Predatory drill holes in the oldest thyasirid bivalve, from the Lower Jurassic of South Germany

Abstract

Drill holes provide valuable information about palaeoecological interactions in fossil ecosystems, but the Jurassic drill‐hole record is scarce. We report circular drill holes in the infaunal bivalve Eothyasira antiqua(Münster in Goldfuss) from the Pliensbachian grey shales of the Amaltheenton Formation of Franconia, southern Germany. The outcrop of the Amaltheenton Formation at Buttenheim yields a rich bivalve fauna (57 species), which represents a typical soft‐bottom community. Eothyasira antiqua, the yet oldest known member of the family Thyasiridae, represents only 1% of the total bivalve specimens from Buttenheim, but shows an exceptionally high frequency of drill holes. With a drilling rate of 81% of the well‐preserved articulated specimens, this occurence is unique compared with other drilled bivalve records from the Mesozoic and even from Recent assemblages. The drill holes resemble those of modern naticids with a larger outer opening and are assigned to the ichnospecies Oichnus paraboloides Bromley. Statistical methods show a non‐random distribution of the drill holes over the shell surface with a concentration on the central flank. The drill‐hole producer was highly efficient in drilling and exhibited a remarkable prey and site‐selective behaviour. The drill holes on the moderately deep infaunal Eothyasira suggest that the predator was also infaunal for at least parts of its life and could cope with poorly oxygenated substrate conditions occurring temporarily in the Amaltheenton environment. Drill‐hole characteristics point to a gastropod as producer. None of the about 50 gastropod taxa of the Amaltheenton Formation can be associated with the drill holes with certainty, but Hayamia reticulata (Münster) seems to be the most likely candidate. The results of this study support the opinion that drill holes are generally a scarce phenomenon in the Early Jurassic generated by rare, probably highly specialised predators. Preservational biases are considered to have been less important.

Drilling predation on Early Jurassic bivalves and behavioral patterns of the presumed gastropod predator—evidence from Pliensbachian soft-bottom deposits of northern Germany

“…Hence, how many times the drilling habit evolved in the early Mesozoic cannot be assessed. The presence of naticid-like drill holes long before the origination of Naticidae (e.g., Ausich and Gurrola 1979; Hoffmeister et al 2004; Klompmaker et al 2016; Karapunar et al 2020a; this study) or muricid-like drill holes long before the origination of Muricidae (Harper et al 1998) corroborates that drilling predation evolved several times in different gastropod groups during Earth's history. The presence of the same drill-hole shapes in the fossil record long before the appearance of modern drilling taxa (e.g., Naticidae and Muricidae) in the fossil record also shows that the shapes of the drill holes are convergent, and identification of the drill-hole producer in the fossil record based solely on drill-hole shape might be erroneous (e.g., interpreting O. paraboloides to be made only by Naticidae).…”

“…The lateral continuation of the same stratigraphic unit (the Amaltheenton Formation) further suggests uninterrupted connection of the same environment in the northern and southern settings, at least during the late Pliensbachian. One argument against the assumption of the same identity is the difference in the range of drill-hole diameters in both settings (0.9–3.4 mm, median 1.76 mm in Gresslya [this study]; 0.7–2.9 mm, median 1.4 mm in Eothyasira [Karapunar et al 2020a]; the median drill-hole sizes are significantly different, Kruskal-Wallis test, H = 18.34, p < 0.05). This difference might indicate different predators in the two settings.…”

“…Previously, Karapunar et al (2020a) used drill-hole diameter as a proxy for size of a potential gastropod predator from the Amaltheenton Formation in southern Germany and discussed the possible identity of the gastropod predator in that size range. They concluded that the most likely candidate is Hayamia reticulata (Münster in Goldfuss, 1844), which occurs in the same horizons with the drilled specimens from that locality.…”

“…The specimens from both regions (southern and northern Germany) exhibit completely perforated holes and occasionally multiple holes. Among the drilled Eothyasira specimens, 14% exhibit two drill holes either on the same valve or on both valves (Karapunar et al 2020a). In the case of Gresslya specimens from the studied localities, 13% exhibit multiple drill holes.…”

“…For comparison, we reexamined Eothyasira antiqua (Münster in Goldfuss, 1841) and Gresslya intermedia (Tate, 1876) specimens from the upper Pliensbachian Amaltheenton Formation at Buttenheim, southern Germany, for trace fossils other than drill holes to explore whether there is any trace fossil associated with drill holes. These specimens were already part of previous studies (Karapunar et al 2020a, b) but not in the context of coprolite and burrowing traces observed in the bivalves of northern Germany.…”