Dipleura Green, 1832

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 : 28-30

Dipleura Green, 1832

Type species: Dipleura dekayi Green, 1832 . From the Middle Devonian Hamilton Group, New York State, USA, by monotypy .

Other species included. Dipleura boliviensis Wolfart, 1968 , D. garratti sp. nov., D. iberica Wenndorf, 1990 , Homalonotus kayseri Thomas, 1905 , H. laevicauda Quenstedt, 1852 , = H. simplex Richter and Richter, 1926 , D. lanvoiensis Morzadec, 1969 , D. praecox Tomczykowa, 1975 , D. sp. (in Richter and Richter, 1943), D. sp. (in Morzadec, 1981), D. sp. nov. (in Wenndorf, 1990).

Range. Upper Silurian (mid Ludlow)-Middle Devonian (Givetian). Revised diagnosis. Cephalon short, length 0.5 to 0.6 times cephalic width, rounded triangular to semicircular in outline. Glabella with sides straight or weakly concave, subparallel to moderately convergent (20º), length 1.1 to 1.25 times width, without lobation in maturity. Preglabellar field of moderate length, 0.15–0.18 times cranidial length. Paraglabellar areas indistinct. Palpebral lobes posteriorly placed, opposite 0.3 to 0.45 cranidial length. Eyes remote, b- b generally 1.7–1.9 times preoccipital glabellar width. Anterior branches of facial sutures describing a broad curve and strongly convergent anteriorly, between 80–100º opposite midlength of preglabellar field. Rostral suture transverse to moderately convex forwards. Ventral surface of rostral plate flat, without projection. Pygidium with weakly defined trilobation, axial furrows very shallow to effaced. Axis with ring furrows moderately impressed to effaced, with weak to distinct posterior swelling, without well defined postaxial ridge. Pleural furrows shallow to effaced, invariably as shallow or shallower than ring furrows. Finely papillate ornament.

Discussion. As with Digonus , Parahomalonotus and Burmeisteria , the type species of Dipleura is one of the youngest members of the genus and differs significantly from older forms. General morphological changes in the genus over time include a trend toward a less tapered glabellar shape with increasingly concave sides, and a decrease in the definition of pygidial segmentation and trilobation. In Middle Devonian species a stronger cephalic convexity is developed, and the contrast in depth between ring furrows and pleural furrows is not as marked. The revised diagnosis differs from previous ones in recognising the limited significance that pygidial outline holds as a generic character. In the case of Dipleura , the oldest (early Ludlow) species, Dipleura garratti from Victoria, has a long, acuminate triangular-shaped pygidium with an acutely pointed tip, in contrast to the short parabolic outline and rounded pygidial tip exhibited by Devonian taxa and considered characteristic of the genus. The upper Ludlow D. praecox from Poland has an intermediate morphology, having a short triangular outline with an angular tip.

Emphasis is placed on several characters listed by Salter (1865), Reed (1918) and Sdzuy (1959) but omitted by Tomcykowa (1975) and Wenndorf (1990). These include the course of the facial and rostral sutures, eye position and weak expression of paraglabellar areas. Several new diagnostic characters are added, including the absence of a projection or keel on the ventral surface of the rostral plate, the presence of a weak to distinct posterior swelling of the pygidial axis and the absence of a well defined postaxial ridge.

