 Based on microsatellite DNA evidence , the sicklefin lemon shark is thought to have diverged from its sister species N. brevirostris 10 – 14 million years ago , when the closure of the Tethys Sea separated lemon sharks in the Indian Ocean from those in the Atlantic . The ancestral lemon shark species may have been N. eurybathrodon , the fossilized teeth of which occur in both the United States and Pakistan . Morphological and molecular phylogenetic analyses suggest that Negaprion is grouped with the whitetip reef shark ( Triaenodon ) and the sliteye shark ( Loxodon ) in occupying an intermediate position on the carcharhinid evolutionary tree , between the most basal genera ( Galeocerdo , Rhizoprionodon , and Scoliodon ) and the most derived ( Carcharhinus and Sphyrna ) . 
