1. Introduction
During the Devonian, trilobites were among the most abundant and diverse groups of marine arthropods, thriving particularly in benthic environments, from shallow to deep waters (Feist Reference Feist1991; Lerosey-Aubril & Feist Reference Lerosey-Aubril, Feist and Talent2012; Bault et al. Reference Bault, Crônier, Allaire and Monnet2022, Reference Bault, Balseiro, Monnet and Crônier2023). However, their biodiversity was severely impacted during the Late Devonian due to a series of environmental perturbations, including major sea-level fluctuations and widespread marine anoxia (Sepkoski Reference Sepkoski and Walliser1996; Bond & Grasby Reference Bond and Grasby2017). Regional taxonomic and biostratigraphic studies of these marine invertebrates are essential for understanding the effects of Late Devonian mass extinction events on their biodiversity, evolutionary patterns and palaeobiogeographic distributions, particularly in shallow, near-shore settings, where the Famennian trilobite record remains relatively scarce (Feist et al. Reference Feist, McNamara, Crônier and Lerosey-Aubril2009; Crônier et al. Reference Crônier, Serobyan, Grigoryan, Witt and Danelian2021). In this regard, the Upper Devonian sedimentary sequences of southern Central Armenia are of particular significance, as they are well-exposed, highly fossiliferous and consist of shallow marine, mixed carbonate-siliciclastic deposits. These sequences belong to the South Armenian Block, which was part of the northern margin of Gondwana during the Devonian (e.g., Dercourt et al. Reference Dercourt, Zonenshain, Ricou, Kazmin, LePichon, Knipper, Grandjacquet, Sbortshikov, Geyssant, Lepvrier, Pechersky, Boulin, Sibuet, Savostin, Sorokhtin, Westphal, Bazhenov, Lauer and Biju-Duval1986; Sosson et al. Reference Sosson, Rolland, Müller, Danelian, Melkonyan, Kekelia, Adamia, Babazadeh, Kangarli, Avagyan, Galoyan, Mosar, Sosson, Kaymakci, Stephenson, Bergerat and Starostenko2010; Nikogosian et al. Reference Nikogosian, Bracco Gartner, Mason, van Hinsbergen, Kuiper, Uwe Kirscher, Matveev, Grigoryan, Grigoryan, Israyelyan, van Bergen, Koornneef, Wijbrans, Davies and Meliksetian2023). More particularly, this realm was part of an extensive carbonate platform located within the tropical zone of the southern hemisphere (Brock & Yazdi Reference Brock and Yazdi2000; Scotese Reference Scotese2014; Golonka Reference Golonka2020).
These sequences are becoming increasingly well-known for their abundant and diverse fossil record (Danelian et al. Reference Danelian, Serobyan, Hairapetian, Tsatryan, Khachatryan, Yeghiazaryan, Avagyan, Hambardzumyan, Kirakosyan, Harutyunyan, Lokteva, Gasparyan and Grigoryan2025), especially their brachiopods (Serobyan et al. Reference Serobyan, Grigoryan, Mottequin, Mayilyan, Crônier and Danelian2019, Reference Serobyan, Danelian, Crônier, Grigoryan and Mottequin2021, Reference Serobyan, Danelian, Crônier, Grigoryan and Mottequin2022a, Reference Serobyan, Danelian, Crônier, Grigoryan and Mottequin2022b, Reference Serobyan, Danelian, Hairapetian, Grigoryan, Crônier, Randon and Mottequin2023, Reference Serobyan, Vinn and Mottequin2024), tentaculitids (Vinn et al. Reference Vinn, Hambardzumyan, Temereva, Grigoryan, Tsatryan, Harutyunyan, Asatryan and Serobyan2025), corals (Sytova et al. Reference Sytova, Chudinova, Ulitina and Akopian1974), bryozoans (Ernst et al. Reference Ernst, Serobyan and Danelian2024), conodonts and fish remains (Grigoryan et al. Reference Grigoryan, Serobyan, Randon, Mayilyan and Danelian2019; Grigoryan & Gatovsky Reference Grigoryan and Gatovsky2020; Ginter et al. Reference Ginter, Hairapetian and Grigorian2011; Tsatryan et al. Reference Tsatryan, Serobyan, Grigoryan, Avagyan, Danelian and Hairapetian2025) and palynological assemblages (Khachatryan et al. Reference Khachatryan, Cascales-Miñana, Danelian, Breuer, Steemans, Grigoryan, Gabrielyan, Hairapetian, Regnier, Kroeck and Serobyan2025; Yeghiazaryan et al. Reference Yeghiazaryan, Kroeck, Danelian, Grigoryan, Regnier and Serobyan2025; Lokteva et al. Reference Lokteva, Kirakosyan, Camina, De Backer, Grigoryan, Danelian and Serobyan2025). Trilobites from Armenia remain poorly known both taxonomically and biostratigraphically; as a result, their palaeoecological, palaeobiogeographic, and evolutionary significance still remain underexplored. Notably, Crônier et al. (Reference Crônier, Serobyan, Grigoryan, Witt and Danelian2021) reported representatives from the families Proetidae and Phacopidae in the Upper Devonian sedimentary sequences of Armenia, including an unidentified proetid pygidium and a few phacopid specimens, among them Omegops cf. accipitrinus (Phillips, Reference Phillips1841). Omegops Struve, Reference Struve1976, represents the youngest known phacopid trilobite genus, occurring in the uppermost Famennian, with the exception of a few records from China (see Zong Reference Zong2023a, Reference Zong2023b). These trilobites persisted into strata equivalent to the Hangenberg Sandstone (Kaiser et al. Reference Kaiser, Becker, Steuber and Aboussalam2011) before their extinction near the Devonian–Carboniferous boundary. In some places such as Belgium, the stratigraphic position of phacopid-bearing rocks has been placed at the base of the Carboniferous due to an incorrect understanding of the Devonian-Carboniferous Boundary in older literature (e.g. Maillieux, Reference Maillieux1933), as discussed in Denayer et al. (Reference Denayer, Prestianni, Mottequin, Poty, Aretz, Herbig, Hartenfels and Amler2019, Reference Denayer, Prestianni, Mottequin, Hance and Poty2021). Phacopids are widely considered as true victims of the Hangenberg Crisis (e.g. Bault Reference Bault2023), and they are of great importance for understanding the end-Devonian extinctions.
