title: The true identity of the supposed giant fossil spider Megarachne.
abstract: Megarachne servinei from the Permo-Carboniferous Bajo de Véliz Formation of San Luis Province, Argentina (32 degrees 17'S, 65 degrees 25'E), was described as a giant mygalomorph spider ('tarantula') and, with its body length of 339mm, the largest known spider ever to have lived on Earth. Its identification as a spider was based on interpretations of the shape of the carapace, the position of the eye tubercle, the anterior protrusion of the carapace as a pair of chelicerae, and the posterior circular structure as the abdomen. X-radiography revealed possible morphology hidden in the matrix: cheliceral fangs, sternum, labium and coxae, and so a reconstruction of Megarachne as a giant spider was presented. Difficulties with the interpretation (unusual cuticular ornament, suture dividing the carapace and spade-like anterior border of the chelicera), together with non-preservation of synapomorphies of Araneae, provoked debate about its interpretation as a spider. Now, the holotype and a new specimen have become available for study. Megarachne is shown to be a bizarre eurypterid ('sea-scorpion'), similar to rare forms known from Carboniferous rocks of Scotland and South Africa, and is the most complete eurypterid so far recorded from Carboniferous strata of South America.
authors.affiliation: School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester M13 9PL, UK. paul.selden@manchester.ac.uk
authors.forename: Paul A
authors.initials: PA
authors.lastname: Selden
authors.forename: José A
authors.initials: JA
authors.lastname: Corronca
authors.forename: Mario A
authors.initials: MA
authors.lastname: Hünicken
mesh_terms: D000818:Animals; D001181:Arthropods / Q000033:anatomy & histology / Q000145:classification*; D053476:Extinction, Biological; D005580:Fossils; D011309:Preservation, Biological