Qualitative analysis
Among the 204 colonial corals assessed in this study, the most common genera were Pamiroseris (145 colonies), Astraeomorpha (4 colonies), Oedalmia (8 colonies) and Chondorcoenia (3 colonies). The rest of the collected samples could not be assigned to a coral genus. All Pamiroseris specimens in our analysis belong to the species Pamiroseris rectilamellosa (Winkler).
Four ichnogenera of bioerosion traces could be identified: Rogerella, Entobia, Trypanites and Gastrochaenolites. Encrustation traces were classified as sponges, bivalves, polychaetes, bryozoans and scleractinian corals.
Rogerella appeared as a slit-like boring with an ovate aperture. It was mostly smaller than 0.3 cm in diameter, usually around 0.1 to 0.2cm. The depth could not be estimated (Fig. 3a, 3b).
Trypanites was categorized by a round aperture with a diameter greater than 0.2cm (Fig. 3c). It appeared mostly as a deep cylindrical boring, although the exact depth could not be estimated. The borings were often associated with encrusting polychaetes and surficial tunnel-like borings (Fig. 4d, Fig. 4e, Fig. 6c, Fig. 6d, Fig. 6e).
A round, surficial boring with a diameter not exceeding 0.2cm was categorized as Entobia. The opening measured usually about 0.1cm. An accumulation of openings in proximity could be found (Fig. 3f).
Gastrochaenolites appeared as an ovate-round boring not greater than 0.3cm in diameter. The formed chamber could not be investigated most of the time. It was distinguished from the other trace fossils by its depth since most of the time Gastrochaenolites appeared deeper than Trypanites or Entobia. If the chamber was visible, it could be investigated as club-shaped with an aperture narrower than the chamber itself (Fig. 3d, Fig. 3g, Fig. 3h, Fig. 5a, Fig. 5b).
For encrustation, the main reef inhabiting organisms, were identified. These consist of sponges, bivalves, polychaetes, bryozoans and solitary scleractinian corals.
Encrusting calcisponges have been found on one specimen and they appeared as round/oval structures up to 0.1 to 0.6cm in diameter and 0.1 to 0.3 cm in height. They had a porous texture with a widespread distribution, partly overgrowing each other (Fig. 4a).
Bivalves have been recognized by the residue or imprints of the shell or a circular recess with raised edges (Fig. 4b, Fig. 4c). They resemble Placunopsis, which is need of taxonomic revision (Todd and Palmer 2002).
Serpulids mostly left surficial tunnel-like buildings or filiform imprints on the coral, where the fossilized serpulid detached from the hardground (Fig. 4d, Fig. 4e). Small circular bryozoan colonies up to 0.3cm in diameter and 0.4cm of maximum height have a dotted texture (Fig. 4f).
Solitary scleractinian corals were sometimes found on the underside of Pamiroseris. Corals are mostly well preserved, with clearly visible septa. The corallites ranged from 0.3cm − 1cm in diameter and 0.3–0.7cm in height (Fig. 4g, Fig. 4h).
Quantitative analysis
Out of the 204 analyzed corals, 51 (25%) were affected by bioerosion or encrustation on the surface of the coral. The underside of the corals showed traces in 177 specimens (87%) (Fig. 7; Table 1).
Table 1
Bioerosion and encrustation data of both the underside and the surface of Pamiroseris rectilamellosa; Avg = Average
| Affected specimens | % specimens | Count pointing (Arithmetic Avg) | % point counting | Weighted average |
Bioerosion surface | 34 | 16.6 | 0.0057 | 0.57 | 0.0009 |
Encrustation surface | 39 | 19.11 | 0.0197 | 1.97 | 0.0058 |
Bioerosion underside | 130 | 63.72 | 0.0075 | 0.75 | 0.0065 |
Encrustation underside | 157 | 76.96 | 0.0315 | 3.15 | 0.0301 |
Bioerosion on the underside was mostly performed by polychaetes (38.6%) and bivalves (27.9%) while sponges had an abundance of 24.5% and barnacles of 8% (Fig. 8). On the surface, Trypanites borings were most abundant (81.7%) whereas almost no other trace fossils could be detected. The underside was more affected than the surface. Entobia, Gastrochaenolites and Trypanites borings differ significantly in the distribution between surface and underside since the underside was more intensively affected in each trace fossil (Fig. 8).
The encrusting taxa on the corals´ underside are dominated by polychaetes, followed by solitary corals and bivalves. Bryozoans and sponges are rare (Fig. 9). Again, the underside is more affected than the surface of the colonial corals. On the surface, bivalves are the main encrusters, followed by sponges and bryozoans (Fig. 9). Only encrusting polychaetes differ significantly in their distribution, which means they encrust the underside to a greater extent than the surface (Fig. 9). In all other encrusting groups, the difference was not as significant.
There is no correlation between encrustation and bioerosion density on the corals´ underside (Fig. 10).
Bioerosion through time
Table 2
Macrobioerosion data for the comparison of bioeroding taxa throughout the geological record
Time | Location | Reference | Sponges | Bivalves | Worms | Cirripeds |
Norian | Nayband, Iran | (Bertling 2000) | | 27.3 | 57 | 15.6 |
Rhaetian | Adnet, Austria | (Bertling 2000) | 11.1 | 25.2 | 60.3 | 3.3 |
Rhaetian | Feichtenstein, Austria | (Bertling 2000) | | 40.3 | 59.6 | |
Carixian | Moulay Idriss, Marocco | (Bertling 2000) | 2.5 | 46.2 | 31.1 | 20.2 |
Domerian | Beni Tadjit, Marocco | (Bertling 2000) | 3.3 | 11 | 50.6 | 35.2 |
Miocene | Mallorca | (Perry 1996) | 75 | 23 | 2 | 0 |
Pleistocene | Falmouth Formation, Jamaica | (Perry 2000) | 64.7 | 8.2 | 25.8 | 0 |
Holocene | Gulf of Eilat, Red Sea | (Loya 1991) | 10.8 | 19.4 | 42 | 0 |
Recent | Gulf of Eilat, Red Sea | (Loya 1991) | 32.1 | 19.8 | 35.3 | 0 |
Recent | Gulf of Eilat, Red Sea | (Loya 1991) | 15.6 | 18.5 | 47.4 | 0 |
Recent | Discovery Bay Jamaica | (Perry 1998) | 81.5 | 2.5 | 15.9 | 0.1 |
Recent | South China Sea | (Chen et al. 2013) | 38.3 | 48 | 5.6 | 8 |
To assess trends in relative bioerosion intensity through the geological record, we analyzed the results of six published studies (Table 2). Bioerosion of bivalves, cirripeds, and polychaetes decreased over time, whereas sponge bioerosion increased over time (Fig. 11). The rank-order of bioerosion traces has significantly changed since the Triassic. For example, worms were significantly more common than cirripeds in the Triassic, whereas today sponges are dominant compared to cirripeds. Including more datasets from the Jurassic and Triassic does not show a difference in the rank order of bioerosional active taxa (supplementary material, Fig. 12). This temporal data shows a gap from 191 Ma to 23 Ma, which is a result of our strict selection of criteria. There are several bioerosion studies e.g. (Bertling 1999; Scasso and Kiessling 2002), which do not report the required quantitative data to be included.