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, 105 (52), 20781-5

Fossil Evidence for the Origin of Spider Spinnerets, and a Proposed Arachnid Order

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Fossil Evidence for the Origin of Spider Spinnerets, and a Proposed Arachnid Order

Paul A Selden et al. Proc Natl Acad Sci U S A.

Abstract

Silk production from opisthosomal glands is a defining characteristic of spiders (Araneae). Silk emerges from spigots (modified setae) borne on spinnerets (modified appendages). Spigots from Attercopus fimbriunguis, from Middle Devonian (386 Ma) strata of Gilboa, New York, were described in 1989 as evidence for the oldest spider and the first use of silk by animals. Slightly younger (374 Ma) material from South Mountain, New York, conspecific with A. fimbriunguis, includes spigots and other evidence that elucidate the evolution of early Araneae and the origin of spider silk. No known Attercopus spigots, including the original specimen, occur on true spinnerets but are arranged along the edges of plates. Spinnerets originated from biramous appendages of opisthosomal somites 4 and 5; although present in Limulus, no other arachnids have opisthosomal appendage homologues on these segments. The spigot arrangement in Attercopus shows a primitive state before the reexpression of the dormant genetic mechanism that gave rise to spinnerets in later spiders. Enigmatic flagellar structures originally described as Arachnida incertae sedis, are shown to be Attercopus anal flagella, as found in Permarachne, also originally described as a spider. An arachnid order, Uraraneida, is erected for a plesion, including these two genera, based on this combination of characters. The inability of Uraraneida precisely to control silk weaving suggests its original use as a wrapping, lining, or homing material.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
Attercopus fimbriunguis, Devonian of New York (localities: G, Gilboa; SM, South Mountain), macerated from matrix with HF and slide-mounted. (A) First-described “spinneret,” G 334.1b.34; darkness of cuticle reflects number of layers, so this fragment is folded over twice. (B) Palpal femur, SM 1.11.12; arrow indicates patch of distinctive spinules. (C) Piece of cuticle from corner of opisthosomal ventral plate showing setae, spigots, and possible silk strand, SM 1.11.4. (D) Close-up of E showing possible silk strand emerging from spigot shaft, SM 1.11.4. (E) Flagellar structure with 12 segments (including possible distalmost) from original Gilboa locality; segments show distal collars and setae, G 334.1a.4. (F) Close-up of cheliceral fang showing a number of holes (arrowed), the most distal of which had been interpreted as a venom-gland opening, G 329.22.9. (Scale bars: 0.5 mm, except F, 0.25 mm.)
Fig. 2.
Fig. 2.
Attercopus fimbriunguis, Devonian of South Mountain, New York, macerated from matrix with HF and slide-mounted. (A) Part of opisthosoma with rows of spigots, SM 1.11.3a. (B) Two flagellar segments emerging from posterior part of opisthosoma, SM 1.11.3b. (Scale bars: 0.5 mm.)
Fig. 3.
Fig. 3.
Paleozoic Araneae and Uraraneida. (A–C) Permarachne novokshonovi, Permian of Russia, PIN 4909/12. (A) Holotype part in rock matrix. (B) Explanatory drawing of A. (C) Close-up of flagellum showing whorls of setae. ch, chelicera; cx, coxa; fe, femur; mt, metatarsus; pa, patella; pl, ventral plate; st, sternum; ta, tarsus; ti, tibia. (D) Palaeothele montceauensis, Carboniferous of France, In 62050a, X-ray CT scan showing appendages buried in the rock matrix; note, anal tubercle (arrowed) is not a flagellum. (Scale bars: B, 1 mm; C and D, 0.1 mm.)

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