For years, the trophy object of choice for the ultra-wealthy was relatively predictable: the Picasso, the Rothko, the rare Patek Philippe watch, maybe a Basquiat large enough to dominate the living room of a newly purchased penthouse, or, more recently, Air Jordans worn by the legend himself at a championship game. But lately, another kind of status symbol has been muscling its way into auction catalogs, gallery exhibitions, and billionaire wish lists: dinosaurs. This summer, Sotheby’s will offer “Gus,” a 67-million-year-old Tyrannosaurus rex skeleton estimated at $20 million to $30 million, the highest estimate ever placed on a dinosaur fossil. The specimen, excavated over several years in South Dakota, stretches roughly 38 feet long and stands more than 12 feet tall. Sotheby’s is billing it as one of the largest and most complete T. rex specimens ever discovered.…