Nectaspida (Nektaspida)

Abstract

The Nectaspida is a group of arthropods, closely related to the widely-known trilobites. For the purposes of this discussion, the taxon is considered to rank as an order within an informal "trilobite clade." They have a trilobite-like arrangement of cephalic limbs, a large pygidium and, in the Liwiidae, thoracic segments. On the other hand, they lack a calcified cuticle or evidence of sutures. The prevailing view is that the Nectaspida are a sister group to the clade which includes the calcified trilobites.

Keywords: evolution, fossil, Arthropoda, Trilobita, Nectaspida, Naraoiidae, Naraoia, Misszhouia, Liwiidae, Liwia, Buenaspis, Soomaspis, Tariccoia

Introduction

The Nectaspida is an order of arthropods, closely related to the widely-known trilobites. Indeed, sometimes the Nectaspida is regarded as an order residing (albeit uneasily) within the Trilobita.

On the one hand they have a trilobite-like arrangement of cephalic limbs, a large pygidium and (in Tariccoia) nonfulcrate thoracic segments. Gould (1989) also mentions the structure of the biramous limbs as another feature in common with true trilobites. On the other, they lack a calcified cuticle, evidence of sutures, and the cephalic shield is shorter than the pygidium – the 'macropygous' condition that is rare among trilobites, and confined exclusively to highly evolved forms (among the Asaphidae and Styginidae). Despite their superficial resemblance, they do not appear to be closely related to the Agnostida.

The prevailing view is that the Nectaspida are a sister group to the clade which includes the calcified trilobites. The revised Treatise adopts the tactic of treating the naraoiids as a family within the class Trilobita, order "uncertain," thereby acknowledging a systematic position within the class, but allowing the formal definition of Trilobita to be based upon the characters of calcified forms (Fortey 1997, p. 294).

At this site, the Nectaspida are not considered to belong within the class Trilobita, but are accommodated instead within an extended "trilobite clade" concept (see below; also fig. 1 in the Arthropoda page).

The order comprises two families: the Naraoiidae and the Liwiidae.

Morphology

Phylogeny and Evolution

Affinities

Fossil Record

Origins

Evolution

Extinction

...

> By now, I think the idea of trilobitomorphs should probably be put to > rest as a convenient storage locker for primitive arthropods that > share a set of primitive characteristics (and therefore shouldn't > stand alone as a clade defined by derived characters). When you look > at the 1959 Treatise and all the strange beasties that were lumped > into Trilobitomorpha, you can see what I mean! >
> Ahhh to one day stroll atop the myriad exfoliating layers of the > Burgess Shale! Yes you are right, I am yearning to one day share your > experience of the Walcott Quarry... > > I would love to learn more about this Silurian Naraoiid! Any pictures?

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ABSTRACTS
10:15am An Unusual Late Silurian arthropod fossil from southern Ontario - delayed fall-out from the Cambrian explosion? David M. Rudkin, Jean-Bernard Caron, and Stuart Milliken.
The families Liwiidae and Naraoiidae, which together include fewer than ten species of non-calcified, Cambro-Ordovician trilobitomorph arthropods, have recently been assigned to a resurrected Order Nektaspida. Nektaspids, along with helmetiids and tegopeltids, comprise a small group of early Palaeozoic euarthropods lacking calcified exoskeletons that figure prominently in discussions on the origin and composition of the Class Trilobita. Here we describe the first known post-Ordovician naraoiid, based on a single specimen from the Upper Silurian (Pridolian) Bertie Formation (Williamsville Member) of southern Ontario. The extension of the range of the Naraoiidae, and the Nektaspida, from the Late Ordovician to the Late Silurian (some 25 million years) has both ecologic and phylogenetic implications.
David M. Rudkin, Department of Palaeobiology; Jean-Bernard Caron, Department of Zoology, University of Toronto;
Stuart Milliken, Brantford, Ontario.
An Unusual Late Silurian Arthropod Fossil from Southern Ontario - Delayed Fall-out from the Cambrian Explosion?

Babcock, L. E., and W. T. Chang. 1997. Comparative taphonomy of two nonmineralized arthropods: Naraoia
(Nektaspida; Early Cambrian, Chengjiang Biota, China) and Limulus (Xiphosurida; Holocene, Atlantic Ocean).
Bulletin of National Museum of Natural Science, 10: 233-250.

(Loren E. Babcock, Associate Professor of Geological Sciences, The Ohio State University)

Systematics

Phylum Arthropoda von Siebold & Stannius 1845

Tactopoda Budd 2001

Supersubphylum Euarthropoda Lankester 1904

Subphylum Arachnomorpha Heider 1913

"Trilobite clade" Cotton & Braddy 2000

Størmer (1944) proposed the name Trilobitomorpha to include the Trilobita and a miscellaneous collection of other Burgess Shale arthropods which he later (Størmer 1959) named the Trilobitoidea.

The name Trilobitomorpha is not much used today, although a case could be made that it is available for naming the informal "trilobite clade" concept.

Order Nectaspida Raymond 1920

Type: Naraoia Walcott 1912

Original Diagnosis: xxx

Description: Small arthropods with cephalic and abdominal shield; subelliptical, trilobate body; cephalon with small sessile eyes; post-abdomen short and narrow with ?single segment and short lanceolate telson bearing lateral spines; first antennae short with numerous setiferous joints; other appendages probably trilobitic. (After Størmer 1959, p. O30.)

