Douglassarachne

Douglassarachne is an extinct genus of arachnid from the Late Carboniferous (Moscovian), known from single species D. acanthopoda. It is known exclusively from one specimen recovered from the Mazon Creek Lagerstätte, Illinois, US.

This arachnid is characterized by having large spines on its legs. It is unknown which group this arachnid specifically belongs to.

Etymology
The genus is named for the Douglass family who donated the specimen to the Field Museum for study.

History
Douglassarachne is known from Mazon Creek which preserves fossils around 308 million years ago. The fossil was preserved in a siderite concretion. In 1980, Bob Masek discovered the fossil at the Pit 15 Northern Mine spoil heap, near Essex, Kankakee County, Illinois. Around 1990 he would sell the fossil to David Douglass.

At this time it, was displayed in the Douglass family's Prehistoric Life Museum. In 2023, when it became apparent that this specimen represented a new species, David Douglass donated the specimen to the Field Museum of Natural History so it could be researched.

The specimen is deposited in the collections of the Field Museum of Natural History with the number PE 91366.

Morphology
The arachnid is characterized by four pairs of legs and a body approximately 15 mm long. The body is divided into an anterior prosoma and a posterior segmented opisthosoma. The prosoma is shielded by a subtriangular, undivided carapace with a slight anterior projection. Chelicerae and pedipalps are not preserved in fossil, suggesting it was either very small or lost during preservation. The legs are robust, with the fourth set slightly thicker, than the others. Each legs had large curved spines on the proximal articles. The opisthosoma is broadly attached to the prosoma, with at least eight visible tergites without ornamentation. The posterior segments form a small anal tubercle, though the total number of segments is unclear.

Classification
The morphology of Douglassarachne does not align with any known arachnid orders. With similarly spiny legs, it superficially resembles certain harvestmen (such as Podoctidae and Lacinius). Overall morphology is also somewhat reminiscent of a mite within order Opilioacarida. However, notable differences in leg structure, body segmentation, and size preclude definitive classification with these groups. It also shares some characteristics with the Pantetrapulmonata lineage, including a distinct coupling between the prosoma and opisthosoma and a series of undifferentiated opisthosomal tergites. However, the absence of preserved chelicerae prevents confirmation of this classification. As such, it is referred to as Arachnida/Pantetrapulmonata incertae sedis due to the lack of definitive apomorphic characters.

Paleoecology
Its spiny legs like some of modern harvestmen suggests that it uses spines for protection from predators. There are multiple taxa from Late Carboniferous with similarly developed spines (such as trigonotarbid Eophrynus or millipede Euphoberia and Myriacantherpestes), which possibly reflects the evolution of their predators.