Keurbos is an extinct genus of enigmatic arthropods from the latest Ordovician (Hirnantian) Soom Shale of South Africa, a Lagerstätten fossil site within the larger Cederberg Formation. The genus contains a single species, Keurbos susanae. It is known from two specimens, one nearly complete and the other a partial body. These specimens, while preserving the internal organs extremely well, barely preserve the exoskeleton. At about 43 centimetres (17 in) long, Keurbos is a large arthropod. Its trunk has 46 segments that decrease in size toward the rear. Its exact phylogenetic placement in relation to other arthropods is uncertain due to the poor preservation of the head and limbs. Keurbos inhabited a cold ocean basin, having lived right after a major glaciation.
Discovery and naming
The Keurbos holotype specimen, C1002, is a well-preserved specimen missing the carapace, legs, and most of its head but preserving soft organs. The paratype, C2044, is a less complete partial body missing the front and back. Both part and counterpart of the two specimens are preserved, with head anatomy being obtained from the holotype's counterpart. These specimens were discovered in the Soom Shale (Cederberg Formation) of South Africa by University of Leicester professor Sarah Gabbott in an expedition around 2000, who nicknamed the species "Sue". The informal name "Keurbosia" was used in reference to this species in artwork published in 2007. The species Keurbos susanae was officially described in 2025 in a paper published in Papers in Palaeontology, by Sarah Gabbott (the lead author), Gregory Edgecombe, Johannes Theron and Richard Aldridge. The genus name, Keurbos, is derived from the name of the farm where the holotype was found. The specific name susanae honours Susan Gabbott (the lead author's mother). Keurbos seems to have been exceptionally rare in the Soom Shale, with only two specimens being recovered over a 20-year timespan and several months of fieldwork. Alongside this, the locality where the holotype was found is now impossible to collect fossils from, due to quarrying activity covering the site in significant overburden.
Description
Unusually, fossils from the Soom Shale are preserved in an "inverted" manner, where soft tissues are exquisitely preserved but hard parts are demineralised and often preserved as moulds. Because of this, no appendages and only a fragmentary head are preserved on Keurbos. It is known from two specimens that preserve most of the body, although the head is only known from a single specimen. The holotype is around 43 cm (17 in) long. The smaller incomplete paratype is half as wide, and around 15 cm (5.9 in) as preserved. The head preserves a partial cephalic shield roughly 0.7 times as wide as long, with a pair of unusual arcing setose (with setae, small hairs in invertebrates) structures preserved alongside various patches of muscle fibres within the head. Several irregular patches are also preserved, one with an unusual granular texture not seen anywhere else on the fossil. The arcing setose structures are likely parts of appendages, with the poor preservation of the head suggesting it was preserved in a tucked position.
Dorsal anatomy
The trunk of Keurbos has 46 segments with each tergite (dorsal portion of the segment) corresponding to one sternite (ventral segment portion). This trunk narrows gently from 8.5 cm (3.3 in) at the front (anterior end) to 5.3 cm (2.1 in) at the posterior. The dorsal exoskeleton is quite inconspicuous except in the paratype where it extends off the main body. Each tergite is defined by a gently curving convex band at the posterior, with a set of narrow, parallel ridges aligned with the body axis also being present on the posterior edge. In the holotype, the cuticle between the tergite boundaries is covered in fine dimples, alongside a patch of finely ridged brown material likely also representing part of the exoskeleton. In the paratype, the posterior (back) tips of each tergite are rounded into overlapping paratergal (part of the tergite extending out from the body) folds, with no sutures present distinguishing them from the main tergite. At the front of these folds, two ridges are present which likely represent more sclerotised (hardened) cuticle, with both being covered in regular, short, stub-like projections. Behind segment 46, the trunk greatly narrows into a rounded posterior margin consisting of at least nine lobe-like structures, each with gently convex margins. Between segment 46 and these lobes, a plate with large tubercles and an arcing margin is preserved. Unfortunately, due to the poor preservation of ventral anatomy and the multiple cracks in this area of the fossil, it cannot be determined if a tail plate was present.
Ventral anatomy
Each segment of Keurbos has a symmetrical series of plates that meet at a Y-shaped structure along the midline. This Y-shaped structure has two prongs anterior (in front) and one shorter one posterior, with each prong having a circular cross-section. On either side of this structure are two subrectangular (rectangular with rounded edges) plates (which reduce in size along with the tapering trunk), likely sternites, with a rough surface and setae along their posterior margin. These setae vary in length and orientation, being longest at the apex (tip) of the convex margin and shorter at the sides. In the holotype, these plates seem to pinch and narrow abaxially (away from the body line), alongside being differentiated between the two halves of the body. In the front half (segments 1–27) they are narrow and do not join at their center in segments where the Y-shaped structure is absent. However, after segment 27 they broaden and join with the Y-shaped structure. This is likely due to a slight change in orientation or split level, rather than any actual change in shape. Outwards from the sternites, two oval to trapezoidal plates are preserved (one pair per segment), being similar to the sternites in having a rough texture and setae on their posterior margin. The holotype's plates are oval, whereas they are more trapezoidal in the paratype. In the holotype, these plates also seem to "merge" with the sternites, appearing contiguous with them.
Vascularized lamellae
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