A Silurian Fish Hash at the Ocean’s Edge Holds An Important Page in the History of Life On Earth
FISH HASH FROM THE EDGE OF THE SEA: A Ludlow Bone Bed Microscope Slide by Edmund Wheeler
A stratum of dark sandy rock that Sir Roderick Impey Murchison (1792–1871) called the Ludlow Bone Bed marks an ancient epoch in Earth’s history. Discovered near Ludlow in Shropshire in 1835 by the local doctor Thomas Lloyd and the Reverend T. T. Lewis, it marks the shift from marine Silurian seas to nearshore and terrestrial conditions that lead into the Old Red Sandstone. Murchison described the bed as looking like gingerbread. When broken, it is packed with thousands of small fragments—the scales, spines, teeth, and bones of some of the earliest known fishes. Victorian microscopists called this material “fish hash.”
Edmund Wheeler of London prepared the slide. Between 1855 and 1884, Wheeler was a leading commercial mounter of microscope slides. He lectured on science, sold microscopes, and offered prepared mounts for sale. By the early 1880s, he claimed a stock of ten thousand slides. The circular label and the red-and-gold paper wrapping are typical of his work. The handwritten oval label reads “Fish Bone Bed Ludlow.”
Under the microscope at fifty diameters, the specimen shows itself to be a microvertebrate lag deposit. The matrix is fine-grained siliconaceous sediment with patches of carbonate. Against this background are the phosphatic fragments. These are composed of calcium carbonates mixed with phosphate. They are the remains of bone and differ from the calcium carbonate deposited by invertebrates. They appear in purple and dark reddish-brown tones. Phosphate minerals (chiefly the carbonate fluorapatite known as francolite or its cryptocrystalline form collophane) are nearly isotropic; under crossed polars they remain dark or show only faint first-order gray. Calcium carbonate shells, by contrast, show high-order interference colors and the twinkling of calcite as the stage is rotated.
On the right-hand side of the field, several long, tapered, longitudinally striated structures stand out. These are acanthodian fin spines. Acanthodians were early jawed fishes sometimes called “spiny sharks.” The genera Onchus murchisoni and Onchus tenuistriatus, both named by Louis Agassiz in the 1830s, are taxonomically distinguished for having such ribbed spines. The parallel longitudinal striations with a pointed distal end are diagnostic. Murchison illustrated comparable spines from the Ludlow Bone Bed in his book titled Siluria, describing his findings.
What is on the slide?
In the image, toward the left, a large, curved, C-shaped dark-brown object is visible. Appears to be a transverse or oblique section through a thelodont scale or dermal denticle. Thelodonts were jawless fishes whose bodies were covered in minute, tooth-like scales. Thelodus parvidens, first described by Agassiz in 1839, is the most abundant thelodont in the lower lenticles of the Ludlow Bone Bed. Concentric growth lines or layered internal structure are typical of these scales in thin section.
Scattered through the center of the field are irregular, segmented, or plate-like fragments in pinkish-purple tones. Most of these are sections of fish bone, compound scales, or additional dermal denticles. A few smaller pieces might represent ostracod valves—tiny bivalved arthropods that occur as a minor component of the bed. Still, most of the dark, colored material is phosphatic vertebrate debris. No graptolites, trilobites, or abundant brachiopod shells are present; the slide is dominated by the fish microfauna that gave the bed its name.
The Ludlow Bone Bed is geologically described as a condensed lag deposit. It formed in a shallow subtidal setting where currents and storm waves washed away finer sediment, concentrating denser phosphatic fragments into thin layers. Modern interpretations link the bed to a relative sea-level fall near the Ludfordian–Přídolí boundary. For much of the nineteenth century it was taken as the base of the Devonian and as the earliest record of fishes. Later work moved the Silurian–Devonian boundary higher, and fishes are now known from older strata, yet the bed remains a reference horizon and a Site of Special Scientific Interest.
Microscopes were important to naturalists
Victorian slide makers such as Wheeler made these microscopic views available to anyone who owned a microscope. Thinly ground sections allowed a naturalist or a member of a microscopical society to study material that Murchison and Agassiz had described from outcrop. The optical distinction between phosphatic and calcitic material, the striations on Onchus spines, and the histology of thelodont scales could all be shown under the polarizer. As such, the slide represents the earliest collection and analysis of geological strata documenting that the Earth had a past inhabited by primitive organisms and a different ecology than contemporary life.
The mounted section of Ludlow Bone Bed materials continues to reward those who study it more than a century and a half after it was mounted. It was a puzzle at its time to explain— a time predating Darwin’s theory of evolution and Leidy’s revolutionary ideas that the geology gradually changed over eons through natural forces.
References
Agassiz, L. (1839). In Murchison, R. I., The Silurian System. London: John Murray.
Bassett, M. G., Lawson, J. D., & White, D. E. (1982). The Downton Series as the fourth Series of the Silurian System. Lethaia, 15, 1–24.
Bracegirdle, B. (1998). Microscopical Mounts and Mounters. Quekett Microscopical Club.
Geological Magazine (2023). Late Silurian event stratigraphy and facies of South Wales and the Welsh Borderland. Cambridge University Press.
JNCC (various). GCR site account: Ludford Lane and Ludford Corner.
Murchison, R. I. (1839). The Silurian System. London: John Murray.
Murchison, R. I. (1854). Siluria. London: John Murray.
Turner, S. (1973). Siluro-Devonian thelodonts from the Welsh Borderland. Journal of the Geological Society, 129, 557–584.
White, E. I. (1950). The vertebrate faunas of the Lower Old Red Sandstone of the Welsh Borders. Bulletin of the British Museum (Natural History), Geology, 1, 51–89.