Non-technical Summary
We report a new stratigraphic section in the Precordillera Basin (western Argentina), at Zanja Honda Creek, San Juan Province, embracing a remarkable record of the Silurian-Devonian transition recognized on the basis of graptolites, brachiopods, trilobites, and stratigraphic correlation. The succession containing the boundary corresponds to particular siliciclastic facies of the uppermost part of the Los Espejos Formation (LEF). Dark shaly mudstones above record the homalonotid trilobite Burmeisteria notica, indicating the Lochkovian-Pragian lower interval of the Talacasto Formation. In coquinites of the base of the column, brachiopods co-occur with trilobites, all characteristic of the Ludlow to Pridolian upper third of the LEF elsewhere. The dalmanitid Pachimocaspis pachimocensis new genus new species has also been recognized from these coquinites. However, this brachiopod and trilobite association disappears in the overlying dark nodular siltstone interval, replaced by a brachiopod association (which indicates an earliest Lochkovian age), and a monospecific, very abundant record of Pachimocaspis pachimocensis n. gen. n. sp. Thus, we recognize a neat faunal turnover around the Silurian-Devonian boundary, both in brachiopods and trilobites, in high paleolatitudinal basins from southwestern Gondwana. Despite its typical glabellar frontal lobe, shape of the visual surface, and morphology of aglabellar furrows and lobes, the pygidium of Pachimocaspis pachimocensis n. gen. n. sp. lacks the usual shape of the family, as well as a caudal spine. Instead, this new taxon exhibits a subelliptical pygidial contour with a smooth margin. In addition, thoracic pleural tips are blunt with variation along the thorax, in contrast with the characteristic, evenly spinose dalmanitid morphology. Such a mosaic combination challenges the systematic position of this new taxon within the family, especially regarding Silurian-Devonian subfamilies usually recognized in basins from high paleolatitudes. Instead, it resembles extra-Gondwanic, early Silurian synphoriines from low paleolatitudes.
Introduction
The documentation of the Silurian-Devonian transition in southwestern Gondwana is critical for discussions on complex paleobiogeographic and evolutionary processes involving its endemic marine faunas. A number of major issues include the recognition of the Silurian Afro-South American paleobiogeographic realm on the basis of brachiopods (Benedetto and Sánchez, Reference Benedetto, Peralta and Sánchez1996), and the turnover into the diversification of endemic brachiopods and trilobites defining the Malvinoxhosan (= Malvinokaffric) Paleobiogeographic Realm during the Early Devonian (Abe and Lieberman, Reference Abe and Lieberman2009, Reference Abe and Lieberman2012; Penn-Clarke and Harper, Reference Penn-Clarke and Harper2023).
In this context, an Early Devonian diversification of dalmanitid trilobites has been hypothesized as an evolutionary burst as least as extraordinary as that exhibited by the calmoniid trilobites, a family interpreted as a hallmark of the Malvinoxhosan Realm (Eldredge and Ormiston, Reference Eldredge, Ormiston, Gray and Boucot1979).
However, the stratigraphic record of the Silurian-Devonian interval is known from just very few localities, usually recording no dalmanitids. On the other hand, stratigraphic sections recording this transition have no accurate biostratigraphic control based on fauna, because the classic early Paleozoic biostratigraphic marker groups (e.g., conodonts and graptolites) virtually vanish after the earliest Devonian in southwestern Gondwana. Some of the best-known localities recording faunas around the Silurian-Devonian transition include the top of the Catavi Formation on the southwestern slope of Cerro Yahachanca, nearly 4 km southeast of Cochabamba, central Bolivia (Edgecombe and Ramsköld, Reference Edgecombe and Ramsköld1994); the top of El Carmen Formation, northwest of Roboré, southeastern Bolivia (Edgecombe and Fortey, Reference Edgecombe and Fortey2000); and the Cerro del Fuerte, Cerro La Chilca and Talacasto sections recording the boundary between the Los Espejos and the Talacasto formations in the Precordillera Basin, western Argentina (Benedetto et al., Reference Benedetto, Racheboeuf, Herrera, Brussa and Toro1992; Carrera et al., Reference Carrera, Montoya, Rustán and Halpern2013; Mestre et al., Reference Mestre, Gomez, Garcias, Corradini, Heredia, Liao and Valenzuela-Rios2017; Gómez et al., Reference Gómez, Mestre, Corradini and Heredia2021, Reference Gómez, Mestre and Heredia2024). Aside from zonation schemes based on regional benthic macrofaunas (e.g., brachiopods and trilobites), the recognition of relative ages has been improved by palynological (e.g., García Muro et al., Reference García-Muro, Rubinstein and Steemans2014, Reference García Muro, Rubinstein and Rustán2017, Reference García Muro, Rubinstein and Steemans2018) and conodont records (Mestre et al., Reference Mestre, Gomez, Garcias, Corradini, Heredia, Liao and Valenzuela-Rios2017; Gómez et al., Reference Gómez, Mestre, Corradini and Heredia2021, Reference Gómez, Mestre and Heredia2024).
The relevance of new latest Silurian–earliest Devonian data has already been commented on in previous taxonomic contributions dealing with Early Devonian dalmanitids from Argentina (Rustán and Vaccari, Reference Rustán and Vaccari2012; Randolfe et al., Reference Randolfe, Rustán and Bignon2020, Reference Randolfe, Rustán and Bignon2022). New findings are critical to check this conservative and cosmopolitan group of trilobites in light of a number of hypotheses on the origin of endemisms in southwestern Gondwana, their evolutionary and biogeographic patterns, and their interpretation in light of evidence coming from other, more geographically restricted, groups like the calmoniids.
Here, we report a new locality recording the Silurian-Devonian transition in particular facies of classical well-known lithostratigraphic units from the Precordillera Basin. Based on Silurian–Early Devonian graptolites, brachiopods, and trilobites, we provide insights on age, faunal turnover, and regional correlation of this new stratigraphic record with coeval classical stratigraphic sections in southern South America.
A new dalmanitid trilobite genus and species is recognized in the Silurian-Pragian in South America, so we comment in some detail on its taxonomic/phylogenetic and biostratigraphic implications. We focus as well on implications of this new trilobite for the geographic distribution of Early Devonian relatives and the early diversification of endemic Malvinoxhosan dalmanitids.
Geological and stratigraphic setting
The studied area corresponds to the Precordillera Basin of San Juan Province, in western Argentina, where the Silurian-Devonian transition has been recognized (Benedetto et al., Reference Benedetto, Racheboeuf, Herrera, Brussa and Toro1992; Mestre et al., Reference Mestre, Gomez, Garcias, Corradini, Heredia, Liao and Valenzuela-Rios2017; Gómez et al., Reference Gómez, Mestre, Corradini and Heredia2021, Reference Gómez, Mestre and Heredia2024) in the uppermost siliciclastics of the Los Espejos Formation (Cuerda, Reference Cuerda1965). This unit overlies in concordance with the Hirnantian-early Silurian La Chilca Formation (Cuerda, Reference Cuerda1965) and is paraconformably covered by the Early Devonian Talacasto Formation (Padula et al., Reference Padula, Rolleri, Mingramm, Criado Roqué, Flores, Baldis and Oswald1967). The La Chilca-Los Espejos formation boundary would correspond to the Wenlock (Benedetto et al., Reference Benedetto, Racheboeuf, Herrera, Brussa and Toro1992).
In general, the Los Espejos Formation consists mainly of a Wenlockian–Lochkovian sedimentary succession (Benedetto et al., Reference Benedetto, Racheboeuf, Herrera, Brussa and Toro1992; Mestre et al., Reference Mestre, Gomez, Garcias, Corradini, Heredia, Liao and Valenzuela-Rios2017; Gómez et al., Reference Gómez, Mestre, Corradini and Heredia2021, Reference Gómez, Mestre and Heredia2024) deposited in three different stages of a transgressive-regressive marine cycle in a foreland basin, evolving from a muddy platform without wave influence, into an inner platform with storm influence, and finally to a transitional shallow platform with high terrigenous supply dominated by storms (Astini and Piovano, Reference Astini and Piovano1992; Astini and Maretto, Reference Astini and Maretto1996). These paleoenvironmental changes allowed Cuerda (Reference Cuerda1969) to subdivide the formation into three members in the stratotype, the Cerro La Chilca section, ~40 km south-southeast of Jáchal City. The lower member consists of iron-rich oolitic sandstones, followed by green to reddish shales with phosphate concretions. The middle member is composed of abundant yellowish-greenish siltstones, with sandstones and brachiopod-dominated coquinas. Finally, the upper member shows abundant green sandstones and multispecific coquinas, yellowish siltstones, and occasional paraconglomerates at the top.
