Digonus wenndorfi, Sandford, 2005

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 : 23-28

Digonus wenndorfi sp. nov.

Trimerus ( Dipleura ?) sp.— Talent, 1964: 49 (pars), pl. 26 figs 1, 2 non text-fig. 6.

Homalonotidae gen. et sp. indet. 1.— Holloway and Neil, 1982: 145, figs 4A-H.

Type material. Holotype NMV P304717 (pygidium) from PL2203 , Thomas locality F3, Parish of Dargile, Heathcote , Victoria ( Fig. 12.17) . Paratypes NMV P304709 – P304716 (cranidia) , NMV P304874 (librigena) , NMV P304872 , P304872 (thoracic segments) , NMV P304717 , NMV P304860 – P304866 , NMV P304868 (pygidia) from PL2203 . Paratype NMV P304913 (librigena) from PL 2327, Heathcote. For localities see Fig. 8 .

Previously figured material. NMV P78295 (ex GSV 39474, pygidium, figured Holloway and Neil, 1982: figs 4E, 4H), NMV P59656 (ex GSV 39200 pygidium, figured Talent, 1964: pl. 26 fig. 2, Holloway and Neil, 1982: figs 4F, 4G), NMV P59655 (ex GSV 39476, pygidium, figured Talent, 1964: pl. 26 fig. 1) from PL 2203. NMV P78293 (thoracic segment, figured Holloway and Neil, 1982: fig. 4C), NMV P78294 (thoracic segment, figured Holloway and Neil, 1982: fig. 4D) from PL 2202, Thomas locality F2, Parish of Dargile, Heathcote. NMV P78292 (cephalon, figured Holloway and Neil, 1982: figs 4A, 4B) from PL 2204, Thomas locality F4, Parish of Dargile, Heathcote. For localities see Fig. 8 .

Registered material. 88 specimens: 2 cephala, 12 cranidia, 4 librigenae, 17 thoracic segments, 53 pygidia. NMV P59655 , P59656 , NMV P78295 , NMV P82882 , NMV P82886 , NMV P304709 – P304717 , NMV P304859 – P304903 from PL2203 . NMV P78293 , P78294 , NMV P82880 , NMV P82883 – P82885 , NMV P304904 from PL2202 . NMV P82941 , NMV P304905 – P304914 from PL2327 . NMV P78292 from PL2204 .

Stratigraphic distribution. Mt Ida Formation, from about 400 m above the base of the Dealba Member up to about the 100 m above the base of the Stoddart Member, Boucotia jana e Assemblage Zone, early Lochkovian. The first appearance of Digonus wenndorfi at PL 2204 is close stratigraphically to the first appearance of Boucotia (see Garratt, 1983: fig. 7) and together support a basal Lochkovian age for these horizons.

Derivation of name. For Dr Klaus-Werner Wenndorf ( Germany), for his contribution to the study of homalonotids.

Diagnosis. Glabella trapezoid, length 1.0-1.2 times width, sides straight and converging at about 20º opposite genae, anterior margin of glabella well defined with medial indentation moderately impressed to indistinct. Glabellar lobation weak to indistinct. Axial furrows moderately impressed opposite genae. Palpebral lobe short, 0.1 times cranidial length, placed with midline opposite 0.5 glabellar length/0.39 cranidial length. Preglabellar field long, length (sag.) 0.22 times cranidial length. Rostral suture transverse. Pygidium triangular, sides moderately convex and converging posteriorly at 85–100º, tip angular. Pygidial axis with width 0.5–0.6 times pygidial width, 11–12 axial rings, ring furrows deep, continuous with raised postaxial ridge. Axial furrows poorly defined opposite first–second axial rings, moderately impressed posteriorly, straight and tapering at 30–40º. Pleural furrows deep. 8–9 pleural ribs, rib-ring medially offset at fifth rib.

Description. Exoskeleton large (maximum length estimated 25 cm from NMV P304860), occipital convexity (tr.) moderate, pygidial convexity (tr.) strong. Dorsal exoskeleton finely granulose.

