New, large actinopterygian fishes from the upper Carboniferous of Nýřany, Czech Republic

Acta Palaeontol. Pol. 69 (3): 501–522, 2024 https://doi.org/10.4202/app.01162.2024 New, large actinopterygian fishes from the upper Carboniferous of Nýřany, Czech Republic PAVEL BARTÁK, MARTIN IVANOV, EVA TIHLAŘÍKOVÁ, MARTIN OLBERT, and VILÉM NEDĚLA Barták, P., Ivanov, M., Tihlaříková, E., Olbert, M., and Neděla, V. 2024. New, large actinopterygian fishes from the upper Carboniferous of Nýřany, Czech Republic. Acta Palaeontologica Polonica 69 (3): 501–522. The lacustrine coal deposits at Nýřany, Czech Republic, yielded a diversified vertebrate assemblage of the Middle Pennsylvanian (Moscovian) age, represented by the remains of early tetrapods, as well as numerous freshwater ichthyofauna, including xenacanthiform sharks, acanthodians, dipnoans, and ray-finned fishes. However, unlike some other upper Carboniferous localities, the actinopterygian diversity is limited to the three small-bodied species, most of them endemic to Nýřany locality and the equivalent strata elsewhere, indicating that the true taxonomic diversity of the group at the locality may be biased. Here we describe first skeletal remains of large actinopterygian fishes from the site, including a new genus and species, Stambergichthys macrodens gen. et sp. nov., which is represented by a well-preserved mandible with teeth. The micro-computed tomographic techniques revealed in the specimen a presence of a complex neurovascular system innerving the teeth and the jaw, and supplying both with blood vessels. The dentition consists of a single row of massive, homodont, conical teeth, which possess simplexodont plicidentine on their base, the characteristics supporting the predatory ecology of the new species. The isolated skeletal remains of large-bodied actinopterygians expand the knowledge on the diversity of the group in Nýřany, and their occurrence in coal deposits of relatively shallow lake indicates they represent allochthonous, poorly known aquatic vertebrate association, likely originating from the braided river system. These findings underline the importance of less complete skeletal materials occurring in wellknown vertebrate assemblages of the upper Carboniferous coal-bearing localities. Key words: Actinopterygii, dentition, continental basins, morphology, neurovascular system, palaeoenvironment, plicidentine, Palaeozoic. Pavel Barták [[email protected]; ORCID: https://orcid.org/0009-0009-3738-4849 ] and Martin Ivanov [ [email protected]; ORCID: https://orcid.org/0000-0001-9108-9239 ], Department of Geological Sciences, Faculty of Science, Masaryk University, Kotlářská 267/2 611 37 Brno, Czech Republic. Eva Tihlaříková [[email protected]; ORCID: https://orcid.org/0000-0002-7983-2971 ], Martin Olbert [[email protected]; ORCID: https://orcid.org/0000-0003-2280-3341 ], and Vilém Neděla [[email protected]; ORCID: https://orcid.org/ 0000-0001-6029-5435 ], Environmental Electron Microscopy Group, Institute of Scientific Instruments of the Czech Academy of Sciences, Královopolská 147, 612 00 Brno, Czech Republic. Received 17 April 2024, accepted 25 July 2024, published online 30 September 2024. Copyright © 2024 P. Barták et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License (for details please see http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Introduction The Nýřany locality is part of a coalfield situated southwest of Pilsen in the Czech Republic, which contains several upper Carboniferous (Middle Pennsylvanian) coal seams, developed within the grey-coloured sequence of fluviolacustrine sediments of the Pilsen Basin. Remnants of vertebrates come from several fossiliferous sapropelic horizons inside the Main Nýřany Coal Seam (Fritsch 1879; Milner 1980; Barták 2020) of the Nýřany Member (Kladno Formation), dated as the late Moscovian (Asturian) age (Opluštil et al. 2016). Their collection began in the second half of the 19th century (Fritsch 1870), with most important findings being derived from the Humboldt and Krimich mines in the vicinity of the Nýřany village. Over the past ca. 150 years, numerous vertebrate body fossils have been described from the site, representing one of the best known late Moscovian faunal assemblages worldwide. Much of the vertebrate skeletal remains are fairly complete, articulated specimens, indicating a rapid burial under the anoxic conditions with minimal effects of transport (Milner 1980). The renowned status of the Nýřany locality undoubtedly arises mainly from the highly diversified and especially well-preserved assemblage of early tetrapods, including 502 ACTA PALAEONTOLOGICA POLONICA 69 (3), 2024 baphetids (Beaumont 1977; Milner et al. 2009), embolomeres (Carroll 1970), various groups of temnospondyls and “lepospondyls” (for the recent overview, see Barták 2020), and the earliest amniotes (Carroll and Baird 1972; Reisz 1975). However, the ichthyofauna is also well-represented and relatively diverse, comprising acanthodians (Zajíc 1988), xenacanthiform sharks (Schneider and Zajíc 1994; Heidtke 1998), dipnoans (Fritsch 1888) and actinopterygian fishes (Štamberg 1978, 1991, 2013). In comparison to the other late Carboniferous localities such as Newsham (Northumberland, UK), Linton (Ohio, USA), Mazon Creek (Illinois, USA), Montceau-les-Mines (France), and Kounov (Czech Republic), the coal-bearing deposits of Nýřany have so far produced surprisingly few actinopterygian species. Although abundant in terms of specimen number, these are currently confined to only three small-bodied forms, the haplolepid Pyritocephalus sculptus and two species of the endemic genus Sceletophorus, i.e., S. biserialis and S. verrucosus, none of them exceeding the total length of 150 mm (Westoll 1944; Gardiner 1967; Štamberg 1978, 1991, 2013). These taxa have been perceived as inhabitants of small and shallow, poorly oxygenated freshwaters (Westoll 1944; Milner 1980), the environmental constraints of which would potentially prevented the settlement by a population of large-bodied actinopterygians known from elsewhere. Here we describe the first skeletal remains of large actinopterygian fishes from the upper Carboniferous of Nýřany, consisting of isolated and fragmentary elements, of which a single lower jaw with well-preserved teeth is described as a new genus and species, Stambergichthys macrodens gen. et sp. nov. The micro-computed tomography of this specimen reveals the presence of a primitive internal organization of the tooth dentine, which is discussed in the context of recent findings of plicidentine in the Actinopterygii. Here described materials not only expand the knowledge on the diversity of the group at the locality, but also contribute significantly to the problematic of faunal associations and palaeoenvironment at the Nýřany locality, and expand the stratigraphic record of the large-bodied actinopterygians in the Central and West Bohemian basins of the Czech Republic. Institutional abbreviations.