The Zanja Honda section, studied herein, is a new fossil locality in the Jáchal River section, located at approximately km 244 of National Route 150 (NR150) adjacent to the east of the Zanja Honda Creek, nearly 15 km to the west-northwest of Jáchal City and 5 km to the west of the Pachimoco locality (Fig. 1). The structural features of this area have been studied by von Gosen (Reference von Gosen1997, fig. 12), who described a complex system of southwest–northeast reverse faults with mainly northwestern vergence, associated with overturned anticlines and synclines. These structural characteristics are recognized at the Zanja Honda area where minor reverse faults produced omissions and repetitions of the sedimentary succession and folded the strata in vertical or overturned angles. However, we identified a nearly 75 m thick overturned succession, with stratigraphic continuity, recording a complete transition between the Los Espejos and the Talacasto formations (Fig. 2). It initiates in the northwestern flank of an anticline to the south of NR150, in a complex core preceded by a fault, and finishes to the north of NR150 adjacent to the Jáchal River (Fig. 2).
Geologic map of the Silurian-Devonian outcrops of the new fossil locality at Zanja Honda Creek area, nearly 15 km to the northwest of Jáchal City in the Precordillera of the San Juan Province, Argentina.
Stratigraphic column of the Silurian-Devonian succession at the Zanja Honda section, involving the upper part of the Los Espejos Formation and the lower part of the Talacasto Formation. The Silurian-Devonian transition is supposed to be near the base of the column in the Los Espejos Formation.
The overlying Talacasto Formation shows its typically dark, muddy, nodular, lowermost interval (early but not earlymost Lochkovian, to Pragian), although the basal boundary is usually covered by NR150. In contrast, the Los Espejos Formation exhibits some important stratigraphic and paleoenvironmental differences regarding its stratotype. Overlying typical coquinite layers of the upper part of the upper member, the uppermost levels of this unit in the Zanja Honda Section abruptly lack the sandstone-shell bed records, displaying instead a dark claystone-siltstone dominance with nodules and gutters, a transition to sandstones, and finally a reddish siltstone interval to the top (Fig. 2). Somewhat similar lithological changes have been noted by Benedetto et al. (Reference Benedetto, Racheboeuf, Herrera, Brussa and Toro1992) and Carrera et al. (Reference Carrera, Montoya, Rustán and Halpern2013) in the Cerro del Fuerte section (located nearly 10–11 km to the southeast of Jáchal City). These authors related the particular upper interval with effects of a regression in the depocenter (supposed to be in the Jáchal region), which developed a restricted nearshore area in the northernmost part of the basin.
Fossils and ages, with focus on brachiopod assemblages
The Los Espejos Formation contains well-preserved shelly faunas dominated by late Silurian Afro-South American Realm brachiopods (Copper et al., Reference Copper, Benedetto and Toro1988; Benedetto, Reference Benedetto1988; Benedetto and Toro, Reference Benedetto and Toro1989b; Benedetto et al., Reference Benedetto, Racheboeuf, Herrera, Brussa and Toro1992, Reference Benedetto, Peralta and Sánchez1996; Benedetto and Sánchez, Reference Benedetto, Sánchez, Copper and Jin1996; Benedetto and Franciosi, Reference Benedetto and Francioisi1998; Benedetto and Montoya, Reference Benedetto and Montoya2015; Leone and Benedetto, Reference Leone and Benedetto2019; Benedetto et al., Reference Benedetto, Lavié and Salas2024). Traditional index fossils, e.g., conodonts and graptolites, however, are generally scarce and poorly diversified, making it difficult to accurately define the age of particular levels within the formation (Lopez et al., Reference Lopez, Conde, Braeckman, Segura, Drovandi, Bueno and Abarca2024). The thick succession exposed at Cerro del Fuerte, is considered the best reference section of the Los Espejos Formation across the Silurian-Devonian boundary, due to its abundant fossil content, exceptional exposures, and stratigraphic continuity.
Benedetto et al. (Reference Benedetto, Racheboeuf, Herrera, Brussa and Toro1992) studied in some detail the rhynchonelliform brachiopods from the Los Espejos Formation at Cerro del Fuerte, recognizing four faunal assemblages, each characterized by a distinctive set of species. The upper third of the formation consists of bioturbated mudstones and hummocky cross-stratified sandstones interpreted as deposited in a storm-dominated muddy shelf (Sánchez et al., Reference Sánchez, Astini and Piovano1991; Astini and Piovano, Reference Astini and Piovano1992). This interval contains the ‘third faunal assemblage’ characterized by Coelospira expansa Benedetto and Toro Reference Benedetto and Toro1989a; Plectodonta (Loxodonta) minima Herrera and Benedetto in Benedetto et al., Reference Benedetto, Racheboeuf, Herrera, Brussa and Toro1992; Salopina sanjuanensis Benedetto in Benedetto et al., Reference Benedetto, Racheboeuf, Herrera, Brussa and Toro1992; Isorthis (Protocortezorthis) cuyanum Benedetto in Benedetto et al., Reference Benedetto, Racheboeuf, Herrera, Brussa and Toro1992; Leptaena argentina Thomas, Reference Thomas1905; Clarkeia alta Benedetto in Benedetto et al., Reference Benedetto, Peralta and Sánchez1996; and Amosina paolae Racheboeuf in Benedetto et al., Reference Benedetto, Racheboeuf, Herrera, Brussa and Toro1992 (Fig. 3). Associated species that are also present in the first and second faunal assemblages are Harringtonina australis Boucot, Reference Boucot1972; Australina jachalensis Clarke, Reference Clarke1913b; and Amosina fuertensis (Kayser, Reference Kayser1897). Because all these species and at least four genera are endemic to the Precordillera Basin, their chronologic value is limited. Among those brachiopods recorded outside the Afro-South American Realm, the subgenus Plectodonta (Plectodonta) Kozłowski, Reference Kozłowski1929 is suggestive of a Pridolian or younger age because most of their species in the North Atlantic Silurian Realm and the Old World Devonian Realm (Boucot and Blodgett, Reference Boucot, Blodgett, Brunton, Cocks and Long2001) are confined to the Pridolian-Lochkovian time span. The subgenus Isorthis (Protocortezorthis) Johnson and Talent, Reference Johnson and Talent1967 is particularly well represented in the upper Silurian and Devonian of Europe and North America, but it also occurs in the Lochkovian and Pragian of New Zealand and Australia. Conodonts from the upper part of the formation belong to the Kockelella variabilis variabilis Biozone, indicating a Ludlow age (Albanesi et al., Reference Albanesi, Beresi and Peralta2006; Heredia et al., Reference Heredia, Mestre, Milana, Díaz-Martínez and Rábano Gutiérrez del Arroyo2007). However, the assemblage recovered recently from the upper part of the Los Espejos Formation at the Talacasto area points to a Pridolian age (Ozarkodina eosteinhornensis Biozone), reaching locally up to the Caudicriodus hesperius Biozone of early Lochkovian age (Mestre et al., Reference Mestre, Gomez, Garcias, Corradini, Heredia, Liao and Valenzuela-Rios2017; Gómez et al., Reference Gómez, Mestre, Corradini and Heredia2021, Reference Gómez, Mestre and Heredia2024). A Pridolian age was also documented by Mestre (Reference Mestre2009) for the top of the Tambolar succession in the southern portion of the basin. This is consistent with the Ludfordian–Pridolian age indicated by the miospores from that same locality (Rubinstein and García-Muro, Reference Rubinstein and García-Muro2013).
