Ovalocephalus aff. obsoletus ( Zhou and Dean, 1986 )

Pour, Mansoureh Ghobadi & Popov, Leonid E., 2009, First report on the occurrence of Neseuretinus and Ovalocephalus trilobites in the Middle Ordovician of Iran, Acta Palaeontologica Polonica 54 (1), pp. 125-133 : 127-130

Ovalocephalus aff. obsoletus ( Zhou and Dean, 1986)

Fig. 2F–H.

Material.—NMW 2004.22G.575, complete enrolled exoskeleton (Gl−9.0, Gw−7.9; ORl−1.3, ORw−3.85, Cw>10.7); NMW 2004.22G.576, incomplete cephalon (Gl−8.0; ORl− 1.3); 2004.22G.577, pygidium (Pl−4.6, Pw−8.7, Al−2.8, Aw− 3.0); NMW 2004.22G.578, pygidium of meraspid (Pl−1.0, Pw−2.4, Al−0.8, Aw−0.9); all from sample F−36, Unit B6,

Section B.

Description.—Exoskeleton elongate suboval, with finely granulated surface. Cranidium about 85% as long as wide with a strongly convex subtriangular glabella, about 115% as long as wide with a broadly rounded anterior margin; axial furrows deep, almost straight, strongly tapering backwards to the occipital furrow. Preoccipital ring short (sag.) with a pair of small preoccipital lobes abaxially. S1 shallow, transverse; S2 and S3 very short (tr.), inclined slightly posteriorly abaxially; S4 poorly defined, situated opposite to the anterior end of palpebral lobes. Occipital ring convex, about one third as long (sag.) as wide (tr.) and about 14–15% as long as the glabella. Palpebral lobes narrow (tr.) with the posterior end about half distance between S1 and S2. Postocular area of fixigena slightly transverse, subtriangular, gently curved downward abaxially. Posterior border convex, uniform in width (sag.); border furrow wide and deep. Librigena with a wide, convex border and holochroal eyes. Hypostome unknown. Thorax with nine thoracic segments.

Pygidium subtrapezoidal in outline, about half as long as wide, with a transverse, almost straight posterior margin; anterior pygidial margin strongly and evenly curved backwards. Axis weakly convex, about one third of maximum pygidial width, gently tapering posteriorly, with four axial rings. Terminal piece not laterally differentiated. Axial furrows distinct in the anterior half of pygidial length, fading posteriorly. Pleural field evenly convex with four pairs of pleural ribs strongly bending backwards abaxially. Three anterior pairs of pleural furrows well defined, fourth pair short (tr.) and shallow.

Discussion.—In the morphology of the pygidium, with pleural ribs strongly curved backwards abaxially and with three posterior ribs lacking free points, the specimens from the Shirgesht Formation resemble Ovalocephalus obsoletus ( Zhou and Dean, 1986) from the Upper Ordovician Chedao Formation (lower Sandbian, Nemagraptus gracilis Biozone ) of Gansu Province, north−west China, but they differ in having longer postocular fixigenae, a slightly more constrained posterior part of the glabella at L1 and L2, and a wider (sag.) posterior border.

Ovalocephalus aff. obsoletus also closely resembles Ovalocephalus intermedius ( Lu and Zhou, 1979) from the lower Darriwilian of Inner Mongolia, north−west China in cranidial morphology, including an entire preoccipital furrow (S1), a relatively short (sag.) and narrow axial ring, and in the absence of S5. However, it differs in having shorter (tr.) S2–S4, in the position of the palpebral lobe posterior margin being slightly posterior to S2, and in finer granulose ornament. The taxonomic significance of these differences cannot be evaluated accurately, because the original description of O. intermedius is based on limited material and pygidial morphology remains unknown. In cranidial morphology, specimens of Ovalocephalus from Iran occupy an intermediate position between O. intermedius and O. obsoletus .

