Research sheds light on life and times of “Fiona” the pregnant ichthyosaur

About 131 million years ago, an 11-foot-long ichthyosaur slammed snout first into the seafloor and was rapidly buried by sediments — a sequence of events that helped preserve not only her skeleton, but that of her unborn baby, along with the remains of her last meal.  

 These details are part of a new study from an international team published in the Journal of Vertebrate Paleontology about the pregnant marine reptile, which the team named Fiona.

The study comes two years after Fiona’s fossilized remains were air-lifted in five pieces from a glacial field in Patagonia and brought to the Natural History Museum Río Seco, in Punta Arenas, Chile, for research.

Fiona is only fully preserved and excavated pregnant ichthyosaur from Chile, and is the only known pregnant ichthyosaur from the Hauterivian, a time period during the Early Cretaceous. Her remains, which are complete and largely intact, provide a detailed look into the anatomy of ichthyosaurs, an apex marine predator that resembles a dolphin of today.

What’s more, the details about how Fiona — and 87 other ichthyosaurs in the glacial field — became buried in the seafloor could be part of a larger story of continental breakup creating new ocean habitats, said study co-author Matt Malkowski, an assistant professor at The University of Texas at Austin’s Jackson School of Geosciences.

Malkowski, in collaboration with researchers at Boise State University, conducted the high-precision isotope dating that determined Fiona to be 131 million years old, a point in Earth’s history where South America was undergoing big changes. The continent was in the process of separating from what is now Africa. Malkowski said the opening up of a narrow oceanic passageway between the continents may have affected global climate, currents and habitats for marine life.

“If you are the apex predator in the ocean at the time, these are all things you care about,” he said. “You care about migration pathways. You care about places that you can hunt and fish and do your thing. You care about places where you can reproduce.”

Malkowski is a sedimentary geologist who focuses on how landscapes evolve over time by analyzing grain samples. Fossils are not in his usual wheelhouse. He got involved with the research when he met the study’s lead author Judith Pardo-Pérez while doing fieldwork in Patagonia’s Torres del Paine National Park.   

The two realized that Malkowski’s research on the larger landscape could help answer more specific questions about Fiona and her environment.

“We were conducting our research with different objectives, they are very synergistic,” Malkowski said. 

Based on how Fiona is preserved in the rock, the researchers determined that her snout burrowed about four inches into the sand when she struck the seafloor. The surrounding rock shows signs that Fiona could have been swept up in a massive flow of sediments during an underwater landslide.

The fact that she was rapidly covered by sediments likely contributed to her excellent preservation, and that of her unborn baby, whose skeleton is also mostly intact. Ichthyosaurs are thought to have given birth to live young. And based on the size of the fetus, the researchers think it was in its final stages of development and was positioned to be born, with its tail pointing toward the birth canal.

Other significant finds inside the fossil include a jumble of small fish vertebrae inside her ribcage – which researchers interpret as her final meal – and signs of a healed injury in her fin bones. Some of the bones are fused together, which may have been caused by an infection.

“The discovery of the Fiona ichthyosaur provides valuable information regarding the species, its paleobiology, and paleoecology,” said Pardo-Pérez, an associate professor at the University of Magallanes and the Cape Horn International Center in Chile. “Moreover, its skeleton will allow us to delve deeper into aspects of its anatomy through new studies and the use of medical technology.”

Researchers are learning a lot from Fiona’s bones. But she is just one of dozens preserved in the glacial field. Based on the geology, it doesn’t look like they all died at once. Preliminary data suggests that multiple landslides occurred in the region over time, leading to multiple mass dying events.

Malkowski’s stratigraphy research could help shed light on this question, in addition to the larger tectonic history of the area.

“We’re conducting an array of geochemical analyses to understand as much as we can about the environment of this ocean basin at the time,” he said. “We're working on teasing out whether or not this is a single event or multiple events and what the triggers were.”  

The research funded by the Chilean Science and Development Agency and the National Science Foundation.