The 150-million-year-old specimen is valued at up to $6 million by Sotheby’s. Some paleontologists worry this auction and earlier ones are driving fossil market speculators.

A version of this article appears in print on May 27, 2025, Section D, Page 2 of the New York edition with the headline: The Chicago Archaeopteryx: This Dinosaur Had Feathers, And Probably Flew ...