Trilobite Origins

Abstract

This page reviews available information about the phylogenetic relationships and possible origins of the Trilobita.

Keywords: Trilobita, trilobite origins, Fallotaspididae, Olenellids, Olenellina, Redlichiida, Eofallotaspis, Profallotaspis

Introduction

The big problem with the earliest known trilobites, is that they are trilobites.

That is to say, their earliest representatives are distinctly and emphatically trilobites, and they do not look like anything else. They provide few clues to which other arthropod groups may be their close relatives, or to their origins.

The earliest occurring trilobites in the fossil record are the Olenellids, members of the order Redlichiida, suborder Olenellina, and particularly the Fallotaspididae. Contenders for "first trilobite" are Eofallotaspis tioutensis Sdzuy 1978 from the Anti-Atlas Mountains in Morocco and Profallotaspis jakutensis Repina 1965 from southeast Siberia.

Although it is true that one or two of the Ediacaran forms such as Spriggina bear a superficial resemblance to the early trilobites, to date the detailed case for such an ancestry is far from compelling.

Affinities

Trilobites are clearly arthropods. They exhibit a number of primitive arthropod features which, together with their very early appearance in the fossil record, have sometimes led to trilobites being considered to lie close to the main arthropod ancestral line. On the other hand, the trilobite eye is highly derived – there is nothing else like it in the arthropod (or any other) lineage – which belies this placement.

Early Trilobite Successions

The incoming of trilobites is recorded in numerous conformable sections around the world. The following table summarises a few.

These taxa belong to the suborder Olenellina, which are distinct among trilobites in primitively lacking facial sutures: though other groups may have secondarily lost facial sutures, Olenellina are believed never to have had them. The unique primitive condition and the circumstantial but voluminous evidence that fossils from this group underlie all others, prompts the view that Olenellina are the ancestral trilobite stock.

Primitive Condition

Fallotaspis ...

Kleptothule ? - A trilobite-like (?olenellinid) arthropod with ogival cephalic region probably containing five segments, one of which is occipital. Parallel-sided and lobed thoracic region with about 27 segments. Caudal region with at least 20 poorly-defined, ankylosed segments. Complete specimens are about 30 mm long and 5-6 mm at their widest point.

Early Arthropod Evolution

The results of a phylogenetic biogeographic analysis of Early Cambrian olenellid trilobites, calibrated by a tectonic event - the breakup of Pannotia at 600 to 550 Ma - provide evidence that "trilobites likely had evolved and begun to diversify minimally by between 550 to 600 Ma. This is a significant period before the trilobites appeared in the fossil record, around 525 to 530 Ma" (Lieberman 2003, p. 231).

Ediacaran 'Arthropods'

Possible ancestral arthropods, such as Parvancorina, are to be found in the Ediacaran fauna. One famous taxon, Spriggina (fig. 2A), in particular, recalls the elongated body shape of Kleptothule. However, Spriggina is also commonly referred to the Annelida and some authorities even question its animal nature.

Another taxon has yet to be formally described and is widely known simply as 'soft-bodied trilobite.' SBT has attained almost mythological status primarily because it hasn't been described, and is known to most researchers solely from an exceedingly poor photograph accompanying an article by James Gehling in the Memoirs of the Geological Society of India, vol. 20. The specimen looks like a trilobite, at least superficially: it has a semicircular head, segmented body, and trilobation.

Early Cambrian Arthropods

Whereas Naraoia is a middle Cambrian form, appearing long after the first 'proper' trilobites, the Polish fossil Liwia probably dates from the Tommotian Stage, prior to the first appearance of true trilobites.

The Chengjiang (China) taxon, Fuxanhuia, was a much simpler arthropod and may have been a lingering descendant of the arthropod stem group.

Mid-Cambrian forms ... Naroia, Tegopelte, etc.

Cuticularisation

It seems likely that the proto-trilobites were not heavily armoured like typical trilobites, perhaps more like the soft bodied trilobite-like organisms from the younger Cambrian lagerstätten (Chengjiang, Sirius Passet, Burgess Shale, etc.)

