Osmolskabole prima ( Osmólska, 1962 )

Lerosey-Aubril, Rudy & Feist, Raimund, 2005, Ontogeny of a new cyrtosymboline trilobite from the Famennian of Morocco, Acta Palaeontologica Polonica 50 (3), pp. 449-464 : 453-459

Osmolskabole prima ( Osmólska, 1962)

Figs. 2 –5.

Cyrtosymbole (Waribole) prima sp. nov; Osmólska 1962: 129, pl. 7: 1–3, text−pl. 6: 5, 6.

Archegonus (Waribole) primus ( Osmólska, 1962) ; Hahn 1965: 242.

Archegonus (Waribole?) primus ( Osmólska, 1962) ; Alberti 1975a: 190, fig. 2: 18–27, fig. 3: 1–7, 9–12.

Pseudowaribole (Pseudowaribole) neptis prima ( Osmólska, 1962) ; Hahn and Hahn 1975: 52.

Cyrtosymbole (Cyrtosymbole) prima ( Osmólska, 1962) ; Alberti 1975b: 45.

Material.—Figured silicified material: 14 cranidia ( UMC−IP 361–374), 8 hypostomes ( UMC−IP 375–382), 1 rostral plate ( UMC−IP 383), 10 librigenae ( UMC−IP 384–393), 2 thoracic segments ( UMC−IP 394–395), 13 pygidia ( UMC−IP 396–408), 7 anaprotaspides ( UMC−IP 409–415), 1 anaprotaspid hypostome ( UMC−IP 416), 5 metaprotaspides ( UMC−IP 417–421); Additional silicified material ( UMC−IP 422): 79 cranidia, 65 hypostomes, 30 librigenae, 27 thoracic segments, 126 pygidia, 49 anaprotaspides, 54 metaprotaspides; Figured calcareous material: 3 cranidia ( UMC−IP 424–426), 1 librigena ( UMC−IP 423), 2 pygidia ( UMC−IP 427–428); Additional calcareous material: 4 cranidia, 4 librigenae, 5 pygidia from Bordj Est ( UMC−IP 429); 8 cranidia, 5 librigenae, 12 pygidia from Bou Tcherafine ( UMC−IP 430); 14 cranidia, 1 hypostome, 12 librigenae, 13 pygidia from Hamar Laghdad ( UMC−IP 431); 1 cranidium from Ouidane Chebbi ( UMC−IP 432), in association with silicified sclerites.

Remarks.—The abundant new material from Morocco corresponds largely to the description of the species given by Osmólska (1962: 130). A few additional adult features and slight differences that might characterize the Moroccan population of the species are summarized as follows:

In late holaspids rather faint posterior branches of S1 are discernable and more glabellar furrows are discernable on the internal mould ( Fig. 2D ). The anterior border is ridgelike, upraised and of nearly equal breadth (sag.) as the preglabellar field. α− α is long and almost straight. Υ is defined by a broad adaxially curving arc. The postocular suture is short, diverging in the early holaspis; a very short straight portion Ɛ− ζ may be present in some late holaspids. A few tubercles are grouped on the medio−posterior margins of both the preoccipital glabella and occipital ring.

Only a single poorly preserved rostral plate was found ( Fig. 3S). It is trapezoidal in outline, bearing four, possibly five terrace ridges; connective sutures convergent backwards (~45°).

Hypostome sub−rectangular, with maximum width (tr.) across shoulders about 65 percent of maximum length; middle body strongly inflated, separated from the lateral and posterior border by deep furrows, weakly divided into anterior and posterior lobes by very faint middle furrows; three pairs of terrace ridges fused antero−medially, running from anterior border to shoulders level and slightly curved outward; no maculae visible; narrow, down−curved (ventrally) anterior rim, separated from the anterior lobe by a break in slope that dies out laterally; sub−trapezoidal anterior wings projecting dorso−laterally; lateral notch rather broad, about 10 percent of hypostomal length (exsag.), and angular; three discontinuous terrace ridges running from the posterior border of the anterior wing to the middle of the wide, about one fifth of maximum hypostomal width (tr.), and outwardly rounded posterior shoulder; posterior border almost straight, inflated and delimited laterally by a pair of tiny forks; doublure narrow (half the width of lateral and posterior borders).