[expand Edgecombe & Ramskold's table of characters]

Stratigraphic Range: Lower Cambrian (Sirius Passet) to Ordovician (Soom Shale)

Discussion: xxx

[? Family ? Orientellidae - find reference]

Family Naraoiidae Walcott 1912

Type: Naraoia Walcott 1912

Description: As above.

Discussion: Walcott originally published this name with the spelling Naraoidae; the name was corrected by Størmer (1959, p. O30).

Genus Naraoia Walcott 1912

Fig. 2.

Type: Naraoia compacta Walcott 1912

Original Diagnosis: xxx

Description: .

Stratigraphic Range: Lower Cambrian (Chengjiang) to Middle Cambrian (Burgess Shale)

Discussion: "When Naraoia was first described, its similarity to an early meraspid trilobite (with cephalon and pygidium, but no thoracic segments) was noted, and was part of an ontogenetic argument for including Naraoia as a 'soft-bodied trilobite'" (from Sam Gon's web page).

Species: Naraoia compacta Walcott 1912; Naraoia halia Simonetta & Delle Cave 1975 (jnr. syn. of Naraoia compacta); Naraoia pammon Simonetta & Delle Cave 1975 (jnr. syn. of Naraoia compacta); Naraoia spinifer Walcott 1931

Genus Misszhouia Chen and Zhou 1997

Type: Misszhouia [Authority]

Original Diagnosis: xxx

Description: .

Stratigraphic Range: Lower Cambrian (Chengjiang)

Discussion: .

Family Liwiidae ? author date

Type: Liwia Dzik and Lendzion 1988

Discussion: "The family Liwiidae is comprised of naraoiid-like species that are distinguished from naraoiids in bearing thoracic tergites. Some, like Buenaspis..., are remarkably similar in general form to some advanced trilobites (e.g., Nileidae), but as in naraoiids, are primarily eyeless, and lack a well-defined axis (in addition to being uncalcified). The Liwiidae are geographically diverse, being recorded from a variety of sites outside of either the Burgess Shale or Chengjiang deposits, including one other Cambrian konservat-lagerstätte, Sirius Passet, North Greenland (where Buenaspis was found). Despite a diversity of form, the Liwiidae are considered most closely related to the Naraoiidae. However it remains to be seem whether the general similarities reflect true homologies" (from Sam Gon's web page).

Genus Liwia Dzik and Lendzion 1988

Type: [Naraoiidae] [Authority]

Original Diagnosis: xxx

Description: .

Stratigraphic Range: Lower to Middle Cambrian (Poland)

Discussion: .

Genus Buenaspis (?)

Type: Buenaspis forteyi [Authority]

Original Diagnosis: xxx

Description: .

Stratigraphic Range: Lower Cambrian (Sirius Passet)

Discussion: .

Genus Soomaspis Fortey & Theron 1995

Type: Soomaspis splendida Fortey & Theron 1995

Original Diagnosis: xxx

Description: .

Stratigraphic Range: Ordovician

Discussion: .

Genus Tariccoia Hammann et al. 1990

Type: Tariccoia arrusensis Hammann et al. 1990

Original Diagnosis: xxx

Description: .

Stratigraphic Range: Ordovician (Sardinia, Italy)

Discussion: .

Conclusion

xxx

Further Information

xxx

Also see the bibliography, below.

References

Budd, Graham E. 2001b: Tardigrades as "Stem Group" Arthropods: The Evidence from the Cambrian Fauna. Zoologischer Anzeiger 240, 265-279.

Chen, J.-Y.; Zhou, G.-Q. 1997: Biology of the Chengjiang Fauna. In Chen, J.-Y.; Chen, Y.-N.; Van Iten, H. (eds.) 1997: The Cambrian Explosion and the Fossil Record. Bulletin of the National Museum of Natural Science, 10. Taichung, pp. 11-105.

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

Edgecombe & Ramsköld 1999

Fortey, R.A.; Theron, J.N. 1995: A New Ordovician Arthropod, Soomaspis, and the Agnostid Problem. Palaeontology 37(4): 841-861.

Fortey, R.A. 1997: Classification. 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.

Gould, Stephen Jay 1989: Wonderful Life. Penguin. 347 pp.

Hammann, W.; Laske, R.; Pillola, G.L. 1990: Tariccoia arrusensis, New Genus New Species, an Unusual Trilobite-Like Arthropod: Rediscovery of the "Phyllocarid" Beds of Taricco (1922) in the Ordovician "Puddinga" Sequence of Sardinia [Italy]. Bollettino della Societa Paleontologica Italiana, 29(2): 163-178.

Størmer, L. 1944: On the Relationships and Phylogeny of Fossil and Recent Arachnomorpha. A comparative study on Arachnida, Xiphosurida, Eurypterida, Trilobita, and other Fossil Arthropoda. Skrifter Utgitt av Det Norske Videnskaps-Academi I Oslo. I. Matematisk-Naturvidenskapelig Klasse 5: 1-158.

Størmer, Leif 1959: Trilobitoidea. In Moore, Raymond C. (ed.) 1959: Treatise on Invertebrate Paleontology: Part O. Arthropoda 1, pp. O23-37.

Walcott, C.D. 1912: Middle Cambrian Branchiopoda, Malacostraca, Trilobita and Merostomata. Smithsonian Miscellaneous Coll. Publ. 2051, v. 57: 145-228.

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, 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.

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.

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.

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, 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.