Dalmanites Barrande, 1852

Sandford, Andrew C. & Holloway, David J., 2006, Early Silurian phacopide trilobites from central Victoria, Australia, Memoirs of Museum Victoria 63 (2), pp. 215-255 : 237-238

Dalmanites Barrande, 1852

Type species. Trilobus caudatus Brünnich, 1781 from the Coalbrookdale Formation (Wenlock) of England .

Dalmanites athamas Öpik, 1953

Figures 13A–K

Dalmanites sp. — Thomas, 1937: 66. Dalmanites athamas Öpik, 1953: 28 , pl. 10, figs 88–91; text-fig. 9 (IV).— Holloway and Sandford, 1993: 97.— Sandford and Holloway, 1998: 916.— Rickards and Sandford, 1998: 751. Dalmanitina (Eudolatites) aborigenum Öpik, 1953: 26 (partim.), pl. 10, fig. 85, text-fig. 9 (III) (non pl. 10, figs 86–87 = Struveria ? sp.). Dalmanites? athamas .— Talent, 1964: 50. ʻ Dalmanitina ʼ aborigenum .— Talent, 1964: 50.

Type material: Holotype NMV P52484 (pygidium, figured Öpik, 1953: pl. 10, fig. 88) and paratype NMV P52485 (fragment of cephalon, figured Öpik, 1953: pl. 10, fig. 89) from PL 2262, Thomas locality F43, Costerfield. Paratype NMV P52486 (fragment of cephalon, figured Öpik, 1953: pl. 10, figs 90–91) from PL 2269, Thomas locality F51, Costerfield. Wapentake Formation.

Other material. NMV P138215 – P138216 from the ʻ Illaenus bandʼ (exact locality unknown), Costerfield . NMV P138217 from PL389 , Costerfield . NMV P52482 (holotype of ʻ Dalmanitina (Eudolatites)ʼ aborigenum ), P138218–P138219 , NMV P138223 from PL2269 , Costerfield . NMV P138220 from PL2263 , Thomas locality F44, Costerfield . NMV P139805 from PL386 , Costerfield. Wapentake Formation. For localities see Thomas (1940) and fig. 9 .

Diagnosis. Dalmanites with very short (sag., exsag.) anterior cephalic border lacking median expansion or process. Glabella about 77% as wide across frontal lobe as long (sag.), frontal lobe comprising a little more than half sagittal length of glabella. Pygidium with 14–15 axial rings and 10 pleural furrows. Axis narrow, comprising 23% maximum pygidial width anteriorly, with apodemes on 1st 10–12 segments. Pleural furrows flat-bottomed, as long (exsag.) as or longer than preceding anterior pleural band, posterior bands much shorter except adaxially and dying out distally on more posterior segments, pleural nodes absent.

Description. Glabella with widths across L1, L3 and frontal lobe approximately in ratio 1:1.6:1.8. Occipital ring, L1 and L2 of equal length, together comprising 35% sagittal glabellar length. S1 and S2 transverse, S1 with apodemal pit extending almost to axial furrow, S2 with slightly narrower (tr.) apodemal pit and shallow extension to axial furrow. L3 as long (exsag.) adaxially as L1 and L2, twice as long at axial furrow. S3 deepest proximally, expanding abaxially and subsequently contracting again toward axial furrow. Frontal lobe transversely elliptical, width 150% length, with short longitudinal depression in posterior half and forwardly expanding median field of muscle scars occupying slightly more than anterior half. Anterior border slightly shorter sagittally than laterally, weakly concave. Anterior branch of facial suture running slightly closer to preglabellar furrow than to anterior cephalic margin. Posterior border furrow lanceolate in outline, gently deflected backward distally, not meeting lateral border furrow.

