Systematic Palaeontology

Order Scorpiones Koch, 1837.

Family Undetermined.

Genus Parioscorpio gen. nov.

Etymology. From Latin, pario, progenitor, and scorpio, scorpion.

Type Species. Parioscorpio venator sp. nov.

Diagnosis. As for P. venator, see below.

Distribution. Silurian (Llandovery, Telychian; c. 437.5-436.5 Ma), Wisconsin, USA.

Parioscorpio venator gen. et sp. nov. Figures 1, 2a and 3).

Etymology. From Latin, venator, hunter.

Types. Holotype, University of Wisconsin Geology Museum, Madison, Wisconsin, UWGM 2162. Paratype, UWGM 2163.

Location. Waukesha Lime and Stone Company west quarry, north of State Highway 164, Waukesha, Wisconsin, USA.

Horizon. Lower part of the Brandon Bridge Formation (Silurian: Llandovery, Telychian).

Diagnosis. Prosoma subtrapezoidal with large eyes situated anterolaterally and ocelli situated anteromedially; pedipalps large, with tibia (fixed finger) elongate, swollen proximally in manus, narrow and recurved distally in ramus; mesosoma moderately wide and much longer than the metasoma, containing 7 dorsal tergites and 7 ventral sternites; sternites 1–2 short (sagitally), length increasing posteriorly. Metasoma excluding telson, approximately 1/3 length of opisthosoma, containing five narrow, subequal, weakly bilobate segments. Telson swollen proximally.

Discussion. Parioscorpio venator gen. et sp. nov. is now the earliest known scorpion. Conodont biostratigraphic zonation22 places the Waukesha Lagerstätte in the Pterospathodus eopennatus Superzone (Telychian Stage; c. 437.5–436.5 Ma). Dolichophonus loudonensis from the Eurypterid Bed (Deerhope Formation) at Gutterford Burn in the Pentland Hills, Scotland15 was previously the earliest described scorpion. Specimens of D. loudonensis are found in association with conodonts assigned to Pterospathodus amorphognathoides and graptolites indicative of the middle Oktavites spiralis Zone to the middle Cyrtograptus lapworthi Zone25,26. Together, conodont and graptolite evidence shows that D. loudonensis occurs in strata correlative with the P. amorphognathoides Zone (P. celloni Superzone; Telychian Stage, c. 435.5–434.5 Ma), which overlies the P. eopennatus Superzone.

Parioscorpio venator gen. et sp. nov. is characterised by a small exoskeleton showing a unique array of characters. Based on earlier studies3,5,27,28,29,30,31,32 some of these characters, such as compound eyes, are primitive (plesiomorphic) for arachnids. Other characters, such as clawed pedipalps and a narrow metasoma terminating in a stinger, are derived (apomorphic). A mesosoma container seven tergites and sternites (Fig. 1c,d), which is observed uniquely in P. venator, is interpreted as a primitive characteristic (Fig. 4). Paleozoic scorpions show a trend toward reducing the number of sternites through time. Six sternites are present in two Silurian species that are younger, Proscorpius osborni28 and Eramoscorpius brucensis6. Most extant and extinct scorpions have five sternites29,30, a condition that had evolved by at least the Carboniferous Period. The large, anterolateral eyes, and anteromedial position of the small medial eyes, also are regarded as plesiomorphic features, as they are present in younger Silurian species such as Allopalaeophonus caledonicus31, Palaeophonus nuncius32, and Proscorpius osborni28. Pectines, which are chemosensory structures present in all extant scorpions, are unknown in most Paleozoic forms including P. venator. This is probably a taphonomic artefact, as pectines are easily lost after death or moulting13,14. In other aspects of external morphology P. venator fits within the range of morphological features exhibited among other, more derived, scorpion taxa. A telson bearing an expanded area for a poison vesicle and a stinger is an apomorphous condition for scorpions27. The holotype of P. venator preserves an incomplete telson, which is folded under the fifth metasomal segment. The proximal portion shows a swelling close to the articulation with the metasoma, inferred to be a poison vesicle, but the more terminal stinger is not evident.

Figure 4 Hypothesis of relationship based on character transformation (number of sternites) among some of the more completely known Paleozoic scorpions, with geologic time scale at left. One important trend in the evolution of early scorpions is a reduction in the number of sternites. P. venator shows the most primitive condition known, seven sternites. There was progressive reduction to six and eventually five sternites. Full size image

Both specimens of P. venator show details of internal anatomy. In the paratype, parting of the rock through the fossil reveals a medial structure interpreted as the gut tract (Fig. 1c,d), and it resembles that of extant scorpions. It is a narrow, simple tube extending from the prosoma to the metasoma. It originates in the anterior prosoma, slightly forward of the inferred position of the mouth.

In the holotype of P. venator, internal structures of the mesosoma and metasoma have been impressed on the thin dorsal cuticle during sediment compaction. They consist of a series of narrow, hourglass-shaped medial structures extending much of the length of the mesosoma (Fig. 1a,b, Supplementary Fig. 1e). Extending laterally from each of the medial structures is a pair of curved, strut-like elements (Fig. 1a,b, Supplementary Fig. 1d). The hourglass-shaped structures of the mesosoma continue into the metasoma as a simple, narrow tube. Neither book lungs nor book gills are evident on the fossil.

Detailed studies of the central architecture of the circulatory and respiratory systems in present-day scorpions33,34 reveal a strikingly similar arrangement to the preserved structures in P. venator (Figs. 1a,b, 2a). The pericardium, which surrounds and houses the heart, comprises a series of narrow, medial hourglass-shaped structures in the mesosoma. Strut-like pulmo-pericardial sinuses project laterally from the pericardium (Fig. 2). In some extant scorpions, these internal medial structures are reflected externally on the dorsal cuticle (Fig. 2c), along with the tergite boundaries. This is evident on P. venator as well. In extant scorpions, the pulmo-pericardial sinuses connect the book lungs with the circulatory system. The book lungs oxygenate the hemolymph (‘blood’) and deliver the oxygenated hemolymph to the pericardium33,34. We infer that the organs of the respiratory-cardiovascular architecture were evolutionarily conservative.

Among extant chelicerates, terrestrial forms such as scorpions tend to be restricted to processing oxygen from air (e.g., by means of book lungs). However, marine xiphosurans, which normally extract oxygen from water by means of external book gills, are nevertheless capable of respiration when they journey onto land to spawn35. The circulatory and respiratory organs of xiphosurans (horseshoe crabs) are equally complex to those of scorpions34,36, and this may contribute to their ability to respire in air and survive on land. Presumably, ancient xiphosurans and arachnid ancestors had a similar capability to venture onto land.

Anatomical details preserved in P. venator suggest that the physiological changes necessary to accommodate a marine-to-terrestrial transition in arachnids occurred early in their evolutionary history. Whether P. venator was a fully terrestrial arthropod is uncertain. The close similarity of its preserved pulmonary-cardiovascular structures with those of extant scorpions and horseshoe crabs hint at the possibility of extended stays on land.