Fossil study sheds light on ancient water-to-land transition

A modern-day microscopic invertebrate, similar to the ostracod fossils found in Vietnam. Credit: Anna33 at English Wikipedia, CC BY-SA 3.0

Palaeobiologists have uncovered the earliest evidence of a major group of marine animals making the transition to estuaries, more than 400 million years ago.

University of Leicester PhD researcher Anna McGairy worked with an international team from the UK, Japan, France and Vietnam, to identify and study fossil remains of ostracods, discovered in rocks near to Dong Van in northern Vietnam.

Ostracods are tiny crustaceans known from thousands of living and fossil species. Today they can be found in a wide range of aquatic settings, from deep oceans to garden ponds, and even damp leaf litter in forests.

The earliest fossil ostracods were all marine, and their transition from seas, to rivers, lakes and ponds is poorly documented in the fossil record. Estuaries are viewed as one of the main routes by which animals have made the ecological transition from seas to freshwater.

The tiny fossils were found in deposits of grey organic-rich mudstone.

The research team’s findings, published in The Royal Society’s Biology Letters, show how ostracods began to swim into estuaries about 420 million years ago during the Silurian Period, beginning their exploration of many new habitats. Their findings help scientists understand how early animals colonised the land, and how terrestrial environments evolved.

Anna McGairy is a PhD researcher at Leicester supported by UKRI’s Central England NERC Training Alliance (CENTA), and corresponding author for the study. She said: “The fossil ostracods are tiny – only a few millimetres long – so you can get lots of them on each piece of rock. In the rocks from Vietnam the ostracods are found along with fossil plant material. It’s the plants that made us think these are not typical marine assemblages of ostracods, but ones living in an estuary.

“This develops our understanding because previously we weren’t sure when ostracods started to transition out of marine environments, and now we know they were probably among the frontrunners of animals moving into freshwater environments. This is about 60 million years before ostracods are known to have lived in freshwater environments, and so the transition may have been more prolonged and gradual than we thought.”

The fossils were collected by members of the team in northern Vietnam in 2018 as part of a Leverhulme Research Fellowship awarded to Leicester’s Professor Mark Williams titled ‘The Early Palaeozoic evolution of NE Vietnam’ (RF-2018-275\4).

Ostracods had colonized estuaries by the late Silurian’ is published in Biology Letters.

View of the Mã Pí Lèng Pass, a mountain road to Dong Van in northern Vietnam, close to where the fossils were discovered. Credit: Toshifumi Komatsu