Thoracic axis weakly convex (tr.), gently tapering backward. Axial rings very weakly convex (sag., exsag.), slightly bowed forward medially. Axial furrow deep. Pleural furrows expanding (exsag.) abaxially to fulcrum, deep, anterior slope much steeper than posterior slope. Anterior pleural bands strongly convex (exsag.), posterior bands becoming flattened toward fulcrum and inclined, poorly differentiated from posterior slope of pleural furrow. Pleural bands with sculpture of small, sparse granules.

Pygidium triangular in outline, shape of posterior termination unknown.Axis with length about 70% maximum pygidial width, tapering uniformly backward, 1st ring with subrectangular medial embayment in posterior edge accommodating pseudo-articulating half ring on 2nd segment, inter-ring furrows shallowing medially and expanded (sag.) except towards back of axis where they become increasingly more poorly defined. Axial furrow narrow but distinct on external surface. 1st pleural furrow straight except distally where it is deflected strongly backwards, subsequent furrows successively more evenly curved and more posteriorly directed, 10th furrow directed exsagittally. 7th pleural rib with distinct forward deflection proximally. Interpleural furrows sharply impressed externally. Doublure narrow, moderately convex abaxially and with a low upturned lip adaxially, with densely distributed small granules.

Remarks. The synonymy of ʻ Dalmanitina (Eudolatites)ʼ aborigenum with Dalmanites athamas was discussed by Sandford and Holloway (1998: 916). The paratype of aborigenum , a pygidial fragment, is not congeneric with the holotype and we assign it to Struveria sp. 2 .

New material of athamas has become available since Öpikʼs (1953) original description but no cephala that are more complete. Öpik considered the species to be close to and possibly a subspecies of Dalmanites wandongensis from the overlying Dargile Formation, but comparison of the available material indicates that the two species are distinct. D. wandongensis was revised by Sandford (2006) and differs from D. athamas in that L1 is shorter than L2 instead of equal in length; the pygidial pleural furrows are shorter (exsag.) and not as flat-bottomed in cross section but form a continous slope with the succeeding posterior pleural band; and the interpleural furrows are weaker.

Ramsköld (1985) recognised a closely related group of European and North American Wenlock–early Ludlow species of Dalmanites including the type species, D. caudatus ( Brünnich, 1781) , as well as D. corrugatus ( Reed, 1901) , D. limulurus ( Green, 1832) , D. myops ( König, 1825) , D. nexilis Salter, 1864 , D. obtusus ( Lindström, 1885) and D. platycaudatus Weller, 1907 . These species share prominent lateral nodes on the first, sixth and seventh thoracic axial rings, and nodes on the pygidial pleural ribs. D. athamas lacks the thoracic and pygidial nodes characterising this group, which also differs from athamas in having a medially expanded anterior cephalic border, and pygidia with shorter (exsag.) pleural furrows and longer (exag.) posterior pleural bands that are subequal in length to the anterior bands.

Figures

Figure 13. A–K, Dalmanites athamas Öpik, 1953. A, NMV P138215, thoracopygon, × 1.5, from the ʻIllaenus bandʼ, Costerfield. B, NMV P138219, pygidium, × 3.5, from PL2269, Costerfield. C, paratype NMV P52485, cranidium, × 2, from PL2262, Costerfield. D, G, NMV P138217, pygidium, × 2, from PL389, Costerfield. E, paratype NMV P52486, cranidium, × 2, from PL2269, Costerfield. F, NMV P138218 pygidium, × 2, from PL2269, Costerfield. H, holotype NMV P52484, pygidium, × 1.5, from PL2262, Costerfield. I, NMV P138223, thorax, enlargement showing granulation, from PL2269, Costerfield. J, NMV P139805, teratological pygidium, × 2, from PL386, Costerfield. K, NMV P52482, incomplete fixigena, holotype of ʻDalmanitina (Eudolatites) aborigenumʼ Öpik, 1953, × 2, from PL2269, Costerfield. L, Struveria sp. 2, NMV P52483, pygidium, paratype of ʻDalmanitina (Eudolatites) aborigenumʼ Öpik, 1953, × 3, from PL2269, Costerfield. (B, D are latex casts)