Bessazoon sp.

Sandford, Andrew C. & Holloway, David J., 2006, Early Silurian phacopide trilobites from central Victoria, Australia, Memoirs of Museum Victoria 63 (2), pp. 215-255 : 240-242

Bessazoon sp.

Figures 14 , 17D–F

Dalmanites .— Rickards and Sandford, 1998: 750.

Material. NMV P139427–P139439, NMV P138276 from PL 1452, Goldie. NMV P139470–P139487 from ʻLancefieldʼ (exact locality unknown). NMV P147769–P147770 from PL 256, Wallan. NMV P312817 from PL 6361, Springfield. Chintin Formation. For locality PL 1452 see Thomas (1960), marked as ʻshelly fossils, Dalmanites ʼ.

Description. Anterior cephalic margin parabolic in outline (fig. 14I). L1 about 75% length of L2. S1 deflected forwards slightly at adaxial end, S2 more or less transverse; S3 shallow, oriented at about 65° to sagittal line, expanding (exsag.) abaxially. Inner ends of S1–S 3 in line (exsag.). Palpebral lobe large, length (exsag.) about 40% sagittal glabellar length, almost semicircular in dorsal outline, anterior margin opposite anterior margin of L3 and posterior margin reaching almost as far back as occipital furrow. Palpebral furrow weak (fig. 17E), palpebral area wide (tr.) and weakly concave.

Hypostome approximately as wide across anterior wings as long (sag.), subparallel sided from back of anterior wing to outer end of posterior border furrow, thereafter narrowing backward and parabolic in outline. Anterior wings small, triangular, length (exsag.) about 15% sagittal length of hypostome. Middle body comprising 80% sagittal length of hypostome, weakly convex transversely and flat sagittally. Maculae indistinct swellings behind weak depressions placed opposite 40% hypostomal length from posterior and halfway between sagittal line and lateral border furrow. Lateral border narrow, approximately 6% width of hypostome at midlength (sag.), lateral border furrow weak in anterior half and moderately impressed in posterior half. Posterior border long, 15% hypostomal length sagittally, posterior border furrow semicircular in outline.

Pygidium triangular in outline, relatively undeformed specimens with length (excluding mucro) approximately 75% estimated maximum width. Axis about 25% maximum pygidial width anteriorly, tapering uniformly backward, with 14 axial rings of which last 6 are poorly defined. Inter-ring furrows 1–9 with deep apodemes extending approximately. 33% width, all inter-ring furrows very shallow medially. Axis continuous posteriorly with postaxial ridge and mucro. Axial furrow deep. Mucro slender, at least 75% length (sag.) of remainder of pygidium. 10 pleural furrows that are directed successively more strongly backward, last one parallel to sagittal axis. Interpleural furrows moderately incised, widening distally. Anterior pleural bands expand slightly and very gradually abaxially, at fulcrum approximately as long (exsag.) as succeeding pleural furrow, slightly elevated above posterior bands distally. Posterior pleural bands subparallel sided except distally where they taper, at fulcrum comprising about 75% length (exsag.) of anterior bands.

Densely distributed small granules present on preserved parts of external surface including occipital ring, thoracic pleural tips and doublure, dorsal surface of pygidium and pygidial doublure.

Remarks. This species can be distinguished from other Silurian dalmanitids from central Victoria by its pygidial pleural morphology, with relatively narrow, trench-like pleural furrows, and anterior bands that are slightly longer (exsag.) than the posterior bands at the fulcrum and only slightly elevated above the posterior bands distally. Dalmanites athamas differs in having longer (exsag.) pleural furrows and anterior pleural bands that are much longer than the posterior bands, whereas D. wandongensis has anterior bands that are slightly shorter (exsag.) than the posterior bands at the fulcrum and strongly elevated above the posterior bands distally. D. wandongensis also differs from the present species in having a pygidium with a well defined axial terminus and a shorter mucro that is very broad at the base, and a hypostome with deeper middle, lateral border and posterior border furrows, and a posterior margin that is transverse in outline medially instead of parabolic.

The specimens are poorly preserved and mostly fragmentary, especially the cephala. Assignment to Bessazoon is based on the finely granulose cephalic ornament, the entire anterior cephalic margin, the curved posterolateral pygidial margin and the long, slender mucro merging with the axial terminus. The type species B. tenuimucronatum differs from the present one in having slightly inflated lateral glabellar lobes, a more strongly curved (exsag.) palpebral lobe, a deeper palpebral furrow, and the posterior pleural bands on the pygidium more expanded and prominent distally than the anterior bands rather than the reverse. B. tigerense from the upper Llandovery of Tasmania has a more strongly curved palpebral lobe, a deeper palpebral furrow, and less robust postaxial ridge and mucro.

Preodontochile Degardin and Pillet, 1984

Type species. Dalmanites (Preodontochile) camprodonensis Degardin and Pillet, 1984 from the central Pyrenees, Spain, by original designation. The precise age of D. (P.) camprodonensis is uncertain, as Degardin and Pillet stated (p. 87) that the species occurs with graptolites of the early Wenlock Monograptus riccartonensis Biozone , but elsewhere (fig. 4) they showed its stratigraphical range as lying in the upper Llandovery.

