Digonus Gürich, 1909

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 : 21-23

Digonus Gürich, 1909

Type species. Homalonotus gigas Roemer, 1843 , from the Lower Devonian (Emsian) Kahleberg Sandstone, Germany, by original designation .

Other species and subspecies included. Burmeisteria (Digonus) antarcticus Saul, 1965 , D. armoricanus Pillet, 1961a , D. asturco Kegel, 1927 , Homalonotus crassicauda Sandberger and Sandberger, 1849 , D. gigas posterior Wenndorf, 1990, H. goniopygaeus Woodward, 1882 , H. (D.) harpyius Richter and Richter, 1932 , H. intermedius Vietor, 1919 , H. laticaudatus Williams and Breger, 1916 , H. (D.) ornatus disornatus Richter and Richter, 1932, D. ornatus linguatus Wenndorf, 1990 , H. ornatus Koch, 1883a , D. comes Chlupác ˘, 1981, H. roemeri de Koninck, 1876 , D. wenndorfi sp. nov., Trimerus zeehanensis Gill, 1949 , D. zemmourensis Pillet, 1961b , Digonus sp. A in Wenndorf (1990).

Other species tentatively included. Digonus collini Renaud, 1942 , Homalonotus derbyi Clarke, 1890 .

Range. Lochkovian-Emsian.

Diagnosis. Cranidium trapezoidal in outline, anterior margin with median cusp. Dorsal section of connective sutures very short. Glabella trapezoidal in outline, sides straight or very weakly concave. Glabellar lobation very weak or absent. Paraglabellar area distinct. Anterior outline of cranidium quadrate, anterior branches of facial suture more or less straight between palpebral lobe and midlength of preglabellar field and converging at an acute angle (20–60º), curving abruptly to the transverse anteriorly. Rostral suture transverse to weakly concave. Ventral surface of rostral plate with thorn, spine or keel. Pygidium triangular to elongate triangular, with wide axis (0.5–0.7 times pygidial width). Axis with weak independent convexity (tr. sect.), postaxial ridge not raised. Pleural and ring furrows deep to very deep, more or less equal in depth. Axial furrow shallow or moderately impressed posteriorly, very shallow or effaced anteriorly, anteriormost ribs and rings fused, with corresponding section of axial furrow effaced or reduced to shallow invagination in posterior margin of segment. Pleural furrows maintaining uniform depth to a point close to border. Border furrow and border poorly defined. Rib-ring offset high (fifth–seventh rib).

Discussion. Previous diagnoses for Digonus are brief. Gürich (1909) listed the truncate and indented outline of the cephalic anterior margin, the angular margin of the antero-lateral corners of the cranidium, the pointed pygidial tip and the weakly tapered, trapezoid glabellar outline. Reed (1918) emphasised the strong expression of the pygidial segmentation as a further diagnostic character. Sdzuy (1959) included the absence of glabellar lobation and angular pleural tips as diagnostic characters although, as he regarded Digonus as a subgenus of Burmeisteria , other characters were incorporated into the generic concept. These included the distinct paraglabellar area, the presence of a ventral process on the rostral plate, a triangular pygidial outline, distinct pygidial trilobation, and a poorly expressed postaxial ridge. Tomczykowa (1975) omitted many of Sdzuy’s characters from her revised diagnosis, closely following Reed. In emending Tomczykowa’s diagnosis, Wenndorf (1990) noted the presence of weak glabellar lobation in some species, and that early representatives of the genus have less elongate pygidial outlines.

The revised diagnosis given here adds substantially to the list of diagnostic characters and qualifies other characters previously used. A substantially restricted concept of the genus is proposed by placing new emphasis on pygidial characters including the relative depth of pygidial ring and pleural furrows, the expression of the axial furrows, the expression of the postaxial ridge and the width and convexity of the pygidial axis. The revised diagnosis emphasises differences between Digonus , Trimerus and Burmeisteria , and results in substantially different assignments of the species listed by Tomczykowa (1975) and Wenndorf (1990).

Tomczykowa (1975) and Wenndorf (1990) considered Digonus to have been derived from Trimerus . The genera are certainly close, many of the characters listed in the diagnosis of Digonus occurring variously in species of Trimerus . Although the presence of a rostral keel or process is considered of primary importance in differentiating the genus from Trimerus , it is the combination of characters that defines Digonus . Other cephalic characters, particularly the degree of glabellar lobation, the quadrate course of the facial and rostral sutures, and the trapezoid glabellar outline, distinguish Digonus from most species of Trimerus . Pygidia of Digonus can generally be distinguished from those of Trimerus by the deeper pleural and ring furrows that are more or less equal in depth, by the shallower axial furrows, and by the less convex axis and postaxial ridge. For the morphologically convergent species, it is the continuity of the axial furrow anteriorly in Trimerus (and the fusion of the anteriormost axial ring and pleural rib in Digonus ) that distinguishes the genera. Hence, Homalonotus crassicauda from the Emsian of Germany, known only from pygidia and previously interpreted as a temporally disjunct representative of Trimerus ( Wenndorf, 1990, Schraut, 2000) clearly belongs in Digonus . The Lochkovian North American H. major Whitfield, 1881 is best assigned to Trimerus rather than to Digonus , because the pygidial axial furrow is deep and distinct anteriorly.

