File:Acadoparadoxides briareus fossil trilobite (Middle Cambrian; Sidi Abdallah ben el Hadj, Morocco) 1 (15267496221).jpg

Acadoparadoxides briareus Geyser, 1993 - fossil trilobite from the Cambrian of Morocco. (centimeter scale) (FMNH 56126, Field Museum of Natural History, Chicago, Illinois, USA)

This species is also known as Paradoxides (Acadoparadoxides) briareus.

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 (Ex: 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).

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). 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; some of these are undisputed junior synonyms of other names on the list).

Classification: Arthropoda, Trilobita, Polymerida, Paradoxididae

Stratigraphy: unrecorded, but probably from the lower Jbel Wawrmast Formation, Cephalopyge notabilis Zone, lower Middle Cambrian

Locality: Sidi Abdallah ben el Hadj, Morocco

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.

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.