The only lichid known to date from Devonian strata of the Montagne Noire is Lobopyge meridionalis (Frech, Reference Frech1887), from light grey Givetian limestones of the Pic de Bissous and which its author attributed to the late Lower Devonian. In an unpublished thesis by Feist (Reference Feist1977), specimens of this age, recovered from various sites around Cabrières, were mistakenly attributed to meridionalis, despite clearly different diagnostic features that differentiate a new taxon, Branikarges euanclarksoni, which is established herein. The material consists of disarticulated adult specimens in limestones, including cranidia, a hypostome and pygidia, but lacking librigenae and thoracic pleurae. The recovery of silicified sclerites from different growth stages, mainly transitory pygidia and some meraspid stage cranidia, hypostomes and librigenae, allows completion of our knowledge of the characteristic morphological traits of the new species and complements known ontogenetic trajectories of the early growth in trochurine lichids.
1. Localities, age and environment
In the Montagne Noire, at the southern edge of the French Massif Central, non-metamorphic Devonian carbonates occur in the southeastern Mont Peyroux/Pic de Bissous nappe complex and in the Cabrières klippes, a chaotic assemblage of detached blocks emplaced by gravity slide (Engel et al. Reference Engel, Feist and Franke1978) (Fig. 1). The successions and facies of these distinct units have been correlated with each other using conodonts and trilobites (Feist Reference Feist1985; Feist et al. Reference Feist, Schönlaub and Bultynck1985). Emsian trilobite communities include the new lichid trilobite species attributed herein to Branikarges Basse & Müller, Reference Basse and Müller2023, B. euanclarksoni, which is present in both structural units (Fig. 1b).
(a) Location of the Cabrières area at the southeastern end of the Montagne Noire, south of the Massif Central. Shaded areas: Prevariscan basement at outcrop. (b) Fossiliferous Devonian trilobite localities with Branikarges euanclarksoni sp. nov. around Cabrières village. Light grey: Cabrières sedimentary klippes with (1) eastern slope of Soureillé d’Izarne hill [43°33′34″; 3°21′15″E] and (2) northern slope of La Serre hill [43°33′27N; 3°21′32″E]; dark grey: nappe area with (3) western slope of Bissounel hill [43°35′41″N; 3°20′15″E].
In the Pic de Bissous nappe area north of Cabrières the new material comprises silicified remains, including early growth stages in cherty limestones of the middle Bissounel Formation of upper Emsian age (Fig. 2, left). In the Cabrières klippes it is present in cherty limestones of the middle Izarne Formation of upper Emsian age (Fig. 2, right). Unlike those of the Bissounel Formation, the trilobite sclerites of the coeval Izarne Formation remained calcareous while tabulate corals and stromatoids were heavily silicified (‘Calcaires à polypiers siliceux’ (auct.)). Interestingly, trilobite associations including the rare, new Branikarges species, but mainly scutelluids, are exclusively accumulated under shelter of silicified tabulate corals (Pedder & Feist Reference Pedder and Feist1998).
Lithofacies and stratigraphical correlation of Emsian Formations between the Mt.Peyroux/Pic de Bissous nappe domain and the Cabrières klippe domain. Lithofacies: biodetrital crinoidal limestone (1a), graded bioclastic limestone (1b), marly limestone (2a), shale (2b), massive Stromatactis limestone with biodetrital clusters (3a), cherty limestone (3b), nodular limestone (4a), dolomite (4b). Dots: occurrences of Branikarges euanclarksoni sp. nov. (localities 1–3).
2. Material and methods
Calcareous material was mechanically prepared and coated with magnesium oxide before photography, while the silicified specimens were recovered using formic acids and imaged using scanning electron micrography. The figured materials (UM-IP 1032–1062) are deposited in the collections of the Institute of Evolution, University Montpellier II, France.
The morphological descriptions follow the terminology of Whittington in Whittington & Kelly (Reference Whittington, Kelly and Kaesler1997).