The primary objective of this study is to document the taxonomy and biostratigraphy of a new species of phacopid trilobite from the uppermost Famennian shallow-water sequences of Central Armenia (Fig. 1a), which represents by far the last known occurrence of the mentioned group in the region prior to their extinction at the Devonian–Carboniferous boundary. This work is based on newly collected fossil material from a locality near the village of Armash (Fig. 1a). In addition to the documentation and description of the new material, we discuss the paleobiogeographic affinities of these Famennian trilobites and the implications for better understanding the evolution and extinction of Late Devonian phacopids.
(a) Schematic geological map (modified after Arakelyan, Reference Arakelyan, Mkrtchian, Vardaniants, Gabrielian, Maghakian and Paffenholz1964) and distribution of the Middle–Upper Paleozoic sequences in the Central Armenia including the location of the studied Armash section. (b) Field view of the Armash section.
2. Devonian – Carboniferous stratigraphy of the Lesser Caucasus
The mid-Paleozoic sequences of the Lesser Caucasus (namely Armenia and Nakhichevan; Fig. 1a) begin with the upper part of the Lower Devonian, which is well exposed in the territory of Nakhichevan (Arakelyan Reference Arakelyan, Mkrtchian, Vardaniants, Gabrielian, Maghakian and Paffenholz1964; Aristov Reference Aristov1994). The Middle Devonian is also primarily documented in Nakhichevan, with tentative reports of Givetian strata from Armenia, near the border with Nakhichevan (Abrahamyan Reference Abrahamyan and Akopian1974; Aristov Reference Aristov1994). However, the age of these Givetian strata in Armenia remains uncertain, as it is based largely on stratigraphic interpretations rather than diagnostic index fossils, such as conodonts. Therefore, their precise dating requires further refinement through fossil-based studies. In contrast, Upper Devonian sequences crop out more extensively in Armenia; they are represented by both Frasnian and Famennian deposits. The Frasnian strata appear to have a more limited distribution and are characterized by siliciclastic sedimentary sequences, including shales, sandstones, and siltstones, with carbonate rocks being relatively minor. The Famennian, however, is more widespread and predominantly consists of carbonate rocks, which are either dominant or at least equal in proportion to the siliciclastic rocks (Abrahamyan Reference Abrahamyan, Mkrtchian, Vardaniants, Gabrielian, Maghakian and Paffenholz1964; Arakelyan Reference Arakelyan, Mkrtchian, Vardaniants, Gabrielian, Maghakian and Paffenholz1964). The Carboniferous is represented by the lower to middle parts of the Mississippian subperiod, covering the Tournaisian and Visean stages (Abrahamyan Reference Abrahamyan, Mkrtchian, Vardaniants, Gabrielian, Maghakian and Paffenholz1964; Aristov Reference Aristov1994; Joachimski et al. Reference Joachimski, Alekseev, Grigoryan and Gatovsky2020). Unlike the Devonian, the Carboniferous sequences in Armenia are mainly composed of carbonate deposits, with siliciclastic rocks being minor in the Tournaisian and Visean intervals (Arakelyan Reference Arakelyan, Mkrtchian, Vardaniants, Gabrielian, Maghakian and Paffenholz1964). The Devonian–Carboniferous succession, approximately 1,000 to 1,500 metres thick, was initially divided by Arakelyan (Reference Arakelyan, Mkrtchian, Vardaniants, Gabrielian, Maghakian and Paffenholz1964) and Abrahamyan (Reference Abrahamyan, Mkrtchian, Vardaniants, Gabrielian, Maghakian and Paffenholz1964) into twelve ‘formations’ (e.g., Gortun, Arshakiaghbyur, Gerankalasy). However, Serobyan et al. (Reference Serobyan, Grigoryan, Mottequin, Mayilyan, Crônier and Danelian2019, Reference Serobyan, Danelian, Crônier, Grigoryan and Mottequin2021) observed that these subdivisions are primarily distinguished by index fossils, particularly brachiopods, rather than by their lithological characteristics. As these subdivisions hold greater biostratigraphic than lithostratigraphic significance, we reinterpret them as horizons rather than formations, as proposed in one of the later works of Arakelyan (Arakelyan et al. Reference Arakelyan, Malxasyan, Mkrtchyan and Paffenholz1975), and following the