Pregnant ichthyosaur dug up 131 million years after she slammed into the seafloor

Palaeontologists have analysed the fossil of a complete 3.5m-long ichthyosaur which was preserved so well that it includes the animal’s last meal as well as its unborn baby.

Nicknamed “Fiona”, the marine reptile’s story is detailed in a paper published in the Journal of Vertebrate Paleontology.

Fiona was airlifted in 5 pieces from the glacial field in southern Chile’s Patagonia region 2 years ago. The remains were transported to the Natural History Museum Río Seco in Chile where the research was conducted.

Another 87 ichthyosaurs have been uncovered in the glacial field, but Fiona is the only fully preserved and excavated pregnant ichthyosaur in Chile. She’s also the only pregnant ichthyosaur found anywhere in the world from this time period.

Ichthyosaurs were large marine reptiles that lived during the time of the dinosaurs.

The largest species may have grown to 25m in length, making them the largest marine reptiles to have existed. Some fossil evidence suggests that ichthyosaurs evolved before dinosaurs and maybe even before the greatest mass extinction event in Earth’s history 252 million years ago.

With bodies like those of modern dolphins, ichthyosaurs were probably the marine reptiles that were best adapted to life underwater.

Despite this, ichthyosaurs went extinct before dinosaurs and other marine reptile groups. They died out about 90 million years ago during the Early Cretaceous period. The exact cause of their extinction is not known, but it was likely bound up with changes in environment.

Fiona lived 131 million years ago during the Early Cretaceous.

When she died, Fiona slammed into the seafloor, burying her snout about 10cm into the sand. The rock around her shows evidence that she was rapidly buried by sediment during an underwater landslide.

This meant she was exquisitely preserved. The palaeontologists identified Fiona’s last meal – a jumble of small fish bones in her ribcage – and a healed injury on her fin.

Fiona’s unborn baby’s skeleton is also mostly intact. Other extraordinary fossils from around the world indicate that ichthyosaurs gave birth to live young. Fiona’s baby was likely in its final stages of development when it and its mother died.

The landslide that killed Fiona and her baby may have been the cause of the deaths of dozens of other of the ichthyosaurs found in the Patagonian glacier field.

It seems that all of these animals died at the same time. Geological surveys suggest that this landslide was one of many in the region during the Early Cretaceous. The palaeontologists believe that this is because of massive changes on the South American continent at the time.

The continent was in the process of separating from what is now Africa. This break up opened a narrow ocean passageway between the continents. Global climate, currents and habitats would have been affected.

“If you are the apex predator in the ocean at the time, these are all things you care about,” says study co-author Matt Malkowski from the University of Texas at Austin’s Jackson School of Geosciences. “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.”

Fiona’s story is part of this bigger picture about continental shifts and the impact that had on the lives of the animals millions of years ago.

“The discovery of the Fiona ichthyosaur provides valuable information regarding the species, its paleobiology, and paleoecology,” says lead author Judith Pardo-Pérez, an associate professor at the University of Magallanes 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.”

“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,” Malkowski adds. “We’re working on teasing out whether or not this is a single event or multiple events and what the triggers were.”