Mucropetraliella cf. robusta

Mucropetraliella cf. robusta

(Canu and Bassler, 1929)

Figure 46 View FIGURE 46

cf. 1929 Petraliella robusta Canu and Bassler , p.

260, pl. 26, fig. 3.

cf. 1957 Mucropetraliella robusta (Canu and Bassler) ; Harmer, p. 717, pl. 46, fig. 13, text-fig. 65.

Figured material. PMC EDM-Collection J.H.B.141a, sample 19083 ( Figure 46A–E View FIGURE 46 ), sample 19135 ( Figure 46F View FIGURE 46 ), sample 19193 ( Figure 46G View FIGURE 46 ), sample 19229 ( Figure 46H View FIGURE 46 ), and sample 19178 ( Figure 46I–J View FIGURE 46 ); Core 19, Daidokutsu cave, Okinawa, Japan, Holocene.

Description. Colony encrusting, multiserial, unilaminar. Autozooids distinct, separated by shallow grooves, irregularly polygonal, elongate (mean ZL/ ZW 1.77). Frontal shield slightly convex, smooth, striated, uniformly pseudoporous, with circular pseudopores 16–35 µm in diameter, indistinct from marginal areolae. Primary orifice subcircular (mean OL/OW 0.92), with a narrow anvil-shaped median denticle on the proximal margin (30–52 µm long by 30–70 µm wide at the tips), occupying one-tenth to one-third of the total width of the orifice, and two acutely triangular condyles placed proximally at about one-third from the orifice proximal margin, curved downward, measuring 26–33 µm in length by 20–43 µm at the base; the median denticle and condyles delimiting two transversely elliptical, slightly asymmetrical sinuses. Two distolateral spine bases, 15–35 µm in diameter. Suboral complex prominent, including a median bi-, tri- to quadrifurcate mucro connected to two lateral ones in a steering wheel shape, imperforate, striated and nodular; central mucro 150–305 µm long by 65–140 µm wide, lateral ones about 175 µm long. Avicularia not observed suborally but likely present and placed atop the mucro branches; two small elliptical avicularia positioned atop the lateral branches of the complex, directed proximally, with a complete crossbar; one of the small avicularia occasionally enlarged with a truncate cup-like rostrum curved upwards, directed proximally or proximolaterally inwards; in one instance, an avicularium observed laterally on the frontal shield on a raised column-like cystid, small and elliptical, directed laterally ( Figure 46F View FIGURE 46 ). Ovicells hyperstomial, prominent, placed on the frontal shield of the distal zooid, occupying approximately one-third of its total length (mean OvL/ZL 0.32), smooth, minutely perforate with pseudopores 4–10 µm in diameter; an area near the orifice appearing imperforate or, at times, covered by incipient secondary calcification ( Figure 46C View FIGURE 46 ). A subelliptical radicular pore chamber situated medially adjacent to the proximal margin of dorsal side of autozooid. Small conical pillars present in the distomedial area of certain autozooids.

Measurements (µm). ZL 818±61, 670–913 (5, 25); ZW 463±43, 368–534 (5, 25); OL 212±13, 189– 236 (5, 19); OW 231±12, 209–253 (5, 19); AvL (small-sized) 52±8, 30–64 (5, 25); AvW (small-sized) 39±4, 32–48 (5, 25); AvL (large-sized) 221±41, 165–304 (5, 13); AvW (large-sized) 125±11, 109–152 (5, 13); OvL 265±13, 252–283 (3, 4); OvW 315±19, 298–342 (3, 4).

Remarks. The extant M. robusta from the Philippines is the most similar species, having a similarly developed suboral complex partially concealing the orifice, a narrow bidentate median denticle, rounded paired latero-oral avicularia, and a small frontal avicularium rarely present near the zooidal boundaries. Our “cf.” attribution is for two reasons. First, the suboral avicularium in our specimens is not preserved, while in M. robusta it is very large and slightly expanded at the distal end. Second, the number of spines differs; they were reported as absent in the original description of M. robusta by Canu and Bassler (1929). However, Harmer (1957) reported 4–6 spines and justified the discrepancy by noting that the previous authors examined dead specimens, which likely had detached spines, with the scars difficult to notice. Additionally, neither reference mentions the enlarged lateral avicularium we observed, which has a very similar shape to the suboral avicularium described by Harmer (1957). Mucropetraliella robusta was reported from water depths of 421 m in the Philippines (Canu and Bassler, 1929) and from 0–45 m in Indonesia (Harmer, 1957).

