(Fig. 1, 3 A, Table 1)
Plocamia ? Hinde & Holmes 1892: 186, Pl. 7, Fig. 51. Janulum sp. Wiedenmayer 1994: 79, Fig. 34, 1– 3.
Material examined. None. Holotype named as the fossil species represented by the microfossil spicule cited in Hinde & Holmes (1892: 217, Pl 7, Fig. 51) as Plocamia ? sp.
Type locality. The exact location of the original material is not specified other than it came from the Oamaru Diatomite member of the Waiareka Volcanic Formation at three possible locations in the Oamaru District (Lautour 1889; Edwards 1991): Cormacks Siding, near Weston (NZ Fossil Record Locality J 41 /f 8888); Jackson’s Paddock, Spring Hill Road, near Weston (NZ Fossil Record Locality J 41 /f 8914); Bain’s Farm, near Weston (NZ Fossil Record Locality J 41 /f 8059).
Age. Runangan (Late Eocene) (36.4 – 34.6 Ma) (Edwards 1991).
Spicules. Megascleres—Strongyle (Fig. 3 A, Table 1) bent abruptly at each end, ends bent to the same side of the spicule. Lightly spined, conical spines perpendicular to the shaft, aligned in short linear spiralling arrays, ends rounded and smooth, 160 µm long x 20 µm thick (modified from Hinde & Holmes 1892).
Substrate, depth range and ecology. A recent interpretation of the Oamaru Diatomite is that it started accumulating in a narrow basin to the east of a formerly active submarine volcanic ridge, at about 100–150 m water depth, under quiet subtropical offshore near-surface waters (Edwards 1991). The phylogenetic diversity of the Oamaru Diatomite is impressive with about 110 sponge species in 43 genera, considered to be an underestimate because Hinde & Holmes (1892) examined material from a restricted location and the sample was very small. The fauna consisted of 10 % hexactinellid sponges and c. 90 % demosponges, and was a mixture of what are typically deep-water and shallow reef genera today (Kelly & Buckeridge 2005).
Etymology. Named as the first record of the genus Janulum in New Zealand, recorded from the Oamaru Diatomite (princeps, first; L.).
Remarks. The Late Eocene microfossil spicule illustrated by Hinde & Holmes (1892) is indistinguishable from the peculiar spined strongyles so characteristic of living species of Janulum (Fig. 2, 3). The length of the illustrated spicule (160 µm) (Fig. 3 A) is shorter than the spicules of Northern Hemisphere J. spinispiculum (c. 200– 240 µm; Table 1) and the spicules of J. imago sp. nov. from the Louisville Seamount Chain (c. 240–320 µm) described below. The spicule was originally described as a “dumb-bell spicule of Plocamia ” (Family Raspailiidae), but the identification was uncertain given that no additional information on spiculation, skeletal architecture or morphology was available at the time.
Because of the striking similarity of the Oamaru Diatomite microfossil spicule to the unique, diagnostic spined strongyles of J. spinispiculum, and because the microfossil spicule is shorter than those of living J. imago sp. nov., we have taken the step of naming the fossil species with confidence. It is interesting to note the absence of spicules resembling those of J. princeps sp. nov., in the recent review of the siliceous sponge fauna of southern Australia, around the same period of the Late Eocene (Łukowiak 2015). Janulum princeps sp. nov. is the first record of the genus in New Zealand, the South Pacific and Southern Ocean waters.