approach of Murphy & Salvador (Reference Murphy and Salvador1999). Moreover, as highlighted in the review by Hambardzumyan et al. (Reference Hambardzumyan, Serobyan, Guillam, Hairapetian, Danelian and Grigoryan2024), the stratigraphic attribution of these horizons, and more particularly those spanning the upper Famennian to lower Tournaisian interval, has been questioned in some previous studies. Notably, Abrahamyan (Reference Abrahamyan, Mkrtchian, Vardaniants, Gabrielian, Maghakian and Paffenholz1964, Reference Abrahamyan and Akopian1974) initially reported that the Arshakiaghbyur Horizon contains a brachiopod- and coral-rich fauna characteristic of both Upper Devonian and Lower Carboniferous strata, and then suggested that it represents a transitional interval between the two periods. In contrast, Arakelyan et al. (Reference Arakelyan, Malxasyan, Mkrtchyan and Paffenholz1975) interpreted the entire horizon as being Tournaisian in age. Aristov (Reference Aristov1994), based primarily on conodont assemblages collected from Nakhichevan (Fig. 1a), followed Abrahamyan’s view and placed the Devonian–Carboniferous boundary (D–CB) within the Arshakiaghbyur Horizon. He correlated its lower to middle part with the uppermost Famennian costatus Conodont Zone of Ziegler (Reference Ziegler1962), and its upper part with the lower Tournaisian sulcata–duplicata zones of Sandberg et al. (Reference Sandberg, Ziegler, Leuteritz and Brill1978). However, Rzhonsnitskaya and Mamedov (Reference Rzhonsnitskaya and Mamedov2000) observed that key conodont species commonly used to constrain the position of the D–CB, such as Polygnathus inornatus, P. communis communis, P. longiposticus, Bispathodus stabilis, and Siphonodella duplicata, have not yet been reported from the Lesser Caucasus. Based on the ostracod and brachiopod records, they suggested to consider the D–CB as probably lying at the base of the overlying Gerankalasy Horizon.
Subsequently, Ginter et al. (Reference Ginter, Hairapetian and Grigorian2011), who documented chondrichthyan microfossils from the Famennian–Tournaisian interval, also reported conodonts from the Arshakiaghbyur Horizon corresponding to the middle expansa–praesulcata interval. In contrast, conodonts recovered from the limestone sequences of the Gerankalasy Horizon indicate an age no younger than the Tournaisian crenulata Conodont Zone. Based on these data, the D–CB can be tentatively placed at the transition between the Arshakiaghbyur and Gerankalasy horizons, pending a more detailed conodont-based investigation of the succession. Given the inconsistencies outlined above, we refrain from attempting to correlate the lithostratigraphy of the studied Armash section with these horizons. However, it is worth noting that the examined section, which consists of a coral- and brachiopod-rich limestone interval, appears to correspond to the Arshakiaghbyur Horizon. In addition to the works mentioned above, the Upper Devonian–Lower Carboniferous biostratigraphy of the Lesser Caucasus has been systematically studied by several other researchers, including Rzhonsnitskaya (Reference Rzhonsnitskaya1948), Abrahamyan (Reference Abrahamyan1954, Reference Abrahamyan1957), Arakelyan (Reference Arakelyan1952), Abrahamyan et al. (Reference Abrahamyan, Arakelyan, Azizbekov, Rzhonsnitskaya, Nalivkin, Rzhonsnitskaya and Markovski1973), Arakelyan et al. (Reference Arakelyan, Malxasyan, Mkrtchyan and Paffenholz1975), Grechishnikova et al. (Reference Grechishnikova, Levitskii and Feliks1980, Reference Grechishnikova, Aristov, Reitlinger and Chizhova1982), Mamedov & Rzhonsnitskaya Reference Mamedov and Rzhonsnitskaja(1985), Grigoryan et al. (Reference Grigoryan, Serobyan, Randon, Mayilyan and Danelian2019), Grigoryan & Gatovsky (Reference Grigoryan and Gatovsky2020), Joachimski et al. (Reference Joachimski, Alekseev, Grigoryan and Gatovsky2020), and Serobyan et al. (Reference Serobyan, Grigoryan, Mottequin, Mayilyan, Crônier and Danelian2019, Reference Serobyan, Danelian, Crônier, Grigoryan and Mottequin2021, Reference Serobyan, Danelian, Crônier, Grigoryan and Mottequin2022a, Reference Serobyan, Danelian, Crônier, Grigoryan and Mottequin2022b, Reference Serobyan, Danelian, Hairapetian, Grigoryan, Crônier, Randon and Mottequin2023).
