Stephen K Donovan (The Netherlands)
It was a dry Saturday in February (2014), but it was blowing a gale such that some gusts stopped me dead in my tracks. My son, Pelham, and I were out for a walk in the Haarlemmermeersebos, which roughly translates as ‘the wood of the lake of Haarlem’. The area where we live, which includes the nearby Amsterdam Schiphol International Airport, is the bed of a lake that was drained over 160 years ago. So it is a flat, featureless, polder landscape (Ministry of Foreign Affairs, 1985, pp. 10-11), apart from what man has put into it; and is criss-crossed by canals and, less commonly, dotted by lakes. The canals in the Haarlemmermeersebos landscape that are intended for water transport are few; rather, most are part of the water management system in a landscape that is below sea level.
In such a landscape, the weekend geologist must look hard for ‘exposures’. Building and decorative stones are always of interest (Donovan, 2014). Beachcombing on the nearby North Sea coast can be rewarding, particularly after storms when Quaternary peat clasts are washed up on the shore (Donovan, 2013). But, in truth, there is more potential for the geomorphologist than the geologist or palaeontologist.
The point of our excursion in a gale was to model palaeontological collecting and to hone our observational skills in the open air. I had discovered a path paved with many hundreds of recent sea shells and rare flint pebbles in the Haarlemmermeersebos (Fig. 1); and I thought it would be both a fun and educational exercise to walk its length, the way one might walk out an interesting bed in a coastal exposure. I was particularly interested in the rare valves of oysters, which commonly support a larger diversity of encrusters and borings than other bivalves. So, another aspect of this exercise was to brush up on my recognition of encrusters and traces. The study of recent traces is neoichnology, hence the title – ichnology is the study of trace fossils.
The ‘exposure’ is long and of the many hundreds or thousands of shells used in the path (Figs. 1 and 2), few were identifiable fragments of oysters, and some of these were too small, broken and friable to be collectable. The shells were all marine in origin and presumably came from dredging offshore. Rare remains of local freshwater mussels were found by the path, close to the canal, and the best specimen was still articulated.
With one exception (Fig. 3A), the oysters that my son and I collected are all fragmentary. Preservation as fragments is not surprising, as the path is used by walkers, cyclists and horses; the surprise must be that any shells are still complete. Specimens discussed and illustrated in this article are deposited in the Naturalis Biodiversity Center, Leiden, the Netherlands, but are numbered arbitrarily here #1 to #11.
Oysters most probably represent Ostrea edulis Linné. Nine specimens preserve evidence of encrustation, boring or both (Fig. 3A-M). The suite of borings and encrusters was not particularly diverse (Table 1), but all might be found in a Cenozoic shell bed.
|Caulostrepsis||Entobia||bryozoan borings||Oichnus||annelid boring indet||encrusting bryozoa||balanid|
Encrusters include part of the basis plate from a balanid barnacle, Balanus sp. (Fig. 3F), and three shells bear parts of what were more extensive colonies of encrusting bryozoans, possibly similar to Membranipora sp. (Fig. 3G, J and K) (Barrett & Jonge, 1958). Two of these valves preserve bryozoans on the inner surfaces of the oysters (Fig. 3G and K), showing that at least some of this infestation by encrusters took place after death. Similar comments might be made concerning the borings on the inner surface of valves (see below).
However, it is the borings that are of the greatest interest to me. The oyster valves preserve three identifiable, invertebrate traces. Entobia isp. is the spoor of boring clionoid sponges. The apertures of the sponges appear as small round holes on the inner and/or outer surfaces of valves, arrayed in lines in some specimens (Fig. 3D, I, K and L). None of the valves has exfoliated on these specimens, so the internal structure of chambers connected by canals was not apparent (compare with, for example, Donovan & Fearnhead, in press, fig. 2).
The second trace was produced by boring annelid worms, Caulostrepsis taeniola Clarke (Fig. 3A-C, H and M). Caulostrepsis are U-shaped borings, with the two limbs separated by a central vane produced by boring polychaete annelids (Bromley, 2004, p. 460). Another trace, not Caulostrepsis (Fig. 3E), is a broad U-shape and was presumably also produced by an annelid. Similar borings may occur in close association with C. taeniola (Fig. 3C).
