Sunday 11 August 2013

A Mygalomorph Spider from the Late Triassic of Italy.

Mygalomorph Spiders (Tarantulas and related species) are considered to be one of the most ancient groups of Spiders. They have two pairs of book lungs (many other Spiders have lost a pair) and downward pointing, rather than opposable fangs, again considered to be a primitive state in Spiders. Many species of Mygalomorph attain large sizes, all have flattened, disk-shaped bodies (rather than the more globular bodies of most other Spiders), and most are ambush predators.

In a paper published in the June 2013 edition of the journal Acta Palaeontologica Polonica, Fabio Dalla Vecchia of the Institut Català de Paleontologia and Paul Selden of the Paleontological Institute and Department of Geology at the University of Kansas, describe a Mygalomorph Spider from the Late Triassic of northeast Italy.

The specimen is described as a new species, and given the name Friularachne rigoi, where 'Friularachne' means spider from Friuli, the region of Italy where it was found, and 'rigoi' honours the finder of the specimen, Roberto Rigo. The Spider is preserved as part and counterpart on a split slab of dolostone (magnesium rich limestone). It has a body-length of 3.48 mm, and a total leg-span of about 8 mm. The specimen is interpreted to be male, due to its enlarged chelicerae, which are used in mating by male Mygalomorph Spiders.

Friularachne rigoi. Part (A), and counterpart (B). Photographs (A1, B1) and explanatory drawings (A2, B2). 1–4, walking legs 1–4. Dalla Vecchia & Selden (2013).

Friularachne rigoi is tentatively assigned to the Atypoidea, Purseweb Spiders and Folding Trapdoor Spiders. If this is correct it will make Friularachne rigoi the oldest known member of the group by a long way, the oldest known Atypoid Spider to date coming from the Early Cretaceous of Mongolia. However this is not an overly surprising diagnosis; Friularachne rigoi is only the fourth Triassic Spider ever discovered, and the second Triassic Mygalomorph. However Mygalomorph Spiders are thought to have diverged from other Spider early in the history of the group, and Atypoid Spiders are similarly thought to have diverged from other Mygalomorphs very early on, so that, despite the poor fossil record of Triassic Spiders, palaeontologists would have predicted that this group were around at this time.

The specimen originates from the Dolomia di Forni Formation in the Rovadia Brook valley on the north slope of the Carnian Prealps. The Dolomia di Forni Formation is a marine dolostone noted for its fossils, which include marine Arthropods and Fish, Plant fragments and terrestrial vertebrates. Many of the Plant specimens show adaptation to a warm arid climate. It is interpreted that the terrestrial components of the Dolomia di Forni Formation assemblage were swept out to sea in flash flood events. 

Map showing the locality where the specimen was found (black Star). Dalla Vecchia & Selden (2013).

Modern Atypoid Spiders build silk-lined burrows, from which they ambush prey. The females, having dispersed from their mothers nest, build a burrow of their own, in which they remain for life, however the adult males leave their burrows in order to search for females, making them much more likely to be swept out to sea in any flash flood event.

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