CHICAGO—After careful consideration, Tyler Keillor made a decision: T. rex had lips. In the debate over dinosaurs’ “oral margin,” one of the country’s top paleoartists has taken sides. Mr. Keillor’s job is to create “flesh models” of dinosaurs and other prehistoric animals based on fossil evidence—giving creatures that died millions of years ago a face, a story and sometimes lips.

Using traditional mediums like clay and resin, as well as CT scanning and 3-D printing, he is at the forefront of incorporating digital art and...