Fossil fish assemblage of the Laguna Formation, Philippines: unveiling the uniqueness of Pleistocene freshwater ecosystems in Southeast Asia

Abstract This study offers a comprehensive analysis and detailed description of the fossil fish assemblage from the Pleistocene Laguna Formation in Luzon Island, Philippines. The fish fossils were collected from the deeper lacustrine facies of the formation, and a total of three fish families were identified. The identification is based on the recognizable synapomorphies in 10 moderately preserved semi-articulated individuals of ray-finned fish specimens, some of which include counterparts. The assemblage is predominantly composed of small clupeiforms of the family Dorosomatidae, accompanied by a gobioid fish (Gobiidae or Oxudercidae) and a synbranchid specimen (Synbranchidae), each represented by a single specimen. This taphocoenosis preserves free-swimming dorosomatids and demersal gobioid and synbranchid, suggesting an autochthonous assemblage with minimal postmortem sorting. Despite all recognized taxa being tolerant to changes in salinity, the environment is inferred to have been freshwater, analogous to modern Taal Lake, where a freshwater sardine thrives. This study represents the first systematic description of freshwater fish fossils in the Philippines and marks the first discovery of these taxa in the tropical West Pacific region.