Figure 1.
Geographic location of the Cabo Cope outcrop (Murcia Region, SE Spain).
Figure 2.
Stratigraphy and biostratinomy of the Cabo Cope outcrop.
(a) A panoramic view of the northwestern wall of the outcrop; (b) A schematic stratigraphic column of the three informal units distinguishable in the outcrop (stratigraphic position of samples indicated with stars); (c) A close-up of matrix-supported skeletal accumulations dominated by abundant tubes of Ditrupa arietina (Lower Unit); (d) A close-up of a monospecific concentration of D. arietina (Middle Unit).
Table 1.
A presence-absence list of macrofossil taxa documented from the three informal stratigraphic units of the Pliocene succession of the Cabo Cope outcrop.
Figure 3.
A schematic summary of biometric measurements used in this study for Ditrupa tubes and tube fragments.
(1) Maximum tube diameter; (2) Relative drillhole location; (3) Maximum drillhole diameter; (4) Minimum drillhole diameter.
Table 2.
Summary of sampling information and resulting data for the 9 samples used in this study.
Figure 4.
Drilled specimens of D. arietina.
(a) A complete specimen with an oval drillhole located anteriorly; (b) A tube fragment with close-to-circular drillhole. Note a second drillhole partially preserved at the posterior end of the specimen (i.e., the tube broke across the drillhole); (c) A tube fragment with close-to-circular drillhole; (d–g) Tube fragments with singular complete oval drillholes; (h) A tube fragment with a complete oval drillhole (note the second, partially preserved drillhole located at the posterior end of the fragment); (i) An unusual tube fragment with two partly superimposed complete drillholes; (j–k) Tube fragments broken across drillholes. Scale bars are 2 mm for (a) and 1 mm for (b–k).
Figure 5.
Stratigraphic changes in quantitative paleoecological patterns.
(a) Abundance of specimens per standardize sample unit (207 g); (b) average specimen size (mean maximum specimen width); (c) drilling frequency (fragmentation-corrected proportion of specimens drilled); and (d) average drillhole size (mean minimum drillhole diameter). Solid small dots represent individual samples and larger gray dots are mean horizon values (arithmetic averages of sample values per horizon). Because horizons 3–5 are represented by one sample each, sample values also represent mean horizon values. See Table 2 for data summary.
Figure 6.
Bivariate scatter plot of total specimen length versus maximum specimen width based on 28 complete specimens.
A solid line represents a reduced major axis regression model. Symbols: r2 = coefficient of determination for the reduced major axis regression (associated p-value for the null hypothesis that the r2 = 0). Inset: Bivariate scatter plot showing negative allometric relationship between specimen size (estimated here as total specimen length) and tube shape (expressed as width-to-length ratio). Small tubes tend to be twice as wide relative to their length when comparing with large tubes.
Figure 7.
Bivariate scatter plot of absolute abundance of specimens plotted against drilling frequency.
A significant negative correlation is observed, with drillholes being less frequent in samples that came from more fossiliferous units. Inset plot shows that the pattern holds when data are plotted by horizon (with values averaged across samples for Horizons 1 and 2), although first differences are not significantly correlated. Symbols, r = Spearman correlation coefficient; rD = Spearman correlation coefficient based on first differences; p = a two-tailed probability of r = 0; pD = a two-tailed probability of rD = 0.
Figure 8.
Size frequency distribution of Ditrupa specimens, with size estimated by the maximum specimen width.
(a) Drilled specimens (n = 233); (b) Undrilled specimens (n = 270). See Table 3 (“pooled data” rows) for data summary and statistical tests.
Figure 9.
Comparison of mean specimen width for drilled and undrilled specimens grouped by stratigraphic horizon.
For Horizons 1 and 2 multiple samples are available, whereas Horizons 3 through 5 are represented by one sample each. Small dots represent individual samples and larger gray dots are mean horizon values (arithmetic averages of sample values per horizon). See Table 3 for data summary and statistical tests.
Figure 10.
Correlation between specimen size (maximum width) and drillhole size.
Drillhole size estimated as (a) minimum drillhole diameter, and (b) maximum drillhole diameter. Data pooled across all samples and horizons. Symbols, r = Spearman correlation coefficient; p = a two-tailed probability of r = 0; n = a number of specimens analyzed. See Table 4 for additional information.
Table 3.
Comparison of size frequency distributions for drilled and undrilled specimens.
Table 4.
Spearman correlation coefficients (upper right numbers) and corresponding p-values for the null hypothesis r = 0 (lower left numbers) for specimen and drillhole-derived variables.
Figure 11.
Bivariate scatter plot of maximum versus minimum drillhole diameter.
Because minimum diameter cannot exceed maximum diameter, values above the diagonal line denoting perfectly circular holes of different sizes (isometric trajectory for circular holes) are not possible. Whereas a significant linear correlation exists between the two variables, the data are visually curvelinear, with larger drillholes displaying more notable departures from circularity (a wide grey arrow illustrates changes in shape of drillholes going away orthogonally from the isometric trajectory for circular holes). Symbols: r = Pearson's correlation coefficient (a value in parenthesis represents Spearman correlation coefficient). Inset: A bivariate plot of the same variable plotted in terms of log-transformed values. A solid line represents a reduced major axis regression model. Symbols: a = slope of the model (associated p-value for the null hypothesis that the slope value is a = 1), r2 = coefficient of determination for the reduced major axis regression (associated p-value for the null hypothesis that the r2 = 0).
Figure 12.
Longitudinal distribution of drillholes plotted for specimens with the anterior end preserved.
Specimen length estimated using the allometric equation relating maximum width and length of complete specimens (see Fig. 6). Drillhole location is expressed proportionally as a distance of drillhole center from the anterior tube edge divided by the total estimated tube length. Thus, the value of 1 indicates a drillhole located at the anterior edge and value of 0.5 indicates a drillhole situated half way along the length of the tube.