—CGS, Czech Geological Survey, Prague, Czech Republic; NHMW, Naturhistorisches Museum Wien, Austria; NMP, National Museum, Prague, Czech Republic; ÚGV PAL, Palaeontological collection of the Department of Geological Sciences, Faculty of Science, Masaryk University, Brno, Czech Republic. Nomenclatural acts.—This published work and the nomenclatural acts it contains have been registered in ZooBank: urn:lsid:zoobank.org:pub:24B92487-A962-474F-B9E2D819D0BCB203 Geological setting The Central and West Bohemian continental basins, extending in the western and central part of the Czech Republic, represent the late Carboniferous fluviolacustrine sedimentary infill deposited on the Neoproterozoic and early Palaeozoic Fig. 1. A. The stratigraphic position of the actinopterygian material described in present study. The stratigraphic scheme is modified from Martínek et al. (2017). B. Permo-Carboniferous continental basins of the Czech Republic with marked location of Nýřany locality. B Age Lithostratigraphy Global stage Regional stage Substage Formation Coal group Asselian Autunian C Líně B Gzhelian Stephanian Saberian Slaný Kasimovian Barruelian Týnec Cantabrian Kladno Moscovian Westphalian Asturian Bolsovian Kladno Kamenný Most Kounov Ledce Hředle Mšec Jelenice Nýřany Radnice Klobuky Zdětín Kounov Mělník Nevřeň Chotíkov Nýřany Touškov Lubná Radnice Plzeň Stratigraphic hiatus A 50 km S Prague Member Stratigraphic hiatus Stratigraphic hiatus Stratigraphic hiatus Duckmantian BARTÁK ET AL.—ACTINOPTERYGIAN FISHES FROM THE UPPER CARBONIFEROUS OF CZECH REPUBLIC 503 basement, and subdivided into Pilsen, Manětín, Radnice, Žihle, Kladno-Rakovník and Mšeno-Roudnice basins, the lithostratigraphic subdivision of which follows the same general terminology (Fig. 1). In the Pilsen Basin, the oldest lithostratigraphic unit represents the Kladno Formation, subdivided into the Radnice and Nýřany members, between which the stratigraphic hiatus related to the Leonian Phase of the Variscan Orogeny, lasting about 3.6 Ma, has been documented (Opluštil et al. 2016; Martínek et al. 2017). The deposits of the Radnice Member are unconformably overlaid by the fluvial sedimentation of the Nýřany Member, consisting of basal conglomerates and dominated by the grey-coloured arcoses, mudstones and claystones, in the Pilsen Basin attaining a thickness of about 290 m (Pešek et al. 1998, 2001). In addition to these deposits, intercalated volcaniclastics and groups of coal seams occur across the Nýřany Member. The zircon samples obtained from the volcanogenic interlayers have been 206Pb/238U dated, providing the absolute age of the Nýřany Member of 308.3–305.9 Ma (Opluštil et al. 2016), corresponding to the late Moscovian (Asturian) to early Kasimovian (Cantabrian) stage, i.e., the boundary of the Middle/Upper Pennsylvanian. The numerous coal seams in the Nýřany Member, representing lacustrine deposits, are usually of relatively low thickness (up to 2.5 m), and subdivided into Touškov, Nýřany, Chotíkov and Nevřeň coal groups (Pešek 1994). The Nýřany group of coal seams consists of two, in some cases three, seams attaining a thickness of about 1 meter, of which the lower, i.e., the Main Nýřany Coal Seam, yielded abundant vertebrate body-fossils. These are derived from several sapropelic horizons at the base of the coal seam, and were historically documented in some coal mines (e.g., Humboldt, Krimich) in the Nýřany and Třemošná coalfields (Fritsch 1879). The sapropelic horizons developed within the Main Nýřany Coal Seam, which produced all vertebrate remains thus far documented at the Nýřany locality, including the material described herein, indicate anoxic conditions in stagnant waters of a relatively shallow freshwater lake (Milner 1980). Material and methods The holotype of Stambergichthys macrodens gen. et sp. nov. (ÚGV PAL00174) is housed in the Palaeontological collections of the Department of Geological Sciences, Masaryk University, Brno (Czech Republic), and consists of an isolated right mandible with teeth in medial view. The isolated and fragmentary elements of two other specimens from the palaeontological collections of the National Museum, Prague, Czech Republic, and the Naturhistorisches Museum Wien, Austria, are referred to indeterminate large actinopterygian fishes. These are represented by the following material: NMP M546, right maxilla with teeth from lateral view and NHMW-Geo-2023/0311/0001, left cleithrum from medial view. All specimens are preserved on a slab of dark coal shale coming from the upper Carboniferous deposits of Nýřany, Czech Republic. In most cases, the material consists of the mineralized bone tissue of a light brown colour, which represents a common type of preservation for the vertebrate body-fossils at the locality. The holotype of S. macrodens gen. et sp. nov. was scanned with the high-resolution, micro-computed tomography device, GE phoenix v|tome|x L240, at the Central European Institute of Technology (CEITEC) in Brno (Czech Republic). The following parameters were used in the course of the specimen scanning: the accelerating voltage of 140 kV, the current of 220 µA, the voxel size of 27 µm, 0.2 mm copper beam filter. The resulting µCT slices were manually segmented in a software ITK-SNAP v. 3.8.0 (Yushkevich et al. 2006) in order to obtain the 3D surface model. The surface morphology of the teeth was imaged using laser scanning confocal microscope Keyence VK-X 1100 with the magnification of 5× and the field of view of 2730 µm; full-ring light mode and laser confocal mode were used. For imaging of micro-morphological details of uncoated tooth samples with a large field of view and depth of field, the Advanced environmental scanning electron microscopy (A-ESEM) was used (Neděla et al. 2020; Stelate et al. 2021). All A-ESEM experi ments were performed at the Institute of Scientific Instruments of the Czech Academy of Sciences using modified microscope QUANTA 650FEG (Thermo Fisher Scientific). The observation conditions were as follows: water