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.

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 to be preserved, including colour banding in gastropods and ‘mother-of-pearl’ in rare nautilus cephalopods.

Ernest St John Burton, a local collector and amateur palaeontologist, recognised various levels within the coastal cliff section and his notation is still used to identify where fossils are found in the cliffs (Burton, 1929). These are A1-A3 (lowest) to L (highest). Burton (1933) also recorded some 400 fossil species throughout the Barton Beds and further taxa have been added to this number ever since. Simplified guide books to the stratigraphy have also been written, including that by Dennis Curry in one of the early series of field guides to specific areas of the country, published by the Geologists’ Association (Curry, 1958). For more detailed information about the area and extensive bibliography, see the splendid online website by West (2010).

Several stretches of the cliffs are protected by blocks of limestone from Purbeck and from the Mendip Hills (see Lewis et al, 2003; Lewis & Donovan, 2008; see also Carboniferous fossils protecting our Eocene coastline: Barton on Sea by David Lewis and Stephen Donovan). Strong points at various locations along the cliffs have ensured a build-up of beach material, further protecting the cliffs. Together, these have slowed the erosion of the cliffs by the sea so that new falls of fresh material onto the beach are less frequent than in the past. The increase in the accumulation of beach material has also covered up the clay ledges of the foreshore, which were an easily accessible source of fossils that did not require digging into the cliffs themselves. However, occasional storms do still uncover parts of these ledges, from time to time.

Sea urchins (or echinoids) can be found in the clays. Commonly, only portions of shell (‘test’) of the ‘irregular’ Schizaster branderianus and Eupatagus hastingiae are found, usually from horizons A2-A3 and F, respectively. Uncommonly, in horizon A3, large accumulations of tests of Schizaster branderianus can be uncovered, sometimes with their spines preserved more or less intact. More commonly, only fragments of test are found, which can be quite difficult to see.

‘Regular’ sea urchin tests are mostly fragmentary, preserved as a few ambulacral and interambulacral plates, but larger portions of test can sometimes be found and are occasionally almost complete (for example, Echinopedina paucituberculata). None of the known regular sea urchins are very large, being barely more than about 10mm in diameter. Irregular sea urchins can be much larger, with a test length for Eupatagus hastingiae of more than 45mm (see Lewis, 1989). Irregular sea urchins are more likely to be found than regular sea urchins and are also likely to be more complete.

Isolated spines can also be found. These include those of the more robust cidarids and more fragile ones from irregular sea-urchins (see Morton, 2011).

Preservation of the sea urchins varies. They can be found either as casts of the internal surface and external moulds of the test, preserved in clay, or the test material itself can be preserved. In one example, a specimen of Eupatagus excentricus (the holotype) has the peristomial buccal plates and periproctal plates preserved.

Some specimens are pyritic and need to be kept dry when stored. The structure and details of some of these tests can be very difficult to see, but application of some kind of varnish can sometimes help, particularly when viewed under a microscope and with suitably adjusted lighting.

Removal of fragile test portions directly from the clay can be facilitated by using a transfer process, either during collecting if there is time, or in the laboratory or home (Lewis, 1986). When the matrix is washed away, the detail on the test can be superb and specimens are sometimes even preserved with their spines.

Fragments of test are found not only in the clays themselves, but also in shelly, lenticular accumulations, which are composed of various other fossils. Careful sieving and picking over the residue is a good method of extraction from these sediments. Because sea urchins are very fragile, tests break up easily after death and just as easily when being collected. So be careful!

