Bioerosion and encrustation on molluscan fossils: A case study from Chehelkaman Formation in the Sheikh section, west of Kopeh Dagh Basin

Macro-borings are macroscopic traces on hard substrates produced by animals that penetrate hard substrates by drilling, grinding, dissolving, and scraping. They are considered as biogenic structures that are produced mechanically or biochemically by guest animals or parasites in hard substrates (Miller, 2007). The survey of such buildings was studied in the present study. Bioerosion is a general process in substrates and widely extended on non-living hard substrates such as rock and woods (Bromley, 1994). However, living substrates can also be used as temporary substrates by different organisms and play the role of inorganic hard substrates, in smaller dimensions. The process of bioerosion on living substrates has long been attracted the attention of many paleontologists, and extensive studies have been carried out to clarify the details of predation on bivalves and, interestingly, most of these studies on Cenozoic fossil communities have been studied (Martinell & Domènech, 1981; Bromley & Martinell, 1991; Kitchell et al., 1981). Many invertebrates can drill. Warme & McHuron (1978) have identified at least 12 species of invertebrates as having the capacity to hunt through the digging of other calcareous shells. According to Stanley (2008), the importance of the hunting process is to the extent that it affects an important part of the diversity of marine faunal specimens. Investigating the location of hunting remains and borings on the hunt can provide information on the behavior of the predator attack. The study of borings in recent decades has become an important topic in many paleontological studies due to the key role of these works in providing important information on the process of hunting, digging animals and even paleontological analyzes (Taylor, 1970; Kitchell et al, 1981; Bromley, 1981). Since borings are generally categorized as trace fossils, they provide valuable evidence of the constructive behavior of the trace that cannot be achieved through the study of body fossils. Besides, the study of these borings, as they are part of the bioerosion process, provides evidence of organisms (Miller, 2007).

Oyster bivalves are the most abundant biological debris present in marine sediments due to the high preservation potential of their shells. Accordingly, these organisms provide the hardest living substrates for different groups of organisms (El-Hedeny, 2005). For this reason, bioerosion is the most important taphonomic process that affects the oyster shells, and so identifying these patterns is important in the study of their taphonomy (El-Hedeny, 2005). On the other hand, most of the oysters, which are semi-infaunal, are in the tidal zone. This group attaches itself to the substrate by special methods to be resistant to the high energy of this region (Stenzel, 1971). Post-mortem oyster shells are also a good substrate for organisms (Farinati & Zavala, 2002). Some carnivorous species can make holes in the outer calcareous skeleton of invertebrates. Interesting observations of some of these activities were observed in the oyster shells of the Chehelkaman Formation in the Sheikh section. These borings extend extensively on the larger left valves of oysters and include vertical, cylindrical, and horizontal traces that developed on the shells. The distribution of encrusters on the gastropods indicates that they are concentrated near the mouth of these organisms. This distribution is apparently due to the high capability of these parts to use nutrients for their growth (Žítt et al., 2003). The vertical and oblique borings that examined in this study were divided into two groups in size. Larger varieties have a maximum depth of 5.15 mm and a maximum diameter of 5.08 mm. Smaller types that are more abundant have a maximum depth of 2.50 mm and a maximum diameter of less than 3 mm. The edges are clear and the walls are smooth. Interior is concave, flat, vertical, or concave-flat. The internal and external openings can also be centered or non-centered. The drilled axis is also at different angles to the surface of the shell, giving rise to different shapes. On this basis, as well as on the geometric structure five different groups A to E can be named. In the Sheikh section, the central part of the oyster shells exhibits the highest concentration of borings. Interestingly these parts are not the thinnest part of the shells. More precisely, it can be seen that these points are located in the muscles situation of the bivalves and these parts have been skillfully targeted by the predator, which assures the immediate opening of the valves (Kelley, 1988). Thin shells break down after drilling, and thus, types of thick shells are more likely to be affected by biological erosion. In older specimens with a thick shell, there is no evidence of attack or predation. Therefore, it is likely that these organisms will not be considered as prey by hunters.

In this study, the oyster fauna and gastropods of the Chehelkaman Formation at Sheikh section were studied for bioerosion and encrustation. Accordingly, five groups borings (A to E) were separated on the oyster shells. Most excavations were observed in Pycnodonte sp. In this study ichnogenus Oichnus (Bromley, 1981) belonged to the Naticid, and Entobia (Bronn, 1837) belonged to the sponges. Maeandropolydora (Voigt, 1965) and Talpina (Von Hagenov, 1840) were attributed to worms' activities. Most excavations in the specimens were concentrated in the central part of the bivalves and the excavations were more selective.