Merodont Hinge Ostracode

Amphidont hinge

Amphidont hinge

Figure 14.22 Descriptive terminology of the ostracode animal (a), including muscle scars (b) and hinge structures (c). (Based on Armstrong & Brasier 2005.)

Articulatory structures are variably developed along the hinge line. Three main types of hinge are known (Fig. 14.22c). Adont hinges lack teeth but have a long median element on the right valve that fits a socket on the left valve. The merodont hinge has long striated terminal elements on the right valve fitting respective sockets on the left valve. Amphidont hinges have short terminal elements with well-developed teeth on the right valve.

The carapace is perforated by canals holding setae that communicate with the exterior. The body is suspended within the carapace, attached by muscles. It is equipped with seven pairs of appendages, three in front of the mouth and four behind. The appendages are specialized, acting as sensory organs, limbs for the capture and processing of food; moreover they allow locomotion and general cleaning and housekeeping within the carapace. The animal has a digestive system, sophisticated genitalia and a nervous system; commonly a median eye is located behind a tubercle.

Sexual dimorphism is common and often reflected in the ostracode carapace (Fig. 14.23). Males commonly have a greater length : height ratio than the females, whereas in some benthic Paleozoic ostracodes the female had a brood pouch in the carapace wall. Females are often called heteromorphs while the males, lacking the brood pouch, are the tecnomorphs.

Ostracodes appeared first during the Early Cambrian. The archaeocopids were a bizarre group of large taxa with distinctive appendages quite different from more typical ostra-codes. The group was short lived, disappearing during the latest Cambrian to earliest Ordovi-cian. The later history of the group shows a number of clear trends: evolution of small size, simpler muscle systems and shorter hinge lines; the functional significance of these changes is not immediately obvious.

Large Leperditicopida and Palaeocopida appeared during the Ordovician, dominating ostracode faunas until the Devonian, when deep-water limestones were locally characterized by the small spiny myocopids. Many new groups appeared near the end of the Paleozoic, but hitherto important groups such as the palaeocopids eventually disappeared in the Triassic after a decline during the Permian.

Figure 14.23 Some ostracode genera: (a) left valve of a male living Limnocythene showing details of appendages (x30); (b, d) left valves of female and male heteromorphs of Beyrichia (Silurian) (x18); (c, e) external and internal views of the left valve of living Patagonacythene (x30); (f) palaeocopid Kelletina (Carboniferous) (x30). (Courtesy of David Siveter.)

Figure 14.23 Some ostracode genera: (a) left valve of a male living Limnocythene showing details of appendages (x30); (b, d) left valves of female and male heteromorphs of Beyrichia (Silurian) (x18); (c, e) external and internal views of the left valve of living Patagonacythene (x30); (f) palaeocopid Kelletina (Carboniferous) (x30). (Courtesy of David Siveter.)

Although Early Jurassic ostracode assemblages are of low diversity the platycopines, cypridaceans and cytheraceans radiated steadily during the Jurassic. By the Cenozoic, the cypridaceans dominated lake environments whereas the cytheraceans were established in marine settings.

Any doubts that real ostracodes did not actually exist in the Paleozoic have been dispelled by some remarkable soft-part preservation, digitally reconstructed from material from the Silurian Lagerstätten at Hereford, England (Siveter et al. 2003). The precise details of the animal's morphology, down to the enormous male copulatory organ, confirm the ostracode identity of the specimen; it seems even very similar to living myodocopids. Lagerstätten, such as the Hereford biota, have provided a remarkable series of windows on arthropod evolution through time, right back to the Cambrian (Box 14.8).

Box 14.8 Exceptional arthropod-dominated faunas

Arthropods are common in a number of Lagerstätten deposits, suggesting that they were much more diverse in the past than the regular fossil record suggests. More than 40% of the animals described from the Mid Cambrian Burgess Shale are arthropods. Apart from typical trilobites such as Olenoi-des there are also soft-bodied taxa, for example Naraoia and the larger Tegopelte. However the commonest and first discovered Burgess arthropod is the elegant, trilobitomorph Marrella. The fauna contains many other arthropods such as Canadaspis, probably the first phyllocariid crustacean. There are many unique arthropods in the fauna that are difficult to classify: Anomalocaris, Emeraldella, Leanchoilia, Odaraia, Sidneyia and Yohoia are not easily aligned with established groups. The small and bizarre Hallucigenia was probably an onychophoran, while Sanctacaris was a stem-group che-licerate. The slightly older faunas at Chengjiang, South China, and Sirius Passet, North Greenland, have also yielded a spectacular array of enigmatic arthropod faunas, further contributing to our knowledge of the Cambrian explosion.

