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Journal of Paleontology 91(4):633–642
boundary. Foresi et al. (2011) assigned the base of the UGLM (and thus the UMPB) to the Burdigalian. Difficulties in giving a chronostratigraphic age to the UMPB bed are exacerbated by the fact that: (1) the base of the Langhian has not been formally defined; (2) there is a significant time gap repre- sented by the hardground at the base of theUMPB (Föllmi et al., 2008); (3) the base of the UMPG across the Maltese Archipe- lago has a diachronous nature (Foresi et al., 2001); and (4) these marker beds can be followed by one or more omission surfaces
phosphatization occurred (Bianucci et al., 2011). By contrast, there is no evidence of transport for the nonphosphatized material that can be considered of autochthonous origin.
Materials and methods
(up to five in all) making exact correlation difficult. In Gozo, five individual horizons constitute the UMPB bed, while only one or two are present in the rest of the Maltese Archipelago (see also Föllmi et al., 2008).
Studied fauna
Malta is known for its rich echinoid fauna, which has been stu- died by numerous authors (e.g., Wright, 1855, 1864; Gregory, 1891; Stefanini, 1908; Lambert, 1909; Cottreau, 1914; Zammit- Maempel, 1969). Challis (1979, 1980), in a study of the paleo- ecology and taxonomy of Maltese echinoids, recognized 13 echinoid biofacies with Echinocyamus showing a wide distribu- tion ranging from lagoonal to offshore facies. The UMPB at the base of the UGLMcontains a macrofauna including the echinoid genera Echinocyamus, Pericosmus, Lovenia, Spatangus, Schizaster, Echinolampas, Brissopsis, Psammechinus,and Sardocidaris. Echinocyamus is particularly common in the basal part of themember, directly on top of the underlying hardground. Theminute clypeasteroids fromthe GLF were first assigned
to the species Fibularia melitensis by Lambert (1909). Cottreau (1914), in his revision of Mediterranean Neogene echinoids, synonymized Fibularia melitensis with Echinocyamus stellatus (Capeder, 1906), a species first reported fromcoeval strata of the northern coast of Sardinia. Subsequent authors including Rose (1974, 1975) and Challis (1980), working on the Maltese echinoid fauna, followed Cottreau (1914). Since more than 30 fibulariid species names appear in the literature of the Mediterranean Miocene, and since 18 of these species names were established by Capeder (1906) alone, it is likely that the diversity of European Neogene fibulariids is overestimated. According to the descriptions of Capeder (1906), Echinocyamus stellatus seems to be the best match for theMaltese specimens.
Sedimentary environment
The investigated echinoids are preserved in two distinct modes, as phosphatized and as nonphosphatized specimens. Phospha- tized tests originate from a period of low sedimentation rate and accumulated at the top of the terminal hardground of theMGLM at a topographic high in NW Gozo. The nonphosphatized specimens, by contrast, derive from a slightly later period when sedimentation rate was higher and phosphatization was absent. Bioturbation is most likely responsible for sediment mixing and time-averaging of the two samples. Age difference between the two samples is poorly constrained because the exact duration of the hiatus linked with phosphatization is unknown. The phosphatized bioclastic sediments are closely associated with the underlying hardground on top of a topographic high on the paleoseafloor, which is interpreted as the site where
Materials.—Fossil tests of the clypeasteroid echinoid Echinocyamus stellatus (Fig. 1) were picked from two bulk sediment samples of the UMPB at the base of the UGLM exposed in the Qolla I-Bajda section at Xwejni Bay, northern coast of Gozo, Malta. The material was collected from the lowermost part of this member, 0.5 to 1.5m above the weath- ered terminal hardground of theMGLM(hatched area in Fig. 3).
Material preparation.—Bulk sediment samples were dry sieved in the field with an effective mesh size of 1.5mm. Sieved resi- duals were processed in the lab by wet sieving through standard sieve sets. Specimens with complete ambitus were cleaned in hydrogen peroxide and an ultrasonic bath. For SEM analysis, tests were additionally cleaned in Rewoqat (Lierl, 1992) to remove fine particles and photographed with a Jeol JSM 6610-LV scanning electron microscope. Specimens were separated into phosphatized and
nonphosphatized specimens. Phosphatized specimens are easily recognizable by their typically brown color and waxy, worn appearance, while the nonphosphatized echinoids are light gray to white and often retain a well-preserved test surface.
Measurements.—Tests are measured using an ocular scale. All deviations of the mean or median are listed as standard devia- tions. The test length is the longitudinal axis, which represents the maximum distance between the anterior and posterior ambitus; test width is the maximum distance between the lateral test margins, perpendicular to the longitudinal axis. Test length and width were compared using Pearson’s correlation coeffi- cient to detect possible allometry. Differences of test length between phosphatized and nonphosphatized specimens were analyzed using a Mann-Whitney U test.
Drill holes.—Drill holes are examined for the: (1) outline; (2) length and width; (3) position of drill hole center; and (4) section profile. The drill hole length is the longest distance between drill hole margins, and drill hole width is the widest distance between margins perpendicular to the drill hole length. Spearman’s rho was used in order to test for correlation of drill hole length and width. Potential differences of the drill hole length between specimen types were examined using a Mann- Whitney-U test. The drilling frequency is defined as the number of drilled
tests, either single or multiple drilled, divided by the total specimen number. Pearson’s chi-square was used to test for differences in drilling frequencies between sample types.
Size and site selectivity.—Spearman’s rho was used to test for size selectivity between test length and drill hole length. Site- dependent drilling preference was explored using four binomial tests comparing drilling frequencies between the (1) oral and aboral side; (2) petal area and ambital disc aboral side;
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