The weird and wonderful of the Ordovician (Part 7): Maclurites bigsbyi – the open-coiled oddity of the Ordovician
Jon Trevelyan (UK)
This is the seventh in my series of short articles on fossils of the Ordovician. Of all the Ordovician gastropods, few are as visually striking as Maclurites bigsbyi. With its large, open-coiled shell and broad, planispiral whorls, the species represents a unique experiment in molluscan shell architecture during the early expansion of marine ecosystems.
Discovery and appearance
Maclurites fossils are well known from North American Ordovician carbonates, particularly from the Trenton and Black River groups. Many specimens reach impressive sizes, with shell diameters exceeding 20cm. The shell coils in a flat spiral, often with a wide umbilicus at the centre, creating a geometry unlike that of most modern gastropods.
The outer shell surface may bear fine growth lines or weak ornament, but the overall impression is one of smooth, sweeping curvature. In life, the animal would have inhabited the last and largest whorl, crawling across firm carbonate substrates or sandy shoals.
Unlike tightly coiled gastropods, the open coil of Maclurites suggests a relatively slow-moving lifestyle, with the broad whorl acting as both support and ballast.
Interpretation and classification
Maclurites bigsbyi belongs to the Macluritidae, an Ordovician-dominant family with unusual coiling patterns and shell structures. These gastropods may represent a lateral branch of early molluscan evolution, experimenting with shell geometries that would later be abandoned.
Their large size and open coiling may have been adaptations to high-energy carbonate settings, where stability and a low centre of gravity offered advantages.
Significance
The distinctive shell of Maclurites makes it an important taxon for interpreting Ordovician carbonate platform environments. Its distribution helps chart shallow-marine conditions across Laurentia, while its unique form provides insight into the ecological diversity of early gastropods.
Beyond its geological significance, Maclurites stands as one of the more charismatic Ordovician invertebrates – instantly recognisable and visually compelling.
Conclusion
With its broad, open coil and substantial size, Maclurites bigsbyi showcases the inventive shell architectures explored by early gastropods. It reflects the dynamic environmental and ecological opportunities that characterised Ordovician carbonate platforms.