Calcareous concretions (or orsten) from the Upper Cambrian of the Baltic area have yielded a phosphatized fauna dominated by stem- and crown-group crustaceans and ostracodes together with agnostid trilobites. Many of these diverse forms were minute, living in microhabitats within or on the muds of the Cambrian seas (Fig. 14.24). These faunas are quite distinct from the earlier Burgess Shale-type faunas and provide a window on a habitat occupied by a wide range of body plans on a microscopic scale, possibly adapted to life below the sediment-water interface. Recent work by Dieter Walossek (Ulm Universität) on, for example, remarkably preserved complete ontogenetic series of Rehbachiella from orsten has helped elucidate the life cycle, habits and functional morphology of these animals. Moreover some of the most remarkable of all the arthropods, the pycnogonids, or sea spiders, are now known from the Cambrian orsten banks, the Silurian Herefordshire fauna and the Devonian Hunsrückschiefer (Budd & Telford 2005).

The Early Devonian faunas of the Hunsrückschiefer of the German Rhineland contain beautifully preserved phyllocariid crustaceans such as Nahecaris, together with a number of other arthropods apparently lacking living counterparts such as Cheloniellon (a large, ovoid creature with a pair of antennae, nine segments and conical telson) or Mimetaster, which is similar to Marrella from the Burgess Shale.

Figure 14.24 Composite of Mid Cambrian and Late Cambrian forms and reconstructions. Lower case letters (a-d), larvae; upper case letters (A-D), adult stages. Distance of sinking into the zone of preservation: 1, short distance; 2, long distance. (Redrawn from Walossek, D. 1993. Fossils and Strata 32.)

Figure 14.24 Composite of Mid Cambrian and Late Cambrian forms and reconstructions. Lower case letters (a-d), larvae; upper case letters (A-D), adult stages. Distance of sinking into the zone of preservation: 1, short distance; 2, long distance. (Redrawn from Walossek, D. 1993. Fossils and Strata 32.)

The Late Carboniferous Mazon Creek fauna of Illinois occurs across two facies associated with a deltaic system. The marine, Essex fauna developed on the delta front and is dominated by fishes, including coelacanths and some of the earliest lampreys. However, huge crustaceans are found together with the weird Tullimonstrum whose affinities are uncertain but might be a heteropod gastropod. The non-marine Braidwood assemblage is a diverse array of arthropods including 140 species of insects together with centipedes, millipedes, scorpions and spider-like arachnids. The fauna, together with a flora of over 300 species of land plant, occupied a lowland swamp milieu between the sea and coal forests. Shrimps and ostracodes apparently inhabited ponds within the swamps.

More recent terrestrial assemblages such as the Montsech fauna from the Lower Cretaceous of northeast Spain have yielded new information on the evolution of spiders. Paul Selden (University of Kansas) has described three web-weaving species equipped to attack an abundant insect life inhabiting settings around coastal lagoons.

It is clear from these extraordinarily well-preserved faunas that numerous ancient communities, marine and non-marine, were dominated by arthropods, just as today.

Review questions

1 The spectacular arthropod faunas of Burgess, Chengjiang and Sirius Passet suggest an early diversification of these ecdysozoan taxa. Was this really evolution's "big bang" or are these arthropods just too weird to comprehend when compared to modern faunas?

2 Trilobites were an integral part of the Paleozoic fauna for over 200 million years yet they finally became extinct at the end of the Permian. What sorts of animals filled their niches in the Modern evolutionary fauna?

3 Trilobites have featured in a number of evolutionary schemes, some showing gradualistic trends and others showing punctuated trends. Are these different patterns correlated with different groups of trilobite or perhaps to their life environments?

4 Insects are and probably were the most numerically dominant life of Earth. Why have they a relatively poor fossil record?

5 Exceptionally-preserved biotas occur sporadically throughout the Phanerozoic. Arthropods are usually well represented.

Briggs, D.E.G., Thomas, A.T. & Fortey, R.A. 1985. Arthropoda. In Murray, J.W. (ed.) Atlas of Invertebrate Macrofossils. Longman, London, pp. 199-229. (A useful, mainly photographic review of the group.)

Clarkson, E.N.K. 1998. Invertebrate Palaeontology and Evolution, 4th edn. Chapman and Hall, London. (An excellent, more advanced text; clearly written and well illustrated.)

Fortey, R. 2000. Trilobite: Eyewitness to Evolution. HarperCollins Publishers, London. (Fascinating personal voyage of discovery.)

Gould, S.J. 1989. Wonderful Life. The Burgess Shale and the Nature of History. Hutchinson Radius, London. (Inspirational analysis of evolution's "big bang".)

Robison, R.A. & Kaesler, R.L. 1987. Phylum Arthrop-oda. In Boardman, R.S., Cheetham, A.H. & Rowell, A.J. (eds) Fossil Invertebrates. Blackwell Scientific Publications, Oxford, UK, pp. 205-69. (A comprehensive, more advanced text with emphasis on taxonomy; extravagantly illustrated.)

Whittington, H.B. 1985. The Burgess Shale. Yale University Press, New Haven, NJ. (Classic description of the Burgess Shale and its fauna.)

