Trimerocephalus (Trifoliops) trifolius ( Osmólska, 1958 ) Crônier, 2003

Crônier, Catherine, 2003, Systematic relationships of the blind phacopine trilobite Trimerocephalus, with a new species from Causses-et-Veyran, Montagne Noire, Acta Palaeontologica Polonica 48 (1), pp. 55-70 : 58-62

publication ID	https://doi.org/ 10.5281/zenodo.13272607
persistent identifier	https://treatment.plazi.org/id/736C8787-5935-FF9F-0A2D-F91DFC04FE78
treatment provided by	Felipe
scientific name	Trimerocephalus (Trifoliops) trifolius ( Osmólska, 1958 )
status	subgen. nov.

Treatment

Trimerocephalus (Trifoliops) trifolius ( Osmólska, 1958) subgen. nov.

Figs. 2A–D, 3A View Fig , 4A, B View Fig .

? Dianops trifolius Osmólska ; Osmólska 1958: 136–138, pl. 4: 2, 3, text−fig. 6.

Trimerocephalus trifolius (Osmólska) ; Osmólska 1963: 507–509, pl. 4: 1–3, 6.

Trimerocephalus? trifolius (Osmólska) ; Chlupač 1977: 123.

Emended diagnosis.—Species belonging to Trimerocephalus (Trifoliops) subgen. nov., showing the following characteristics: cephalon with a narrow, trilobed outline; frontal lobe plumbs above the frontal border; S2 and S3 present; presence of ocular protuberances; marked elongation (sag.) of the vincular furrow.

New material.—The calcareous material is composed of six cephala, from the Concours−le−Haut old quarry at Causses−et−Veyran, in the Montagne Noire, France ( Fig. 1 View Fig ).

Remarks.—The trilobed cephalon, corresponding to the distal widening of the frontal border at the level of the genal angles, with a sagittal and transverse elongation of the vincular furrow, is similar to the configuration of Trimerocephalus? trifolius ( Osmólska 1958, 1963). In addition to previous descriptions, it may be noted that deep slightly outwardly concave dorsal furrows diverge about 60º and widen just before and close to the antero−lateral angle making indentations; the frontal lobe does not overhang the wide frontal border; the glabellar furrows S3 and S2, well marked on internal moulds, are visible on the surface of exoskeletons; ocular protuberances (relics of ancestral ocular lobe) are present at the level of the anterior genal angle; the submarginal facial suture, detectable from the mid−length of the lateral border, traverses the border obliquely and comes back up on the cheeks, forming a

crescent only slightly narrower than the lateral border, and continuing into the dorsal furrow, elongated at this level, and cutting through the ocular protuberance; the lateral border at the anterior angle is inclined strongly backward; the vincular furrow is very broad (tr.), deep, constant, simple, delimited by sharp ridges and abnormally widened (sag.); the curve of the vincular furrow rearwards is about 115º, the posterior band of the doublure is narrow (sag.) and flat; the ratio between the maximum width of the vincular furrow/ posterior band is about 0.32. The ventral view (ZPAL Tr. D. I/39) figured by Osmólska in 1963, has nearly the same ratio (0.33).

Osmólska (1958) mentioned that S2 and S3 are indistinct, S1 is discontinuous and the antero−lateral angles are gently truncated. But remarks added by Osmólska in 1963 from new material demonstrate opposite features. According to her (H. Osmólska personal communication 2002), (1) the left glabellar angle on the holotype ( Osmólska 1958: pl. 4: 2) is less “slightly truncated” than the right one on the paratype ( Osmólska 1958: pl. 4: 3); thus it seems, that this character may be either preservation−dependent, or just due to an intraspecific variability; (2) glabellar furrow S1 is rather continuous, although the mesial sections are shallower than the distal ones. This difference is clearly due to preservation; (3) glabellar furrows S2 and S3 are not clearly perceptible; the surface of the glabellar exoskeleton is poorly preserved. Thus, it may be another taxonomically worthless character.

This species differs from other species of the genus Trimerocephalus in possessing the trilobed cephalic outline: the frontal border widens distally near the antero−lateral angles; the configuration of the vincular furrow shows sagittal and transverse elongation. The specimens here described probably belong to a separate genus.

Developmental instars.—The material includes a few ontogenetic instars, from the early holaspid period. This has allowed at least partial description of the morphogenesis of Trimerocephalus (Trifoliops) trifolius subgen. nov. In order to establish the size distribution in Trimerocephalus (Trifoliops) trifolius subgen. nov., the widths as a function of length of 6 cephala in dorsal view, have been measured with a Nikon mesuroscope (0.001 mm), and have been plotted on a bivariate diagram ( Fig. 5 View Fig ). Relative proportions of cephalon and glabella remain constant (y = ax + b; cephalon: r = 0.996, p <0.001***; glabella: r = 0.995, p <0.001***) whatever the degree of development of individuals. Nevertheless, it is not possible to distinguish many instars; the sample is too poor for this.

