A Large Marine Life Fossil Millions of Years Old!

This is a really awesome fossil found by my grandfather, years ago, who had been a geologist. I inherited some of his prized collection of personal finds. I'm trying to do some downsizing & have decided to find new homes for some of my collectables, including some of my grandfather's fossils (please keep your eyes open for future listings of other collectable items, including a few more fossils). Not being real knowledgeable about fossils, all I can really tell you about this piece is that I think it's likely, fossilized marine life. I've seen other, similar looking items sell for hundreds of dollars! This is a very nice display size piece, that measures 10" X 5.5" x 2" , & was likely found in the Southwestern part of the US. Please feel free to ask questions.