(1–7) Brachiopods from the lower shell beds, Los Espejos Formation, Zanja Honda: (1) Amosina pediculorum Racheboeuf in Benedetto et al., Reference Benedetto, Racheboeuf, Herrera, Brussa and Toro1992, INGEO-PI 2303, ventral valve exterior; (2) Harringtonina australis Boucot, Reference Boucot1972, INGEO-PI 2293a, ventral valve exterior; (3, 4) Australina jachalensis Clarke, Reference Clarke1913b: (3) INGEO-PI 2284a, dorsal valve exterior; (4) INGEO-PI 2284c, ventral valve internal mold; (5) Clarkeia aff. Clarkeia bodenbenderi (Kayser, Reference Kayser1897), INGEO-PI 2294, dorsal valve exterior; (6, 7) Castellaroina aff. Castellaroina fuertensis Benedetto and Montoya, Reference Benedetto and Montoya2015: (6) INGEO-PI 2293b, ventral valve exterior; (7) INGEO-PI 2389, ventral valve internal mold. (8–15) Brachiopods from the upper beds N1-N11, Los Espejos Formation, Zanja Honda: (8) Isorthis (Protocortezorthis) sp. indet., INGEO-PI 2290, dorsal valve internal mold; (9) Leptaena sp. indet., INGEO-PI 2284b, ventral valve exterior; (10) Australina sp. indet., INGEO-PI 2300, dorsal valve internal mold; (11, 12) cf. Septothyris sp. indet., INGEO-PI 2285, internal mold of conjoined shell: (11) ventral view; (12) dorsal view; (13–15) Orthostrophia meridionalis Benedetto in Benedetto et al., Reference Benedetto, Racheboeuf, Herrera, Brussa and Toro1992: (13) INGEO-PI 2288, internal mold of dorsal valve; (14) INGEO-PI 2296, ventral valve exterior, latex cast; (15) INGEO-PI 2291, ventral valve interior. Scale bars = 2 mm.
The fourth brachiopod assemblage recognized by Benedetto et al. (Reference Benedetto, Racheboeuf, Herrera, Brussa and Toro1992) is restricted to an interval near the summit (not topmost) of the Cerro del Fuerte stratigraphic succession, consisting of reddish to purple bioturbated mudstones, fine-grained sandstones, and ferruginous mudstones. Fossils come mainly from marly nodules at the base followed by extensively bioturbated mudstones containing scattered articulated brachiopod shells and corals (Carrera et al., Reference Carrera, Montoya, Rustán and Halpern2013). It should be noted that this peculiar lithofacies is absent in most of the localities in which the Silurian-Devonian transition is exposed. Besides the Cerro del Fuerte section, it has been recognized in the Cerro La Chilca section and in the Zanja Honda section studied in this paper. At Cerro del Fuerte, the assemblage from this interval is characterized by a quite different array of taxa denoting that a significant faunal turnover took place between the third and the fourth brachiopod assemblage. This biostratigraphic change is marked by numerous extinctions and the appearance of genera and species previously unrecorded (Benedetto, Reference Benedetto2007). The fourth brachiopod assemblage consists of Orthostrophia meridionalis Benedetto in Benedetto et al., Reference Benedetto, Racheboeuf, Herrera, Brussa and Toro1992; Isorthis (Tyersella) megamyaria Benedetto in Benedetto et al., Reference Benedetto, Racheboeuf, Herrera, Brussa and Toro1992; Molongella keideli Benedetto in Benedetto et al., Reference Benedetto, Racheboeuf, Herrera, Brussa and Toro1992; Australocoelia intermedia Benedetto and Herrera in Benedetto et al., Reference Benedetto, Racheboeuf, Herrera, Brussa and Toro1992; Coelospira sp. indet.; Leptaena sp. indet.; and Anoplia sp. indet. Associated species are Amosina paolae and Clarkeia cf. Clarkeia alta, also present in the underlying levels. Concerning the age, Orthostrophia Hall, Reference Hall1883 is present in the Pridolian-Lochkovian of Bohemia, North America, Australia, and North China, whereas Molongella Savage, Reference Savage1974 is confined to the Pridolian-Pragian interval of Bohemia and Australia. Australocoelia Boucot, Reference Boucot1972 is restricted to the Early Devonian of the Malvinoxhosan Realm. In most features, the species Australocoelia intermedia is morphologically intermediate between the Silurian Harringtonina australis and its probable descendent Australocoelia tourteloti Boucot and Gill, Reference Boucot and Gill1956, which is abundant during the Early Devonian of the Precordillera Basin. The presence of the widespread chonetoid Anoplia Hall and Clarke, Reference Hall and Clarke1892 is significant because it is confined to the Lower Devonian (Lochkovian–upper Emsian) according to Herrera (Reference Herrera1995). Finally, a number of terebratulids have been collected from the same beds. A preliminary revision of this material suggests the genus Septothyris Cooper and Williams, Reference Cooper and Williams1935, which ranges from the Givetian–Frasnian of Bolivia, Africa, Libya, and eastern North America (Lee et al., Reference Lee, Mackinnon, Smiornova, Baker, Jin, Sun and Moore2006). Recently, Mestre et al. (Reference Mestre, Gomez, Garcias, Corradini, Heredia, Liao and Valenzuela-Rios2017) and Gómez et al. (Reference Gómez, Mestre, Corradini and Heredia2021, Reference Gómez, Mestre and Heredia2024) recorded the conodont Caudicriodus hesperius (Klapper and Murphy, Reference Klapper and Murphy1975) in the uppermost beds of the Los Espejos Formation, which marks the base of the Devonian. Thus, all biostratigraphic evidence led us to confirm the Early Devonian (Lochkovian) age for the fourth assemblage.
The other locality where the Los Espejos Formation culminates with a comparable purple interval is Cerro La Chilca. These beds have yielded a fauna dominated by Australocoelia intermedia and a low diversity palynomorph assemblage of Early Devonian age (García Muro et al., Reference García Muro, Rubinstein and Rustán2017).
In the studied Zanja Honda stratigraphic section, two different brachiopod assemblages have been recognized. The lower one comes from reworked, densely packed skeletal concentrations lying immediately above the hinge of a fold, at the base of the stratigraphic succession surveyed. It includes Harringtonina australis; Leptaena argentina; Castellaroina aff. Castellaroina fuertensis Benedetto and Montoya, Reference Benedetto and Montoya2015; Amosina pediculorum Racheboeuf in Benedetto et al., Reference Benedetto, Racheboeuf, Herrera, Brussa and Toro1992; Clarkeia aff. Clarkeia bodenbenderi (Kayser, Reference Kayser1897); Isorthis (Protocortezorthis) sp. indet.; and Australina jachalensis (Fig. 3.1–3.7). This fauna has no species in common with the overlying succession, except for the long-lived Australina jachalensis. Rather, the assemblage displays a mixture of species present in both the second and third brachiopod assemblages recognized by Benedetto et al. (Reference Benedetto, Racheboeuf, Herrera, Brussa and Toro1992). It shares with the second assemblage, likely of Ludlow age (Garcia Muro et al., Reference García Muro, Rubinstein and Steemans2018), a species of Clarkeia Kozłowski, Reference Kozłowski1923 very similar to Clarkeia bodenbenderi, and Castellaroina aff. Castellaroina fuertensis, both restricted to these levels. On the other hand, Leptaena argentina, Amosina pediculorum, and Isorthis (Protocortezorthis) are typical of the third assemblage which, as stated above, has been considered to be Pridolian in age in absence of other evidence. The dark noduliferous siltstone beds are characterized by a different assemblage dominated by Orthostrophia meridionalis, Isorthis sp. indet., Australina sp. indet., Amosina sp. indet., and a new species of Leptaena Dalman, Reference Dalman1828 similar to that recorded in the uppermost reddish beds of the Cerro del Fuerte succession. Interestingly, small specimens of the terebratulid Septothyris are present in level N3 (Fig. 3.11, 3.12). The presence of a representative of this order indicates Devonian age, because the record of the centronellidine terebratulid Brachyzyga? obsoleta Modzalevskaya, Reference Modzalevskaya2007 in the upper Silurian of central Asia (Tadzhikistan) does not seem to be conclusive. To date, the oldest well-dated members of this order are from the Lochkovian of Podolia (Ukraine) and Novaya Zemlya (Russia) according to Curry and Brunton (Reference Curry, Brunton, Moore and Part2007) and Modzalevskaya (Reference Modzalevskaya2007). In summary, this upper brachiopod assemblage, even though less diverse, is comparable to the ‘fourth assemblage’ from Cerro del Fuerte supporting an Early Devonian age.