The complete individual (ZPAL Tr. D. I/16) described by Osmólska (1963) agrees well with these specimens from the Montagne Noire ( Fig. 5 View Fig ).

Early post−larval instars.—The cephalon, 0.43 cm long is narrower with a ratios L/w about 0.77 than in older stages. The L/w ratios of the hardly inflated glabella is about 0.90. The frontal outline is more closely parabolic and more pointed anteriorly than in older stages. The frontal lobe hides the anterior border that is slightly bent backwards by contrast with older stages where the anterior border is no longer bent backwards. Slightly outward concave dorsal furrows diverge about 60º. The glabella widens anteriorly from S1. The base of the glabella corresponds to 45% of the total width (tr.). The ratios wG/wC is about 0.68. The glabellar furrows S2 and S3 are faint. The narrow cheeks show more marked ocular protuberances. The facial suture is no longer functional: the librigenae are linked to the cephalic doublure. At the level of the dorsal furrows and the anterior genal angles, there is a widening in front of ocular protuberance.

Trimerocephalus (Trifoliops) nigritus subgen. et sp. nov. Figs. 3B View Fig , 6A–H View Fig .

Holotype: Cephalon USTL−CC018, Fig. 6A View Fig .

Type locality: Old quarry of Concours−le−Haut near Causses−et−Veyran. Montagne Noire, France ( Fig. 1 View Fig ).

Type horizon: Red cephalopod limestones, lower member of “griottes” limestonesformation,Lower rhomboidea Subzone ,lowerFamennian.

Derivation of the name: After Latin equivalent of “Noire” (black): niger. The name refers to the Montagne Noire where specimens have been sampled.

Material.—The calcareous material is composed of 10 cephala and 3 fragmentary pygidia, from the Concours−le−Haut near Causses−et−Veyran old quarry, in the Montagne Noire, France ( Fig. 1 View Fig ).

Diagnosis.—Species belonging to Trimerocephalus (Trifoliops) subgen. nov., showing the following characteristics: cephalic outline wide and trilobed; glabella with a broadly parabolic frontal outline; frontal lobe overhanging the frontal border; S2 and S3 absent. Slight thickening along the whole length (tr.) of the posterior pygidial margin.

Description.—Cephalon with a wide and trilobed outline. Ratios L/w about 0.55.

In dorsal view, the ratio L/w of the poorly inflated glabella is about 0.73. The frontal outline is broadly parabolic. Deep, slightly outwardly concave dorsal furrows diverge at about 55º and widen slightly just before the antero−lateral angle. The frontal lobe overhangs the uninflated frontal border. Glabellar furrows S3 and S2 are very faint and often indistinct. The median portion of the preoccipital furrow is shallow. The preoccipital ring is not inflated. The lateral preoccipital lobes are slightly differentiated, whereas the occipital furrow is more clearly differentiated. The strongly inflated occipital ring is almost twice as broad (sag.) as the preoccipital ring. Lateral occipital lobes are not differentiated. No ocular protuberance and no visual area are developed at the level of the anterior genal angle. The submarginal facial suture cuts the cheeks forming a crescent narrower than the lateral border, and continuing along the dorsal furrow. The cheeks are uniformly and poorly inflated. The frontal furrow is well defined but clearly shallower than the lateral and posterior furrows. The lateral and posterior furrows are not interrupted at the level of the genal angle, only reduced in depth. The lateral border at the anterior angle is inclined strongly backward. Lateral and posterior borders widen at their junction and are inclined slightly backward. The genal angle is rounded.

In lateral view, the frontal outline of the glabella is moderately arched and posteriorly is slightly convex dorsally. The outline of the preoccipital ring is straight. The strongly convex occipital ring is as high as the glabella at its maximum of convexity. The uninflated frontal border is oriented inward.

In frontal view, the glabella outline is semicircular.

In ventral view, transversally, the vincular furrow is very broad (tr.), fairly deep, constant, simple, delimited by sharp ridges and slightly widened (sag.) in comparison with the genus Trimerocephalus sensu stricto. The curve of the vincular furrow backwards is about 135º. The posterior band of the doublure is narrow (sag.) and flat. The ratio between the maximum width of the vincular furrow/posterior band is about 0.47.

Pygidium with a transverse and lenticular shape.

