A New Reconstruction of Plesiosaurs Based on an Exceptionally Complete Specimen

Plesiosaurs (Plesiosauria) were one of the most iconic groups of marine reptiles during the Mesozoic Era. Their evolutionary history began in the Triassic and continued until the mass extinction at the end of the Cretaceous, spanning roughly 140 million years. Fossils of these animals have been discovered across the globe, indicating that they were widely distributed and highly diverse. Their general body plan is familiar: a long neck, a compact torso, and four powerful flippers adapted for life in the ocean.

Yet despite their fame, the actual external appearance of plesiosaurs has remained uncertain. While other marine reptile groups have gradually acquired more detailed life reconstructions through discoveries of soft tissues and skin impressions, plesiosaurs have lagged behind in this regard. The reason is simple. Although skeletal fossils are abundant, preserved soft tissues are extremely rare. To date, only eight plesiosaur specimens with preserved soft tissue traces have been scientifically documented.

Reconstruction of the new specimen MH 7, Courtesy of Joschua Knüppe

A Remarkably Complete Fossil from Germany

A recent study describes an exceptionally complete plesiosaur specimen from the Posidonienschiefer Formation in Germany. The fossil, catalogued as MH 7, dates to the Early Jurassic. What makes this specimen particularly valuable is its remarkable state of preservation. Not only is the skeleton nearly complete, but extensive skin impressions are preserved around the tail and along the trailing edge of the right forelimb.

Even more extraordinary, the skin traces on the tail contain remnants of melanosomes. These microscopic structures are associated with pigmentation and rarely survive fossilization. Their preservation makes the specimen an especially important source of information for reconstructing the appearance and biology of plesiosaurs.

The fossilized skin also shows clear layering, allowing researchers to identify structures corresponding to the epidermal layers found in modern amniotes. These include the stratum corneum, the stratum spinosum, and the stratum basale.

Photograph of specimen MH 7（Image source：Klaus Nilkens/Urwelt-Museum Hauff， CC BY 4.0 ）

Analysis of the Soft Tissue Structure

Microscopic examination reveals that the stratum spinosum of the tail skin in specimen MH 7 reaches a thickness of approximately 215 micrometers, whereas the stratum corneum is relatively thin, measuring about 15 to 25 micrometers. Within the stratum spinosum, fossilized keratinocytes are preserved. These cells are roughly circular in shape, about 20 micrometers in diameter, and contain dark central regions interpreted as remnants of cell nuclei. The stratum corneum consists of densely packed keratinized cells. Melanosomes are scattered throughout the outer layers of the skin.

Tail skin of MH 7, with the black dots in D and E representing dispersed melanosomes（Image source：Mark, M et al， CC BY 4.0 ）

In contrast, the right forelimb of MH 7 preserves numerous small, irregularly shaped structures that appear roughly triangular. Researchers interpret these as scales. Unlike the soft, scale-less skin around the tail, these scales have smooth surfaces and lack melanosomes. Their morphology closely resembles the epidermal scales seen in marine reptiles such as mosasaurs and sea turtles.

Right forelimb of MH 7, with the sub-triangular scales visible in A（Image source：Mark, M et al， CC BY 4.0 ）

Earlier hypotheses suggested that the scarcity of plesiosaur skin fossils might be explained by unusually thin epidermis, comparable to that of modern snakes, which would rapidly decay after death. However, the new study shows that the combined thickness of the epidermal layers in MH 7 reaches at least about 250 micrometers. This is comparable to the skin thickness of modern sea turtles and therefore relatively robust.

The rarity of plesiosaur skin fossils may instead reflect environmental conditions. Plesiosaur carcasses may have been less likely to settle in low-oxygen seabed environments that favor the preservation of soft tissues.

Possible Functions of the Skin Structures

The fossil evidence from MH 7 reveals an interesting pattern: the tail skin lacks scales, while the flippers bear them. This suggests that the scales on the flippers served a particular functional role.

One interpretation is that these scales stiffened the flippers. By reinforcing the trailing edge of the flippers, the scales may have helped maintain stability during swimming, thereby improving hydrodynamic efficiency.

Another possibility involves feeding behavior. Previous research on another plesiosaur specimen, MB.R.1992, revealed preserved stomach contents consisting of coarse sediment mixed with gastropods and cephalopods. This suggests that some plesiosaurs fed near the seafloor, scooping up sediment along with prey and swallowing the mixture. Comparable feeding behavior can be observed in certain modern dolphins and sea turtles.

If plesiosaurs frequently moved along the seabed while feeding, scales on the flippers could have served a protective role. They may have shielded the flipper surfaces from abrasion while the animals interacted with sediment and hard substrates.

Soft tissue specimens of other plesiosaurs; F and G show the gut contents of MB.R.1992, including gastropods and belemnites（Image source：Mark, M et al， CC BY 4.0 ）

Conclusion

The new findings demonstrate that plesiosaurs retained reptilian scales even after fully adapting to life in the ocean. This evolutionary pattern contrasts sharply with that of other marine reptiles such as ichthyosaurs and marine crocodylomorphs, which reduced or lost their scales to minimize drag in the water.

The presence of scales on the flippers of plesiosaurs suggests that these structures served important functional purposes. Their combination of smooth body skin and localized scaling may have contributed to the ecological success of plesiosaurs, helping them thrive as one of the most enduring marine reptile lineages of the Mesozoic oceans.

Author: Bai Leng

Reference:

Mark, M., Sjövall, P., Kear, B. P., Jarenmark, M., Eriksson, M. E., Sachs S., Nilkens, K., Beeck, M. O. D., Lindgren, J. (2025). Skin, scales, and cells in a Jurassic plesiosaur. Current Biology.

（Paid content. Unauthorized reproduction or use is prohibited.）