Two oral spines, as seen in our specimens, were described for M. bifidata Tilbrook, 2006 and M. bispinata Liu, 2001 , from the Solomon Islands and China Sea, respectively, but they lack avicularia; M. asymmetrica Hayward and Cook, 1983 , may have 2–3 spines, suboral avicularia at the base of a simpler mucro, and large lateral avicularia of different shape. The south-west Atlantic M. reticulata Figuerola, Gordon and Cristobo, 2018 has two oral spines, but also a peculiar suboral mucro forming a variable reticulate net of tubular kenozooids.

Compared to other Mucropetraliella species reported from Japan, the Recent Mucropetraliella japonica (Ortmann, 1890) , and the Pliocene M. shibikawaensis Hayami, 1975 and M. tenuis Hayami, 1975 all lack spines. Spines are absent also in some Australian species: M. elleri (MacGillivray, 1869) has a simple short mucro and numerous avicularia sparse on the frontal surface; M. halei (Livingstone, 1928) has a simple mucro, numerous avicularia aligned on the lateral margins of the frontal shield, and avicularia instead of spines at the distolateral corners of the orifice; M. nodulosa Stach, 1936 is full of rounded tubercles around the orifice and on the frontal surface; and M. ligulata Stach, 1936 is characterized by its ligulate avicularia (Gordon, 1989b).

Spines are also absent in the following species from the Philippines: M. philippinensis (Canu and Bassler, 1929) which has large frontolateral spatulate avicularia; M. verrucosa (Canu and Bassler, 1929) has the aperture surrounded by small avicularia; and M. trita (Canu and Bassler, 1929) which lacks a mucro. Mucropetraliella malwanensis Sonar, Wayal and Badve, 2021 from the Holocene of India, has no spines, a simple mucro, and differently shaped lyrula and condyles. Two species from the Indian Ocean, M. multiaviculariata (Thornerly, 1912) and M. laccadivensis (Robertson, 1921) , also lack spines; the former species is also characterized by supernumerary avicularia on the ovicell, around the orifice and along zooidal margins, with a large frontal avicularium directed proximally. Mucropetraliella armata ( Waters, 1913) from East Africa also lacks spines (Hayward and Cook, 1983).

A group of species has up to four spines: M. capricornensis Tilbrook, Hayward and Gordon, 2001 , from Vanuatu, has a wide, shallow median denticle, a short and simple mucro bearing an oval avicularium, and lateral avicularia, laterally directed, on cystids that are not much elevated; M. loculifera Harmer, 1957 , from Indonesian stations of the Siboga Expedition, has 2–4 spines and a great number of small oval avicularia all aligned along the lateral margins of the zooid and surrounding the orifice, plus a large pointed avicularium budded more centrally on the frontal, obliquely oriented and directed proximolaterally, occupying one proximolateral corner of the zooid, and a broad lyrula with very pointed lateral corners; M. albirostris (Canu and Bassler, 1927) from Hawaii, has a straight and smooth mucro, and lateral avicularia directed laterally; and M. magnifica (Busk, 1884) also from Hawaii, lacks a median denticle and has hexagonal pores in the ovicell; M. neozelanica (Livingstone, 1929) , from New Zealand, lacks an avicularian complex (Gordon, 1989b); M. tuberosa (Busk, 1884) , from east Australia, has only a large suboral avicularium, and no lateral avicularia.

Several species have a greater number of oral spines. The Australian M. bennetti (Livingstone, 1926) has 7–9 (most commonly eight) thin oral spines, a small suboral mucro and associated avicularium, a large median denticle occupying one-third of orifice width (Tilbrook and Cook, 2004). The Philippine M. echinata (Canu and Bassler, 1929) has 12 oral spines. The western Pacific M. gaudialis d’Hondt, 1986 has eight spines and a large suboral avicularium with a spatuliform and curved rostrum.

A species of Mucropetraliella most reported from the Recent of Japan (Hirose, 2010; Arakawa, 2024) is M. thenardii (Audouin, 1826) . The specimen from Sri-Lanka illustrated by Tilbrook and Cook (2005, figure 11A, B) has a similar general aspect to M. cf. robusta but differs in the orientation of the avicularia and the shape of the median denticle and condyles. In the original illustration by Savigny (Audouin, 1826; d’Hondt, 2006), the mucro is slender and trifurcate (similar to Figure 46F, G View FIGURE 46 ), but no avicularia or additional conical processes are shown. Harmer (1957) described numerous varieties under the name M. thenardii , making it difficult to understand what the real M. thenardii is, as with all species described by Audouin (1826), until a neotype is chosen. However, the choice of a neotype is problematic considering that the original localities are not well defined and sometimes it is not even clear whether it is a site in the Mediterranean or the Red Sea.