Librigena with large, kidney−shaped eye, provided with a particularly broad visual surface, composed of about 275 lenses, sloping moderately abaxially, and surrounded by a narrow eye−socle around the anterior half of the eye that widens posteriorly to form a smooth triangular surface between eye and posterior border furrow; genal field rather wide and of equal size from front to rear, separated from lateral border by a deep, broad lateral border furrow that shallows posteriorly, smooth anteriorly but bearing a group of small tubercles on a narrow elevated area near the boundary between it and the posterior triangular eye socle; lateral border inflated, widening slightly anteriorly, bearing four well−defined terrace ridges; rather short genal spine (less than one−third of the maximum exsagittal length of the librigena) with broad base, flattened roughly dorso−ventrally and bearing four to five terrace ridges; broad posterior border separated from genal field and eye socle by a deep and slightly backwardly curved posterior border furrow. Doublure incurved, thus creating a tubular structure in association with lateral border, of equal width with latter except anteriorly, where it widens; four terrace ridges present. In lateral view, relief high.

Only a few remains of thoracic segments were found ( Fig. 4H, I). As far as it is known, axial ring rather large (about one third of the maximum transversal width of the thoracic segment), more convex anteriorly than posteriorly; articulating half ring well developed (less than one third of the maximum sagittal length of the axial ring) and separated from a more or less visible preannulus by a deep (in particular medially) articulating furrow; preannulus, when visible, separated from postannulus by a faint intra−annular furrow; no apodemal pits obvious; axial furrows rather deep; ring sockets very small; pleura divided by a break in slope (about 45°) at its mid−width (tr.) into a backwardly directed inner portion and an outer portion that is rounded at its extremity (with the exception of a tiny rearwardly directed spine) and bearing a small projection at the antero−lateral corner of the flange; deep pleural furrow shallows adaxially and does not reach the distal end of the pleura; doublure of the axial ring wide, extending forward almost as far as the articulating half ring; pleural doublure limited to the posterior and the lateral parts of the outer portion and exhibiting a small panderian notch.

The mean width of Moroccan pygidia is a little larger than that of the holotype. In adult specimens the pygidial axis carries 8 axial rings and a terminal piece. The posterior border furrow is medially interrupted by the postaxial ridge, which continues onto border. Posterior border extends in breadth (sag.) that of the postaxial field (the border furrow included). On the pygidium, a single row of tiny tubercles is aligned along the posterior edge of both posterior pleural bands and the central parts of the axial rings (see Fig. 2F ). In addition, other tiny nodules are widely spaced and randomly dispersed on the remaining parts of the pleural fields and the border.

Ontogeny.—The discovery of a wide size−range of cranidia, pygidia and to a lesser degree of librigenae in addition to protaspid remains of Osmolskabole prima enables us to describe the first almost complete growth series of a cyrtosymboline trilobite.

Protaspid period

Fig. 5.

Pre−metamorphic stage.—More than 50 anaprotaspid remains were recovered. The scatter plot of maximum length

(sag.) versus maximum width (tr.) of the well−preserved specimens is shown in Fig. 6. Only one pre−metamorphic stage is recognized. All are bulbous, dorsally sub−ovoid, slightly longer than wide, length (sag.) 0.590 –0.661 mm, width (tr.) 0.512 –0.568 mm; axis long (about 85 percent of the maximum sagittal length of the larva) and subdivided into a glabella and a protopygidial axis by a faint, often inconspicuous transverse furrow located just posterior to a large node that we suppose to be occipital; glabella expands (tr.) forward in its posterior third (sag.) then tapers anteriorly, reaching a pair of usually obvious anterior pits close to the margin; no preglabellar furrow visible; short triangular protopygidial axis, devoid of transverse furrows and slightly rounded postero−medially; exceptionally the axial transverse furrow seems to continue slightly onto the pleural field partially separating a protocranidium and a protopygidium; three pairs of short, sharp and conical spines project from margin; sub−horizontal anterior pair that projects antero−laterally at about 12 percent length of anaprotaspis, where facial suture crosses doublure; middle pair of spines projects postero−laterally and sometimes slightly dorsally from about mid−length of anaprotaspis; posterior pair of spines slightly laterally, backward and strongly ventrally directed from 90 percent length of anaprotaspis, with spines separated by roughly 30 percent width of the anaprotaspis. Ventrally, incurved doublure, slightly narrower postero−medially and gently decreasing in width anteriorly. Librigena unknown, but it can be supposed with respect to the facial suture that it is sub−marginal, narrow (almost entirely constituted of doublure), lacking a genal spine and restricted to anterior 30 to 40 percent of anaprotaspis. Rostral plate unknown. Hypostome known from a single poorly preserved specimen (Fig. 5H); it is elongate, ovoid in outline except anteriorly where it is sub−rectangular; one (but possibly three) pair of short lateral spines projecting lateroventrally; postero−median spine broken; no furrows distinct ventrally; dorsally, narrow doublure all around the border except anteriorly where it decreases gradually in width; laterally, hypostome almost flat except anteriorly where it is bent dorsally.

