Baby sea turtles are cute. They fascinate us as they dig out of their sandy nests and make a run for the sea. But they face instant danger from voracious birds and other predators – so we cheer them on, hoping that they will “make it” to the relative safety of their natural habitat, the ocean.
Those that are successful in this journey will not return to land until they are breeding adults, usually around 20 years old, to bury their own eggs. This means that they only make tracks on land as hatchlings for the first few minutes of life. And most of these tracks are quickly washed away.
This makes finding the first ever fossil baby sea turtle trackways all the more surprising.
We are part of a research team from South Africa, the US and Canada that recently had an article published in the journal Quaternary Research revealing the existence of baby sea turtle trackways dating back about 100 000 years, on what is today South Africa’s Cape South Coast. This is an area that stretches eastward along the coast from Cape Town.
These trackways date from a time in the Pleistocene Epoch, when our species, Homo sapiens, was emerging. The Pleistocene was characterised by large fluctuations in global climate. Ice sheets repeatedly advanced and retreated over much of the northern hemisphere, as the climate cooled and warmed. These changes are reflected along the Cape South Coast by evidence of fluctuating ocean levels, which at times rose to as much 13 metres above present-day levels.
Because nothing like these fossil trackways has ever been described before, we have coined the scientific term “Marineropodidae” meaning “seafarer foot traces” to describe the family of remarkable trackways that we have encountered.
And we have created the genus and species names Australochelichnus agulhasii and Marinerichnus latus for the two distinctively different trackway patterns we have found, that resemble those made today by hatchling Loggerhead and Leatherback turtles respectively.
These trackways preserve an incredibly brief moment in time. More importantly, they tell us about ancient climates, and how turtle breeding ranges have changed over the millennia. Today these turtles are found nesting over 1 000 km to the northeast, around the St Lucia coast and near the border with Mozambique. But they rarely come ashore along the Cape South Coast, and successful nesting appears exceptionally rare. We infer that the climate and sand temperatures were warmer when the tracks were made.
Our team of trackers, in searching the beautiful beaches of the Cape South Coast for Pleistocene fossil footprints, have already found tracks of humans, lion, rhinoceros, elephant, giraffe, two species of buffalo, a giant horse, and various birds.
The turtle tracks were spotted by our co-author Jan De Vynck and his colleagues on a large rock surface in 2016. These were seven nearly parallel trails, and the team could deduce that the trackmakers had been heading southwards, towards the sea. Since then three other sites have been found, two of them within a few kilometres of the first site. The third site is about 100 km to the east, and possibly shows evidence of the nest from which the hatchlings emerged.
Almost all previously-known fossil turtle tracks were made by fresh water species that lived in lakes and ponds. And most of these have been found and studied in the Northern Hemisphere, in Europe, North America and Asia. There have been reports of giant sea turtle tracks from Jurassic rocks in western Europe. But these were made by large adults touching the sea bed with their paddles while swimming.
From then on there was absolutely no known trace of any kind of sea turtle activity for the next 150 million years, until these fossil hatchling trackways were discovered in South Africa.
One question related to our find is how the trackways were preserved. Most likely, we suggest, wind blew fine dry sand over the beach surface where the tracks had recently been made in wet sand, preserving the tracks and later creating a separation layer. This allowed for their fortuitous discovery 100 000 years later.
Such tracks contribute to an ecological census of the diverse fauna that inhabited the coast when the human species was young and lived close to nature. The ecology has changed over the last 100 000 years due to natural climate change as well as human hunting activity. Tracks and traces complement the body fossil record in helping to document these changes.