Trilobites are so named for the three longitudinal lobes: 4 right pleural lobe 5 axial lobe 6 left pleural lobe the antennae and legs are not shown in these diagrams. In future studies, Hopkins is planning to add growth-rate data on different, more diverse-looking trilobite species to her models. The trilobite body is divided into three major sections (tagmata): 1 cephalon 2 thorax 3 pygidium. But while Elrathia kingii was smaller in early development and went through fewer molts before adulthood, it had faster growth rates, ultimately reaching sizes on par with Aulacopleura konincki, the largest of which are about 4 centimeters long. The two species look very similar and Hopkins found that they also grow in similar ways: for example, the growth of the trunk - the area immediately below the trilobite's head made up of segments that increase with age - was controlled by a growth gradient, with those that were younger and closer to the back of the body undergoing faster growth. Favorites: Roots Remain, Ancient Kingdom, Clandestiny, Jaguar God Least Favorites: Sultans Curse, Show Yourself. Hopkins estimated the growth rate and compared it to previously published data on a different trilobite, Aulacopleura konincki - the first time two trilobite species have been compared in this way. By the end of the trip, they had collected about 500 specimens - many of them juveniles, which can be as small as half a millimeter long - from a section of outcrop just 1.5 meters (about 5 feet) long. So in May 2018, Hopkins spent five days in Utah with a crew consisting of Museum staff and volunteers at a new fossil site said to preserve bucketloads of Elrathia kingii. "I needed material that I could collect from as small a section as possible that included a lot of juveniles." "There are tons of specimens of Elrathia kingii out there but most of them are adults, and data from exactly where they were collected is inconsistent," Hopkins said. But to calculate the species' growth rate, scientists need fossils representing all stages of the animal's life - and lots of them. Like insects, they molted throughout their lifetimes, leaving clues to how they changed during development. Their fossilized exoskeletons are preserved in sites all over the world, from the United States to China. There are some people who only collect trilobites showing predatory pathology, said Terry Abbott, a trilobite collector based in Delta, Utah. They were incredibly diverse, with more than 20,000 described species. Trilobites are a group of extinct marine arthropods - distantly related to the horseshoe crab - that lived for almost 300 million years. "Growth-rate studies like this one can help us tackle some of the big-picture questions: How did some trilobites get so big? What was the environmental context for that? And how did body size evolve over the evolutionary history of the clade?" Some never got bigger than about a centimeter, while the largest on record is 72 centimeters (28 inches)," said Melanie Hopkins, an associate curator in the Museum's Division of Paleontology and the study's author. "There's quite a big size range among trilobites.
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