Age interpretations based on brachiopod data and conodonts from other localities have recently been confirmed by the record of the graptolite Slovinograptus Urbanek, Reference Urbanek1997 in the coquinites of the very base of the column surveyed at Zanja Honda section, which would correspond to the Silurian-Devonian transition (Lopez et al., Reference Lopez, Rustán and Benedetto2025).
The reddish uppermost interval of the Los Espejos Formation in the Zanja Honda section is barren of fossils, as stated previously, and is overlaid by the typically dark shaly mudstones of the base of the Early Devonian Talacasto Formation, nearly covered by NR150. Although the very base of this unit yielded no fossils, nearly 100 m to the north of the route, the homalonotid trilobite Burmeisteria notica (Clarke, Reference Clarke1913a) has been recorded (Fig. 4), thus reliably indicating the Lochkovian-Pragian (Rustán et al., Reference Rustán, Waisfeld and Vaccari2020), in stratigraphic continuity.
Burmeisteria notica (Clarke, Reference Clarke1913a), INGEO-PI 1920, thoracopygidium from the (most probably) Pragian, greenish black muddy nodular lower interval of the Talacasto Formation at Zanja Honda section (north of National Route 150), San Juan Province, Argentina: (1) dorsal view; (2) ventral view; (3) lateral view. Scale bars = 10 mm.
Materials and methods
Fossils were prepared using pneumatic jacks and needles. Latex casts of external molds were inked and whitened with ammonium chloride (NH4Cl) for taking pictures under raking light using a Canon Power Shot S50 digital camera mounted on a Leica MZ75 binocular loupe.
Repositories and institutional abbreviations
Specimens are housed in the collection of the Geological Research Institute Dr. Emiliano Pedro Aparicio at University of San Juan (UNSJ), trilobites numbered as INGEO-PI-068, 128, 129, 1920, and 2249–2283, and brachiopods numbered from INGEO-PI-2284 to 2304.
Systematic paleontology (by J. J. Rustán and E. A. Randolfe)
Technical terms are abbreviated as traditional in trilobite descriptions: transverse/transversely (tr.), sagittal/sagittally (sag.), exsagittal/exsagittally (exsag.). In the description of the pygidial terminal piece, we counted any furrow that indicates segmentation, including those barely impressed (see discussions by Campbell, Reference Campbell1977). Morphological terminology follows Whittington and Kelly (Reference Whittington, Kelly and Kaesler1997).
Order Phacopida Salter, Reference Salter1864
Suborder Phacopina Struve, Reference Struve and Moore1959
Superfamily Dalmanitoidea Vogdes, Reference Vogdes1890
Family Dalmanitidae Vogdes, Reference Vogdes1890
Subfamily Synphoriinae Delo, Reference Delo1935
Remarks
The taxonomic definition of Dalmanitinae and Synphoriinae, the most supported subfamilies recognized in post-Ordovician dalmanitids (Campbell, Reference Campbell1977; Holloway, Reference Holloway1981; Holloway and de Carvalho, Reference Holloway and de Carvalho2009), has been long discussed. Originally, Synphoriinae was erected as a subfamily of Dalmanitidae (Delo, Reference Delo1935). However, it was later synonymized with Dalmanitinae (Richter and Richter, Reference Richter and Richter1942), although Lespérance and Bourque (Reference Lespérance and Bourque1971) recognized it again when reviewing material from the Devonian of North America and was considered an independent family by Lespérance (Reference Lespérance1975). Campbell (Reference Campbell1977) and Holloway (Reference Holloway1981) came back to the previous concept of Synphoriinae as a valid subfamily of Dalmanitidae and established the current diagnosis of Synphoriinae and Dalmanitinae. In later years, trilobites of the Early Devonian from southwestern Gondwana (Malvinoxhosan) challenged the distinction between Synphoriinae and Dalmanitinae. Amazonaspis de Carvalho and Fonseca, 2007; Chacomurus Braniša and Vaněk, Reference Braniša and Vaněk1973; Dalmanitoides Delo, Reference Delo1935; and Fenestraspis Braniša and Vaněk, Reference Braniša and Vaněk1973, were included in Synphoriinae because of some affinities with Devonian synphoriines of North America (Delo, Reference Delo1935; Braniša and Vaněk, Reference Braniša and Vaněk1973). However, most of them have repeatedly been reassigned to different subfamilies due to the combination of diagnostic characters of both Dalmanitinae and Synphoriinae (Lespérance, Reference Lespérance1975; Edgecombe, Reference Edgecombe1993; de Carvalho and Fonseca, Reference de Carvalho and da Fonseca2007; Holloway and de Carvalho, Reference Holloway and de Carvalho2009; Rustán and Vaccari, Reference Rustán and Vaccari2012). New taxa with combinations of diagnostic characters from both subfamilies were recently recognized in the Malvinoxhosan Realm based on data from Argentina, coming back again to the issue of the taxonomic/phylogenetic validity of these groupings (Randolfe et al., Reference Randolfe, Rustán and Bignon2022).
Because a detailed analysis of the subfamilies is beyond the scope of this study, we follow taxonomic proposals given by Campbell (Reference Campbell1977) and Holloway (Reference Holloway1981), together with additional taxonomic criteria drawn by Holloway and de Carvalho (Reference Holloway and de Carvalho2009), thus including Pachimocaspis n. gen. provisionally as a member of the Synphoriinae.
Genus Pachimocaspis new genus
Type species
Pachimocaspis pachimocensis n. gen. n. sp., by monotypy.
Diagnosis
As for type species by monotypy.
Occurrence
Upper part of the Los Espejos Formation (uppermost Silurian–lower Lochkovian) at Zanja Honda Creek section, National Route 150 next to the Jáchal River section, 5 km to the west of the Pachimoco locality, San Juan Province, Antequera region, Oruro Department, Bolivia, from rubble (Kozłowski, Reference Kozłowski1923, p. 39, table 2, fig. 5), Silurian?–Devonian? Lower part of the Talacasto Formation, Pragian? at Las Aguaditas Creek section, southwest of Jáchal City (see Rustán and Balseiro, Reference Rustán and Balseiro2016).
Etymology
From Pachimoco, a classic locality on the Jáchal River section located nearly 10 km to the west-northwest of Jáchal City and 5 km to the east of the Zanja Honda fossil locality, San Juan Province, Argentina.
Remarks
Pachimocaspis n. gen. can be differentiated from other dalmanitids by the combination of the ventral convexity of the frontal cephalic doublure, no vincular furrow, the development of the genal spines, the truncated thoracic pleural tips, and the shape of the pygidium, which is devoid of a caudal spine. It shares some characters with a group of Wenlockian dalmanitids considered closely related (Storey et al., Reference Storey, Thomas and Owens2016) including Delops Rickards, Reference Rickards1965 (as synonym of Lygdozoon Holloway, Reference Holloway1981), Struveria Rickards, Reference Rickards1965, and Rickardsia Storey, Thomas, and Owens, Reference Storey, Thomas and Owens2016. Pachimocaspis n. gen. shares with them a similar pygidial outline, devoid of a caudal spine, and the morphology of the glabellar frontal lobe. The most similar is Delops, sharing the subtle epiborder furrow, the maximum lateral extent of the interpleural furrows (this character has variability at the species level in Delops, with some species exhibiting interpleural furrows reaching the pygidial margin), and the short postaxial ridge. Pachimocaspis n. gen. differs from Delops in a larger (sag.) L1, the vestigial cephalic anterior border (in Delops visible in dorsal view), the S2 reaching the axial furrow, and the longer (exsag.) and wider (tr.) genal spines reaching at least half of the thorax and with a deep longitudinal furrow continuing from the subtle epiborder furrow. In contrast, when present (Holloway, Reference Holloway1981), genal spines of Delops species are short (exsag.). Additionally (although not included in its diagnosis), Delops has an occipital node, absent in Pachimocaspis n. gen. The longer (exsag.) genal spines and the vestigial cephalic anterior border of Pachimocaspis n. gen. more closely resemble Rickardsia. This genus could be differentiated by the taller eyes, devoid of a rounded rim, the deeper and wider (tr.) lateral border furrow in contact with the posterior border furrow, the presence of an occipital node, the ornamentation composed of wider tubercles, the slightly bifurcated S1 in the inner end (although Storey et al. in 2016, described it without bifurcation), and the pygidial interpleural furrows reaching a pygidial margin, which is denticulate. Struveria, in turn, is comparable to Pachimocaspis n. gen. in relative sizes (sag.) of the L1 and the L2, although it differs notoriously in the deeper and wider (tr.) lateral border furrow in contact with the posterior border furrow, the adaxially bifurcated S1, the presence of an occipital node, and the interpleural pygidial furrows reaching the pygidial margin. The genal spines of Struveria are poorly known but could be of similar size to those of Pachimocaspis n. gen. The discussed genera were considered the oldest (early Silurian) members of the typically Devonian subfamily Synphoriinae (Holloway, Reference Holloway1981), of disputed validity, hence the provisional subfamilial assignment of Pachimocaspis n. gen.