vapour pressure of 300 Pa, beam accelerating voltage of 20 kV, a probe cur rent of 80 pA, and a working distance of 12 mm. Micrographs were recorded using wide field aperture detector WFAD for A-ESEM (Bačovský et al. 2022). Macrographic/large field images were made by composing micrographs using Maps soft ware (Thermo Fisher Scientific). The additional morphological details of the specimen were imaged with Leica DMC5400 (20 mpx) digital camera connected to the Leica MZ-16 stereomicroscope. Systematic palaeontology Gnathostomata Gegenbaur, 1874 Osteichthyes Huxley, 1880 Actinopterygii Cope, 1887 sensu Goodrich, 1930 Genus Stambergichthys nov. ZooBankLSID: urn:lsid:zoobank.org:act:07AE2773-07C0-45AA-BE3B44FFB4C2DE0A Etymology: In honour of Stanislav Štamberg (Museum of Eastern Bohemia, Hradec Králové, Czech Republic) for his long-term research on the Permo-Carboniferous actinopterygians from the European basins. Gender is masculine. Type species: Stambergichthys macrodens gen. et sp. nov., by monotypy; see below. Diagnosis.—As for type and only known species. Stratigraphic and geographic range.—Moscovian (Middle Pennsylvanian) of Nýřany, near Pilsen, Czech Republic. 504 ACTA PALAEONTOLOGICA POLONICA 69 (3), 2024 Stambergichthys macrodens sp. nov. Figs. 2–5, 6A. ZooBankLSID: urn:lsid:zoobank.org:act:9068DCBE-3FC8-4507-A25085EFEC9E7591 Etymology: From Greek μακρóς (makrós), meaning “long” or “large”, and Latin dens, i.e., “tooth”; in reference to the conspicuous teeth of the holotype. Holotype: ÚGV PAL00174, an isolated right mandible with teeth exposed in medial view (Fig. 2A1). Type locality: Nýřany, coal mine 13 km southwest of Pilsen, Czech Republic. Type horizon: Main Nýřany Coal Seam, Nýřany Member, Kladno Formation, Pilsen Basin. A volcanic ash bed located ca. 85 m above the base of the Nýřany Member has been 206Pb/238U dated to 307.05 Ma ± 0.16 Ma, corresponding to the latest Moscovian stage (Asturian substage) of the Middle Pennsylvanian (Opluštil et al. 2016). Diagnosis.—A large actinopterygian fish (estimated total length 600–700 mm) distinguished from all other early actinopterygians by the following unique combination of characters: well-developed posterodorsal process of mandible; external dermal sculpture consists of anastomosing ridges and grooves; single row of large, smooth, homodont, conelike teeth with bulbous bases; monocuspid marginal tooth apices lateromedially compressed and distinctly curved medially; subtle mesial cutting edge without serration restricted to apical portion of teeth. Stambergichthys macrodens gen. et sp. nov. differs from Acrolepis sedgwicki Agassiz, 1833, in more densely arranged teeth larger in size, and from Acropholis stensioei Aldinger, 1937, and Plegmolepis kochi Aldinger, 1937, by larger and more robust teeth fewer in number. Stambergichthys macrodens gen. et sp. nov. differs from Acrolepis gigas (Frič, 1877) by more widely rounded posteroventral part of the mandible, deeper middle portion of the mandible, angular not extending far anteriorly, and by the presence of anastomosing ridges and grooves on the dentary external surface. Stambergichthys macrodens gen. et sp. nov. differs from Brazilichthys macrognathus Cox & Hutchinson, 1991, Progyrolepis specio sus (Frič, 1875), Progyrolepis heyleri Poplin, 1999, Usclasichthys macrodens Heyler, 1977, Zaborichthys fragmentalis Štamberg, 1989, and NMMNH P-77557 (the unnamed large actinopterygian from Tinajas Member, New Mexico; Harris and Lucas 2017) by homodont dentition arranged in a single row close together, with slightly bulbous bases and medially recurved, flattened tooth tips. It further differs from Progyrolepis speciosus, Progyrolepis heyleri, and U. macrodens by smooth surface of marginal teeth, and from Z. fragmentalis by well-developed posterodorsal process of mandible. Stambergichthys macrodens gen. et sp. nov. differs from Brachydegma caelatum Dunkle, 1939, by more closely spaced teeth with bulbous bases, which extend far posteriorly to the base of the posterodorsal process. Description.—Mandible: The holotype of Stambergich thys macrodens gen. et sp. nov. consists of the isolated right mandible with teeth, exposed in the rock matrix in medial view (Fig. 2A1). The preserved portion of the mandibular ramus measures 52 mm and it is incomplete anteriorly. Most of its posterior part is present in the form of mineralized bone tissue, although the small portion corresponding to the posterodorsal process is preserved as an imprint of the lateral surface (Fig. 2A1). The mandibular ramus is disrupted by the numerous cracks, some of which are obliquely directed and resulted in both the dorsoventral and mediolateral displacement within the jaw fragment. Similarly, the bone medial surface of the mandible is damaged in the anterior region and along the ventral margin of most of the teeth, resulting in the exposure of the internal structures of the dentary (Fig. 2A1, A3). The general form of the mandible is boomerang-like, with the posteroventral border of the jaw smoothly curved dorsally to form a distinct posterodorsal process of quadrangular shape, which is elevated above the marginal dentition of the mandible. The preserved portion of the mandible is markedly deep in the central part, and gradually tapers in both the anterior and posterior directions. The majority of the lower jaw is formed by the dentary. It forms the entire anterior region of the jaw laterally and extends posteriorly slightly behind the level of the marginal teeth, where it contacts the angular and surangular, although the sutures cannot be clearly observed (Figs. 2A1–A3, 6A). The lateral surface of the dentary, imaged through the µCT, reveals the presence of a sculpture consisting of a system of elongate, anastomosing ridges and grooves (Fig. 2A2). The ornamentation extends along the entire length of the dentary and is confined to its central part, being absent in the ventral, as well as dorsal regions. It is currently unclear, to what degree this feature is affected by the taphonomy and the µCT processing, or whether the sculpture distribution represents a natural condition. The medial surface of the dentary is deeply excavated and smooth for most of its extent (Fig. 2A1, A3). It has a prominent medial wall just ventral to the tooth row, of which surface is damaged and exposes the internal structures of the dentary, including the vascularised bone tissue (bone of attachment), as well as a set of thin canals partly filled with a white aluminosilicate