The revised concept of the genus excludes a number of species that have been previously assigned to Dipleura (see Wenndorf, 1990: table 1, Tomczykowa, 1975: table 3). These include Burmeisteria (Digonus) accraensis from West Africa, Homalonotus clarkei from Bolivia, Dipleura salteri Morris, 1988 from England and Trimerus (Dipleura) fornix Haas, 1968 from Turkey. Generic assignment of the two former species is discussed under Digonus . D. salteri is tentatively assigned to Trimerus (Ramiotis) , discussed below. T. (D.) fornix is assigned below to Wenndorfia , erected for many of the species previously assigned to Parahomalonotus . Cephalic outline, the stronger segmentation of the pygidial axis compared to the pleural field, and the definition of the pygidial axial furrow posteriorly were considered by Wenndorf (1990) to be important generic characters in distinguishing Dipleura from morphologically convergent species of Parahomalonotus . In restricting Parahomalonotus to a group close to the type species, the distinction between the two genera is marked. In particular, the strong glabellar and pygidial axial convexity, strong glabellar lobation and strong pygidial segmentation easily separate members of Parahomalonotus s.s. from those of Dipleura . The problem of convergence addressed by Wenndorf re-emerges however, between Dipleura and species assigned below to Wenndorfia . Dipleura and Wenndorfia share many characters including a weakly tapering glabellar outline, effaced glabellar lobation, a posterior eye position, a preglabellar field of moderate length, an effaced pygidial postaxial ridge and pygidial outlines that are for the most part rounded parabolic. For the latter character triangular outlines are exceptional, expressed in early representatives of each genus such as D. garratti and W. lilydalensis ( Gill, 1949) . Many species assigned to Wenndorfia can be immediately distinguished from Dipleura in having distinct pygidial segmentation, with pleural furrows being moderately to deeply impressed. Such species include W. multicostatus ( Koch, 1883a) , W. miloni ( Renaud, 1942) , W. elegans (Tomcykowa, 1975) , W. bostoviensis ( Tomczykowa, 1975) , W. plana junior ( Wenndorf, 1990) and W. sp. (= Digonus vialai in Tomczykowa, 1975). However, in a number of taxa the pygidial axial and pleural furrows are weakly impressed to indistinct, including W. plana plana ( Koch, 1883a) , W. forbesi ( Rouault, 1855) and W. fornix . Assignment of these convergent species to Wenndorfia is indicated by consistent differences in the course of the facial sutures between Dipleura and Wenndorfia . In Dipleura , the facial sutures tend to be more strongly convergent and the rostral suture more forwardly convex, giving a more parabolic or V-shaped outline to the anterior margin of the cranidium. By contrast, the rostral suture of Wenndorfia is transverse or weakly convex forwards. The anterior branches of the facial sutures tend to be less strongly convergent posteriorly, abruptly curved medially and more strongly convergent anteriorly, giving the anterior margin of the cranidium a more rounded, U-shaped outline.

Reed (1918) suggested that Dipleura was derived from Digonus . More recent phylogenies are in accord in deriving Dipleura directly from Trimerus , although there is disagreement in the chronology of this transition. Sdzuy (1957: fig. 3) considered Dipleura to have originated from Trimerus at the Siluro-Devonian boundary. Tomczykowa (1975) established an earlier origin for Dipleura , and suggested two species from the upper Ludlow of Poland, Dipleura praecox Tomczykowa, 1975 and Trimerus permutus Tomczykowa, 1978 from the immediately underlying beds, to be closely related and to represent a transitional link between the genera. An earlier origin for Dipleura is indicated by its occurrence in lower Ludlow strata of Victoria, supporting Wenndorf’s (1990: fig. 6) suggestion of a Wenlock origin for the genus.

Dipleura is presumably derived from Lower Silurian Trimerus . Lower Silurian Trimerus are variably assigned in this work to T. ( Trimerus ) or to T. (Ramiotis). T. ( Trimerus ) is characterised by a suite of distinctive cephalic and pygidial features, notably a glabellar outline markedly expanded across L1-L1 (tr.), raised glabellar profile, the strongly-defined glabellar lobation, L1-L2 muscle scars, sagittal ridge, exsagittal furrows and paraglabellar areas, the long and weakly concave (tr.) preglabellar field, and the produced pygidial tip. In the absence of most of these features species assigned to T. (Ramiotis) more closely resemble Dipleura . Llandovery species of T. (Ramiotis) exhibit features that show a strong resemblance to those typical of Dipleura , sharing trapezoid glabellar outlines, shorter preglabellar fields and marked contrast between the depth of the ring furrows and pleural furrows, with the latter very shallow to almost effaced.

Sdzuy (1957) interpreted Homalonotus (Trimerus) mongolicus Tchernycheva, 1937 from the Upper Silurian of Mongolia as an intermediate form between Dipleura and Trimerus . Sdzuy emphasised the posterior eye position, the rounded pygidial outline and the marked contrast between the depth of the ring furrows and almost effaced pleural furrows to support his interpretation. The significance of these characters is overstated by Sduzy. The markedly weak pleural furrows and rounded pygidial outline of mongolicus emphasised by Sdzuy are not features unique to Dipleura and occur amongst species of T. (Ramiotis) and T. (Edgillia). The eyes of mongolicus are clearly placed directly opposite the glabellar midlength ( Tchernycheva, 1937: pl. 1 fig. 8). The rounded pygidial outline emphasised by Sdzuy is contradicted by Tchernycheva’s description of the pygidium as “triangular, usually elongated, terminates in a thick spine” ( Tchernycheva, 1937: 25). In strong contrast to Dipleura , the species exhibits a very long preglabellar field (0.3 cranidial length) and the anterior branches of the facial sutures are relatively weakly convergent (60˚). Tchernycheva’s description of other features including a subquadrate, weakly tapering glabellar outline, an evenly vaulted glabellar profile, weak S1 and indistinct S2 and S3, 13 pygidial axial rings, shallow pygidial pleural furrows, the absence of a postaxial ridge (“rhachis falling short of posterior margin”) and the pygidial proportions (length 0.92 width) indicate assignment to T. (Edgillia).