“Bloody Mary”: The Complete Tyrannosaur Fossil That Finally Settles the Nanotyrannus vs. T. rex Debate

In October 2025, paleontologists Lindsay Zanno and James Napoli delivered the decisive blow in one of dinosaur science’s most bitter disputes. Their Nature study on the “Bloody Mary” specimen (NCSM 40000) — the near-complete tyrannosauroid from Montana’s famous Dueling Dinosaurs — proves it is not a juvenile Tyrannosaurus rex, but a fully grown adult of the long-debated species Nanotyrannus lancensis.

Discovered in 2006 in the Hell Creek Formation, the fossil pair — a Triceratops and its smaller adversary — remained in legal limbo for years. When the North Carolina Museum of Natural Sciences finally acquired and prepared the tyrannosaur in 2020, researchers gained an unprecedented 98% complete skeleton. Histological analysis of the bones revealed the animal died at approximately 20 years old, with fully mature bone tissue and no remaining growth zones typical of rapidly growing juveniles.

Key differences seal the case: Bloody Mary possesses fewer vertebrae, more teeth, and limb proportions incompatible with any known T. rex growth series. At roughly 5 meters long and under 700 kg, it was a lean, long-armed predator — roughly one-tenth the mᴀss of an adult T. rex.

“This specimen is the Rosetta Stone,” Zanno stated. “It is the first complete skeleton of Nanotyrannus, and it is unequivocally adult.” The team also recognized a second species, N. lethaeus, based on the “Jane” specimen.

The findings confirm two distinct tyrannosauroid apex predators coexisted in Late Cretaceous North America. After two decades of heated debate, “Bloody Mary” has spoken: Nanotyrannus was real, and the “teen T. rex” hypothesis is ᴅᴇᴀᴅ.