3. Systematic palaeontology
Order Lichida Moore, Reference Moore and Moore1959
Family Lichidae Hawle & Corda, Reference Hawle and Corda1847
Subfamily Trochurinae Pfleger, Reference Pfleger1936
Genus Branikarges Basse & Müller, Reference Basse and Müller2023
Type species: Lichas Branikensis Barrande, Reference Barrande1872 by original designation (Barrande Reference Barrande1872, p. 43), from the Dvorce-Prokop Limestone, Pragian, Czech Republic. Lectotype: pygidium, NMP IT1378, selected Přibyl & Erben (Reference Přibyl and Erben1952, p. 147; figured Barrande (Reference Barrande1872, pl. 16, fig. 31), Přibyl & Erben (Reference Přibyl and Erben1952, pl. 11, fig. 6), Vaněk (Reference Vanĕk1959, pl. 7, fig. 10) and Thomas & Holloway (Reference Thomas and Holloway1988, pl. 13, fig. 283).
Discussion: The concept of Lobopyge Přibyl & Erben (Reference Přibyl and Erben1952) was based primarily on the arrangement of the anterior pygidial pleural segments, whose pleural bands extend into the marginal spines, and is therefore distinct from Acanthopyge Hawle & Corda, Reference Hawle and Corda1847. This concept has been followed until recently by many authors, although they considered Lobopyge simply as a subgenus of Acanthopyge (e.g., Chatterton & Wright Reference Chatterton and Wright1986; Ebach & Ahyong Reference Ebach and Ahyong2001; Thomas & Holloway Reference Thomas and Holloway1988). Özdikmen (Reference Özdikmen2009) recognised that Lobopyge Přibyl & Erben (Reference Přibyl and Erben1952) constitutes a junior homonym of the millipede Lobopyge Attems, Reference Attems1951, and he proposed Belenopyge Pek & Vanĕk, Reference Pek and Vanĕk1991 as a substitute name. Van Viersen (Reference Van Viersen2021) rejected the subjective synonymy of ‘Lobopyge Přibyl & Erben (Reference Přibyl and Erben1952)’ and Belenopyge, claimed by Ebach & Ahyong, and argued for the independent status of Belenopyge. He suggested Huginarges Basse in Basse & Müller Reference Basse and Müller2004 as a replacement name for the invalid trilobite genus Lobopyge despite the fact that the two taxa are based on different generic concepts, as typified by their respective type species. In this regard, Basse & Müller (Reference Basse and Müller2023) pointed out that Huginarges does not exhibit the distally fused pleural bands that define ‘Lobopyge Přibyl & Erben (Reference Přibyl and Erben1952)’ in its original concept and it is therefore not representative. Basse & Müller (Reference Basse and Müller2023) thus proposed the new name Branikarges as a substitute name for the typical ‘Lobopyge of Přibyl & Erben (Reference Přibyl and Erben1952)’. At the same time, they restricted Branikarges to the type species Lichas branikensis Barrande, Reference Barrande1872 and its close allies, while mainly Middle Devonian descendants with a longer pygidial axis and a broader, flatter posterolateral pleural field previously assigned to Acanthopyge or A. (Lobopyge) (e.g., A. (L.) pervasta Pribyl et al., Reference Přibyl, Vanĕk and Hörbinger1986) were referred to Rheicarges Basse & Müller, Reference Basse and Müller2023 with Rheicarges decheni (Holzapfel, Reference Holzapfel1895) as the type species. Given the restricted concept of Branikarges defined by Basse & Müller (Reference Basse and Müller2023), the diagnosis of A. (Lobopyge) given by Thomas & Holloway (Reference Thomas and Holloway1988, p. 225) is applied herein for Branikarges, but with the addition of a narrow, convex pleural field between the axis and posterolateral margin.
