Trimerus (Trimerus) vomer ( Chapman, 1912 )

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 : 35-38

Trimerus (Trimerus) vomer ( Chapman, 1912)

Figure 15

Homalonotus vomer Chapman, 1912: 298 , pl. 62 figs 2, 3, pl. 63 fig.

2, non pl. 63 fig. 1 (= Trimerus (Ramiotis) otisi sp. nov.) Homalonotus (Trimerus) vomer .— Reed, 1918: 323. Trimerus vomer . — Gill, 1949: 65, text-fig. 1C non 1B (= Trimerus

(Ramiotis) otisi sp. nov.).— Tomczykowa, 1975: 11. — Wenndorf,

1990: 16.— Schraut, 2000: 383.

Type material. Holotype NMV P12301 (cephalon and displaced thorax, figured Chapman, 1912, pl. 62 fig. 1, 2, Gill, 1949 a, text-fig. 1C, Fig. 15.4 herein) from “Wandong”, Victoria. Paratype NMV P12302 (thoracopygon, figured Chapman, 1912, pl. 63 fig. 2 from “Wandong”. Paratype NMV P12303 (cephalon, figured Chapman, 1912, pl. 63 fig. 1, Gill, 1949 a, text-fig. 1B) from “Wandong”. For localities see Fig. 11 .

Registered material. 55 specimens. NMV P455 , NMV P529 , NMV P16415 , NMV P136623 – P136627 , NMV P136629 – P136633 , NMV P137158 , NMV P137291 , NMV P137910 from “Wandong”. NMV P136612 – P136622 , NMV P136628 , NMV P136680 , NMV P137120 , P137121 , NMV P137126 – P137137 from PL286 , Williams locality F22, Kilmore East, Victoria. NMV P136634 – P136636 from “Kilmore East–Sunday Creek Road”, Kilmore East. NMV P136637 from “Langford Park”, Kilmore East. NMV P136846 , NMV P137887 from “Kilmore”, Victoria. NMV P137122 – P137124 , NMV P137144 , NMV P138649 from “Kilmore-Wandong”. NMV P137144 from PL868 , Kilmore East. Yan Yean Formation. NMV P136638 from “Heathcote”, Victoria. For localities see Fig. 11 .

Revised diagnosis. Cranidial width 1.5 times length. Glabella strongly raised, length equal to width, strongly bell shaped in outline, much narrower anteriorly than posteriorly, anterior margin well defined, with deep medial indentation. S1-S3 distinct, S1 with deeply impressed apodeme at adaxial end. Paraglabellar area very strongly defined. Preglabellar field very long, 0.3 times cranidial length, weakly concave (tr. sect.). Palpebral lobes placed opposite 0.5 cranidial length (0.7 glabellar length). Weak but distinct eye ridges. Anterior branches of the facial suture straight, converging at about 55˚ to a point opposite midlength of preglabellar field. Rostral suture transverse. Dorsal section of rostral plate triangular, long, 0.1 cephalic length. Ventral section of rostral plate elongate, width 0.85 times length, connective sutures straight and converging posteriorly at 35˚. Pygidium triangular, length equalling width, with broad based, acutely pointed tip. Axial furrows moderately to deeply impressed. Axial width (measured across second ring) 0.37 times pygidial width. Axis raised, markedly swollen posteriorly. Wide postaxial ridge. 10–12 axial rings. 8 pleural ribs with nineth rib reduced or absent. Pygidial pleural and ring furrows deep, of equal depth. Rib-ring medial offset at fifth rib.

Description. Exoskeleton maximum length estimated 12 cm long. Exoskeleton granulate, more densely in the alae, finely tuberculate thoracic segments.

