Homalonotus König, 1825

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 : 14-15

Homalonotus König, 1825

Homalonotus König, 1825:104 .

Koenigia Salter, 1865: 119.

T ype species. Homalonotus knightii König, 1825 from the Ludlow of England, by monotypy. The type species has been recorded from Britain, Germany, Poland and Canada, although its the relationship to the Canadian H. dawsoni is not clearly established. Hall (1860) described dawsoni solely from pygidia, and his diagnosis clearly allies the species with knightii and the Swedish H. rhinotropis . McLearn (1924) suggested that differences in pygidial morphology easily distinguish dawsoni from knightii , citing the posterior projection of the pygidial axis, narrower proportions and less convex profile of the lateral margin as distinguishing the type species. On these criteria McLearn identified specimens from the McAdam and Moydart Formations at Arasaig, Nova Scotia as knightii , and those from the slightly younger Stonehouse Formation in the same area as dawsoni . The cephalic morphology of dawsoni was described by Dawson (1868, 1877) and McLearn. McLearn noted that the two cephala known from the McAdam and Moydart Formations were very similar to those of dawsoni from the Stonehouse Formation. Indeed, a relatively undeformed cephalon from the Moydart Formation examined by the author is clearly attributable to dawsoni rather than knightii . The specimen is comparable to the Stonehouse Formation cephala in the anterior placement of the eyes and more elongate and more-weakly tapering glabellar shape, distinguishing the Canadian species from the type species. The specimen also shows the acutely pointed lateral cusps of the anterior margin, not previously documented. Whether the differences in pygidial morphology between the McAdam and Moydart Formation populations are significant is a question that cannot be resolved without detailed re-examination of the faunas.

Other species included. Homalonotus dawsoni Hall, 1860 , H. rhinotropis Angelin, 1852 , H. williamsi sp. nov., H. talenti sp. nov.

Range. Ludlow, possibly early Pr˘ídolí. England, Germany, Poland,

Sweden, eastern North America and south-eastern Australia.

Revised diagnosis. Cephalon much wider than long. Glabella long (length 1.1–1.3 times width), tapering forward weakly to moderately (15–30º), sides straight, lobation weak to indistinct. Paraglabellar area distinct. Preglabellar furrow present, of variable depth (shallow to very deeply impressed). Preglabellar field very short (<0.08 times cranidial length). Anterior margin of cephalon tricuspid with a strongly folded (M-shaped) profile in dorsal view. Central cusp triangular, and strongly convex downwards in anterior view. Facial suture and rostral suture meeting at invagination of anterior margin such that connective suture is absent dorsally. Eyes forwardly placed, opposite 0.55–0.7 glabellar length. Pygidial lateral border furrow distinct, anteriorly lateral border swollen, lip-like, fused ventrally with rolled doublure and continuous with long (exsag.) articulating facet.

Discussion. Sdzuy’s (1959) diagnosis of Homalonotus reiterates the characters listed by Reed (1918), omitting only the ‘angular course of the anterior branch of the facial suture’. Additional characters listed by Sdzuy include features of the glabella (indistinct lobation, trapezoidal shape), the rostral plate (lacking a process), the folding of the anterior margin, and the presence of a cephalic border (i.e. that defined by the preglabellar furrow). The preglabellar furrow is variable in depth, being very deeply impressed in H. williamsi and H. talenti , moderately impressed in H. dawsoni (see McLearn, 1924: pl. 27, fig. 14) and H. rhinotropis (see Angelin, 1878: pl. 20, fig. 1). In the type species, the depth of the preglabellar furrow varies from shallow (e.g. Tomczykowa, 1975: pl. 1, figs 1, 3) to moderately impressed (e.g. Salter, 1865: pl. 12, fig. 2) and suggests the presence/absence rather than the depth of the furrow is of significance.

The assignment of two new Australian species to the genus indicates a somewhat broader range of morphologies for the genus than represented by the northern hemisphere species, particularly in features of the pygidium. Homalonotus knightii , H. rhinotropis and H. dawsoni share a distinct pygidial morphology characterised by raised, long pleural ribs that are continuous with the axial rings (separated by a deep, continuous, pleural and ring furrows), effaced axial furrows, by a fused postaxial ridge and posterior area of pleural field, and by a strongly acuminate tip. In contrast, the pygidial axial furrows of H. williamsi and H. talenti are only moderately impressed (Figs 10.3–10.11). In williamsi the axis is raised, defining distinct pygidial trilobation, the postaxial ridge is not fused with the posterior area of the pleural field, whilst in talenti the pleural and ring furrows are shallow and the posterior tip is rounded. The revised diagnosis given above accommodates these morphologies by excluding all pygidial characters listed in Sdzuy’s (1959) and Tomczykowa’s (1975) diagnoses, including the ‘acuminate triangular and acutely tipped pygidial outline’, the ‘weakly defined trilobation’, the ‘posterior fusion of axis and pleural field’ and the ‘distinct segmentation’. Nevertheless, the pygidial morphology characterising the contemporary northern-hemisphere species defines a species group distinct from the Australian taxa and reflects a degree of provincialism not otherwise seen in homalonotid distribution.