Species assigned to Branikarges: B. altirhachis (Chernysheva, Reference Chernysheva1951), Upper Silurian, Kuznetsk basin, Siberia; B. australiformis (Chatterton, Johnson & Campbell, Reference Chatterton, Johnson and Campbell1979), Pragian, New South Wales; B. australis (McCoy, Reference McCoy1876), Pragian, Victoria; B. branikensis (Barrande, Reference Barrande1872), Pragian, Prague Basin; B. aff. branikensis (Barrande, Reference Barrande1872), Emsian, Uzbekistan; B. ?brevis (Maximova, Reference Maximova1968), Lower Devonian, Kazakhstan; B. campbelli (Chatterton & Wright, Reference Chatterton and Wright1986), Emsian, New South Wales; B. consanguineus (Clarke, Reference Clarke1894), Lochkovian, New York; B. contusus (Hall & Clarke, Reference Hall and Clarke1888), Emsian, New York; B. bassei (Chatterton & Gibb, Reference Chatterton and Gibb2010), Emsian–Eifelian transition, Maider, SE Morocco; B. bellerophon (Flick, Reference Flick2021), Emsian–Eifelian transition, eastern Rhenish Slate Mts.; B. docekali (Vaněk, Reference Vanĕk1959), lower Emsian, Prague Basin; B. edgecombei (Ebach, Reference Ebach2002), Lochkovian, New South Wales; B.? erinaceus (Haas, Reference Haas1968), Emsian, Bosporus region; B. estevei (Corbacho & Kier, Reference Corbacho and Kier2013), Pragian, Maider, SE Morocco; B. euanclarksoni sp. nov., Emsian, Montagne Noire; B. hexapteryx (Přibyl & Erben, Reference Přibyl and Erben1952), Pragian, eastern Rhenish Slate Mts; B. hirsutus (Fletcher, Reference Fletcher1850, Wenlock, Dudley; B. kerfornei (Renaud, Reference Renaud1930) emend. Feist & Morzadec (Reference Feist and Morzadec2025), Pragian, Brittany; B.? limbatus (Maximova, Reference Maximova1968), Lower Devonian, Kazakhstan; B.? longiaxis (Maximova, Reference Maximova1968), Lower Devonian, Kazakhstan; B. orientalis (Wu, Reference Wu1977), Upper Silurian, southwest China; B. parvus (Barrande, Reference Barrande1846) Pragian, Prague Basin; B. peneaui (Pillet, Reference Pillet1973), Emsian, Brittany; B. pragensis (Bouček, Reference Bouček1933), Lochkovian, Prague Basin; B.? pulex (Haas, Reference Haas1968), Emsian, Bosporus region; B. pristinus (Campbell & Chatterton, Reference Campbell and Chatterton2006), Ashgill, northwestern Canada; B. pusillus (Angelin, Reference Angelin1854), Silurian, Gotland; B. pustulosus (Morzadec, Reference Morzadec1983), Emsian, Brittany; B. richteri (Vanĕk, Reference Vanĕk1959), Lochkovian, Prague Basin; B. rohri (Perry & Chatterton, Reference Perry and Chatterton1977), Wenlock, Canadian Arctic; B. sibiricus (Chernysheva, Reference Chernysheva1951), Middle Devonian, Kuznetsk basin, Siberia; B. sinuatus (Ratte, Reference Ratte1886), early Lower Devonian, New South Wales; B. trinodis (Ellermann, Reference Ellermann1992), Pragian, Carnic Alps; B. n. sp. of Basse & Müller (Reference Basse and Müller2023), Pragian-Emsian transition, Cantabrian Mts.; B. sp. of Chatterton Johnson & Campbell (Reference Chatterton, Johnson and Campbell1979), Emsian, New South Wales.; B. sp. of Holloway & Neil (Reference Holloway and Neil1982), Lochkovian, Victoria, B. sp. of Campbell & Chatterton (Reference Campbell and Chatterton2006), Wenlock, northwestern Canada.
Branikarges euanclarksoni sp. nov.
Holotype: Pygidium, UM-IP 1034, La Serre (locality 2), Montagne Noire, upper Emsian, Figure 3f–h.
Branikarges euanclarksoni sp. nov. from the middle Izarne Formation, upper Emsian. (a, j) cranidium, UM-IP 1032, dorsal (a) and lateral (j) views; (b–c) small cranidium, UM-IP 1033, dorsal (b) and frontal (c) views; (d–e) hypostome, UM-IP 1035 with incomplete posterior border, ventral (d) and lateral (e) views; (f–h) holotype pygidium, UM-IP 1034, lateral (f), posterior (g) and dorsal (h) views; (i) fragmentary pygidium, UM-IP 1036, dorsal view; (k) fragmentary cranidium, UM-IP 1038, dorsal view; (l) incomplete pygidium, UM-IP 1037, latex cast of external mould, dorsal view. Scale bars represent 1 mm.
Paratypes: Cranidia UM-IP 1032–1033, 1038, Soureillé d’Izarne (locality 1), UM-IP 1032, Figure 3a, j; UM-IP 1033, Figure 3b, c; UM-IP 1038, Figure 3k; hypostome UM-IP 1035, La Serre (locality 2) Figure 3d–e; pygidium UM-IP 1036, Soureillé d’Izarne (locality 1), Figure 3i; pygidium UM-IP 1037, La Serre (locality 2), Figure 3l.