Ovalocephalus aff. obsoletus belongs to the “Species group 2 of Ovalocephalus ” after Dean and Zhou (1988), in having the entire preoccipital ring (S1). Iranian specimens differ from Ovalocephalus primitivus extraneus ( Lu and Zhou, 1979) from the Middle Ordovician of South China, in having a shorter (sag.) axial ring not exceeding 15% of glabellar length, more anterior position of the palpebral lobes with posterior ends situated about half distance between S1 and S2, and in the absence of S5. The pygidium of Ovalocephalus aff. obsoletus differs from most of the Upper Ordovician species of the genus, e.g., Ovalocephalus yangtzeensis ( Lu, 1974) and Ovalocephalus ovatus ( Sheng, 1964) , in having pleural ribs bending strongly backwards. In width of the glabellar base (about 36% of cranidial width), Ovalocephalus aff. obsoletus occupies an intermediate position between Ovalocephalus primitivus (37–39%) and the Upper Ordovician species of the genus (34% and less), but it cannot be proved statistically significant from the available material (for details see also Yuan et al. 2003).

The pygidium of Ovalocephalus aff. obsoletus has some similarities to Ovalocephalus plewesae Fortey, 1997 from the Upper Ordovician Pa Kae Formation of southern Thailand, but it differs strongly in cranidial morphology, including a rounded, not pointed anterior margin of the glabella and obtuse genal angles of fixigenae lacking any trace of genal spines.

The only complete exoskeleton of Ovalocephalus aff. obsoletus has nine thoracic segments, whereas ten thoracic segments are typical for holaspides of Ovalocephalus . However, in length (about 2.5 mm), the specimen from Iran is comparable or slightly longer than complete exoskeletons of Description of Iranian specimens.—Cephalon with maxi−

holaspides illustrated by Lu and Zhou (1979: pl. 2: 8, 11; pl. mum length about 65–70% of maximum width at the level of

4: 4). It is probable that the Iranian specimen represents a late L1. Glabella excluding the occipital ring truncated conical in meraspid, but in view of the significance of heterochrony in outline, as long as wide with maximum width at mid point of the evolution of Ovalocephalus , demonstrated convincingly L1, and about 70% as long as the cranidium. Anterior gla−

by Yuan et al. (2003), a loss of thoracic segments in some bellar margin anteriorly slightly convex to almost transverse species cannot be excluded. with curved margins, defined by a deep preglabellar furrow with a pair of distinct fossulae at abaxial terminations. Axial

Suborder Calymenina Swinnerton, 1915 furrows deep, tapering anteriorly, becoming almost straight

Superfamily Calymenoidea Burmeister, 1843 anterior of L1. L1 expanded laterally, almost hemispherical

Family Calymenidae Burmeister, 1843 with evenly rounded outer margins. S1 deep, straight and

Subfamily Reedocalymeninae Hupe, 1955 slightly bent posteriorly anterior to L1, then strongly curved posteriorly adaxially. S2 and S3 deep, straight, slightly in−

Genus Neseuretinus Dean, 1967 clined posteriorly adaxially. Occipital ring about 25% of

Type species: Neseuretus (Neseuretinus) turcicus Dean, 1967 ; Upper glabellar length and about 85% of maximum glabellar width,

Ordovician, Sandbian, lower shale member of Bedian Formation, tapering near the outer margins. Occipital furrow deep, trans−

south−eastern Turkey. verse medially, slightly curved posterior to L1. Preglabellar

Remarks.—In several publications an acute genal angle of field strongly swollen medially, about 30–35% of cephalic

fixigena in Neseuretinus was interpreted as bearing a short length and about 65–75% as wide as the glabella. Anterior

genal spine ( Hammann and Leone 1997: text−fig. 28; Peng et cranidial border pointed, subtriangular, arched in transverse

al. 2000; Turvey 2005). However, this was based on imper− profile. Anterior border furrow wide (sag.), well defined,

fectly preserved cranidia without librigenae. The study of shallow medially, slightly deepened abaxially. Rostral plate

well preserved complete specimens of Neseuretinus from

Iran shows convincingly that the species has evenly rounded slightly narrower (tr.) than the anterior cranidial border.

