Abstract

†Protosyngnathus sumatrensis is known from six specimens from Eocene lacustrine deposits of the Sangkarawang Formation of Sumatra, Indonesia. The fish was named almost 150 years ago, but has never been well described or figured. Probably because of this, it has not been included in phylogenies of extant pipefishes and seahorses (Syngnathiformes). A detailed description is provided, along with a new phylogenetic analysis of morphological characters that allows the Eocene fish to be confidently placed with the extant families Syngnathidae (pipefishes and seahorses) and Solenostomidae (ghost seahorses) along with the Paleocene marine fossil †Eekaulostomus cuevasae from Mexico. The vast majority of these fishes are marine inhabitants, with a few syngnathids entering brackish or freshwater habitats. †Protosyngnathus documents the occurrence of syngnathiform fishes in a lacustrine environment, indicating that this lineage invaded and became adapted to the freshwater environment early in the evolution of this group.