In a quiet corner of a German museum, a fossil lay waiting for nearly half a century. It had been carefully excavated from the world-famous Posidonia Shale near Holzmaden in 1978, cataloged, and placed in the collections of the Stuttgart State Museum of Natural History. For decades, it sat largely unnoticed by the wider scientific community—until now.
In a stunning revelation, paleontologists have reexamined this specimen and identified it as a previously unknown species of ancient marine reptile. Named Plesionectes longicollum, meaning “long-necked near-swimmer,” this sleek, long-necked creature swam Earth’s oceans during the Jurassic Period nearly 183 million years ago, at a time when massive predators, ancient reefs, and ammonites filled a vibrant underwater world.
The full scientific account of this remarkable find was recently published in PeerJ by Sven Sachs of the Naturkunde-Museum Bielefeld and Dr. Daniel Madzia of the Polish Academy of Sciences.
A New Chapter in the Plesiosaur Story
Plesionectes longicollum belongs to a group of marine reptiles known as plesiosauroids—iconic creatures characterized by their compact bodies, paddle-like limbs, and dramatically long necks. Plesiosaurs have captivated public imagination for generations, often evoked in images of sea monsters and prehistoric predators. Yet, the actual story of their evolution and diversity remains far more complex and nuanced than once believed.
This discovery significantly expands what we know about plesiosaur evolution. The specimen, despite being from a juvenile individual, exhibits a combination of anatomical traits never before documented in the fossil record. Its skeletal proportions and unique vertebral structures were distinct enough to warrant classification not only as a new species, but as a new genus altogether.
“This specimen has been in collections for decades, but previous studies never fully explored its distinctive anatomy,” said Sachs. “Our detailed examination revealed an unusual combination of skeletal features that clearly distinguish it from all previously known plesiosaurs.”
Impeccable Preservation from an Ancient Seafloor
What makes this fossil particularly compelling is not only its rarity, but its state of preservation. Unlike many fragmentary remains, the Plesionectes specimen—cataloged as SMNS 51945—is nearly complete. Its fossilized bones are laid out in articulation, and soft-tissue impressions are even visible in certain regions, a hallmark of the exceptional fossilization conditions in the Posidonia Shale.
These rocks were deposited during the Early Jurassic Toarcian stage in a shallow marine environment that became starved of oxygen—an “anoxic event” that killed much of the seafloor life and simultaneously created ideal conditions for preserving carcasses in exquisite detail. The Holzmaden fossil beds are world-renowned for this reason, often producing fossils with remarkable clarity, from ichthyosaurs with preserved skin outlines to ancient squids with visible ink sacs.
Now, Plesionectes longicollum joins this elite company, offering scientists a rare and vivid glimpse into Jurassic seas.
A Time of Crisis and Change Beneath the Waves
The early Toarcian period, during which Plesionectes lived, was a time of profound environmental upheaval. The planet experienced a major oceanic anoxic event (OAE), likely triggered by intense volcanic activity, climate change, and disruptions in the global carbon cycle. Widespread deoxygenation in the oceans led to mass extinctions in marine ecosystems, altering food webs and evolutionary trajectories.
“This discovery adds another piece to the puzzle of marine ecosystem evolution during a critical time in Earth’s history,” explained Dr. Madzia. “The early Toarcian period was marked by significant environmental changes, and the survival and emergence of new marine reptile lineages during this interval sheds light on how life adapted to these challenges.”
In this context, Plesionectes longicollum is more than just a new species—it is a data point in a larger story of resilience and adaptation, a snapshot of life navigating through a time of planetary crisis.
Youthful, But Not Ordinary
Though the fossil represents a relatively young individual, its distinctive features were not the result of incomplete development. Paleontologists took great care to distinguish juvenile characteristics from those that were evolutionary adaptations. This distinction is crucial; immature bones can sometimes mimic novel features, but in this case, the skeletal traits held strong across detailed comparative analysis.
Key anatomical hallmarks—such as the proportions of the neck vertebrae, the structure of the pectoral girdle, and the configuration of the paddles—set Plesionectes apart from its plesiosaur cousins. Its unusually long neck, which contains more vertebrae than most related forms, hints at specialized swimming strategies or perhaps even unique hunting behaviors.
The Posidonia Shale: A Window into a Vanished Sea
The Posidonia Shale Formation remains one of the most celebrated fossil sites on Earth. Located in southwestern Germany, this geological formation has yielded an unparalleled abundance of well-preserved marine reptiles, fishes, invertebrates, and even plants. Its deposits provide a nearly cinematic view of Early Jurassic marine life.
Prior to this discovery, scientists had identified five plesiosaur species from the Holzmaden area, representing all three major plesiosaur lineages. The addition of Plesionectes longicollum deepens the known diversity of this ecosystem, suggesting that the waters of the Jurassic Tethys Ocean were teeming with more species than previously thought.
“This is the oldest plesiosaur yet discovered from Holzmaden,” said Sachs. “Its presence suggests that these reptiles diversified earlier and in greater numbers in this region than we had imagined.”
Scientific Patience and Rediscovery
One of the most remarkable aspects of this discovery is that it comes not from a recent expedition, but from a specimen collected almost 50 years ago. The fossil had been resting in the museum’s collection, its secrets locked within stone until modern science and persistent curiosity brought it back into the spotlight.
This is not unusual in paleontology. Countless fossils in museum collections await reexamination. As new tools, techniques, and comparative frameworks develop, old bones can yield fresh insights. Sometimes, it is not the field, but the archive, that holds the next breakthrough.
The study by Sachs and Madzia underscores the value of revisiting museum collections with fresh eyes and modern understanding. It is a testament to the importance of preserving and curating these scientific treasures—and to the unexpected stories that can still be told by silent fossils.
A Glimpse into the Unknown
With every new species unearthed from Earth’s deep past, our understanding of ancient life becomes a little sharper, a little richer. But it also grows more humbling. Evolution is not a simple ladder, nor is extinction the end of the story. The world of Plesionectes longicollum was dynamic, dangerous, and full of life pushing the limits of survival in the face of global change.
As scientists continue to uncover new fossils from places like Holzmaden, they reveal not only the creatures that once lived but the intricate and delicate webs of ecosystems that supported them. These discoveries remind us that life is adaptable, innovative, and often more diverse than we could ever imagine.
For now, Plesionectes longicollum swims no more, but its bones tell a tale that stretches across eons—a tale of oceans that breathed and choked, of creatures that evolved and vanished, and of a species—our own—straining to understand the past before it forgets us.
More information: An unusual early-diverging plesiosauroid from the Lower Jurassic Posidonia Shale of Holzmaden, Germany, PeerJ (2025). DOI: 10.7717/peerj.19665