genetic loci recovered the genera Hippocampus and Idiotropiscis as sister taxa, suggesting that seahorses and pygmy pipehorses are a monophyletic group and hence share a common evolutionary origin. However, the same phylogeny indicates that if the subfamily Hippocampinae is accepted as valid, then the pipefish subfamily Syngnathinae is paraphyletic, because the former is not a sister group of the latter, but is nested within it. Hippocampinae includes only seahorses, pygmy pipehorses are placed into the pipefish subfamily Syngnathinae This classification system disregards both genetic data and the morphological characters shared by seahorses and pygmy pipehorses. As seahorses have a sister taxon relationship with Idiotropiscis, and other pygmy pipehorse genera are likely basal to this group (given their more pipefish-like appearance), this classification would also make the Syngnathinae paraphyletic. Hippocampinae includes only seahorses, pygmy pipehorses are placed into own subfamily This classification places all pygmy pipehorses into the subfamily Acentronurinae. Based on the nuclear DNA phylogeny, the exclusion of the seahorses from this group likely makes it paraphyletic. However, such a placement is partially supported by an alternative molecular phylogeny that is based on a combination of nuclear and mitochondrial markers and that recovered a group comprising pygmy pipehorses and several pipefishes as a sister lineage of Hippocampus. As the genus Hippocampus consists of two morphologically distinct forms, it has been suggested that it should be split into two distinct genera, Hippocampus and a new genus comprising the pygmy seahorses. Pygmy seahorses have a single gill opening on the back of the head (instead of two on the sides as in normal seahorses), and the males brood their young inside their trunk, instead of in a pouch on the tail. A molecular phylogeny confirms that the pygmy seahorses are a monophyletic sister lineage of all other seahorses. Evolution and fossil record The morphology of pygmy pipehorses suggests that they are an evolutionary link between pipefishes and seahorses, and that seahorses are upright-swimming pygmy pipehorses. Molecular dating indicates that Hippocampus and Idiotropiscis diverged from a common ancestor during the Late Oligocene. During this time, tectonic events in the Indo-West Pacific resulted in the formation of shallow-water areas, which considerably changed marine habitats in this region. Particularly important was the establishment of vast seagrass meadows where there had previously been deeper water. This has led to speculation that the earliest seahorses managed to establish themselves as a new species because, unlike pygmy pipehorses,