Paleontologists in Uruguay have discovered large, poorly preserved and incomplete skulls, as well as many associated bones, of ancient aquatic reptiles called mesosaurs. These new specimens indicate gigantism in mature mesosaurs that reached more than twice the size of previously described adults and specimens of the type.
Mesosaurs are small to medium-sized aquatic or semi-aquatic amniotes that lived in Gondwana during the Early Permian. Photo credit: Roman Yevseyev / Graciela Pinheiro.
Mesosaurs They are a group of small aquatic reptiles that lived in southern Africa and South America during the Permian Period, about 299 to 270 million years ago.
These creatures were the first known aquatic reptiles, and appear to have returned to an aquatic lifestyle from more terrestrial ancestors.
“Mesosaurs are often described as small to medium-sized aquatic animals, partly due to their long bodies and long tails that exceeded the length of the rest of the body,” said Dr. Graciela Pinheiro from the University of La Repubblica and colleagues.
“In addition, their long-snouted skull consists of very thin cranial bones, bearing numerous needle-like marginal teeth.”
“Despite the fragility of some of the skull bones, mesosaur remains are abundant in Gondwana’s Permo-Carboniferous shale and silt deposits, including well-preserved, nearly complete skeletons with articulated skulls and partially preserved specimens that still retain soft tissue that remains recognizable.”
“This exceptional preservation has led to the Mangrolo Formation in Uruguay and the Irati Formation in Brazil being named Konservat-Lagerstätten of Gondwana,” they added.
“Furthermore, mesosaur fossils from these units include different ontogenetic stages, from unborn individuals to juveniles and adults.”
“Therefore, the growth evolution of mesosauria is well documented, with body lengths ranging from 10 or 12 cm (3.9-4.7 in) in juveniles to 80 or 90 cm (2.6-3 ft) in young adults.”
“The average length of a mesosaur was about 70 cm (2.3 feet),” they said.
“Previous studies suggest that mesosaurs are semi-aquatic animals rather than fully aquatic.”
“In addition, in recent studies on mesosaur taxonomy and paleobiology, more than 1,000 specimens have been examined, including small and large individuals belonging to Mesosaurus tenuidens“.
“The data also indicate that mesosaurs, in their mature stage, evolved more terrestrial habits.”
A series of embryonic mesosaur skulls documented from specimens found in the Mangrolo Conservat-Lagerstadt Formation in Uruguay. Scale bar – 1 cm. Image credit: Pinheiro et al., doi: 10.3390/fossils3010001.
Dr. Pineiro and co-authors discovered and examined the remains of the largest known mesosaurs documented to date.
The specimens include two fragmentary skulls, a dorsal vertebra, part of a tail, an isolated rib and several other bones.
They came from the areas of Picada de Coelho and El Baron Fe Mangrolo formation In northern Uruguay.
Paleontologists compared the new fossils to a large sample of the same bones Mesosaurus tenuidens.
Their analyzes revealed that the size of mesosaurs from the Mangrolo Formation exceeds the typical size distribution previously recorded for Mesosaurus tenuidens.
These individuals were four to five times larger than the average standard deviation, with skull lengths between 15 and 20 cm (5.9–7.9 in) and total body sizes between 1.5 and 2.5 meters (4.9–8.2 ft).
“To explore the importance of such large mesosaurs in the population of the Mangrolo Formation, we took into account the potential influence of Bergmann’s rule,” the researchers said.
“Although this is an important ancient geographical principle in the study of isolated populations of the same or different species whose body size may have been affected by changes in temperature and food availability, including competition for nutrients, it does not explain the presence of such large quantities of Living organisms. Specimens of mesosaurs in the Mangrolo Formation.”
“Therefore, we conclude that the large variation in body size found in mesosaurs corresponds to ontogenetic stage and growth pattern,” they said.
“The previously recognized smaller body sizes likely represent assemblages dominated by early juveniles, subadults, and young, which may reflect catastrophic mass mortality events that interfere with the natural attritional addition of carcasses of animals that died of various causes.”
“The bentonite layers and gypsum crystals found in the Mangrolo Formation result from tangential inputs of waterfalls above the lake derived from increased tectonic activity associated with the formation of Pangea.”
“These volcanic events together with the observed gradual drying out of the Irati-Whitehill Sea and accompanying desertification are suggested to be the main cause that may explain the extinction of the mesosaur, at least in the Uruguay region of the Paraná Basin.”
the Results It was published in the magazine Fossil studies.
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Graciela Pinheiro et al. 2025. The largest mesosaurs ever known: evidence from scant records. Phos. stud 3 (1): 1; doi: 10.3390/fossils3010001