3. Material and methods
The studied material comes from an approximately 10 m-thick continuous carbonate sequence, primarily composed of thin bedded marly limestones, rich in brachiopods and isolated corals (Fig. 1b). The section is located 5-6 km north-northeast of the Armash village (Ararat Province), on the southern slope of the Urts Mountain Range (Fig. 1a). The studied sequence is part of a much thicker Devonian–Carboniferous section exposed in the same area, known as the ‘Armash section’, and previously described by Arakelyan (Reference Arakelyan, Mkrtchian, Vardaniants, Gabrielian, Maghakian and Paffenholz1964) and Abrahamyan (Reference Abrahamyan, Mkrtchian, Vardaniants, Gabrielian, Maghakian and Paffenholz1964). These authors inaccurately attributed this part of the exposed section to the Carboniferous strata. The age of the studied sequence is determined by the presence of extracted conodonts (this study; e.g. 24/1-6; Fig. 2). Although it is difficult to distinguish subdivisions of the expansa Zone within this sequence, we identified Icriodus raymondi Sandberg & Ziegler, Reference Sandberg and Ziegler1979 in the lower part of the studied section (level 24/2), Polygnathus inornatus Branson & Mehl, Reference Branson and Mehl1934 in the levels 24/1, 24/4 and 24/5, and Siphonodella praesulcata Sandberg, 1972 (in Sandberg et al. Reference Sandberg, Streel and Scott1972) in the upper part of this section (level 24/5) (Figs 2–3). The First Appearance Datum (FAD) of Siphonodella praesulcata marks the uppermost Famennian praesulcata Zone, corresponding to the middle of the Bispathodus ultimus Zone (Spalletta et al. Reference Spalletta, Perri, Over and Corradini2017). In contrast, Icriodus raymondi has its Last Appearance Datum (LAD) in the Middle expansa, which corresponds to the Bispathodus costatus Zone. Therefore, it is reasonable to assume that the base of the section spans the Middle to Upper expansa Zone (i.e., upper Famennian), while the upper part of it (including horizon 24/5) covers the uppermost Famennian praesulcata Zone.
Stratigraphic log of the Armash section showing the Upper Devonian in calcareous facies. The calcareous sandstone could represent the Hangenberg Sandstone event bed (HS).
Scanning electron micrographs of selected conodont species from the Armash section. (a) Icriodus raymondi Sandberg & Ziegler, Reference Sandberg and Ziegler1979; IGS/Ar/C2-1, level 24/2. (b–c) Polygnathus inornatus Branson & Mehl, Reference Branson and Mehl1934; (b) IGS/Ar/C2-2, level 24/1; (c) IGS/Ar/C2-3, level 24/5. (d) Siphonodella praesulcata Sandberg, 1972; IGS/Ar/C2-4, level 24/5. Scale bars = 300 µm (a, b), 400 µm (c, d).
With respect to the geological structure of the studied area, we encounter here almost east-west (e.g. SE-100°) trending strata, which are notable for their subvertical dips (around 80°) towards the North or South, in places where they are strongly deformed. Looking at the overall picture, it is obvious that layer 24/4 and the other parts coincide with the southern, steep limb of the local anticline. In the core of the latter, mainly Upper Devonian terrigenous-carbonate strata are exposed, while Permian and/or Carboniferous (?) carbonate levels are widespread on the flanks (Fig. 1a).
The trilobite specimens were recovered from an interval corresponding to samples 24/3 to 24/4 (Fig. 2). Further samples may provide additional conodont species, including index bispathodids, which could help refine the age assignment. The trilobite material assigned to the Phacopidae family includes disarticulated molting remains as well as partially enrolled specimens. Phacopid remains are more or less well-preserved with some distortion due to some abrasion features as a result of sediment transport. Most of the specimens were collected from a soft, weathered surface, which yielded well-preserved specimens with minimal sediment. Almost complete specimens were coated with magnesium oxide or ammonium chloride sublimate before being photographed with a Nikon D850 camera, fitted with the Nikkor 60 mm f/2.8 G ED macro lens.
All studied specimens are deposited at the Geological Museum of the Institute of Geological Sciences of the National Academy of Sciences of Armenia, Yerevan (IGS/Ar), unless otherwise stated.
4. Systematic palaeontology (C. Crônier)
Morphological terminology follows that of Whittington & Kelly (Reference Whittington, Kelly and Kaesler1997).
Order PHACOPIDA Salter, Reference Salter1864
Superfamily PHACOPOIDEA Hawle & Corda, Reference Hawle and Corda1847
Family PHACOPIDAE Hawle & Corda, Reference Hawle and Corda1847
Subfamily PHACOPINAE Hawle & Corda, Reference Hawle and Corda1847
Genus Omegops Struve, Reference Struve1976
Type species . Calymene accipitrina Phillips, Reference Phillips1841; Upper Devonian Pilton Beds, England.
