Scientists find unique fossil of turtle on Danish island thought to be in transition from land to sea

In evolutionary terms what happens when a terrestrial organism adapts to a marine environment? The recent discovery of a unique fossil turtle provides an unusual example of one such organism in transition from land to sea. 

The finding in rock sediment in a northern Denmark island dates to the Eocene Period, about 54 million years ago. The fossil is unusual because it is a partial but well-preserved hard-shelled turtle with intact soft tissue remains.

A team of scientists from Sweden, Denmark and North Carolina describes the fossil and its importance in the journal Nature's Scientific Reports, Dec. 31, 2022.

The fossil turtle is a stem cheloniid, one of a family of large marine turtles that have paddle-like forelimb flippers. What makes this fossil different, the researchers report, is that "unlike its scaly living descendants," it combines scaleless limbs with a bony carapace covered in scutes." Scutes are hard plates that cover the turtle shell. They are shed and replaced like snake skin when the turtle grows. 

The authors state their findings show the adaptation process from land to a marine environment "was more complex than hitherto appreciated, and included at least one evolutionary lineage with a mosaic of integumental features not seen in any living turtle."

However, because the fossil is incomplete and one of its kind, the authors caution, their findings remain hypothetical and "need to be validated by additional soft-tissue specimens of similar age."

An abundant fossil area

The fossil was found in what's called the Fur Formation in Denmark, which is sediment composed of diatoms and clay mixed with volcanic ash. (Fur is the name of the island.) The island area's cliffs and quarries have ideal conditions for fossil preservation, and are the source of many fossilized insects, fish, reptiles and birds, as well as plants.

Four turtle fossils have been found at the location, but only two had preserved soft tissue.

The abundance of fossils in the location reflect "the organismal diversity immediately after the most pronounced greenhouse event of the Cenozoic, " the researchers state, in the Paleocene-Eocene Thermal Maximum.

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Randolph Glenn De La Garza, et al. "An ancestral hard-shelled sea turtle with a mosaic of soft skin and scutes," Scientific Reports, Dec. 31, 2022. https://doi.org/10.1038/s41598-022-26941-1

An interview with Randolph De La Garza

The importance of finding a fossil with both a skeleton and preserved soft parts 

Current Science Daily asked lead author Randolph De La Garza, Ph.D, to discuss the unusual ancestral turtle fossil. His work was done while he was a doctoral student at Lund University in Sweden in the Department of Geology.

Why is this fossilized sea turtle so important?

Fossilization is a rare event for most animals. The chances of its occurring are higher if the organism has hard body parts, like a skeleton, but the fossilization of soft tissues like the skin is exceedingly rarer since it is relatively easy for this tissue to rot.

Very special circumstances are required for fossilization of soft tissue to happen, such as a stagnant seafloor. This is a problem for scientists because while skeletons give us information about many aspects of the animal’s biology, like diet and evolution, they do not, however, show the whole story about the animal when it was alive. Most of the other important information, like color, behavior, sex, is largely determined by soft tissue, which is unfortunately lost to time.

Whenever soft tissues are preserved, one can appreciate a greater depth of information about an animal’s biology than what would otherwise only be speculation when working only with skeletal remains.

Where was the fossil found?

The sea turtle was found in northern Denmark, on an island called Mors. It was originally discovered in one of the local mine pits in a block of rock that had fallen from the walls of the pit. One of our co-authors saw some of the bones when looking at the side of the rock. He knew from experience that it was a sea turtle, so he sent it to another coauthor to prepare the fossil.

When we excavated the fossil, we discovered not only that an entire half of the turtle was present, but also that it had soft tissues as well.

What kind of adaptations are needed for a turtle to go from terrestrial animal to a sea animal, as this one did about 54 million years ago?

Well, “sea turtles,” as in their taxonomic group Panchelonioidea, have been around for some time before this one appeared in the fossil record. The oldest confirmed sea turtle fossil is around 120 million years in age.

The Danish fossil sea turtle is part of an extinct side lineage that inherited the basic adaptations for living their lives underwater from these ancient marine turtles. These adaptations include:

• Shells that became flattened, thinner and heart shaped (sometimes oval in others), which helped streamline the animal so that it could easily swim through the water.

• Fingers and toes became stretched out while their limbs became more stiffened. This transformed their feet into paddles, which they use to propel themselves through water.

• They have special glands to remove excess salt from their bodies.

• Many of the turtles evolved specialized jaws for feeding underwater, with some like the hawksbill, having a beaklike mouth to eat sponges.

• The leatherback sea turtle, which belongs to a different family than the other “hard-shelled” sea turtles, takes it a step further by evolving unique adaptations for life in the open seas, also known as the “pelagic” environment. These adaptations include growing to large sizes, having completely scaleless skin as adults, (hatchlings have scales), having blubber, or fatty subcutaneous tissue that helps the turtle retain body heat, having a higher body temperature than what is normally expected for reptiles, and having a shell that lacks a carapace, the upper part of the turtle’s shell.

What did this fossil show that was different?

This fossil turtle is different from modern sea turtles in that it seems to show scaleless flippers like the leatherback turtle, yet the fossil still has scales on its shell like the other sea turtles. This probably indicates that this turtle was transitioning to open ocean habitats.

Can you say something about the different types of turtles and their scales and turtle evolution?

There are around 13-14 families of turtles, with most having shells that are covered in several tough scales called scutes. These scutes provide a sort of armor to protect the animal from predators and the environment.

Depending on the species of turtle, most also have additional scales that cover their faces, feet and lower arms, while their neck, tails and shoulders often have soft, flexible skin.

Tortoises and “hard-shelled” sea turtles are notable for having thick, tough scales, most likely because they live in hazardous environments, for example, in habitats with sharp rocks present.

However, a few other families of turtles are unusual in that they lost most or even all of their scales, including the scutes on their shells. Such turtles include the soft-shelled turtles, pig-nosed turtle and the leatherback sea turtle. The first two, the soft-shelled and pig-nosed turtles, spend almost their entire lives in freshwater.

The leatherback turtles live in the open water, where they also take long, frequent deep dives to look for prey. It appears that these turtles lost their scales to give them a hydrodynamic edge by allowing them to have flexible bodies and paddles, which allows them to be more maneuverable while underwater.

What are your future research plans?

I would like to look at other fossils of reptiles, including more sea turtles, that also have soft tissue present and see how their skins are preserved. The evolution of scales was an important innovation for the conquest of the land by reptiles, but little is known on how much their scales have changed throughout their evolutionary history.

For example it is known that some marine-dwelling reptiles like the ichthyosaur completely lost their scales in favor of a smooth, rubbery skin. How did these dolphin-like reptiles transition from a scaly, land-based ancestor to an aquatic descendent with no scales? How their skin changed, and whether the leatherback sea turtle also had similar changes, are poorly known.

What else would you like to highlight about this fossil?

This turtle is quite the survivor. There are puncture marks along the edges of the shell that look like tooth marks from a crocodile or some other large animal with cone-shaped teeth. Part of the outermost bones on the shell are mangled. However, the bone did partially heal for some extended time. This was one lucky turtle that survived an attack from a predator.