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

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 (CT) 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 recognised, 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. Whilst 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 postdates the origins of silk, and may be 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.

蜘蛛是一类历史悠久的重要动物类群。我们对其起源的细节仍知之甚少，对其干群（stem group）的认知匮乏，也尚未明确蜘蛛演化过程中性状获得的序列。本文记述了一件产自法国蒙索莱米讷（Montceau-les-Mines）晚石炭世（斯蒂芬期，约3.05~2.99亿年）的全新蛛形纲化石——新属新种Idmonarachne brasieri gen. et sp. nov.。该化石保存在菱铁质结核（siderite concretion）中，呈三维立体保存状态，可通过实验室方法与同步辐射相位衬度计算机断层扫描（CT）技术进行重建。其中同步辐射CT扫描为首例针对菱铁质结核寄主化石的此类研究，使我们得以解析其精细的解剖学结构。尽管该化石的外形与蜘蛛极为相似，但它缺少蜘蛛目（Araneae）的关键鉴定特征：后腹部（opisthosoma）腹面的纺器（spinnerets）。同时，它也不具备新近被确立的、与蜘蛛亲缘关系较近的泥盆纪-二叠纪乌拉拉蛛目（Uraraneida）所拥有的鞭状尾节（flagelliform telson）。分支系统学分析（cladistic analysis）显示，这件新化石是蜘蛛类群的姊妹群（sister group），蜘蛛干群由乌拉拉蛛类与I. brasieri组成。尽管我们无法在该化石中确认纺管（spigots）的存在，但重建的系统发育（phylogeny）关系表明，蜘蛛干群演化过程中最早出现的性状是丝腺分泌功能。在此之后，依次发生了鞭状尾节的退化，以及利用纺器纺丝的能力的演化。这一最终的创新特征界定了真正的蜘蛛，其出现时间远晚于丝分泌功能的起源，这或许是蜘蛛类群得以繁盛的关键因素。蒙索莱米讷遗址此前曾出土过带有纺器的中纺亚目（Mesothelae）蜘蛛化石。显然，晚古生代的蜘蛛与多种已接近蜘蛛类群特征的古生代蛛形纲类群共存，但这些类群并不具备冠群（crown group）蜘蛛的全部独有衍征（autapomorphies）。