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).
A nice large specimen of Paradoxides minor is shown below from the Middle Cambrian of Bohemia. Some workers refer to this species as Hydrocephalus minor or Paradoxides (Hydrocephalus) minor. It was first named & described & illustrated by Chris Boeck in 1828 as Trilobites minor (see Boeck, 1828, pp. 27-30, pl. 2, figs. 12-14).
This species is often placed in the genus or subgenus Hydrocephalus, named by Joachim Barrande in 1846 for paradoxidid trilobites having noticeably inflated glabellas in early ontogenetic stages (hydrocephaly is a condition in some human fetuses or infants involving swelling of the head due to abnormal fluid buildup). This feature is not apparent in later holaspids, so many authors consider the use of Hydrocephalus at the genus level problematic.
Classification: Arthropoda, Trilobita, Polymerida, Paradoxididae
Stratigraphy: upper Jince Formation, Paradoxides gracilis zone, upper Middle Cambrian
Locality: Jince area, Stredocesky Region, Bohemia, Czech Republic
Paradoxides minor (Boeck, 1828) complete exoskeleton (13.9 cm long), but lacking the librigenae (free cheeks), from the Jince Formation (Middle Cambrian) of Jince, Bohemia.
Paradoxides minor (Boeck, 1828) cranidium attached to thorax (width of cranidium at posterior end of palpebral lobes: 5.1 cm across) from the Jince Formation (Middle Cambrian) of Jince, Bohemia.
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.
Paradoxides minor (Boeck, 1828) pygidium attached to thorax (tail is 1.6 cm across) from the Jince Formation (Middle Cambrian) of Jince, Bohemia.
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.
Paradoxides minor (Boeck, 1828) labrum (hypostome), or mouthplate (2.6 cm across at its widest) from the Jince Formation (Middle Cambrian) of Jince, Bohemia.
Barrande, J. 1846. Notice Préliminaire sur le Systčme Silurien et les Trilobites de Bohęme. Leipzig, Germany. J.B. Hisrschfeld. 97 pp.
Boeck, C. 1828 (not 1827). Notitser til Lćren 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.
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.