Ivops wallanensis, Sandford & Holloway, 2006

Sandford, Andrew C. & Holloway, David J., 2006, Early Silurian phacopide trilobites from central Victoria, Australia, Memoirs of Museum Victoria 63 (2), pp. 215-255 : 233

Ivops wallanensis sp. nov.

Figure 11

Ananaspis .— Rickards and Sandford, 1998: 752.

Ananaspis .— Sandford and Holloway, 1998: 915.

Type material. Holotype NMV P139323 (enrolled cephalothorax) . Paratypes NMV P139324 (dorsal exoskeleton, pygidium displaced) , NMV P139325 (enrolled cephalothorax) , NMV P139326 (thoracopygon) , NMV P139328 (partly enrolled dorsal exoskeleton) , NMV P138230 (dorsal exoskeleton) , NMV P138231 (incomplete cephalothorax). All from PL206 , Wallan. Bylands Siltstone .

Other material. NMV P138232, NMV P139327, P147056 from PL 206, Wallan. For locality see Sandford and Holloway (1998: textfig. 1).

Derivation of name. In reference to the type locality.

Diagnosis. As for genus.

Description. Exoskeleton of known maximum length 30 mm. Cephalon about 60% as long (sag.) as wide, anterior outline with greatest curvature medially and in front of anterior extremity of eye. In frontal view anterior margin moderately arched upward medially, glabella strongly convex and evenly rounded. Glabella comprising about 30% maximum cephalic width posteriorly, width across frontal lobe about twice width at occipital ring and equal to sagittal length.Axial furrow diverging forward at about 70° from S1 to anterior end of palpebral furrow, thereafter diverging slightly more weakly to widest part of frontal lobe. Occipital ring high medially and with small, well-developed, obliquely oriented lobe laterally. Occipital furrow deep, transverse medially. L1 slightly narrower (tr.) than occipital ring, as high medially as composite lobe, lateral node smaller than lateral lobe of occipital ring. S1 expanding (exsag.) adaxially or bifid, posterior section transverse and weakly continuous medially, anterior section curving forward and rapidly dying out. L2 almost as short (exsag.) laterally as L1 and about 60% length of L3. S2 meeting axial furrow more or less opposite posterior edge of palpebral lobe, directed anteromedially and shallowing rapidly, weakly arcuate. S3 with anterior and posterior branches confluent opposite about 40% glabellar length (sag.) from posterior, anterior branch extending forward to opposite about 60% glabellar length (sag.) from posterior. Preglabellar furrow distinct across entire width (tr.) of glabella on internal moulds, decreasing in length (exsag.) adaxially in front of lateral part of frontal lobe. Posterior border and border furrow deflected rather strongly backward beyond fulcrum to genal angle. Palpebral lobe narrow (tr.), lenticular in outline, raised above palpebral area, palpebral furrow weakly curved. Visual surface with lenses arranged regularly in files anteriorly but less regularly posteriorly, lens formula ( NMV P139328, figs 11H–I, K) from front 4566525654?3?. Librigenal field subvertical below eye, lateral border steeply inclined. Posterior branch of facial suture transverse across genal field and deflected backward at about 45° across border. Cephalic doublure flat sagittally, vincular furrow forming distinct bevelled edge anteriorly, deep posterolaterally. Hypostomal suture transverse medially.

Thorax of 11 segments. Axis comprising 30% segmental width (tr.), strongly convex, rings transverse medially, with short exsagittal incisions in anterior margin defining obliquely forwardly directed lateral lobes. Pleurae with well-rounded tips and deep pleural furrows that are situated at midlength (exsag.) of segment at fulcrum and terminate distally at edges of articulating facets.

Pygidium lenticular, length (sag.) about 60% maximum width. Axis with 5 well defined rings and 1 or 2 very short (sag.) and weak ones posteriorly, axial terminus well rounded. Pleurae weakly convex (tr.), with 4 distinct pleural furrows and 2 very weak ones posteriorly. Border not present.

Remarks. The above description is based mainly on the least deformed specimens (e.g. figs 11B–C, E, J). S2 and S3 appear to vary markedly in depth, being very weak on most specimens but moderately impressed on both internal and external moulds of NMV P138230 (figs 11A, G). However, the apodemal pit in the lateral part of S2 is always present.

Figures

Figure 11. Ivops wallanensis gen. et sp. nov., from PL206, Wallan. A, G, paratype NMV P138230, dorsal exoskeleton; A, × 4.3; G, × 3.2. B, F, J, L, paratype NMV P139324, exoskeleton with displaced pygidium; B, view of pygidium, × 6.5; F, view of cephalon, × 3.8; J, view of pygidium, × 5.4; L, view of cephalic doublure, × 3.8.C, E, holotype NMV P139323,enrolled exoskeleton, view of cephalon, × 3.8.D, paratype NMV P139325, cephalon, × 3.8. H–I, K, paratype NMV P139328, partly enrolled exoskeleton, view of cephalon, × 4.3. (A, C–F, J–K are internal moulds).