Pachimocaspis pachimocensis new genus new species
Pachimocaspis pachimocensis n. gen. n. sp., cephalic morphology, earliest Devonian Los Espejos Formation; Zanja Honda section, San Juan Province, Argentina: (1–9) INGEO-PI 2249, holotype, nearly complete cephalon: (1) dorsal view; (2) oblique view (note the wide tubercles in the frontal lobe); (3) lateral view; (4) frontal view; (5) oblique view; (6) lateral view (note caecal ornamentation on the librigenal field); (7) ventral view; (8, 9) details of the ocular region (note the pitted furrow adjacent to the rim of the top of the visual surface, postsutural furrow, and caecal ornamentation); (10–15) INGEO-PI 2250, paratype, partially preserved cephalon: (10) dorsal view; (11) oblique view; (12) lateral view; (13) frontal view; (14) oblique dorsal view; (15) detail of the ocular region. Scale bars = 10 mm.
Pachimocaspis pachimocensis n. gen. n. sp., cephalic morphology, earliest Devonian Los Espejos Formation; Zanja Honda section, San Juan Province, Argentina: (1–4) INGEO-PI 2251, paratype, internal mold of partial cephalon: (1) dorsal view; (2) lateral view; (3) ventral view; (4) oblique view; (5) INGEO-PI 2252, paratype, internal mold of a partially preserved cranidium; (6, 7) INGEO-PI 2253, paratype, partial cephalon: (6) lateral view; (7) detail of the eye; (8–10) INGEO-PI 2254, paratype, internal mold of partial cephalon: (8) dorsal view; (9) lateral view (note the ventral convexity of the doublure); (10) frontal view (note the ventral convexity of the doublure); (11) INGEO-PI 2262, paratype, cephalon and associated thoracic segments, detail of the eye; (12, 14, 15) INGEO-PI 2256, paratype, latex cast of partial cephalon; note the course of the lateral cephalic furrow and depth and width (exsag.) of glabellar furrows: (12) detail of the eye; (14) dorsal view; (15) oblique dorsal view; (13) INGEO-PI 2255, paratype, glabella in dorsal view preserving carapace. Scale bars = 10 mm.
Pachimocaspis pachimocensis n. gen. n. sp., thoracic and pygidial morphology: (1–7, 9–19) earliest Devonian Los Espejos Formation, Zanja Honda section, San Juan Province, Argentina: (1–3) INGEO-PI 2257, paratype, nearly complete thorax with remains of cephalon and pygidium attached: (1) dorsal view; (2) laterodorsal view; (3) posterior view; (4, 5) INGEO-PI 2258, paratype, partial thorax with nearly complete pygidium attached: (4) dorsal view; (5) lateral view; (6) INGEO-PI 2259, paratype, detail of distal pleurae in lateral view (note the tip with oblique truncation); (7) INGEO-PI 2260, paratype, detail of distal pleura in lateral view (note the tip with oblique truncation); (9, 11–13) INGEO-PI 128, paratype, nearly complete pygidium: (9) posterolateral view; (11) dorsal view; (12) lateral view; (13) posterior view; (10) INGEO-PI 2262, paratype, detail of distal pleura in lateral view (note the tip with oblique truncation); (14–16) INGEO-PI 2263, paratype, nearly complete pygidium: (14) dorsal view; (15) lateral view; (16) posterior view; (17–19) INGEO-PI 2264, paratype, nearly complete pygidium: (17) dorsal view; (18) lateral view; (19) posterior view. (8) INGEO-PI 2261, paratype, internal mold of partial pygidium, Pragian of the lower part of the Talacasto Formation at Las Aguaditas Creek section (see Rustán and Balseiro, Reference Rustán and Balseiro2016), San Juan Province, Argentina. Scale bars = 10 mm.
Type specimens
Holotype, INGEO-PI-2249, 9 m above the base of the Zanja Honda section, upper part of the Los Espejos Formation, Lochkovian. Paratypes, INGEO-PI-068, 128, 129, 1920, 2250-2283, from the Zanja Honda section, upper part of the Los Espejos Formation, uppermost Silurian-Lochkovian.
Diagnosis
Dalmanitid with frontal glabellar lobe inflated, strongly overhanging anteriorly and evenly convex in lateral and frontal view; ventral doublure long (sag. and exsag.), clearly convex ventrally with no expressed vincular furrow; dorsal profile of the glabella slightly convex in lateral view; cephalic border vestigial frontally; lateral cephalic furrow wide (tr.) and shallow, disappearing shortly after crossing the suture posterior branch without joining the posterior cephalic furrow; lateral border with subtle epiborder furrow; SO with subtle furrows running laterally; S2 narrowing adjacent to the axial furrow; eyes relatively low and elongated (exsag.), with visual surface of ~35 vertical rows with 12 lenses maximum each; visual surface bending and narrowing posteriorly and upward, with a rounded rim all around its margin; short (tr.) and subtle postocular furrow adjacent to and behind the suture; genal spine long, smooth, stout, and thick distally in cross section. Thoracic pleural tips expanded, thick, and truncated. Pygidium with evenly subparabolic marginal shape, with rounded posterior margin, with maximum width (tr.) at level of the second axial ring; pigaxis with 11 axial rings, particularly wide (tr.) in the anterior part (reaching nearly 30% of maximum total pygidial width), with nearly straight axial furrows, discrete post axial ridge flanked by one or two nearly straight ribs subparallel to the median axis; pleural fields with nine or ten pygopleurae (including subtle ones flanking the postaxial ridge), pygidial border wide (tr.), barely concave dorsally without border furrow, anterior and posterior pleural bands of similar length (sag., exsag.), interpleural furrows well-impressed, incised, slightly convex anteriorly, exhibiting subtle apodemal deepening and widening locally next to the pygidial border, becoming shallow wide traces deflecting posteriorly; pleural furrows shallowing and disappearing rapidly before reaching the pygidial border.
Occurrence
As for the genus.
