Cyrtosymbole escoti (von Koenen, 1886 )

Feist, Raimund & Lerosey-Aubril, Rudy, 2005, The type species of Cyrtosymbole and the oldest (Famennian) cyrtosymboline trilobites, Acta Palaeontologica Polonica 50 (3), pp. 465-475 : 467-468

Cyrtosymbole escoti (von Koenen, 1886)

Figs. 2, 3.

Dechenella Escoti sp. nov.; von Koenen 1886: 164.

Dechenella sp. nov.; von Koenen 1886: 164.

Dechenella Escoti von Koenen, 1886 ; Frech 1887: 438.

Cyrtosymbole Escoti von Koenen, 1886 ; Richter 1913: 381, pl. 22: 17, 18.

Cyrtosymbole escoti von Koenen, 1886 ; Perna 1915: 45.

Cyrtosymbole (Cyrtosymbole) escoti von Koenen, 1886 ; Richter and Richter 1926: 31.

Cyrtosymbole (Cyrtosymbole) escoti von Koenen, 1886 ; Richter, Richter and Struve 1959: O389–O391

Cyrtosymbole (Cyrtosymbole) escoti von Koenen, 1886 ; Osmólska 1962: 70.

Cyrtosymbole (Cyrtosymbole) escoti von Koenen, 1886 ; Chlupáč 1966: 22.

Cyrtosymbole (Cyrtosymbole) escoti von Koenen, 1886 ; Alberti 1975: 209, fig. 1: 1–9.

Cyrtosymbole (Cyrtosymbole) escoti von Koenen, 1886 ; Alberti 1976: 40.

Cyrtosymbole (Cyrtosymbole) escoti von Koenen, 1886 ; Feist 1977: 64, pl. 6: 15, 16.

Cyrtosymbole (Cyrtosymbole) escoti von Koenen, 1886 ; Yuan 1988: 16.

Lectotype.—Selected Alberti 1975: 209; cranidium, Gö 420−1, Fig. 2F (herein), from Famennian, lower member of Griotte Limestone Formation, La Serre Hill at 2.5 km SW of Cabrières, southeastern Montagne Noire. Paralectotypes (syntypes from de Rouville’s collection): cranidia—Gö 420−2, Gö 420−3, Gö 420−4; librigena—Gö−11; pygidia—Gö 420−5, Gö 420−6/7, Gö 420−8, Gö 420−9/10, from Famennian, lower member of Griotte Limestone Formation, La Tourière Hill, 1.7 km W of Cabrières.

Material.—Comprises from La Tourière: cranidia—UMC−IP320 ( Fig. 2A), UMC−IP321 ( Fig. 2B), UMC−IP322 ( Fig. 2C), UMC−IP323 ( Fig. 2D), UMC−IP324 ( Fig. 2E), UMC−IP325 ( Fig. 2F), UMC−IP326 ( Fig. 2G), UMC−IP328 ( Fig. 2I), UMC−IP329 ( Fig. 2J), UMC−IP330 ( Fig. 2K), UMC−IP331 ( Fig. 2L), UMC−IP332 (unfigured); cephala—UMC−IP333 ( Fig. 3A), UMC−IP334 (unfigured); hypostomes—UMC−IP335 ( Fig. 3B), UMC−IP336 ( Fig. 3C), UMC−IP337 ( Fig. 3D), UMC−IP338 (unfigured); librigenae—UMC−IP339 ( Fig. 3E), UMC−IP340 ( Fig. 3F), UMC−IP341 ( Fig. 3G), UMC−IP342 ( Fig. 3H), UMC−IP343 (unfigured); pygidia—UMC−IP344 ( Fig. 3I), UMC−IP345 ( Fig. 3K), UMC−IP346 ( Fig. 3L), UMC−IP347 ( Fig. 3M), UMC−IP348 ( Fig. 3N), UMC−IP349 ( Fig. 3O), UMC−IP350 ( Fig. 3P), UMC−IP351 ( Fig. 3Q), UMC−IP353 ( Fig. 3S), UMC−IP354 ( Fig. 3T), UMC−IP355 ( Fig. 3U), UMC−IP357 (unfigured). From Peret: cranidia— UMC−IP327 ( Fig. 2H), UMC−IP358 (unfigured); librigenae— UMC−IP359 (unfigured); pygidia—UMC−IP352 ( Fig. 3R), UMC−IP356 ( Fig. 3V), UMC−360 (unfigured).

Revised diagnosis.—Type−species of Cyrtosymbole , with anterior part of cranidium strongly declined; long glabella, widest (tr.) at base, with frontal lobe extending as far as deep border furrow in late holaspids; weakly convex, flat lying, modestly arched border; palpebral lobes of moderate size; postocular sutures with Ɛ− ζ long, extending from opposite anterior third of L1 to occipital furrow, parallel to axial furrows in late holaspids; low, reniform visual field; genal spines outwardly directed, with broad bases; hypostome narrow, with low posterior lobe and narrow postero−lateral borders; thoracic segments with low pre−annulus; pygidium with strongly vaulted pleural fields, moderately long axis with 7 + 1 low axial rings, well delimited by continuous, deep ring furrows; 7 pleural ribs with distinct pleural bands; border furrow tends to be progressively effaced from anterior to posterior from early to late growth stages; in latter, posterior bor− der is continuously declined behind axis.

Differentiated from C. gotica Richter and Richter, 1926 , C. crebra Perna, 1915 , and C. playfordi Feist and Becker, 1997 by: anteriorly strongly downwards flexed cranidium with relatively flat anterior border and narrower, more anteriorwards placed palpebral lobes; from C. vates Alberti, 1976 by: preglabellar field absent; from C. pusilla ( Gürich, 1896) , C. nana Richter, 1913 , and C. niazensis Feist, 2003 by: glabella widest at base and fewer number of pygidial axial rings.

