Trimerus (Ramiotis) otisi, 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 : 49-53

Trimerus (Ramiotis) otisi sp. nov.

Figure 20

Homalonotus vomer .— Chapman, 1912: 298, pl. 63 fig. 1, non pl. 62 figs 2, 3, pl. 63 fig. 2.

Trimerus vomer .— Gill, 1949: 65, text–fig. 1B, non text–fig. 1C.

Trimerus cf. lilydalensis .— Williams, 1964: 285, table 1.

Trimerus sp. — Garratt, 1968: 164, chart 1.

Trimerus cf. vomer .— Garratt, 1968: 164, chart 1.

Type material. Holotype NMV P304741 (cephalon and displaced thoracopygon) from PL1898 , Eden Park , Victoria ( Fig. 20.5) . Paratypes NMV P304727 (cephalon and displaced thoracopygon) , NMV P304722 – P304724 (cephala) , NMV P304726 (cranidium) , NMV P304718 – P304720 (pygidia) from PL6633 , Eden Park . Paratypes NMV P304739 , NMV P304797 (incomplete cephalothoraxes) , NMV P304738 (thoracopygon) , NMV P304742 (pygidium) from PL1898 . Paratypes NMV P304730 , NMV P304733 , P304734 (cephala) , NMV P304729 (pygidium) from PL6632 , Eden Park. Paratype NMV P304737 (pygidium) from PL 300, Clonbinane, Victoria. For localities see Fig. 11 .

Registered material. 141 specimens: 4 dorsal exoskeletons, 3 dorsal exoskeletons with displaced cephala, 3 articulated cephalothoraxes, 2 thoraxes with displaced cephala, 15 cephala, 22 cranidia, 3 librigenae, 1 hypostome, 8 thoracic segments, 4 articulated thoraxes, 10 articulat- ed thoracopygons, 66 pygidia. Eden Park: NMV P304766–P304770 from PL 6627, Jutson locality VIII. NMV P304743, NMV P304775– P304778 probably from PL 6627. NMV P304771–P304773 from PL 6628. NMV P304832–P304836 from PL 6629, Williams locality F130. NMV P304745, NMV P304837–P304840 from PL 6630. NMV P304738–P304742, NMV P304779–P304799 from PL 1898. NMV P304832–P304838 from PL 6631. NMV P304728–P304736, NMV P304807–P304822 from PL 6632. NMV P304718–P304727, NMV P304747–P304765 from PL 6633. NMV P304800–P304806 from PL 6635. NMV P304744 from “Eden Park”. Unregistered specimens from PL 6634 and PL 6616, Williams locality X12. Upper Plenty–Clonbinane area, Victoria: NMV P304774 from PL 6636, Jutson locality III=Williams locality F12, Upper Plenty. NMV P304746, NMV P304830, P304831 from PL 6638, Kenley locality 32g, Upper Plenty. NMV P304844–P304846 from PL 6639, Upper Plenty. NMV P304841–P304843 from PL 6642, Williams locality G23, Wandong. NMV P304737, NMV P304823–P304826 from PL 300. NMV P304827–P304829 from PL 1782, Clonbinane. In addition, homalonotids recorded by Garratt (1968) from PL 6618, Jutson locality IX=Williams locality F90, Eden Park and by Williams (1964) from Williams locality F41, Clonbinane undoubtedly refer to Trimerus (Ramiotis) otisi , although specimens from these localities are not represented in the MOV collections. Humevale: NMV P304851 from PL 6646, Williams locality W17. Unregistered specimen from PL 6648. For localities see Fig. 11 .

Stratigraphic distribution. Eden Park Formation, from 350 m above the base to the top of the unit, Notoparmella plentiensis Assemblage Zone , late Ludlow.

