Fossil evidence for the origin of spider spinnerets, and a proposed arachnid order

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.Araneae ͉ Chelicerata ͉ Devonian ͉ Paleozoic ͉ silk

Tarantulas cling to smooth vertical surfaces by secreting silk from their feet

“…The presence of tarsal silk secretion in the three phylogenetically disparate (Pérez-Miles et al, 1996) tarantula genera examined, plus the Costa Rican zebra tarantula, argues against such a piecemeal process. Both the different suggestions for the origin of tarsal silk secretion are consistent with the homology of legs and spinnerets as arthropod appendages (Shultz, 1987;Shear et al, 1989;Damen et al, 2002;Selden et al, 2008). As not all tarantulas are large, some being less than 1mm in length (Coddington and Levi, 1991), it should be possible to disambiguate the influence of size and primitiveness on tarsal silk secretion by determining whether small representatives of the Theraposidae family can secrete silk from their tarsi.…”

“…The earliest known silk-secreting animal was the spider Attercopus from the Devonian period (Selden et al, 2008), which secreted sheets of silk not from spinnerets but from spigots arranged along the edges of an abdominal plate on body segments 4 and 5. The spinnerets themselves originated later from biramous appendages on the same segments (Damen et al, 2002;Selden et al, 2008). In adult spiders the silk-secreting spigots on the spinnerets do not resemble generalized mechanosensory setae but have large bases important for the transition of the stored liquid silk protein to a long silk fibre.…”

“…In fact, a defining feature of all spiders is the production of silk, synthesized by specialist glands inside the opisthsoma (abdomen) and then extruded from the spinnerets on the abdomen. However, tarsal silk secretion would provide strong support for the hypothesis that spider spinnerets are derived from modified limbs (Shultz, 1987;Shear et al, 1989;Damen et al, 2002;Selden et al, 2008), and direct silk secretion from a setae on a limb without any complex storage organ may represent the ancestral condition, with silk production from abdominal spinnerets being the product of much later evolution. The ability of spiders to secrete silk from their feet would show that silk production is controlled by developmental modules able to be expressed in a variety of body parts (Selden et al, 2008).…”

Cross-bracing uncalibrated nodes in molecular dating improves congruence of fossil and molecular age estimates

“…Due to the patchiness and amplicon brevity of the latter data set, in addition to the present study's emphasis on basal chelicerate relationships, the sequences reported by Starrett et al [37] were not included here. Both of these studies ( [37,42]) utilized the Middle Devonian fossil Attercopus fimbriunguis (382.7 Ma [43])-a putative member of a spider stem-group [44]-to constrain the basal split of spiders, but neither data set included a representative of Mesothelae, the lineage sister to all remaining spiders. Apropos, both Ayoub et al [42] and Starrett et al [37] recovered Devonian ages (i.e., the calibration itself) for the split between Araneomorphae and Mygalomorphae, a de facto overestimate stemming from the application of the fossil date to a derived (internal) node.…”

Arthropod cuticles from the upper Viséan (Mississippian) of eastern Germany

“…4A-D) bears in its surface structure similarities to the scorpion cuticle depicted in Stankiewicz et al (1998, fig. 2g) and also to that of Attercopus fimbriunguis (see Selden et al 2008, their fig. 1), an arachnid probably in the evolutionary lineage leading towards modern araneans.…”