Triproetus Kobayashi and Hamada, 1979

Fortey, Richard A. & Heward, Alan P., 2015, A new, morphologically diverse Permian trilobite fauna from Oman, Acta Palaeontologica Polonica 60 (1), pp. 201-216 : 214-215

Genus Triproetus Kobayashi and Hamada, 1979

Type species: Neoproetus (Triproetus) subovalis Kobayashi and Hamada, 1979 ; Asselian (Permian), northern Thailand .

Discussion.—The distribution of Triproetus has been summarised by Owens (2003) who also reassigned to this genus some species that had been placed elsewhere. We follow his interpretation herein, which allows for considerable variation in the size of the basal glabellar lobes. These are smallest in the new species described below. There also appears to be variation in the length of the genal spines. All species share a comparatively short and transverse pygidium with usually six pleural ribs.

Triproetus bonbon sp. nov.

Fig. 5A–E.

Etymology: The perfectly enrolled trilobite resembles a boiled sweet bonbon).

Type material: Holotype: nearly complete enrolled exoskeleton, NHMUKPI It 29103. Paratypes: enrolled exoskeletons, NHMUKPI It 29104, 29105; pygidium, NHMUKPI It 29141, 29142, 29143, unfigured; free cheek, NHMUKPI It 29106, 29144, unfigured.

Type horizon: Kungurian–Roadian (Permian).

Type locality: Allochthonous limestone Wadi Khawr al Jaramah , Oman. An appropriate free cheek is from the Roadian of Jebel Qarari .

Diagnosis.— Triproteus with small, prominent, subcircular basal glabellar lobes; preglabellar furrow almost effaced medially; genal spines stout; fine tuberculate sculpture on glabella.

Description.—This is probably the best-preserved material of the genus known. The articulated specimens are perfectly and tightly enrolled to a subspherical state. Genal spines are broken off in this material, but an isolated free cheek proves that they were stout, triangular. Cephalon with arched outline, 1.6 times as wide as long (dorsal view, approx. allowance for damaged genal spines). Glabella is swollen anteriorly and bulges forwards to slightly overhang the anterior border in lateral aspect. The pyriform anterior glabellar lobe has maximum transverse width just behind the anterior cephalic margin, this being close to the sagittal length of the lobe in dorsal view. The occipital ring is one-third of this length medially, and its transverse width is slightly less than the maximum glabellar width. It is wider medially, where it slopes downwards forwards, and carries a small medial occipital tubercle posterior to the mid-point of the ring. Basal lobes of the glabella small but prominent, the inflated part nearly circular, with weaker parts extending further forwards, the whole outlined by deep furrows on all sides, and linked by the mid part of the occipital furrow. Deep axial furrows slightly sinuous at level of eyes, shallowing and curving outwards anteriorly, and preglabellar furrows shallowing still further after curving around round anterolateral corners of glabella to fade out almost completely at the mid line. Anterior border continues downward slope of front of glabella and carries about ten fine transverse raised lines. Anterior fixed cheeks reduced to narrow selvage. Posterior fixed cheeks spinelike, with acute distal border portions, anteriorly converging at a low angle to the sag. line before describing outer edge of prominent elevated palpebral lobes; thence diverging outwards around glabella before cutting obliquely across anterior border seen in anterior view (Fig. 5A 3). Fine tuberculate sculpture on glabellar surface.

Free cheek steeply attached and sloping down in life position. It is divided into pleural field and a steep border of similar widths, the latter wider than the cranidial border and widening still further backwards where it forms a nearly vertical “wall” at the cephalic edge. The border furrow is a deep notch in the profile of cheek and continues into the proximal part of the genal spine. Posterior border furrow is quite deep, but fades rapidly towards the genal angle. In plan view this angle is about 60°. The curved, high eye lobe is half as long (exsag.) as the frontal lobe of the glabella (sag.) in dorsal view. The base of the eye is marked by a furrow that follows its outline, but there is no inflated eye socle. Lateral border also carries raised lines parallel to its margin, extending into genal spine.

Thorax of nine segments. Each segment is similar and thorax does not taper backwards. Axial ring convex (tr.) wid- er than pleura, with very prominent “doubled” annulus and preannulus. Anterior pleural convex band one-third width (exsag.) of posterior band, the former concealed beneath down turned pleural tips during enrolment. Pleural tips blunt. Facets telescoped during enrolment, and at least half pleural width. During enrolment the tips of the anterior five segments are tucked beneath the posterior cephalic border. and lie alongside the steep lateral border, showing that its doublure must be reflexed steeply upwards. Curvature of the enrolled thoracic axial rings continues into the pygidial axis, which therefore has a curved profile.

Pygidium 60% as long as wide (most ditomopygids 80% or more). Convex axis just wider than pleural lobes at front, tapering gently to a short, broadly rounded terminal piece. 11 or perhaps 12 long (tr.) narrow (sag.) rings are present, three or four times wider than sag. length. Only 6–7 pleural ribs are present, decreasing in transverse width, the last very short. Structure of the first three ribs is like that of thoracic pleurae with narrow posterior band. Deep inter-rib furrows tend to look a little concave posteriorly. Neither axis nor ribs extend on to convex and downward sloping border, the bor- der furrow mostly being a sharp change in slope. The border has a narrow marginal convex rim. Weak tuberculate surface sculpture especially on axis.

Discussion.—The new species is similar in most features to the type species from Thailand, Triproetus subovalis Kobayashi and Hamada, 1979 , although the type material is fragmentary. However, the small, rounded posterior glabellar lobes of the Oman species differ from the more generally inflated triangular lobes of the former, while the pygidial border of the type species is conspicuously more convex than is the case in T. bonbon , and its anterior pygidial pleural ribs are unfurrowed. Owens (2003: 383) reassigned Paladin (Neokaskia) borealis Kobayashi, 1987 , from the Kapp Starostin Formation, Svalbard, to Triproetus , which is of similar age to the species reported here. Bruton (1999) provided a full description of this species showing it is indeed similar to the new species, but obviously differs in its larger basal glabellar lobes, reduced genal spines, and pygidium having a wider border, more tapering axis and deep border furrow. Bruton (1999: pl. 2: 1–3) additionally figured an enrolled specimen of Triproetus under open nomenclature from the Qarari Limestone of northeastern Oman. This specimen has large basal glabellar lobes; and the pygidial border does not slope so steeply as in T. bonbon and there seem to be 8, or possibly 9 axial rings. Although closely similar it is unlikely to be conspecific with T. bonbon . Owens (1983: pl. 5: 12, 13) reillustrated the type specimen of Griffithides brevicauda Gheyselinck, 1937 , a species from south central Timor that has subsequently been variously accommodated in the genera Paladin , Paladin (Neokaskia) or Triproetus by later authors, but is very similar to the species described above and must be congeneric. This specimen is apparently slightly distorted such that the basal glabellar lobe on the left is larger—or at least more clearly seen—than that on the right, and this character cannot objectively be used in relation to T. bonbon , which it closely resembles otherwise. A persuasive point of difference between T. brevicaudus and T. bonbon is the relatively large size of the eye lobe of the former, such that there is room for very little genal field altogether; Gheyselinck (1937) does not mention a genal spine on T. brevicaudus , but it was conceivably broken off. Viewed dorsally, the cephalic border of T. brevicaudus is wider than that of T. bonbon . Pygidia of these two species are also very similar, but there seem to be only ten axial rings in T. brevicaudus .