Wenndorfia, 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 : 55-56

Wenndorfia gen. nov.

Type species. Homalonotus mutabilis Koch, 1880 from the Lower Devonian (upper Emsian) of Germany .

Other species included. Parahomalonotus angusticostatus Tomczykowa, 1975 , P. bostoviensis Tomczykowa, 1975 , Digonus elegans Tomczykowa, 1975 , Homalonotus expansus Hector, 1876 (= H. (Burmeisteria) huttoni Allan, 1935 , = D. margaritifer Wenndorf, 1990 ), H. forbesi Rouault, 1855 , Dipleura fornix Haas, 1968 , Trimerus lilydalensis Gill, 1949 , H. miloni Renaud, 1942 , H. multicostatus Koch, 1883a , H. mutabilis Koch, 1880 , T. novus Tomczykowa, 1975, H. obtusus Sandberger and Sandberger, 1849 , P. planus junior Wenndorf, 1990 , H. planus Sandberger, 1849 , “ P. sp. nov. A” in Wenndorf, 1990, “ D. sp.” in Le Menn et al., 1976.

Range. Lower Devonian (Lochkovian-Emsian).

Diagnosis. Glabella of moderate height to very low, weakly tapering. Lobation indistinct. Anterior margin of cranidium U-shaped and more or less concentric with rounded anterior margin of glabella, anterior branches of facial suture broadly curved and strongly convergent anteriorly, rostral suture short (tr.), transverse or continuous in curvature with anterior section of facial suture. Eyes placed posteriorly, opposite 0.3–0.45 cephalic length. Sides of glabella more or less straight and parallel, varying to strongly concave. Ventral surface of rostral plate with low or prominent anterior node. Pygidial outline parabolic with rounded tip, but triangular with pointed tip in early species. Pygidium with axial furrows very shallow to indistinct, axis without independent convexity, wide (0.4–0.65 time pygidial width anteriorly), strongly tapering (furrows converging at 30–45˚), not reaching margin but terminating at about 0.85–0.95 times pygidial length, semicircular terminal piece short (sag.), slightly raised, but often indistinct from postaxial field.

Discussion. Wenndorfia has been erected to accommodate many species previously assigned to Parahomalonotus (see above). In establishing Parahomalonotus, Reed (1918) listed a number of characters in the diagnosis that are not apparent in the type species Homalonotus gervillei DeVerneuil, 1850 from the Emsian of France. Reed described the “facial sutures uniting in a regular wide arched commissure close to (the) anterior margin” as diagnostic of the genus ( Reed, 1918: 326). However, in gervillei the anterior cranidial margin is quadrate, the rostral suture being transverse and the anterior branches of the facial sutures subparallel (see Pillet, 1973: fig. 120). Reed’s original diagnosis also states a pygidial entire margin and more or less indistinct trilobation as diagnostic. However, in contradiction to these definitive parameters, trilobation is well defined by the raised pygidial axis of gervillei , and the postaxial ridge extends posteriorly as a short posteromedial spine (see Pillet, 1973, pl. 37 fig. 5).

The revised diagnosis for Parahomalonotus s.s. (see above) emphasises the distinctive features of P. gervillei , and hence differs markedly from that given by Reed (1918). Of species assigned to Wenndorfia, Reed assigned Homalonotus planus , H. obtusus and H. multicostatus to Parahomalonotus with question. In the course of their facial sutures and in pygidial features, these species are in closer agreement with Reed’s diagnosis of Parahomalonotus than with the type gervillei . These species further differ from gervillei in having pygidial axes that are much wider anteriorly, taper more strongly, are not independently convex, lack postaxial ridges, and terminate between 0.85 and 0.95 pygidial length, and glabellae that are very low and lack distinct lobation.

Species included in Wenndorfia were previously assigned variously to Digonus , Dipleura , Parahomalonotus and Trimerus . Wenndorfia is most reliably distinguished from these genera by the rounded outline of the anterior cranidial margin, concentric to the strongly rounded glabellar anterior margin. This morphology contrasts most strongly with the angular, quadrate anterior cranidial margin of Digonus . In pygidial morphology early representatives of Wenndorfia and Digonus are alike, although marked differences between these groups are manifest in later representatives in trends toward short, rounded outlines, and elongate, triangular outlines with acute processes respectively. Later representatives of Wenndorfia further differ from Digonus in exhibiting moderately to strongly concave glabellar sides.

