Stephen K Donovan (The Netherlands) and David AT Harper (UK)
The most obvious manifestations of geological materials in the urban environment are building and facing stones, and similar rocks used in street furniture, such as kerbstones. As a Londoner, SKD was impressed as a boy by the massive kerbstones that he saw in the City and locally where he lived. It was only as his knowledge of geology grew that he discovered such stones to be truly exotic, being largely crystalline rocks (mostly granites in the broad sense) and probably derived from the southwest or the north of the British Isles. A field guide to the kerbstones of London would have accelerated his education in geology at that time.
More satisfactorily to palaeontologists, such as the authors of this article, are building stones that are fossiliferous. We have particular interests in the palaeontology of Palaeozoic limestones. These are common building stones and street furniture in many cities in the Netherlands (and elsewhere). These rocks are all imported (Van Ruiten and Donovan, 2018; Dr Bernard Mottequin, email to DATH, 9 May 2018) and are mainly Mississippian, although there are some limestones of Devonian age here and there (Van Roekel, 2007; Reumer, 2016).
However, the Mississippian limestones are the more widespread and contain abundant fossils, from the well-known, such as bryozoans (Donovan and Wyse Jackson, 2018), brachiopods, crinoids, and rugose and tabulate corals (Van Ruiten and Donovan, 2018) to the more exotic, such as rostroconch molluscs (Donovan and Madern, 2016).
This article presents our first exploration of the brachiopods in these building stones. For a pocket guide to brachiopods, we recommend Rudwick (1970), now a little long in the tooth, but packed with information. Clarkson (1998), together with Benton and Harper (2009), will also be useful. We have chosen one group amongst the Mississippian articulated brachiopods as being particularly distinct when viewed in section (Figs. 1 to 3), a group that were large, thick-shelled and, in consequence, obvious fossils, namely the productids. Productids are locally common fossils in the Mississippian of northern Europe (for example, Nolan et al., 2017) and have unusual shell geometry. In this article, we focus on Mississippian limestones in the Netherlands, imported from elsewhere, probably Belgium, but productids are identifiable in building stones in Britain as well (for example, Donovan, in press, fig. 3B, C).
The productids are notable for the geometry of their shells. We are used to finding the disarticulated valves of infaunal bivalve molluscs washed up on the beach. These molluscs are biconvex. That is, both valves are more-or-less domed mirror images of each other; the plane of symmetry passes between the left and the right valves. The whole shell is more or less globular or seed-shaped – in short, streamlined for moving through wet sand or mud. Most brachiopods are similarly biconvex, although they have well-known differences in symmetry from bivalve molluscs (Harper, 2005). Some epifaunal bivalves have a more flattened valve resting on a deeply-curved valve – oysters are the commonest example – but these are easily recognised as such.
Productids belong to the Strophomenata group of articulated brachiopods. Distinctive features include a straight hingeline and large size; and productids are among the largest of brachiopods. What unites strophomenates as a distinct group is the curvature of the shells. The larger valve bears the pedicle attachment structure (Rudwick, 1970; Clarkson, 1998), at least when juvenile, although it is commonly lost in adulthood. The ventral valve is convex. The other valve – the dorsal valve – is the unusual feature; it is concave and sits inside the large ventral valve (Figs. 1B, C, 2C and E, and 3 all show this well), leaving only a limited space for the soft parts. In longitudinal section – posterior (umbo) to anterior (commissure) – the shape of the shell is like a comma or a section through the head of a ladle (Fig. 3 shows this well).
Sections transversely across the shell appear as two concentric circles (best seen in Figs. 1A, B and D, and 2D and F). The thicker outer circle is the pedicle valve in section; the thinner-shelled dorsal valve is inside. These orientations are easily modelled – cup both hands and put the knuckles of the right hand (= ‘dorsal valve’) in the palm of the left hand (= ‘ventral valve’).
Although articulated brachiopods are typically epifaunal (that is, living on the surface of a substrate, for example, rocks), there is consistent evidence from productids preserved in life position that they were passively infaunal (that is, living in the substrate of a body of water, especially a soft sea bottom). An infaunal bivalve has a streamlined shell that can be moved up and down within the sediment by using a muscular foot. They are infaunal, but vagile (that, they are able disperse or move from place to place). Brachiopods are not molluscan and have no foot. Among the brachiopods, only inarticulated brachiopods such as lingulids have solved the problem of how to be actively infaunal, with a smooth, long, essentially biconvex shell and a large, muscular pedicle, whose function is analogous to that of the foot in bivalves.
Adult productids lacked these adaptations: they lacked a pedicle and were poorly-shaped for sliding through the sediment. Instead, they lay on the seafloor and were passively infaunal, that is, buried in life. The shell was stable, with the thicker, heavier ventral valve lowermost and the commissure elevated above the sediment surface. Some productids were more securely anchored by spines on the ventral valve, which penetrated the sediment (Rudwick, 1970, figs 46, 47). That these brachiopods were not buried alive is explained by their size and the comma-like section of the shell. Although most of the shell may have been buried, the long commissure was elevated above the sediment surface like a snorkel and could still access clean water, which could be drawn down into the body space for respiration and feeding by the lophophore (that is, a structure with the shape of a horseshoe with tentacles around the mouth).
This brings us back to the specimens figured in this article. Perhaps because of what we are used to seeing on the beach, it is common to see a fossil shell bed and assume that the shells represent a more or less transported (= death) assemblage (allochthonous). Yet there is ample evidence that the shelly associations of productids shown in Figs. 1 and 2 are essentially in situ, that is, life assemblages (autochthonous). The principal evidence is that so many shells are obviously articulated with both valves in place.
Transport would almost certainly have disarticulated them and the evidence of multiple specimens decreases the likelihood of reworking to, essentially, zero. Furthermore, most of the shells are sectioned transversely, because they have similar orientations, leaving a valve-in-valve structure (Figs. 1 and 2). From this, we interpret that the surfaces of the kerbstones are parallel to bedding. (Bedding must have been massive, as there is rarely evidence of thin beds in the road-side of a kerbstone.) Some specimens do show longitudinal sections, as discussed above, and these presumably toppled over onto their sides in life.
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