Cetopirus complanatus Morch 1853

Cetopirus complanatus (Mörch, 1853)

Fig. 2

Lepas complanata polytalamia Chemnitz, 1785: 325, pl. 99, figs. 845–846 Lepas balaenaris Spengler, 1790: 187

Coronula balaenaris Dufresne, 1802: 473, pl. 30, figs. 2–4 Cetopirus balaenaris Ranzani, 1817: 87

Lepas quinquevalvis Mawe, 1823: 5, pl. 2, fig. 3 Coronula balanarum Blainville, 1824: 380

Polylepas (Cetopirus) vulgaris Gray, 1825: 105 Ceteopirus [sic] complanatus Mörch, 1853: 67

Coronula darwini Stebbing, 1910: 572

Coronula complanata Pilsbry, 1916: 276, pl. 63, figs. 1–3a Coronula complanatus Hiro, 1936: 62

Range and distribution. Middle Pleistocene to Recent. Known as fossil from late Middle Pleistocene deposits of South Africa (this work), Upper Pleistocene deposits of Spain (Álvarez-Fernández et al. 2014), and Holocene deposits of Argentina (Pastorino & Griffin 1996) and The Netherlands (Holthuis et al. 1998). Currently known as an exclusive phoront of Eubalaena Gray, 1864, living in temperate seas between the (sub-)arctic regions and about 25° latitude.

Material. A single isolated rostral compartment kept at the Mossel Bay Archaeology Project Cultural Resources Management (MAPCRM) Laboratory of the Bartolomeu Dias Museum (Mossel Bay, South Africa) under accession number MAPCRM 85138.

Occurrence. Late Middle Pleistocene (164 ± 12 ka) cemented deposit (LC-MSA Lower horizon of Marean et al. 2007) in cave site PP13B (Pinnacle Point, South Africa).

Description and remarks. MAPCRM 85138 (Fig. 2) is an isolated rostral compartment broken transversely to the ribs, a few mm below the basis of the sheath, so that it lacks its lowermost portion.

The shape of MAPCRM 85138 suggests a significantly convex, dome-shaped shell. The sheath is smooth, depicting a barrel-shaped body chamber. The basal edge of the sheath does not project freely. Judging from the geometry of the fossa for the accomodation of the ala, the latter should have been thin and square. The compound radius is very thick (i.e., almost as thick as the whole compartment) and, in its median portion, its reaches the basis of the sheath. The sutural face of the radius presents closely spaced and branched septa which are inclined with respect to the outer edge of the radius. These sutural septa include both short septa radiating outward and long septa radiating inward from a main septum running along the outer edge of the radius. The external radius is horizontally (i.e., transversely) striated. Radial buttresses (i.e., ribs) terminating in T-shaped flanges project from the paries forming a solid outer wall. These ribs are ornamented by both transverse growth ridges (externally) and longitudinal striae (both externally and internally). The ribs bifurcate near to the basis of the sheath in an apparently symmetrical way. The terminal transverse loops of the ribs exhibit thin and well-spaced septa which connect the opposite inner faces of the inner lamina (i.e., the terminal transverse loops of the ribs display longitudinally elongated tubes); such a tubes-and-septa pattern also characterises the very central portion of each loop.

MAPCRM 85138 mainly differs from Coronula diadema by showing: (i) a narrow and very thick radius; (ii) a sub-straight sheath whose basal edge does not project freely; (iii) a shallow fossa for the accomodation of a thin and square ala; (iv) flat and broad ribs which bifurcate in an apparently symmetrical way; and (v) terminal transverse loops of the ribs that exhibit longitudinally elongated tubes. In turn, all the above reported features of MAPCRM 85138 match perfectly the various descriptions of the hard parts of Cetopirus complanatus given by Davis (1972), Pastorino & Griffin (1996) and Seilacher (2005). On the whole, these features distinguish Cetopirus complanatus from any other fossil and extant whale barnacle species known to date, including the early Pleistocene species Cetopirus fragilis Collareta et al., 2016a, which features a rather thin radius and a different inner structure of the terminal transverse loops of the ribs.