Colonising skeletal substrates: Encrusters and borers from the Upper Jurassic oyster shell beds of Central Poland

Michał Zatoń, Adrian Szewczuk and Mirosława Kuziomko-Szewczuk (Poland)

Skeletons of live and dead marine animals very often serve as a secondary hard substrate for various organisms that either encrust it (encrusters) or bore into it (borers). The terminology for encrusters and borers varies. However, following Paul Taylor and Mark Wilson’s latest and widely accepted terminology for organisms inhabiting hard substrate, those associated with skeletal substrates should be referred to as episkeletobionts and endoskeletobionts, respectively. Another, more convenient term is ‘sclerobionts’, as it refers to any organisms (encrusters or borers) inhabiting any hard substrate (biogenic or lithic, including skeletons of both live and dead organisms).

Sclerobionts are diverse in marine ecosystems today, being represented by such various groups as sponges, corals, bivalves, polychaetes, bryozoans and brachiopods. They were also diverse in the past, being represented by different taxa during different epochs. And like today, they inhabited various kind of hard substrate, such as stones, shells and reefs, creating specific benthic communities that changed and evolved over time. Sclerobiotic assemblages are also very useful in ecological studies.

As encrusters firmly adhere or cement to, and borers drill into, the substrates they colonise, they are always found as fossils where they had actually colonised. And as they play a crucial role in ecosystems, being both constructive (for example, as a component of reef frame-builders – the so called ‘binders’) and destructive (for example, as reef bio-eroders), there is a growing scientific interest in their ecology, which focuses on both Recent and past environments.

In this article, we will discuss examples from the late Jurassic (Kimmeridgian) sclerobiotic assemblages colonising oyster shells. The late Jurassic was also a time when modern-looking encrusting communities had become well-established.

Location and geological background

The Kimmeridgian oyster beds are currently well-exposed in two quarries at Małogoszcz and Wierzbica, located at the borders of the Holy Cross Mountains in Central Poland (Figs. 1A to C). The examples used in this article are oyster shells from the Małogoszcz quarry. However, similar oysters and sclerobionts can also be found in Wierzbica.

Fig. 1A. Map of Poland with Jurassic deposits indicated after removal of Cainozoic cover. HCM, Holy Cross Mountains; B. Exposure at the Wierzbica quarry. Oyster beds occur above the white line; and C. View on the oyster beds at the Małogoszcz Quarry. For scale, see the backpacks in the lower right corner.

The active quarry at Małogoszcz is situated in the south-western border of the Holy Cross Mountains (Fig. 1A). It is located at the village of Małogoszcz and belongs to the cement mill.

In the quarry, the Upper Jurassic (Kimmeridgian) is capped by Cretaceous (Upper Albian) sandstones. The Kimmeridgian is represented by diverse lithologies, from shallow water oolitic limestones, through to shelly limestones, shell beds and shales. The exposed section clearly displays the development of shallow-water carbonate platform passing into siliciclastic shelf deposits.

The oyster beds at the Małogoszcz quarry (Fig. 1C) are referred to as the Skorków Lumachelle lithostratigraphical unit, which is confined to the Lower Kimmeridgian. The whole unit is about 25m thick, but the oyster beds alone attain a thickness of about 18m. Their lower part (about 3m thick) is formed by an accumulation of oysters dominated by the species, Actinostreon gregareum (the so-called Actinostreon shell bed) (Fig. 2A). The Actinostreon shell bed also occurs in the Wierzbica quarry (Fig. 2B).

Fig. 2A. *Actinostreon* shell bed at the Wierzbica quarry; B. Lower valves of *Actinostreon* attached to a congener and being encrusted by smaller individuals.

These beds include other ostreid bivalves, such as the very common, but small Nanogyra, and the very rare ‘Liostrea’, Deltoideum and Gryphaea. As one goes up through the unit, Actinostreon oysters gradually become less common and are finally replaced by Nanogyra. Apart from oysters, the shell beds are also very rich in other fossils, like brachiopods, gastropods, crustaceans, corals, echinoderms (echinoids, crinoids and asteroids), fish and reptile remains.

The data on sclerobionts presented below come from the Actinostreon shell beds.

Borers

Traces left by boring organisms in the Actinostreon shell beds from Małogoszcz are not very diverse, being represented by only three ichnogenera: Gastrochaenolites, Ropalonaria and Talpina. However, these are not the only animal traces present in the deposits. Other fossils found outside the shell bed, display traces on Actinostreon shells belonging to the Entobia ichnogenus, and these will be briefly discussed below. The traces are the only evidence of organisms that were originally soft-bodied or had skeletons (shells) that were aragonitic and therefore dissolved, leaving no evidence of the borers themselves.

Entobia

This consists of a network of thin canals and chambers (Fig. 3A).

These borings have been common since the Jurassic, but were also present during the Palaeozoic. They were produced by sponges that bored into the carbonate substrate, which was both biogenic and lithic in nature. A living example of a boring sponge is Cliona.

Gastrochaenolites

These flask-shaped traces, consisting of a chamber and a neck leading to the surface, are very common in the fossil record and also on rocky shores today (Fig. 3B). They are produced by boring bivalves. Many kinds of Gastrochaenolites borings exist and, by comparison with some recent borings, they can be assigned to particular bivalves, such as Lithophaga – a boring bivalve very common in shallow waters.