Species included. Omegops accipitrinus (Phillips, Reference Phillips1841), Famennian (do VI), Belgium, England, France, Germany, Morocco (see Richter & Richter Reference Richter and Richter1933; Malvesy et al. Reference Malvesy, Morzadec and Feist1999; Feist et al. Reference Feist, Cornée, Corradini, Hartenfelds, Aretz and Girard2021; Crônier et al. Reference Crônier, Denayer, Feist and Mottequin2025); O. cf. accipitrinus (Phillips, Reference Phillips1841), Famennian, Armenia (see Crônier et al. Reference Crônier, Serobyan, Grigoryan, Witt and Danelian2021); O. cornelius (Richter & Richter, Reference Richter and Richter1933), Famennian (do VI), Germany, Central Iran (see Mistiaen et al. Reference Mistiaen, Gholamalian, Gourvennec, Plusquellec, Bigey, Brice, Feist, Feist, Ghobadi Pour, Kebria-EE, Milhau, Nicollin, Rohart, Vachard and Yazdi2000; Weber Reference Weber2000); O. cf. cornelius (Richter & Richter, Reference Richter and Richter1933), late Famennian, Eastern Iran (see Feist et al. Reference Feist, Yazdi and Becker2003); O. ?cornelius (Richter & Richter, Reference Richter and Richter1933), late Famennian (Strunian), Afghanistan (see Ghobadi Pour et al. Reference Ghobadi Pour, Popov, Omrani and Omrani2018); O. insolatus (Struve, Reference Struve1976), Famennian (do VI), Morocco; O. maretiolensis (Richter & Richter, Reference Richter and Richter1933), Famennian (do VI), Belgium (see Crônier et al. Reference Crônier, Denayer, Feist and Mottequin2025); O. mobilis (Xiang, Reference Xiang, Teichert, Liu and Chen1981), late Famennian, NW-China (Xinjiang); O. multisegmentatus (Weber, Reference Weber1937), Famennian (? do VI), Kazakhstan (= sp. K, see Struve Reference Struve1976); O. paiensis Farsan, Reference Farsan1998, late Famennian (Strunian), Afghanistan; O. tilabadensis Ghobadi Pour et al., Reference Ghobadi Pour, Popov, Omrani and Omrani2018, late Famennian, northern Iran; O. sp. M (see Struve Reference Struve1976, p. 435–436), Famennian (do VI), Moravia; O. sp. T (see Struve Reference Struve1976, p. 438–439), Famennian (do VI), Morocco; O. sp. (see Yuan & Xiang Reference Yuan and Xiang1998), Famennian (do VI), South China (Guangxi).
Questionable species . Omegops mobilis (Xiang, Reference Xiang, Teichert, Liu and Chen1981), middle–late Famennian, northwest China (see Crônier et al. Reference Crônier, Denayer, Feist and Mottequin2025 for discussion on species).
Remarks. Omegops is characterized by the distinctive configuration of its intercalating ring (see Struve Reference Struve1976). According to Struve’s diagnosis (Reference Struve1976), the intercalating ring is typically erased and depressed, or only faintly visible in grazing light. This genus is typically found in the uppermost Famennian (Crônier & François Reference Crônier and François2014).
From Armenia, Crônier et al. (Reference Crônier, Serobyan, Grigoryan, Witt and Danelian2021) described an isolated cephalon from Abrahamyan’s collection, housed at IGSNASRAGM/PS, though the exact locality was not provided. The specimen, nearly complete but partially exfoliated, was identified as Omegops cf. accipitrinus (Phillips, Reference Phillips1841), a phacopid trilobite from the Upper Famennian. Omegops accipitrinus represents an index fossil for the Upper Devonian (late Famennian, Strunian, Wocklumeria Zone).
Omegops armeniensis sp. nov.
Phacopid trilobites from the Armash section, Armenia. (a–q) Omegops armeniensis sp. nov. (a–c) IGS/Ar-T1, cephalon in frontal, dorsal and lateral views, level 24/3. (d) IGS/Ar-T9, detail of a left eye, level 24/4. (e) IGS/Ar-T10, detail of a left eye, level 24/4. (f–h) IGS/Ar-T4, exfoliated cephalon in lateral, frontal and dorsal views, level 24/3. (i–k) IGS/Ar-T3, exfoliated enrolled exoskeleton in frontal, dorsal and lateral views, level 24/3. (l–p) IGS/Ar-T2, partly exfoliated enrolled exoskeleton, cephalon in dorsal, frontal views, pygidium in dorsal view, cephalon in lateral view, level 24/3. Scale bars: 5 mm.
Phacopid trilobites from the Armash section, Armenia. (a–n) Omegops armeniensis sp. nov. (a–c) IGS/Ar-T5, cephalon in dorsal, lateral and frontal views, level 24/3. (d) IGS/Ar-T7, cephalon in lateral view, level 24/3. (e–i) IGS/Ar-T6, cephalon in left and right lateral, ventral, dorsal and frontal views, level 24/3. (j–l) IGS/Ar-T8, thoracopygon in dorsal, lateral and frontal views, level 24/4. (m–n) IGS/Ar-T7, exuvia in oblique views, detail of the left eye and the cephalic doublure, level 24/3. Scale bars: 5 mm.
Etymology. The species name is derived from Armenia, the country of origin where the new species was discovered.
Holotype. Cephalon, IGS/Ar-T1 (Fig. 4a–c).
Paratypes. Enrolled exoskeletons, IGS/Ar-T2 (Fig. 4l–p) and IGS/Ar-T3 (Fig. 4i–k); cephala IGS/Ar-T4 (Fig. 4f–h), IGS/Ar-T5 (Fig. 5a–c), IGS/Ar-T6 (Fig. 5e–i); exuvia IGS/Ar-T7 (Fig. 5d, m–n); thoracopygon IGS/Ar-T8 (Fig. 5j–l); isolated eyes IGS/Ar-T9 (Fig. 4d) and IGS/Ar-T10 (Fig. 4e).
Type locality. Armash section, south-central Armenia.
Type stratum. Famennian (thin-bedded coral- and brachiopod-rich limestones), Middle-Upper expansa Zone to praesulcata Zone, Do VI, Late Devonian.
