This species was recovered from slightly earlier deposits (Llandovery age) in the Pentland Hills of Scotland and could be distinguished from S. acuminata by the more elongated telson (also not as broad in the parts furthest back), thinner telson spike and a slightly different, tapering, body shape that tapers evenly the whole way instead of suddenly narrowing near the seventh segment as in S.

[8][9] The type specimen of S. dubia is a badly preserved carapace, with fragments of various degrees of completion of the first eleven segments found associated.

Only one specimen, the anterior part of a carapace with the compound eyes placed on the margin, is known and though it does resemble Slimonia, it could also potentially be referred to Hughmilleria or even represent the carapace of Salteropterus abbreviatus (a closely related slimonid eurypterid known only from the telson and metastoma, a large plate part of the abdomen).

[10] The fossils were recovered from deposits of Pridoli (Late Silurian) age in Herefordshire, England and suggest that the species grew to 12 cm (5 in) in length.

[11] In 1973, another species of Slimonia was named by Kjellesvig-Waering based on one single fossil recovered by Eduardo Rodriguez from the Kirusillas Formation, of Ludlow-Pridoli (Late Silurian) age, in Cochabamba, Bolivia.

Named S. boliviana, the holotype (BLV15, deposited at the National Museum of Natural History of France) comprises a well-preserved telson typical of the genus, being laterally inflated and with a dagger-like terminal point.

[11] Historically Slimonia was first considered a member of the Pterygotidae, until it was reclassified alongside Hughmilleria and other genera to the family Hughmilleriidae in 1951 by Erik N.

The cladogram below is simplified from a study by O. Erik Tetlie (2007),[15] and showcases the position of Slimonia relative to the rest of the Eurypterina suborder of eurypterids, with the Stylonurina suborder as an outgroup.Stylonurina Megalograptoidea Eurypteroidea Carcinosomatoidea Waeringopteroidea Adelophthalmoidea Hughmilleria Herefordopterus Slimonia Pterygotidae In 2017, W. Scott Persons IV and John Acorn reported finding an S. acuminata specimen, MB.A 863, in the Patrick Burn Formation of Scotland, dated to the Telychian, around 430 million years ago.

The specimen was a complete and articulated series of telsonal, postabdominal and preabdominal segments, and it showed a very strong lateral curvature in the postabdomen.

Lastly, they argued that the serrations on the telson would most likely be attachment points for setae that would have aided the animal in sensing the water flow to make steering much easier.

[18] Slimonia was very similar to the basal pterygotid Erettopterus in terms of visual acuity, with the number of lenses being comparable to those of Pterygotus and Jaekelopterus and possessing an IOA between 2 and 3 (which is higher than the IOA of Pterygotus and Jaekelopterus, suggesting that the visual acuity of Slimonia was good, but not as good as in the derived pterygotids).

Telychian deposits in the Pentland Hills, where remains of S. dubia have been found, preserve fossils of a large amount of other eurypterids, including Drepanopterus pentlandicus, Laurieipterus elegans, Parastylonurus ornatus, Hardieopterus macrophthalmus, Carcinosoma scoticus, Stoermeropterus conicus and Pentlandopterus minor.

S. acuminata has been found associated with Nanahughmilleria lanceolata, Hardieopterus lanarkensis, Eusarcana obesus, Parastylonurus sigmoidalis, Carcinosoma scorpionis and Erettopterus bilobus[19][20] and S. stylops have been found associated with Nanahughmilleria pygmaea, Eusarcana salteri, Hardieopterus megalops, Erettopterus brodiei, E. gigas, Hughmilleria banksi, Eurypterus cephalaspis and Pterygotus ludensis.

[24] Like many eurypterid species, Slimonia acuminata requires a modern re-description to properly establish defining traits and characteristics.

Some traits that appear to be unique to S. acuminata have been described based on specimens housed at the Doncaster Museum and Art Gallery, including rows of pustules (bulges) along the marginal rim of the body and appendages.

Similar pustule rows have been discovered in the other eurypterid Drepanopterus abonensis, a sweep-feeder that used the marginal rim to search the substrate of its living environment for prey.