Ropalonaria

These traces consist of a network of more or less perpendicularly arranged tiny borings (Fig. 3C). They consist of oval chambers and long, thin canals. Ropalonaria traces were left by soft-bodied bryozoa (“moss animals”). The oval chambers were occupied by feeding individuals called zooids and the canals are evidence of a common tissue (stolon) connecting the zooids in the colony.

Talpina

These borings are formed by thin, ramified (that is, splitting into branches) canals penetrating the shells, but with openings on the shell surface (Fig. 3D). The borings are interpreted as being made by suspension-feeding animals called phoronids, which are very close relatives of brachiopods.

Encrusters

Encrusters colonising the oyster shells from Małogoszcz are represented by several different groups of filter-feeding organisms, ranging from some as small as foraminifera to others that were as large as bivalves.

Foraminifera

These small-chambered protozoans are here represented by the genus Placopsilina, which has a characteristic rough shell (called a test), composed of tiny sediment particles cemented together (Fig. 4A).

Fig. 4. Encrusting organisms preserved on oyster shells: A. Foraminifer *Placopsilina*; B. Calcisponge; C. Sedentary polychaete tube of *Glomerula* encrusting both bryozoan colonies (small arrow) and oyster valve (long arrow); D. Branches of a uniserial colony of a bryozoan *Stomatopora*; E. Ribbon-like colony of a bryozoan *Oncousoecia*, with a brood chamber developed (arrow); and F. A small thecideid brachiopod.

The foraminifers can occur as single individuals or as groups on the oyster shell exteriors.

Porifera

Sponges are represented by tiny disc to sheet-like skeletons (Fig. 4B) adhering to the oyster shells as individual specimens. They belong to forms with calcareous skeletons, called calcisponges.

Bivalvia

These organisms are here represented by oysters cementing to the host Actinostreon shells (Figs. 2B and 4C). They are preserved solely as lower attachment valves, the upper one having been disarticulated and washed away. However, as may be deduced from the ornamentation and shape of larger individuals, they belong to small Nanogyra, ‘Liostrea’ and species of Actinostreon, settling on parental shells. Very often, they occur in groups, as both small (young) and larger (more mature) individuals.

Annelida

These are represented by sedentary polychaetes (“serpulid worms”), preserved in the form of long calcitic tubes, growing either in one direction or showing irregular growth patterns. They may be circular, quadrate or triangular in cross-section, representing different genera, such as Glomerula (Fig. 4C), Nogrobs or Propomatoceros. Very often, they overgrow the neighbouring encrusters (Fig. 4C) or are overgrown themselves. As in modern environments, here they can also occur in large numbers on a single shell.

Bryozoa

These colonial “moss animals” possess calcareous skeletons. They are represented by three forms of colonies:

Uniserial colonies, consisting of long connecting and bifurcating tubes, represented by a common genus, Stomatopora (Fig. 4D).
Ribbon-like colonies, consisting of a few series of zooecia (skeletons of particular individual zooids) that may become larger and then smaller, creating an irregular pattern of colony growth. They are formed by the genus, Oncousoecia (Fig. 4E).
Discoidal or plate-like colonies, the individuals of which grow in all directions (Fig. 4C).

Unless the brood chambers (convex structures of different shapes on the colony surface, used for brooding the larvae) are preserved, these colonies cannot be determined even to the genus level. In these cases, we refer to them as belonging to the genus ‘Berenicea’. These latter kinds of colony are the most common bryozoans on the oyster shells at Małogoszcz.

Brachiopoda

Brachiopods occur as tiny (up to 5mm) encrusting individuals belonging to a group called thecideids (Fig. 4F). They are preserved as complete shells or just the attachment valves. They too occur in groups on the exterior of the oyster shell.

Environment

The Actinostreon shell beds originated in a shallow-water, marine environment during periods of low rates of sedimentation. In such conditions, the oysters could colonise the seafloor to form dense populations. It is known that Actinostreon oysters were capable of developing different life positions in response to different sedimentary conditions. In higher rates of sedimentation, they attached themselves to semi-infaunal bivalves and grew upward; whereas, in lower sedimentation rates, they cemented themselves to other objects (including the Actinostreon shells and valves) on the seafloor. Similar life habits were also developed by Deltoideum oysters and these different strategies are preserved in the shell beds.

The high frequency of articulated specimens associated with fragmentary ones and their chaotic orientation may indicate that the shell beds are parautochtonous (that is, displaced from their original position) remnants of the original oyster banks. Such orientation and fragmentation of the oyster shells were most probably caused by episodic storms.

Therefore, the sclerobionts were able to colonise both the shells of living oysters before any disturbance, as well as isolated valves and shell fragments of dead individuals lying on the seafloor as a result of storm-induced displacement. Such small, isolated shells and valves were certainly prone to further physical or biological disturbance, leading to their being overturned and the previous encrusters being smothered. When this happened, a new round of colonisation could then take place. This is evidenced by the encrustation of both the exteriors and interiors of the oyster valves.

The rich faunal assemblages present within the shell beds point to a well-oxygenated environment. The accumulated oyster shells on the seafloor created suitable conditions for sclerobionts, because such ‘build-ups’ offered many niches for different organisms. Encrusters, such as bryozoans, serpulids or thecideide brachiopods, are well-known for their cryptic nature (so-called coelobiotic organisms), preferentially colonising rather sheltered and gloomy habitats. Other ones, like oysters and foraminifers, are known to colonise exposed surfaces.

Colonising skeletal substrates: Encrusters and borers from the Upper Jurassic oyster shell beds of Central Poland

Location and geological background