Type horizon and locality: Dark grey sparitic cherty limestone, middle Izarne Formation, northern slope of La Serre hill, 2,300 m south of Cabrières village (Fig. 1b, locality 2). Paratype locality: eastern slope of Soureillé d’Izarne hill (Fig. 1b, locality 1).
Other non-type material: silicified early growth stages from cherty limestones of the middle Bissounel Formation, western slope of Bissounel hill (Fig. 1b, locality 3), UM-IP 1039–1062.
Etymology: Named in honour and memory of Euan N. K. Clarkson for his fundamental contribution to trilobite research.
Diagnosis: Cranidium with medially reduced anterior border without lateral protuberances, low median glabellar lobe defined by straight, slightly diverging longitudinal furrows, asymmetrical teardrop-shaped bullar lobes, medially reduced preoccipital furrow, nodular sculpture without distinct pairs of prominent tubercles on median lobe. Hypostome with short trapezoidal middle body and very strong median swelling on the broad posterior border. Pygidium with narrow axis, high lateral profile, with paired tubercles medially and a single prominent tubercle on the posterior end, short postaxial field with low triangular postaxial ridge reaching the margin, pleurae extending into robust spines of equal width, narrow convex posterolateral pleural field (exsag.) without a border; margin between second and third marginal spines is straight.
Description: Cranidium is wide parabolic in its anterior outline, slightly depressed antero-medially, anterior border reduced to a narrow band medially, slightly widening abaxially without protuberances, moderately convex (sag.). Preglabellar furrow is continuously deep. Axial and longitudinal furrows deeply impressed, the latter becoming shallower across L1. Longitudinal furrows straight, very slightly diverging forwards: width of the median glabella lobe at the posterior ends of bullar lobes is 84 % of its width at the anterior end of bullar lobes. Its base has an obtusely rounded posterior outline, the maximum width of bullar lobes being 70 % of that of median lobe. Bullar lobes strongly swollen, asymmetrical, twice as long as wide, teardrop-shaped narrowing anteriorly, maximum width opposite the mid-length of median lobe, maximum length being 68 % of that of median lobe. In lateral view, the profile of median lobe is regularly arched, does not overhang anterior border; bullar lobes advanced, remaining slightly below median lobe. In frontal view, profile of median lobe slightly arched, barely higher than bullar lobes and palpebral lobes. Posterolateral lobes uniformly swollen. Palpebral lobes swollen, elevated at 45° and located opposite posterior third of bullar lobes. Preoccipital lobes triangular. Occipital ring narrow (sag.). Medial preoccipital furrow is reduced (sag.). Sculpture of scattered tubercles of different sizes; on bullar lobes their size increases from anterior to posterior, becoming very coarse posteriorly. Posterolateral lobes are surmounted by a prominent tubercle. Median glabellar lobe with tubercles of different sizes without distinction of prominent paired tubercles.
Hypostome sub-rectangular, with a straight and abaxially curved median anterior outline, very narrow anterior rim, pointed anterior wings, short subtrapezoidal, slightly arched and tuberculate middle body, short shallow middle furrows. Posterolateral corners of posterior lobe of middle body are swollen. Posterior border pitted, with a very strong anteromedian swelling without pits, and a row of prominent tubercles along anterolateral and median border furrows. Posterior and lateral borders gently embayed, provided with numerous terrace ridges.