secondary mineralization. These canals are interpreted here as a part of the mandibular neurovascular system innerving the teeth and the mandible (see below). The narrow Meckelian groove is partly preserved in the anteroventral portion of the dentary, extending more posteriorly to form a large Meckelian fossa bordered dorsally by a prominent medial wall of the bone. The angular forms the caudal border of the mandibular posterodorsal process, and constricts the lateral extent of the dentary, although the exact boundary between the two bones is unclear (Figs. 2A2, A3, 6A). The lateral extent of the angular on the posterodorsal process of the mandible is apparent from the imprint of its lateral dermal sculpture, consisting of thin and relatively densely arranged ridges (Figs. 2A1). The angular is confined to the posterior border of the process and is extended anteriorly in the dorsal region, while ventrally it has a subquadrangular shape (Figs. 2A2, 6A). The smooth lateral surface anterior to the angular in posterodorsal part of the bone probably corresponds to the surangular, and BARTÁK ET AL.—ACTINOPTERYGIAN FISHES FROM THE UPPER CARBONIFEROUS OF CZECH REPUBLIC 505 A1 2 A 4 A 3 A 10 mm Fig. 2. The actinopterygian fish Stambergichthys macrodens gen. et sp. nov. from Moscovian, Middle Pennsylvanian, Nýřany Member, Kladno Formation, Nýřany, Czech Republic. The right mandible of the holotype, ÚGV PAL00174, as preserved in medial view (A1). µCT surface model of the mandible in lateral (A2), medial (A3), and dorsal (A4) views. The green colour in A3 and A4 represents the sedimentary infill of neurovascular mandibular canal. Dashed lines with arrows indicate fractures where displacement has occurred. 506 ACTA PALAEONTOLOGICA POLONICA 69 (3), 2024 represents the area of overlap with the posterior region of the maxillary postorbital plate (Figs. 2A1, A2, 6A). Dentition: The marginal dentition is implanted in shallow depressions of the acrodont form and consists of a single row of large, conical teeth (Fig. 2A4). There is no evidence on the presence of a set of smaller lateral teeth common in many other early actinopterygians (Poplin and Heyler 1993). In addition to the nine more or less complete marginal teeth, seven more tooth positions are present in the preserved portion of the mandibular ramus, making the total count no less than sixteen teeth in the lower jaw of S. macrodens gen. et sp. nov. The teeth are homodont and set close together; the distance between the two adjacent tooth positions is only 0.8 mm. The largest teeth are 3.5 mm high and 2 mm long on the base. The dimensions of the marginal dentition are relatively constant throughout the length of the jaw, and only minor size decreasing is perceptible in the posterior region of the tooth row. The proportions of the teeth are relatively large in comparison to the dorsoventral depth of the jaw ramus, which reaches about 2.5 to 3.5 times of their apico-basal height. The general form of the teeth is cone-like and resembles the large laniary teeth of other Palaeozoic predatory actinopterygians (Fig. 3A1–A13; Poplin 1999; Štamberg 2018, 2020). The teeth are set perpendicular to the mandibular ramus and are upright along their entire height. The base is suboval in the cross section and slightly bulbous both mediolaterally and mesiodistally. A subtle mesial cutting edge with no serration is confined to the medially curved upper-most part of the tooth crown as a result of the lateral tooth compression (Fig. 3A3–A5, A12, A13). The surface of the teeth is smooth throughout their height and shows no presence of ridges or microsculpture in a form of tubercles and protuberances seen in some other actinopterygian forms (Richter 1983; Poplin 1999; Štamberg 2018, 2020). The smooth enameloid layer is present in the specimen and occupies about 40% of the tooth height in the apical part (Fig. 3A1, A2, A6–A11). Its bright surface forms a sharp transition between it and the collar enamel of the tooth shaft, and can be seen as a very thin, superficial layer in the µCT transversal and axial slices of the teeth (Figs. 3A15, 4A6; Germain et al. 2016). The internal structure of the marginal dentition, uncovered by the µCT, shows the presence of a large pulp cavity surrounded by a moderately thick layer of the dentine (Figs. 3A15, 4A1–A5). In the cross section, this dentine layer displays the presence of a simple form of plicidentine, which is restricted to the very base of the teeth and cannot be observed in any form (e.g., grooves and ridges) on the external surface of the implanted teeth. The individual dentine folds are not ramified and do not extend to the centre of the large pulp cavity of the teeth. This simple form of the dentine infolding corresponds well to the simplexodont type of plicidentine reported recently in some actinopterygian and sarcopterygian fishes (Meunier et al. 2015a, b). Neurovascular system: The X-ray µCT imaging techniques revealed inside the mandibular ramus of S. macrodens gen. et sp. nov. the presence of a slender bony canal, located immediately below the marginal dentition, which is interpreted here as a passage for the mandibular branch of the trigeminal nerve (V, ramus mandibularis trigemini; Figs. 2A1, A3, 3A14–A16, 5). The continuity of the mandibular canal has been disrupted by the numerous fractures and short displacements that occur in the dentary of the specimen. The descending branch of the mandibular nerve, coming from the trigeminal ganglion, probably entered the mandibular ramus of S. macrodens gen. et sp. nov. in its posterior region, as reported in some extant lower actinopterygians (Allis 1922; Piotrowski and Northcutt 1996). The posterior-most portion of the preserved part of the mandibu lar canal splits into four or five subparallel branches which more anteriorly, at the level of the posterior-most teeth, pass into the lateral ramus of the mandibular canal. The lateral ramus alters in morphology from nodular to rod-like along its course, and is accompanied by the subparallel medial ramus in the posterior part of the mandible, which retains the form of a simple bar throughout its length. Both rami merge at the level of the tenth posterior-most tooth position to form a single main ramus that passes into the more anterior portion of the mandible. Its form is well exposed in the longitudinal section at the medial surface of the dentary due to the breakage of the bone (Figs. 2A1, A3, 5A4). Both rami of the mandibular canal posteriorly, as well as a single main ramus anteriorly, split into several branches towards the dentary teeth to form a dental plexus, through which the nerves and blood vessels passed, supplying the pulp cavities. In the anterior portion of the jaw fragment, a robust branch of the mandibular neurovascular canal extends posterolaterally from its main trunk and terminates in several smaller branches leading to the pores on the lateral surface of the dentary which might have innerved the external surface of the mandible. No mandibular sensory canal of the lateral line system (Leuzinger et al. 2020) could be clearly observed in the dentary of S. macrodens gen. et sp. nov., although a few narrow, elongate, parallel canaliculi present in the more ventral position of the jaw ramus might be related to the cephalic sensory system. Remarks.