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An Ordovician Pycnogonid (Sea Spider) with Serially Subdivided ‘Head’ Region

Published online by Cambridge University Press:  15 October 2015

David M. Rudkin ,
Michael B. Cuggy ,
Graham A. Young  and
Deborah P. Thompson
David M. Rudkin
Affiliation:
Department of Natural History (Palaeobiology) Royal Ontario Museum, Toronto, Ontario, Canada M5S 2C6,
Michael B. Cuggy
Affiliation:
Department of Geological Sciences, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 5E2,
Graham A. Young
Affiliation:
Geology & Paleontology, The Manitoba Museum, Winnipeg, Manitoba Canada R3B 0N2,
Deborah P. Thompson
Affiliation:
Geology & Paleontology, The Manitoba Museum, Winnipeg, Manitoba Canada R3B 0N2,
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Abstract

The bizarre morphology of living Pycnogonida, known colloquially as sea spiders, has long fueled dissent over their status within the arthropods. Pycnogonids figure prominently in recent analyses of anterior limb homologies and ancestral crown-group euarthropod relationships, with support for the concept of Pycnogonida as sister taxon to Euchelicerata now contested by proponents of a more basal position between Radiodonta and all other arthropods. A challenge to further elucidation of their phylogenetic position is the exceptional rarity and disjunct distribution of pycnogonids in the fossil record, due largely to their fragile unmineralized exoskeletons. New fossil discoveries therefore have the potential to add significantly to knowledge of their evolution, paleoecology, and paleobiogeography. Here we report the first known occurrence of fossil pycnogonids from rocks of Ordovician age, bridging a 65 Myr gap between controversial late Cambrian larval forms and a single documented Silurian specimen. The new taxon, Palaeomarachne granulata n. gen. n. sp., from the Upper Ordovician (ca. 450 Ma) William Lake Konservat-Lagerstätte deposit in Manitoba, Canada, is also the first reported from Laurentia. It is the only record thus far of a fossil sea spider in rocks of demonstrably shallow marine origin. Four incomplete, partially disarticulated molts represent a relatively large, robust animal with a series of five segment-like elements in a ‘head’ region that does not incorporate the first of four preserved limb-bearing trunk segments. This unique pattern may reflect the plesiomorphic condition prior to complete fusion of anterior ‘head’ elements and first trunk segment to form a cephalosoma, as seen in all eupycnogonids. Palaeomarachne granulata is interpreted as occupying a basal stem-group position in the Pycnogonida.

Type
Research Article
Information
Journal of Paleontology , Volume 87 , Issue 3 , May 2013 , pp. 395 - 405
Copyright
Copyright © The Paleontological Society 

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