Megarachne servinei
from the Permo-Carboniferous Bajo de Véliz Formation of San Luis Province, Argentina (32° 17′ S, 65° 25′ E), was described as a giant mygalomorph spider (‘tarantula’) and, with its body length of 339 mm, 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.
The continuous sequence of Maastrichtian to Paleocene sediments exposed in the Sierra Dorotea area in southwestern Santa Cruz (Argentina) contains a rich molluscan fauna with many elements characteristic of the Weddellian Faunistic Province. The presence in this fauna of genera such as Taioma, Heteroterma, Fyfea, Zemacies, and Priscaphander suggests close affinities with faunas of similar age from New Zealand, further supporting the existence of continuous shallow-sea conditions along the southern margin of the Pacific Ocean during the end of the Cretaceous and beginning of the Tertiary. In this paper 25 species are described, of which six are new: Pseudofax costellatus n. sp., Taioma patagonica n. sp., Heteroterma elegans n. sp., Fyfea beui n. sp., Priscaphander sanjosensis n. sp., and Priscaphander bracaccinii n. sp.
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