Marine bivalves\u00a0lost around three-quarters of their species\u00a0during this mass extinction, which marked the end of the Cretaceous Period. My colleagues\u00a0and I\u00a0\u2013 each of us paleobiologists studying biodiversity \u2013 expected that losing so many species would have severely cut down the variety of roles that bivalves play within their environments, what we call their \u201cmodes of life.\u201d<\/p>\n
But, as we\u00a0explain in a study\u00a0published in the journal Sciences Advances, that wasn\u2019t the case. In assessing the fossils of thousands of bivalve species, we found that at least one species from nearly all their modes of life, no matter how rare or specialized, squeaked through the extinction event.<\/p>\n
Statistically, that shouldn\u2019t have happened. Kill 70% of bivalve species, even at random, and some modes of life should disappear.<\/p>\n
Most bivalves happily burrow into the sand and mud, feeding on phytoplankton they strain from the water. But others have\u00a0adopted\u00a0chemosymbionts\u00a0and\u00a0photosymbionts\u00a0\u2013 bacteria and algae that produce nutrients for the bivalves from chemicals or sunlight in exchange for housing. A few have\u00a0even become carnivorous. Some groups, including the oysters, can lay down\u00a0a tough cement\u00a0that hardens underwater, and mussels hold onto rocks by\u00a0spinning silken threads.<\/p>\n
We thought surely these more specialized modes of life would have been snuffed out by the effects of the asteroid\u2019s impact, including dust and debris likely blocking sunlight and disrupting a huge part of the bivalves\u2019 food chain: photosynthetic algae and bacteria. Instead, most persisted, although biodiversity was forever scrambled as a new ecological landscape emerged. Species that were once dominant struggled, while evolutionary newcomers rose in their place.<\/p>\n
The reasons some species survived and others didn\u2019t leave many questions to explore. Those that filtered phytoplankton from the water column suffered some of the highest species losses, but so did species that fed on organic scraps and didn\u2019t rely as much on the Sun\u2019s energy. Narrow geographic distributions and different metabolisms may have contributed to these extinction patterns.<\/p>\n
Biodiversity bounces back<\/strong><\/p>\n Life rebounded from each of the\u00a0Big Five mass extinctions\u00a0throughout Earth\u2019s history, eventually punching through past diversity highs. The rich fossil record and\u00a0spectacular ecological diversity of bivalves\u00a0gives us a terrific opportunity to study these rebounds to understand how ecosystems and global biodiversity rebuild in the wake of extinctions.<\/p>\n The extinction caused by the asteroid strike knocked down some thriving modes of life and opened the door for others to dominate the new landscape.<\/p>\n While many people lament the loss of the dinosaurs, we malacologists miss the\u00a0rudists.<\/p>\n These bizarrely shaped bivalves resembled giant ice cream cones, sometimes reaching more than 3 feet (1 meter) in size, and they dominated the shallow, tropical Mesozoic seas as massive aggregations of contorted individuals, similar to today\u2019s coral reefs. At least a few\u00a0harbored photosymbiotic algae, which provided them with nutrients and spurred their growth, much like modern corals.<\/p>\n Today, giant clams (Tridacna)\u00a0and their relatives\u00a0fill parts of these unique photosymbiotic lifestyles once occupied by the rudists, but they lack the rudists\u2019 astonishing species diversity.<\/p>\n Mass extinctions clearly upend the status quo. Now, our ocean floors are dominated by clams burrowed into sand and mud, the quahogs, cockles and their relatives \u2013 a scene far different from that of the seafloor 66 million years ago.<\/p>\n New winners in a scrambled ecosystem<\/strong><\/p>\n Ecological traits alone didn\u2019t fully predict extinction patterns, nor do they entirely explain the rebound. We also see that simply surviving a mass extinction didn\u2019t necessarily provide a leg up as species diversified within their old and sometimes new modes of life \u2013 and few of those new modes dominate the ecological landscape today.<\/p>\n Like the rudists,\u00a0trigoniid bivalves had lots of different species\u00a0prior to the extinction event. These highly ornamented clams built parts of their shells with a super strong biomaterial called\u00a0nacre\u00a0\u2013 think iridescent pearls \u2013 and had fractally interlocking hinges holding their two valves together.<\/p>\n But despite surviving the extinction, which should have placed them in a prime position to accumulate species again,\u00a0their diversification sputtered. Other types of bivalves that made a living in the same way proliferated instead, relegating this once mighty and global group to a handful of species now found only off the coast of Australia.<\/p>\n Lessons for today\u2019s oceans<\/strong><\/p>\n These unexpected patterns of extinction and survival may offer lessons for the future.<\/p>\n The fossil record shows us that biodiversity has definite breaking points, usually during a perfect storm of\u00a0climatic and environmental upheaval. It\u2019s not just that species are lost, but the ecological landscape is overturned.<\/p>\n Many scientists believe the current biodiversity crisis may cascade into a\u00a0sixth mass extinction, this one driven by human activities that are changing ecosystems and the global climate. Corals, whose reefs are home to\u00a0nearly a quarter\u00a0of known marine species, have faced\u00a0mass bleaching events\u00a0as warming ocean water puts their future at risk. Acidification as the oceans absorb more carbon dioxide can also\u00a0weaken the shells\u00a0of organisms crucial to the ocean food web.<\/p>\n Findings like ours suggest that, in the future, the rebound from extinction events will likely result in very different mixes of species and their modes of life in the oceans. And the result\u00a0may not align with human needs\u00a0if species providing the bulk of ecosystem services are driven genetically or functionally extinct.<\/p>\n The global oceans and their inhabitants are complex, and, as our team\u2019s latest research shows, it is difficult to predict the trajectory of biodiversity as it rebounds \u2013 even when extinction pressures are reduced.<\/p>\n Billions of people\u00a0depend on the ocean for food. As the\u00a0history recorded\u00a0by the world\u2019s bivalves shows, the upending of the pecking order \u2013 the number of species in each mode of life \u2013 won\u2019t necessarily settle into an arrangement that can feed as many people the next time around.<\/p>\n Stewart Edie is a Research Geologist and Curator of Paleobiology, Smithsonian Institution.<\/p>\n
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