Slight differences are observed in certain larvae. For example, in most of the specimens dorsal features consist only of anterior pits, whereas rare specimens display discernable axial and even a transverse furrow. This seems to be due to different qualities of silicification. It is also possible to observe slight differences in the shape of the larvae that can be more or less elongated. The magnitude of these shape differences is so small that it is not possible to know whether they are due to natural inter−individual variations or to some extent to taphonomic influences. In all cases, these minor differences do not warrant any suggestion that more than a single pre−metamorphic stage occurs.

Note that the presence of a transverse furrow on the axis, which in some cases runs abaxially on the pleural field, should preclude the use of the term “anaprotaspis” for this first larval stage. Nevertheless, it seems more judicious to conserve this term in particular for comparative purposes, because the globular first protaspid stage in proetoid trilobite ontogenies is usually referred to as an anaprotaspis. Possession of these features is exceptional and, although it seems improbable, we cannot rule out that their presence or absence may correspond to variation within the population.

Post−metamorphic stage.—About 60 metaprotaspid remains were found in our sample. Most of them consist of broken specimens, where only the protopygidium and the posterior part of the protocranidium can be observed. Nevertheless, eight specimens are sufficiently well preserved to permit the recognition of a single metaprotaspid stage in O. prima ( Fig. 6). It is slightly elongate, 1.05–1.21 mm in sagittal length, maximum width (located at the mid−length of the protopygidium) 0.84–0.90 mm; glabella rather long (about three quarters of the protocranidial sagittal length), maximum width across L1, very gently tapering anteriorly and moderately rounded antero−medially; axial furrow large and shallow, mostly weakly impressed against L1; S1 shallow to inconspicuous, strongly curved back to meet S0 and defining low L1; possible shallow and short S2; occipital furrow deep and wide; occipital ring narrowing abaxially and bearing a large (about one half of the maximum transversal width of the occipital ring and almost equal to occipital sagittal length) and rather high occipital node; faint anterior border furrow curved back abaxially but also medially where it merges with the small depression occurring immediately in front of the glabella in a plectrum; anterior border smooth; α− α short (roughly equal to the transversal width of the anterior glabella) and slightly backwardly−curved abaxially; α− β rather long and strongly diverging backward (~40°); β− Υ short (about half α− β length) and sub−parallel; Υ(+ undefined δ)−Ɛ straight, short and diverging backward (~25°); no eye ridge discernable; Ɛ(+undefined ζ)−η diverging backward

Width (mm)

(~30°); η− ω less divergent posteriorly; posterior border furrow wide, shallow and almost straight; posterior border moderately inflated; junction between the protocranidium and the protopygidium marked by a deep furrow (especially on the pleurae), the inner two thirds of which is almost straight whereas the outer third strongly curves backward abaxially. Protopygidium elliptical, margins entire with a faint posterior notch; axial furrow deep; axis short (about one half of the maximum sagittal length of the pygidium), one or two axial rings distinct; one pleural and interpleural furrow sometimes visible; post−axial field short (one third of the sagittal length of the post−axial region); flat border, separated from pleural field by an abrupt break in slope, rather broad (two thirds of the sagittal length of the post−axial region), but narrowing antero−laterally. Doublure flat, of equal width and corresponding to the pygidial border. In lateral view, larvae asymmetrically vaulted with the anterior third strongly sloping