lateral genal margins of the cephalon. Postocular fixigenae subtriangluar, steeply sloping downward abaxially with acute genal angles. Palpebral lobes

Neseuretinus birmanicus ( Reed, 1906) short, raised above the glabella, with a posterior margin situ−

Fig. 2A–E. ated opposite the mid point of L2 and the anterior margin sit−

1906 Calymene birmanica sp. nov.; Reed 1906: 71, pl. 5: 27. uated slightly posterior to L3. Weakly defined, transverse

1978 Calymensun tingi (Sun); Kolobova in Sokolov and Yolkin 1978: eye ridge occasionally present. Posterior border furrow deep,

133, pl. 26: 1–5. transverse; posterior border convex, slightly widening

2004 Neseuretus ghavideli sp. nov.; Bruton in Bruton et al. 2004: 140,

pl. 10: 5–13. (exsag.) abaxially. Librigenae sloping strongly abaxially

2005 Neseuretinus birmanicus (Reed) ; Turvey 2005: 560, pl. 2: 1–9 with an evenly convex border widening posteriorly.

(see Turvey 2005 for synonymy prior 2004). Thorax with strongly convex axis of about one third tho−

Lectotype.—Cranidium ( GSI 8341 ), selected by Turvey racic width and deep pleural furrows. It was described in de−

(2005: 560), Upper Ordovician, Sandbian (lower Caradoc), tail by Bruton ( Bruton et al. 2004).

Naungkangyi Group, Kunkaw, northern Shan States, Myan− Pygidium about 90% as long as wide. Axis strongly con−

mar ( Burma). vex, subconical, about 90% as long as the pygidium and with

Material.—Nine complete enrolled exoskeletons from sample width about half of maximum pygidial width. Axis with 8 ax−

F−35, including NMW 2004.22G.567, 568, 570, 571–573, ial rings and a small terminal piece with a rounded posterior

579–581; five variably preserved fragments of enrolled exo− margin. Axial furrows deep, converge posteriorly. Broadly skeletons from sample F−35, including NMW 2004.22G.582– arched postaxial ridge present between the terminal piece

586; two thorax+pygidium, including NMW 2004.22G.574 and posterior pygidial margin. Pleural field convex adax−

from sample F−36 and 2004.22G.588 from sample F−35; one ially, steeply declined abaxially, with six pleural ribs; the thorax from sample F−35, NMW 2004.22G.587; one pygi− most posterior rib weakly defined. First three pairs of pleural dium from sample F−36, NMW 2004.22G.569; all from Units ribs slightly inclined posteriorly in proximity of the axial fur−

B5 and B6, Section B. rows, then strongly curved posteriorly with interpleural furrows originating at the bending point. Posterior pleural ribs strongly inclined posteriorly for entire length.

External surface of cephalon covered by coarse tubercles with fine granules in interspaces, surface of thorax and pygidium finely granulated.

Remarks.—Affinities of Neseuretinus birmanicus were discussed in detail by Turvey (2005). Study of the Iranian specimens reveals that they are identical in all characters to Neseuretus ghavideli Bruton (in Bruton et al. 2004), including the proportions of the cranidium, cephalon and pygidium; a long, swollen preglabellar field defined by the anterior border furrow; asymmetrically arranged tubercles and granules on the external surface; a subconical pygidial axis with up to 8 axial rings plus terminal piece and 6 pleural ribs reaching the posterior margin. This synonymous affinity was also pointed out by Turvey (2005) and Tuvrey and Siveter (2007).

Specimens of Calymene aff. birmanica from the Upper Ordovician of the Panj river basin, Pamirs, as described and illustrated by Weber (1948: pl. 10: 26–28) are probably conspecific with Vietnamia pamirica ( Balashova, 1966) are not related to N. birmanicus .

Some minor differences in proportions as recorded in the description of the Iranian specimens by comparison with previously described specimens from Burma and Uzbekistan are mainly due to imperfect preservation of previously described disarticulated pygidia and cranidia, often distorted and strongly flattened.