Studied material. The studied material comes from the Ararat Province, south-central Armenia (Fig. 1a), and more specifically from two calcareous levels (levels 24/3 and 24/4) of the Armash section (Fig. 2), which also contain rugose corals, brachiopods, gastropods, and conodonts. The material includes the holotype, three mostly exfoliated enrolled exoskeletons, three fragmented cephala, three complex visuals, one exuvia showing a Salterian moulting pattern, and one fragmented and mostly exfoliated thoraco-pygidium. The material is often incomplete and partially or largely exfoliated, sometimes being difficult to prepare for observations. Due to insufficient preservation of the cephala, additional material is required. However, the phacopid trilobites are assigned to Omegops Struve, Reference Struve1976, a common taxon in the upper Famennian, based on the cephalic shape and ornamentation.
Diagnosis. Cephalon narrow; glabella with weakly divergent axial furrows; median portion of L1 with few tubercles; visual complex with 15 vertical lens files and maximum six lenses per file; inter-lensar sclera thickened dorsally and more lightly ventrally; subocular and postocular pads with tubercles. Pygidium rather long; pygidial axis composed of seven clear rings plus terminal piece; pygidial pleural field with six well-defined ribs; deep pleural furrows. Coarse pustular and dense tubercles on cephalon. Dominant pits on the lateral border. Pygidium with a fine granulation.
Description. Cephalon: Length/width ratio about 0.66. Dorsal view: Glabellar ratio of width at L1/maximum width of frontal lobe about 0.51, wide at the base. Glabella lowered, bounded by relatively deep and divergent (56°) axial furrows. Frontal outline parabolic anteriorly. Frontal lobe barely overhanging a distinct preglabellar furrow. S2 and S3 indistinct. S1 continuous and hardly curved in its middle part, subparallel to S0, becoming deeper in its distal portions. L1 shorter (sag.) than L0 (half the length of L0), with a slightly inflated and wide (tr.) median portion relative to its total width. L1 with slightly transverse lateral lobes, separated from its median part by a weak exsagittal furrow. S0 distinct, almost transverse, hardly curved in its middle portion. L0 relatively short (tr.), with lateral lobes not defined. Reniform visual surface, with 15 dorso-ventral files of lenses and a maximum of six lenses per file, smaller lenses in the anterior part. Inter-lens sclera thickened dorsally and more lightly ventrally. Palpebral furrow distinct and strongly convex (abax.). Exsagittal length of the postocular genal field less than half the length (exsag.) of the posterior border. Lateral view: Glabella sloping obliquely and then dorsoventrally in its anterior part. Outline of L1 slightly curved. L0 as high as the glabella at its maximum convexity. Anterior border short, ridge-like, projecting slightly forwards. Marginulation extending virtually to the genal angle. Posterior border furrow deep and narrow. Lateral border furrow moderately narrow and shallow at the front of the eye, becoming narrower towards the back. Subocular librigenal field distinct at the back of the eye with tubercles forming a subocular pad, fading towards the front of the eye. Posterolateral border furrow distinct, and the posterolateral border widens at the genal angle. Postocular genal field swollen, with tubercles forming a postocular pad. Frontal view: Glabellar outline semicircular, becoming oblique laterally (divergence of 49°). Palpebral area lower than the inflated palpebral lobe. Ventral view: Vincular furrow relatively deep and wide (sag.), broadly and evenly curved. Posterior band of the cephalic doublure long (sag.), with a hypostomal suture transverse medially. Pygidium: length/width ratio about 59% (excluding the half ring), with its maximum width (tr.) in front of its midlength (sag.) in dorsal view. Posterior outline broadly rounded. Pygidial axis very long (sag.) and relatively wide (tr.), strongly tapered, with almost complete terminal closure. At least seven distinct rings in the pygidial axis. Pygidial pleural field with six distinct ribs delimited by deep pleural furrows. Interpleural furrows indistinct. Pseudo-articulating half rings suggested by expanded inter-rings furrows. Sculpture: Cephalon covered with coarse pustular tubercles lacking granules, with coarsest elements larger in diameter than lenses in eye, dense across the entire composite lobe. Few small pits added to tubercles on the frontal lobe. Dominant pits on the lateral border Aligned tubercles on the palpebral lobe. Posterior band of the cephalic doublure with fine short ridges. Pygidium with a fine granulation. Short scaly ridges and pits laterally on the ventral portion of the anterior border.
Comparison. The holotype is relatively well-preserved and complete. Although the associated cephala and complete exoskeletons are less well-preserved, they nonetheless contribute valuable information, for completing the species description. The sizes range from approximately 12 to 30 in width and from 7 to 20 in length, showing a typically linear increase (Fig. 7a). The size range is consistent with that of other Omegops species; size is not a discriminating character at the species level (Fig. 7b). The morphological features observed and discussed in the description support the assignment of this species to the genus Omegops and enable comparisons with all other globally known Omegops species of equivalent age.
Schematic representation of (a) four visual surfaces in Omegops accipitrinus (Phillips, Reference Phillips1841) and (b) seven visual surfaces in Omegops armeniensis sp. nov., following the method of Thomas (Reference Thomas1998). Front of visual surface is left; numbers below drawing denominate individual dorso-ventral files, counting from the front (1-15); roman numerals denote successive horizontal rows; numbers in boxes indicate a surface having that lens present in all visual surfaces.