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Fig. 5. Osmolskabole prima . Middle Famennian (to IV, Upper Palmatolepis trachytera to Palmatolepis postera Zones ). Korb−el−Atil (“West”) and Ouidane Chebbi, southeast of Erfoud, Tafilalt, Morocco. A–G. Anaprotaspides. A. Well−preserved specimen, UMC−IP 409 in dorsal view (A 1); A 2, detail of the posterior half showing the occipital node (arrow); A 3, detail of the occipital node (apex broken?). B. Well−preserved specimen, UMC−IP 410 in dorsal (B 1) (note the pair of anterior pits, the dorsal furrows and the occipital node) and lateral (B 2) views (posterior on the left, note the occipital node). C. Specimen showing deep anterior pits, UMC−IP 411 in dorsal view (C 1); C 2, detail of the left anterior pit. D. UMC−IP 412 in dorsal view. E. UMC−IP 413 in ventral view. F. UMC−IP 414 in ventral view. G. Specimen showing a pair of swollen areas (arrows), UMC−IP 415 in ventral view (G 1); G 2, detail of the swollen areas. H. Anaprotaspid hypostome, UMC−IP 416 in ventral view. I–M. Metaprotaspides. I. Specimen UMC−IP 417 in ventral view (anterior and right parts broken) showing the protopygidial doublure. J. Specimen UMC−IP 418 in dorsal view (anterior part of the protocranidium and protopygidial border broken). K. Complete specimen (see the large occipital node), UMC−IP 419 in dorsal view. L. Specimen UMC−IP 420 in dorsal view (right side partially broken). M. Specimen UMC−IP 421 in dorsal view (protopygidial border broken; note the anterior border furrow that merges medially in a plectrum). All figures are scanning electron micrographs of silicified specimens. Scale bars 0.25 mm, except A 3 and C 2 for which are 50 µm.

Length (mm) downward anteriorly and posterior two thirds more gently flexed rearward.

Meraspid and holaspid period.—The smallest meraspid cranidium found is 0.83 mm long (sag.) ( Fig. 3A). Although it is almost impossible to determine which meraspid degree it represents, we speculate that it belongs to the very early meraspid period. When compared to metaprotaspid specimens, it exhibits the following shape changes: glabella rather long (about three quarters of the protocranidial sagittal length); axial furrow narrower, deeper and more convergent forward; S1 deeper; L1 more inflated; occipital furrow deeper; occipital ring broader (tr.); plectrum better defined (deeper and narrower transversally); anterior border narrower (sag.), slightly more inflated and protruding anteriorly; α− α moderately longer and becoming straight; α− β divergence increased (~45°); β− Υ more elongated and curved outward; appearance of δ defining a narrow (tr.) but rather long (exsag.) palpebral lobe with backwardly diverging (~35°) Υ− δ and moderately backwardly converging (~5°) δ− Ɛ of equal length; Ɛ(+undefined ζ)−η more strongly divergent backwards (~40°); V− ω less divergent backwards (~15°).

From this early meraspid stage to the oldest holaspid one, the following developmental traits can be observed: glabella elongating (sag.), greatly enlarging (tr.), rounding antero−medially and becoming constricted at the level of Υ; axial furrows deepening at first, but shallowing in the latest stages; S1 deepening in the earliest stages but rapidly shallowing thereafter, brief appearance of a short anterior branch running adaxially; L1 initially inflating greatly, then flattening in the second half of the ontogeny; brief appearance of short straight S2 and S3; S0 slightly shallows; occipital ring continues to widen (tr.) medially but still more laterally (exsag.) thus becoming less narrowed abaxially; occipital node strongly decreasing in relative size and height; plectrum shortens (sag.) but also narrows (tr.), and shallows before being substituted rather late in ontogeny by a short preglabellar field (e.g., it is reduced but still present on specimen of the Fig. 3G); anterior border furrow deepening but remaining still broad throughout ontogeny; anterior border progressively inflating, rapidly developing one then successively two, three, four and even five terrace ridges; α− α continues to extend in length (tr.) throughout almost all the ontogenetic development; α− β shortening slightly and becoming more divergent, subdivided into a strongly divergent anterior part and a less divergent posterior one; β− Υ still elongating in early ontogeny, and becoming progressively more convergent; palpebral lobe enlarges (tr.) due to outward migration of δ and inward migration of Υ, and to a lesser degree of Ɛ, and shifts backward in the late ontogenetic stages; Υ− δ more diverging and moderately elongating; δ− Ɛ more converging; appearance of ζ; Ɛ− ζ less diverging, becoming almost straight in latest stages, and slightly elongating; ζ− η more diverging backward; posterior border furrow shallowing. In lateral view, cranidia increasingly arched; glabella flattening dorso−ventrally (especially the occipital ring) but expanding forward; anterior border greatly inflating.

