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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.

 
 

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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.

That they are extinct prevents molecular analysis, but cladistic analyses based on morphological features (e.g. Wills et al. 1998) place trilobites, together with Naraoia and Molaria, and adjacent to several other Burgess Shale forms, within the Arachnomorpha.
"The groups phylogenetically closest to the Trilobita are the Helmetiidae (Helmetia, Kuamaia, Rhombicalvaria), Tegopeltidae (Tegopelte, Saperion), and Naraoiidae (Naraoia, Liwia, Tariccoia, Soomaspis, Misszhouia). These four clades share uniquely derived characters, in particular their style of thoracic/pygidial tagmosis, indicating that they form a monophyletic group within the Arachnata. Some detailed characters present in these taxa, such as the division of the exopod into proximal and distal lobes (the former bearing lamellar setae and the latter fringed with bristles) and hinging of the exopod along the basis, are more generally distributed, also being present in xandarellids and other Cambrian forms, such as Emeraldella. The bilobate exopod appears to be very general within Arachnata (e.g., present in Sanctacaris). These appendage characters are, however, lacking in Agnostus. Evaluation of agnostid relationships should include all available evidence, without ad hoc dismissal of taxa or characters. Within the trilobite-allied clade, several features (including an anterior sternite and a notched anteromedian cephalic margin; drop-shaped ventral eyes set in an exoskeletal bulge) support the Helmetiidae and Tegopeltidae as sister taxa. Naraoiid monophyly is well-supported, forcing notions of a "nektaspid" grade as "soft-bodied trilobite" ancestors to be abandoned. The three-taxon statement (Naraoiidae) (Trilobita) (Helmetiidae + Tegopeltidae) remains unresolved, with different characters favouring each resolution: the style of hypostome attachment and edge-to-edge tergite articulation are shared by helmetiids and trilobites, whereas an ovate pre-hypostomal sternite that bears paired "frontal organs" is apparently unique to naraoiids, helmetiids, and tegopeltids. New data on larval appendages for Naraoia reveal major, possibly metamorphic, morphological changes in ontogeny that should be considered in determining character polarity in the the trilobite-allied arachnates" (Edgecombe & Ramsköld 1997, Abstract).

Early Trilobite Successions

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

Siberia Northern Poland Scandanavia Anti-Atlas (Morocco) East California East Yunnan (China)
  Schmidtiellus, ?Fallotaspis, Holmia Holmia mobergi, ?Wanneria lundgreni   Daguinaspis

Judomia

Fallotaspis

 

Yunnanocephalus, Malungia, Eoredlichia

Wutingaspis, Tsunyidiscus (=Mianxiandiscus)

Archaeaspis, Pagetiellus anabarus  
Holmia inusitata
    Parabadiella
Fallotaspis     Daguinaspis    
Profallotaspis jakutensis  
Schmidtiellus mickwitzi
     
      Fallotaspis tazemmourtensis    
      Eofallotaspis    
Shim1Pel.gif (799


            bytes) Colour Key       
Order Agnostida
     Suborder Eodiscina
Order Redlichiida
     Suborder Olenellina

     Suborder Redlichiina

Table 1: Approximate correlation of the first appearing trilobite faunas from six important basal Cambrian regions. The earliest occurences are shown at the bottom of the table; the youngest at the top. (Adapted from Brasier 1989 and Shergold 1997.)

As shown, the earliest forms are probably those from Siberia, Scandanavia and Morocco, particularly Eofallotaspis tioutensis Sdzuy, Eofallotaspis prima Sdzuy, followed by Fallotaspis tazemmourtensis, Schmidtiellus mickwitzi, and Profallotaspis jacutensis Repina.

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

The following inventory of characteristics is proposed (Fortey and Owens 1997, p. 249) as the primitive condition for trilobites: body with low dorso-ventral convexity, typically oval; cephalon with genal spines present; eyes long and extending close to the glabella anteriorly; thorax composed of a large number of spinose segments, of which the third may be macropleural, nonfulcrate articulation allowing “considerable arching of the body, if not complete enrolment”; pygidium small but appearing to comprise more than one segment.

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.

(A) thmFallotaspisDiagram.jpg (15508


        bytes)     (B) Kleptothule rasmusseni (2916


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Fig. 1: (A) Fallotaspis typica (Hupe 1953) – One of the earliest occurring trilobites from the Lower Cambrian of Morocco. Overall length approximately 5 cm. After Moore 1959, fig. 133(5).

(B) Kleptothule rasmusseni Budd 1995 – An unusual trilobite-like arthropod from the Lower Cambrian Sirius Passet fauna of Greenland. Overall length is approximately 3 cm. After Budd 1995, fig. 2.

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'

cf. Parvancorina and Naraoia (Fortey et al. 1996)

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.

(A) Spriggina floundersi (37177


        bytes)     (B) <SBT?>

Fig. 2: (A) Spriggina floundersi Glaessner – An Ediacaran from the Vendian Pound Quartzite of Ediacara, South Australia. Overall length about 10 cm. Specimen from the Yale collection (YPM 63257). [Image courtesy of the Peabody Museum of Natural History, Yale University.]

(B) try to get a usable image of Gehling's 'soft bodied trilobite'

Early Cambrian Arthropods

Naraoia ...

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.

 
(A) Naraoia compacta (34386


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Fig. 3: (A) Naraoia compacta Walcott – A Burgess Shale (Middle Cambrian) specimen from the Yale collection (YPM 5869). Length approximately 3 cm. [Image courtesy of the Peabody Museum of Natural History, Yale University.]

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.


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