Almost a spider: a 305-million-year-old fossil arachnid and spider origins.

Russell J Garwood,Michael Drakopoulos,Robert C Atwood,Nghia T Vo,Jason A Dunlop,Paul A Selden,Alan R T Spencer

doi:10.1098/rspb.2016.0125

Abstract

Spiders are an important animal group, with a long history. Details of their origins remain limited, with little knowledge of their stem group, and no insights into the sequence of character acquisition during spider evolution. We describe a new fossil arachnid, Idmonarachne brasieri gen. et sp. nov. from the Late Carboniferous (Stephanian, ca 305–299 Ma) of Montceau-les-Mines, France. It is three-dimensionally preserved within a siderite concretion, allowing both laboratory- and synchrotron-based phase-contrast computed tomography reconstruction. The latter is a first for siderite-hosted fossils and has allowed us to investigate fine anatomical details. Although distinctly spider-like in habitus, this remarkable fossil lacks a key diagnostic character of Araneae: spinnerets on the underside of the opisthosoma. It also lacks a flagelliform telson found in the recently recognized, spider-related, Devonian–Permian Uraraneida. Cladistic analysis resolves our new fossil as sister group to the spiders: the spider stem-group comprises the uraraneids and I. brasieri. While we are unable to demonstrate the presence of spigots in this fossil, the recovered phylogeny suggests the earliest character to evolve on the spider stem-group is the secretion of silk. This would have been followed by the loss of a flagelliform telson, and then the ability to spin silk using spinnerets. This last innovation defines the true spiders, significantly post-dates the origins of silk, and may be a key to the group's success. The Montceau-les-Mines locality has previously yielded a mesothele spider (with spinnerets). Evidently, Late Palaeozoic spiders lived alongside Palaeozoic arachnid grades which approached the spider condition, but did not express the full suite of crown-group autapomorphies.

Highlights

Spiders (Arachnida: Araneae) are a diverse and successful arthropod clade, which can be traced back ca 315 Ma to the Late Carboniferous [1]
Idmonarachne is clearly a pantetrapulmonate arachnid on the basis of its clasp-knife chelicerae and general habitus—an assertion supported by the cladistic analysis presented
The prosomal region is spider-like. This is seen in both the relatively large, forward-projecting chelicerae, with an oblique articulation, similar to the plagiognathic condition seen in mesothele spiders [17], and the nature of the prosomal appendages, as discussed in the morphological interpretation

Summary

Introduction

Spiders (Arachnida: Araneae) are a diverse and successful arthropod clade, which can be traced back ca 315 Ma to the Late Carboniferous [1]. One of the oldest reported spiders was found in the Late Carboniferous (ca 305 Ma) deposits of Montceau-les-Mines in France. This important Konservat-Lagerstatte has yielded scorpions [3], harvestmen [4,5] and members of the extinct order Trigonotarbida [6], in addition to other invertebrates [7,8], vertebrates [9,10] and plants [11]. Concretions, fine anatomical details can be recovered that allow precise systematic placement. We place the new species using a cladistic analysis, and discuss its impact on our understanding of spider origins

Results

Discussion

Findings

A Paleozoic stem group to mite harvestmen