Cephalic ratios of (a) Omegops armeniensis sp. nov. and (b) seven Omegops species and one Clarksonops species. Sizes are based on specimens either illustrated or reported in the literature: O. accipitrinus and O. maretiolensis from Crônier et al. (Reference Crônier, Denayer, Feist and Mottequin2025), O. tilabadensis from Ghobadi Pour et al. (Reference Ghobadi Pour, Popov, Omrani and Omrani2018), O. mobilis and Clarksonops junggariensis from Zong (Reference Zong2023a), O. cornelius from Weber (Reference Weber2000, p. 543), and O. paiensis from Farsan (Reference Farsan1998, p. 25).
Omegops armeniensis sp. nov. shares with the type species O. accipitrinus a visual surface with 15 dorso-ventral files, a relatively wider occipital ring (tr.), and intervening pits that dominate in appearance on the lateral border. However, Omegops armeniensis sp. nov. differs by having a maximum of six lenses per vertical file (only four in O. accipitrinus), a subocular and lateral border furrow separately distinguishable, with an intervening area that is inflated and tuberculated (appearance of an expanded furrow anteriorly in O. accipitrinus), a postocular pad covered with several tubercles (not smooth or only with a few tubercles in O. accipitrinus), and less elongated (tr.) lateral preoccipital lobes. While the Armenian species exhibits six pleural rib pairs, O. accipitrinus from Belgium seems to possess only five distinct pleural rib pairs (Crônier et al. Reference Crônier, Denayer, Feist and Mottequin2025).
Omegops armeniensis sp. nov. resembles to some extent O. maretiolensis, known from Belgium, based on a visual surface with 15 dorso-ventral files, a postocular pad covered with several coarse tubercles, and a pygidial axis with at least seven distinct rings and six pleural ribs. Omegops armeniensis sp. nov. differs from O. maretiolensis by having a maximum of six lenses per vertical file (only four in O. maretiolensis), intervening pits that dominate in appearance on the lateral border (a row of coarse tubercles in O. maretiolensis), a subocular and lateral border furrow that are separately distinguishable, with an intervening area that is inflated and tuberculated (appearance of an expanded furrow anteriorly in O. maretiolensis), and less elongated (tr.) lateral preoccipital lobes.
Omegops armeniensis sp. nov. differs from O. cornelius from Europe and Iran by having a maximum of six lenses per vertical file (only four in O. cornelius), intervening pits that dominate in appearance on the lateral border (coarse tubercles in O. cornelius), and six pleural rib pairs (only five in O. cornelius).
Omegops armeniensis sp. nov. shares with O. tilabadensis from northern Iran a visual surface with 15 dorso-ventral files, coarse pustular tubercles lacking granules on the glabella, and a subocular pad with coarse tubercles. However, the former species differs primarily in having a maximum of six lenses per vertical file (only four in O. tilabadensis), and six distinct pleural ribs (only five in O. tilabadensis).
Omegops armeniensis sp. nov. is distinguishable from O. paiensis from Afghanistan by having coarse pustular tubercles that lack granules on the glabella (less coarse and pustular in O. paiensis), a maximum of six lenses per vertical file (seven in O. paiensis), an inter-lens sclera thickened dorsally and ventrally (only dorsally in O. paiensis), and a pygidial axis with six distinct pleural ribs (only 4-5 in O. paiensis).
Omegops armeniensis sp. nov. differs from O. mobilis from northwest China by having a maximum of six lenses per vertical file (only four in O. mobilis; see Zong, Reference Zong2023a, figs 2.2-2.3), a subocular and lateral border furrow separately distinguishable, with an intervening area that is inflated and tuberculated (appearance of an expanded furrow anteriorly in O. mobilis; see Zong, Reference Zong2023a, figs 2.2-2.3). Moreover, the Armenian specimens possess a postocular pad covered with several tubercles, whereas it is smooth in O. mobilis (see Zong, Reference Zong2023a, figs 2.1-2.3, 2.7), and a pygidial pleural field with six distinct ribs, whilst O. mobilis displays only five.
Omegops armeniensis sp. nov. shows some morphological features close to Clarksonops junggariensis Crônier & Waters (Reference Crônier and Waters2023), known from northwest China (see Crônier et al. Reference Crônier, Denayer, Feist and Mottequin2025 for discussion on species), such as the reniform visual surface with 15 dorso-ventral files of lenses and intervening pits that dominate in appearance on the lateral border. However, the new species differs from Clarksonops junggariensis by having a preoccipital ring that is erased and depressed, or barely detectable in oblique light, six lenses per vertical file (five lenses per vertical file in Clarksonops junggariensis; see Zong, Reference Zong2023a, figs 4.4, 4.9), a subocular and lateral border furrow separately distinguishable, with an intervening area that is inflated and tuberculated (appearance of an expanded furrow anteriorly covered by sparse fine granules in Clarksonops junggariensis; see Zong, Reference Zong2023a, figs 4.4, 4.9), a postocular pad covered with several tubercles (only sparse granules in Clarksonops junggariensis; see Zong, Reference Zong2023a, figs 4.4, 4.9), and coarse, flattened tubercles on the anterior glabellar lobe (smaller tubercles with granules in Clarksonops junggariensis; see Zong, Reference Zong2023a, figs 4.14).
