Parillaenus liluensis
Creators: name: Fortey, Richard A.
name: Wernette, Shelly J.
name: Hughes, Nigel C.
Publisher: name: Zenodo
Year: 2022
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Trilobita
Corynexochida
Illaenidae
Parillaenus
Parillaenus liluensis

description: Parillaenus liluensis (Reed, 1915)
Figs 14.1–7
1915 Illaenus liluensis sp. nov.; Reed, p. 37–38, pl. 7, figs 4–7.
1915 Holometopus wimani sp. nov.; Reed, p. 39–40, pl. 7, figs 10–12.
Material. Lectotype (selected herein); cranidium from Nam Tu above Lilu, Fig. 14.3 (Reed, 1915, pl. 7, fig. 4), GSI 11536. Other Material: Complete dorsal skeleton from Lilu, Figs 14.5, 6 (Reed, 1915, pl. 7, fig. 5), GSI 11537; cranidium from Lilu, Fig. 8.8 (Reed, 1915, pl. 7, fig. 12), GSI 115432; cranidia from Lilu, Fig. 14.1 and unfigured here (Reed, 1915, pl. 7, figs 10,11), GSI 11541-2 respectively; pygidium with partial thorax from Lilu, Fig. 14.4 (Reed, 1915, pl. 7, fig. 7), GSI 11538; hypostome from Lilu, Fig. 14.7 (Reed, 1915, pl. 7, figs 5,6), GSI 11537. All specimens from Upper Naungkangyi Beds (Katian) around Lilu. Reed (1915, p. 40) gives several localities for this species, but states in the plate legend that figured specimens of Holometopus wimani are from “ 1 mile north of Lilu” which must be the type locality for that species. The locality for the lectotype of Illaenus liluensis is “Right bank of Nam Tu above Lilu on path to Manping.”
Discussion. Classification of illaenid trilobites is difficult and controversial, as with other trilobites tending to effacement. The species discussed here is placed in Parillaenus because of its similarity to Parillaenus creber Hammann, 1992. We are indebted to Sofia Pereira for pointing out that Hammann & Leone (2007, table, p. 109) revised this assignment to Illaenus? creber. The proposition of Parillaenus in Jaanusson (1954, p. 574) was perfunctory, with the sole distinguishing character being the simple arcuate inner margin of the pygidial doublure. On the type species of Parillaenus the proportion of the dorsal exoskeleton occupied by the axis is greater than in the species from Myanmar (Jaanusson in Moore, 1959), but it is unclear whether this feature has systematic importance. There is no opportunity in this work to revise generic concepts of late Ordovician illaenids, and it is recognized that the assignment of Reed’s Illaenus liluensis to Parillaenus could well change when a comparative study is carried out.
The casts made of small specimens of Holometopus limbatus Reed, 1915, are imperfect and do not add to Reed’s description, while other, mostly larger specimens of Illaenus Iiluensis can only be refigured from Reed (1915). Reed directly compared the small cranidia with Holometopus limbatus Angelin, 1854, which subsequently became the type species of Raymondaspis Přibyl in Prantl and Přibyl (1949), a member of Styginidae Vogdes, 1890. The narrow, pestle-shaped glabella with well-defined occipital ring, but no other glabellar furrows, does resemble that of R. limbata, the type material of which was revised by Poulsen (1969). The Burmese species has a glabella that extends almost to the front of the cranidium, where the preglabellar furrow is effaced, while R. limbata has a distinct cranidial border. However, there are some characters that indicate that these smaller cranidia should not be referred to Raymondaspis. The width of the fixed cheeks (tr.) is much greater than in most species attributed to Raymondaspis, particularly posteriorly, and as far as can be judged the palpebral lobes are far out and relatively gently curved, rather than almost circular, and close to the glabella, as in well-preserved species attributed to Raymondaspis; the latter is also generally an early- to mid-Ordovician genus. Scandinavian species discussed by Nielsen (1995) indicate that Raymondaspis comprises a closely similar group of species of this age with a Baltic-Laurentian distribution. The larger specimens attributed to Illaenus liluensis by Reed (e.g. 1915, pl. 7, fig. 4) differ principally from the small ones attributed to I. wimani by the anteriorly effaced axial furrows and loss of the occipital furrow, and we believe that these differences are attributable to ontogeny. The two supposed species have been found together in a locality described by Reed (1915) as “1 Mile north of Lilu”. Suggestive ontogenetic comparisons with the Burmese species can be drawn with several trilobites described by Hammann (1992) from the Ashgill (Katian) of Spain, including Cekovia perplexa perplexa Hammann and Parillaenus creber Hammann. As adults these species lack a cranidial border, have anteriorly effaced axial furrows, and wider fixed cheeks that are more convex (tr.) than in Raymondaspi s, and the palpebral lobes, placed far out, are not strongly curved. These are like Illaenus liluensis. However, smaller cranidia (Hammann, 1992, pl 4, fig. 4; pl. 9, figs 10-12) much more resemble wimani in their comparatively well-defined pestle shaped glabellas and visible occipital furrows. The small, nearly complete individual of Illaenus liluensis figured by Reed (1915, pl. 7, fig. 5) has a glabella more like that of wimani than the larger specimens attributed to I. liluensis. The mature specimens from Burma more closely resemble Parillaenus creber than Cekovia, notably in having wider cranidia and relatively large palpebral lobes as adults. The pygidium figured by Reed (1915, pl. 7, fig. 7), although not large, is very similar to that of Parillaenus creber Hammann in its very short (sag.) obtusely triangular pygidial axis. Hence our attribution to Parillaenus here, with the reservations noted above that the genus may well be further clarified. Zhou et al. (1984) and Zhou & Zhen (2008, p. 252) assigned some “Caradoc to early Ashgill” Chinese illaenids to Parillaenus, mostly fragmentary material previously attributed to Illaenus. The name liluensis appears before wimani in Reed’s (1915) monograph and is therefore selected as the preferred name in this work. A second species of “ Holometopus ”, H. orientalis, from the Hwe Maung Beds was erected by Reed (1915) on the basis of one, poorly preserved cranidium, refigured in Fig. 6.10. It does seem possible that this is a styginid with posteriorly positioned palpebral lobes, and this specimen is tentatively referred to Stygina.
description: Published as part of Fortey, Richard A., Wernette, Shelly J. & Hughes, Nigel C., 2022, Revision of F. R. C. Reed's Ordovician trilobite types from Myanmar (Burma) and western Yunnan Province, China, pp. 301-356 in Zootaxa 5162 (4) on pages 343-345, DOI: 10.11646/zootaxa.5162.4.1, http://zenodo.org/record/6810290