Actinopterygii

Barták, Pavel, Ivanov, Martin, Tihlaříková, Eva, Olbert, Martin & Neděla, Vilém, 2024, New, large actinopterygian fishes from the upper Carboniferous of Nýřany, Czech Republic, Acta Palaeontologica Polonica 69 (3), pp. 501-522 : 511-512

Actinopterygii indet.

Fig. 7 .

Material.—NMP M546, isolated right maxilla with teeth in lateral view and a possible infraorbital bone; NHMW- Geo-2023/0311/0001, isolated left cleithrum in medial view, both from the Middle Pennsylvanian (Moscovian) of the Nýřany locality, Czech Republic.

Description.—NMP M546 represents an isolated right maxilla exposed in lateral view with an associated fragment of a possible infraorbital bone ( Fig. 7A ). The preserved portion of the maxilla is fairly large and measures about 61 mm. The elongated suborbital ramus is 40 mm long and it appears to be broken transversally in the middle part, resulting in a slight displacement of its anterior portion ventrally relative to the bone posterior part. The anterior portion of the suborbital ramus is slightly curved and terminates in a rounded tip with lateral rugosity. The postorbital plate of the maxilla is well developed and consists of a high and rounded dorsal part, preserved as an imprint of the bone medial surface, and a ventral projection extending below the toothed ventral border. The latter structure appears to be incomplete, based on the sharply bevelled posteroventral margin of the bone. The postorbital plate is elongated dorsoventrally, short anteroposteriorly, and forms nearly right angle with both the dorsal and ventral borders of the suborbital ramus. The length/height ratio of its preserved part is equal to 0.9. The lateral surface of the maxilla is in the posterior part formed by the granular ornamentation, while its medial surface consists of fine radial grooves. The maxilla preserves about twelve tooth imprints, which are restricted to the ventral margin of the suborbital ramus and do not extend behind the anterior level of the postorbital plate. The largest teeth are located in the central part of the toothed ventral margin and reach about 3 mm in height and 2 mm in length at the base. The posterior-most teeth rapidly decrease in size and reach only about half the height of the largest teeth. There is also some indication on the tooth reduction towards the anterior end of the suborbital ramus, although it is unclear whether teeth were truly absent in this part of the bone. The dentition is homodont and consists of a single row of densely spaced, cone-like to bulbous upright teeth, with some of them showing a longitudinal groove along the centre of the tooth shaft. In large and well-preserved teeth, the ratio of the tooth height/longitudinal diameter varies from about 1.4 to 1.6 and the distance between adjacent tooth positions/ tooth longitudinal diameter is about 0.2. An elongated bone fragment of subrectangular shape located dorsal to the maxillary suborbital ramus is tentatively interpreted here as an infraorbital based on its form and position.

NHMW-Geo-2023/0311/0001 represents a fragment of the isolated left cleithrum exposed in medial view, which can be clearly referred to a large actinopterygian fish Fig. 7B ). It forms a massive dermal bone of the pectoral girdle, with a preserved portion of the element reaching 88 mm in height and 41 mm in length. The principal parts of the preserved cleithrum fragment consist of an elongate dorsal process and a broadly expanded central region. The ventral and posterior parts of the bone, including the notch for the connection of the pectoral fin and the ventral lamina, are broken-off in the specimen. The dorsal process is narrow, triangular in shape and distinctly elongate; it measures 43 mm in height and its base is 11 mm long. The longitudinal axis of the process is directed anterodorsally, and it gradually narrows along its vertical length to a pointed tip, which bears an oblique, rugose surface. A prominent ridge present on the medial side of the cleithrum in many actinopterygian forms (e.g., Pearson and Westoll 1979; Long 1988; Štamberg 2007) extends along the midline of the dorsal process and continues ventrally on the medial surface of the expanded central region, dividing the bone into two parts. The posterior part of the dorsal process is narrow and consists of the convex margin with remnants of the medial lamina forming a caudal border of a shallow groove posterior to the medial ridge of the cleithrum. The medial lamina is broken-off along most of its length, so its original extent is unknown in the specimen. The anterior margin of the dorsal process is slightly concave, and the bone surface in front of the medial ridge is moderately convex, particularly on its base. The expanded central region of the bone is strongly extended anteriorly, and its convex margin joins the dorsal process of the cleithrum at an almost right angle. The medial ridge, extending from the dorsal process of the cleithrum ventrally to the expanded central region, curves at the mid-length of the bone anteriorly, where it gradually diminishes and is replaced by the distinct groove directed anteroventrally. The medial surface of the cleithrum is roughened at some places, and comprises of numerous very fine pits and grooves.

