Digonus zeehanensis ( Gill, 1949 )

Sandford, Andrew C., 2005, Homalonotid trilobites from the Silurian and Lower Devonian of south-eastern Australia and New Zealand (Arthropoda: Trilobita: Homalonotidae), Memoirs of Museum Victoria 62 (1), pp. 1-66 : 28

Digonus zeehanensis ( Gill, 1949)

Figure 13

Trimerus zeehanensis Gill, 1949: 70 , pl. 9 figs 1, 2, 4, text-fig. 1D.— Tomczykowa, 1975: 11. — Wenndorf, 1990: 16. — Holloway and Sandford, 1993: 93. — Schraut, 2000: 382.

Type material. Holotype NMV P14590 (cranidium, counterpart previously registered NMV P14591 , Fig. 13.7) and paratype NMV P14592 (pygidium, counterpart previously registered NMV P145913 ) from PL1726 , Gill and Banks locality 16, Zeehan , Tasmania.

Registered material. 67 specimens: 2 cephala, 13 cranidia, 12 librigenae, 2 hypostomes, 11 thoracic segments, 27 pygidia. NMV P14787 , P14788 (counterparts), NMV P14789 , NMV P14791 , NMV P304648 – P304656 , NMV P304662 , NMV P304664 – P304708 from PL1726 . NMV P304657 from “Little Henty River”, Zeehan. NMV P304661 from “near Little Henty River”, Zeehan. NMV P304663 from PL1725 , Gill and Banks’ locality 15, Zeehan. NMV P304660 , NMV P304658 from PL6653 , Zeehan. NMV P7561 , NMV P304659 from “Mt Zeehan”, Zeehan .

Stratigraphic distribution. Bell Shale, Boucotia australis

Assemblage Zone, mid-late Lochkovian.

Diagnosis. Cephalon trapezoid, sides straight, anterior margin tricuspate, with short (0.05 times cephalic length) but wide (length 0.25 times width) median cusp, lateral cusps about half? the length of the median cusp. Glabella trapezoid, sides weakly concave and converging at about 20º, length about 1.2 times width, anterior margin well defined, lobation weakly defined. Axial furrows moderately impressed. Palpebral lobe placed with midline opposite 0.52 glabellar length/0.42 cranidial length. Preglabellar field with length (sag.) 0.17 times cranidial length. Anterior branches of facial suture straight and converging at about 50º, curving abruptly to the exsagittal anteriorly. Rostral suture transverse. Dorsal surface of rostral plate triangular, short, length 0.15 times width. Hypostome with middle furrow deep adaxially, shallow abaxially. Posterior border of hypostome with long lateral lobes of length 0.17 times hypostomal length. Pygidium short, length 0.68 times width, sides moderately convex and converging posteriorly at about 110º. Pygidial axis 0.45 times pygidial width, 12 axial rings, continuous with low postaxial ridge. Axial furrows tapering at about 30º, moderately impressed posteriorly, not continuous anteriorly opposite first ring. Pleural and ring furrows deep. 8 ribs, rib-ring medially offset at fifth rib. Dorsal exoskeleton with coarse pitting.

Discussion. The Bell Shale is strongly sheared, resulting in wide variations in shapes and proportions of specimens. Because Gill (1949) described the species from a limited number of specimens, examination of a larger population reveals a number of inaccuracies in his description. The concavity of the preglabellar field and the upturning of the anterior cranidial margin on the holotype ( Fig. 13.7), considered diagnostic of the species by Gill, are features attributable to tectonic distortion and are absent from other topotype cranidia. Gill recorded lateral glabellar furrows as absent, but they are weakly defined on most specimens. The pygidial axial furrows are moderately impressed rather than poorly defined. Despite the abundance of cephalic and pygidial tergites, thoracic pleurae are unusually rare in the Bell Shale. From the fragments available, there is no indication of spines on the thorax as suggested by Gill.

Tomczykowa (1975) and Wenndorf (1990) assigned Gill’s species to Trimerus , but the quadrate course of the anterior branches of the facial and rostral sutures, the equally deep pygidial ring and pleural furrows and the effacement of the axial furrow anteriorly are more consistent with an assignment to Digonus . Pygidia have a much wider axis (average width 0.46 pygidial width) than recorded by Gill, who cited the axial width to be one third the pygidial width. However, as noted above, the longer glabellar proportions, more distinctly expressed glabellar lobation, shorter pygidial proportions with a relatively narrower axis and a more obtusely-angled pygidial tip suggest that D. zeehanensis represents a less derived morphology compared to other Digonus . In these features, zeehanensis is most closely comparable to D. roemeri from the basal Lochkovian of France.

Environmental notes. Digonus zeehanensis dominates the trilobite fauna of the Bell Shale. At the best-sampled locality ( PL 1726), the relative abundance of D. zeehanensis is>90%. The fauna is typical of homalonotid-dominated assemblages, being low diversity and otherwise comprising a proetid, a cheirurid and a dalmanitid (Gill, 1950). The latter suggests alignment with the deeper water Trimerus delphinocephalus-Dalmanites limulurus Community of Mikulic, 1999. The proportion of articulated specimens in populations of zeehanensis is low. Of the 67 specimens, all are isolated tergites with the exception of three cephala. Breakage is high in the sampled populations (Br=27%). Specimens occur on bedding planes in association with other fossil concentrations or in more sparsely fossiliferous layers between the lag bands. The taphonomy is indicative of taphofacies TIII and together with the lithofacies and biofacies suggests a setting around maximum storm wave base.