Plakinidae Schulze, 1880

Łukowiak, Magdalena, 2015, Late Eocene siliceous sponge fauna of southern Australia: reconstruction based on loose spicules record, Zootaxa 3917 (1), pp. 1-65 : 42-45

Family Plakinidae Schulze, 1880

There were some very rare plakinid spicules found among the studied material that belong to the genus Placinolopha Topsent, 1897 ( Figs. 27 A–H). One can distinguish several distinct types among these spicules: the amphiclads/anadiaenes with two opposed cladomes (made of two clads) curved backward in the same plane ( Figs. 27 A, B), the amphiclads (anadiaenes) with two opposed cladomes curved backward but the cladomes twisted in relation to one another at about 90° ( Figs. 27 C, D). Sometimes the clad ends are divided dichotomously ( Fig. 27 D), sometimes the amphiclads have long shaft endings with cladomes divided into 4 on their both ends ( Figs. 27 F, G), and finally there is also a mix of both of these morphotypes ( Fig. 27 E). All of these anchorate spicules are similar, both in morphology and size, to those occurring in the Recent species Placinolopha sarai Lévi & Lévi, 1989 ( Fig. 28 A) and Placinolopha moncharmonti ( Sarà, 1960) (compare also with Sarà, 1960, fig. 1A). The spicules described here are big and exceed 500–600 µm of length which makes them almost as big as the Recent P. sarai spicules that are of size 770–880 µm in length ( Lévi & Lévi 1989). The spicules of P. moncharmonti described by Sarà (1960) are smaller, exceeding about 400 µm of length. P. s ar a i is known from Eastern Philippines and P. moncharmonti from the Mediterranean (van Soest et al. 2013). Considering their distribution and their size, it is most probable that the studied Late Eocene spicules belong to Placinolopha cf. sarai or to another closely related fossil species.

It is also worth mentioning that spicules resembling amphiclads with cladomes divided in 4 occur in the tetillid Paratetilla aruensis Hentschel, 1912 known today from Indonesia (van Soest et al. 2013) but contrary to Placinolopha ’s spicules, those belonging to Paratetilla have cladomes divided trichotomously (compare Hentchel 1912, pl. 18, fig. 11) and are much larger.

Apart from amphiclads, there are also rare plakinid lophocalthrops and triods occurring in the studied samples ( Figs. 27 I–L). These distally lophate spicules of about 500–1000 µm in size resemble acanthocalthrops of Recent Placinolopha , especially those described by Muricy & Díaz (2002, fig. 2I). They mostly resemble the spicules of Placinolopha bedoti Topsent, 1897 from the Banda Sea and the Indonesian Exclusive Economic Zone ( Fig. 28 C). Admittedly, there is another species of Placinolopha recorded from around Australia (see Atlas of Living Australia)— P. sp i no s a [( Kirkpatrick, 1900), now Plakina australis ( Gray, 1867a) ] but nevertheless, the spicules resemble more closely those of P. bedoti .

In both cases, however, the size of described spicules (even the biggest category) is smaller than the size of fossil spicules that are about 275–300 µm large. However, despite the differences in size of the spicules, the close resemblance of these morphotypes supports attribution of the Eocene spicules to Placinolopha cf. bedoti .

Plakinid calthrops were rarely recognized in the Jurassic, Cretaceous, or the Miocene of the Bahamas (Burky 1978). Similar triods were described from the Miocene of Portugal but assigned, with some doubts, to Pachastrellidae or Calthropellidae (compare with Pisera et al. 2006, fig. 10A). On the other hand, Reid (1970) suggests that plakinid-related sponges were already noted in the Cambrian, but it is an open question. The spicules of Placinolopha bedoti- type were, for example, described from the Early Pliocene deposits of the New Caledonia Basin ( Locker & Martini 1986), but the amphiclads of Placinolopha cf. sarai have not been reported so far in the fossil record.

Figures

FIGURE 27. Spicules of sponges of the order Homosclerophorida; A – G—Amphiclads of Placinolopha cf. sarai Lévi & Lévi, 1989, family Plakinidae; H—Amphiclad, fragment; I – K—Lophocaltrops of Placinolopha cf. bedoti, family Plakinidae; L—Triod, probably of Placinolopha cf. bedoti, family Plakinidae.

FIGURE 28. Recent plakinid spicules; A, B—Amphiclads of Placinolopha sarai (redrawn from Lévi & Lévi 1989); C—Lophocaltrop of Placinolopha bedoti (redrawn from Topsent 1897).