Douglassarachne

Douglassarachne
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Arthropoda
Subphylum:	Chelicerata
Class:	Arachnida
Clade:	Pantetrapulmonata
Genus:	†Douglassarachne Selden and Dunlop, 2024
Species:	†D. acanthopoda
Binomial name
†Douglassarachne acanthopoda Selden and Dunlop, 2024

Douglassarachne acanthopoda is an extinct arachnid species from the Late Carboniferous. It is the only species in the genus Douglassarachne. it is known exclusively from one specimen recovered from the Mazon Creek Lagerstätte, Illinois, USA.

The fossil’s morphology does not align with any known arachnid orders, but it superficially resembles certain harvestmen and mites.

Etymology

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

History

Douglassarachne lived around 3.08 million years ago in the coal forests of North America. 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.

Fossil description

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 either very small or lost during preservation. The legs are robust, with the fourth set slightly thicker, than the others. The legs feature large curved macrospines 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. The fossil’s morphology does not align with any known arachnid orders, but it superficially resembles certain harvestmen and mites from the Opilioacarida group. However, notable differences in leg structure, body segmentation, and size preclude definitive classification with these groups. The fossil 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, the fossil is referred to as Arachnida / Pantetrapulmonata incertae sedis due to the lack of definitive apomorphic characters.

Morphological description

The large arachnid fossil features a subovate body with a total length, including the anal tubercle, of approximately 15.4 mm. The subtriangular carapace measures 5.2 mm in length and 7.2 mm in width (ratio 0.73). The cephalic area is subtrapezoidal, 2.2 mm long and 3.0 mm wide (ratio 0.73), with curved anterior-lateral margins and a straight posterior edge marked by a line of tubercles. A suboval raised region, possibly an ocular tubercle, is located in the median part of the cephalic area, which is finely tuberculate, with less tuberculation on the lateral areas.

The leg lengths, including femur length and maximum preserved length, are as follows:

Leg 1: Femur 6.7 mm, Total 16.1 mm

Leg 2: Femur 6.8 mm, Total 17.9 mm

Leg 3: Femur 7.0 mm, Total 22.7 mm

Leg 4: Femur 8.8 mm, Total 24.6 mm

The leg formula, based on the femora lengths from longest to shortest, is 4321. The trochanters are spineless, with well-demarcated femora, though other podomere joints are indistinct. Femur 4 is gently curved following the lateral edge of the abdomen. The femora and more distal podomeres have macrospines about 2 mm in length, tapering distally and curving towards the distal end of the leg, arranged in rows of approximately four on the lateral and dorsal sides of the femora, becoming more numerous and closer together on the distal podomeres.

The opisthosoma is suboval, 9.7 mm in length and 9.1 mm in width (ratio 0.56), with tergites bearing variously recurved anterior margins and straighter lateral margins. The sagittal lengths of the tergites are as follows:

1. 1.4 mm
2. 1.1 mm
3. 1.2 mm
4. 1.2 mm
5. 1.0 mm
6. 1.0 mm
7. 1.2 mm

The mean sagittal length of the tergites is 1.1 mm. The median and lateral areas are distinct but not divided into separate plates, with a finely tuberculate cuticle and few tubercles along the posterior edges of the tergites. The posteriormost sternite features a bilobed structure postero-medially, and the anal tubercle is 1.2 mm long and 2.1 mm wide (ratio 0.55).

References

•  This article incorporates text from a free content work. Licensed under CC BY 4.0. Text taken from A remarkable spiny arachnid from the Pennsylvanian Mazon Creek Lagerstätte, Illinois​, Paul A. Selden and Jason A. Dunlop, Journal of Paleontology.