biostudies-literatureUnknown10(1)Wendruff AJScorpions are among the first animals to have become fully terrestrialised. Their early fossil record is limited, and fundamental questions, including how and when they adapted to life on land, have been difficult to answer. Here we describe a new exceptionally preserved fossil scorpion from the Waukesha Biota (early Silurian, ca. 437.5-436.5 Ma) of Wisconsin, USA. This is the earliest scorpion yet reported, and it shows a combination of primitive marine chelicerate and derived arachnid characteristics. Elements of the circulatory, respiratory, and digestive systems are preserved, and they are essentially indistinguishable from those of present-day scorpions but share similarities with marine relatives. At this early point in arachnid evolution, physiological changes concomitant with the mScientific reports14https://www.ebi.ac.uk/biostudies/studies/S-EPMC6965631biostudies-literatureA Silurian ancestral scorpion with fossilised internal anatomy illustrating a pathway to arachnid terrestrialisation.PMC6965631Wirkner CSKluessendorf JBabcock LEWendruff AJMikulic DGfalseA Silurian ancestral scorpion with fossilised internal anatomy illustrating a pathway to arachnid terrestrialisation.Scorpions are among the first animals to have become fully terrestrialised. Their early fossil record is limited, and fundamental questions, including how and when they adapted to life on land, have been difficult to answer. Here we describe a new exceptionally preserved fossil scorpion from the Waukesha Biota (early Silurian, ca. 437.5-436.5 Ma) of Wisconsin, USA. This is the earliest scorpion yet reported, and it shows a combination of primitive marine chelicerate and derived arachnid characteristics. Elements of the circulatory, respiratory, and digestive systems are preserved, and they are essentially indistinguishable from those of present-day scorpions but share similarities with marine relatives. At this early point in arachnid evolution, physiological changes concomitant with the m2020-01-01T00:00:00Z2020 Jan2025-04-26T03:04:48.162Z2020-05-22T08:17:56ZS-EPMC69656313194918510.1038/s41598-019-56010-z