The Burgess Shale arthropod exhibits characters in common with trilobites, most notably, limb morphology, a cephalon, and hypostome.
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|When the Burgess Shale arthropod Naraoia compacta was examined and redescribed (Whittington 1977), it was very similar to trilobites in limb morphology, in the possession of a hypostome and cephalon, and in cephalic limb-number. However, the cephalon was eyeless, transverse, ovoid, and largely lacking dorsal features, it lacked any discernable thoracic segments, and perhaps most fundamentally, the exoskeleton was not hardened by calcium carbonate as in trilobites (the specimen shown at left had a fold in the cephalon indicating its flexibility). The trunk was prominent and much longer than the cephalon, which was quite different from the typically micropygous primitive trilobite pygidium. Nor was Naraoia compacta the only representative of this kind of animal. The Chinese Chengjiang deposits also yielded at least two species of Naraoia, as well as another closely-related genus in the family Naraoiidae, Misszhouia (see Chen & Zhou 1997, Zhang et al 2007). Examination of Cambrian and Ordovician specimens added species in the family Liwiidae that are similar to Naraoiidae, but bear thoracic segments. Finally, a Silurian species of Naraoia (N. bertiensis Caron et al 2004) extended the naraoid clade well beyond their Cambrian heyday. Below are some figures of representative naraoiids/liiwids. |
Similarity of limbs between Naraoiids and Trilobites
The similarity between the limbs of naraoiids and primitive trilobites is quite striking. Based on limb structure alone it would seem incontrovertible that trilobites and naraoiids belong to the same clade. The limb reconstructions of the naraoiid Misszhouia longicaudata and the trilobite Eoredlichia intermedia demonstrate this startling similarity quite clearly.
Order Nektaspida of the Class Trilobita?
Based on the similarities to trilobites (particularly limb similarities), Whittington (1985) considered Naraoia and its relatives to be members of an order of trilobites (Nektaspida) that lacks a calcified exoskeleton (he called them "soft-bodied trilobites"). Some analyses place species such as Naraoia, Misszhouia, Liwia, Buenaspis, Tariccoia, and Soomaspis closer to trilobites than to any other known arthropod clade, although they lack some important trilobite features. In his discussion of the relationship between naraoiids and Trilobita, Fortey (1997) suggested that the handful of naraoiids did not require ordinal status, and they would be treated as trilobites of uncertain ordinal status, based on their many similarities to calcified species.
Developmental origins of agnostids and naraoiids
NORMAL MODE: normal growth rate, normal development rate, normal onset of maturation
The normal progression of growth and development in a typical trilobite begins with a protaspis of 0 segments (far left), a series of meraspid stages with increasing numbers of segments, leading ultimately to a holaspid with the adult number of segments (far right).
AGNOSTID MODE: normal growth rate, normal development rate, early onset of maturation
Via progenesis, normal protaspid to meraspid metamorphoses occur, but sexual maturity is reached in the early meraspid larval stage of a normal trilobite, leading to mature, but miniscule individuals with few segments, the typical state of Agnostida.
NARAOIID MODE: higher growth rate, inhibited development, normal onset of maturation
In contrast, hypermorphosis in naraoiids results in a higher growth rate, but development is greatly suppressed, leading to large adults that resemble gigantic meraspid larvae with zero or few thoracic segments.
Fortey and Theron (1994) suggested that because of the distinctly different developmental routes to produce naraoiids vs agnostids, their superficial similarities (few thoracic segments) were not sufficient to argue for a close phylogenetic relationship. Their differences (e.g., small size in Agnostida, non-calcified exoskeleton and macropygy in Naraoiidae) argued they belonged to distinct clades. Agnostida arguably represents a secondary loss of eyes, reduction in size and segment number, and specialization of limb form and function within the trilobite clade, while the Naraoiidae never developed dorsal eyes, calcified exoskeletons, or the numerous segments of the typical trilobite, yet share the primitive limb form of most arachnomorphs, including trilobites, helmetiids, tegopeltids, xandarellids, and naraoiids.
Chen, J. & G. Zhou. 1997. Biology of the Chengjiang Fauna. in The Cambrian Explosion and the Fossil Record. Bulletin of the National Museum of Natural Science 10:11-106.
Cotton, T.J., and S.J. Braddy. 2000. A "big hand" for the chelicerates? Phylogeny of arachnomorph arthropods and the origins of the Chelicerata. Young Systematists Forum.
Edgecombe, G. D. & L. Ramskøld. 1999. Relationships of Cambrian Arachnata and the systematic position of Trilobita.
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Fortey, R. A., and J. N. Theron. 1994. A new Ordovician arthropod, Soomaspis, and the agnostid problem. Palaeontology 37(4):841-61.
Fortey, R.A. 1997. Classification. In Kaesler, R. L., ed. Treatise on Invertebrate Paleontology, Part O, Arthropoda 1, Trilobita, revised. Volume 1: Introduction, Order Agnostida, Order Redlichiida. xxiv + 530 pp., 309 figs. The Geological Society of America, Inc. & The University of Kansas. Boulder, Colorado & Lawrence, Kansas.
Whittington, H. B. 1977. The middle Cambrian trilobite, Naraoia, Burgess Shale, British Columbia. Philosophical Transactions of the Royal Society of London, Series B. 280:400-43.
Whittington, H. B. 1985. Tegopelte gigas, a second soft-bodied trilobite from the Burgess Shale, British Columbia. Journal of Paleontology 59:1251-74.
Whittington, H. B. 1992. Trilobites. The Boydell Press, Woodbridge, UK. 145 pp., 120 plates.
Wills, M.A., D.E.G. Briggs, R.A. Fortey, M. Wilkinson & P.H.A. Sneath. 1998. An arthropod phylogeny based on fossil and Recent taxa. In: G.D. Edgecomb, ed. Arthropod Fossils and Phylogeny. Columbia University Press, N.Y.
Zhang, X.-L., D.-G. Shu, & D.H. Erwin. 2007. Cambrian naraoiids (Arthropoda): Morphology, ontogeny, systematics, and evolutionary relationships. Journal of Paleontology 81 (sp68):1–52