''Palaeopleurosaurus'' is an extinct

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial n ...

diapsid Diapsids ("two arches") are a clade of sauropsids, distinguished from more primitive eureptiles by the presence of two holes, known as temporal fenestrae, in each side of their skulls. The group first appeared about three hundred million years ...

reptiles belonging to the group Sphenodontia.Dupret, V. (2004). The pleurosaurs: anatomy and phylogeny. ''Revue de Paléobiologie'', 9: 61-8

/ref> ''Palaeopleurosaurus'' fossils were discovered in

Germany Germany, officially the Federal Republic of Germany (FRG),, is a country in Central Europe. It is the most populous member state of the European Union. Germany lies between the Baltic and North Sea to the north and the Alps to the sou ...

(Holzmaden and Kerkhofen) from the Lower

Toarcian The Toarcian is, in the ICS' geologic timescale, an age and stage in the Early or Lower Jurassic. It spans the time between 182.7 Ma (million years ago) and 174.1 Ma. It follows the Pliensbachian and is followed by the Aalenian. The Toarc ...

(

Early Jurassic The Early Jurassic Epoch (in chronostratigraphy corresponding to the Lower Jurassic Series) is the earliest of three epochs of the Jurassic Period. The Early Jurassic starts immediately after the Triassic-Jurassic extinction event, 201.3 Ma ...

Posidonia Shale The Posidonia Shale (german: Posidonienschiefer, also called Schistes Bitumineux in Luxembourg) geologically known as the Sachrang Formation, is an Early Jurassic (Toarcian) geological formation of southwestern and northeast Germany, northern Swi ...

, dated to about 182-180 million years ago.R. L. Carroll, R. L. (1985). ''A pleurosaur from the Lower Jurassic and the taxonomic position of the Sphenodontida''. Palaeontographica Abteilung A, 1985. Volume 189. Pages 1-28. The holotype come from the Middle Section, just under the Seagrasschiefer section (=Bifrons subzone) on both localities, probably linked with the coeval measured sea regression. Initially, ''Paleopleurosaurus'' was described to represent an intermediate morphology between Late Triassic-Lower Jurassic terrestrial sphenodontia and the late Jurassic fully aquatic ''

Pleurosaurus ''Pleurosaurus'' (meaning "side lizard") is an extinct genus of diapsid reptiles belonging to the group Sphenodontia, extinct relatives of the modern tuatara. ''Pleurosaurus'' fossils were discovered in the Solnhofen Limestone of Bavaria, G ...

''. The study of a new specimen provided additional information, corroborating the existence of an early lineage of elongated sphenodontians, although with retained characters of terrestrial genera such as '' Kallimodon''.

Paleobiology

''Palaeopleurosaurus'' evidences that there was a slightly skeletal specialization for an aquatic lifestyle, achieved through the Jurassic gradually on pleurosaurs, a lifestyle supported by increased bone density in its

gastralia Gastralia (singular gastralium) are dermal bones found in the ventral body wall of modern crocodilians and tuatara, and many prehistoric tetrapods. They are found between the sternum and pelvis, and do not articulate with the vertebrae. In thes ...

. It has similarities with other marine reptiles, such with members of

Sauropterygia Sauropterygia ("lizard flippers") is an extinct taxon of diverse, aquatic reptiles that developed from terrestrial ancestors soon after the end-Permian extinction and flourished during the Triassic before all except for the Plesiosauria bec ...

the presence of a defined suture between the centrum and the neural arch, along with reducted sternum. However, the lack of increase in rib and femur bone density suggests that the lineage of ''Palaeopleurosaurus'' became gradually adapted to aquatic lifestyles, not as adapted as ''

'', as show limited morphological evidence of adaptation to a complete aquatic lifestyle, defined by no

Osteosclerosis Osteosclerosis is a disorder that is characterized by abnormal hardening of bone and an elevation in bone density. It may predominantly affect the medullary portion and/or cortex of bone. Plain radiographs are a valuable tool for detecting and ...

and the lack of

Pachyostosis Pachyostosis is a non-pathological condition in vertebrate animals in which the bones experience a thickening, generally caused by extra layers of lamellar bone. It often occurs together with bone densification (osteosclerosis), reducing inner ca ...

, except for a thicker shaft region in the humerus, that is as narrow as in terrestrial rhynchocephalians, such as the terrestrial ''

Clevosaurus ''Clevosaurus'' (meaning "Gloucester lizard") is an extinct genus of rhynchocephalian reptile from the Late Triassic and the Early Jurassic periods. Species of ''Clevosaurus'' were widespread across Pangaea, and have been found on all continents ...

''.Carroll RL, Wild R (1994) Marine members of the Sphenodontia. In: Fraser N, Sues H-D (eds) In the shadow of the dinosaurs—early Mesozoic tetrapods. Cambridge University Press, Cambridge, pp. 70–83 ''Palaeopleurosaurus'' probably was still able to walk on land, for example for Oviposition. Recent studies suggest a shorter lifespan than modern

Tuatara Tuatara (''Sphenodon punctatus'') are reptiles endemic to New Zealand. Despite their close resemblance to lizards, they are part of a distinct lineage, the order Rhynchocephalia. The name ''tuatara'' is derived from the Māori language and ...

, based on irregular spacing of growth marks.

Histology Histology, also known as microscopic anatomy or microanatomy, is the branch of biology which studies the microscopic anatomy of biological tissues. Histology is the microscopic counterpart to gross anatomy, which looks at larger structures vi ...

shows that the number of growth rings in ''Palaeopleurosaurus'' was smaller than the living

tuatara Tuatara (''Sphenodon punctatus'') are reptiles endemic to New Zealand. Despite their close resemblance to lizards, they are part of a distinct lineage, the order Rhynchocephalia. The name ''tuatara'' is derived from the Māori language and ...

, possibly indicating a shorter lifespan.

References

Sphenodontia Prehistoric reptile genera Toarcian life Jurassic lepidosaurs Jurassic Germany Fossils of Germany Posidonia Shale Fossil taxa described in 1985 {{jurassic-reptile-stub