5. Discussion and conclusions
The genus Omegops is palaeogeographically widespread, comprising several species that exhibit subtle, but distinct morphological features. While the number of dorso-ventral files of lenses remains constant across the genus Omegops, a key distinguishing feature among its species is the variation in the number of lenses per file. For instance, species such as Omegops accipitrinus, O. maretiolensis, O. cornelius, O. mobilis, and O. tilabadensis typically have a maximum of four lenses per vertical file, whereas species like O. armeniensis and O. paiensis possess a higher number of lenses. This variation in the number of lenses per file is often used to differentiate between species. The differences in the number of lenses in trilobite eyes could reflect adaptations to specific environmental conditions, although further studies are needed to confirm this hypothesis (Schoenemann Reference Schoenemann2025). Among arthropods, species with a relatively large number of lenses, which might indicate a more developed visual system, could be associated with life preference in clearer or more variably illuminated environments. Conversely, a reduced number of lenses could be linked to more specifically enlightened conditions (Fortey Reference Fortey2004; Clarkson et al. Reference Clarkson, Levi-Setti and Horváth2006; Vannier et al. Reference Vannier, Schoenemann, Gillot, Charbonnier and Clarkson2016).
The postocular pad represents another key feature observed across Omegops species and shows substantial variation, as first noted by Struve (Reference Struve1976). For example, species such as Omegops armeniensis, O. tilabadensis, O. maretiolensis, and O. cornelius, have postocular pads covered with coarse tubercles, while others, like O. accipitrinus and O. mobilis, display smoother surfaces or fewer tubercles. These morphological differences in the postocular pad may reflect environmental factors, such as variations in the nature of the substrate in which they inhabited.
Among the species exhibiting small but distinct features, Omegops armeniensis sp. nov. and O. accipitrinus are characterized by intervening pits along the lateral border, which dominate in appearance. In contrast, species such as Omegops maretiolensis and O. cornelius, exhibit coarse tubercles.
The pygidium and its number of pleural ribs also serve as important distinguishing features among species. For instance, Omegops armeniensis and O. maretiolensis possess six pairs of pleural ribs, whereas O. tilabadensis, O. paiensis, O. mobilis, O. accipitrinus, and O. cornelius exhibit only five. This characteristic can be used to differentiate closely related species with otherwise similar morphologies. However, it remains unclear whether the number of pleural ribs is linked to functional morphology, serving as a protective mechanism or contributing to locomotion efficiency, particularly in species inhabiting different environmental conditions. The observed variation in pygidial structure could indicate different evolutionary pressures or responses to varying ecological conditions, but the functional significance for such structures remains a topic of ongoing research.
Based on small but consistent differences in the number of pygidial pleural ribs, Ghobadi Pour et al. (Reference Ghobadi Pour, Popov, Omrani and Omrani2018) suggested the existence of two geographically isolated Omegops lineages that diverged during the pre-Strunian period. One lineage includes taxa from the Middle East and Northwest China (Junggar), characterized by four to five pairs of pleural ribs. The other comprises taxa from Western Europe and North Africa, which typically exhibit six or more pleural ribs. However, Omegops accipitrinus from Belgium challenges this trend, with only five pairs of pleural ribs, suggesting a more complex morphological variation than initially recognized. Furthermore, the newly illustrated specimens of Omegops armeniensis sp. nov. from Armenia, also possesses six pairs of pygidial ribs, providing further insights into the morphological diversity within these geographically isolated lineages.
Crônier & François (Reference Crônier and François2014) identified a bathymetrical gradient in the distribution of Famennian phacopid taxa, with Omegops being restricted to shallow-water deposits, a pattern also reported earlier by Chlupáč (Reference Chlupáč1977). This distribution is evident along both the South Laurussian and Northern and peri-Gondwanan margins. Omegops associations are typically found in shallow-water clastic limestones, likely representing deposition originated from lower shoreface to upper offshore environments during the late Famennian.
This pattern, as established by Crônier & François (Reference Crônier and François2014), is also evident in the studied Armenian section, where Omegops armeniensis sp. nov. is a major constituent of the benthic fauna, alongside abundant brachiopods and corals, inhabiting a limestone substrate that is rich in bioclasts within a shallow shelf nearshore setting. Additionally, it is worth noting that most documented occurrences of Omegops are restricted to tropical and subtropical palaeolatitudes. In terms of their demise, all phacopid genera became extinct during the end-Famennian Hangenberg Crisis, making the Famennian a critical interval marked by significant taxonomic turnovers and the decline in phacopid diversity. Chronostratigraphically, the Upper Devonian trilobite-bearing strata of the Armash section should correspond to a late Famennian pre-Hangenberg age, prior to the kockeli Zone.
Acknowledgements
This work is a contribution to the French project ‘Contrat de Plan Etat-Région ECRIN’, and to the French CNRS UMR 8198 Evo-Eco-Paleo. This study was supported by the Higher Education and Science Committee of the Ministry of Education, Science, Culture, and Sports of the Republic of Armenia (Research project 22RL-016). We thank A. Van Viersen (the Netherlands), an anonymous reviewer, and Y. Candela, the editor, for their helpful comments and suggestions. We extend our sincere gratitude to Dr. Vachik Hairapetian (Islamic Azad University, Isfahan) for his invaluable taxonomic consultations with conodont identifications. We are also indebted to Karine Asatryan (Institute of Chemical Physics after A. B. Nalbandyan) for conducting the SEM photographic work on conodonts. Special thanks to Ashot Simonyan, an amateur palaeontologist, who first indicated us the presence of trilobites in the studied area.
Competing interests
The authors declare none.