The pygidium is parabolic in posterior outline (without spines), length represents 60 % of width. Its narrow axis reaches half its width and 74 % of its length (without spines) and is as wide as long (articulating half-ring included). Its lateral profile is strongly arched, less so transversely. Two axial rings are present behind narrow articulating half-ring. The anterior ring is slightly wider (sag.) and higher than the posterior one, the first inter-ring furrow is straight, deep, and widens slightly distally. The second inter-ring furrow is continuous, but shallow medially, and slightly curved anteriorly. Remnants of at least three other axial rings, with inconspicuous inter-ring furrows, are discernible by alternating groups of paired nodules. A single robust nodule is present medially above steep posterior slope of axis. Posterior end of axis is transverse, only slightly curved, separated from low postaxial ridge by faint furrow. Axial furrows moderately convergent, deeply impressed, somewhat shallower behind axis, connected by deep furrows alongside broad and flat triangular postaxial ridge to its end at posterior margin of pygidium. Length of postaxial field = 30 % of axial length. Pleural field narrow, reaching half axial width. Anterior border long and straight, obtusely backward curved distally. There are two prominent pleurae, separated by fairly deep interpleural furrows, immediately curved backward at axial furrow, and extended distally by moderately long border spines. They are subdivided adaxially by deep slit-like pleural furrows, which remain narrow and die out at the base of the border spines. The anterior pleural bands are individually swollen, slightly lower and half as wide as posterior bands, and outlined by a single row of nodules. Border spines are robust, equal in width and height to the distal pleurae, slightly curved upwards proximally and spreading moderately distally. Posterolateral pleural field between second interpleural furrow and post-axial ridge is narrow, strongly convex, of constant width as it runs continuously into short broad-based third border spines; these are close together and separated by a narrow embayment. Posterolateral margin between second and third border spines is long, straight, without border, with rounded edges. Sculpture consists of tubercles of various sizes, arranged in rows on the axial rings, pleural bands and post-axial field. Paired tubercles are displayed on crest of axis. A culminating tubercle is displayed medially at posterior end of axis. Posterior pleural field and postaxial ridge are covered with a few tubercles.
4. Discussion
Since the Silurian various representatives of Branikarges have been found worldwide and diversified particularly during the Lower Devonian. Sub-contemporaneous Emsian species of B. euanclarksoni share various morphological features with the new species, more so when comparing cranidia than pygidia.
The Barrandian Prague Basin is the classic Lower Devonian area where typical Branikarges are most diverse, represented by five taxa including the type species B. branikensis (Barrande, Reference Barrande1872). Among them, B. docekali (Vanĕk, Reference Vanĕk1959) is the only taxon from the lower Emsian, slightly older than the new species. Compared to B. euanclarksoni, it has a broader median glabellar lobe with distinctly outwardly curved longitudinal furrows, a very low pygidial axis, and wider pleural fields.
Among representatives of Branikarges from the late Lower Devonian of the southeastern Rhenish Slate Mountains, the new species resembles Branikarges bellerophon (Flick, Reference Flick2021) from Greifenstein (Emsian–Eifelian transition). It differs from it by a narrower and lower median glabella lobe, higher and more extended bullar lobes and, in the pygidium, shorter and thicker border spines and a straight posterior margin between second and third spines. Branikarges is represented in the Armorican Massif by B. peneaui (Pillet, Reference Pillet1973) at Erbray and B. pustulosus (Morzadec, Reference Morzadec1983) from the Rade de Brest, both of Emsian age. B. peneaui, of which only one cranidium is known, has a particular outline of its median glabella lobe which diverges posteriorly. B. pustulosus shares with B. euanclarksoni the outlines of the median glabellar lobe and the bullar lobes but it has a much sagittally longer occipital lobe and occipital furrow. It is distinguished by its cranidial sculpture which consists of rather coarse round tubercles, arranged in pairs on the median glabellar lobe and at its posterior end by the presence of a single prominent tubercle. The pygidium has narrower marginal spines, a relatively shorter trapezoidal axis and a raised, parallel-sided post-axial ridge.
The Branikarges n. sp. of Basse & Müller (Reference Basse and Müller2023) is the first representative of the genus from the Lower Devonian (Pragian–Emsian transition) of the Cantabrian Mountains. The undescribed pygidium shares the outline and sculpture of the axis with the new species. It is distinguished by longer and deeper pleural furrows, shorter outwardly stretched marginal spines, a significantly wider embayment between the third border spines and a stout, parallel-sided postaxial ridge.
In Emsian–Eifelian transitional beds at Djebel Issemour, in the Maider region of southeastern Morocco, occurs Branikarges bassei (Chatterton & Gibb, Reference Chatterton and Gibb2010), which shares with B. euanclarksoni sculptural patterns with rows of paired tubercles on the pygidial axis. However, in B. bassei, the bullar lobes are distinctly smaller and lower, and the median glabellar lobe is much more arched and wider transversely than in the Montagne Noire species. Additionally, the pygidium of B. bassei has deeper and wider pleural furrows, as well as more flared and more outwardly stretched marginal spines.
