ensure the fossil remains accessible to the public and to science.

The Society of Vertebrate Paleontology, which represents scientists, scholars and students in the field, argued that specimens of this significance should be housed in museums or research institutions where they can be properly studied and preserved for future generations.

"Our hope is that the new owner recognises the extraordinary scientific and educational value of Gus the T-rex and that they aim to keep it in the public trust by immediately donating it to an accredited natural history museum," said Kristi Curry Rogers, the society's president-elect. "That outcome would ensure that this remarkable specimen continues to advance science, rather than becoming unavailable for study."

Whether the mystery buyer intends to loan, donate or keep the fossil privately remains unknown. The identity of the winning bidder, who participated by phone, has not been disclosed.