—The fragmentary nature of the type specimen of S. macrodens gen. et sp. nov. merits detailed account of its actinopterygian characteristics before the taxonomic assignment at the specific level and comparison to other taxa can be addressed. The thin enameloid layer confined to the apical portion of the teeth represents the only actinopterygian synapomorphy present in the holotype of S. macrodens gen. et sp. nov. (Ørvig 1978b; Patterson 1982). Although a number of other characteristics of the jaw fragment have a wider distribution among the early osteichthyians, all of these features occur to varying degrees in early actinopterygians, further supporting its attribution to this group. These include (i) simplexodont plicidentine at the tooth base, (ii) mandibular ramus dorsoventrally deep in posterior part, (iii) smoothly curved posteroventral margin of mandible, (iv) prominent posterodorsal process of quadrangular shape, (v) laterally flattened mandibular ramus, (vi) external dermal sculpture formed by elongated ridges and grooves. BARTÁK ET AL.—ACTINOPTERYGIAN FISHES FROM THE UPPER CARBONIFEROUS OF CZECH REPUBLIC 507 A thin enameloid layer is clearly present in the dentition of S. macrodens gen. et sp. nov. (Figs. 3A1, A2, A15, 4A6) and, unlike chondrichthyans where the enameloid covers most of the tooth shaft (e.g., Jambura et al. 2019), it is confined to the apical portion of the teeth, forming the characteristic acrodin caps of actinopterygians. Furthermore, the simplexodont form of plicidentine is present at the base of the marginal teeth, which is now known to be widely distributed among 4 A 5 AA 1 2 A 3 A 250 mµ 1 mm 6 A 7 A8 A 3 A 9 A10 A11 A 12 A 13 A 14 A15 A16 A 17 A 14 A15 A 1 A2 A 16 A 1 mm 1 mm 1 mm 1 mm 1 mm 800 mµ 200 mµ100 mµ100 mµ 10 mm 1 mm 1 mm 1 mm Fig. 3. The actinopterygian fish Stambergichthys macrodens gen. et sp. nov. from Moscovian, Middle Pennsylvanian, Nýřany Member, Kladno Formation, Nýřany, Czech Republic. Marginal dentition and transversal sections of the right mandible of the holotype, ÚGV PAL00174. Photographs of the marginal teeth in medial view (A1, A2). Anterior-most preserved tooth showing apically restricted cutting edge in mesial view (A3, A4, A5). Close up images from A-ESEM showing apical portion of the marginal teeth in medial view (A6, A7, A8). Morphology of the well-preserved anterior marginal teeth from A-ESEM in medial view (A9, A10, A11). 3D model of the third anterior-most preserved tooth displayed through the confocal microscopy showing mesial cutting edge (A12, A13). Transversal sections through the mandibular ramus (A14, A15, A16). Interpretative drawing of the mandible showing the course of slices and details of the teeth depicted in other figures (A17). Abbreviations: ct, cutting edge; en, enameloid; mc, mandibular neurovascular canal; mcb, mandibular neurovascular canal branch; Mg, Meckelian groove; pc, pulp cavity; pmcb, posterior mandibular neurovascular canal branches. Scale bars for A12 and A13 not available. 508 ACTA PALAEONTOLOGICA POLONICA 69 (3), 2024 the various actinopterygian forms (see Viviani et al. 2022 for the recent summary), although not necessarily confined to them (Meunier et al. 2015b). The marginal teeth arranged in two series have been interpreted to represent a plesiomorphic condition for the actinopterygians, with the large, cone-like teeth placed medially in respect to the diminutive lateral dentition (Poplin and Heyler 1993; Lund and Poplin 1997). Although this characteristic indeed frequently occurs in the Palaeozoic forms, the presence of a single marginal tooth row present in S. macrodens gen. et sp. nov. does not preclude its attribution to Actinopterygii, as it is currently known from many other early actinopterygian genera (e.g., Dunkle 1939; Lund 2000; Poplin and Lund 2000; Hamel 2005; Štamberg 2016; Stack et al. 2021; Argyriou et al. 2022), and is considered to represent an apomorphic state of this group (Lund and Poplin 1997). The mandible of S. macrodens gen. et sp. nov. is deepest in its posterior part in front of the posterodorsal process, and it gradually narrows both anteriorly and posterodorsally along the smoothly curved ventral margin of the bone (Figs. 2, 6A). The same general form of the lower jaw can be seen in many Palaeozoic actinopterygians, and the resemblance is especially evident in taxa such as Paratarrasius hibbardi Lund & Melton, 1982, “Elonichthys” hypsilepis Hay, 1900, Progyrolepis heyleri, Eurynotus crenatus Agassiz, 1835, and Brachydegma caelatum, in which the pronounced posterodorsal process is well exposed (Lund and Melton 1982; Schultze and Bardack 1987; Poplin 1999; Štamberg 2018; Friedman et al. 2019; Argyriou et al. 2022). Moreover, the mandible is markedly flattened mediolaterally, as present in early osteichthyians (Botella et al. 2007; Zhu et al. 2009), as well as sarcopterygian (e.g., Jeffery 2002; Porro et al. 2015a) and actinopterygian fishes (e.g., Arratia and Cloutier 1996; Giles et al. 2015; Figueroa et al. 2019), but absent in early tetrapods (Ahlberg and Clack 1998; Porro et al. 2015b). In early actinopterygians, the form of dermal sculpture on the external surface of the jaws and skull roof varies most commonly from the granular to ridged, both capped with a ganoine layer (Ørvig 1978a, b). The ridges may be relatively coarse and pronounced (e.g., Lund 2000; Mickle et al. 2009), or rather fine and vermicular, as in Progyrolepis (Štamberg 1991, 2018; Poplin 1999). Although the presence of the ganoine cannot be readily confirmed in the mandible of S. macrodens gen. et sp. nov., the dermal sculpture on the dentary conforms to the first pattern, whereas the second condition