Origin of spiders and their spinning organs illuminated by mid-Cretaceous amber fossils.

Diying Huang,Yitong Su,Chenyang Cai,Fangyuan Xia,Gustavo Hormiga,Zongjun Yin,Gonzalo Giribet

doi:10.1038/s41559-018-0475-9

Abstract

Understanding the genealogical relationships among the arachnid orders is an onerous task, but fossils have aided in anchoring some branches of the arachnid tree of life. The discovery of Palaeozoic fossils with characters found in both extant spiders and other arachnids provided evidence for a series of extinctions of what was thought to be a grade, Uraraneida, that led to modern spiders. Here, we report two extraordinarily well-preserved Mesozoic members of Uraraneida with a segmented abdomen, multi-articulate spinnerets with well-defined spigots, modified male palps, spider-like chelicerae and a uropygid-like telson. The new fossils, belonging to the species Chimerarachne yingi, were analysed phylogenetically in a large data matrix of extant and extinct arachnids under a diverse regime of analytical conditions, most of which resulted in placing Uraraneida as the sister clade of Araneae (spiders). The phylogenetic placement of this arachnid fossil extends the presence of spinnerets and modified palps more basally in the arachnid tree than was previously thought. Ecologically, the new fossil extends the record of Uraraneida 170 million years towards the present, thus showing that uraraneids and spiders co-existed for a large fraction of their evolutionary history.

Highlights

Palaeogeography, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, People’s Republic of China. 4Lingpoge Amber Museum, Shanghai 201108, People’s Republic of China. 5Museum of Comparative Zoology & Department of Organismic and Evolutionary Biology, Harvard University, 26 Oxford Street, Cambridge, MA 02138, USA
Salient characters preserved in some of these non-spider Palaeozoic arachnids [2,4,5,6,7,9,10] include one or more of the following features: a segmented opisthosoma, similar to that found in modern Mesothelae spiders; silk spigots; and a flagellum similar to that of extant Uropygi and Palpigradi
Fig. 9), cooccurs with a collembolan. Both amber pieces are deposited in the Nanjing Institute of Geology and Palaeontology, CAS, Nanjing

Summary

Introduction

The discovery of Palaeozoic fossils with characters found in both, extant spiders and other arachnids, provided evidence for a series of extinctions of what was thought to be a grade, Uraraneida, that led to modern spiders. Uraraneida and its relatives are known from a handful of Palaeozoic fossils from the Devonian—Permian, in the genera Attercopus and Permarachne. This order of arachnids has long thought to have missed the Palaeozoic/Mesozoic transition and uraraneids have been interpreted as a transitional form between spiders and their flagellate tetrapulmonate counterparts (i.e., Uropygi). Devonian strata from Schoharie County of a supposed conspecific 6, or a single compression fossil of Permarachne novokshonovi, from the Permian of the Ural Mountains, Russia 8

Methods

Results

Conclusion