Opabinia. Found in the Burgess Shale in British Columbia Canada. It's not an arthropod. How can it even be a bilateria? With five eyes? It's still unassigned to any group. No one knows. A transitional species?

http://www.nmnh.si.edu/paleo/images/fopabin.gif

http://www.nmnh.si.edu/paleo/images/dopabin.gif

Source: http://www.nmnh.si.edu/paleo/shale/popabin.htm
http://evolution.berkeley.edu/evolibrary/article/0_0_0/cambrian_06

Has to be related to Anamalocaris?:
http://www.learningfamily.net/reiser/9809-nathisttour/images/anamalocaris.jpg

Source: http://www.learningfamily.net/reiser/9809-nathisttour/0_cambrian.htm

See also:http://dml.cmnh.org/2001Mar/msg00025.html
http://omniknow.com/common/wiki.php?in=en&term=Opabinia

Anamalocaris:

http://www.learningfamily.net/reiser/9809-nathisttour/images/anamalocaris.jpg

I did not see how old it is. Does anyone know?

The Burgess shale fauna are about 500 mya, so that's mid-Cambrian.

Classification of organisms: living and fossil.
Anderson C.Biosystems. 1993;31(2-3):99-109.
Abstract
The classification advanced herein provides a framework to which any organism can be readily placed and offers comprehensive coverage of the entire biological spectrum. The viruses are incorporated as a kingdom of organisms and a rationale advanced for a 6-kingdom system. Many seldom-classified fossil organisms are included, with a significant number of them classified at an advanced level. Individual phyla are recognized for the fossil genera Tribrachidium, Amiskwia, Dinomischus, Anomalocaris, Tullimonstrum, Hallucigenia, Opabinia and Pikaia. Other seldom-recognized phyla include viruses, conularids, loriciferans, tentaculites, hyoliths, scleritophores, myzostomans, acorn worms, conodonts, monoblastozoans and volborthellids. The notion that failures (geologically) cannot be phyla is rejected. A system of kingdom-related and rank-related suffixes is incorporated with the flexible opportunity, if deemed necessary, to advance or reduce taxa by re-suffixing. Other features of the complete classification of Anderson (Classification of Organisms: Living and Fossil, Golden Crowns Press, Lancaster, OH, 1992) include the elimination of duplicate taxa, incorporation of geologic ranges and number of species for many groups, and extensive usage of common names, which are completely indexed.