The distinctive morphology of the pygidial lateral border is listed as an additional diagnostic character. The complex anterior cephalic margin of Homalonotus corresponds to a complex coaptive morphology. Laterally, the swollen, lip-like section of the pygidial border-doublure defines the overlap of the librigena, the pygidial border-doublure fitting against the cephalic doublure. The pygidial doublure is narrow posterior to the lip-like section, and at this point the doublure of the enrolled pygidium emerged from underneath the anterior cephalic border at the invagination of the anterior margin. The anterior margin of the rostral process fitted against the posterior margin of the pygidial doublure, as indicated by the matching convexity of these structures (in anterior/posterior view) in H. talenti ( Figs 9.5b, 10.7c).

Several of the cephalic features listed as generic characters by Sdzuy (1959) are excluded from the revised diagnosis, including the ‘indistinct glabellar lobation’ and the ‘median point on the rostral suture’. These characters are excluded to accommodate the Australian species, that lack a median point on the rostral suture, whilst Homalonotus williamsi exhibits weak glabellar lobation. Sdzuy noted the difficulty in interpreting the anterior border of Homalonotus . However, the junction of the facial suture and rostral suture at the invagination of the anterior margin (and the absence of the connective suture dorsally) is clearly evident on williamsi , and is comparable to the morphology of the anterior margin of H. rhinotropis described by Moberg and Grönwall (1909).

Sdzuy (1957), Tomczykowa (1975) and Thomas (1977) considered Homalonotus to have been derived from Trimerus and interpreted H. (T.) johannis Salter, 1865 as transitional between the genera. Wenndorf (1990) expressed uncertainty on this derivation of Homalonotus . Reed (1918) tentatively placed johannis in Homalonotus , primarily on account of its tricuspid, folded anterior margin and supposed pygidial similarities, but noted that long preglabellar field of johannis differed markedly from that of H. knightii . Although Prantl and Pr˘ibyl (1948) suggested a new genus or subgenus be erected for johannis, Sdzuy (1957) retained the species in Homalonotus . The length of the preglabellar field is only one of many differences between knightii and johannis indicating that these species are not related. Whilst johannis shares with the northern hemisphere species of Homalonotus ( knightii , H. dawsoni and H. rhinotropis ) deeply impressed pygidial pleural and ring furrows and an elongate, acutely tipped triangular outline, it lacks the effaced trilobation and fused postaxial area characterising those species. More importantly, johannis lacks the swollen lip-like pygidial border shared also by the Australian species and considered diagnostic of the genus. The species also differs markedly from Homalonotus in having a highly derived cranidial morphology. The raised glabella which is markedly expanded across L1-L1 (tr.) and has deep S1 apodemes and a distinct sagittal ridge, the strong expression of the medial indentation of the anterior glabellar margin and paraglabellar areas, and the long and weakly convex (tr.) preglabellar field indicates assignment of Salter’s species to Trimerus (Trimerus) , in agreement with Tomczykowa’s (1975) and Morris’ (1988) assignment.

Of the species assigned in this work to Homalonotus , H. williamsi appears to be the least derived. H. williamsi retains distinct glabellar lobation, the axial furrows are moderately impressed, the pygidial axis is raised posteriorly and the postaxial area is not fused with the pleural field. In these respects williamsi shares features of Trimerus possibly reflecting the relationship suggested by Sdzuy, Tomczykowa and Thomas. The interpretation of williamsi as morphologically primitive supports the derivation of Homalonotus from T. (Ramiotis) rather than from the more derived T. ( Trimerus ). Of species assigned to T. (Ramiotis), williamsi bears closest resemblance to T. (R.) otisi ( Fig. 20), which bears a well-developed tricuspid cephalic margin comparable to that of T. (T.) johannis , though more weakly folded. As with several other Upper Silurian species of T. (Ramiotis), otisi also a exhibits a well defined pygidial border furrow and swollen lip-like border comparable to that of Homalonotus , a morphology otherwise only poorly developed or absent in other groups including T. ( Trimerus ) and T. (Edgillia). T. (R.) otisi lacks the strongly derived glabellar features of johannis , sharing with Homalonotus the elongated, weakly tapering glabella with weak lobation. In these respects otisi is a more likely candidate than johannis in representing an intermediate morphology between Trimerus and Homalonotus , although its interpretation as an ancestral species is not in accord with its stratigraphic position, otisi appearing in strata immediately overlying those yielding williamsi .