Mucropetraliella cf. tubulifera

Mucropetraliella cf. tubulifera

(Canu and Bassler, 1929)

Figure 47 View FIGURE 47

cf. 1929 Petraliella tubulifera Canu and Bassler , p. 264, pl. 27, figs. 7–12.

Figured material. PMC EDM-Collection J.H.B.142a, sample 19035 ( Figure 47A–E View FIGURE 47 ) and sample 19095 ( Figure 47F View FIGURE 47 ); Core 19, Daidokutsu cave, Okinawa, Japan, Holocene.

Description. Colony encrusting, multiserial, unilaminar. Autozooids distinct, separated by shallow grooves, irregularly polygonal, elongate (mean ZL/ ZW 1.62). Frontal shield slightly convex, sculptured, uniformly pseudoporous, with circular pseudopores 15–40 µm in diameter, indistinct from marginal areolae, funnel-shaped, or centred in a polygonal mesh. Primary orifice subcircular (mean OL/OW 1.06), with a narrow trapezoidal median denticle on the proximal margin (c. 30 µm long by 40 µm wide at the base), occupying one-fifth of the total width of the orifice, and two acutely triangular condyles positioned proximally, very close (c. 40 µm) to the proximal margin of the orifice, pointing medially, measuring 20–26 µm in length by 17–23 µm at the base; the median denticle and condyles defining two transversely subcircular, slightly asymmetrical sinuses. Four to six distolateral spine bases, 19–39 µm in diameter. Suboral complex prominent, including a median tri- to quadri-furcate mucro connected to two lateral ones in a steering wheel shape, imperforate, nodular, striated, creating a polygonal pattern; central mucro 190–360 µm long, partially obscuring the orifice in frontal view, with branches reaching 150–240 µm in length; lateral mucrones about 80–160 µm long; additional subconical mucrones 75–150 µm long, developing either below the main suboral mucro complex or sparse on the frontal shield, being either straight or slightly curved. Avicularia and ovicells not observed. A circular radicular pore chamber situated in one corner of the proximal margin of the dorsal side of the autozooid. Small conical pillars present in the distomedial area of certain autozooids.

Measurements (µm). ZL 717±69, 614–822 (3, 12); ZW 443±37, 376–489 (3, 12); OL 215±18, 202– 235 (2, 3); OW 203±13, 194–218 (2, 3).

Remarks. Comparisons between these specimens and known species of Mucropetraliella featuring up to six oral spines will always be partial because crucial distinguishing characters—the shape, position, and size of avicularia—are not yet developed or preserved in our samples. Therefore, we use “cf.” for the identification of this species. Based on other Mucropetraliella species, including the congener found in Daidokutsu cave, we would expect avicularia to have developed atop the mucrones in this species as well. Hence, the Recent M. tubulifera (Canu and Bassler, 1929) from the Philippines is the most similar, having a variable number of spines (3–6 in the nominal species compared with 4–6 observed in the present specimens), and a similarly developed suboral complex. The avicularian complex in M. tubulifera consists of “two laterally salient avicularia, a suboral enormous avicularian mucro bifurcated or branching adorned with very salient tubules” (Canu and Bassler, 1929, p. 264). The “tubules” likely correspond to the frontal subconical mucrones observed in our specimens, either connected with the mucro complex or sparse on the frontal shield. Similar structures are also visible, sparsely distributed on the frontal shield in the original illustration (Canu and Bassler, 1929, pl. 27, figures 10, 12). In the Philippines, Mucropetraliella tubulifera was reported at five stations of increasing depth, from 37 m down to 622 m. Both the ornamentation of the shield and the complexity of the suboral mucro were observed to increase with the depth of the water (Canu and Bassler, 1929).

Other Mucropetraliella species with up to six spines are less similar. Mucropetraliella unimucronata Hayami, 1975 , from the Pliocene of Japan, has 5–6 stout oral spines but only a simple suboral mucro. Among Recent Australian species, M. biaviculata ( Waters, 1887) has six spines but a diminutive suboral avicularian complex (Gordon, 1989b); M. serrata (Livingstone, 1926) has six oral spines, large frontal avicularia and a suboral complex consisting either of only the avicularium or a short, narrow mucro; M. vultur (Hincks, 1882) has six spines and a bifid mucro with a pair of avicularia on the sides, in addition to latero-oral avicularia directed laterally or slightly proximolaterally. Among the other Recent species from the Philippines, M. robusta (Canu and Bassler, 1929) was originally described as lacking spines, although Harmer (1957) reported 4–6 spines and doubted their genuine absence because Canu and Bassler (1929) examined dead specimens that may have only preserved the spine bases, which could be easily overlooked. This species has a very massive suboral complex partially concealing the orifice, similar to M. cf. tubulifera , but there is no mention of additional “tubules” in its description. Conversely, M. robusta is characterized by paired latero-oral avicularia and frontal avicularia near the zooidal margins, like those in the other Mucropetraliella species found in Daidokutsu cave that we identified as M. cf. robusta (see description above).