Bryozoans in the English Chalk

Dr Paul D Taylor (UK) 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. 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 … Read More

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Humble flint sea urchins and the stories they tell

Joe Shimmin (UK) Flint is a very hard-wearing rock from the chalk of the Upper Cretaceous. Whole beaches made of flint pebbles can be found many miles away from the chalk strata that the nodules originated in, owing to the rock’s ability to withstand the processes that destroy other rocks quickly. Flint sea urchins are especially hard-wearing, as their rounded shapes require a lot of force to damage, while less-rounded flints tend to break up over time if subjected to high-energy environments, such as beaches and fast-flowing rivers. Because of this robustness, it is possible to find flint urchins, which have undergone some very interesting journeys before being collected, adding to their interest for fossil hunters. Fig. 1. The hardness of flint and the rounded shape of flint urchins make them extremely robust fossils. All flints start off within chalk strata. Where these strata are exposed at the coast or in quarries and cuttings, it is possible to collect flint sea urchins, which, at first, look very much as if they are preserved like every other urchin found in chalk. They have a white calcite-replaced test and all that can be seen of the flint within is a slight blueish tint or maybe a glimpse of the nodule through the anal or oral apertures. Of course, flints can also be found that partially or fully envelop an urchin and, in these cases, highly aesthetic display pieces can sometimes occur. Fig. 2. Two of these pristine fossil urchins, extracted straight from … Read More

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Jamaica’s geodiversity (Part 2): Highlights from the Neogene

Stephen K Donovan (The Netherlands) and Trevor A Jackson (Trinidad) This is the second and concluding part of our introduction to Jamaica’s geodiversity. Here, we are concerned with more Neogene ‘highlights’ dating from the Middle or Late Miocene, about 10mya, when the island became, once again, sub-aerially exposed. The glossary provided in Part 1, as well as the maps (Donovan & Jackson, 2012, figs 1 and 2), are also relevant to this article and first appearance of the relevant terms in the text are highlighted in bold italics. Highlights 1 to 5 were discussed in Part 1 and 6 to 12 are described below. Highlight 6. Wait-A-Bit Cave Jamaica is a land of caves and sinkholes (Fincham, 1977). About two thirds of the rocks exposed at the surface of the island are limestones, which are soluble in acidic groundwaters, that is, those that are more or less rich in dissolved CO2. The percolation of these waters ‘excavated’ extensive cave systems throughout Jamaica, mainly by dissolution, since the island was sub-aerially exposed about 10mya (Miller, 2004). Wait-a-Bit Cave, south of Green Town in the parish of Trelawny (Fig. 1), is unusual among these myriad caves for reasons apart from its euphonious name. Fig. 1. Cave survey and selected passage cross-sections (A-A’ to G-G’) of the Wait-a-Bit Cave, parish of Trelawny, Jamaica (after Miller & Donovan, 1996, text-fig. 2). The thick dashed line to the west of E’, and south of F’ and G’, marks the edge of the limestone overhang from … Read More

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Fossil folklore: Echinoderms

Dr Paul D Taylor (UK) The distinct five-fold – or pentameral – symmetry of echinoderms makes them particularly striking fossils. Some even have a vaguely mystical appearance. Modern echinoderms – starfish (asteroids), sea urchins (echinoids), feather stars and sea lilies (crinoids), sea cucumbers (holothurians) and brittle stars (ophiuroids) – are all animals of the oceans. As no echinoderms inhabit freshwater environments, it is difficult to envisage what ancient people living far distant from the coast and who had never visited the sea might have thought when finding a fossil echinoderm with peculiar star-like marks on its surface. How could such a stone have been formed? What was its significance? Did the star markings point to a heavenly origin? Could the stone possess magical or mystical properties? Even today, many folklore beliefs about echinoderms persist. For example, the echinoid, Eurhodia matleyi, is found in west-central Jamaica around Stettin, where it can be abundant on bedding planes of the Eocene Yellow Limestone Group. These fossils are locally referred to as ‘lucky stones’, because of the distinctive star-shaped pattern of the ambulacra (SK Donovan, pers. comm, July 2003). Fossil echinoderms must have seemed worthy of collecting and treasuring regardless of how they were viewed. Indeed, some were even worn as amulets to protect against evil. Not surprisingly, echinoderms have a folklore that is matched only by that of ammonites (see Fossil folklore: Ammonites). Pre- and unscientific beliefs about various kinds of fossil echinoderms abound and a plethora of folklore names have been given … Read More

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Fossil sea urchins from the Middle Eocene of the Bartonian of Christchurch Bay, UK

David N Lewis (UK) The spectacular fossil gastropods and the teeth of sharks – found at the type locality of the Middle Eocene Bartonian in Christchurch Bay (Hampshire and Dorset) – overshadow the other fauna and flora found there. However, among the ‘Cinderella’ groups are the echinoids (sea urchins). Several kinds, both ‘irregular’ and ‘regular’, can be found, some preserved with superb detail. Fig. 1. Sketch maps to show the location of Barton-on-Sea (modified after Lewis & Donovan, 2008).The coastal holiday resorts of Christchurch Bay, near the New Forest, include Highcliffe to the west, Milford-on-Sea to the east, and the well-known Barton Cliffs of Barton-on-Sea between the two (Fig. 1). All lie within the Hampshire Basin of southern England. This coastal stretch is famous for its extensive range of well-preserved Eocene fossils found in the sea cliffs and on the foreshore. The most fossiliferous area is sometimes referred to simply as ‘Barton’, and the clays and sands in which the fossils are found as the ‘Barton Beds’. Of particular interest to fossil collectors, students and holiday-makers alike are the abundant fossil molluscs and the teeth of sharks. However, there are other fossils too, including plants, microfossils, a wide variety of other invertebrates such as bryozoans, brachiopods, corals, crabs, echinoderms (brittle-stars, starfish and sea urchins) and worms, and vertebrates including fishes, reptiles and rare mammals (see Hooker, 1986). Trace fossils can also be seen in the clay sequences. In fact, some of the clays allow considerable fine detail of the fossils … Read More

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Diadematoid echinoids: A cryptic part of the tropical fossil record

Stephen K Donovan (The Netherlands) The writers of holiday brochures invariably fail to mention, let alone emphasise, the bad points of a location. For example, I’ve lived in both Jamaica and the Netherlands, and, for me, the thing that unites these two countries is the number of mosquitoes. However, as mosquitoes aren’t a good sales point with tourists, they are carefully ignored in holiday brochures and advertisements. Another Caribbean critter that doesn’t get mentioned until you actually arrive and want to go for a dip in the sea is the sea urchin, known in Jamaica as a sea egg. In truth, any danger to the unwary swimmer comes from the few species of regular echinoid that have long, pointed spines. These are found in many shallow water habitats, but are best concealed (and, therefore, most dangerous to the swimmer) in seagrass beds. The best protection from these echinoids is to wear an old pair of training shoes that you’d be happy to dispose of at the end of the vacation. However, without prior knowledge, who would take such a thing on holiday with them? Among these echinoids, the one most likely to ruin your holiday is the black, long-spined urchin, Diadema antillarum (Phillipi) in the Caribbean; which is broadly similar in morphology to the figured specimen, Diadema setosum (Leske) from Indonesia (Fig. 1). These echinoids have relatively small bodies, but numerous long, needle-like spines. These are, essentially, single calcite crystals. The unsuspecting swimmer treading on such an urchin will have … Read More

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