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A remarkably preserved crab trapped in amber is forcing scientists to rethink when amphibious crabs first evolved. Here\u2019s why this fossil shouldn\u2019t exist.<\/p>\n
Luque et al., doi:10.1126\/sciadv.abj568<\/p>\n
If a scientist used the word \u201czombie\u201d to describe a fossil, know that they aren\u2019t trying to be poetic. Usually, they mean that it looks so fresh and intact that it could easily have stepped straight out of the past. This is precisely the case with Cretapsara athanata: a crab fossilized in Burmese amber 100 million years ago, during the age of the dinosaurs.<\/p>\n
What makes Cretapsara so intriguing is that, for decades, scientists believed that fully amphibious \u201ctrue crabs\u201d didn\u2019t appear until much later in Earth\u2019s history. Similarly, they believed that early crab evolution was to be strictly marine; any claim otherwise bordered on speculation. <\/p>\n
Then this tiny, amber-entombed crustacean appeared.<\/p>\n
A Crab So Well Preserved That It Shouldn\u2019t Exist<\/p>\n
The fossil was formally described in a 2021 study<\/a> published in Science Advances, by researcher Javier Luque and his colleagues. After conducting micro-CT scans on the specimen, they discovered a crab preserved so flawlessly<\/a> that it was unlike anything ever seen before. Astonishingly, its legs, claws, antennae, compound eyes and delicate mouthparts were all still intact. <\/p>\n It\u2019s worth noting that crabs almost never fossilize well. Their exoskeletons fall apart quickly, and soft tissue preservation is nearly impossible. This is precisely why Luque and his colleagues were so surprised by Cretapsara.<\/p>\n Evidently, amber made this much easier. As sticky resin flowed from ancient trees, it trapped organisms rapidly. In turn, they\u2019re sealed off from decay indefinitely. However, there\u2019s little reason for a crab to encounter tree resin, given that they\u2019re marine creatures; resin flows on land. <\/p>\n This is what made the idea of a modern-looking brachyuran crab preserved in amber once seem biologically impossible. Yet the fossil was unmistakably real. It was lifelike, intact and surprisingly advanced for its age.<\/p>\n Cretapsara was just a mere 2\u20133 millimeters long, which made it more than small enough to easily become ensnared in tree resin. However, despite its small size, its limbs and sensory structures suggested unexpected sophistication. <\/p>\n Eventually, the researchers named it Cretapsara athanata \u2014 meaning \u201cthe immortal spirit of the clouds and waters.\u201d<\/p>\n The Evolutionary Shock: A \u2018Modern\u2019 Crab In The Mid-Cretaceous<\/p>\n Crab evolution is complicated. As 2017 research<\/a> from Biological Journal of the Linnean Society explains, this is because the group has undergone \u201ccarcinization.\u201d This refers to the various different ways that the evolution of crab-like forms has repeated across different lineages. Because of this process, scientists came to agree that amphibious or semi-terrestrial crabs likely didn\u2019t emerge until well after the mid-Cretaceous. However, Cretapsara overhauled this.<\/p>\n Based on its morphology (especially its legs, claws and respiratory structures), Luque and his research team concluded that this species most likely lived in both water and on land. It clearly wasn\u2019t fully terrestrial, but it also couldn\u2019t have been strictly marine, either. This means that Cretapsara represented an early experiment in amphibious living<\/a> tens of millions of years earlier than expected.<\/p>\n Cretapsara athanata, a modern-looking eubrachyuran crab in Burmese amber. (A) Whole amber sample with crab inclusion in ventral view. (B) Close-up of ventral carapace. (C) Whole amber sample with crab inclusion in dorsal view. (D) Close-up of dorsal carapace. White arrows in (B) and (D) indicate the detached left fifth leg or pereopod. Photos by L.X. Figure by J.L.<\/p>\n Luque et al., doi:10.1126\/sciadv.abj568<\/p>\n For evolutionary biologists, this fossil dropped a missing chapter into the historical record. It revealed that crabs were exploring ecological niches previously assumed to be off-limits during the age of dinosaurs. These marine arthropods were testing the boundaries of land use long before models predicted.<\/p>\n Why The \u2018Zombie Crab\u2019 Was Once Considered A Myth<\/p>\n The zombie crab wasn\u2019t mythological in a folklore sense; rather, its existence was considered a myth based on previous scientific assumptions. Specifically, before its discovery, researchers believed that:<\/p>\n Perfectly preserved mid-Cretaceous crabs shouldn\u2019t appear in amber, considering how unlikely it would be for a marine organisms rarely meet tree resinSoft tissue from a 100-million-year-old crustacean couldn\u2019t possibly be fossilized in such exquisite detail<\/p>\n Yet Cretapsara defied these expectations. Amber preservation of aquatic creatures is so rare that, even today, discoveries of fossils like these are met with scrutiny. Some turn out to be composite fossils or mining contaminants \u2014 but this was not the case for Cretapsara. Radiometric dating, micro-CT imaging and resin chemistry all confirmed that it was, indeed, a tiny crab frozen in resin, perfectly and impossibly whole.<\/p>\n By all scientific standards, it was a \u201czombie fossil\u201d: a creature reappearing after being presumed absent from the historical timeline.<\/p>\n Why This Crab Matters Today<\/p>\n What makes Cretapsara scientifically significant isn\u2019t just its astonishing beauty and age. Rather, its discovery offered us invaluable insights regarding ancient ecosystems. Namely, the amber fossil suggests that:<\/p>\n Amphibious niches emerged much earlier than we believed. Trees overhanging shallow waters likely created resin-rich zones. This likely offered small crustaceans areas in which they could forage.Crabs experimented with land use in the mid-Cretaceous. This pushes back the timeline for non-marine adaptations.Biodiversity was much richer and more complex than once believed. Transitional species most likely occupied dynamic environments where land and water blended.Amber can preserve aquatic life under the right conditions. Cretapsara\u2019s discovery offers us hope that more \u201cimpossible\u201d specimens can be found in the future.<\/p>\n Fossils like Cretapsara overhaul major parts of our understanding of ancient ecosystems and how they shifted with climate, geography and evolutionary pressure. Transitions like these, from water to land, have profoundly shaped life on earth \u2014 and repeatedly, too. Understanding these transitions is essential for scientists to model ancient species\u2019 resilience and adaptation.<\/p>\n Do you have an eye for nature photography? Join my Nature Photography Club<\/a> and take your photos to the next level.<\/p>\n