A third trace, more difficult to see, covers the valve in Fig. 3M. Many of the valves bear scratch marks produced by being crushed underfoot on the path, but this specimens has a series of numerous, fine, narrow lens-shaped structures. At first glance, these appeared to be scratch marks, as occur on some other valves, but they are penetrative and are more probably the product of a boring bryozoan. The ichnotaxonomy of bryozoan borings and boring bryozoans is confused (Häntzschel, 1975, pp. W136-W137; Bromley, 2004, pp. 463-464), and these structures are referred to as indeterminate bryozoan borings (bryozoan borings indet.). However, these structures are new to me; although they have a fossil record, I had never encountered them before.
I also collected a left valve of Spisula solida (Linné) (Tebble, 1976, p. 132, text-fig. 9; Wiese & Richling, 2008) that I noticed as including a boring not seen in the oysters, namely Oichnus paraboloides Bromley (Fig. 3N), a small, round, parabolic hole near the umbo (that is, the often somewhat protruding part of a bivalve shell formed when the animal was a juvenile). This was undoubtedly a predatory boring, whereas those in the oysters – Entobia isp., C. taeniola and the bryozoan borings indet. – are all domiciles. In modern shallow water marine environments, the most probable producer of O. paraboloides is a predatory naticid gastropod. Although no naticids (or any other marine snails) were observed in the ‘oyster path’, they would have been much less common than bivalves and their inflated shells would make them prone to crushing underfoot.
So, what did I gain from this eclectic exercise? I look upon such an urban geological (in the loosest sense) exercise, as being as necessary as practice in any pastime. Look upon it as an equivalent to a cricket net or tuning a musical instrument. Count the benefits. It got me out of the house, and my son and I enjoyed the fresh air and company, even if it was blowing a gale. The walking was good, made even better by it having a purpose. Keeping the eye in, like a batsman, is an important part of fieldwork. As I’ve got older and my eyes have slowly deteriorated, I’ve compensated by increasing my knowledge base and, therefore, my search pattern (my main research interest is echinoderms, not oysters).
The few oysters that were found reminded me of so many days collecting, the pursuit of an elusive fossil X. In my study, the specimens needed cleaning (a light wash with tap water) and drying. Then came identification, interpretation, photography and, lastly, the pleasure of writing this short article. And I also encountered bryozoan borings for the first time. All good, no bad.
Special thanks go to my son, Pelham, for his help, companionship and observations in the field.
|How to get there|
|If you are in Amsterdam Schiphol International Airport and would like to visit this site, transport is frequent and easy to use. Take the articulated red bus, route #300, towards Haarlem Centraal Station. This leaves from just outside the main terminal. It is nine stops to Spaarne Ziekenhaus, which is the closest stop to the views in Fig. 2. Alternately, go one stop further to Vijfhuizen and walk back through the Haarlemmermeersebos – you may need a slightly more detailed map than Fig. 1, at least for the Vijfhuizen end.|
Barrett, J. & Yonge, C.M. 1958 (1977 reprint). Collins Pocket Guide to the Sea Shore. Collins, London, 272 pp.
Bromley, R.G. 2004. A stratigraphy of marine bioerosion. In McIlroy, D. (ed.), The Application of Ichnology to Palaeoenvironmental and Stratigraphic Analysis. Geological Society Special Publication, 228: 455-479.
Donovan, S.K. 2013. A distinctive bioglyph and its producer: Recent Gastrochaenolites Leymerie in a peat pebble, North Sea coast of the Netherlands. Ichnos, 20: 109-111.
Donovan, S.K. 2014 (in press). Urban geology: A sunny Sunday in Hoofddorp. Deposits.
Häntzschel, W. 1975. Trace fossils and problematica (2nd edition, revised and enlarged). In Teichert, C. (ed.), Treatise on Invertebrate Paleontology, Part W, Miscellanea, Supplement 1. Geological Society of America, Boulder, and University of Kansas Press, Lawrence, xxi + 269 pp.
Ministry of Foreign Affairs. 1985. Compact Geography of the Netherlands. Ministry of Foreign Affairs, The Hague, 43 pp.
Tebble, N. 1976. British Bivalve Seashells. Second edition. Her Majesty’s Stationery Office, Edinburgh, 212 pp.
Wiese, V. & Richling, I. 2008. Schelpen van het Nederlandse strand. Haus der Natur – Cismar, Germany, 2 pp.