FIGURE 1. Location map of Puerto Lobos in the southern part of the San Matías Gulf, Patagonia, Argentina.

figure1a 

FIGURE 2. Sampling: (1) view of the modern beach of Puerto Lobos showing a boulder field at low tide (distance from the boulder to the steps is approximately 15 m). (2) detail of the 50 x 50 cm quadrate used for sampling mollusk specimens. (3) Detail of the boulder field (from side to side the picture measures ca. 5 m).

figure2a 

FIGURE 3. (1). External view of a specimen of Nacella magellanica with a drilling on the posterior part (CEGH-UNC-26872). (2). Internal view of a specimen of Nacella magellanica with a drilling on the posterior part, above the U-shaped muscle scar (CEGH-UNC-26873). (3). Modeling of limpet shell as cone and truncated cones to estimate the proportion of the area represented by the different sectors. To the left, a real representation of a limpet shell. The U-shaped muscle scar (U-scar) is visible from the inside. To the right, the model composed of one cone and two truncated cones.

 figure3a

FIGURE 4. Photographs of drill holes. (1) Successful muricid drill hole (Oichnus simplex) (internal minimum diameter of the hole: 1.76 mm; CEGH-UNC-26874). (2) Unsuccessful muricid drill hole (external minimum diameter of the hole: 1.75 mm; CEGH-UNC-26875). (3 and 4), Octopus drill hole (Oichnus ovalis) (internal minimum and maximum diameters of the hole: 0.15 and 0.27 mm, respectively. Both images correspond to the same specimen, CEGH-UNC-26876).

figure4 

FIGURE 5. Scanning electron microscope images of drill holes on Nacella magellanica shells. (1). Complete and functional Oichnus simplex (CEGH-UNC-26877). (2). Complete non-functional O. simplex (CEGH-UNC-26877). 3. Complete and functional Oichnus ovalis (CEGH-UNC-26876). (4-6). Incomplete O. simplex. (4). Note the flat nature of the bottom. (5) and (6) magnifications of a drill hole wall with probable radular rasp marks. (4-6, CEGH-UNC-26878).

figure5 

FIGURE 6. Box plot of anterior-posterior length (APL) of drilled (n=36) and undrilled (n=59) Nacella magellanica shells. Undrilled shells are slightly larger although non significantly different from drilled ones (Wilcoxon sum ranks test, p= 0.284; t-test, p= 0.179).

figure6 

FIGURE 7. Dispersion plot between drill hole minimum inner diameter (mm) and length (mm) of the limpet N. magellanica.

figure7 

FIGURE 8. Stacked bars with the distribution of observed vs. expected frequencies (uniform distribution) of drill holes in sectors along a dorso ventral direction. Data as percentages.

figure8