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Ancient soft-bodied organisms are rarely preserved in the fossil record. Even more unusual is the preservation of younger life stages, particularly for invertebrates whose juveniles are fairly small, grow up quickly, and represent only a fraction of the total population. However, an exceptionally well-preserved fossil deposit in southwest China near Kunming is yielding hordes of soft-bodied creatures and juveniles in its mudstone layers. A research team led by paleontologist Xianfeng Yang of Yunnan University has uncovered thousands of marine invertebrate fossils—many of them juveniles—that were buried in a Cambrian-era sea approximately 518 million years ago.
Among the 118 species found in the deposit—including seventeen new to science— most are the ancestors of today’s arthropods, sponges, algae, and marine worms. There were also a few hard-bodied animals including the ancestors of jawless fishes. So far, two fossil-rich layers have yielded 2,846 specimens, with more than 90 percent of them coming from the lower layer known as the Haiyan Lagerstätte. This layer is also where researchers unearthed the majority of soft-bodied creatures and juveniles, turning up stunningly preserved features such as compound eyes, tentacles, and digestive tracts.
The surprising concentration of so many eggs, larvae, juveniles, and subadults in one place has led researchers to suggest that it was a “paleo-nursery,” where prehistoric marine life could take refuge from predators and reproduce or molt in its relatively protected waters. The team surmises that a paleo-nursery could have been quickly buried in sediment by a massive storm or earthquake event. However, the presence of several fossil-containing layers in this outcrop led the research team to consider another possibility—that this area of prehistoric sea was subjected to rapid environmental changes, perhaps in oxygen or salinity, creating boom-and-bust cycles. In that scenario, marine life would periodically flood in and reproduce when environmental conditions were favorable, but get wiped out when catastrophic natural events changed conditions.
The discovery of this fossil deposit invites further research, such as studying the development and evolution of Cambrian marine life in new detail, according to Pennsylvania State University paleontologist and co-author Julien Kimmig. The Cambrian marks the emergence of many animal taxa. “Understanding not only what animals were present, but how they grew from larval to adult stages and what morphological traits they might share will give us a better understanding of the relationships of modern and ancient animals,” said Kimmig. (Nature Ecology and Evolution)