Paper No. 6
Presentation Time: 3:00 PM


LAMSDELL, James C.1, STEIN, Martin2, OLESON, Jørgen2 and SELDEN, Paul A.3, (1)Department of Geology and Geophysics, Yale University, Kline Geology Laboratory, 210 Whitney Avenue, New Haven, CT 06511, (2)Natural History Museum of Denmark, Universitetsparken 15, København, DK-2100, Denmark, (3)Paleontological Institute, University of Kansas, 1475 Jayhawk Blvd, Lindley Hall, Rm 120, Lawrence, KS 66045,

The original Arachnomorpha hypothesis placed trilobites and a number of predominantly early Palaeozoic arthropod taxa, including megacheirans, in the chelicerate stem lineage. As an alternative hypothesis, Lamellipedia (Arachnomorpha excluding megacheirans) was originally interpreted as a paraphyletic grade of derivatives of the chelicerate stem lineage but,are increasingly being discussed as possible stem-lineage mandibulates under a modified lamellipedian hypothesis.

Here, we present a broad analysis aimed at testing the Arachnomorpha hypotheses versus the modified lamellipedian hypothesis. Most of the over 80 included taxa are extinct, and the more than 170 characters coded focus on external morphology recognizable in the fossils. Euchelicerates are represented by three extant and 14 extinct species spanning synziphosurines, xiphosurids, chasmataspids, eurypterids, and scorpions. Three extant myriapods are included in acknowledgement of the Paradoxopoda versus Mandibulata controversy. Crustacea is represented by five extant and seven extinct taxa, including cephalocarids, branchiopods, mystacocarids, remipedes, and malacostracans, as well as four supposed derivatives of the crustacean stem lineage. Lamellipedians are represented by marrellomorphs, 16 artiopod species, three of which are trilobites, as well as Cheloniellon calmani, Sidneyia inexpectans, Squamacula clypeata, and Aglaspis spinifer. Five megacheirans were included. Also, several controversial or problematic taxa were included, such as Canadaspis perfecta, Agnostus pisiformis, Sanctacaris uncata, Kiisortoqia soperi, and Sarotrocercus oblita. The stem-lineage euarthropod Shankouia zhengei was selected as outgroup. The matrix was run in TNT.

Results strongly support the position of all ingroup taxa except Fuxianhuia protensa within a well-supported Euarthropoda. Arthropods with a great appendage and Kiisortoqia are well supported as derivatives of the stem lineage of a monophyletic Chelicerata (Pycnogonida + Euchelicerata). Canadaspis is retrieved as an early derivative of the mandibulate stem lineage, and artiopods are rendered a paraphyletic grade giving rise to Crustacea. These results falsify the Arachnomorpha hypothesis, but are more in line with the modified lamellipedian hypothesis.