Seahorses (Syngnathidae: Hippocampus) today are common throughout the world's tropical, subtropical and temperate marine regions (Kuiter 2000), but fossil seahorses are exceptionally rare. The genus is represented at only two well-documented sites, namely Tunjice in Slovenia (Middle Miocene, approx. 13 Myr; Žalohar et al. 2009) and Marecchia in Italy (Late Pliocene, approx. 3 Myr; Sorbini 1988). In both cases, the fossil seahorses are considered to be morphologically similar to certain extant species rather than being primitive transitional forms, suggesting that seahorses must have evolved earlier.
The genus Hippocampus is one of four genera in the subfamily Hippocampinae, which also includes three genera of pygmy pipehorses (Acentronura, Amphelikturus and Idiotropiscis) that some authors treat as a single genus (Kuiter 2004). Pygmy pipehorses are morphologically very similar to seahorses, but all lack the upright posture. This suggests that they could be a surviving evolutionary link between seahorses and the remaining members of the family Syngnathidae, all of which have a horizontal posture. The species of the temperate Australian pygmy pipehorse genus Idiotropiscis are by far the most seahorse-like in appearance (Kuiter 2004), and the time when these shared a common ancestor with the seahorses is therefore likely to be close to the time when the seahorses’ upright posture evolved. An Australasian origin of seahorses is supported by the fact that the most basal and second most basal Hippocampus lineages occur in the Indo-West Pacific and in Australia, respectively (Teske et al. 2004).
Central to understanding why the seahorses’ upright posture was favoured by natural selection is to determine what environmental conditions prevailed in Australasia during the time when seahorses and pygmy pipehorses diverged from their common ancestor. To this end, we reconstructed phylogenetic relationships among species of Hippocampus and Idiotropiscis, determined their placement among six other syngnathid genera representing the major evolutionary lineages of the family Syngnathidae (Wilson et al. 2003) and dated the split between seahorses and pygmy pipehorses using a relaxed molecular clock.