Today I Learned: PH Geologists Found the First-Ever Global Record of a Swamp Eel Fossil
Esquire Philippines
24 January 2024
"The fossil nobody wants," said Allan Gil Fernando of the National Institute of Geological Sciences, University of the Philippines Diliman.
The piece of rock was initially discarded and ignored by a student during fieldwork at the fish fossil locality site in Antipolo, Rizal, in 2013. The fossil specimen isn't complete, after all. However, the geologist, being the self-admitted hoarder that he is, stored it in his office without knowing its identity and significance. More than 10 years later, the specimen in question was identified to be the first-ever global record of a swamp eel fossil.
Freshwater Fossils Found in Pleistocene Laguna Formation
The research, "Fossil fish assemblage of the Laguna Formation, Philippines: unveiling the uniqueness of Pleistocene freshwater ecosystems in Southeast Asia," was published in the Swiss Journal of Paleontology in January 2025. It was a collaborative effort among Filipino scientists from the National Museum of the Philippines' Geology and Paleontology Division (Abigael Castro and Jaan Ruy Conrad Nogot), University of the Philippines Diliman's National Institute of Geological Sciences (Allan Gil Fernando) as well as the School of Archaeology (Clarence Magtoto), and Mines and Geoscience Bureau (Kevin Garas). Tomas Prikryl, the lead researcher, is from the Institute of Geology of the Czech Academy of Sciences; Dominique Mediodia and Chien-Hsiang Lin from Taiwan joined them. The group attempted to make great strides in Philippine and Southeast Asian paleoichthyology (the study of prehistoric fish).
"The fossil record in the Philippines, particularly concerning fish, is notably sparse. [... This limited evidence has provided only fragmented insights into the past biodiversity and evolutionary history of the region’s freshwater ecosystems. The absence of freshwater fossil records leaves a significant gap in our understanding of how this ecosystem has changed over time, especially in response to climatic and geological events," Prikryl et al. wrote.
Their objective was to document a collection of freshwater fish fossils from the Pleistocene Laguna Formation in Antipolo, Rizal Province. This included a total of 14 articulated and semi-articulated specimens, which were deposited at the National Institute of Geological Sciences - University of the Philippines and at the National Museum of the Philippines.
Living specimens were compared to the fish fossil specimens, and their physical features—from length to dorsal profile—were documented. We'll not go into the specifics, but the scientists went as far as recognizing the prehistoric fish's jaws and measuring the skull. These are important not only in identifying the species, but also in studying the environment of their habitat, which in this case was the Laguna Lake.
"The fish fossil assemblage of the Pleistocene Laguna Formation is predominantly composed of the families Dorosomatidae, a Gobiidae vel Oxudercidae, and Synbranchidae. These fossils represent the first records of freshwater fish in the Philippines and the first discovery of these taxa in the tropical West Pacific region. Notably, this study also provides the first-ever record of a synbranchid fossil globally," the researchers concluded.
What This Means for Philippine Ecosystems
Aside from establishing several firsts, this discovery represented the first systematic study of freshwater fish fossils in the Philippines and provided valuable data that can be used to compare with modern ecosystems.
First, it provided solid evidence that the distribution of Herklotsichthys (a genus of herrings found mostly in Southeast Asia and Australia, with one species each in the Persian Gulf, the Red Sea, and the western Indian Ocean) was prehistorically restricted in the Indo-Pacific region.
Second, the presence of dorosomatid fish fossils in the lake suggests a habitat transition from marine to freshwater in the Pleistocene period. This is similar to Taal Lake, where fish species adapted from brackish to freshwater. The presence of demersal swamp eels and gobioid fishes also suggests that the Pleistocene Laguna Lake once had a silty and muddy bottom.
Third, the gobioid fossil from the Pleistocene showed their adaptability through all those periods in history. Up to 89 species of Gobiidae have been recorded in the Philippines, with 20 of them endemic (or found only in the country).
Fourth, the identification of the Pleistocene synbranchid fossil suggested that the swamp eel has an Indo-Pacific origin.
"These newly discovered freshwater fish fossils from the Pleistocene Laguna Formation in the Philippines not only provided evidence of their presence in the West Pacific region but also offered new insights into the origin and historical geographic distribution of these taxa," they said.