Sea urchins from the Barton Beds

The following are the currently known species of sea urchins from the Barton Beds. More may yet be discovered (see Lewis, 1989 p. 7, Morton, 2012), namely ‘Regular’ sea urchins:

• Indeterminate cidarid (Fig. 2a); originally described as Cidaris websteriana Forbes, 1852 – spines only; known from horizons A3-H. Other regular taxa known from spines include diadematids;
• Echinopedina paucituberculata Lewis, 1989 (Fig. 3a) – tests, both fragmental and pyritic internal casts; known from horizon J;
• Porosoma? sp (Fig. 2b-h) – spines and test fragments; known from horizons A3 and E;
• Coelopleurus sp (Fig. 3c, d) – test fragments and an incomplete test; known from horizon E; and
• Ambipleurus quaylei Lewis, 1989 (Fig. 3b) – incomplete tests and fragments; collected from a shell-drift accumulation in horizon A3.

 ‘Irregular’ sea urchins:

• Schizaster (Hypselaster) branderianus (Forbes, 1852) (Fig. 4) – casts, moulds, some well-preserved but incomplete tests; known from horizons A2-A3, though they have been reported from other horizons;
• Eupatagus (Eupatagus) excentricus (Gregory, 1891) (Fig. 5) – the holotype is an almost complete test with periproctal and peristomial plates; probably from horizon A3;
• Eupatagus (Eupatagus) hastingiae Forbes, 1852 (Fig. 6) – nearly complete but frequently crushed tests, some with radioles; known from horizon F;
• Atelospatangus grandituberculatus (Lewis, 1989) (Fig. 7); also referred to Maretia grignonensis (Desmarest) by Gregory in 1891, and
• Spatangus omalii Galeotti by Forbes in 1852 – well-preserved tests and numerous test fragments; known from horizons A2-A3.

And spines – from regular and irregular echinoids; known from various horizons.

Acknowledgement

My thanks to Phil Crabb of the Photographic Studio of the Natural History Museum for the photographs. I also thank Dr SK Donovan for helpful comments.

References

Burton, E. St. J. 1929. The horizons of Bryozoa (Polyzoa) in the Upper Eocene of Hampshire. Quarterly Journal of the Geological Society of London, 85, 223-239.

Burton, E. St. J. 1933. Faunal horizons of the Barton Beds in Hampshire. Proceedings of the Geologists’ Association, 44, 131-167.

Curry, D. 1958. Itinerary IV. The Barton Area in Curry, D. & Wisden, E. 1958 Geologists’ Association Guides No. 14: Geology of the Southampton Area: 1-16. Colchester.

Forbes, E. 1852. Monograph of the Echinodermata of the British Tertiaries. Monograph of the Palaeontographical Society, London: v-vii, 1-36, pls. 1-4.
Gregory, J. W. 1891. A revision of the British fossil Cainozoic Echinoidea. Proceedings of the Geologists’ Association, 12: 16-60, pls. 1-2.

Hooker, J. J. 1986. Mammals from the Bartonian (middle/late Eocene) of the Hampshire Basin, southern England. Bulletin of the British Museum (Natural History) 39 (4): 191-478, pls. 1-35.

Lewis, D. N. 1986. A method for the extraction of fossil echinoids preserved in clay. Tertiary Research, 7, 125-127, pl. 1.

Lewis, D. N. 1989. Fossil Echinoidea from the Barton Beds (Eocene, Bartonian) of the type locality at Barton-on-Sea in the Hampshire basin, England. Tertiary Research, 11, 1-47, pls. 1-7.

Lewis, D. N. & Donovan, S. K. 2008. Carboniferous fossils protecting our Eocene coastline. Deposits Magazine 14, 8-10.

Lewis, D. N., Donovan, S. K. & Sawford, P., 2003. Fossil echinoderms from the Carboniferous Limestone sea defence blocks at Barton-on-Sea, Hampshire, southern England. Proceedings of the Geologists’ Association, 114, 307-317.
Morton, A. 2012. A Collection of Eocene and Oligocene Fossils. Internet site: www.dmap.co.uk/fossils.

West, I. 2010. Barton and Highcliffe, Eocene Strata: Geology of the Wessex Coast of southern England. Internet site: www.soton.ac.uk/~imw/barton.