Asaphellus isabelae, Arcerito & Waisfeld & Balseiro, 2015

Arcerito, Facundo René Meroi, Waisfeld, Beatriz & Balseiro, Diego, 2015, Diversification of Asaphellus Callaway, 1877 (Asaphidae: Trilobita) during the Tremadocian in South West Gondwana (Cordillera Oriental, Argentina), Geodiversitas 37 (2), pp. 131-150 : 144-145

Asaphellus isabelae n. sp.

( Figs 7G ; 9B ; 10 )

Asaphellus catamarcensis – Waisfeld & Vaccari 2003: pl. 22, fig. 5; non Kobayashi, 1935.

HOLOTYPE. — CEGH-UNC 25751 ( Fig. 10E ). Type horizon DLA 28 , lower Tr 2, Quebrada del Arenal creek, lower Rupasca Member , Santa Rosita Formation , Santa Victoria Group.

MATERIAL. — 140 fragmentary specimens, including 1 partly complete individual, 20 cranidia, 48 pygidia, 28 free cheeks, 7 specimens with thoracic segments, and 12 hypostomas. Illustrated material CEGH-UNC 25749-25761.

OCCURRENCE AND AGE. — DLA 28, DLA 29, DLA 30, lower part of the Rupasca Member, Quebrada del Arenal, K. teiichii Zone , middle Tremadocian (Tr2); reworked boulder of the Cardonal Formation, Pascha-Incamayo area, K. meridionalis Zone , lower Tremadocian (Tr1).

ETYMOLOGY. — After FR Meroi Acerito’s grandmother Isabel Olivares.

D IAGNOSIS

Flat and large species of Asaphellus . Cuticle very thin, high degree of smoothness and effacement, except for the lightly impressed preglabellar furrow, pygidial axis indistinct.

DESCRIPTION

Exoskeleton large, subelliptical and flat. Isopygous with flat cephalic and pygidial borders. Cephalon semicircular, with wide border, diminishing in width towards the genal angle. Doublure subparallel to the margin. Cranidium depressed or flat; axial furrow indistinct. Glabella almost indistinguishable, faint traces of convex anterior preglabellar furrow in some specimens ( Fig. 10E, F ). Wide and subtriangular (exsag.) posterolateral projection. Posterior border and occipital ring reduced, poorly defined, and very close to posterior margin. Palpebral lobes of medium size, located at cranidial midlength. Anterior section of facial suture diverges gently forwards, bends smoothly adaxially, and converges medially to form an obtuse point sagittally in an ogive. Posterior section of facial suture curves backwards, bends downwards and slightly inwards at the level of the posterior border.

Free cheek long and relatively narrow, subtriangular with rounded external margin. Broad-based and long genal spine (55% of the total librigena length), slightly divergent from external margin.

Hypostoma elongated, oval in outline (sag.), with rounded posterior margin. Wide posterolateral border, with fine terraced lines. Median body widest (tr.) at midlength, anterior lobe convex (sag., tr.), posterior lobe short (sag.), crescentic, maculae transverse, sharply impressed.

Thorax incompletely known; almost indistinguishable from A. stenorhachis but less convex. Axis weakly convex, about 35% of the total width (tr.), defined by well impressed and slightly backward-tapering furrows; articulating furrow narrow and deep; articulating half-ring short (tr.). Pleural region moderately wide (about 65% of thoracic maximum width); pleural furrow deeper and wider medially, runs toward the anterior margin of pleura, shallower and narrower adaxially, lightly impressed abaxially. Pleural extremities obliquely truncated and backward-facing, faint marked pleural fulcra.

Pygidium of very low convexity, 57% wider than long (n=14), border and border furrow indistinct. Doublure evenly wide, about 10% length of pygidium (n = 11), subparallel to the pygidial margin along its extension. Anterior half-ring narrow (sag.), slightly convex forward.Articulating facet subtriangular, long (tr.). Anterior half-rib defined by narrow and shallow pleural furrow, remaining pleural furrows and ribs not visible.

