Oryctocephalus burgessensis (ROM 49962). Complete small individual; a presumed carcass with free cheeks in place. Specimen length = 5.5 mm. Specimen dry – direct light (left) and coated with ammonium chloride sublimate to show details (right). Walcott Quarry talus.
© ROYAL ONTARIO MUSEUM. PHOTOS: JEAN-BERNARD CARON
Trilobites are extinct euarthropods, probably stem lineage representatives of the Mandibulata, which includes crustaceans, myriapods, and hexapods (Scholtz and Edgecombe, 2006).
Oryctocephalus – from the Greek oryktos, “dug” or “burrowed,” and kephalos, “head.”
reynoldsi – after Mr. S. H. Reynolds, who collected and donated the type specimen to the Woodwardian Museum of the University of Cambridge (now in the Sedgwick Museum of Earth Sciences).
Burgess Shale and vicinity: Oryctocephalus burgessensis Resser, 1938.
Other deposits: many other species worldwide.
The Trilobite Beds and smaller localities on Mount Stephen. The Walcott Quarry on Fossil Ridge.
The genus Oryctocephalus was established by Charles Walcott in 1886 to include the species O. primus, based on isolated cranidia and pygidia from the Middle Cambrian of Nevada. Reed named and described O. reynoldsi in 1899 from a complete specimen (including the articulated thorax), probably collected at the Mount Stephen Trilobite Beds. In the same year as Reed’s paper appeared, G. F. Matthew also had a publication in press, describing O. walkeri from collections on Mount Stephen. Although minor differences between O. reynoldsiand O. walkeri were noted (Matthew, 1899), they are almost certainly one and the same, and Reed’s name has publication priority. In 1938, Resser erected a new species, O. burgessensis, for specimens from the Walcott Quarry. Rasetti (1951) illustrated O. reynoldsiand O. burgessensis and named another new species, O. matthewi, from both localities. Whittington reassessed the Burgess Shale species of Oryctocephalus in 1995, and found that Rasetti’s O. matthewi was indistinguishable from O. reynoldsi.
Hard parts: both Oryctocephalus reynoldsi and O. burgessensis are small trilobites, with adult exoskeletons generally 15-20 mm long, excluding pygidial spines. Dorsal shields are ovoid in outline, slightly narrower posteriorly. O. reynoldsi has a broad semicircular cephalon, with the genal angles drawn out and back into long slender spines extending almost to the pygidium. The distinctive glabella widens slightly forwards to a rounded front at the anterior border. Three pairs of pits lie forward of the occipital ring, just inside the axial furrows; the posterior pair is joined by a shallow transverse furrow. Faint eye ridges swing back from near the front of the glabella to the long crescentic eye lobes far out on the cheeks. The thorax contains seven wide segments with strong, curving pleural furrows and long terminal spines directed obliquely rearward. The unmistakable pygidium is semicircular, narrower than the cephalon, with a tapering axis of five rings and a terminal piece ending well inside the posterior margin. Six radially disposed pleurae all end in spines, the fourth pair being much broader at the base and very long, directed out and back to at least twice the length of the pygidium. The short fifth and sixth spine pairs extend straight back. O. burgessensis can be distinguished mainly by its subtly shorter genal and fourth pygidial spines; the genal spine also appears to arise slightly farther forward than in O. reynoldsi.
Unmineralized anatomy: not known
Rare, both on Mount Stephen and on Fossil Ridge.
Very similar species of Oryctocephalus are found in Middle Cambrian rocks of deeper water origin in many places around the world, suggesting that these cosmopolitan trilobites typically inhabited open ocean settings.
MATTHEW, G. F. 1899. Studies on Cambrian faunas, No. 3. Upper Cambrian Fauna of Mount Stephen, British Columbia: The trilobites and worms. Transactions of the Royal Society of Canada, Series 2, Vol. 5, Section IV: 39-66.
RASETTI, F. 1951. Middle Cambrian stratigraphy and faunas of the Canadian Rocky Mountains. Smithsonian Miscellaneous Collections, 116 (5): 1-277.
REED, F. R. C. 1899. Woodwardian Museum Notes: a new trilobite from Mount Stephen, Field, B.C. Geological Magazine, New Series (Decade 4), 6: 358-361.
RESSER, C. E. 1938. Fourth contribution to nomenclature of Cambrian fossils. Smithsonian Miscellaneous Collections, 97: 1-43.
SCHOLTZ, G. AND G. D. EDGECOMBE. 2006. The evolution of arthropod heads: reconciling morphological, developmental and palaeontological evidence. Development Genes and Evolution, 216: 395-415.
WALCOTT, C. D. 1886. Second contribution to the studies on the Cambrian faunas of North America. Bulletin of the US Geological Survey, 30: 1-255.
WHITTINGTON, H. B. 1995. Oryctocephalid trilobites from the Cambrian of North America. Palaeontology, 38: 543-562.
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