Cuticularised trilobites appear right at the end of the Cambrian explosion. However they obviously had ancestors, for their dictinctive trackways (such as the ichnogenus Cruziana) extend right down to the earliest Cambrian beds.

The Agnostina Problem

Are agnostids trilobites?

Early Evolution

Possible Ancestry

Genetic Studies

Calibrated gene analyses ...

Conclusion

References

Brasier, M.D. 1989: Towards a Biostratigraphy of the Earliest Skeletal Biotas. In Cowie, J.W. and Brasier, M.D. (eds.) The Precambrian-Cambrian Boundary, pp. 40-74. Clarendon Press.

Budd, Graham E. 1995: Kleptothule rasmusseni gen. et sp. nov.: an ?Olenellinid-Like Trilobite from the Sirius Passet Fauna (Buen Formation, Lower Cambrian, North Greenland). Transactions of the Royal Society of Edinburgh: Earth Sciences, 86: 1-12.

Edgecombe, Gregory D.; Ramsköld, Lars 1997: Phylogenetic Relationships of Trilobite-Allied Arachnata. Second International Trilobite Conference, Brock University, St. Catharines, Ontario.

Fortey, Richard A.; Owens, Robert M. 1997: Evolutionary History. In Whittington, H.B. et al. 1997: Treatise on Invertebrate Paleontology Part O Arthropoda 1 Trilobita, Revised, Volume 1: Introduction, Order Agnostida, Order Redlichiida. The Geological Society of America and The University of Kansas.

Shergold, John H. 1997: Explanatory Notes for the Cambrian Correlation Chart. In Whittington, H.B. et al. 1997: Treatise on Invertebrate Paleontology Part O Arthropoda 1 Trilobita, Revised, Volume 1: Introduction, Order Agnostida, Order Redlichiida. The Geological Society of America and The University of Kansas.

Wills, M.A.; Briggs, D.E.G.; Fortey, R.A. 1997: Evolutionary Correlates of Arthropod Tagmosis: Scrambled Legs. In Fortey, R.A.; Thomas R.H. (eds.): Arthropod Relationships. Systematics Association Special Volume Series 55.

Bibliography

Babcock, Loren E. (1990) Phylogenetic relationships among Cambrian trilobites. Geological Society of America Abstracts with Programs. vol. 22, no. 7, pp. 265.

Collins, Desmond H. (1985) A new Burgess Shale type fauna in the Middle Cambrian Stephen Formation on Mt. Stephen, British Columbia. Geological Society of America Abstracts with Programs. vol. 17, no. 7, pp. 550.

Dzik, J., and K. Lendzion (1988) The oldest arthropods of the East European platform: Lethaia, vol. 21, pp. 29-38.

Hou, Xianguang (1993) The arthropod Naraoia from the Lower Cambrian Chengjiang fauna. In Mikael Siverson (ed.), p. 12, Lundadagarna I; Historik geologi och paleontologi; III abstracts [Lundadagarna I; Historical geology and paleontology; III abstracts.] Lund Publications in Geology. vol. 109 University of Lund, Department of Geology. Lund, Sweden.

Lieberman, B.S. 2003: Taking the pulse of the Cambrian radiation. Integrative and Comparitive Biology 43: 229-237.

Robison, R. A. (1984) New occurrences of the unusual trilobite Naraoia from the Cambrian of Idaho and Utah. University of Kansas Paleontological Contributions Paper. no. 112, pp. 1-8.

Whittington, H. B. (1977) The Middle Cambrian trilobite, Naraoia, Burgess Shale, British Columbia. Philosophical Transactions of the Royal Society of London, Series B: Biological Sciences. vol. 280, no. 974, pp. 409-443.

Whittington, Harry B. (1985) Tegopelte gigas, a second soft-bodied trilobite from the Burgess Shale, Middle Cambrian, British Columbia. Journal of Paleontology. vol. 59, no. 5, pp. 1251-1274.

Zhang, Wen T., and Hou, Xian G. (1985) Preliminary notes on the occurrence of the unusual trilobite Naraoia in Asia. Gushengwu Xuebao, Acta Palaeontologica Sinica. vol. 24, no. 6, pp. 591-595.