Abstract
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 marine-to-terrestrial transition must have occurred but, remarkably, structural change in the circulatory or respiratory systems appear negligible. Whereas there is no unambiguous evidence that this early scorpion was terrestrial, this evidence suggests that ancestral scorpions were likely capable of forays onto land, a behavior similar to that of extant horseshoe crabs.
Highlights
Scorpions are among the first animals to have become fully terrestrialised
Stomathecae, trichobothria, and respiratory structures are rarely preserved as fossils, and in many examples the non-preservation of such structures likely has a taphonomic origin[13,14]
Fossils are preserved in a 12 cm layer of thinly laminated, fine-grained dolostone deposited during a transgressive episode over the eroded dolostones of the Schoolcraft and Burnt Bluff formations[23]
Summary
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. Scorpions include some of the earliest animals to have become fully terrestrial[1,2]. Arguments concerning whether early scorpions were terrestrial or marine have been based principally on stratigraphic context[9] and preserved anatomical features. All Silurian scorpion fossils, and most from the Paleozoic, have been recovered from nearshore to marginal-marine strata.
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