Digonus wenndorfi and D. zeehanensis from the lower Lochkovian of south-eastern Australia are the best known of the early representatives of Digonus and support the derivation of this genus from Trimerus . D. wenndorfi and the French D. roemeri are the earliest Digonus , occurring immediately above the Silurian-Devonian boundary and suggesting a latest Silurian origin for the genus. In cranidial features, wenndorfi and zeehanensis show close affinities to the D. ornatus group from the middle Pragian-Emsian of Europe rather than to the D. gigas group (see Wenndorf, 1990) whose members share longer preglabellar fields. Weak glabellar lobation and short pygidia with markedly obtuse (120-130º) angular tips set wenndorfi and zeehanensis apart from later members of the genus, with the exception of several species ( D. antarcticus and D. ornatus disornatus ) that retain weak lobation, and several others that have similarly short pygidia (e.g. D. armoricanus and D. crassicauda ). In addition to weak glabellar lobation and short pygidial proportions, wenndorfi also exhibits a moderately raised pygidial postaxial ridge ( Figs 12.6, 12.12–12.18). In these features wenndorfi can be considered a ‘primitive’ Digonus , with relict features of Trimerus retained. D. zeehanensis bears a short (sag.) but wide (tr.) median cephalic cusp ( Fig. 13.1) intermediate in morphology between the large semicircular cusps of Trimerus on species such as T. (Ramiotis) otisi and T. (Trimerus) johannis , and the small, acute cusps of Emsian Digonus . The anteriorly distinct pygidial axial furrows of roemeri (see Morzadec, 1986: pl. 32 figs 1, 4, 6–7) and (to a lesser extent) wenndorfi , can also be interpreted as a Trimerus - like feature, although the wide pygidial axis and equally deep pleural and ring furrows indicates their affinities with other Digonus .

Emphasising the significance of the transverse anterior margin of the cranidium and the depth of the pygidial pleural and ring furrows, Tomczykowa (1975) derived Digonus from Trimerus , suggesting T. (Trimerus) johannis as a transitional morphology between the genera. Thomas (1977) doubted this suggestion, emphasising the difference in age between T. (T.) johannis (late Wenlock) and the first appearance of Digonus (earliest Lochkovian). Wenndorf (1990) interpreted European Lochkovian species such as Homalonotus vialai Gosselet in Gosselet et al., 1912 and H. roemeri as representing early Digonus morphologies, noting similarities to the Polish Ludlow T. permutus Tomczykowa, 1978 (= T. lobatus Tomcykowa, 1975 non Prouty, 1923 in Swartz and Prouty). A more plausible transitional morphology is represented by the upper Ludlow T. (Ramiotis) otisi . The similarities of this species to contemporary Homalonotus have been discussed above, but its similarities to species of Digonus are more marked. In addition to having a tricuspid cephalic margin, otisi exhibits a subquadrate course of the facial and rostral sutures, weak glabellar lobation, straight-sided glabellar outline, deep pleural and equally deep ring furrows, and a pygidial axial furrow that is indistinct anteriorly ( Fig. 20). In the latter feature, otisi is an exception amongst congeners. However, the flat rostral plate, the long glabellar proportions, the narrow proportions of the pygidial axis and the raised postaxial ridge demonstrate its close affinities to Trimerus .

Digonus wenndorfi and D. zeehanensis demonstrate the conservative Digonus cephalic morphology to have been already established in the earliest Lochkovian. European species considered by Tomcykowa (1975) as representing early Digonus morphologies, including Homalonotus vialai , D. bostoviensis and D. elegans , are variably assigned below to Parahomalonotus and Wenndorfia , and exhibit few of the diagnostic cephalic or pygidial features of Digonus . In particular, the anterior branches of the facial sutures of these species are broadly curved and converge at a strongly obtuse angle opposite the preglabellar field, giving the anterior margin of the cranidium a more rounded/parabolic outline. Burmeisteria ( Digonus ?) delattrei Pillet and Waterlot, 1982 from the Emsian of France exhibits few features typical of Digonus . It has a raised pygidial axis, a raised postaxial ridge with a posteriorly projecting spine, anteriorly distinct axial furrows and a moderately convex (tr.), elongate glabella. The pygidial morphology, including the posteriorly projecting axis, indicates assignment to Burmeisterella . Wenndorf (1990) questionably assigned the Brazilian Homalonotus oiara Hartt and Rathbun, 1875 to Digonus , but their emphasis on the concavity of the sides of the glabella is not compatible with the quadrate glabellar outline considered here as diagnostic of Digonus . The forward eye position exhibited by the single cranidium known suggests affinities with Burmeisteria rather than with Dipleura .

