Fossil bryozoan

from southern Ontario, Canada

Here are two photographs of a colonial bryozoan, a fossil collected from the banks of the Trent river downstream of Campbellford, Ontario. This striking sample was collected loose just above the west end of the Percy Reach, and appears to be derived from the local Verulam Formation (Carson, 1979). Such fossils, 10 to 50 mm in diameter, are often seen in outcrop along the banks of the Trent river, including the section downstream of Campbellford illustrated in the Introduction to Local Geology. The limestone outcrops in the region are of Ordovician age, roughly 460 million years old.

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1. Left: Fossiliferous limestone of Ordovician age, with brachiopod shells and a large bryozoan mass.

2. Right: The bryozoan, a hemispherical colonial form such as Prasopora or Mesotrypa, is 25 mm in diameter. A thin section of rock from the banks of the Trent (see Local Features) contains a slice of a bryozoan with internal structures consistent with Prasopora.

Many tonnes of rock were displaced in canal digging and maintenance prior to the completion of the Trent-Severn waterway in 1918: this sample was probably near its bedrock source. It was collected on the west side of the waterway above Lock 8, north of Percy Boom, at the west end of the Percy Reach, southwest Seymour Township. The specimen is a 17x8x2-cm slab with a beautiful hemispherical bryozoan, 25 mm wide and 10 mm high, in biosparite (limestone rich in fossil fragments) with abundant brachiopods and minor tube- and mat-like bryozoans.

"Rock of the Month # 11, posted for May 2002" ---

sample 1984. Photography by Karyn Gorra of the Department of Geology, University of Toronto.


The local limestones contain layers rich in fossil debris, comprising fragments and whole skeletons of brachiopods, crinoids, bryozoans and other life forms. Fossils of these groups are widespread around the world, not limited to a relatively small area such as southern Ontario. Thus the bryozoan genus Prasopora is found also in the Girvan district of Ayrshire, Scotland, in strata of late Ordovician, Caradoc age (Anon, 1969, pp.58-59). These fossils and the assemblages they comprise are essential tools for students of stratigraphy, the scientific reconstruction of the progression of geological time.

Bryozoans are found from Ordovician to Recent times. Bryozoans (Moore et al., 1952, pp.156-198) are aquatic, and mainly marine, with a much smaller number of freshwater representatives. Most are colonial and sessile, with calcareous skeletons. There are some 4,000 modern species, and as many as 15,000 fossil forms (Barnes, 1968, pp.588-599). The earliest known fossil freshwater bryozoa are found in the upper Triassic of South Africa (Kohring and Hornig, 1998). Individual bryozoans average <1 mm in size, but form colonies which are often 25 mm or more in diameter, and as much as 60 cm across in the case of some fossil forms. They survive today at all latitudes and at depths as great as 5.5 km. An older term, polyzoan, refers to the colonial tendency of the animals. The word bryozoan means `moss animal', referring to the tufted, moss-like form of some modern types such as the cheilostome Bugula. Bryozoans generally prefer clear water in shallow seas: a rocky sea bottom is ideal, where shells of other invertebrates afford places of attachment. Their food is microscopic creatures such as diatoms and radiolarians.

The order Trepostomata is limited to the Paleozoic era, when its members were abundant, forming stony colonies and reef structures which contributed to the bulk of many sedimentary formations (Moore, 1953, pp.G90-97). The colonies are always calcareous and may exceed 50 cm in thickness and diameter. Trepostome colonies, whether in large masses or branching growths, are zoned from an inner immature region to an outer mature region, whence the name from trepos (change). The trepostomes form the majority of the so-called monticuliporoids, which were for a long time regarded as corals (they display elevated clusters of cells on their surfaces, known as monticules).

The Verulam Formation was named by Liberty for a township on the south shore of Sturgeon Lake, which extends northeast from Lindsay to Bobcaygeon. Liberty (1969) later provided a detailed description of the strata and their fauna. Formerly known, at least in part, as the `Prasopora Beds', the Verulam Formation is up to 70 metres (230 feet) thick. The faunal list for the Verulam (ibid., pp.184-191) includes many bryozoans, brachiopods, bivalves, gastropods, trilobites and other forms, including bryozoan species of genera such as Dekayella, Dekayia, Favositella, Hallopora, Homotrypa, Mesotrypa, Pachydictya, Prasopora, Prasoporina and Rhinidictya.


ANON (1969) British Palaeozoic Fossils. British Museum (Natural History), London, 3rd edition, 208pp.

BARNES,RD (1968) Invertebrate Zoology. W.B. Saunders Company, Philadelphia, 2nd edition, 743pp.

CARSON,DM (1979) Paleozoic geology of the Peterborough- Campbellford area, southern Ontario. OGS Misc.Pap. 90, 146-148.

KOHRING,RR and HORNIG,ACF (1998) The earliest freshwater Bryozoa: evidence from the upper Triassic Molteno Formation (South Africa). J.Afr.Earth Sci. 27 no.1A, 125.

LIBERTY,BA (1969) Palaeozoic Geology of the Lake Simcoe area, Ontario. GSC Memoir 355, 201pp. plus map and chart.

MOORE,RC (editor) (1953) Treatise on Invertebrate Paleontology, Part G, Bryozoa. Geol.Soc.Amer. / University of Kansas Press, 253pp.

MOORE,RC, LALLICKER,CG and FISCHER,AG (1952) Invertebrate Fossils. McGraw-Hill, 766pp.

Graham Wilson, for May 2002, last revised 02 July 2002, format amended 25 October 2013.

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