Arizona State Dinosaur: Sonorasaurus thompsoni

Richard Thompson's 1994 find set off years of careful excavation in difficult terrain. The Aravaipa Canyon region of Graham County — remote, deeply eroded desert in southeastern Arizona — is exactly the kind of landscape where bones erode unnoticed for millions of years before someone happens to walk past at the right moment. Thompson understood what he was seeing and made the call that started everything.

Ron Ratkevich of the Arizona-Sonora Desert Museum led the scientific work and formally described the new species in 1998. The genus name Sonorasaurus honors the Sonoran Desert — a region shared between the American Southwest and northwestern Mexico. The species name thompsoni is a direct acknowledgment that a student hiker, not a professional paleontologist, is responsible for one of Arizona's most significant fossil discoveries.

By 93 million years ago — the Turonian stage of the Late Cretaceous — brachiosaurid sauropods had become rare across North America. Most dinosaur faunas of that era on the continent were dominated by hadrosaurs and ceratopsians. Sonorasaurus represents a lineage persisting in the American Southwest long after its family had declined elsewhere on the continent. That's the scientific value: not just that it's big, but that it was still here when it probably shouldn't have been.

The Turonian is also one of the less-documented intervals of North American dinosaur history. There's a well-known gap in the fossil record between the Morrison Formation faunas of the Jurassic and the better-understood latest Cretaceous — Tyrannosaurus, Triceratops, and their contemporaries. Sonorasaurus, recovered from the Turney Ranch Formation of southeastern Arizona, is one of the few animals that fills any part of that gap for the Southwest specifically.

The Turney Ranch Formation itself has not been extensively excavated. Sonorasaurus remains its most significant find. Its presence suggests that the Cretaceous Southwest hosted a distinct dinosaur community — one that preserved conditions allowing late-surviving sauropods to persist — but the picture is still fragmentary.