Episode 394 The Mangrullo Formation of Uruguay
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\\nToday we\\u2019re going back about 280 million\\nyears, to what is now Uruguay in South America.
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\\n280 million years ago puts us in the early part of the\\nPermian Period. Gondwana, the huge southern continent, was in the process of\\ncolliding with North America and Eurasia to form the supercontinent of Pangaea.\\nSouth America, Africa, Antarctica, India, and Australia had all been attached\\nto each other in Gondwana for several hundred million years, and the extensive\\nglaciers that occupied parts of all those continents were probably still\\npresent in at least in highlands in southern South America and South Africa, as\\nwell as Antarctica.
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\\nBut the area that is now in Uruguay was probably in cool,\\ntemperate latitudes, something like New Zealand or Seattle today. The\\nconnection between southern South America and South Africa was a lowland,\\npartially covered by a shallow arm of the sea or perhaps a broad, brackish\\nlagoon at the estuary of a major river system that was likely fed in part by\\nglacial meltwater from adjacent mountains. We know the water was shallow\\nbecause the rocks preserve ripple marks produced by wave action or currents.
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\\nThe basin must have been near the shore because delicate\\nfossils such as insect wings and plants are among the remnants. It looks like\\nthis shallow sea or lagoon became cut off from the ocean, allowing the waters\\nto become both more salty, even hypersaline, and anoxic, as the separation\\nrestricted inflows of water, either fresh or marine, that could have continued\\nto oxygenate the basin. In the absence of oxygen, excellent preservation of\\nmaterials that fell to the basin floor began, and there were few or no\\nscavenging animals to disrupt the bodies.
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\\nThe rocks of the Mangrullo Formation, as it\\u2019s called today,\\ninclude limestones and siltstones, but the most important for fossil\\npreservation are probably the extremely fine-grained claystones and oil shales.\\nThese rocks contain some of the best preserved fossil mesosaurs known anywhere.\\nThat\\u2019s mesosaurs, not the perhaps more well-known mosasaurs, which are large\\nwhale-like marine reptiles that lived during Cretaceous time. Here, we\\u2019re in\\nthe Permian, well before the first dinosaurs.
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\\nMesosaurs were aquatic reptiles, and they are the earliest\\nknown. They evolved from land reptiles and were among the first to return to\\nthe water to adopt an aquatic or amphibious lifestyle. They were once thought\\nto be part of a sister group to reptiles, a separate branch of amniotes, which\\nare animals that lay their eggs on land or bear them inside the mother, like\\nmost mammals do. In that scheme, mesosaurs and reptiles would have diverged\\nfrom a common, earlier ancestor. But more recent studies categorize them as\\nreptiles that split off from the main genetic stem early in the history of the\\nclass, so they\\u2019re pretty distant cousins to dinosaurs and all modern reptiles,\\nbut they\\u2019re still reptiles. There is ongoing debate among evolutionary\\npaleontologists as to exactly where mesosaurs fit.
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| Mesosaur by Nobu Tamura (Creative Commons license & source) |
\\nMesosaurs were aquatic reptiles, and they are the earliest\\nknown. They evolved from land reptiles and were among the first to return to\\nthe water to adopt an aquatic or amphibious lifestyle. They were once thought\\nto be part of a sister group to reptiles, a separate branch of amniotes, which\\nare animals that lay their eggs on land or bear them inside the mother, like\\nmost mammals do. In that scheme, mesosaurs and reptiles would have diverged\\nfrom a common, earlier ancestor. But more recent studies categorize them as\\nreptiles that split off from the main genetic stem early in the history of the\\nclass, so they\\u2019re pretty distant cousins to dinosaurs and all modern reptiles,\\nbut they\\u2019re still reptiles. There is ongoing debate among evolutionary\\npaleontologists as to exactly where mesosaurs fit.
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\\nThe fossils in Uruguay are so well preserved that we can\\nidentify the gut materials of mesosaurs, and we know they mostly ate\\ncrustaceans, aquatic invertebrates related to crabs, shrimp, and lobsters. The\\npreservation is so exceptional that in some cases, soft body parts are\\npreserved including major nerves and blood vessels in mesosaurs and stomachs\\nand external appendages in the crustaceans. The earliest known amniote embryos\\nalso come from these fossil beds.
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\\nMesosaurs had a short run in terms of their geologic\\nhistory, only about 30 million years. They were extinct about 270 million years\\nago, well before the great extinction event at the end of the Permian, 250\\nmillion years ago. But the presence of coastal-dwelling mesosaurs in both South\\nAmerica and Africa was a contributing idea in the early development of the\\ntheory of continental drift, since it was presumed that they could not have\\ncrossed the Atlantic Ocean as it is today.
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\\n\\n\\u2014Richard I. Gibson
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