Relationships between Digonus and Burmeisteria have been variously interpreted. Sdzuy (1959) considered Digonus to be a subgenus of Burmeisteria . Tomczykowa (1975) listed several differences between the taxa, emphasising the absence of spines and glabellar lobation in Digonus to support its independent status. Cooper (1982) argued that the range of variation in the course of the rostral suture, the degree of glabellar lobation and the glabellar outline in the highly polymorphic type species B. herschelii ( Murchison, 1839) overlapped with morphologies typical of Digonus . Following the emphasis placed by Sdzuy on these characters, Cooper regarded Digonus as junior synonym of Burmeisteria . Wenndorf (1990) questioned many of the differences listed by Tomczykowa, noting a convergence in morphologies between Burmeisteria and Digonus , but nevertheless maintaining their independent status. Kennedy (1994) noted that the pygidial doublure of Digonus , unlike that of Burmeisteria , is often narrower and bears a narrow groove in the margin of the pygidial doublure. This distinction between the genera also applies to Burmeisterella , which Kennedy considered as a junior synomym of Burmeisteria . The independent status of Burmeisterella is supported here, with high generic significance accorded to the very short (sag.) preglabellar field and the morphology of the pygidial axis, which is raised high rather than fused with the pleural field posteriorly, and extends posteriorly over the margin as a narrow-based to spinose tip. In addition, the pygidium of Burmeisterella has a more convex lateral margin, and always bears a pair of prominent, large nodes or spines on the anteriormost pleural rib. The independent status of Burmeisteria and Digonus is supported in this work. The taxa can be readily distinguished morphologically, despite the extreme polymorphism exhibited by herschelii . In Burmeisteria , the pygidial axial furrow is impressed anteriorly, and the pygidial pleural and ring furrows are typically shallower than those of Digonus and shallow posteriorly. Burmeisteria is further characterised by a high number of pygidial segments (>13 axial rings). Cranidia of Burmeisteria and Digonus may exhibit morphological convergence, but a glabellar outline markedly expanded across L1-L1 (tr.) or with concave sides, and well defined lateral glabellar furrows immediately distinguish Burmeisteria when present.

Following Hennig (1965), Wenndorf suggested that the mutually exclusive palaeogeographic ranges of Burmeisteria and Digonus represented a significant taxonomic feature supporting their independent status. Digonus wenndorfi , D. zeehanensis and D. antarctica extend the range of the genus into temperate palaeolatitudes of north-eastern Gondwana. However, the palaeogeographic ranges of the genera remain mutually exclusive. B. herschelii is restricted to lower palaeolatitudes of southern Gondwana, occurring in southern Africa, the Falkland Islands and South America. The reassignment of several South American taxa to Burmeisteria emphasises the provincialism of the genus (see Table 1), perhaps as early as the Late Silurian. Homalonotus linares Salter, 1861 , assigned below to Burmeisteria (see Trimerus ) occurs in the Late Silurian of Bolivia and Uruguay.

Affinities of the Lochkovian Homalonotus noticus Clarke, 1913 (?= H. caorsi Mendez-Alzola, 1938 ) from South America and possibly South Africa (see Cooper, 1982) are with Burmeisteria . Reed (1918) initially assigned the species to Burmeisteria but later reassigned it to Digonus ( Reed, 1925) . Sdzuy (1957) suggested that the species represents a transitional form between Burmeisteria and Digonus . The distinct S1-S3, the narrow pygidial axis and the anteriorly continuous pygidial axial furrow are incompatible with assignment to Digonus . Tomczykowa (1975) and Wenndorf (1990) assigned H. noticus to Trimerus , but the presence of a rostral projection, a sinusoidal rostral suture and high pygidial segmentation (up to 13 axial rings) excludes the species from the genus as defined here.

Homalonotus clarkei from the Lower Devonian South America (?= H. buqueti Mendez-Alzola, 1938 ,?= H. spatulirostris Mendez-Alzola, 1938 ), has been variably assigned to Digonus (Wolfart, 1968) , to Trimerus (with question, Tomczykowa, 1975), to Burmeisteria ( Cooper, 1982) and to Dipleura ( Wenndorf, 1990) . The strongly folded anterior cephalic margin excludes assignment to Trimerus , as this taxon is characterised in this work by having a flat rostral plate. The very shallow pygidial pleural furrows and somewhat deeper ring furrows of clarkei are typical of Dipleura , but the tricuspid anterior cephalic margin, the quadrate and medially concave anterior cranidial margin, and the strong forward tapering of the glabella do not conform to this assignment. The weakly expressed pygidial segmentation excludes the species from Digonus as defined here. The generic affinities of the species are uncertain.

Burmeisteria (Digonus) accraensis from the Devonian of Ghana exhibits weak pygidial furrows, excluding the species from Digonus as defined in this work. The taxon has been variably assigned to Trimerus ( Tomczykowa, 1975) and to Dipleura (with question, Wenndorf, 1990), but the presence of a rostral process (see Saul, 1967: pl. 143 fig. 6) excludes the species from these genera.. The species is assigned here to Burmeisteria .