is present on the external surface of the angular (Fig. 2A1). All the characteristics discussed above therefore indicate that the holotype of S. macrodens gen. et sp. nov. represents an actinopterygian fish. The mandible of S. macrodens gen. et sp. nov. is in general form similar to that of other large-bodied predatory genera like Brazilichthys macrognathus (Cox and Hutchinson 1991; Figueroa et al. 2019; Figueroa and Andrews 2023), B 4 A5 A6 A A1 2 A 3 A 7 A A1 2 A3 A 6 A 5 A 4 A 3 mm 3 mm 3 mm 1 mm 1 mm 1 mm Eusthenopteron Cheirolepis Polypterus Hoplias Arapaima Stambergichthys Sarcopterygii Actinopterygii Teleostei 10 mm Fig. 4. The actinopterygian fish Stambergichthys macrodens gen. et sp. nov. from Moscovian, Middle Pennsylvanian, Nýřany Member, Kladno Formation, Nýřany, Czech Republic. Cross section through marginal dentition of the holotype, ÚGV PAL00174, showing the morphology and distribution of plicidentine. A. µCT axial slices at different levels of marginal teeth (A1, A2, A3). Interpretative drawings of cross sections at the base (A4), in the middle (A5) and close to apical tip (A6) of a single tooth. Interpretative drawing of the specimen showing the course of slices in other figures (A7). B. Phylogenetic diagram showing distribution and morphology of plicidentine in marginal teeth of selected extinct and extant actinopterygian fishes. Abbreviations: de, dentine; df, dentine fold; en, enameloid; pc, pulp cavity; vb, vascularised bone (bone of attachment). BARTÁK ET AL.—ACTINOPTERYGIAN FISHES FROM THE UPPER CARBONIFEROUS OF CZECH REPUBLIC 509 Brachydegma caelatum (Dunkle 1939; Argyriou et al. 2022), Rastrolepis riojaensis López-Arbarello et al., 2006, and acrolepids (Aldinger 1937; Gardiner 1963; Štamberg 1991, 2018; Poplin 1999; López-Arbarello et al. 2010), having elongated mandibular ramus tapering anteriorly, and the pronounced posterior articulating region elevated above the marginal tooth row. The well-preserved mandibles from the lower Permian of France and Brazil reveal that the prominent posterodorsal region of the lower jaw in Progyrolepis heyleri and Brazilichthys macrognathus consists of both angular and surangular, whereas the dentary is posteriorly rounded and is not involved in its construction (Štamberg 2018; Figueroa and Andrews 2023). Although the sutures are difficult to be traced in S. macrodens gen. et sp. nov., the congruent arrangement of these elements can also be assumed for this region (Fig. 6A). The preserved portion of the jaw fragment of S. macrodens gen. et sp. nov. measures 52 mm. However, the mandible is clearly incomplete anteriorly, and we assume that about one-quarter to one-third of the lower jaw might be missing (Fig. 6A). If this estimation is correct, then the complete mandible of the specimen could have reached about 70–80 mm in length. Based on the similarly sized lower jaws, Štamberg (1991) and Poplin (1999) estimated the total body length of Progyrolepis at about 600–700 mm. Although no information on body proportions, such as the ratio of skull length to total body length, are available in S. macrodens gen. et sp. nov., we assume the estimated length of the specimen could be roughly similar to that of Progyrolepis speciosus and Progyrolepis heyleri. In the Permo-Carboniferous continental basins of the Czech Republic, several actinopterygian species reaching a size comparable to S. macrodens gen. et sp. nov. are known, including Acrolepis gigas, Progyrolepis speciosus, and Zaborichthys fragmentalis, the material all of which was first hand re-examined for comparison purposes. The lectotype of Acrolepis gigas is an almost complete specimen preserved as part and counterpart in a large siderite concretion, which includes, among others, the skull and mandible, and measures 1250 mm in total length (Štamberg 1991). The mandible of A. gigas shows the general plesiomorphic morphology shared with similar actinopterygians of reasonable size, but is much larger compared to that of S. macrodens gen. et sp. nov., reaching a total length of at least 120 mm (Fig. 6B; Štamberg 2006). The mandibular ramus is very slender and straight anterior to the posterodorsal A 12 A 4 A 3 A 10 mm B A 10 mm 20 mm Fig. 5. The actinopterygian fish Stambergichthys macrodens gen. et sp. nov. from Moscovian, Middle Pennsylvanian, Nýřany Member, Kladno Formation, Nýřany, Czech Republic. Neurovascular system in the right mandible of the holotype, ÚGV PAL00174, displayed through the µCT. The mandible in medial (A1), lateral (A2), and dorsolateral (A3) views. Reconstruction of the mandibular neurovascular system in lateral view (A4). Green colour represents the canals of mandibular neurovascular system, yellow colour marks the position of lateral pores. Abbreviations: dp, dental plexus; mcb, mandibular neurovascular canal branch; mr, mandibular canal ramus; pca, parallel canaliculi; pmcb, posterior mandibular neurovascular canal branches. Fig. 6. The comparison of the mandible of actinopterygian fishes (A) Stambergichthys macrodens gen. et sp. nov. (Moscovian, Middle Pennsylvanian, Nýřany Member, Kladno Formation, Nýřany, Czech Republic) and (B) Acrolepis gigas (Frič, 1877) (Gzhelian, Upper Pennsylvanian, Mšec Member, Slaný Formation, Žilov, Czech Republic). A. ÚGV PAL00174, holotype, the reconstruction of the right mandible (reversed) in lateral view. B. NMP M125, lectotype, the photography of the left mandible in lateral view. 516 ACTA PALAEONTOLOGICA POLONICA 69 (3), 2024 according to Lopez et al. (2008), the fish remains are derived from the lowermost part of the Tuilliѐres-Loiras Formation, represented by the black shale deposits, indicating a deepwater lacustrine environment with anoxic conditions near the bottom (Lopez et al. 2008; Pochat and Van Den Driessche 2011). Finally, the medium-sized (<400 mm total length) elonichthyid Rhabdolepis macropterus is known from the lower Permian limnic deposits of the Saar-Nahe Basin, Germany (Gardiner 1963; Schindler 2018b). Its frequent occurrence has been documented from the clay ironstone facies of the Humberg Black Shale, which are generally considered to contain the autochthonous fauna in the deepest part of the extensive Humberg-Lebach Lake (Boy 1987; Schoch 2009, 2014). Thus, based on the overview presented above, it could be concluded that the large freshwater actinopterygians are commonly found in deepwater environments across the PermoCarboniferous continental basins in the central Pangea (Fig. 8). Although the occurrence of such fishes