Cranidia with elongate and short forms. Elongate form with cranidial width about 1.6 times length, short form with cranidial width about 1.9 times length. Glabella with length 0.78 times cranidial length, anterior margin very broadly rounded to transverse. Elongate form with glabellar length 1.2 times width, short form with glabellar length equal to width. Occipital ring with length 0.1 times glabellar length. Occipital furrow deeply impressed, with weak forward flexure medially. Glabellar lobation (best seen on NMV P304715 and NMV P304710) with L1 0.28 and L2 0.15 times glabellar length respectively. S1 straight and directed posteromedially at about 17º from the transverse, S2 weakly convex forwards and transverse, S3 indistinct. In some specimens (e.g. NMV P304710, P304711) S1 moderately impressed for short distance adjacent to axial furrow. Axial furrows very shallow and directed diagonally opposite occipital ring. Paraglabellar area weakly defined (best seen on NMV P304711). Length (exsag.) of posterior border equal to occipital length adaxially, lengthening slightly abax-ially. Posterior border furrow transverse, very wide, moderately impressed, terminating distally. Postocular fixigenal area very short, length (exsag.) 0.16 times cranidial length. Palpebral lobes placed remotely (b- b 1.67 times preoccipital glabellar width/0.7 times cranidial width). Palpebral furrow weak to moderately impressed. Preocular fixigenal area of moderate width, 0.17 times b- b narrowing slightly anteriorly. Preglabellar field weakly concave (tr. sect.). Anterior branches of facial suture angular, directed exsagittaly for a short distance adjacent to the eye (to a point opposite 0.7 glabellar length), anteriorly converging at about 40º in elongate forms and 60º in short forms. Librigena without distinct border furrow or lateral border. In anterior view anterior margin of librigenae moderately convex.

Thorax with axial furrows indistinct. Pleural furrows wide and deep.

Pygidia with long form and narrow form, sides converging posteriorly at about 85º in narrow form and 100º in wide form. Pygidial axis with width about 0.5 times pygidial width in wide form, 0.6 in narrow form. Pygidial axis reaching to about 0.85 times pygidial length, raised, continuous posteriorly with postaxial ridge. Postaxial ridge wide, width 0.3 times axial width. Axial furrows straight and tapering at about 33º in narrow form, 38º in wide form, curving to the exsagittal posteriorly. Pleural furrows not reaching margin. Border furrow and border not defined. In posterior view posterior margin of pygidium horizontal. In lateral view dorsal profile of axis inclined at about 30º to the horizontal, postaxial ridge very steeply inclined.

Discussion. Digonus wenndorfi is abundant and dominates the trilobite fauna at several localities. Except for a cephalon ( Holloway and Neil, 1982: figs 4A, 4B) the species is known only from isolated tergites. Relative proportions of many features (eg. glabellar and pygidial length/width ratios) are difficult to ascertain due to deformation. Nevertheless, two distinct forms of wenndorfi can be recognised, elongate and short morphs differing in the relative proportions of the cephalon, glabella and pygidium. These morphs are not related to size, nor are they products of deformation as their ranges in size overlap, and they occur together on the same bedding plane ( Fig. 3 ). The differing morphological proportions within the population are strongly bimodal rather than continuous. Pygidial length-width ratios range between 0.76–0.93 (average 0.86) in the short morphs, in the long morphs between 1.11–1.12. Similarly, glabellar length-width ratios range between 0.96–1.00 (average 0.99) in the short morphs, in the long morphs between 1.15–1.23 (average 1.19). Similar observations have been made for populations of D. gigas and attributed to sexual dimorphism ( Dahmer, 1914), although Wenndorf (1990: table 31) demonstrated, with a much larger sample size of 67 pygidia, that the variation in gigas is continuous rather than bimodal, with intermediate forms being the most numerous, and so not reflecting sexual dimorphism. Although the sample size for wenndorfi is much smaller (11 measurable specimens) the complete absence of intermediate forms (e.g. pygidia with length-width ratios range between 0.93–1.1) is statistically significant. Whether the bimodality reflects sexual dimorphism is uncertain, although the narrower morphs strongly outnumber the wider morphs and so suggest that it does not.

