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Those measurements make it the largest soft-shelled egg ever found and the second largest egg from any animal.<\/p>\n
Fossil egg in an unlikely place<\/p>\n
At first the fossil did not look like an egg at all. It was a leathery, folded object buried in Antarctic sediment and it reminded researchers of a deflated bag.<\/p>\n
Under a microscope, thin sections of the fossil revealed a delicate wall only a fraction of a millimeter thick.\u00a0<\/p>\n
That wall lacked obvious pores and instead showed stacked layers, giving it the texture of a modern lizard or snake egg rather than the thick, chalky shells many people picture for dinosaur eggs.<\/p>\n
The work was led by Lucas Legendre, a paleontologist at the University of Texas at Austin<\/a>. His research focuses on fossil eggs and how reptile reproduction evolved over deep time.<\/p>\n Eventually the team assigned the formal name Antarcticoolithus bradyi, the official fossil label for this unusual egg, to the specimen.\u00a0<\/p>\n The slow, careful reconstruction of its shape showed that the shell had collapsed after hatching, which is why the fossil looks like an empty sack instead of a neatly rounded egg.<\/p>\n How giant reptiles reproduced<\/p>\n Before this discovery, large marine reptiles<\/a> such as mosasaurs, huge lizards that hunted in ancient oceans, were widely thought to give birth to live young.\u00a0<\/p>\n Earlier work<\/a> on tiny mosasaur skulls from open ocean rocks suggested that some of these lizards gave birth far from shore instead of coming onto beaches to lay eggs.<\/p>\n The Antarctic egg points to a different strategy. Its thin, flexible shell suggests that at least one marine reptile laid soft-shelled eggs in the water, with the young hatching almost at once rather than sitting in a nest for weeks.<\/p>\n The egg came from an animal comparable in size to a large dinosaur, yet its structure showed none of the typical features seen in dinosaur eggs<\/a>. <\/p>\n It was also noted for its unusual combination of size and form, which set it apart from any known fossil egg type.<\/p>\n Across reptiles as a whole, live offspring-bearing called viviparity, in which mothers retain embryos until birth, has evolved many times but rarely leaves clear fossils.\u00a0<\/p>\n This Antarctic egg hints that some marine reptiles may have used a mixed approach, with mothers carrying young almost to term and then releasing an egg that hatched quickly in the water.<\/p>\n Who laid this fossil egg?<\/p>\n Near the egg, researchers found bones from Kaikaifilu hervei, a large species of mosasaur known from the same rock formation on Seymour Island.\u00a0<\/p>\n A detailed description<\/a> of this animal shows that it reached about 33 feet in length, making it the biggest known top predator from Antarctic seas of that time.<\/p>\n The egg\u2019s estimated parent length of more than 23 feet, based on comparisons with 259 modern reptile species, fits comfortably within that size range.\u00a0<\/p>\n That match, along with the closeness of the fossils<\/a>, makes Kaikaifilu a strong candidate as the egg layer, even if the link cannot yet be proven.<\/p>\n The area also preserves small bones from young mosasaurs and from plesiosaurs, long necked marine reptiles with flippers, suggesting that the region functioned as a nursery.\u00a0<\/p>\n In such a setting, freshly laid eggs that hatched almost at once would have released mobile babies directly into sheltered coastal waters.<\/p>\n For decades, almost all known fossil eggs from dinosaurs and other ancient reptiles had thick, mineral-rich shells.\u00a0<\/p>\n That record made scientists think that hard shells were the ancestral pattern, and that softer eggs were rare exceptions.<\/p>\n That view has started to change. An independent analysis<\/a> that examined eggs from the plant-eating dinosaurs Protoceratops and Mussaurus found that their shells were leathery and flexible, not rigid like a bird-egg.<\/p>\n The research team behind that work concluded that soft-shells were likely present in the earliest dinosaurs and that rigid shells evolved several times in separate lineages.\u00a0<\/p>\n A museum report for the public explained that these early dinosaur eggs probably resembled turtle eggs, with leathery coverings that could be buried in soil or sand.\u00a0<\/p>\n The Antarctic egg slots into this emerging picture, extending the reach of soft-shells into giant marine reptiles living near the poles.<\/p>\n Lessons from Antarcticoolithus bradyi<\/p>\n Soft-eggs almost never survive long enough to fossilize because bacteria and scavengers destroy them quickly.\u00a0<\/p>\n The preservation of this one suggests that the sedimentary environment, layers of mud and sand laid down in a shallow sea, rapidly buried the egg and shielded it from decay.<\/p>\n Antarctica\u2019s climate was warmer at the time, with ice-free coasts and productive seas, even though the region still sat within the polar circle.\u00a0<\/p>\n Those conditions, combined with steady sediment buildup, turned parts of the seafloor around Seymour Island into natural vaults for delicate remains.<\/p>\n Well-preserved embryos<\/a> of Protoceratops from Mongolia show how entire nests can sometimes be locked in stone.\u00a0<\/p>\n In a similar way, the Antarctic egg and nearby juvenile marine reptiles offer a snapshot of how life began for some of the largest predators in southern oceans.<\/p>\n Each new find could tighten the connection between egg type, nesting behavior, and environment, revealing how life cycles adapted to cold, seasonal light near the ancient South Pole.<\/p>\n The study is published in Nature<\/a>.<\/p>\n \u2014\u2013<\/p>\n Like what you read? Subscribe to our newsletter<\/a> for engaging articles, exclusive content, and the latest updates.<\/p>\n Check us out on EarthSnap<\/a>, a free app brought to you by Eric Ralls<\/a> and Earth.com.<\/p>\n \u2014\u2013<\/p>\n","protected":false},"excerpt":{"rendered":"About 68-million-years ago, during the Late Cretaceous, a giant fossil egg was laid in Antarctica. The excavated remains…\n","protected":false},"author":2,"featured_media":641377,"comment_status":"","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[8],"tags":[933,70,16,15],"class_list":{"0":"post-647170","1":"post","2":"type-post","3":"status-publish","4":"format-standard","5":"has-post-thumbnail","7":"category-science","8":"tag-animals","9":"tag-science","10":"tag-uk","11":"tag-united-kingdom"},"share_on_mastodon":{"url":"https:\/\/pubeurope.com\/@uk\/115760113869544559","error":""},"_links":{"self":[{"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/posts\/647170","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/comments?post=647170"}],"version-history":[{"count":0,"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/posts\/647170\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/media\/641377"}],"wp:attachment":[{"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/media?parent=647170"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/categories?post=647170"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/tags?post=647170"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}![\"Enormous](\"https:\/\/www.europesays.com\/uk\/wp-content\/uploads\/2025\/12\/marine-reptile_fossil-egg_Antarcticoolithus-bradyi_the-thing_Nature_1s.webp.webp\")
<\/a>Enormous 68-million-year-old fossil egg from Antarcticoolithus bradyi nicknamed \u2018The Thing\u2019 is excavated in Antarctica. Credit: Nature. Click image to enlarge.Soft-shells in the deep past<\/p>\n