Description
Cephalon parabolic in dorsal view, with length (sag.)/width (tr.) index ~0.6. Preglabellar area vestigial; preglabellar field not developed; shallow and delicately incised preglabellar furrow. Glabella pyriform-shaped, with maximum width (tr.) slightly located posterior of the middle part of the frontal lobe, which is nearly 0.6 of maximum cephalic width (tr.). Axial furrows weakly divergent anteriorly (~15°) from the posterior part to the junction with S1, then slightly more rapidly diverging to become strongly divergent from S3 to the cephalic border, shallowest adjacent to the anterior part of the eye. Posterior part of the glabella with no S1 to S3 furrowing medially, nearly flat-topped in lateral view (barely more convex and higher in the rear part), slightly convex in frontal view. SO nearly straight, slightly anteriorly convex sagittally, trending to be concave, deeper and wider (exsag.) laterally, narrow (sag., exsag.) and shallow in general. S1 slightly concave anteriorly, evenly wide (exsag.) and deep, barely shallowing in contact with axial furrows. S2 nearly straight, slightly posterolaterally oriented, longer (tr.) than S1, barely shallowing in contact with axial furrows. S3 nearly straight to barely convex anteriorly, anterolaterally oriented, effacing adjacent to the anterior part of the eye in contact with axial furrows and facial suture. LO wide (sag., exsag), slightly narrower (exsag.) laterally, barely concave anterolaterally, with very gentle convex dorsal top in lateral view, slightly higher than the posterior part of the glabella in lateral view, very convex in posterior view, with subtle furrows running along the lateral parts slightly posteriorly located, corresponding to 13–14% of glabellar total length (sag.). L1 shorter (exsag.) than LO and L2, scarcely inflated; anterior side slightly convex posteriorly and slightly concave anteriorly, corresponding to 10% of glabellar total length (sag.). L2 slightly longer (exsag.) than L1, nearly subrectangular, slightly wider (exsag.) toward the median region, barely less inflated near the axial furrows, corresponding to 13–14% of glabellar total length (sag.). L3 larger (exsag.) than L2, rapidly expanded laterally, evenly inflated dorsally. Glabellar frontal lobe subrhombic in dorsal view, strongly convex and sloping anteriorly but not protruding forward to the anterior margin in lateral view, with a convexity maximum to the front of the middle part in lateral view, scattered tubercles very shallow and wide in the anterior dorsal part, each one with a faint central depression, no posterior median impression. Anterior cephalic doublure long (sag.), with length ~15% of total cephalic length (sag.), nearly convex ventrally in frontal view (Fig. 5.4).
Eye nearly kidney-shaped in dorsal view, with posterior edge opposite of middle part of L1 and anterior edge opposite of the contact between axial furrows and S3. Eye maximum length (exsag.) ~35% of cephalic total length (sag.), and maximum width (tr., from axial furrow to lateralmost visual surface) ~17% of cephalon total width (tr.). Ocular surface taller in the anterior part, gently convex and subvertical in general, more vertically oriented and elevated in the posterior part, which faces the rear; in general, bounded exteriorly by a rounded rim mainly at the bottom and the top (Fig. 5.9), the latter continued as a ridge behind the eye between the facial suture and a postsutural furrow (Fig. 5.8, 5.9). Lenses regularly ordered in ~35 vertical rows with a maximum of ~12 lenses per row. Lens formula is not warranted. Subocular furrow shallow and narrow (exsag.). Palpebral area elevated in comparison with remaining fixigenal field, slightly convex dorsally in posterior view. Palpebral furrow wide (tr.), well-impressed, and evenly convex laterally, continuing laterally and downward behind the eye and the suture into a postsutural furrow. Marginal furrow on the palpebral lobe adjacent to the internal side of the rim of the top of the visual surface, narrow (tr.) and incised, with a series of subtle pits.
Postocular area of the fixigena narrow (exsag.), slightly convex and steeply inclined laterally, with a postsutural ridge and associated postocular furrow, widening laterally and downward developing subtle caecal ornamentation, with widespread genal area evenly convex dorsally with no trace of connection between lateral and posterior border furrows but receiving the more deepening posterior end of the lateral cephalic furrow behind the posterior branch of the facial suture (Fig. 5.12). Posterior border furrow deep and narrow (exsag.), nearly straight, disappearing suddenly at genal area. Posterior cephalic border narrow adjacent to axial furrow then widening laterally and downward, evenly convex dorsally.
Anterior branch of facial suture running subparallel to axial furrows, then circumscribing the glabellar frontal lobe. Posterior branch of facial suture running with strong convexity anterolaterally from the posterior part of the eye, turning rapidly backward after crossing the lateral cephalic furrow, reaching the lateral cephalic margin clearly behind the eye. Librigenal field narrow (tr.) and steeply sloped downward, with caecal ornamentation. Lateral border furrow broad (tr.) and shallow, moreso on librigenal field than on fixigenal field, not clearly parallel to the lateral margin but approximately straight and oblique in dorsal view, defining a wide (tr.) and nearly concave lateral border with a subtle epiborder furrow.
Hypostome unknown.
Thorax of 11 segments, considerably wide (tr.), with axial furrows slightly convex laterally (tr.), converging progressively backward, deep and narrow (tr.), shallowing and narrowing posteriorly. Axial rings considerably short (sag., exsag.) with length (sag.) ~17–18% of its width (tr.); ~36–37% of total thoracic width (tr.) measured across the seventh thoracic segment of INGEO-PI 2257; with anterior and posterior sides nearly straight (tr.); more convex dorsally in the median part (sag.) in transverse view, nearly flat-topped in lateral view; with lateral portions slightly wider (exsag.) and nearly subquadrate ending at junction with axial furrows; separated from the articulating half ring by an apparently deep and narrow (sag., exsag.) articulating furrow. Pleural lobe (measured at seventh segment of specimen INGEO-PI 2257) with width (tr.) ~32–33% of total thoracic width in dorsal view, with proximal region (between axial furrow and fulcrum) of > 50% of total width of pleural lobe (measured in posterior view), slightly and evenly convex dorsally; distal region (between fulcrum and thoracic margin) abruptly sloped laterally, trending to lose dorsal convexity distally. Pleural furrow in contact with axial furrow adaxially, narrow (exsag.) and incised, slightly concave forward, running posterolaterally, and then becoming a faint trace entering the articulating facet beyond fulcrum. Anterior pleural band fusiform (with anterior margin evenly convex forward and more anteriorly directed acuminated ends at the junction with the axial furrow, and at fulcrum covered by the overlapped anterior thoracic segment distally), almost entirely overlapping the posterior pleural band of the anterior thoracic segment. Posterior pleural band wider (exsag.) than anterior pleural band, with its visible part in dorsal view (that part not covered by the anterior pleural band of the posterior thoracic segment) with minimum width (exsag.) at its midlength. Distal pleural part beyond fulcrum progressively widening (exsag.), with posterior edge trending to be concave and anterior one convex forward, articulating facet roughly developed with slender spatulated shape and finally granulated surface; pleural tips thick, obliquely truncated (Fig. 7.1, 7.2).
Pygidium broadly subtriangular in dorsal view, with sharp smooth margin, length (sag.)/width (tr.) index ~0.6, with maximum width (tr.) at level of second axial ring. Axial furrows nearly straight to barely convex laterally, very narrow (tr.) and moderately deep, diverging from the sagittal plane ~20–25°. Pygidial axis wide (tr.), ~30% of the maximum pygidial width (tr.) at a level of the second axial ring, with maximum dorsal convexity (tr.) medially in posterior view and lateral parts trending to be slightly concave; even and barely convex dorsal profile in lateral view, with ~12 well-defined axial rings, with a subtle postaxial ridge. Axial rings subrectangular in dorsal view, inter-ring furrows narrow (sag.) and well impressed, subtly deeper laterally. Pleural field with nine or ten pygopleurae, gradually less defined backward and more posteriorly directed, to become the last two or three nearly parallel to the sagittal axis; wide (tr.) pygidial border becoming nearly concave dorsally just adjacent to the margin in posterior view. Interpleural furrows well impressed, incised, convex forward, locally widening (exsag.) and deepening just adjacent to the pygidial border becoming a discrete elongated depression very asymmetrical in cross section (exsag.), then continuing to enter the border as a faint trace oriented backward (Fig. 7.15). Pleural furrows evenly wider (exsag.), deeper and convex forward, symmetrical in cross section, disappearing just next to the pygidial border. Anterior pleural bands elongated (tr.) and fusiform, and posterior pleural bands like a strip narrower in its midlength. Mucro or caudal spine absent.
Etymology
From Pachimoco locality, as for the genus.