In dorsal view, the anterior margin is straight (tr.). Dorsal furrows are wide and more or less deep. The rachis, arched in relief, shows a convexity decreasing progressively backwards. The rachis is short (sag.) and wide (tr.). Its posterior extremity is rounded. It has 4 well differentiated rings, well delimited by wide axial furrows. The terminal piece is delimited by less marked furrows. Pleural and interpleural furrows sharply distinct anteriorly, becoming a little less distinct at the posterior portion of the pygidium. They delimit distinct, moderately inflated half−ribs. These furrows do not reach the posterior border, delimiting a laterally widening limbus. The limbus is twice as broad (sag.) as the terminal piece of the rachis. The posterior margin, lacking a pleural spine, shows a very slight forward inflection in its median portion and a thickening along all its length (tr.). Angles of lateral and posterior border are about 70º.

In frontal view, the posterior margin is strongly widened transversally and moderately elongated sagittally, and its ends are inclined posteriorly.

Sculpture —Both cephalon and pygidium show a fine and dense granulation, except on the doublure of the vincular furrow that is smooth.

Remarks.—The dorsal view of this form may be compared to that of Trimerocephalus? trifolius ( Osmólska 1958, 1963). Nevertheless, the frontal lobe overhangs the inwardly oriented frontal border, the configuration of the vincular furrow showing only a moderate elongation (sag.) and differing from those of the genus Trimerocephalus sensu stricto.

Developmental instars.—The material collected includes a few ontogenetic instars, from the early holaspid period. This has allowed to a partial description of the morphogenesis of Trimerocephalus (Trifoliops) nigritus subgen. et sp. nov. In order to establish the size distribution in Trimerocephalus (Trifoliops) nigritus subgen. et sp. nov., the width as a function of length of 10 cephala in dorsal view, has been measured with a Nikon mesuroscope (0.001 mm), and plotted on a bivariate diagram ( Fig. 5 View Fig ). Whatever the degree of development, relative proportions of cephalon and glabella remain constant (y = ax + b; cephalon: r = 0.995, p <0.001***; glabella: r = 0.995, p <0.001***). Nevertheless, it is not possible to distinguish many instars; the sample is insuficuent for this.

Early post−larval instars.—The cephalon, 0.38 cm long, is moderately broad with a L/w ratios about 0.58. The L/w ratios of the moderately inflated glabella is about 0.76. The frontal outline is more closely parabolic and more pointed anteriorly. Slightly outwardly concave dorsal furrows diverge at about 55–60º. The glabella widens anteriorly from S1. The base of the glabella corresponds to 46% of the total width (tr.). The ratios wG/wC is about 0.64. Glabellar furrows S2 and S3 are indistinct. The narrow cheeks show no ocular protuberances. The facial suture is no longer functional: the librigenae are linked to the cephalic doublure.

Conclusion and discussion.—The dimensions and the principal morphologic traits that differ between these two species Trimerocephalus (Trifoliops) trifolius subgen. nov. and Trimerocephalus (Trifoliops) nigritus subgen. et sp. nov. are given in Appendix 1.

Morphological changes which obviously took place between the early and late post−larval instars include: (1) cephalon relatively wider in older stages due to convexity change and lateral extension of the glabella; (2) decrease of the length/width ratios of the cephalon and the glabella, whereas the base/width ratios of the glabella increase; (3) decrease of the ocular protuberances (testimony to a relic of an ancestral ocular lobe) in a mature stage of Trimerocephalus (Trifoliops) trifolius subgen. nov.; (4) orientation of the anterior border slightly bent backwards in larval becomes and no longer bent backward in older stages.

These ontogenetic shape changes, notably including the widening (tr.) of the glabella, have also been observed in Phacops spedeni Chatterton, 1971 and Trimerocephalus lelievrei Crônier and Feist, 1997 , thus emphasizing the close phylogenetic relationship between these taxa. However, whereas in the ancestral Phacops the visual complex enlarges and migrates posteriorly, a real eye is never developed in the descendant Trimerocephalus . An ocular protuberance, without any visual area, develops and decreases during the ontogenetic development of Trimerocephalus (Trifoliops) trifolius subgen. nov. This delay in development results from the effects of paedomorphosis ( Feist 1995; Crônier and Feist 1997).

Within the assemblage from the Upper Devonian of Montagne Noire, two distinct morphotypes have been identified in the same bed, depending on two important cephalic characters: absence/presence of ocular protuberances and a vincular furrow moderately or strongly elongated sagittally. These two morphotypes seem to have no intermediary forms and show a morphological differentiation comparable to all development instars. The main shape difference manifested in the youngest specimen and maintained during ontogenetic development is a structural difference.

The occurrence of Trimerocephalus (Trifoliops) trifolius subgen. nov. in Holy Cross Mts. ( Osmólska 1958, 1963) and Montagne Noire ( France), underlines the close geographic relationship.