Paleontologists in Uruguay have discovered large, poorly preserved and incomplete skulls, as well as several associated bones, of ancient aquatic reptiles called mesosaurs. These new specimens suggest the existence of gigantism in mature mesosaurs reaching more than twice the size of previously described adults and type specimens.
Mesosaurs are small to mid-sized aquatic to semiaquatic amniotes that lived in Gondwana during the Early Permian epoch. Image credit: Roman Yevseyev / Graciela Piñeiro.
Mesosaurs are a group of small aquatic reptiles that lived in South Africa and South America during the Permian period, roughly 299 to 270 million years ago.
These creatures were the first known aquatic reptiles, having apparently returned to an aquatic lifestyle from more terrestrial ancestors.
“Mesosaurs are often described as small to medium-sized aquatic amniotes, partly due to their elongated bodies and long tails that exceed the length of the rest of the body,” said Dr. Graciela Piñeiro from the Universidad de la República and colleagues.
“Additionally, their long-snouted skull is composed of very thin cranial bones, and it bears numerous needle-like, marginal teeth.”
“Despite the fragility of some of their cranial bones, mesosaur remains are abundant in the Gondwanan Permo-Carboniferous shale and silt deposits of Gondwana, including well-preserved, nearly complete skeletons with articulated skulls and partially preserved specimens that retain still recognizable soft tissues.”
“This exceptional preservation has led to the designation of the Mangrullo Formation of Uruguay and the Iratí Formation of Brazil as Konservat-Lagerstätten for Gondwana,” they added.
“Furthermore, mesosaur fossils from these units include different ontogenetic stages, from unborn individuals to juveniles and adults.”
“Therefore, mesosaur ontogeny is well-documented, with body lengths ranging from 10 or 12 cm (3.9-4.7 inches) in hatchlings to 80 or 90 cm (2.6-3 feet) in young adults.”
“The average mesosaur length is approximately 70 cm (2.3 feet),” they said.
“Previous studies suggested that mesosaurs are semiaquatic rather than fully aquatic amniotes.”
“In addition, in recent studies on mesosaur taxonomy and paleobiology, over 1,000 specimens where examined, including both small and large individuals belonging to Mesosaurus tenuidens.”
“The data also suggested that, at its mature stage, mesosaurs developed more terrestrial habits.”
Mesosaur ontogenetic skull series documented from specimens found in the Mangrullo Formation Konservat-Lagerstätte of Uruguay. Scale bar – 1 cm. Image credit: Piñeiro et al., doi: 10.3390/fossils3010001.
Dr. Piñeiro and co-authors discovered and examined the remains of the largest known mesosaurs documented so far.
The specimens include two fragmentary skulls, a dorsal vertebra, a fragment of the tail, an isolated rib and several other bones.
They came from the Picada de Cuello and El Baron localities of the Mangrullo Formation in northern Uruguay.
The paleontologists compared the new fossils to a large sample of the same bones of Mesosaurus tenuidens.
Their analyses revealed that the size of the mesosaurs from the Mangrullo Formation exceeds the typical size distribution previously recorded for Mesosaurus tenuidens.
These individuals were larger than the mean by four to five times its standard deviations, with skulls measuring between 15 and 20 cm (5.9-7.9 inches) in length and a total body size between 1.5 and 2.5 m (4.9-8.2 feet).
“To explore the significance of such large mesosaurs in the population of the Mangrullo Formation, we considered the potential impact of Bergmann’s rule,” the researchers said.
“Although this is an important paleogeographic principle in the study of isolated populations of the same species or distinct taxa 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 specimens of mesosaurs in the Mangrullo Formation.”
“Therefore, we conclude that the great variability in body size found in mesosaurs corresponds to their ontogenetic stage and growth pattern,” they said.
“The previously recognized smaller body sizes likely represent assemblages dominated by early juveniles, subadults and young adults, potentially reflecting catastrophic mass mortality events that interfere with the attritional normal addition of carcasses of animals that died by varied causes.”
“Bentonite layers and gypsum crystals found in the Mangrullo Formation result from the occasional input of ashfalls over the lagoon derived from the increased tectonic activity related to the Pangea formation.”
“These volcanic events combined with the observed gradual drought in the Iratí-Whitehill sea and the concomitant desertification are suggested as the main trigger that might explain the mesosaur extinction, at least in the Uruguayan region of the Paraná Basin.”
The findings were published in the journal Fossil Studies.
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Graciela Piñeiro et al. 2025. The Largest Mesosaurs Ever Known: Evidence from Scanty Records. Foss. Stud 3 (1): 1; doi: 10.3390/fossils3010001