Cephalon triangular, length 0.8 width, in transverse and sagittal section posteriorly convex, anteriorly concave, lateral margins converging at 70-80˚, genal angles and anterior tip rounded. Glabella 0.6 cephalic length, strongly bell shaped, narrowing markedly opposite palpebral area, posterior width equal to length and twice anterior width, in section (tr., sag.) weakly convex posteriorly, less so anteriorly. Occipital ring transverse, lower than glabella, 0.1 glabellar length, slightly wider at medial apex, in transverse section arched at medial apex, sloping at a uniform gradient abaxially, not interrupted by axial furrow and thus continuous with posterior border. Occipital furrow deeply impressed, strongly deflected forwards medially and less so abaxially, wider at medial apex. S1 a wide shallow furrow directed inwards backwards between 0.2 and 0.4 preoccipital glabellar width, deeper at extremities. L1 weakly defined as an independently convex, triangular lobe, widest abaxially, maximum exsagittal length 0.3 glabellar length. S2 and S3 weakly defined as wide shallow depressions not connected with axial furrows. L2 and L3 undefined. Frontal lobe impressed medially by a short (sag.) wide notch. Axial furrow developed posteriorly as large semicircular alae as long as and placed opposite L1. Anteriorly axial furrow very wide, deepening opposite antero-lateral corner of glabella. Preglabellar furrow indistinct, defined as a change in height from the frontal lobe to the frontal area. Frontal area long (sag.), 0.4 glabellar length. Medial area increasingly concave anteriorly, margins increasingly convex posteriorly in transverse section. Genae inflated. Fixigenae wide (tr.), approximately 0.5 preoccipital glabellar width at midline. Palpebral area wide and convex (tr.), palpebral lobe inclined at 45˚, semicircular, 0.25 glabellar length (sag.), placed anterior of and adjacent to glabellar transverse midline, weak eye-ridge directed inwards forwards to a point opposite S3. Anterior to palpebral area fixigenae sloping down to frontal area, not extending anteriorly as far as glabella. Posterior to palpebral lobe fixigenae wide (exsag.), 0.33 glabellar length, narrowing abaxially, strongly convex. Posterior border furrow wide, shallowing and curving forwards abaxially. Posterior border lower than genal area, increasingly convex (exsag.) and widening to twice occipital ring width (sag.) abaxially. Genal angle broadly rounded. Anterior facial suture converging forwards at about 60˚, meeting connective suture very close to margin at a point opposite 2/3 sagittal length of frontal area (measured from glabella). Connective sutures short on dorsal surface, diverging forwards at 90˚. Rostral suture transverse. Posterior branch of facial suture transverse adaxially, curving backwards abaxially, crossing margin just anterior of genal angle. Librigenae triangular, convex, steep. Eye small, raised above genal surface. Lateral border furrow defined as an angular change in slope, less distinct toward genal angle. Lateral border narrowing anteriorly by a factor of four from genal angle, flat and horizontal. Lateral border and furrow indistinct anterior to genae. Anterior facial sutures converging at 60˚, close to margin and curving inwards anteriorly, meeting connective suture on a transverse line 0.3 sagittal length of frontal area from anterior tip. Connective suture short on dorsal surface, diverging at 90˚. Ventral portion of rostral plate flat, elongate pentangular, long (sag.), 0.4 cranidial length (extending to anterior margin of glabella), maximum width 0.8 length, placed at 0.75 rostral length. Connective sutures straight, converging posteriorly at 35˚, posterior margin transverse, 0.4 maximum width. Dorsal portion of rostral plate triangular to weakly crescentic, flat, very short (sag.) length 0.2 width between posterolateral corners and 0.2 ventral length.Anterior margin straight abaxially, each side converging at 110˚, medially rounded. Rostral suture convergent at 170˚ laterally (to one third maximum width), transverse medially. Connective sutures short (exsag.) apparently marginal between posterolateral corners of dorsal portion and anterolateral corners of ventral portion. Doublure posterior margin transverse, as wide as rostral plate adjacent to connective suture but narrowing adaxially. Adjacent to lateral margin doublure concave in section such that it closely parallels the dorsal exoskeleton.

Thorax 11 segments. Axis cigar shaped, weakly convex (tr.), wide, 0.7 (max.) thoracic width in dorsal view. Pleural field narrow in dorsal view, 0.15 thoracic width, convex (tr.), flexed to subvertical, twice as long abaxial of flexure than adaxial. Axial rings convex (sag.), transverse, wider medially than abaxially. Pleural furrow continues onto rings, deep slot-like, defining anterior band wider abaxially. Axis distinguished from pleural field by independent convexity and by a depressed area in the posterior edge of the pleurae. Pleural furrow divides narrow anterior band from posterior band twice as wide, shallows towards tip. Abaxially of flexure pleurae widen slightly, almost wholly occupied by articulating facet, which has a rounded tip.

Pygidial proportions as for cephalon. Trilobation distinct.Anteriorly axis as wide as pleurae, tapering uniformly, 0.8 pygidial length, in transverse section axis weakly convex anteriorly to strongly convex posteriorly. In sagittal section axis concave anteriorly, posteriorly with long prominent swelling and strongly concave. 9–10 distinct rings, swelling 0.25 axial length and very feintly ringed. Axial rings very weakly convex (sag.) and weakly convex-forwards. Anterior ring furrow higher than others, second ring with pseudo-articulating halfring. Rings 1–7 of uniform width, posterior rings (8+) as narrow ridges Posterior of axis border strongly arched (tr. sect.) and high. Axial furrow shallow and wide, Pleural field uniformly convex. Pleural furrows distinct. 9 ribs, wider than rings, and widening abaxially, flat (exsag.). Interpleural furrows not distinct. Border narrow, concave, widening posteriorly.

