Parahomalonotus Reed, 1918

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 : 32-34

Parahomalonotus Reed, 1918

Type species. Homalonotus gervillei De Verneuil, 1850 . From the Pragian of France, Turkey, Spain, Morocco, Romania and Turkey, by original designation .

Other species included. Parahomalonotus diablintianus Morzadec, 1976 , Homalonotus vialai Gosselet, 1912 (in Gosselet et al.), P. sp. A (= P. gervillei in Haas, 1968), P. sp. B (= P. aff. gervillei in Wenndorf, 1990).

Other species tentatively included. Digonus sp. cf. E in Morzadec, 1986.

Range. Lower Devonian.

Revised diagnosis. Glabella moderately to strongly convex (tr. sect.) and narrow, with length 1.2-1.6 times width. Axial furrows moderately to deeply impressed. Lobation distinct, S1-S3 moderately to deeply impressed. Pygidial axis narrow (0.35–0.4 times pygidial width anteriorly), weakly tapering (~25–30˚), with moderate to strong convexity independent of the pleural convexity, raised posteriorly, continuous with wide postaxial ridge reaching posterior margin, pygidial tip with small point or short slender spine.

Discussion. The restricted diagnosis of Parahomalonotus defines a tight group distinct from Wenndorfia , to which many species previously assigned to Parahomalonotus have been assigned. Species assigned to Parahomalonotus also share weak glabellar tapering (0–18˚), a preglabellar field of moderate (0.2) to short (0.05) length, eyes placed posteriorly (opposite 0.35–0.5 cranidial length), subparallel to moderately convergent anterior branches of facial suture, a transverse rostral suture, an obtusely angled (130–150˚) to rounded pygidial tip and equally well defined axial rings and pleural ribs.

Pygidial morphology of Parahomalonotus is distinct from that of Wenndorfia and is conservative, with the exception of P. diablintianus in which pygidial ring and pleural furrows are relatively shallow and the pygidial outline distinctly rounded. In cephalic features, the younger late Pragian-Emsian forms (P. sp. A, P. gervillei ) differ from earlier species in their stronger glabellar convexity and lobation, and greatly reduced preglabellar fields. Of the earlier taxa, P. vialai shows the closest affinities with the type species and represents an intermediate morphology in which glabellar furrows and lobation are more moderately expressed.

To the species Parahomalonotus sp. A illustrated by Haas (1968: pl. 30 figs 1–4 as P. gervillei ) from the Emsian of Turkey can be added at least one of the cranidia (pl. 29 fig. 26) described as Dipleura fornix , differing from other specimens of fornix in having a transverse glabellar anterior margin and straight rather than concave glabellar sides.

Trimerus Green, 1832

Type species. Trimerus delphinocephalus Green, 1832 from the Wenlock Rochester Shale , New York State, by monotypy .

Subgenera included. Trimerus (Trimerus) Green, 1832 , T. (Ramiotis) subgen. nov., T. (Edgillia) subgen. nov.

Range. Llandovery-Lochkovian.

Revised diagnosis. Cephalon triangular to rounded pentagonal in outline, length 0.6–0.7 times width, anterior margin entire, trilobed or tricusped. Preglabellar field of moderate length to very long, 0.16-0.35 times cranidial length, flat or weakly concave (tr.) medially. Glabella generally with distinct lobation and median indentation of anterior margin. Eyes placed forwardly, with midline of palpebral lobes opposite 0.5–0.75 glabellar length (0.38–0.5 cranidial length) and medially on genae (b- b 1.3-1.7 times preoccipital glabellar width). Anterior branch of facial suture more or less straight and moderately convergent (45–60˚) between eye and a point opposite midlength of preglabellar field. Rostral suture on dorsal surface, transverse or arching forwards evenly or with a median angle. Paraglabellar areas large. Ventral surface of rostral plate flat or weakly convex, without process. Thoracic axial furrows poorly defined. Pygidium with axis of moderate width (0.3–0.6 times pygidial width), not reaching posterior margin, axial furrows moderately impressed, axial ring and pleural furrows distinct but of variable depth, 6–12 rings, 5–10 ribs.

Discussion. The revised diagnosis incorporates and quantifies most features listed in more recent diagnoses of Trimerus . New characters include the relatively forward position of the eye, the straight course of the anterior branch of the facial suture, and the moderate axial width. Early diagnoses, such as Salter’s (1865), predominantly list characters of a very general nature.

Three species groups are recognised in this work and defined as the subgenera Trimerus (Trimerus) , T. (Ramiotis) and T. (Edgillia). Previous diagnoses of Trimerus emphasise the well-defined glabellar lobation, triangular pygidial outline and acuminate pygidial tip (i.e. a mucro) exhibited by the type species and shared with closely related species including T. (T.) cylindricus , T. (T.) vomer ( Chapman, 1912) and T. (T.) johannis . These features and a suite of other distinctive glabellar features are considered here to be diagnostic only of the typical subgenus T. ( Trimerus ). The forwardly arcuate course of the rostral suture and the short length (sag.) of the dorsal section of the rostral plate noted in previous diagnoses of Trimerus ( Tomczykowa, 1975, Thomas in Curtis and Lane, 1998) is considered here to be only of specific significance for T. (T.) delphinocephalus .

