Evolution and fossil record
Anatomical evidence, supported by molecular, physical, and genetic evidence, demonstrates that seahorses are highly modified pipefish. The fossil record of seahorses, however, is very sparse. The best known and best studied fossils are specimens of Hippocampus guttulatus (though literature more commonly refers to them under the synonym of H. ramulosus), from the Marecchia River formation of Rimini Province, Italy, dating back to the Lower Pliocene, about 3 million years ago. The earliest known seahorse fossils are of two pipefish-like species, H. sarmaticus and H. slovenicus, from the coprolitic horizon of Tunjice Hills, a middle Miocene lagerstätte in Slovenia dating back about 13 million years.[22]
Molecular dating implies that pipefish and seahorses diverged during the Late Oligocene. This has led to speculation that seahorses evolved in response to large areas of shallow water, newly created as the result of tectonic events. The shallow water would have allowed the expansion of seagrass habitats that served as camouflage for the seahorses’ upright posture.[23] These tectonic changes occurred in the western Pacific Ocean, pointing to an origin there, with molecular data suggesting two later, separate invasions of the Atlantic Ocean.[24] In 2016, a study published in Nature found the seahorse genome to be the most rapidly evolving fish genome studied so far.[25]
The evolution of seahorses from pipefish may have been an adaptation related to the biomechanics of prey capture. The unique posture of the seahorse allows them to capture small shrimps at larger distances than the pipefish is capable of.[26]