A new mid-Silurian aquatic scorpion-one step closer to land?

Abstract

One of the oldest known fossil scorpions, a new species from the mid-Silurian Eramosa Formation (430 myr) of Ontario, Canada, exhibits several surprising features. The depositional environment and associated biota indicate a marine habitat; however, the leg morphology of this scorpion, which has a short tarsus in common with all Recent scorpions, suggests that a key adaptation for terrestrial locomotion, the ability to support its weight on a subterminal 'foot', appeared remarkably early in the scorpion fossil record. Specimens are preserved intact and undisturbed in a splayed posture typical of moults rather than carcasses. We postulate that these animals were aquatic, but occasionally ventured into extremely shallow water, or onto a transient subaerially exposed surface while moulting, before returning to deeper water. Shed exuviae were preserved in situ by rapid overgrowth of bacterial biofilm.

Keywords: Silurian; behaviour; exuviae; locomotion; new species; scorpion.

Figure 1.

Eramoscorpius brucensis gen. et sp. nov. All scale bars, 10 mm. (a,c,d,f,g,h) Complete to near complete specimens demonstrating the range of sizes; (a,c,d and f) appear to represent a single size class; (f,h) well-preserved telson with aculeus; (g,h) the largest and the smallest complete individuals respectively. (b,e) Detail of coxosternal area demonstrating the lack of coxapophyses (I and II). Modern (terrestrial) scorpions all have projections of the anterior coxae. Short pectines (arrows) are clearly evident on (b) and faintly visible on (e). (i). Detail of walking legs III and IV demonstrating claws and short tarsus with respect to basitarsus. (a,b), Ventral aspect, holotype ROM53247; (c) dorsal, ROM49276; (d,e) ventral, ROM60063, the only specimen with no original exoskeleton preserved; (f) ROM56751; (g) ROM58778; (h) dorsal ROM58777; (f,g) inferred to be ventral because of the lack of obvious carinae on the last mesosomal somite; (i) ROM50048. (j) Sketch of scorpion leg IV comparing three Silurian fossils with a Recent scorpion. ta, tarsus; ba, basitarsus; ti, tibia; pa, patella. From left to right: Palaeophonus with stubby ‘crab-like’ leg, with all elements about the same length; Proscorpius with tarsus distinctly longer than basitarsus (both slightly younger than Eramoscorpius); Eramoscorpius with tarsus distinctly shorter than basitarsus; Buthus, a modern scorpion with tarsus distinctly shorter than basitarsus. The Eramoscorpius leg is well on its way to being modern with its short tarsus, but is not as gracile as a typical modern scorpion leg, which shows a notable lengthening of the patella. (Online version in colour.)

Figure 2.

Idealized reconstruction of Eramoscorpius brucensis gen. et sp. nov. in dorsal (a) and ventral (b) view based on a composite of available material.