In southern Uzbekistan Branikarges is represented by a single pygidium referred to Acanthopyge (Lobopyge) aff. branikensis (Barrande, Reference Barrande1872), present in the Emsian of the Zerafsha Range (Owens et al. Reference Owens, Ivanova, Popov and Feist2010). This pygidium resembles B. euanclarksoni in the outline of its axis and the pattern of its tuberculate sculpture, but it has wider and deeper pleural furrows, a broader posterolateral pleural field and a narrow postaxial ridge that does not reach the posterior margin.
In the Emsian of the Bosporus region, B.? erinaceus (Haas, Reference Haas1968) and B.? pulex (Haas, Reference Haas1968) are distinguished from B. euanclarksoni by a much narrower cranidium, lower and shorter bullar lobes, and more protruding anterior margin. The pygidium of the former is distinctive: the posterior pleural bands extend into rather short marginal spines and the pleural furrows reach the margin. These features are not typical for Branikarges.
Questionable Lower Devonian Branikarges are represented in Kazakhstan by B.? limbatus (Maximova, Reference Maximova1968), B.? brevis (Maximova, Reference Maximova1968) and B.? longiaxis (Maximova, Reference Maximova1968). These are not closely related to B. euanclarksoni: B.? limbatus has a rather wide arcuate cranidial border and, in the pygidium, a sagittally very long post-axial field without a post-axial ridge, and very short marginal spines; B.? brevis has a very long pygidial axis subdivided by longitudinal furrows; B.? longiaxis has a long axis with an indistinct post-axial ridge and a wide and flat postaxial region.
Among the numerous Australian representatives of Branikarges from the Lower Devonian only B. cambelli (Chatterton & Wright, Reference Chatterton and Wright1986) is contemporary with B. euanclarksoni. It differs from the new species in many respects. In particular, its anterior cranidial border has distally pointed extensions, which are absent in B. euanclarksoni. Also B. campbelli has a rather coarse tubercular sculpture, with prominent pairs of tubercles on the median glabellar lobe, its pygidial border spines are strongly directed backwards and the third pair of spines are rather small, and unlike B. euanclarksoni additional spines are displayed between the second and third major border spines.
In eastern North America Branikarges is represented by B. consanguineus (Clarke, Reference Clarke1894) from the early Lower Devonian New Scotland Limestone (see Whittington Reference Whittington1956) and by B. contusus (Hall & Clarke, Reference Hall and Clarke1888) from the Emsian Onondaga limestone (see Ludvigsen Reference Ludvigsen1987). The former species differs from B. euanclarksoni in having a narrower median glabellar lobe and symmetrical bullar lobes, a pitted hypostome and, in the pygidium, a narrower elevated postaxial ridge and diverging border spines. B. contusus is distinguished by its rather narrow, parallel-sided median glabellar lobe and symmetrical bullar lobes. The pygidium has broader anterior pleural bands, much longer pleural furrows and a higher post-axial ridge in comparison with B. euanclarksoni.
5. Patterns of early growth
Early growth stages are known from various Silurian trochurine lichids (e.g., Chatterton & Speyer Reference Chatterton, Speyer and Kaesler1997, Campbell & Chatterton Reference Campbell and Chatterton2009), while in the early Lower Devonian a single protaspis was investigated by Whittington (Reference Whittington1956). From the late Lower Devonian, Chatterton (Reference Chatterton1971) published the most complete ontogenetic sequence of Acanthopyge (Jasperia) bifida (Edgell, Reference Edgell1955) from the Receptaculites Limestone Formation near Yass, New South Wales. This locality is contemporaneous with the middle Bissounel Formation which has produced silicified early growth stages attributed to B. euanclarksoni. This new material includes, besides a questionable protaspis fragment, several meraspid cranidia, hypostomes, librigenae and transitory pygidia which are illustrated herein in Figures 4 and 5. The specimens were recovered from the type locality for the middle part of the Bissounel Formation (Fig. 1b) which has also yielded series of silicified growth stages of various trilobite taxa (Feist Reference Feist1970; Feist & Lerosey-Aubril Reference Feist, Lerosey-Aubril, Rabano, Gozalo and Garcia-Bellido2008; Lerosey-Aubril et al. Reference Lerosey-Aubril, Feist and Chatterton2008).
