CRUSTACEAN FOSSILS FROM THE LATE TRIASSIC WOLFVILLE FORMATION, MINAS BASIN, NS

Abstract

Conchostraca from the Late Triassic Wolfville Formation near Medford Beach, Southern Minas Basin,NS

 

Barry Cameron2, Simon Gould3

 

Fossiliferous blocks of finely laminated, mudcracked siltstones and claystones occur along the coastline near Medford Beach between the Pereau River and Kingsport on the western shores of the Minas Basin, Nova Scotia. Similar rocks were first reported 100 years ago by Haycock (1901) as containing ostracodes. These fossil identifications were reinterpreted as two forms of the conchostracan Estheria ovata (=Cyzicus) by Powers (1916). One of us (Gould) rediscovered these fossiliferous rocks this past summer. Analysis of our new material indicates that there are at least 2 species of bivalved conchostracans (Crustacea) that we provisionally refer to Cyzicus sp. and ?Cornia sp. They are moderately well preserved as molds and as calcite replacements of the original chitinous exoskeleton and are associated with less common Darwinula-like freshwater ostracodes and fish scales. Although conchostracans and ostracodes have been previously reported from the overlying Blomidon Formation, this is the first report of Cornia from the Fundy Basin. This locality is about 1 km south of Paddy Island where the contact between the Wolfville and Blomidon formations is well exposed.

 

These fossils are associated with seacliffs up to 10 m high that expose fluvial channel systems of the upper part of the middle Wolfville Formation. Although no in situ fossiliferous beds were located, similar but discontinuous lithologies are interbedded with sandstones and conglomerates in the adjacent outcrop. Lithologically similar angular intraformational rip-up clasts up to 1 m long occur at the base of thick channel sandstones. These laminated siltstones and claystones were probably deposited in small interfluvial floodplain ponds or abandoned channels that were subsequently reworked after early lithification. These fossiliferous siltstones are interpreted as having fallen from the adjacent outcrop and are not considered displaced beds from the Blomidon Formation, as suggested by Haycock (1901) and Powers (1916). The Blomidon Formation contains different species and genera of conchostracans.

 

Cyzicus sp. is the larger and more elongate of the two forms present, ranging (n = 20) from about 4.3 to 10 mm long and about 2.6 to 5.5 mm wide. It has a long, straight hinge line and numerous growth lines. Cornia sp. is subrounded, convex, ranges (n = 20) from about 1.4 to 4.3 mm in length and about 1.0 to 3.2 mm in width, has fewer growth lines, lacks umbonal growth lines, and often has a short umbonal node or spine. Previously reported occurrences of Cyzicus include the Late Triassic Blomidon Formation of Nova Scotia as well as Late Triassic and Early Jurassic formations in many of the Newark basins in the Eastern United States. Cornia has been reported from Early Jurassic rocks in the Culpepper and Hartford basins in Virginia and Connecticut, respectively. A more comprehensive taxonomic study of the Wolfville and Blomidon conchostracans of Nova Scotia is in progress.

 1Funded in part by Acadia University and by SHELL UK EXPRO NBU2Dept. of Geology, Acadia University, Wolfville, Nova Scotia B0P 1X03Department of Geology and Petroleum Geology, University of Aberdeen, Meston Building, Kings College, Aberdeen, AB24 3UE, Scotland, UK

Dimensions of Fossil Crustaceans from the Wolfville Formation

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Length

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Ostracodes

?Cornia

Cyzicus

Conchostracans

The fossil assemblage includes plant fragments, isolated fish scales, a few ostracodes (Crustacea) that were washed into mudcrack fillings, and two different kinds of conchostracans (branchiopod crustaceans). These fossils are important because fossils are rare in the Wolfville Formation and because conchostracans are significant biostratigraphic indicators in nonmarine sequences. Conchostracans are the most common component of this assemblage.

The Fossil Assemblage

Unidentified wood fragments Unidentified fish scales Ostracodes (?Darwinula sp.)

Low magnification views of conchostracans which are chitinous, bivalved crustaceans (Phylum Arthropoda) that are included in the Class Branchiopoda. Typically, their valves are preserved as molds or calcitic replacements and usually they are parallel to the bedding surfaces.

The fossils occur in silty and fine sandy, thinly laminated claystone. Scale at left is in mm.

Some of the conchostracans are uncompressed and oriented vertically with their dorsal surfaces pointing upward in the sediment. This appears to be a "life position," which suggests that at least Cyzicus (Euestheria) sp., the larger species, was a burrower.

TWO KINDS OF CONCHOSTRACANS:

Cyzicus (Euestheria) sp. has an ovate to subrectangular carapace, asymmetrically placed umbo, numerous growth lines, a length up to 10 mm, and a width up to 5.5 mm. Scales in photos below are in mm.

The carapace of Cornia sp. is subcircular to subrectangular in outline and less than half the size of that of Cyzicus (Euestheria) sp., ranging from about 1.5 to 4.5 mm in length and about 1 to 3 mm in width. It is characterized by a small tubercle or spine rising from the center of the initial valve which is now the umbonal region. The umbo is usually subcentral in location, but can have an anterior position. Growth lines are prominent and not too numerous.

small spine on umbo

The fossils are also associated with mudcracked claystone layers.