He was the first American to devote significant time and attention to vertebrate paleontology and was one of the most important contributors to the field in the early nineteenth century.

[1][2] Prior to the time of Harlan, it was common practice to publish only a genus name for a fossil animal that was new to science.

He made an error in describing a species called Osteopera platycephala based on the skull of Agouti paca.

In 1834, Harlan described and named Basilosaurus ("king lizard"), a genus of early whale, erroneously assuming he had found a Plesiosaurus-like dinosaur.

[10] In 1839 he visited Europe again and received a plaster copy of Mosasaurus hoffmannii from the Muséum d’Histoire Naturelle that is now in the Academy of Natural Sciences of Philadelphia.