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Bathyuriscus rotundatus

A very common Mount Stephen trilobite

Line drawing of Bathyuriscus rotundatus.

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Bathyuriscus rotundatus (figures 4 and 5) illustrated by Rominger (1887) as Embolimus rotundata.

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Taxonomy

Kingdom:

Animalia

Phylum:

Arthropoda

Class:

Trilobita (Order: Corynexochida)

Affinity:

Trilobites are extinct euarthropods, probably stem lineage representatives of the Mandibulata, which includes crustaceans, myriapods, and hexapods (Scholtz and Edgecombe, 2006).

Species name:

Bathyuriscus rotundatus

Described by:

Rominger

Description date:

1887

Etymology:

Bathyuriscus – a variation of the earlier trilobite genus name Bathyurus, originally based on the Greek bathys, “deep,” and the Greek oura, “tail,” thus, a trilobite with a deep tail.

rotundatus – from the Latin rotundus, “round,” presumably alluding to the rounded outline of the dorsal shield.

Type Specimens:

Type status under review – UMMP 4884 (9 specimens), University of Michigan Museum of Paleontology, Ann Arbor, Michigan, USA.

Other species:

Burgess Shale and vicinity: Bathyuriscus adaeus Walcott, 1916, from several localities higher in the Bathyuriscus-Elrathina Zone on Mount Stephen, Mount Odaray, and Park Mountain.

Other deposits: other species of Bathyuriscus have been described from numerous localities elsewhere in the Cambrian of North America.

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Age

Period:

Middle Cambrian, Bathyuriscus–Elrathina Zone (approximately 505 million years ago).

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Localities

Principal localities:

The Trilobite Beds and other localities on Mount Stephen. Fossil Ridge in sections stratigraphically below the Walcott Quarry.

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History of Research

Brief history of research:

Bathyuriscus rotundatus was first described in the same 1887 publication as several other important Mount Stephen trilobites. Carl Rominger initially used the name Embolimus rotundata for partial specimens of this trilobite, and named a second similar species in his collection Embolimus spinosa (now known as Zacanthoides romingeri). In 1908, Walcott revised Rominger's original species name to yield the combination Bathyuriscus rotundatus, still in use today (Walcott, 1908). Along with the co-occurring Elrathina cordillerae, B. rotundatus is a signature fossil for the Middle Cambrian Bathyuriscus-Elrathina Zone in the southern Canadian Rockies.

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Description

Morphology:

Hard parts: adult dorsal exoskeletons may be up to 5 cm long and are narrowly oval in outline, with a semicircular cephalon, a thorax of nine segments ending in blade-like tips with short spines, and a semicircular pygidium without spines.

The long glabella reaches almost to the anterior cephalic border; the posterior portion is narrow and parallel-sided, while the anterior third expands rapidly forward. There are four pairs of lateral glabellar furrows, with the two front pairs angled forward and the posterior pair directed obliquely back. The eyes are relatively long and lie close to the glabella. Broad free cheeks are extended back into short genal spines. The pygidium is slightly smaller than the cephalon, with a well-defined narrow axial lobe of five rings and a terminal piece; four pairs of pygidial ribs are usually visible. The exoskeleton is mostly smooth externally, but very well preserved specimens may show faint anastomosing ridges on the free cheeks.

Unmineralized anatomy: not known.

Abundance:

Extremely common in the Mount Stephen Trilobite Beds, where it rivals Ogygopsis klotzi in abundance.

Maximum size:

50 mm

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Ecology

Life habits:

Epibenthic, mobile

Feeding strategies:

Carnivorous or deposit feeder

Ecological Interpretations:

Bathyuriscus rotundatus was a mobile epibenthic trilobite. Because we have no direct evidence of limb structure, its feeding habits are uncertain. It may have been a deposit feeder and opportunistic scavenger. Like Ogygopsis, Bathyuriscus may occur as fully intact individuals (probably carcasses), with the free cheeks missing, inverted, or rotated (presumed moults), and as scattered pieces. Some show evidence of healed injuries that may be predation scars (Rudkin, 2009).

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References

Bibliography:

RASETTI, F. 1951. Middle Cambrian stratigraphy and faunas of the Canadian Rocky Mountains. Smithsonian Miscellaneous Collections, 116 (5): 1-277.

ROMINGER, C. 1887. Description of primordial fossils from Mount Stephens, N. W. Territory of Canada. Proceedings of the Academy of Natural Sciences of Philadelphia, 1887: 12-19.

RUDKIN, D. M. 2009. The Mount Stephen Trilobite Beds, pp. 90-102. In J.-B. Caron and D. Rudkin (eds.), A Burgess Shale Primer - History, Geology, and Research Highlights. The Burgess Shale Consortium, Toronto.

SCHOLTZ, G. AND EDGECOMBE, G. D. 2006. The evolution of arthropod heads: reconciling morphological, developmental and palaeontological evidence. Development Genes and Evolution, 216: 395-415.

WALCOTT, C. D. 1888. Cambrian fossils from Mount Stephens, Northwest Territory of Canada. American Journal of Science, series 3: 163-166.

WALCOTT, C. D. 1908. Mount Stephen rocks and fossils. Canadian Alpine Journal, 1: 232-248.

WALCOTT, C. D. 1916. Cambrian Geology and Paleontology III. Cambrian Trilobites. Smithsonian Miscellaneous Collections, 64(5): 303-456.

Other links:

http://www.trilobites.info/ordcorynexochida.htm

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