REMARKS

Asaphellus isabelae n. sp. appears to represent an extreme in effacement, flatness, and reduction of the cuticle thickness within the group. The latter character is noteworthy, as even the sediment roughness is reproduced on the surface of the exoskeleton. Despite effacement complicates taxonomy, the extreme smoothness and flatness might be considered as an autoapomorphic character for this species.

Such a combination of characters has seldom been recorded in other Asaphellus species. For example, Asaphellus yanheensis Yin in Yin & Shanji, 1978 ( Peng 1990: pl. 7, figs 6-13) from the Nantsinkwan Formation (Tremadocian, NW Hunan, China) exhibits a thorough effacement, with blurred furrows either in the cranidia or pygidia. Though the pygidium of the Chinese species exhibits great resemblance to A. isabelae n. sp., the cranidium of the former differs in several significant respects, such as a better defined anterior ogive, a median node that is placed more forwardly, a subparallel preocular facial sutures, and postocular sutures running straight backwards.

Other species also show effacement either on the cranidium or the pygidium. Asaphellus tungtzuensis ( Sheng 1958) ( Peng 1990: pl. 8, figs 1, 2) from the Tremadocian of north west Hunan resembles A. isabelae n. sp. in the general effacement of the cranidium, however, the pygidium shows traces of axial furrows delimiting a weak axis along its length. The cranidium of Asaphellus cf. A. trinodosus Chang, 1949 ( Jell & Stait 1985) from the Florentine Valley Formation (Tremadocian, Tasmania) is largely effaced; however, the pygidial axial furrows distinctly outline a convex axis. Among the Argentinean species of Asaphellus , only A. stenorhachis (see above) exhibits a general effacement though not to the degree of that of A. isabelae n. sp. Asaphellus stenorhachis differs from the new species in the higher convexity mainly of the pygidium, a better definition of the pygidial axis in its anterior third, and the presence of some furrows, lightly impressed but discernible (for instance the border furrows in the cephalon and pygidium).

Figures

FIG. 7. — A, B, Asaphellus sp. 2, pygidia in dorsal view: A, CEGH-UNC 25763; B, CEGH-UNC 25764; C-F, Asaphellus sp. 1, CEGH-UNC 25765-25768; C, pygidium, latex cast, dorsal view, CEGH-UNC 25765; D, pygidium, dorsal view, CEGH-UNC 25766; E, pygidium, dorsal view, CEGH-UNC 25765; F, pygidium and thorax, dorsal view,CEGH-UNC 25767;G, A. isabelae n. sp., articulated juvenile, latex cast, dorsal view,CEGH-UNC 25762. Scale bars: A-F, 0.5 cm; G, 0.25 cm. All specimens come from the Quebrada del Arenal, Santa Rosita Formation, Santa Victoria Group: A, B, G, from DLA 28; C, E, from DLA 25; D, F, from DLA 20.

FIG. 9. — A, reconstruction of A. stenorhachis (Harrington, 1938); B, reconstruction of A. isabelae n. sp. Drawing by Santiago Druetta.

FIG. 10. — Asaphellus isabelae n. sp.: A, hypostoma, latex cast, dorsal view, CEGH-UNC 25749; B, hypostoma, dorsal view, CEGH-UNC 25752; C, right free cheek, dorsal view, CEGH-UNC 25750; D, left free cheek, latex cast, dorsal view, CEGH-UNC 25756; E, holotype cranidium, dorsal view, CEGH-UNC 25751; F, cranidium, latex cast, dorsal view, CEGH-UNC 25753; G, left free check, dorsal view, CEGH-UNC 25754; H-J, L, pygidia, dorsal view; H, CEGH-UNC 25755; I, CEGH-UNC 25758; J, CEGH-UNC 25757; L, CEGH-UNC 25760; K, pygidium, latex cast, dorsal view, CEGH-UNC 25759; M, almost complete moult, dorsal view, CEGH-UNC 25761. All specimens come from the Quebrada del Arenal, Santa Rosita Formation, Santa Victoria Group; A, C-E, G-J, L, M, come from DLA 28; B, F, K, come from DLA 30. Scale bars: A, B, 0.25 cm; C-M, 0.5 cm.