Rostral morphology is recognised in this work as a reliable character distinguishing Wenndorfia from Trimerus and Dipleura . The rostral plate of Wenndorfia exhibits a diamondshaped outline and exhibits a variably developed antero-medial node, whereas rostral plates of Trimerus and Dipleura are elongate pentangular and pentangular in outline respectively, and lack rostral processes.

Developmental trends in the pygidial morphology of Wenndorfia , manifest in the replacement of triangular outlines by rounded outlines in the mid-Pragian, are comparable to developmental trends in Dipleura in the Late Silurian. However, pygidia of species assigned to Wenndorfia vary markedly in the depth of the pleural and ring furrows. In a number of species the pleural and ring furrows are very shallow to indistinct, and approach morphologies exhibited by species of Dipleura . The distinction of these morphologically convergent species relies on the course of the cephalic sutures, as discussed above (see Dipleura ). Pygidia of Trimerus (Ramiotis) and T. ( Trimerus ) are distinguished from the Wenndorfia pygidial morphology in having a distinct postaxial ridge.

Wenndorfia mutabilis , unlike most other species of the genus, is abundant and its dorsal and ventral morphology is both completely known and recently redescribed (see Wenndorf, 1990). The species exhibits the rounded anterior cranidial margin, the weakly expressed pygidial trilobation and the entire pygidial margin included in Reed’s original diagnosis of Parahomalonotus and in the diagnosis of Wenndorfia . Other features exhibited by mutabilis and considered typical of Wenndorfia include the strongly rounded anterior margin of glabella, the posteriorly placed eyes, the small node on the ventral surface of rostral plate, and the wide, strongly tapering axis terminating at about 0.85 pygidial length. W. mutabilis is distinguished from its congeners in having a glabella of moderate height with strongly concave sides, a wide palebral area, and a shorter pygidial outline with shallow pygidial ring and pleural furrows.

Lochkovian-lower Pragian Wenndorfia differ from upper Pragian-Eifelian species in variously exhibiting features that can be regarded as underived. These features include a trapezoid glabellar outline, the expression of glabellar lobation, moderately impressed pygidial axial furrows, deep pleural and ring furrows, and a triangular pygidial outline with an obtusely angled tip. In these respects these early species represent a transitional morphologies with early Digonus . Early representatives of Wenndorfia include several Polish homalonotids originally described as species of Digonus by Tomczykowa (1975) and regarded by Wenndorf (1990) as representing an early Digonus morphology. Lacking in these Polish species is the quadrate anterior cranidial outline exhibited by the earliest Digonus (see above, D. wenndorfi ). The strongly rounded anterior glabellar margin, the concentric cranidial margin and posteriorly placed eyes indicate assignment of these species to Wenndorfia . The upper Lochkovian W. bostoviensis from Poland is the earliest species assigned to Wenndorfia . W. bostoviensis differs from W. mutabilis and other Wenndorfia in having and a short triangular pygidial outline. This pygidial morphology suggests affinities of the earliest Wenndorfia to the earliest Digonus .

Tomczkowa (1975) assigned homalonotids from upper Lochkovian-lower Pragian strata immediately overlying Wenndorfia bostoviensis to Digonus vialai . The number of specimens representing these two morphologies is limited, with four cranidia representing vialai and two representing bostoviensis . Tomczkowa distinguished vialai from bostoviensis by its trapezoid rather than rectangular glabellar outline, but the significance of these supposed differences cannot be determined without reference to larger populations. In all other respects it is difficult to distinguish vialai from bostoviensis , and the specimens are best considered conspecific. Similarly, Tomczykowa described Parahomalonotus angusticostatus from the lower Pragian of Poland using a limited collection of pygidia, only one complete. Tomczykowa (pl. 6 figs 1–4) assigned pygidia from a slightly lower horizon to Parahomalonotus forbesi ( Rouault, 1855) , distinguished from those of angusticostatus in having fewer axial rings and ribs and shallower pleural and ring furrows. However, the pygidia figured do not differ markedly from angusticostatus and are considered to be conspecific.