The smallest meraspid hypostome found is 0.65 mm long (sag.) ( Fig. 3O). Few shape changes can be observed during ontogeny: posterior part of the middle body inflating; lateral and posterior furrows initially represented by evident breaks in slope that narrow and deepen; initially two short terrace ridges, then appearance of a third, all of which extend progressively farther backwards; anterior rim narrows especially medially, less and less down−curved (ventrally); anterior wings enlarging (exsag.) and projecting more laterally; lateral notch more differentiated, especially from the anterior wings; appearance of third discontinuous terrace ridges on the anterior wings, all of which running farther backwards; posterior shoulder differentiating from the lateral border by the appearance of an increasingly well marked lateral constriction; lateral borders, initially converging backward become sub−parallel; posterior border narrowing (sag.), inflating and straightening (initially posteriorly concave); postero−lateral spines initially elongated, shortening slightly in the latest stages; doublure narrowing medially.

Concerning librigenae, the following shape changes can be observed during meraspid and holaspid periods: genal field enlarging (tr.) and expanding backwardly posterior to the eye; genal spine becoming progressively flattened dorso−ventrally (initially flattened laterally) and shortening; doublure initially bearing two, then three and four terrace ridges. The general trend during ontogeny is a flattening of the librigena, giving rise to the relative enlargement of the librigenal field and the shift in the orientation of the flattening genal spine.

The smallest transitory pygidium is only 0.65 mm long (sag.) ( Fig. 4N). It differs from the protopygidium in the following features: sub−parabolic due to the great enlargement of the postero−medial pygidial border; postero−medial notch enlarged (sag.); axial furrows deepened; axis elongated (sag.) and becoming pointed posteriorly (initially rounded); five axial rings visible; four pairs of obvious ribs, defined by five pairs of broad pleural furrows; only one pair of shallow to inconspicuous interpleural furrows. Doublure shape changes reflect that of the pygidial border. In lateral view, axis and pleurae higher, axial rings also become particularly high.

From this smallest specimen to adults, the following trends can be noticed: pygidial outline evolves from sub−parabolic to semi−elliptical; postero−medial notch progressively disappearing; axis enlarging (tr.) and becoming rounded posteriorly; eight inter−ring furrows in meraspid specimens that moderately shallow in largest specimens; post−axial field slightly decreasing in width (sag.); border furrow deepens except in the latest stages where it secondarily shallows; narrow and low post−axial ridge progressively appearing in late ontogeny; pleural field decreasing in width (tr.) posteriorly but greatly enlarging anteriorly, its distal part becoming less and less declined; eight to nine apparent ribs in youngest meraspides; pleural and interpleural furrows shallowing and enlarging in particular in the largest specimens; border enlarging in early ontogeny but secondarily narrows in later stages, inflating until the almost fading−away of the border furrow. Doublure becomes enrolled and bears progressively more terrace ridges. In lateral view, axis, pleural field and, to a lesser degree, axial rings and post−axial field flatten. In summary, the major ontogenetic changes in the pygidia result in a general flattening of the exoskeleton.

Figures

Fig. 2. Osmolskabole prima. Middle Famennian (to IV, Upper Palmatolepis trachytera to Palmatolepis postera Zones). A. Left librigena, Hamar Laghdad, UMC−IP423 in dorsal view (see the small tubercles). B. Cranidium, Hamar Laghdad, UMC−IP424 in dorsal view. C. Cranidium, Hamar Laghdad, UMC−IP425 in dorsal (C1) and posterior (C2) views. D. Largest cranidium, Hamar Laghdad, UMC−IP426 in dorsal (D1) and lateral (D2) views. E. Pygidium, Ouidane Chebbi, UMC−IP427 in posterior (E1) and dorsal (E2) views. F. Pygidium, Hamar Laghdad, UMC−IP428 in dorsal (F1) and lateral (F2) views. All figures are digital photographs of calcareous specimens. Scale bars 0.5 mm.