Remarks
Pachimocaspis pachimocensis n. gen. n. sp. exhibits some characters unusual for dalmanitids. In particular, the rounded rim of the eye (Fig. 5.9) is a character of intrageneric variability rarely developed in other dalmanitids as in Pachimocaspis pachimocensis n. gen. n. sp. Of the previously compared Wenlockian dalmanitids, Delops anoplos (Holloway, Reference Holloway1981) has a robust rounded rim but only around the lower margin of the eye. Fenestraspis amauta Braniša and Vaněk, Reference Braniša and Vaněk1973, a proposed synphoriine from the Early Devonian of Bolivia (Holloway and de Carvalho, Reference Holloway and de Carvalho2009), has a better-developed eye rim on top of the visual surface (clearly laterally overhanging), but it does not continue at the bottom of the eye. Holloway and de Carvalho (Reference Holloway and de Carvalho2009) provided some insight into the function of the upper rim in F. amauta following proposals by Fortey and Chatterton (Reference Fortey and Chatterton2003) and Chatterton et al. (Reference Chatterton, Fortey and Brett2006). Based on Erbenochile erbeni (Alberti, Reference Alberti1981), a dalmanitid from the Emsian of Morocco, this structure was proposed as an ‘eyeshade,’ shielding the visual surface from glare caused by light coming from directly above (Fortey and Chatterton, Reference Fortey and Chatterton2003). In Pachimocaspis pachimocensis n. gen. n. sp., this function is unlikely, because the extension of the rim would not cover almost any portion of the visual surface. Whatever their function, eye rims have evolved independently several times within the Dalmanitidae.
Another interesting feature of Pachimocaspis pachimocensis n. gen. n. sp. is the variation of pleural tips along the thorax of an individual, which was an important feature in discussions about the validity of the subfamily Synphoriinae (Holloway, Reference Holloway1981). The thorax is poorly known in most species considered part of this subfamily. Campbell (Reference Campbell1977) proposed three morphologies for the thoracic pleural tips in post-Ordovician dalmanitids. When each segment has a pointed tip, this pattern was considered diagnostic of Dalmanitinae. In Synphoriinae, Campbell (Reference Campbell1977) differentiated a synphoriinid-type when rounded pleural tips are invariably present and a less defined trypaulinitid-type, with anterior segments truncated and posterior ones pointed. Holloway (Reference Holloway1981) rediagnosed Dalmanitinae and Synphoriinae by adding a state for the thoracic pleural tips of synphoriines with the anteriormost pleural tips rounded and the posterior ones truncated or pointed. This last proposal included Delops and Lygdozoon (considered a junior synonym of Delops by Storey et al., Reference Storey, Thomas and Owens2016) as Silurian representatives of the subfamily. These diagnostic thoracic morphologies were based on a few species with known pleural tips. Delops nobilis (Thomas, Reference Thomas1900), currently reassigned to Rickardsia by Storey et al. (Reference Storey, Thomas and Owens2016), has the posteriormost pleural tips pointed; and Delops collatum (Holloway, Reference Holloway1981) has the posteriormost pleural tips truncated. However, Holloway (Reference Holloway1981) described D. collatum based on a few isolated thoracic segments. The proposal of the anterior segments as being rounded was based on the assumption that the anterior segments should suffer more movement during enrollment, as a preferable condition (Holloway, Reference Holloway1981). In the case of Pachimocaspis pachimocensis n. gen. n. sp., the pleural tips of the anterior thoracic segments are preserved in a few specimens. These segments look slightly rounder than those more clearly truncated posterior ones (Fig. 7.6, 7.7). This agrees with the variable morphology of the pleural tips in a single species as proposed by Holloway (Reference Holloway1981). Morphological variation of the pleural tips along the thorax is a remarkable feature shared between Pachimocaspis pachimocensis n. gen. n. sp. and the Wenlockian putative synphoriines.
Discussion of Pachimocaspis pachimocensis n. gen. n. sp.
Stratigraphic and geographic distribution around the Silurian-Devonian transition
Pachimocaspis pachimocensis n. gen. n. sp. is recorded as low in the coquinite underlying the base of the column surveyed at Zanja Honda section, which records brachiopods of the ‘third assemblage’ of Benedetto et al. (Reference Benedetto, Racheboeuf, Herrera, Brussa and Toro1992), e.g., specimen INGEO-PI 2270. These kinds of coquinas are typical sedimentological features of the Silurian (Ludlow–Pridolian) upper part of the Los Espejos Formation in different localities (Sánchez et al., Reference Sánchez, Astini and Piovano1991; Astini and Piovano, Reference Astini and Piovano1992). They recorded anywhere a typical trilobite association dominated by the phacopid Paciphacops argentinus (Thomas, Reference Thomas1905), the homalonotid ‘Homalonotus’ kayseri (Thomas, Reference Thomas1905), the calmoniid Andinacaste espejensis Edgecombe, Waisfeld, and Vaccari, Reference Edgecombe, Waisfeld and Vaccari1994 and the dalmanitid ‘Kasachstania’ gerardoi Edgecombe and Ramsköld, Reference Edgecombe and Ramsköld1994. This trilobite association is unknown in layers younger than the latest Silurian. Because Pachimocaspis pachimocensis n. gen. n. sp. is present in these coquinas, we conclude that it ranges since the Silurian. Pachimocaspis pachimocensis n. gen. n. sp. is also present in coquina lenses of similar taxonomic composition at the very base of the surveyed stratigraphic column. These typical coquinites recorded the graptolite Slovinograptus (Lopez et al., Reference Lopez, Rustán and Benedetto2025), so that they reach the Silurian-Devonian transition in accordance with conodont and brachiopod evidence elsewhere (Mestre et al., Reference Mestre, Gomez, Garcias, Corradini, Heredia, Liao and Valenzuela-Rios2017; Gómez et al., Reference Gómez, Mestre, Corradini and Heredia2021, Reference Gómez, Mestre and Heredia2024).
Although poorly preserved in the marginal regions, we referred to Pachimocaspis pachimocensis n. gen. n. sp. an additional pygidium (INGEO-PI 2261; Fig. 7.8), coming from the dark muddy lower part of the Talacasto Formation at Las Aguaditas Creek section, to the southwest of Jáchal City. This specimen suggests that Pachimocaspis pachimocensis n. gen. n. sp. ranges upward into the Pragian, integrating a typical trilobite association recorded in the lower (not the lowermost) part of the Talacasto Formation, which includes Talacastops zarelae Edgecombe, Waisfeld, and Vaccari, Reference Edgecombe, Waisfeld and Vaccari1994; Tormesiscus hildae Waisfeld, Edgecombe, and Vaccari, Reference Waisfeld, Edgecombe and Vaccari1994; Echidnops taphomimus Rustán and Balseiro, Reference Rustán and Balseiro2016; Aguaditaspis mediaspina Randolfe, Rustán, and Bignon, Reference Randolfe, Rustán and Bignon2022; and Reedops cf. Reedops bronni (Barrande, Reference Barrande1846); see Holloway and Rustán (Reference Holloway and Rustán2012) and Rustán and Peralta (Reference Rustán and Peralta2024).
Kozłowski (Reference Kozłowski1923, p. 39, table 2, fig. 5) reported a pygidium collected from rubble from the Antequera region, Oruro Department, Bolivia, which is indistinguishable from the Argentinian Pachimocaspis pachimocensis n. gen. n. sp. specimens reported herein. Kozłowski (Reference Kozłowski1923, p. 39) noticed the particular “primitive” (sic) morphology of this specimen, which referred with doubts to the Devonian because he stated that strata of this age cropping out in the surroundings of Antequera were “nearly barren of fossils.” Antequera is the main city of the homonymous mining district, to the east of the Poopó Lake in Oruro. According to Bolivian Geological Survey mapping, the ranges to the east of the Poopó Lake include Silurian to Devonian sedimentary rocks so that the record of the Silurian-Devonian transition is present in this area. Suárez Riglos et al. (Reference Suárez Riglos, Dalenz Farjat and Perez Guarachi1994) and Dalenz Farjat (Reference Dalenz Farjat2005) reported Silurian fossiliferous localities recording the upper Silurian brachiopod Clarkeia, nearly 60 km to the south of Antequera near Challapata (Huari to Condo localities). Lithostratigraphic units recorded there comprise the Silurian Uncía and Silurian–lowest Devonian Catavi formations. According to these authors, the main fossiliferous layers would correspond to the Lochkovian of the Catavi Formation, which has independently been correlated to the uppermost part of the Los Espejos Formation based on similar trilobite content at Cochabamba (Edgecombe and Ramsköld, Reference Edgecombe and Ramsköld1994). Unfortunately, although the upper part of the Catavi Formation would record the Silurian-Devonian transition, at this point we are not able to surely identify any Silurian-Devonian unit as the most probable candidate for the bearing-levels of the Kozłowski’s pygidium so that the precise geographic and stratigraphic provenance of this Bolivian specimen is doubtful to date. Moreover, after consulting the Muséum National d’Histoire Naturelle, where the bulk of the Kozłowski´s collection of the Devonian from Bolivia is housed, this specimen is considered lost by us.
