The Oldest Plesiosaur in Town – Rhaeticosaurus mertensi

Following the end-Permian mass extinction event, the world’s ecosystems took several million years to recover.  In marine environments, just as on land, the mass extinction event led to devastating losses, it has been estimated that 57% of marine families died out.  However, as the Triassic progressed, a number of terrestrial reptiles adapted to marine habitats and new, diverse ecosystems evolved.  It had long been suspected that the Plesiosauria (the long-necked Plesiosaurs and the big-headed Pliosaurs), the most diverse and longest-lived of all the extinct marine reptile groups, had their origins in the Triassic, but the fossil evidence for basal Plesiosaurs was somewhat lacking.  However, the discovery of a partially articulated fossil in a clay pit, close to the village of Bonenburg in North Rhine-Westphalia (Germany), has helped to plug a gap in the fossil record.

The Fossilised Remains of the World’s Oldest Plesiosaur

Rhaeticosaurus fossil (A) with line drawing below (B).

Rhaeticosaurus fossil (A) with line drawing (B).

Picture Credit: Georg Oleschinski

The fossil discovery marks the first Plesiosaur specimen to be recovered from Triassic-aged rocks.  It is the oldest Plesiosaur to be found to date, the only one which dates from the Triassic Period.

Intriguingly, a study of cross-sections of some of the larger fossilised bones in the 2.37-metre-long skeleton, support previous research that suggests these marine reptiles grew rapidly and were (most likely), warm-blooded.  The new species has been named Rhaeticosaurus mertensi, (ree-ti-co-sore-us mur-ten-see), the genus name comes from the last faunal stage of the Triassic (the Rhaetian), the trivial name honours  private collector Michael Mertens, who made the initial fossil discovery.

201 Million-Year-Old Fossil

Michael Mertens discovered the specimen in 2013, some of the neck bones had been lost but the majority of the skeleton was in situ.  The resulting excavation, study and publication in the academic journal “Science Advances”, is a credit to the parties involved, namely Herr Mertens, the natural heritage protection agency, the Münster museum, and scientists from various institutes including Bonn University, the Osaka Museum of Natural History, the University of Tokyo and the Natural History Museum of Los Angeles County, amongst others.

Co-author of the Scientific Paper Tanja Wintrich with the Fossil Finder Michael Mertens

Rhaeticosaurus fossil discovery.

PhD student Tanja Wintrich with Michael Mertens show where the fossil was found.

Picture Credit: Professor Martin Sander (University of Bonn)

The Long-lived and Diverse Plesiosauria

In a press release from Bonn University, Plesiosaurs are described as especially effective swimmers.  They evolved a unique, four-limbed propulsion using broad flippers, in essence, “flying underwater”.

One of the authors of the scientific paper Professor Martin Sander explained:

“Instead of laboriously pushing the water out of the way with their paddles, Plesiosaurs were gliding elegantly along with limbs modified to underwater wings.  Their small head was placed on a long, streamlined neck.  The stout body contained strong muscles keeping those wings in motion.  Compared to the other marine reptiles, the tail was short because it was only used for steering.  This evolutionary design was very successful, but curiously it did not evolve again after the extinction of the Plesiosaurs.”

An Illustration of a Typical Long-necked Plesiosaur

Plesiosaurus.

An illustration of a Plesiosaurus.

Bone Histology Suggests Rapid Growth and Potential Endothermy

The Triassic Plesiosaur already has the typical long-necked Plesiosaur bauplan and it was, like most of its descendants, a pelagic piscivore (an active swimmer, hunting fish).  Analysis of the bone structure indicates that the specimen represents a juvenile, one that was growing rapidly.  Thin cross-sections of fossil bone were compared to Jurassic and Cretaceous specimens and the team’s findings support the hypothesis that to grow this quickly, these reptiles needed to be warm-blooded.

Professor Sander stated:

“Plesiosaurs apparently grew extremely fast before reaching maturity.  Since Plesiosaurs spread quickly all over the world, they must have been able to regulate their body temperature to be able to invade cooler parts of the ocean.”

The Hind Leg Bones of Rhaeticosaurus mertensi

Hind leg bones of Rhaeticosaurus.

Left femur (f), tibia (ti) and fibula (fi). The proximal femur is a cast because the original was sectioned for histology (scale bar = 1 cm).

Picture Credit: Science Advances

In the photograph (above), the part of the femur (f) is a cast as this bone was cross-sectioned as part of the bone study.

Filling a Gap in the Fossil Record

The evolution of the Plesiosauria is poorly understood.  They are probably descended from a group of long-necked, marine reptiles known as Pistosaurs, fossils of which are associated with Middle to Late Triassic deposits.  An example of a Pistosaur is Bobosaurus (B. forojuliensis) from the Rio del Lago Formation of Italy (Carnian faunal stage of the Triassic).  However, Bobosaurus lived some thirty million years before Rhaeticosaurus evolved.  This German fossil discovery helps to fill in a little of the temporal gap in the fossil record of this successful lineage.  Rhaeticosaurus has been assigned to a basal position within the Pliosauridae family and its discovery reveals that the diversification of the Plesiosauria was a Triassic event and a number of genera survived the end Triassic extinction into the Jurassic.  The researchers conclude that the bone histology of this Late Triassic marine reptile suggests that the evolution of fast growth and an elevated metabolic rate were adaptations to an active, pelagic life-style foraging in open water.

Articulated Cervical Vertebrae (C) and Elements from the Left Front Limb (D)

Neck bones (c) and forearm, hand bones of Rhaeticosaurus.

Cervical vertebrae (C) and the left radius (ra), a phalanx (ph) and a (cr) carpal element (D).

Picture Credit: Science Advances

The new specimen corroborates the hypothesis that the open ocean life of Plesiosaurians facilitated their survival of the end-Triassic extinction.

The scientific paper: “A Triassic Plesiosaurian Skeleton and Bone Histology Inform on Evolution of a Unique Body Plan” by Tanja Wintrich, Shoji Hayashi, Alexandra Houssaye, Yasuhisa Nakajima and P. Martin Sander published in the journal “Science Advances”.

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