''Idmonarachne'' is an extinct genus of

arachnid Arachnids are arthropods in the Class (biology), class Arachnida () of the subphylum Chelicerata. Arachnida includes, among others, spiders, scorpions, ticks, mites, pseudoscorpions, opiliones, harvestmen, Solifugae, camel spiders, Amblypygi, wh ...

s, containing one species, ''Idmonarachne brasieri''. It is related to uraraneids and

spider Spiders (order (biology), order Araneae) are air-breathing arthropods that have eight limbs, chelicerae with fangs generally able to inject venom, and spinnerets that extrude spider silk, silk. They are the largest order of arachnids and ran ...

Fossil

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...

assigned to this genus was found at

Montceau-les-Mines Montceau-les-Mines () is a Communes of France, commune in the Saône-et-Loire Departments of France, department in the Regions of France, region of Bourgogne-Franche-Comté in eastern France. It is the second-largest commune of the metropolitan C ...

, France, in

ironstone Ironstone is a sedimentary rock, either deposited directly as a ferruginous sediment or created by chemical replacement, that contains a substantial proportion of an iron ore compound from which iron (Fe) can be smelted commercially. Not to be c ...

concretion deposits of

Late Carboniferous Late or LATE may refer to: Everyday usage * Tardy, or late, not being on time * Late (or the late) may refer to a person who is dead Music * Late (The 77s album), ''Late'' (The 77s album), 2000 * Late (Alvin Batiste album), 1993 * Late!, a pseudo ...

( Stephanian) age, about 305–299 million years old. Montceau fossils are generally preserved in such a way that fine details can be observed and three-dimensional analysis is possible. In the case of ''Idmonarachne'',

computerized tomography A computed tomography scan (CT scan), formerly called computed axial tomography scan (CAT scan), is a medical imaging technique used to obtain detailed internal images of the body. The personnel that perform CT scans are called radiographers or ...

was used to construct a "virtual fossil".

Description

The total body length of the fossil is around 10.5 mm, with the preserved part of the carapace of the

cephalothorax The cephalothorax, also called prosoma in some groups, is a tagma of various arthropods, comprising the head and the thorax fused together, as distinct from the abdomen behind. (The terms ''prosoma'' and ''opisthosoma'' are equivalent to ''cepha ...

about 5 mm long and the

opisthosoma The opisthosoma is the posterior part of the body in some arthropods, behind the prosoma ( cephalothorax). It is a distinctive feature of the subphylum Chelicerata (arachnids, horseshoe crabs and others). Although it is similar in most respects ...

(abdomen) about 6 mm long. The eight walking legs are more-or-less uniform in appearance, with the fourth leg longest at about 8.5 mm and the first shortest at about 6.5 mm. The legs terminate in at least two claws. The two

pedipalp Pedipalps (commonly shortened to palps or palpi) are the secondary pair of forward appendages among Chelicerata, chelicerates – a group of arthropods including spiders, scorpions, horseshoe crabs, and sea spiders. The pedipalps are lateral to ...

s are slightly shorter than the legs, at just under 4 mm long. The chelicerae are shaped like a clasp-knife, i.e. with the fang at the end folding back into the part to which it is attached. The opisthosoma shows at least seven sections on the upper surface (tergites), each with a flattened W-shaped profile composed of a wider upwardly curved central plate and two side plates. The lower surface of the opisthosoma bears eight visible plates. No

spinnerets A spinneret is a silk-spinning organ of a spider or the larva of an insect. Some adult insects also have spinnerets, such as those borne on the forelegs of Embioptera. Spinnerets are usually on the underside of a spider's opisthosoma, and ar ...

telson The telson () is the hindmost division of the body of an arthropod. Depending on the definition, the telson is either considered to be the final segment (biology), segment of the arthropod body, or an additional division that is not a true segm ...

or terminal spike (flagellum) are present.

Taxonomy

The fossil was placed in a new genus, ''Idmonarachne'', as the species ''I. brasieri''. The genus name is based on the Arachne myth in which Idmon is her father, and is intended to show the position of the genus as a close relative of spiders. The species name honours Martin David Brasier for his contributions to the study of ancient life. The authors did not place the genus in a family or order.

Phylogeny

Based on its overall morphology, ''Idmonarachne'' was considered to belong to the Serikodiastida, a clade of tetrapulmonate

s capable of making and using silk, although the presence of silk-producing spigots was not demonstrated. Like uraraneids, it lacked

, but it did not have a flagellum, thus resembling spiders. A cladogram placed ''Idmonarachne'' between uraraenids and spiders: The Late Carboniferous appears to be a time when there was a greater diversity of tetrapulmonate arachnids.

References

{{Taxonbar, from=Q23660866 Carboniferous arachnids Monotypic arachnid genera Carboniferous arthropods of Europe Carboniferous France Fossils of France Fossil taxa described in 2016