Triarthrus canadensis, Whitby Formation, southern Ontario
photo courtesy K. Brett
Dec. 2001:
Triarthrus:
Triarthrus canadensis, Whitby Formation, southern Ontario
photo courtesy K. Brett
Triarthrus GREEN is an Ordovician olenid trilobite predominantly from North America. It is restricted to the upper Caradoc and Ashgill ages. Triarthrus is one of the more common trilobites in Upper Ordovician deep water black shale facies of eastern North America. Ludvigsen and Tuffnell (1983) provided a revision of most of the species of Triarthrus. Many species have been assigned to Triarthrus in the past, of which they allocated 10 to the genus Triarthrus, 13 to Porterfieldia COOPER, and 5 to Bienvillia CLARK. Only the North American species of Triarthrus will be considered here for the moment. Below is a list of the 10 species currently assigned to Triarthrus (8 of which occur in N. A.).
North American species:
Triarthrus beckii GREEN, 1832. The type species. Upper Caradoc
and Ashgill, N.Y. and Kentucky.
Triarthrus eatoni (HALL, 1838). Upper Caradoc-Ashgill, N.Y.,
Ontario, and Quebec
Triarthrus canadensis SMITH, 1861. Ashgill, Ontario
Triarthrus rougensis PARKS, 1921. Ashgill, Ontario
Triarthrus spinosus BILLINGS, 1857. Ashgill, Ontario, Quebec,
and N.Y.
Triarthrus billingsi BARRANDE, 1872. Ashgill?, Quebec
Triarthrus glaber BILLINGS, 1859. Ashgill, Quebec
Triarthrus huguesensis FOERSTE, 1924. Ashgill, Quebec
Non-North American species:
Triarthrus linnarssoni THORSLUND, 1940. Norway and Sweden
Triarthrus sichuansis LU & CHANG, 1974. China
Triarthrus latissimus Månsson, 1998. Sweden – a new
species
All other species of Triarthus have been
reassigned to either Porterfieldia, Bienvillia, Parabolina,
Parabolinella,
or are too poorly preserved to determine their status.
The locality information given above is far from
complete, as many of the species may occur throughout Ontario, Quebec,
New York, and possibly Pennsylvania. For example, T. eatoni, T. spinosus,
T. rougensis, and T. canadensis all seem to occur in the Whitby
Formation of southern Ontario, as well as in equivalent strata in the Billings
Shale of the Ottawa-St. Lawrence Lowlands of Ontario and Quebec. Many of
these can also be found in the Utica Shale of New York. Triarthrus beckii,
which occurs mainly in New York, may also occur in Quebec and possibly
Ontario.
The exoskeletons occuring
in these units are flat, and preserved in a black calcareous to bituminous
shale. Usually they occur as abundant partials although complete specimens
are fairly common, with some death layers having been discovered (Ottawa
area, N.Y., Quebec, southern Ontario). Many multiple death slabs have been
found, some exceeding 20 complete specimens on the same slab.
Triarthrus
are one of the more affordable complete trilobites for collectors. Both
the part (positive) and counterpart (negative) impressions are attractive.
They are typically preserved as flattened impressions on black shale. Juvenile
holaspids and even meraspids have been found in the black shale layers
as well. Triarthrus is one of the most studied of all trilobite
species. Studies include: taxonomic, phylogenetic, morphometric, and stratigraphic
aspects.
Triarthrus species are distinguished mainly
by cephalic features. Below are some basic differences to distinguish the
species:
Triarthrus beckii GREEN –short palpebral lobe, located far
forward opposite 3p and 4p lobes.
Triarthrus eatoni (HALL) –long palpebral lobes, midpoint
opposite the 2s furrow.
Because of the difference
in position of the palpebral lobes, the fixed cheeks on these 2 species
are of different lengths and shapes. These 2 species are somewhat difficult
to tell apart except for this feature. Furthermore, I have observed many
specimens which have axial nodes down the axis. Some T. eatoni show
this feature, while others do not. Are these the same species?
Both T. beckii
and T. eatoni lack genal, occipital, and thoracic spines.
Triarthrus canadensis SMITH –has short, stout, diverging
genal spines.
Triarthrus rougensis PARKS –slender genal spines and a spine
on the 9th thoracic segment.
Triarthrus spinosus BILLINGS –slender genal spines, an occipital
spine, and spines on the 8th-10th thoracic segments.
Triarthrus billingsi BARRANDE –forwardly expanding glabella
and a greater number of thoracic segments. Only known from 1 specimen.
Triarthrus glaber BILLINGS –similar to T. canadensis,
but lacks genal spines. Free cheeks large and expanding forward which produces
an
indentation in front
of the cephalon.
Triarthrus huguesensis FOERSTE –long palpebral lobes like
T.
rougensis. Only known from poorly preserved cranidia.
References:
Brett K. and Rudkin, D. (1997) Ordovician stratigraphy and trilobite faunas of south central Ontario. Field trip 3, Guidebook, 2nd International Trilobite Conference, Brock University, St. Catharines, Ontario.
Cisne, J. L. 1975. Anatomy of Triarthrus and the relationships of the Trilobita. Fossils & Strata 4: 45-63.
Ludvigsen, R. and Tuffnell, P. (1994) The last olenacean trilobite: Triarthrus in the Whitby Formation (Upper Ordovician) of southern Ontario. New York State Museum Bulletin 481, p. 183-212.
Ludvigsen, R. and Tuffnell, P. A. (1983) A revision of the Ordovician olenid trilobite Triarthrus. Geol. Mag. 120 (6), pp. 567-577
Månsson, K. 1998: Middle Ordovician olenid trilobites (Triarthrus
Green and Porterfieldia Cooper) from Jämtland, central Sweden.
Transactions of the Royal Society of Edinburgh: Earth Sciences 89, 47-62
Triarthrus rougensis, Billings fm., Ottawa, Ontario.
Photo courtesy K. Brett
T.
spinosus, Billings Fm., Ottawa, Ontario
photo courtesy K. Brett
T.spinosus,
Billings Fm., Ottawa, Ontario.
photo courtesy K. Brett
Triarthrus beckii, Utica Shale Fm., Portneuf Co., Quebec
Photo, courtesy Hanman
More Triarthrus at Frank Habets' Page. Frank has specimens
of T. eatoni,
T.
rougensis,
T.
spinosus, T.
canadensis, and T.
glaber.
A brief description and some fine photographs of various Triarthrus
species at Deadbugs.com
Some Triarthrus for sale at: Hanmansfossils.com
A Triarthrus
beckii in the collections at PRI (Paleontological Research Institution).
A couple of T. spinosus for sale at Xfossil.com
A reconstruction
at the Ottawa Museum.
T.
eatoni from the Cincinnati area, at the University of Georgia Stratigraphy
Lab.
Triarthrus from the famous Beecher's
trilobite bed at the Peabody Museum, Yale. Lots of nice photos!
The distribution
of Triarthrus in New York, at Union College.
An abstract
by Kim et al. on the evolution of Triarthrus.
Dr.
John L. Cisne's Page. Although there are no photos of Triarthrus,
there is a list of Dr. Cisne's publications, some of which are studies
on Triarthrus and it's applicatuion to coencorrelation, gradient
analysis of fossil communities.
Kristina
Månsson's doctoral dissertation (Lund University) abstract.