at Nýřany indeed could indicate a presence of deep and extensive water of body, as previously suggested by Milner (1980), we note that palaeobiological evidence alone is ambiguous and insufficient to interpret depositional environment, and other lines of evidence should be taken in account. As noted above, sedimentological data support neither the presence of deepwater environment laterally to the shallow swamp lake, nor subsidence of the basin, allowing the accumulation of a large water column. Despite this, an allochthonous aquatic vertebrate association apparently occurs at the locality (Table 1). These taxa can be recognized based on the criteria established by Milner (1980), including the relative abundance of specimens, degree of their completeness and articulation, functional morphology, as well as comparison to other faunal associations from elsewhere. The large xenacanthiform shark Orthacanthus bohemicus has been described at the locality from some skull and postcranial skeleton materials, isolated dorsal spines and numerous isolated teeth (Fritsch 1889; Heidtke 1998; SolerGijón 2004). Previous authors noted the wide geographic and stratigraphic distribution of this and other xenacanthiform genera across the Permo-Carboniferous limnic basins of the Central and Western Europe, and proposed a palaeogeographical scenario of their continental dispersion through the river channels interconnecting these basins (Schneider and Zajíc 1994; Schneider 1996; Luccisano et al. 2022). This is supported by the recent isotopic analyses of xenacanthiform teeth and spines (Fisher et al. 2013; Luccisano et al. 2023), indicating that large-bodied forms like Orthacanthus were restricted to large and deep freshwater lakes and river sysTable 1. The list of aquatic taxa considered in this study to be allochthonous at the locality of Nýřany (Czech Republic). Taxon Material References Xenacanthiformes Orthacanthus bohemicus Frič, 1875 two fairly complete juvenile individuals; articulated skull with mandibles and pectoral girdle; mandible with skull elements; pelvic girdle; numerous isolated spines and teeth Fritsch (1889, 1907); Heidtke (1998); SolerGijón (2004) Actinopterygii Actinopterygii indet. isolated maxilla with infraorbital, cleithrum this study Stambergichthys macrodens gen. et sp. nov. isolated mandible with teeth this study Dipnoi Sagenodus sp. isolated scales Fritsch (1888); Watson and Gill (1923) Tetrapoda Baphetes orientalis Milner et al., 2009 two articulated skulls, mandibles and pectoral girdle Steen (1938); Beaumont (1977); Milner et al. (2009) Diplovertebron punctatum Frič, 1879 disarticulated cranial and postcranial elements Fritsch (1885); Carroll (1970); Klembara et al. (2014) mountains land mass shallow sea present absent large-bodied acnopterygians 7 1 2 4 6 5 3 Pangea Paleotethys Fig. 8. The late Carboniferous (Pennsylvanian) palaeogeographic map showing the occurrence of large-bodied actinopterygians in selected freshwater or marine-influenced localities within continental basins of central Pangea. 1, Nýřany, Czech Republic; 2, Mšec, Czech Republic; 3, Newsham, Northumberland, UK; 4, Montceau-les-Mines, France; 5, Linton, Ohio, USA; 6, Mazon Creek, Illinois, USA; 7, Tinajas, New Mexico, USA. BARTÁK ET AL.—ACTINOPTERYGIAN FISHES FROM THE UPPER CARBONIFEROUS OF CZECH REPUBLIC 517 tems. Similarly, the dipnoan fish Sagenodus sp., known in Nýřany from several isolated scales (Fritsch 1888; Watson and Gill 1923), has been documented not only from various European continental basins (Fritsch 1888; Schultze 1993; Olive et al. 2012; Beeby et al. 2020), but also from numerous localities in the USA (Schultze and Chorn 1997). The latter authors proposed that the genus Sagenodus was euryhaline and spread from the North America to Europe via a marine connection, followed by its dispersal across the European continental basins through the river channels. Its remains are frequently found at Newsham (Beeby et al. 2020) and occur, among others, in the Mšec and Kounov members (Fritsch 1888; Lojka et al. 2009), indicating the genus might have preferred a deepwater environment, although not being necessarily restricted to it. The aquatic specializations of the baphetoids, in Nýřany represented by Baphetes orientalis, are well supported by the various morphological adaptations on the skull and mandible (Beaumont 1977; Beaumont and Smithson 1998). Westoll (1944) noted that members of the group often co-occur with haplolepids in deposits interpreted as stagnant shallow waters, while Milner (1987) assumed the baphetoids inhabited coastal brackish deepwater environments, although he pointed to their possible wide range of ecological adaptations. Although the Baphetoidea represents a rare faunal component in both the Linton (Hook and Baird 1986) and Nýřany (Milner 1980) locality, the complete articulated skulls have been reported from both sites (Romer 1930; Steen 1938; Beaumont 1977; Milner et al. 2009), indicating that their materials did not undergo a long transport to the place of deposition. We hypothesize that the scarce occurrences of the baphetoids in Nýřany reflect their ecological preferences, and these forms might have indeed primarily inhabited the river systems, with only occasional excursions to the relatively shallow lake to forage or breed. Finally, Diplovertebron punctatum is currently known from a single incomplete specimen (Fritsch 1885: pl. 52) which represents scattered skeletal remains of a possible embolomere (Carroll 1970; Klembara et al. 2014). The nature of its preservation, as well as sparse occurrence, indicate it is allochthonous at the locality (Milner 1980), although too incomplete to infer its possible habitat. The allochthonous aquatic vertebrate association documented in deposits of the relatively shallow lake of Nýřany thus does not require the presence of a nearby large and deep freshwater lake, as previously hypothesized (Milner 1980), but it can equally be interpreted as originating from the braided river system known to occur across the Nýřany Member (Opluštil et al. 2005). In context of this, the mandible of Stambergichthys macrodens gen. et sp. nov. and remains of the same or other large actinopterygian fishes in the locality are interpreted here also as derived from a river channel that could have been connected to the shallow lake for some time (Pešek 1994; Pešek et al. 1998). It is also interesting to point out here that, although largely incomplete, the allochthonous aquatic