Remarks. Many of the characters listed as diagnostic of Trimerus (Trimerus) are strongly expressed in T. (T.) vomer , including the definition of S1 and L1 and L2 swellings, the paraglabellar area, the length of the preglabellar field and the bell-shape of the glabella. T. (T.) vomer also displays a number of unusual characters including distinct eye ridges and deep S1 apodemes, and further differs from other members of Trimerus (Trimerus) in having a particularly long rostral plate (dorsally), and very forwardly placed eyes. The deep S1 apodemes are one of the most distinctive features of vomer . The orientation and position of these apodemes appears to be somewhat variable, however, being transverse on some specimens, diagonal on others, and sagittal on others. These differences may well be attributable to deformation, as is suggested by the correlation of glabellar proportions with apodeme orientation. Specimens with tectonically shortened glabellae (L:W~0.8) have transverse apodemes, those with tectonically elongated glabellae (L:W~1.1) have sagittally directed apodemes. The predeformational orientation was probably diagonal, as in the holotype.

Although with many unique characters, Trimerus (Trimerus) vomer is most closely comparable to T. (T.) johannis from the Wenlock of Wales. These species share a transverse rostral suture, a markedly raised and very strongly expanded (tr.) glabella (more exaggerated in vomer ), very distinct and large paraglabellar areas and a distinctly concave preglabellar field. The deep ring furrows and pleural furrows, the strong swellings on the pygidial axis posteriorly (more prominent on vomer ) and the acute process defining the pygidial tip distinguish vomer and johannis from other species of T. ( Trimerus ). These two species differ in that vomer has more forwardly placed eyes, distinct eye ridges, a longer preglabellar field, an entire cephalic anterior margin, deep apodemes on S1 and distinct S2-S3 (fused to define a shallow exsagittal furrow in johannis ).

As noted by Chapman (1912), Trimerus (Trimerus) vomer is also close to the type species, T. (T.) delphinocephalus . Feint eye ridges and very weak L1 apodemes have been observed ( Whittington, 1993, fig. 2a) on delphinocephalus , and are comparable to those on vomer ( Figs 15.3–15.4) The most conspicuous differences include the much wider pygidial axis, shallower pygidial furrows, the convex rostral suture, the angular (rather than straight) connective sutures, the low glabella and and the less elongate pygidial outline of delphinocephalus .

The cephalon designated as paratype by Chapman and considered by him to be a juvenile form is not conspecific with the holotype. The specimen (NMV P12303, see Fig. 20.1) lacks the bell shaped glabellar outline and deep lateral glabellar furrows of Trimerus (Trimerus) vomer , further differing from it in having distinct subocular and sutural furrows. The specimen is assigned in this work to T. (Ramiotis) otisi . Aspects of Gill’s (1949) redescription of vomer are clearly based on this paratype, notably the description of the axial and glabellar furrows as shallow and faint (respectively).

Documenting an increasingly distal eye position in a succession of Victorian homalonotids, Gill suggested a migration of the Trimerus eye through time. In addition to the problem that that the width of the librigenae and eye position for T. (Trimerus) vomer is measured from the paratype cephalon of T. (Ramiotis) otisi , Gill’s model is flawed firstly, in that only two of the species in Gill’s model can be assigned to Trimerus , secondly, in that the relative stratigraphic positions of the species in Gill’s model are incorrect.

Environmental notes. Although no complete exoskeletons are known from the population of Trimerus (Trimerus) vomer , the articulation ratio of the population as a whole is high (Ar=28%). T. (T.) vomer occurs in a fine siltstone lithology at all of the few localities from which it is known. The holotype is a cephalon displaced from an articulated thorax ( Fig. 15.4), interpreted as a moult assemblage. As for T. (Edgillia) kinglakensis , articulated thoracopygons, cephalothoraxes and partly disarticulated thoraxes are interpreted as exuvial configurations (taphofacies TIV). An outer shelf deep-water habitat below maximum storm wave base is suggested for vomer .

Figures

Figure 11. Geological sketch map of the Springfield-Kinglake West area showing Llandovery-Lochkovian fossil localities yielding homalonotids. For other fossil localities see also Jutson (1908, pl. 3), Thomas (1960), Talent (1964, fig. 1), Williams (1964, fig. 2), Garratt (1972, 1977), Rickards and Sandford (1998, fig. 6), Sandford (2002, fig. 1C; 2005, figs 2–3).