Bryozoans in the English Chalk

We are very fortunate in Britain to host one of the most remarkable deposits in the entire geological record, the Chalk. The Late Cretaceous Chalk (with a capital ‘C’) is an extremely pure limestone, famous for the White Cliffs of Dover and responsible for the landscape of rolling hills and dry valleys, forming the ‘downs’ and ‘wolds’ that stretch through England from Devon in the southwest, to Yorkshire in the northeast. The economic importance of the Chalk to the early human inhabitants of Britain was enormous because the flints contained within it could be fashioned into axe heads and hard cutting tools.

Why is the Chalk so special geologically? It is a rare example of a pelagic sediment – an open ocean sediment – that was deposited over the continental shelf. This occurred at a time when global sea-level was high and the supply of terrigenous clastic sediment into the sea was minimal. The Chalk is an oceanic ooze composed mainly of the disaggregated plates – coccoliths – of coccolithophores, planktonic microalgae with exquisitely engineered skeletons of calcite. Unfathomable numbers of coccolithophores sank to the seabed over a period of some 35 million years to produce the thick accumulation of Chalk that today extends over northern Europe and into western Asia.

Fig. 1. Bryozoans encrusting an echinoid test from the Upper Chalk of Chatham, Kent.

The Chalk is a favourite hunting ground for fossil collectors, yielding beautifully preserved specimens, especially of echinoids. But closer inspection of the Chalk shows that the dominant macrofossils are often bryozoans. These colony-forming invertebrates are not only present in great abundance, but are also very diverse (Taylor, 2002a), probably exceeding a thousand species in the English Chalk alone. When I started work almost 40 years ago as a bryozoan researcher at the Natural History Museum, London (then called the British Museum (Natural History)), I was struck by the fact that almost one-third of the drawers in the fossil bryozoan collection were occupied by specimens from the Chalk. Many of the Chalk bryozoans in this huge collection encrust the irregular echinoids Echinocorys (Fig. 1) and Micraster, some undoubtedly acquired incidentally from collectors, who decided to pass their ‘imperfect’ echinoids to the bryozoan curator rather than attempting to scrape-off the offending bryozoans. In fact, a very high proportion of Echinocorys tests from the British Chalk are encrusted by bryozoans – just check your own collection.

In addition to the encrusting bryozoans, the Chalk is a rich source of broken branches from bush- or palm-like bryozoan colonies. A lot of these bryozoans in the NHMUK were collected during the nineteenth and early twentieth centuries at a time when the countryside was dotted with small chalk pits available to avid enthusiasts. Prominent among the band of Chalk aficionados were Arthur Rowe (1858–1926), a medical doctor who devised the early macrofossil zonation of the Chalk, and Christopher Gaster (1878–1963), an energetic, non-vocational geologist, who published several important papers on the Chalk of southern England. Working as a warder at Chatham Prison, William Gamble also sold a large number of specimens to the museum (Fig. 1). It is easy, though perhaps entirely fanciful, to imagine Gamble ordering the prisoners under his charge to break-up boulders of Chalk so that he could search for bryozoans and other fossils. Solicitor Reginald Marr Brydone (1873-1943) was another collector of Chalk bryozoans who, unlike the others mentioned here, published numerous papers, describing in all more than 400 new species of bryozoans. Brydone’s massive collection of Chalk bryozoans and other fossils went to the Sedgwick Museum in Cambridge.

Fig. 2. Scanning electron microscope images of two common colony-forms found among cyclostome bryozoans from the Chalk: (A) sheet-like colony of the ‘Berenicea’ type (Upper Chalk, Northfleet, Kent); (B) runner-like colony Voigtopora (Campanian, Mousehold, Norfolk).

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