Silicified meraspid stage cranidia, librigenae and hypostomes of Branikarges euanclarksoni sp. nov. from the middle Bissounel Formation, upper Emsian. All illustrations are scanning electron micrographs. (a) Early meraspid cranidium, UM-IP 1039. (b) Fragment of anterior meraspid cranidium, UM-IP 1040. (c) Early meraspid cranidium, UM-IP 1041. (d) Early meraspid librigena, UM-IP 1042. (e) Meraspid hypostome, UM-IP 1043. (f) Meraspid hypostome UM-IP 1044, dorsal (internal) view. (g) Fragment of antero-lateral meraspid cranidium UM-IP 1045. (h) Late meraspid librigena UM-IP 1046. (i) Late meraspid cranidium UM-IP 1047, posteriorly tilted. (j) Fragment of late meraspid cranidium UM-IP 1048. (k) Late meraspid cranidium UM-IP 1049. (l) Late meraspid cranidium UM-IP 1050. Scale bars represent 0.2 mm (a–g) and 0.3 mm (h–l).
Silicified protaspid (?) and transitory pygidia of Branikarges euanclarksoni sp. nov. from the middle Bissounel Formation, upper Emsian. All illustrations are scanning electron micrographs. (a) Fragmentary protaspis, UM-IP 1051. (b) Transitory pygidium UM-IP 1052, meraspid stage 0. (c) Fragment of transitory pygidium, UM-IP1053, meraspid stage 4. (d–e) Transitory pygidium, UM-IP 1054, ventral view (d) showing the presence of third pair of protopygidial border spines and dorsal (e) view, meraspid stage 5. (f) Fragmentary transitory pygidium UM-IP 1055, meraspid stage 7. (g, j) Fragmentary transitory pygidium, UM-IP 1056, meraspid stage 5. (h) Transitory pygidium, UM-IP 1057, meraspid stage 7. (i) Transitory pygidium, UM-IP 1058, meraspid stage 9. (k) Transitory pygidium, UM-IP 1059, meraspid stage 9. (l) Fragmentary transitory pygidium, UM-IP 1060, meraspid stage 9. (m) Fragmentary transitory pygidium, UM-IP 1061, meraspid stage 10. (n) Pygidium, UM-IP 1062, initial holaspid stage. Scale bars represent 0.2 mm (a–h, j) and 0.3 mm (i, k–n).
The fragmentary protaspis (?) (Fig. 5a) has five pairs of border spines on the protopygidium, but unlike the metaprotaspis of A. (J.) bifida and Branikarges consanguineus (Clarke, Reference Clarke1894), the spines of the most posterior pair are atypically distant to each other. At least three pairs of spines are developed on the narrow protocranidial axis. No preoccipital lateral lobes are discernible. The poor preservation of the specimen makes interpretation of morphological features difficult and does not allow its assignment to the protaspis period with certainty.
In the smallest meraspid cranidium (Fig. 4a), measuring 1.1 mm long, swollen anterolateral cranidial lobes are developed behind the broad eye ridges, and reach one third of the length of the protoglabella. The median glabellar lobe widens anteriorly and bears three pairs of major spines. There is a median occipital spine flanked by a pair of spines on the posterior margin, pairs of spines on the posterior fixigenae and a pair of spines on each eye ridge. In later stages, the spines on the bullar lobes increase from anterior to posterior. As the bullar lobes grow anteriorly in later meraspides, the eye ridges disappear in front of them but they remain laterally. The librigenae are almost identical with those of A. (J.) bifida, notably in the presence of a deep notch behind the genal spine and the projection of spines on the adaxial posterior margin. The 1.2-mm-long meraspid stage hypostomes (Fig. 4e, f) are transversely rectangular and have straight converging lateral borders that are divided by four slits similar to smaller hypostomes of that species.
The smallest six-segmented transitory pygidium (Fig. 5b) corresponds to meraspid stage M0. It is very similar to the slightly smaller (0.6 mm versus 0.68 mm length) M0 of A. (J.) bifida in terms of the lateral projection of the pleural spines of the first segment, the straight anterior margin and the relatively long articulating half-ring. Unlike in A. (J.) bifida, no spinules are visible on the posterior margins of the pleural spines. The axial rings show an alternating spinous development: the first, third and fifth axial rings are provided with pairs of progressively posteriorly projecting spines, while these are absent on the second, fourth and sixth rings, where only low nodules are discernible. The pleural segments are provided alternately with two spines and one distal spine.