A second group comprising other Silurian species of Trimerus can be recognised, and is described in this work as T. (Ramiotis). Members of this group lack the produced pygidial tip and distinctive glabellar features of T. ( Trimerus ). The group is typified by a new upper Llandovery species from Victoria, T. (R.) rickardsi . The species assigned to this group share a suite of characters including less acute to obtusely angled pygidial tips lacking a mucro, and an elongate, weakly tapering, more or less straight-sided glabella with weakly defined lobation. Tomczykowa (1975) included a slight degree of tapering of the glabella in her diagnosis of Trimerus , but this character is peculiar to T. (Ramiotis). The degree of glabellar tapering in Trimerus varies between the strongly tapered (~55˚) and posteriorly expanded morphologies of T. ( Trimerus ), moderately tapered trapezoid morphologies of T. (Ramiotis), and very weakly tapered (<15˚) subquadrate morphologies of T. (Edgillia), the latter erected in this work for a group of Upper Silurian-Lower Devonian species. T. (Ramiotis) is further distinguished from T. ( Trimerus ) by a tendency towards a shorter preglabellar field, shorter pygidial proportions and more weakly defined pygidial segmentation.

Trimerus (Edgillia) can be distinguished from the Silurian subgenera by a distinctive morphology typified by the Victorian T. (E.) kinglakensis ( Gill, 1949) . The species assigned share a very weakly tapered, subquadrate glabellar outline, weakly defined glabellar lobation, and lack the raised postaxial ridge present on most Silurian representatives of Trimerus .

Sdzuy (1959) considered Trimerus to comprise also the subgenus Dipleura . The revised diagnosis of Dipleura is in accord with Tomczykowa (1975) and Wenndorf (1990) in recognising Dipleura and Trimerus as independent genera. Sdzuy’s diagnosis of T. ( Trimerus ) emphasises differences from Dipleura , listing the longer cephalic proportions, a more tapering glabella, distinct glabellar lobation and paraglabellar areas, a forwardly convex rostral suture, an acuminate pygidial tip, and more distinct pygidial segmentation. Diagnoses of Trimerus by Thomas (in Curtis and Lane, 1998) and Tomczykowa (1975) closely follow Sdzuy’s brief subgeneric diagnosis, but incorporate only some of the features listed by him. Importantly, both Thomas and Tomczykowa omitted the morphology of the rostral plate as a diagnostic character. Holloway and Neil (1982) suggested the significance of homalonotid coaptative morphologies, and in accord with this view the absence of a rostral process is reinstated in the diagnosis. In emphasising this feature and in the presence of other differences, a number of Lower Devonian species previously assigned to Trimerus are excluded, including Homalonotus noticus from South America and South Africa), Burmeisteria (Digonus) accraensis from Ghana and Trimerus novus Tomczykowa, 1975 from Poland.

Edgecombe and Fortey (2000) reassigned Homalonotus linares Salter, 1861 from the Pr˘ídolí of Bolivia to Trimerus . The species exhibits a number of features that are not compatible with this assignment. The axial ring count (14–16) is higher than any species assigned to Trimerus (ranging 6–12), the width of the axis (up to 0.7 width) is greater than in any species assigned to Trimerus (ranging 0.3–0.6 width), the pygidium is longer (length/width ratio 1.1–1.2) than all species of Trimerus (length/width ranging 0.6–1.0) except for T. (Trimerus) cylindricus Salter (1865) , and the cephalic proportions are longer (“ transverse width slightly exceeding length ”: Edgecombe and Fortey, 2000: 332) than any species of Trimerus (length/width ranging 0.6–0.7), complementing its elongate pygidial proportions. These features are more compatible with assignment to Burmeisteria . The type species, B. herschelii exhibits comparably high pygidial segmentation (16–17 axial rings, 9–11 pleural ribs), elongate pygidial proportions and a wide pygidial axis. Although the course of the rostral suture and the morphology of the rostral plate (including the presence of a ventral process) for B. linares remains uncertain, there is a suggestion of moderate convexity of the rostral plate in one of the cranidia illustrated by Edgecombe and Fortey (2000: pl. 2 fig. 16).

In addition to the several species reassigned to Burmeisteria and Digonus (discussed above), a considerable number of species assigned to Trimerus by Tomczykowa (1975) and Wenndorf (1990) are assigned to other genera. T. lilydalensis is assigned below to Wenndorfia . Pygidia from the upper Pragian of Morocco, identified by Schraut (2000) as T. sp. cf. crassicauda are too fragmentary for confident assignment to Trimerus , although the shallower axial ring and pleural furrows preclude assignment to the German species. The relationships of a number of poorly documented taxa previously assigned to Trimerus are uncertain, including Homalonotus acuminatus Tromelin and Lebesconte, 1876 and the French H. leheri Barrois, 1886 .