Remarks. Degardin and Pillet (1984) erected the monotypic Dalmanites (Preodontochile) for a poorly known species represented by few and poorly preserved specimens including a single crushed cranidium, an isolated thoracic segment and a number of pygidia that are mostly incomplete posteriorly. The material permits only a limited assessment of Preodontochile , but as diagnostic of their subgenus Degardin and Pillet listed the small eye, the course of the anterior branch of the facial suture, and the multisegmented pygidium with a short, blunt mucro. They likened the facial suture both to that of Dalmanites in being situated in close proximity to the glabella and to that of Odontochile in being separated from the glabella by the preglabellar furrow and a narrow band of the anterior cephalic border. However, as noted by Whittington and Campbell (1967), the distinction between Dalmanites and Odontochile on the basis of the anterior cephalic morphology is not as clear-cut as stated by Richter, Richter and Struve (1959). Several Wenlock– Ludlow species of Dalmanites , including the Swedish D. imbricatulus ( Angelin, 1851) (see Ramsköld, 1985), the North American D. puticulifrons Whittington and Campbell, 1967 and D. rutellum Campbell, 1967 , and the Australian D. wandongensis , have a narrow band of the anterior border enclosed by the facial suture on the cranidium. This condition is also present in the Victorian Llandovery dalmanitid described below as Preodontochile springfieldensis (see Fig. 16A ), which further resembles P. camprodonensis in the greatly reduced eye situated far forwards opposite L3, the robust genal spine, the short, bluntly pointed mucro and the finely granulose sculpture. We consider this combination of characters to be of generic significance. With better understanding of P. camprodonensis other cephalic features of P. springfieldensis may also prove to be diagnostic of the genus, such as the uniformly narrow (sag., exsag.) anterior border that is less than half the width of the lateral border and lacks a median projection. In its pygidium with a large number of segments and narrow axis, P. camprodonensis resembles late Silurian–Devonian species assigned to Odontochile and closely related genera. Degardin and Pillet (1984) considered a large number of pygidial segments as diagnostic of Preodontochile , but in view of the otherwise close similarity of the type species with the more poorly segmented P. springfieldensis the number of pygidial segments is here regarded as only of specific significance.

Other dalmanitid genera known from the Llandovery include Bessazoon (see above), Daytonia Holloway, 1981 and Prodontochile Kobayashi and Hamada, 1971 , all of which are easily distinguished from Preodontochile by their much larger eyes. In addition, Bessazoon differs from Preodontochile in having the facial suture tightly enclosing the glabella, and a long, slender mucro connected to the pygidial axis by a postaxial ridge; Daytonia has the occipital ring markedly reduced in length abaxially, S1 bifurcate adaxially and converging slightly with S2 abaxially, the anterior cephalic border as wide as the lateral border, short and slender genal spines, and a pygidium with parabolic outline and a tiny mucro joined to the axis by a postaxial ridge; and Prodontochile has a narrower lateral cephalic border and slender genal spines.

Wenlock–early Ludlow species of Dalmanites assigned by Ramsköld (1985) to his group around the type species D. caudatus differ from Preodontochile in having a large eye extending from opposite L3 to opposite L1, an anterior cephalic border with a well-developed medial process, tuberculate sculpture on the glabella, prominent lateral nodes on some thoracic axial rings, pleural nodes on some pygidial segments, and a long pygidial mucro.

Figures

Figure 14. Bessazoon sp. A, NMV P139479, pygidium, × 2, from ʻLancefieldʼ. B, NMV P139481, pygidium, × 3.5, from ʻLancefieldʼ. C, NMV P139475, pygidium, from ʻLancefieldʼ. D, NMV P139474, pygidium, × 2.5, from ʻLancefieldʼ. E, NMV P139477, pygidium with doublure exposed, × 6, from ʻLancefieldʼ. F, NMV P139471, crushed cephalon, × 2, from ʻLancefieldʼ. G, NMV P139427, hypostome, × 3, from PL1452, Lancefield. H, J, NMV P139438, pygidium, from PL1452, Lancefield; H, enlargement of anterolateral region showing granular sculpture, × 8; J, × 2. I, NMV P139439, crushed cephalic doublure, × 1.9, from PL1452, Lancefield. K, thoracic segment, enlargement showing granulose ornament on pleural tip, × 8, from ʻLancefieldʼ. L, NMV P139470, fragment of cephalon, × 3, from ʻLancefieldʼ. M, NMV P139473, pygidium, × 2, from ʻLancefieldʼ. (E–F are internal moulds).

Figure 16. Preodontochile springfieldensis sp. nov., from PL1369, Springfield.A, E–F, holotype NMV P312070, × 2. B, paratype NMV P312074, pygidium, × 2. C, G, paratype NMV P312075, thoracopygon; C, × 3; G, × 2.7. D, paratype NMV P312072, cranidium and displaced cheek, × 5. H, paratype NMV P139350, enlargement of eye, × 8. I, NMV P139351, three thoracic segments, × 1.5. (B–C, E are internal moulds).

Figure 17. A, C, Struveria sp. 2, from PL206, Wallan. A, NMV P138209, pygidium, × 2. C, NMV P139337, cranidium,× 5.5.B, Dalmanitidae indet., NMV P127957, partial view of incomplete thorax, × 0.9, from old Costerfield Antimony Mine, Costerfield. D–F, Bessazoon sp. D, NMV P147769, pygidium (fragment), × 1.8, from PL256, Wallan. E, NMV P312817, cranidium (fragment), × 2.2, from PL6361, Springfield. F, NMV P147770, pygidium (fragment), × 2.4, from PL256, Wallan.