Diagnosis. Cephalon with length 0.7 times width, anterior margin tricuspid. Glabella trapezoid, length about 1.17 times width, sides converging at about 25˚, anterior margin strongly defined, transverse. Glabellar lobation moderately well defined. Palpebral lobe placed opposite 0.6 glabellar length (0.5 cranidial length). Moderately impressed sutural and sub-ocular furrows, weak eye ridges. Preglabellar field 0.2 times cranidial length. Anterior branch of facial suture straight poster-iorly, converging at about 55˚, then anteriorly curving strongly to the transverse, rostral suture transverse. Dorsal surface of rostral plate semicircular, without connective sutures, equal in length to preglabellar field. Ventral surface of rostral plate with length equalling width, connective sutures moderately convex and converging posteriorly at about 90˚. Pygidium triangular, short, length 0.6 times width, with sides weakly convex and converging posteriorly at about 110˚ to an obtusely pointed tip. Pygidial axis narrow, width (measured across second ring) 0.3 times pygidial width, 10 axial rings, ring furrows deep. 7–8 pleural ribs, pleural furrows as deep as ring furrows and uniform in depth to lateral border furrow, rib-ring medially offset at fifth rib. Axial furrows moderately impressed posteriorly, poorly defined anteriorly, postaxial ridge posteriorly raised. Lateral border furrow well defined, border a raised ridge.

Discussion. The distinctive tricuspid cephalic margin immediately distinguishes Trimerus (Ramiotis) otisi from all other members of the genus. The pygidium of otisi is also distinguishes the species from many congeners, with deep pleural and ring furrows and the anteriorly indistinct axial furrow shared only with the Bolivian T. ( R.) sp. A. The Bolivian species also displays a well defined border furrow and border comparable to that of otisi and T. (R.) thomasi , but the species differs in having longer pygidial proportions (L:W~0.9) and a wider pygidial axis (width 0.5 pygidial width). The very short pygidial proportions of otisi are closest to those of T. (R.) iani , that also shares comparable cephalic features including moderate length of the preglabellar field, longer glabellar proportions, a glabellar outline weakly expanded across L1-L1 (tr.), and a forward eye position (opposite 0.5 cranidial length). In addition to the tricuspid margin, otisi differs from iani in having distinct subocular furrows and eye ridges.

The tricuspid cephalic margin of Trimerus (Ramiotis) otisi is comparable to that of Digonus , as is the quadrate course of the preocular facial sutures and the very weak expression of the pygidial axial furrows anteriorly, with up to the third rib fused with the axial rings. The species also resembles Digonus in the very deep and equally deep pleural and ring furrows. The moderately well defined glabellar lobation and glabellar outline weakly expanded across L1-L1 (tr.) are comparable to early Digonus morphologies best represented by D. zeehanensis , and in these respects otisi is the best candidate as an intermediate species between Trimerus and Digonus . Significant features precluding the assignment of otisi to Digonus include the lack of a ventral process on the rostral plate, the very narrow pygidial axis, and the strongly raised postaxial ridge. T. (R.) otisi and Digonus further differ in the expression of the dorsal sections of the connective sutures (short in Digonus but absent in otisi ), and in the size of the medial cephalic process (small in Digonus but very large in otisi ). In the large size of the medial cephalic cusp, otisi is most closely comparable to T. (T.) johannis . However, marked differences in cephalic morphology between tricuspid T. ( Trimerus ) (represented by johannis ), T. (Ramiotis) (represented by otisi ) and Homalonotus suggest that the tricuspid cephalic margin was independently derived in these groups.

The paratype cephalon of Homalonotus vomer ( NMV P12303, Fig. 20.1 and figured Chapman, 1912, pl. 63 fig. 1, Gill, 1949, text-fig. 1B) belongs to Trimerus (Ramiotis) otisi , and is designated a paratype of the new species. Chapman (1912) considered the specimen to be a juvenile form of vomer , but did not otherwise compare it with the holotype cephalon. Although Chapman’s description of vomer seems to be based entirely on the holotype, Gill (1949) included features of the paratype in his redescription of the species. Chapman and Gill recorded the localities of the paratype and the holotype of vomer only as “Wandong”. The holotype was almost certainly collected from PL 286 (Williams locality F22, Wandong), where the species occurs in relative abundance and in an identical lithology to that of the holotype. As noted by Gill, the paratype occurs in a yellow-brown, fine grained sandstone. T. (R.) otisi has been collected from similar lithologies in the Merriang Syncline to the east of Wandong, and it is presumed the specimen was collected in that area, and probably in the vicinity of PL 300, Comet Creek Mine, Clonbinane.

It is remarkable that although Trimerus (Ramiotis) otisi is very common in the Eden Park Formation in the Merriang Syncline between Merriang and Clonbinane, trilobites (predominantly homalonotid) are exceedingly rare in the equivalent horizons of the nearby Kinglake Basin sequence.

Environmental notes. See taphonomy and biofacies. Trimerus (Ramiotis) otisi is considered to have inhabited mid shelf environments.

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).