Pelmatozoan arms from the Middle Cambrian of Australia: bridging the gap between brachioles and brachials?
Article first published online: 3 FEB 2009
© 2009 The Authors, Journal compilation © 2009 The Lethaia Foundation
Volume 42, Issue 3, pages 283–296, September 2009
How to Cite
CLAUSEN, S., JELL, P. A., LEGRAIN, X. and SMITH, A. B. (2009), Pelmatozoan arms from the Middle Cambrian of Australia: bridging the gap between brachioles and brachials?. Lethaia, 42: 283–296. doi: 10.1111/j.1502-3931.2008.00145.x
- Issue published online: 23 JUL 2009
- Article first published online: 3 FEB 2009
- manuscript received on 10/3/2008, manuscript accepted on 19/8/2008.
- functional anatomy;
The early Middle Cambrian Monastery Creek Phosphorite (Beetle Creek Formation, Queensland, Australia) contains an assemblage of disarticulated echinoderm ossicles that are exquisitely preserved. Amongst this material we recognize pelmatozoan brachials, radials, basals and holomeric columnals. Although we cannot reconstruct the complete animal with precision, these elements represent the oldest known pelmatozoan with crinoid-like appendages. Key elements include isotomously to heterotomously branched uniserial appendage plates with a tripartite adoral food groove, a longitudinal central canal interpreted as housing entoneural nerve, and differentiated articulation facets. There are also epispire-bearing radials bearing one to four arm insertion-facets, each one pierced by a central neural canal. These canals run internal towards the oral area beneath the external food groove. Co-occuring material includes single truncated cone-shaped basals and holomeric columnals, both with a similar articulation pattern, and irregular, epispire-bearing thecal plates. This mosaic of crinozoan (uniserial isotomous to heterotomous arms with neural canal), blastozoan (epispire-bearing thecal plates, appendage leading to oral thecal food groove without direct connection with body cavity) and apomorphic characters (circumoral instead of basal entoneural plexus) is unexpected and demonstrates that crinoid-like pelmatozoans with uniserial, branched arms appeared significantly earlier than the Tremadocian, when the first articulated crinoid skeletons are found. It also raises questions about the polyphyletic appearance of feeding appendages among pelmatozoan echinoderms.