Minutella cf. minuta ( Cooper, 1981 )

Simon, Eric, Lüter, Carsten, Logan, Alan & Mottequin, Bernard, 2018, Recent thecideide brachiopods (Thecideida, Thecideoidea) from northern Sulawesi (Indonesia) with discovery of a new Thecidellina species (Thecidellinidae), Zootaxa 4526 (4), pp. 481-515 : 495-496

Minutella cf. minuta ( Cooper, 1981) (Lembeh form)

Text-Fig. 2; Pl. 7, Figs 1a–g, 2

? 1981 Thecidellina minuta: Cooper , p. 62; pl. 6, figs. 27–40.

? 2013 Minutella cf. minuta: Simon & Hoffmann , p. 405; pl. 1, figs. 1–5, pl. 2, figs. 1–8.

? 2016 Minutella cf. minuta ( Cooper, 1981) : Simon et al., p. 5.

Material. Available specimens were collected in the shipwreck from sieved sediment or attached on oyster shells and preserved in ethanol. Specimens in ethanol, although numerous (111), are reserved for biological studies. The dried or dead specimens of this species are represented in this station with a lower quality level of preservation. The dried material is represented by 15 articulated shells, 19 dorsal and 6 ventral valves.

Description. The shell is elongate, drop-like with a pointed umbo in outline, strongly biconvex in lateral and anterior views. In posterior view, the bottom of the shell (Pl. 7, Fig. 2) is flat when visible (the shell is often attached by its posterior side). The anterior commissure is rectimarginate whereas the lateral commissure is straight or slightly concave dorsally.

The ventral valve has a smooth external surface with many fine growth lines. The transversely striated triangular interarea (Pl. 7, Fig. 1g) is flat or presents a very unlikely concavity and flanks a wide, dorsally convex rugideltidium (sensu Logan and Baker 2013). The smooth peripheral rim is relatively wide and the peripheral ridge has one or two rows of small tubercles (Pl. 7, Fig. 1g). The ventral valve floor is smooth. The hemispondylium is made of two plates with narrow and acutely pointed anterior prongs. The cyrtomatodont teeth are short, thick and blunt. They are covered with secondary fibers. The hinge is straight.

The smooth dorsal valve is lid-like with many concentric growth lines and has a posterior part slightly convex at the level of the circular protegulum that has a granulate surface. The peripheral rim is quite wide and clearly defined. The peripheral ridge is heavily tuberculate (Pl. 7, Fig. 1d). The median septum is thin, widening in its anterior part. The ventral surface of the septum is narrowly canaliculated and its lateral sides are irregularly spinous (Pl. 7, Fig. 1a, 1e). Brachial cavities (or brood pouches) are covered with canopies similar to spicular lattices leaving free interstices of irregular shape (Pl. 7, Fig. 1a–e). Canopying spicules are not very numerous and relatively thick with a finely granulated surface (Pl. 7, Fig. 1b). The cardinal process is curved backwards. Inner socket ridges are very thick and covered with secondary shell material. The brachial bridge, with a denticulate ventral crest, is convex posteriorly. Lophophore muscle scars are strong and clearly visible on its anterior surface (Pl. 7, Fig. 1d).

Comparison with Minutella minuta ( Cooper, 1981) and Minutella cf. minuta (in Simon & Hoffmann 2013)

The specimens from Lembeh as all Pacific Minutella representatives (see Bitner 2009, p. 18, fig. 13A–I; Hoffmann and Lüter 2010, pp. 150–152) are extremely similar to the specimens of M. minima sensu stricto from Samper Bank, Madagascar, as described by Cooper (1981). Some details are however different between the “populations”.

In Lembeh, spicules of the canopies are generally coarser. The calcitic pole has also a different outline (Pl. 7, Fig. 1e). It has very poorly developed lateral outgrowths and also a posterior outgrowth reduced to a tiny and sharp triangle (Pl. 7, Fig. 1f).

The specimens of M. cf. minuta ( Indonesia) from Donggala in the Strait of Makassar described in Simon & Hoffmann (2013) have a longer interarea with a narrower rugideltidium. They have a thin lamellar calcitic pole with a quite extended posterior outgrowth ( Simon & Hoffmann 2013, pl. 1, fig. 4d, 4g–h). Their canopies are made with slender and more numerous spicules leaving more regular free cavities (pl. 1, fig. 4e; pl. 2, fig. 1a–b).

In M. minuta ( Cooper, 1981) from Samper Bank (see pp. 61–62) the interarea is generally shorter with a relatively wider rugideltidium (see Cooper 1981, pl. 6, figs. 27–28). The canopies of brood pouches have similar irregular cavities (see Cooper 1981, pl. 6, figs. 37–38, 40). Also, in M. minuta from Samper Bank (see Cooper 1981, pp. 61–62; pl. 6, figs. 27–40) the calcitic pole is thicker with a short posterior outgrowth but larger lateral outgrowths.

However, these comparisons concern characters that remain affected by variability both in M. minuta ( Cooper, 1981) and in all M. cf. minuta from Pacific origin. These variations should be studied statistically on very large samples in order to evaluate the value of such morphological features. But predominantly, living specimens of all “populations” of M. minuta and M. cf. minuta from the Indo-Pacific must be available to benefit from molecular investigations based on studies of the small subunit of the nuclear ribosomal RNA gene (18S rDNA).

Ontogeny. The shell development for this species has been studied and illustrated in detail by Simon & Hoffmann (2013, p. 425–427; pl. 2, figs. 1–8) and can be consulted.