Urban geology: a rostroconch in Hoofddorp

Part of my job is to provide service teaching for the University of Leiden. The university lacks a geology department, but my colleagues and I provide tuition in stratigraphy and palaeontology for life science students at the undergraduate and masters degree level. One of my favourite practical classes is a building stones tour of a part of Leiden that is rich in Mississippian (Lower Carboniferous) limestones, which are packed with fossils. These have been used for facing stones, external stairs and paving slabs. Many have been in place for some hundreds of years and many have been etched by slow solution by rainwater as a result. Common fossils include crinoid columnals, tabulate and rugose corals, brachiopods, and molluscs (Donovan, 2016; van Ruiten and Donovan, in review). These are most commonly seen in two dimensions and random sections, a different view of life to what the life scientists are usually accustomed.

Figure 1
Fig. 1 Outline map of the Beukenhorst area of Hoofddorp, the Netherlands (modified after Donovan, 2014, fig. 1). Some roads are largely omitted for clarity. The railway (trellised-line) is one stop from Amsterdam Schiphol Airport to the east; similarly, the elevated bus lane of the Zuid Tangent (routes 300 and 310) come from Schiphol. Key: * = Mississippian limestone with rostroconch (see Fig. 2A); + = railway or bus station; B = Beukenhorst Zuid Tangent station; H = Hoofddorp railway and Zuid Tangent station.

One group of fossils in these rocks were a mystery until recently, but we now know they are sections through rostroconchs (Donovan and Madern, 2016, p. 349), an extinct group of Palaeozoic molluscs. Rostroconchs were formerly considered to be an ancient group of bivalves and they are certainly bivalve-like in appearance, but lack an articulation of interlocking teeth and a ligament. That is, the shell is a univalve, a one-piece structure. I had only seen the sections of rostroconchs in building stones in Leiden. It was therefore gratifying, shortly after publication of these fossils, to receive an email from Jelle Reumer recording them from building stones in Middelburg, elsewhere in the Netherlands.

This brings me to the Hoofddorp rostroconch, also in Mississippian limestone. Unlike the buildings in Leiden discussed above, the specimen from Hoofddorp is from a recent development, in an area that was a farmer’s field when I first moved there in 2001. The site (Figs. 1 and 2A) is southwest of Hoofddorp railway station, just one stop from Schiphol International Airport. The site is a long reflecting pool (Fig. 2A), in the centre of an area of office buildings sandwiched between two roads that parallel the Zuid Tangent bus lane to the northeast (the roads are not shown in full in Fig. 1). The specimen (Fig. 2B) was found in the limestones edging the pool, on the left in Fig. 2A, and closer to the end furthest from the station.

Figure 2
Fig 2 Images of the locality of Mississippian (Lower Carboniferous) limestones near Hoofddorp station, the Netherlands. (A) The view of the reflecting pool, looking in the direction of Hoofddorp station (hidden by a crane, a hotel and offices). The rostroconch was found in the limestones to the left that are edging the pool. (B) The Hoofddorp rostroconch. Scale in centimetres.

That this specimen is a rostroconch is best demonstrated by comparing it with a complete shell (Fig. 3). Fig. 3 demonstrates the gross similarity to a bivalve. The section illustrated in Fig. 2B is close to those illustrated by Donovan and Madern (2016, fig. 1). Finding this specimen, less than two kilometres walk from my home, was like bumping into an old friend unexpectedly.

This leads me to a pertinent comment and a complimentary question. I will not be surprised if any or all of you recognise rostroconchs in your local building stones. Good luck. I feel that I have been a little dense in only recognising them now. But this leads to my second comment – what other ‘unidentifiable’ fossils are there in building stones in your local area? I am confident that someone in the Deposits ‘brains trust’ will be able to shed light on its true affinity.

Figure 3
Fig 3 Detail of a Mississippian (Lower Carboniferous) rostroconch, Pleurorhynchus fusiformis M’Coy (see Amler and Rogalla 2004), a nomen dubium, presumed natural size, from Anon (1885-1890, plate ‘Steinkohlenformation 1’). This figure is based on M’Coy’s original illustrations. Anterior towards the top of the page.

References

Amler, M.R.W. & Rogalla, N.S. 2004. History and nomenclature of the Conocardioidea (Mollusca: Rostroconchia). Paläontologische Zeitschrift, 78: 307—322.

Anon. 1885-1890. Meyers Konservations – Lexikon: eine Encyklopädie des allgemeinen Wissens, 4. Bibliographisches Institut, Leipzig.

Donovan, S.K. 2014. Urban geology: A sunny Sunday in Hoofddorp. Deposits, 38: 8-10.

Donovan, S.K. 2016. A mollusc-coral interaction in a paving slab, Leiden, the Netherlands. Bulletin of the Mizunami Fossil Museum, 42: 45-46.

Donovan, S.K. & Madern, P.A. 2016. Rostroconchs in Leiden. Swiss Journal of Palaeontology, 135: 349-352.

Ruiten, D.M. van & Donovan, S.K. (in review with the Bulletin of the Mizunami Fossil Museum). Provenance, systematics and palaeoecology of Mississippian (Lower Carboniferous) corals (subclasses Rugosa, Tabulata) preserved in an urban environment, Leiden, the Netherlands. Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen.


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