Remarks.— NMP M546 was originally briefly described and figured by Fritsch (1885: pl. 61: 5) as a putative palatal bone associated with the early tetrapod “ Gaudrya latistoma ”, although a possible affinity to actinopterygian fishes was also noted. The morphology of this isolated element is reassessed here and interpreted as belonging to the right maxilla of a large actinopterygian fish. Fritsch (1885) noted the resemblance of NMP M546 to “ Amblypterus -like” actinopterygians, i.e., various amblypterids and a sceletophorid sensu Štamberg 2013). The postorbital plate of the maxilla has a rounded dorsal margin and is anteroposteriorly short compared to its height, the condition which indeed most resembles to amblypterids ( Gad 1988; Dietze 1999; Heyler 2000), Sphaerolepis Frič, 1877 , and Sceletophorus Fritsch, 1894 ( Gardiner 1967; Štamberg 1991, 2013) and gonatodids ( Traquair 1907; Gardiner 1967; Elliott 2018). However, the dentition of NMP M546 markedly differs from the distinctive tubular marginal teeth of Amblypteridae , although exhibits some resemblance with their more robust, cone-like coronoid teeth, and it neither can be associated with species of Sphaerolepis , Sceletophorus , and Gonatodidae based on the details of the dentition ( Gardiner 1967; Štamberg 2020). In addition, the large dimensions of the maxilla do not support its attribution to any of the taxa mentioned above. The distinct rounded morphology of the postorbital plate differs from the large tetragonal-shaped bone present in most other large-bodied genera and likely represents a unique feature of the specimen, although this morphology might be affected by taphonomic processes to some degree. The slightly bulbous tooth shafts, oriented perpendicular to the suborbital ramus, are densely spaced and arranged in a single row, as also documented in S. macrodens gen. et sp. nov. In addition, both the teeth proportions and dimensions of the maxilla would also support the attribution to the new taxon. However, its unambiguous reference to this species is precluded by the absence of overlapping material, thus until the more complete specimens are discovered, it is retained here as an indeterminate actinopterygian fish.

The general morphology of the cleithrum in NHMW- Geo-2023/0311/0001 conforms that of other Palaeozoic early actinopterygians,and most resembles to Progyrolepis heyleri , Brachydegma caelatum , Angatubichthys mendesi Figueiredo & Carvalho, 2004 , and Elonichthys germari Giebel, 1848 ( Figueiredo and Carvalho 2004; Schindler 2018a; Štamberg 2018; Argyriou et al. 2022) in that it has elongated and upright dorsal process, and distinctly expanded central region. The dorsal process differs in Cosmoptychius striatus (Agassiz, 1835) , in which it has been reconstructed as markedly arched anteriorly ( Gardiner 1963). The anterior border of the dorsal process and the expanded central region form an almost right angle, the condition that differs from the smooth, concave transition between the two parts seen in Progyrolepis heyleri and other forms, where the bone region is well-documented (e.g., Pearson and Westoll 1979; Gardiner 1984; Štamberg 2007, 2018; Coates and Tietjen 2019; Argyriou et al. 2022), but somewhat resembles Elonichthys germari ( Schindler 2018a) . This and the anteroventrally curved medial ridge of the expanded central region, which gradually diminishes to be replaced by a deep groove, may represent the distinctive characters of the specimen. Furthermore, the dimensions of the preserved dorsoventral part of the cleithrum ( 88 mm in height) indicate a very large individual, of which length would have probably exceeded that of most Palaeozoic actinopterygian fishes. For example, the complete cleithrum of Progyrolepis heyleri reached only about 50 mm in total height ( Štamberg 2018). Based on a rather peculiar morphology of the bone not seen in most other early actinopterygians, there is a possibility that the specimen could belong to a larger individual of S. macrodens gen. et sp. nov. However, due to the lack of more complete material, it is currently not possible to compare NHMW-Geo-2023/0311/0001 with the holotype of the latter, thus it is provisionally retained here as an indeterminate large-bodied actinopterygian.

Figures

Fig. 7. Isolated skeletal elements referred to indeterminate large actinopterygian fishes from Moscovian, Middle Pennsylvanian, Nýřany Member, Kladno Formation, Nýřany, Czech Republic. A. NMP M546, right maxilla in lateral view, photography (A1) and interpretative drawing (A2). B. NHMW- Geo-2023/0311/0001, left cleithrum in medial view, photography (B1) and interpretative drawing (B2).