Paradoxides spinosus - fossil trilobites from the Cambrian of Bohemia. (YPM 72967, Yale University's Peabody Museum of Natural History, New Haven, Connecticut, USA)
Trilobites are extinct marine arthropods. They first appear in Lower Cambrian rocks and the entire group went extinct at the end of the Permian. Trilobites had a calcitic exoskeleton and nonmineralizing parts underneath (legs, gills, gut, etc.). The calcite skeleton is most commonly preserved in the fossil record, although soft-part preservation is known in some trilobites (Examples: Burgess Shale and Hunsruck Slate). Trilobites had a head (cephalon), a body of many segments (thorax), and a tail (pygidium). Molts and carcasses usually fell apart quickly - most trilobite fossils are isolated parts of the head (cranidium and free cheeks), individual thoracic segments, or isolated pygidia. The name "trilobite" was introduced in 1771 by Johann Ernst Immanuel Walch and refers to the tripartite division of the trilobite body - it has a central axial lobe that runs longitudinally from the head to the tail, plus two side lobes (pleural lobes).
Seen here are paradoxidid trilobite exoskeletons from the Jince area (pronounced "In-Say") in Bohemia. Paradoxidids were some of the largest trilobites ever. They are relatively common large trilobites in Middle Cambrian rocks in many parts of the world. About 150 species and subspecies names are available for Paradoxides (sensu lato), which includes some junior synonyms (subjective & objective) and some homonyms. Generic-level taxonomy of paradoxidid trilobites continues to be in a state of confusion, despite the family treatment by Dean & Rushton (1997) (see also, e.g., comments by Fletcher et al., 2005). Several genus-level or subgenus-level names are used by various authors to refer to groups of Paradoxides-like species that may or may not be morphologically distinctive (Examples: Paradoxides Brongniart, 1822, Bucephalites Thompson, 1834, Hydrocephalus Barrande, 1846, Phlysacium Corda in Hawle & Corda, 1847, Phanoptes Corda in Hawle & Corda, 1847, Plutonia, Hicks, 1871, Plutonides Hicks, 1895, Eccaparadoxides Šnajdr, 1957, Acadoparadoxides Šnajdr, 1957, Vinicella Šnajdr, 1957, Eoparadoxides Solovev, 1969, Baltoparadoxides Šnajdr, 1986, Macrocerca Pillet in Courtessole et al., 1988, Rejkocephalus Kordule, 1990, Mawddachites Fletcher, 2007; some of these are undisputed junior synonyms of other names on the list).
Characters of the cephalon/cranidium of paradoxidid trilobites are often used for defining new species. This is standard practice for most trilobites if the complete exoskeleton is unknown. Unfortunately, the paradoxidid head does not have consistently diagnostic features for species distinctions. Ideally, paradoxidid trilobite species should be defined based on all characters of the cephalon, thorax, and pygidium. Typically, only the cranidium is represented in a collection. However, the pygidium appears to be the most diagnostic sclerite for distinguishing the numerous species of Paradoxides (sensu lato). Pygidial morphology does vary somewhat within paradoxidid species, but its overall construction does seem more taxonomically useful than cranidial characters.
Classification: Animalia, Arthropoda, Trilobita, Polymerida, Paradoxididae
Stratigraphy: Jince Formation, lower Middle Cambrian
Locality: vicinity of Jince, Stredocesky Region, Bohemia, Czech Republic
----------------
References cited:
Barrande, J. 1846. Notice Préliminaire sur le Système Silurien et les Trilobites de Bohême. Leipzig, Germany. J.B. Hisrschfeld. 97 pp.
Boeck, C. 1828 (not 1827). Notitser til Læren onm Trilobiterne. Magazin for Naturvidenskaberne 8: 11-44, pl. 2.
Brongniart, A. & A.-G. Desmarest. 1822. Histoire Naturelle des Crustacés Fossiles. Paris. F.-G. Levrault, Libraire. 154 pp. 11 pls.
Courtessole, R., J. Pillet & D. Vizcaino. 1988. Stratigraphie et Paleontologie du Cambrien Moyen Greseux de la Montagne Noire (Versant Meridional). Carcassonne, France. 55 pp. 8 pls.
Dean, W.T. & A.W.A. Rushton. 1997. Superfamily Paradoxidoidea. pp. 470-481 in Treatise on Invertebrate Paleontology, Part O, Arthropoda 1, Trilobita, Revised, volume 1: Introduction, Order Agnostida, Order Redlichiida. Boulder, Colorado & Lawrence, Kansas. Geological Society of America & University of Kansas.
Fletcher, T.P., G. Theokritoff, G.Stinson Lord, & G. Zeoli. 2005. The early paradoxidid harlani trilobite fauna of Massachusetts and its correlatives in Newfoundland, Morocco, and Spain. Journal of Paleontology 79: 312-336.
Fletcher, T.P. 2007. Correlating the zones of 'Paradoxides hicksii' and 'Paradoxides davidis' in Cambrian Series 3. Memoirs of the Association of Australasian Palaeontologists 33: 35-56.
Hawle, I. & A.J.C. Corda. 1847. Prodrom einer Monographie der Böhmischen Trilobiten. Prague. J.G. Calve'sche Buchhandlung. 176 pp. 7 pls.
Hicks, H. 1871. Descriptions of new species of fossils from the Longmynd rocks of St. David’s. Quarterly Journal of the Geological Society of London 27: 399-402, pls. 15-16.
Hicks, H. 1895. On the genus Plutonides (non Plutonia) from the Cambrian rocks of St. David’s. Geological Magazine, Series 4 2: 230-231.
Kordule, V. 1990. Rejkocephalus, a new paradoxid genus from the Middle Cambrian of Bohemia (Trilobita). Vestník Ústredniho Ústavu Geologického 65: 55-60, 2 pls.
Šnajdr, M. 1957. O novych trilobitech z ceskeho kambria. Vestník Ústredniho Ústavu Geologického 32: 235-244, 2 pls.
Šnajdr, M. 1986. Two new paradoxid trilobites from the Jince Formation (Middle Cambrian, Czechoslovakia). Vestník Ústredniho Ústavu Geologického 61: 169-174, pls. 1-2.
Solovev, I.A. 1969. Novye vidy Paradoxides (Trilobity) iz goryuchikh slantsev amginskogo yarusa severnoy Yakutii. Uchenye Zapiski Paleontologiya i Biostratigrafiya, Nauchno-Issledovatelskiy Insitut Geologii Arktiki 25: 9-20, 5 pls.
Sternberg, K.M. 1833. Rede des Prasidenten Kaspar Grafen von Sternberg, Beilage III. Verhandlungen der Gesellschaft des Vaterlandischen Museum in Bohmen 11: 45-55, pl. II.