A fragmentary transitory pygidium (Fig. 5c) probably corresponds to meraspid stage M4. It is of the same size and shows the same pattern of alternating spines along the axis as the M4 of A. (J.) bifida. The two anterior pleural segments (S5–6) have prominent spines distally and small spines adaxially, while the posterior segments S7 to S10 show alternating prominent spines and small spines, as observed in the A. (J.) bifida M4 meraspis.
Several specimens (Fig. 5d–e, g, j) correspond to meraspid stage M5. Their general shape is more laterally extended with pleural spines stretching outward and less curved posteriorly than in A. (J.) bifida.
Thoracic segments S11 to S6 are present, characterised by alternating paired spines on the axial rings and prominent spines on the distal pleurae (segments 11, 9, 7) and spineless segments provided with tubercles (segments 10, 8, 6). The protopygidium (Fig. 5d–e) shows three pairs of pleural spines with one pair on the axial ring and the pleurae of P2. The posterior end of the axis is well differentiated and less pointed than in A. (J.) bifida, and the postaxial field between the tip of the axis and the posterior margin (23 % of the axis, including the articulating half-ring) is much shorter. The postaxial field and P3 are devoid of spines, and P3 bears tubercles. The doublure is slightly wider adaxially than antero-distally. Thin terrace lines are barely perceptible.
Two fragmentary specimens with four protothoracic segments and a complete protopygidium represent meraspid stage M7 (Fig. 5f, h). Paired axial spines and prominent spines on the distal pleural segments are present on segments S2, S4 and P2; the other segments have tubercles. A single spine occurs at the posterior end of the protopygidial axis.
Stage M9 is characterised by the presence of two protothoracic segments (S10 and S11) still attached to the protopygidium (Fig. 5i–l). As in A. (J.) bifida, the axial ring of S11 has a pair of prominent spines, and the outer end of the pleural segment has a large protruding spine. In the anterior segment (S10), the axial ring bears a single transverse row of tubercles, and the pleural segment has a small spine at mid-length. This pattern of alternating spines continues in the axial rings of the protopygidium corresponding to the two anterior segments. At this stage, anterior pleural bands are discernible in the two anterior pleural segments of the protopygidium. Unlike in A. (J.) bifida, the prominent spines of the pleural segment of P2 are positioned more adaxially and close to the axial furrow. Other pairs of spines are discernible on the axis and may correspond to posterior segments. The end of the axis bears medially a strong, posteriorly directed spine, which persists until the adult stage, but whose size diminishes. The posterior contour of the axis, conical in earlier stages, becomes parabolic and surrounded by a continuous axial furrow. At this stage, the axis of the protopygidium takes on an adult form. A slight, swollen postaxial ridge appears, but is not triangular.
In the postaxial field, a pair of tubercles is located just behind the tip of the axis, on either side of the postaxial ridge. This could indicate the presence of a fused segment between P2 and P3.
The M10 stage (Fig. 5m) is characterised by the presence of the last protothoracic segment (S11), still fused with the unarticulated protopygidium. It displays the strong pair of spines on the axis, as in earlier stages. P1 has a prominent axial ring with a single row of tubercles. Behind it, a low axial ring is separated from the rest of the axis by a slight but continuous inter-ring furrow that disappears medially in the adult stage. In the early holaspis (Fig. 5n), this second axial ring bears a strong pair of spines that diminish in size during adult development but remain present.
6. Conclusions
The new species is a representative of Branikarges at the end of the Early Devonian. Typical Emsian Branikarges species remain specifically less diverse than in the earlier Devonian and are less cosmopolitan as they appear to be restricted to terranes related to north Gondwana. This could be due to sampling bias and to the fact that the same taxa originally attributed to Acanthopyge and assigned to A. (Lobopyge) by Thomas & Holloway (Reference Thomas and Holloway1988) can now be assigned to Rheicarges Basse & Müller, Reference Basse and Müller2023. For the first time, early growth stages of Branikarges were recovered. A nearly complete meraspid developmental sequence is recognised by applying the successful model of marker spine distribution on alternate pleurae developed by Chatterton (Reference Chatterton1971).
Acknowledgements
The paper profited from the detailed corrections and constructive remarks provided by the reviewers Derek Siveter and Arnaud Bignon. I am grateful to them and to the editors Alan Owen and Susie Cox for their efficient help.
Competing interests
The author declares none.