vertebrate association of Nýřany seems to resemble in general composition to some other Carboniferous localities interpreted as deepwater environments (Fig. 9). This faunal concordance can be perceived as partly artificial and reflecting a wide distribution of its constituents in various depositional settings. In other words, certain aquatic vertebrates (e.g., xenacanthiform sharks, actinopterygian and dipnoan fishes) might have used the river channels primarily to reach the more suitable destinations such as deep freshwater basins, where their occurrence is indeed well-documented. This might also be the case of large-bodied actinopterygians like Progyrolepis, i.e., Progyrolepis speciosus and Progyrolepis heyleri, which have been recorded in the Permo-Carboniferous basins of the Czech Republic (Štamberg 1991, 2013; Lojka et al. 2009), 1 4 5 3 2 1 2 3 4 5 6 7 8 4 3 2 1 5 3 4 6 7 5 Strepsodus sauroides Orthacanthus gibbosus Pholiderpeton attheyi Megalocephalus pachycephalus Acrolepis hopkinsi Sagenodus inaequalis Acanthodopsis wardi platysomids Rhabdoderma elegans Actinopterygii indet. Orthacanthus bohemicus Baphetes orientalis Diplovertebron punctatum Stambergichthys macrodens Sagenodus sp. Orthacanthus kounoviensis Acrolepis gigas Progyrolepis speciosus Zaborichthys fragmentalis Sagenodus sp. Plicatodus plicatus Acanthodes fritschi small-bodied actinoptery gians 7 8 6 1 2 3 5 4 2 1 5 3 6 7 Mšec Newsham Nýřany 4 1 2 Bashkirian Moscovian Gzhelian Fig. 9. The comparison of Pennsylvanian aquatic vertebrate associations of deepwater environment (Newsham and Mšec) with allochthonous aquatic vertebrate association of Nýřany, interpreted here as originating from braided river system. Black silhouettes represent shared faunal components at the genus level. Faunal lists based on Lojka et al. (2009) for Mšec, on various sources for Newsham (note that listed fauna of Newsham is not exhaustive). 518 ACTA PALAEONTOLOGICA POLONICA 69 (3), 2024 France (Poplin 1999; Štamberg 2018; Štamberg and Steyer 2021) and Spain (Soler-Gijón and Díez Ruiz 2023). The absence of S. macrodens gen. et sp. nov. from other continental basins of Europe may thus be related either to its endemism, sampling bias, or the simple lack of European basins of the corresponding age. The dentition of S. macrodens gen. et sp. nov. consists of large, massive, uniform teeth with pointed tips and smooth surface, which are arranged in a single row, and clearly display the carnivorous adaptations. The well-developed simplexodont form of plicidentine, found in all teeth of the jaw, indicates the dentition was adapted to withstand the high pressure loads associated with prey capturing. These morphological and structural features, combined with the relatively large dimensions of the species (estimated total length up to 700 mm), support the interpretation it occupied high trophic levels in the ecosystems it inhabited. However, the remains of much larger actinopterygians are documented in Nýřany (NHMW-Geo-2023/0311/0001; the estimated total body length probably no less than 1200 mm). Due to the lack of overlapping material, it currently cannot be resolved whether this material can be attributed to S. macrodens gen. et sp. nov., or it represents another so far unknown largebodied species. In any case, this form would have represented a gigantic actinopterygian fish likely occupying topmost positions of the food web (Fig. 9), and probably able to compete with large-bodied xenacanthiform sharks and early tetrapods co-occurring at the locality. Conclusions The isolated skeletal remains of large actinopterygian fishes are reported here for the first time from the Moscovian, Middle Pennsylvanian of Nýřany locality, Czech Republic. These include a new genus and species, Stambergichthys macrodens gen. et sp. nov., based on a single lower jaw with well-preserved teeth, which has been scanned through the micro-computed tomography and shows a presence of a complex internal neurovascular system innerving the lower jaw and teeth, and supplying both with blood vessels. The marginal dentition of the new species exhibits a single row of massive, homodont and conical teeth with the presence of simplexodont plicidentine on their base, the features that support its predatory ecology. The occurrence of isolated and fragmentary remains of large-bodied actinopterygians in the coal deposits of a relatively shallow lake indicates that their remains were probably derived from the braided river system flowing through the alluvial plain of the Nýřany Member, and represent the oldest stratigraphic record of large actinopterygian fishes in the Permo-Carboniferous sediments of Czech Republic. These results underline a potential taxonomic and palaeoecological importance of some isolated and fragmentary specimens found in coal-bearing deposits of the Central and Western Europe, being a part of the classic and well-known late Carboniferous vertebrate assemblages. Acknowledgements We are indebted to Nela Doláková (Masaryk University, Brno, Czech Republic), Ursula Göhlich (NHMW), Boris Ekrt (NMP), and Petr Budil (CGS) for the permission to study the materials in their care and assistance in the collections. Stanislav Štamberg (Museum of Eastern Bohemia, Hradec Králové, Czech Republic), Frederik Spindler (Dinosaurier Museum Altmühltal, Denkendorf, Germany) and Arjan Mann (National Museum of Natural History, Smithsonian Institution, Washington, USA) are thanked for the fruitful discussion on the taxonomic assignment of the Nýřany material. We are also thankful to Jakub Sakala and Stanislav Opluštil (both Charles University, Prague, Czech Republic), as well as Jaroslav Zajíc (Czech Academy of Sciences, Prague, Czech Republic) for the discussion on the palaeoenvironment of the Nýřany locality. Valuable comments from two reviewers, Adriana López-Arbarello (Ludwig-Maximilians-Universität München, Germany) and Sam Giles (University of Michigan, USA), helped to greatly improve the initial draft of the manuscript. This study was supported by the Specific Research Project MUNI/A/1261/2022 of the Faculty of Science at Masaryk University in Brno and the Czech Science Foundation (grant no. GA22-25799S). Author contributions’ PB, conceived the study, conducted analyses, wrote the text and prepared figures; MI, conceived and supervised the study, wrote the text; ET, MO, conducted analyses; VN, conducted analyses and wrote the text. References Agassiz, L. 1833–1844. 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