FEATURE ARTICLE
Virtual Fossils from 425 Million-year-old Volcanic Ash
A set of exceptionally preserved but difficult-to-extract fossils reveals the diverse creatures from a Silurian sea-floor community
Derek E. Briggs, Derek J. Siveter, David J. Siveter, Mark D. Sutton
Silurian Time Capsules
We found these fossils in the rocks of Herefordshire, a county that lies on the historic borderland between England and Wales. In the late 18th and early 19th centuries, this area was the stomping ground for many geological pioneers, including one who has been jokingly dubbed the "King of Siluria," Sir Roderick Impey Murchison. In 1839, he wrote a monumental treatise, titled The Silurian System, in which he coined the name for this geological period. He named it after the Silures, an ancient tribe that inhabited this part of Britain during Roman times.
Herefordshire owes much to its underlying geology. It boasts a beautiful, tranquil landscape of gently rolling hills, small-scale escarpments and open river valleys, which have attracted artists and men of letters down the ages. Some modern-day geologists have gravitated to the place, too.
One, Robert J. King, a noted mineralogist and retired curator at the Department of Geology of the University of Leicester, visited Herefordshire while vacationing in the area in the summer of 1990. Intrigued by the geology, he returned later that year to collect rock samples. On splitting open a hard, near-spherical concretion that had rolled to the quarry floor at one site, he caught sight of some sparkling mineralization that seemed to preserve a fossil.
King collected nine concretions, four of which revealed fossils when he cracked them open, and in December 1990 he donated these specimens to the collections at Leicester. In the fall of 1994, King's successor as curator, Roy G. Clements, asked one of us (David Siveter) to look at one of these finds. It showed something quite unexpected under the microscope—an arthropod with limbs preserved. That discovery prompted the involvement of another one of us (Derek Siveter), David's twin brother and fellow paleontologist, who is a specialist in both arthropods and Silurian geology.
Derek photographed the material at the University of Oxford, and together with his brother and King visited Herefordshire in December 1994 to find the source of these concretions. The following April, Derek contacted another one of us (Briggs), a specialist in exceptionally preserved fossils. Unlike most of the examples that Briggs had studied, the Herefordshire fossils he was shown were not visually spectacular in hand specimens, and the mineral that gives them their sparkle, common calcite, would normally not invite a second glance. This rather uninteresting appearance is undoubtedly the main reason why this wonderful fossil cache lay hidden from so many generations of geologists.
The Herefordshire fossils were deposited 425 million years ago within a marine basin that extended across what is now central England into Wales. This basin first formed some 120 million years earlier, at the beginning of the Cambrian Period. The fossils are preserved in a soft, cream-colored volcanic ash that mixed with some of the normal marine sediment.
This ash deposit is known from just the one locality, where it is exposed over a distance of about 30 meters. Measuring more than a meter thick in places, it is unusual compared with other ash deposits in this region. The hard concretions that carry the fossils vary in size from that of a cherry to something as big as a grapefruit. They seem to have formed randomly throughout the deposit. Even now, the ashy sediment around them is largely unconsolidated and can be dug by hand without great difficulty.
The volcanic ash that engulfed the animals was deposited on top of a thin layer of mud covering thick limestones, the remnants of a reef that was effectively dead and had probably sunk well beneath the waves. Indeed, the animals that became fossilized here likely lived 100 to 200 meters down, below the depth to which light penetrates. We know this because we found no vestiges of photosynthetic algae, which are common in contemporaneous rocks laid down at shallower points on the seafloor to the east.
It is not clear whether a volcanic eruption entombed these Silurian animals directly, Pompeii-style. Perhaps they were buried many years after the explosion, covered in ash that had been transported along the seabed by a fast-moving bottom current. In any event, what is clear is that some very special circumstances allowed for their remarkable preservation.
The first was the immediate precipitation of clay minerals around the dead organisms, which decayed over time, leaving empty spaces behind. The mineral calcite (a form of calcium carbonate) then filled these natural molds, faithfully replicating the shape of the animals. Even spines and other structures just a few microns across were preserved in this way.
At about the same time, the hard round concretions began to form, being cemented by calcite. Thanks to the early hardening of these Silurian time capsules in this way, the fossils were not squashed as the ash layer slowly compacted. We can be sure of this because the fossils appear undeformed and because the concretions are spherical rather than pancake shaped.
Freshly exposed concretions are hard with a blue- to gray-colored core. The Herefordshire concretions are unusual in that they do not correspond in size to the fossils they contain. And the fossils are commonly not at the center—so the nucleus of the concretions must have been something other than the fossil itself. Just what it was remains a mystery, however, because no trace remains.
We have carried out fieldwork at this site for several days most summers since 1996. On most of these occasions, we hired excavators who used earth-moving equipment to strip off the overlying shale and expose the volcanic deposit fully. They scooped the ash up in the bucket of their backhoe and tipped it out slowly, at which point we collected the hard, round concretions by hand from the piles of dumped sediment. We carefully mined the deposit in this way, amassing more then 4,000 samples, which are held in the Oxford University Museum of Natural History.
In our announcement of the discovery to the scientific community (a 1996 article in Nature), we highlighted the rarity of soft-bodied fossils from the 100-million-year interval following the Cambrian. We provided brief descriptions of a small arthropod and of a number of other animals that we then thought were worms. We noted the unusual setting—calcite-filled voids in concretions in a volcanic ash laid down in the sea—and suggested that deposits like this could provide an important new source of data on the history of life. But at that point we were relying on random cross sections to view the ancient creatures preserved in these rocks. Little did we know at the time how much more there was to learn.
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