The subquadrate outline of the anterior part of the cranidium, the weakly tapered trapezoid glabellar outline, the wide pygidial axis, and the depth of the pygidial ring and pleural furrows clearly indicate assignment to Digonus . D. wenndorfi differs markedly from the D. gigas group and more closely resembles the D. ornatus group in having a relatively longer preglabellar field, wide fixigenae anteriorly, and a transverse rather than weakly concave anterior cranidial margin.

Digonus wenndorfi differs from most Pragian-Emsian species of Digonus in lacking the elongate, straight-sided triangular outline and a long produced tip characteristic of the type and other taxa such as D. ornatus and D. intermedius . In having shorter pygidial proportions, wenndorfi more closely resembles other Lochkovian species, including D. laticaudatus from North America, D. armoricanus from France, and D. roemeri from Europe. Compared to Pragian-Emsian species, Lochkovian species of Digonus lack some of the features characterising Digonus . These species can be interpreted as reflecting ancestral morphologies, as might be expected in the earliest representatives of a genus. ‘ Trimerus -like’ features including the raised postaxial ridge and weak glabellar lobation of wenndorfi can be interpreted in this context.

Of Lochkovian species of Digonus , only D. zeehanensis from Tasmania (redescribed below) is known sufficiently for detailed comparison with D. wenndorfi . These taxa are very close, but the latter differs in having slightly concave glabellar sides, a longer glabella, more distinctly expressed glabellar lobation, a shorter preglabellar field, a much shorter pygidium with a relatively narrower pygidial axis, a more obtusely-angled pygidial tip, and a lower postaxial ridge ( Fig. 13). With the exception of the latter, these features suggest that zeehanensis represents a less derived morphology.

Digonus roemer i is a near-contemporary of D. wenndorfi , recorded from the base of the Lochkovian (hesperius zone) in Europe (Chlupác˘ et al., 2000). The species is poorly documented and, as noted by Tomczykowa (1975), a number of taxa have been assigned to it. Topotypes of D. roemeri ( Morzadec, 1986: pl. 32 figs 1, 4, 6–10) share with wenndorfi short pygidial proportions and the equally deep pygidial pleural and ring furrows typical of Digonus . D. roemeri differs from wenndorfi in that the pygidial axis is wider and the postaxial ridge is not raised. In contrast to other species assigned to the genus, pygidia of roemeri and the comparable D. laticaudatus exhibit anteriorly continuous pygidial axial furrows, interpreted here to reflect the ancestral morphology.

Many species assigned to Digonus are poorly documented and comparison with D. wenndorfi is difficult. From Europe, D. harpyius is known only from a rostral plate. The Emsian D. goniopygaeus from England and D. crassicauda from Germany are known only from pygidia. Despite the comparable short proportions, the pygidium of crassicauda differs from that of wenndorfi in having a weakly defined postaxial ridge and a narrow, recurved flange defining the pygidial tip ( Wenndorf, 1990: pl. 9 figs 1–3), as in other Emsian Digonus . The Antarctic D. antarcticus differs markedly from wenndorfi in having weakly expressed pygidial axial furrows, exceptionally deep pleural furrows and a poorly expressed postaxial ridge ( Saul, 1965: pl. 17 figs 1–11).

Holloway and Neil (1982) considered specimens of Digonus wenndorfi to be close to Trimerus lilydalensis Gill, 1949 , also from the Lochkovian of central Victoria. The availability of many new specimens of wenndorfi and a revision of Gill’s species does not support this comparison, with the latter assigned to the new genus Wenndorfia .

Figures

Figure 3. Sandstone slab from PL2204 (Thomas’ F4, Parish of Dargile), Heathcote with bedding plane showing numerous isolated pygidia and cranidia of Digonus wenndorfi in various orientations and with large bivalves and brachiopods indicative of Boucot and Johnson’s (1967) ‘big shell’ community.

Figure 8. Geological sketch map of the Heathcote area showing Wenlock-Lochkovian fossil localities yielding homalonotids. For other fossil localities see also Thomas (1940a, 1940b, 1941, 1956), Talent (1964, fig. 1), Sandford (2002, fig. 1A; 2005, fig. 4).