References

Armstrong, H.A. & Brasier, M.D. 2005. Microfossils, 2nd edn. Blackwell Publishing, Oxford, UK.

Babcock, L.E. 1993. Trilobite malformations and the fossil record of behavioral symmetry. Journal of Paleontology 67, 217-29.

Barrande, J. 1852. Systèm Silurien du Centre de la Bohème. Recherches Paléontologiques, Vol. 1, Planches, Crustacés, Trilobites. Prague and Paris.

Briggs, D.E.G., Fortey, R.A. & Wills, M.A. 1993. How big was the Cambrian evolutionary explosion? A taxonomic and morphological comparison of Cambrian and Recent arthropods. In Lees, D.R. & Edwards, D. (eds) Evolutionary Patterns and Processes. Linnean Society of London, London, pp. 33-44.

Bruton, D.L. & Haas, W. 2003. Making Phacops come alive. Special Papers in Palaeontology 70, 331-47.

Why?

Further reading

Budd, G.E. & Telford M.J. 2005. Along came a sea spider. Nature 437, 1099-102.

Clarkson, E.N.K. 1979. The visual system of trilobites. Palaeontology 22, 1-22.

Clarkson, E.N.K. 1998. Invertebrate Palaeontology and Evolution, 4th edn. Chapman and Hall, London.

Clarkson, E.N.K., Ahlberg, A. & Taylor, C.M. 1998. Faunal dynamics and microevolutionary investigations in the Cambrian Olenus Zone at Andrarum, Skáne, Sweden. GFF 120, 257-67.

Cotton, T.J. & Braddy, S.J. 2004. The phylogeny of arachnomorph arthropods and the origin of the Che-licerata. Transactions of the Royal Society of Edinburgh: Earth Sciences 94, 169-93.

Edgecombe, G.D. & Ramskold, L. 1999. Relationships of Cambrian Arachnata and the systematic position of Trilobita. Journal of Paleontology 73, 263-87.

Fortey, R.A. & Owens, R.M. 1990. Trilobites. In McNamara, K.J. (ed.) Evolutionary Trends. University of Arizona Press, Tucson, pp. 121-42.

Glenner, H., Thomsen, P.F., Hebsgaard, M.B., Sorensen, M.V. & Willerslev, E. 2006. The origin of insects. Science 314, 1883-4.

Gould, S.J. 1989. Wonderful Life. The Burgess Shale and the nature of history. W.W. Norton & Co., New York.

Fortey, R.A. & Chatterton, B. 2003. A Devonian trilo-bite with an eyeshade. Science 301, 1689.

Johnson, E.W., Briggs, D.E.G., Suthren, R.J., Wright, J.L. & Tunnicliff, S.P. 1994. Non-marine arthropod traces from the subaerial Ordovician Borrowdale Volcanic Group, English Lake District. Geological Magazine 131, 395-406.

Labandeira, C.C. 2006. The four phases of plant-arthropod associations in deep time. Geologica Acta 4, 409-38.

Lauridsen, B.W. & Nielsen, A.T. 2005. The Upper Cambrian trilobite Olenus at Andrarum, Sweden: a case of interative evolution? Palaeontology 48, 1041-56.

Lin Jih-Pai, Gon III, S.M., Gehling, J.G. et al. 2006. A Parvancorina-like arthropod from the Cambrian of South China. Historical Biology 18, 33-45.

MacNaughton, R.B., Cole, J.M., Dalrymple, R.W., Braddy, S.J., Briggs, D.E.G. & Lukie, T.D. 2002. First steps on land: Arthropod trackways in Cam-brian-Ordovician eolian sandstone, southeastern Ontario. Geology 30, 391-4.

McKinney, F.K. 1991. Exercises in Invertebrate Paleontology. Blackwell Scientific Publications, Oxford, UK.

Owen, A.W. 1985. Trilobite abnormalities. Transactions of the Royal Society of Edinburgh: Earth Sciences 76, 255-72.

Penalver, E., Grimaldi, D.A. & Declos, X. 2006. Early Cretaceous spider web with its prey. Science 312, 1761.

Penalver, E. & Grimaldi, D. 2006. Assemblages of mammalian hair and blood-feeding midges (Insecta: Diptera: Psychodidae: Phlebotominae) in Miocene amber. Transactions of the Royal Society of Edinburgh: Earth Sciences 96, 177-95.

Siveter, D.J., Sutton, M.D., Briggs, D.E.G. & Siveter, D.J. 2003. An ostracode crustacean with soft parts from the Lower Silurian. Science 302, 174951.

Wilson, H.M. & Anderson, L.I. 2004. Morphology and taxonomy of Paleozoic millipedes (Diplopoda: Chilognatha: Archipolypoda) from Scotland. Journal of Paleontology 78, 169-84.

Wootton, R.J., Kukalova-Peck, J., Newman, D.J.S. & Muzon, J. 1998. Smart engineering in the mid-Carboniferous: how well could Palaeozoic dragon-flies fly? Science 282, 749-51.

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