All in all, a Ludlow? to probably Pragian stratigraphic range is assumed for Pachimocaspis pachimocensis n. gen. n. sp.
Taxonomic and phylogenetic implications
Pachimocaspis pachimocensis n. gen. n. sp. has a remarkable relevance in the discussion concerning the taxonomic definition of Dalmanitinae and Synphoriinae and their phylogenetic relationship with the nearly Ordovician subfamilies Mucronaspidinae Holloway, Reference Holloway1981 and Dalmanitininae Destombes, Reference Destombes1972. Usually treated as sister groups, Holloway (Reference Holloway1981) proposed that the origins of Dalmanitinae and Synphoriinae lie in different Ordovician subfamilies. Thus, Dalmanitinae was considered more closely related to Mucronaspidinae and Synphoriinae to Dalmanitininae. The most important character of this relationship was the thoracic pleural tips, because Holloway (Reference Holloway1981) considered that the rounded or truncated pleural tips of some synphoriines were homologous to the characteristically truncated pleural tips of the Dalmanitininae. On the other hand, Mucronaspidinae and Dalmanitinae share pointed pleural tips. One of the problems mentioned by Holloway (Reference Holloway1981) was the absence of Dalmanitininae in the Ordovician in the regions in which Synphoriinae was present during the Silurian and Devonian. In addition, he highlighted the temporal gap between Dalmanitininae and the first synphoriines.
Pachimocaspis n. gen. at first sight seems to support Holloway’s (Reference Holloway1981) proposal that Synphoriinae is more related to Dalmanitininae than to Dalmanitinae, because Dalmanitininae is present in southwestern Gondwana. Nevertheless, in southwestern Gondwana, the Hirnantian Dalmanitina subandina Monaldi and Bosso, Reference Monaldi and Boso1987 is virtually the only one representative of Dalmanitininae and there is an extensive Silurian gap. The only mention of a Silurian Synphoriinae from South America is a cranidium in open nomenclature from the Los Espejos Formation (Waisfeld et al., Reference Waisfeld, Toro and Brussa1988), which was considered in Dalmanitinae by Holloway and Scott (Reference Holloway and Scott2023) and clearly differs from Pachimocaspis n. gen. The remaining Silurian dalmanitids of South America are reliably either Dalmanitinae or Mucronaspidinae (Baldis et al., Reference Baldis, Benedetto, Blasco and Martel1976; Baldis and Hansen, Reference Baldis and Hansen1980; Jell and Adrain, Reference Jell and Adrain2002; Tortello et al., Reference Tortello, Uriz, Alfaro, Cingolani, Bidone and Galeano Inchausti2012). Therefore, the synphoriine Silurian record gap in southwestern Gondwana is conspicuous. The similarities of Pachimocaspis pachimocensis n. gen. n. sp. with Silurian close relatives from low paleolatitudes move to hypothesize that Synphoriinae sensu stricto might be present in Malvinoxhosan basins, although a phylogenetic analysis of the entire Dalmanitidae, with focus on synphoriines is needed to unravel this issue.
Jumping into the Malvinoxhosan (southwestern Gondwanan) radiation of Dalmanitidae
Trilobite faunas from the Early Devonian of southwestern Gondwana have been characterized as noticeably endemic since their first reports (Clarke, Reference Clarke1913b; Richter and Richter, Reference Richter and Richter1942), which allowed recognition of a major Early–Middle Devonian paleobiogeographical region of cold waters in high paleolatitudes (Dowding and Ebach, Reference Dowding and Ebach2018). This region, known as the Malvinoxhosan (= Malvinokaffric) Realm (Penn Clarke and Harper, Reference Penn-Clarke and Harper2023), embraced relatively isolated basins and was considered a natural laboratory for analyzing evolutionary and paleobiogeographical patterns and processes (Lieberman et al., Reference Lieberman, Edgecombe and Eldredge1991; Lieberman, Reference Lieberman1993). The earliest hypothesis, accounting for extraordinary morphological disparity in Malvinoxhosan trilobites, proposed Devonian regional diversifications from previous Silurian cosmopolitan stocks (Eldredge and Ormiston, Reference Eldredge, Ormiston, Gray and Boucot1979). The bulk of studies focused on the calmoniids, an extremely diversified late Silurian–Middle Devonian family, virtually endemic to Malvinoxhosan basins (Eldredge and Braniša, Reference Eldredge and Braniša1980; Abe and Lieberman, Reference Abe and Lieberman2009, Reference Abe and Lieberman2012). However, other trilobite families include some widely-distributed representatives as in the case of phacopids (Holloway and Rustán, Reference Holloway and Rustán2012; Rustán and Balseiro, Reference Rustán and Balseiro2016; Rustán and Peralta, Reference Rustán and Peralta2024). Dalmanitids in particular were considered mostly endemic and remarkably diversified in Malvinoxhosan basins, similar to calmoniids (Eldredge and Ormiston, Reference Eldredge, Ormiston, Gray and Boucot1979). Originally, most of the information derived from several pioneer works (e.g. Clarke, Reference Clarke1890; Kozłowski, Reference Kozłowski1923; Swartz, Reference Swartz1925; Wolfart, Reference Wolfart1968) that assigned species to classic genera like Dalmanites Barrande, Reference Barrande1852 or Odontochile Hawle and Corda, Reference Hawle and Corda1847, hid a strong paleobiogeographic signal of this family in southwestern Gondwana. However, several new genera, endemic to this area, were subsequently erected (e.g., Braniša and Vaněk, Reference Braniša and Vaněk1973) or resulted from taxonomic revisions with focus on morphological characters rare for dalmanitids (Edgecombe, Reference Edgecombe1993; de Carvalho and Fonseca, Reference de Carvalho and da Fonseca2007; Holloway and de Carvalho, Reference Holloway and de Carvalho2009, Reference Holloway and de Carvalho2010; Rustán and Vaccari, Reference Rustán and Vaccari2012; Randolfe et al., Reference Randolfe, Rustán and Bignon2022). These characters included robust dorsal spines (e.g. Dalmanitoides, Fenestraspis, and Aguaditaspis Randolfe, Rustán, and Bignon, Reference Randolfe, Rustán and Bignon2022), a distinctly inflated frontal lobe (Francovichia Braniša and Vaněk, Reference Braniša and Vaněk1973), bifid caudal spines (e.g., Chacomurus), and intersegmental fenestrae (e.g., Fenestraspis). Only Dalmanites seems to be a widely distributed taxon, but the few referred specimens, mainly from the Devonian of Bolivia (e.g., from Kozłowski, Reference Kozłowski1923) deserve exhaustive taxonomic revision. It became clear that Malvinoxhosan dalmanitids are important for diversification studies because of their distinctive characters and the highly endemic distribution. Pachimocaspis pachimocensis n. gen. n. sp., with a unique morphology more closely resembling early Silurian synphoriines from low paleolatitudes than to late Silurian-Early Devonian Malvinoxhosan dalmanitids, represents a paradox. It appears to be a typical early Silurian synphoriine ‘jumping’ the Silurian gap record to fall, by dispersal, into the scenario of an Early Devonian evolutionary burst of endemic dalmanitids in high paleolatitudinal basins from Gondwana.
Acknowledgments
A. Dalenz Farjat and G. Ríos Cordero provided insights into stratigraphic provenance of the pygidium from the region of Antequera, Bolivia, illustrated by Kozłowski (Reference Kozłowski1923). The authors acknowledge financial support from the Facultad de Ciencias Exactas Físicas y Naturales of the University of San Juan, National Research Council of Argentina (CONICET), University of La Rioja project 2017-6993 granted to JJR; and the Agencia Nacional de Promoción Científica y Tecnológica PICT 2017-3095 granted to JJR. Thanks are extended to the reviewers, A. van Viersen and F. Pérez Peris, and the editorial staff for their helpful comments and suggestions that significantly improved the final version of the manuscript.
Competing interests
The authors declare none.
