Conciliterga

Reconstruction of Kuamaia lata, from Hou and Bergström (1997). Note that the reconstructed appearance of the eyes is probably erroneous, as explained below.

Belongs within: Artiopoda.

The Conciliterga are a group of soft-bodied arthropods related to trilobites known from the Lower and Middle Cambrian Burgess Shale and Chengjiang faunas.

The concilitergans: sitting next to trilobites
Published 20 October 2019

The last few decades have seen a vast increase in our understanding of life during the early Cambrian. Long one of the most famous groups of invertebrates of the Palaeozoic, the trilobites are now known to have shared their early environment with a number of related lineages that bore some resemblance in overall appearance but lacked their mineralisation of the exoskeleton. One such group was labelled by Hou & Bergström (1997) as the Conciliterga.

Reconstruction of Tegopelte gigas, copyright Marianne Collins.

Concilitergans are a group of flattened marine arthropods known from the early and middle Cambrian of a number of parts of the world, including North America, China and Australia. Most species were ovoid in shape (like a typical trilobite), tapering somewhat towards the rear and often ending in a point. An Australian species, Australimicola spriggi, was more elongate in form and ended in a pair of terminal spines. Some were quite sizable; one species, Tegopelte gigas, reached nearly a foot in length and was one of the largest known animals of its time. Concilitergans also resembled trilobites in possessing a more or less semi-circular head shield followed by a series of regular segments and often a final larger pygidial segment. Towards the front of the body, the segment boundaries were anteriorly reflexed (Paterson et al. 2012). In a number of species, the body segmentation was more prominent medially than laterally with the tergites overlapping slightly down the mid-line but not along the edges. A pair of antennae arose from the underside of the head near the front. In most species, with the exception of Australimicola, a pair of prominent teardrop-shaped bulges was also present dorsally near the front of the head. These bulges were interpreted as a pair of dorsal eyes by Hou & Bergström (1997) but re-interpreted by Edgecombe & Ramsköld (1999) as raised areas of the exoskeleton that provided accomodation for the actual eyes located on the underside of the head.

Phylogenetic analyses have confirmed a close relationship between concilitergans and trilobites (Edgecombe & Ramsköld 1999) and the two groups probably resembled each other in life-style. With their ovoid shape, flattened body and down-cast eyes, concilitergans were also not dissimilar in overall conformation to modern cockroaches and a comparison is tempting. Study of trackways attributed to Tegopelte, owing to their size and structure, indicated that it mostly walked with a slow, low gait but was also capable of adopting a higher, faster gait for quickly skimming across the sediment surface (Minter et al. 2012). It should be noted that while news reports on the latter study (like this one) repeatedly refer to Tegopelte as a predator, the original paper consistently describes it as a “predator or scavenger”. One can imagine concilitergans crawling along the sea-bed, picking up fragments of organic matter and scavenging on the remains of the less fortunate. Eventually, though, their lack of armament compared to their longer-surviving allies might have been their downfall as they were less prepared to deal with the diversification of active predators as the Cambrian progressed.

Systematics of Conciliterga

Synapomorphies (from Cotton & Braddy 2004, not including Australimicola): Dorsal bulge present in exoskeleton accomodating drop-shaped ventral eyes; anteromedian margin of cephalon notched, accomodating strongly sclerotised plate; distal lobe of exopod large, teardrop-shaped, with long attachment to proximal lobe; joints between posterior tergites functional, anterior ones variably fused. (Running data matrix from Legg et al. (2013) through TNT identifies only one synapomorphy for Conciliterga including Australimicola: mediolateral spines present.)

<==Conciliterga [Helmetiida]
    |--AustralimicolaLSE13
    `--+--+--SkioldiaLSE13
       |  `--+--Saperion glumaceum Hou et al. 1991LSE13, CB04
       |     `--Tegopelte Simonetta & Delle Cave 1975LSE13, CB04
       |          `--T. gigas Simonetta & Delle Cave 1975CB04
       `--+--Nathorstia Walcott 1912B95
          `--HelmetiidaeCB04
               |--Helmetia Walcott 1918CB04
               |    `--H. expansa Walcott 1918CB04
               |--Rhombicalvaria Hou 1987CB04, B95
               |    `--R. acantha Hou 1987CB04
               `--Kuamaia Hou 1987CB04, B95
                    |--K. lata Hou 1987CB04
                    `--K. muricata Hou & Bergström 1997CB04

*Type species of generic name indicated

References

[B95] Bousfield, E. L. 1995. A contribution to the natural classification of Lower and Middle Cambrian arthropods: Food-gathering and feeding mechanisms. Amphipacifica 2: 3–34.

[CB04] Cotton, T. J., & S. J. Braddy. 2004. The phylogeny of arachnomorph arthropods and the origin of the Chelicerata. Transactions of the Royal Society of Edinburgh: Earth Sciences 94: 169–193.

Edgecombe, G. D., & L. Ramsköld. 1999. Relationships of Cambrian Arachnata and the systematic position of Trilobita. Journal of Paleontology 73 (2): 263–287.

Hou X. & J. Bergström. 1997. Arthropods of the Lower Cambrian Chengjiang fauna, southwest China. Fossils and Strata 45: 1–116.

[LSE13] Legg, D. A., M. D. Sutton & G. D. Edgecombe. 2013. Arthropod fossil data increase congruence of morphological and molecular phylogenies. Nature Communications 4: 2485.

Minter, N. J., M. G. Mángano & J.-B. Caron. 2012. Skimming the surface with Burgess Shale arthropod locomotion. Proceedings of the Royal Society of London Series B—Biological Sciences 279: 1613–1620.

Paterson, J. R., D. C. García-Bellido & G. D. Edgecombe. 2012. New artiopodan arthropods from the Early Cambrian Emu Bay Shale Konservat-Lagerstätte of South Australia. Journal of Paleontology 86 (2): 340–357.

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