Description.—Based on largest holaspid exoskeletons (sagittal length of cranidia> 4 mm; pygidia> 3mm). Glabella long, subconical (length/width ratio of preoccipital glabella: 1.27), frontal lobe narrow, parabolic in outline, non−constricted antero−laterally, moderately vaulted transversely, more so antero−medially than laterally in anterior view; in longitudinal profile strongly downturned anteriorly, terminating opposite Υ, less vaulted to the rear; circumscribed by axial furrows of even depth that run parallel from occipital furrow to S1, then converge slightly towards anterior. Glabellar furrows narrow, moderately deep, slightly curved backwards; S1 markedly impressed, bifurcated with transversely directed anterior branch half as long as posterior; posterior branch obliquely curved backwards with narrowing distal ends that do not extend to occipital furrow; S2 and S3 narrow and shallow; S4 not discernable. Lateral glabellar lobes not inflated; L1 markedly longer (exsag.) than wide (tr.) at base. Occipital furrow deep, medially widened and curved forward. Occipital ring not inflated, slightly vaulted (sag.), higher than base of preoccipital glabella, narrowing laterally (exsag.), narrowed (tr.) behind posterior border furrow (exsag.) by inward curvature of axial furrows; anterior margin slightly notched opposite posterior ends of S1; lateral occipital lobes absent.

Deep, narrow anterior border furrow and broad, upturned, weakly convex border which is flexed evenly backwards laterally; border weakly vaulted (sag.) with two narrow terrace ridges on its anterior part. Palpebral lobes relatively narrow and short, forwardly placed (Ɛ a short distance behind S1), a little lower than glabella and slightly inclined from axial furrows in longitudinal profile, provided with thickened margins; anterior part of palpebral suture (δ− Υ) longer and of lesser curvature than posterior (δ− Ɛ). Preocular facial sutures (Υ− β) shorter than postocular (Ɛ− ζ), straight and moderately divergent, then sharply adaxially incurved on anterior border into long β− α portion. Posterior sutures with long Ɛ− ζ portion parallel to axial furrows, which turns strongly abaxially on posterior cephalic border. Posterior fixigenae rather narrow, Ɛ being situated closer to axial furrow than Υ; anterior fixigenae strongly declined, weakly vaulted. Posterior cephalic border narrow, subcylindrical in section (exsag.).

Librigenae broad, with field weakly convex, defined by deep border furrows and wide, semicylindrical border with four continuous terrace ridges. Lateral and posterior borders merging at genal angle and continue onto broad, relatively short genal spine. Eye semicircular, relatively low, with visual surface inclined laterally at 45° in life position, carrying less than 150 lenses; eye−socle absent; eye circumscribed by broad, shallow sub−ocular furrow, with tiny, ovoid weakly inflated sub−ocular platforms adjacent to the facial suture in front of and behind the eye−lobe.

Hypostome narrow subrectangular; middle body well defined by deep furrows around it, anterior lobe ovoid, inflated, merging medially with low, weakly vaulted posterior lobe; middle furrows straight, well marked, directed towards posterior, and lack maculae; anterior margin straight, with narrow upturned anterior rim, markedly separated from middle body by straight anterior furrow; declined, broadly rounded anterior wings; lateral border narrows at lateral notch; shoulder narrow, only modestly outwardly curved; lateral borders flat, of even width, straight, convergent, before merging with curved posterior border; tiny spines protruding below postero−lateral corners.

Sculpture of minute, dense granules on axial parts of the cranidium; granules increasing in size towards posterior, being present on anterior fixigenae but absent on palpebral lobes, posterior fixigenae and borders. Granules present over entire librigenal field, becoming coarser adaxially, and form an obvious single row, where they are slightly enlarged, along the margin of the otherwise smooth sub−ocular groove. Anterior fixigenal and abaxial librigenal fields pitted. Terrace ridges on hypostomal margins and around centre of anterior lobe; posterior lobe smooth.

Thoracic segment with high axial ring in transverse profile, moderately vaulted sagittally, less so laterally, subdivided into antero−laterally enlarged (exsag.) postannulus and rather narrow, short (tr.), depressed preannulus; articulating half ring flat−topped, markedly shorter (tr.) than axial ring. Pleura as broad (tr.) as axial ring, displaying a horizontal inner portion that is broader (tr.) than declined, abaxially pointed outer portion.

Pygidium with relatively narrow, elliptical postero−lateral outline (mean length/width index = 0.63), evenly curved postero−lateral margin, high in posterior and lateral views. Maximum breadth of axis slightly less than that of pleural area; axis strongly vaulted in transverse profile, gently declined in lateral view, but with terminal piece remaining elevated above postaxial area; well defined by straight, slowly converging axial furrows and narrow parabolic postaxial furrow; 7+1 low, moderately inflated axial rings, well defined by anteriorly curved, continuously and evenly marked ring furrows, diminishing in depth from front to rear. Pleural area slightly vaulted adaxially, strongly declined abaxial of fulcrum, considerably higher than axis in lateral view; broad border sloping strongly downwards, separated from inner pleural area by weak border furrow that shallows towards posterior; postaxial area vaulted with short, triangular, weakly inflated, post axial ridge. 6+1 pleurae, slightly curved towards posterior, extending as far as border furrow, markedly separated from each other by narrow interpleural furrows that widen abaxially at border furrow, and subdivided by deeper and wider pleural furrows into weakly vaulted bands, the posterior of which are adaxially twice as wide (exsag.) as anterior ones. Pygidial sculpture of fine, dense granulation on entire exoskeleton, with anastomosing terrace ridges along border.