From sharks to giraffes, many of Earth’s biggest and most magnificent species are threatened with extinction. A new study of the fossil record indicates that once large vertebrates disappear, evolution cannot quickly restore them — for tens of millions of years, most animals remain small.

The study, published Thursday in Science, emerged from research carried out by Lauren Sallan, a paleontologist at the University of Pennsylvania.

Studying fish that lived during the Mississippian Period, from 359 million to 323 million years ago, she noticed that they were substantially smaller than their ancestors.

“It piqued my curiosity,” Dr. Sallan said in an interview. “Why are these fish so small?” (She earned a nickname from her fellow paleontologists: the Sardine Queen.)