Nation ‘The tipping point:’ First T. rex mass death site in southern US, found in Utah, strengthens evidence of pack behavior April 19, 2021April 20, 2021 K. Sophie Will, St. George Spectrum & Daily News Comments Off on ‘The tipping point:’ First T. rex mass death site in southern US, found in Utah, strengthens evidence of pack behavior The pack of four, possibly five, T. rexes likely died in a seasonal flood after a slow-burn fire between 66 and 100 million years ago, scientists say.