Scales of a new species of Teleosteomorpha from the continental Aptian of the south of South America are studied. These neopterygians are from the La Cantera Formation in central Argentina, and were previously identified as Pholidophoriformes. They present ganoid scales; most of them are rhombic with well-developed peg-and-socket articulations and possessing a smooth surface. They have a straight posterior margin, but occasionally, some scales of the flank have a sinuous posterior margin with one or two serrations. The shape of the scales varies along the body from large, rectangular and deeper than long scales behind the head to the preanal region to smaller and rhomboidal scales in the caudal region. There are a few horizontal rows along the flank and about 32 lateral line scales. Thick, round ganoid scales are present in the prepelvic region close to the ventral margin. The round and rhombic scales present growth lines, which form concentric ridges on the external side. A characteristic row of deep scales forms the dorsal margin on each side of the body; a row of median ridge scales is not present. This is a unique feature of the studied fishes. Scutes covered with unornamented ganoine precede the pelvic, dorsal, and anal fins, as well as the dorsal and ventral margins of the caudal fin. The posterior margin of the dorsal lobe of the caudal fin is formed by a single line of scales, which continues and covers the base of the first principal caudal ray. Histological studies reveal a lepisosteoid-scale type with multiple ganoine layers, lack of dentine, and the presence of canaliculi of Williamson. The macro- and micromorphology of the scales shows features that are found in other teleosteomorphs, but also in other neopterygians.
Cretaceous continental actinopterygian faunas from southern South America are scarce compared to marine ones. Records from the Southern Hemisphere have been less studied than those of the Northern Hemisphere (Arratia and Cione, 1996; Arratia, 2004; López-Arbarello, 2004). The La Cantera Formation represents upper Aptian continental deposits from western central Argentina. Among its biodiversity is a group of ganoid neopterygians (López-Arbarello et al., 2002; Arcucci et al., 2009, 2015). After more than 30 years of collecting, fishes are the only vertebrates recorded, but they have never been studied in detail (Castillo-Elías, 2011; Castillo-Elías et al., 2012; Giordano and Arratia, 2011, 2013).
Neopterygians from La Cantera Formation have been assigned to the order Pholidophoriformes (Berg, 1937) since 1969 (Flores, 1969). However, such assignment was based on unreliable generalized characters found in many actinopterygians (Arratia, 2000, 2013), such as specimen size, the presence of bones covered by ganoine, and ganoid scales. These were the common traits for identifying “pholidophoriforms”, a group now interpreted as non-monophyletic (Arratia, 2000, 2013). Our current studies suggest that the fishes from the La Cantera Formation are not “pholidophoriforms”. Their cranial and postcranial features represent a new combination of characters that indicates new species of neopterygians. At least two of the species from the La Cantera Formation are within the Teleosteomorpha (Giordano, 2015), and one of them is the subject of study here.
Scales of fishes can provide valid taxonomic and phylogenetic information for different taxa (e.g., Schultze, 1966, 1996, 2015; Sire and Meunier, 1994; Richter and Smith, 1995; Meunier and Brito, 2004; Vullo et al., 2009; Arratia and Schultze, 2012). Detailed descriptions (micro- and macromorphology) of these structures could be a tool for helping to clarify taxonomic assignments and possible relationships of fishes from the La Cantera Formation.
The material examined in this study originates from the type locality of the
La Cantera Formation in western central Argentina (32
Specimens studied in this work are small, approximately 5.5 cm in total length. Their body flanks are totally covered by ganoid scales, and they have a hemiheterocercal caudal fin (Fig. 2). Fishes studied herein have characters that place them within the clade Teleosteomorpha (Arratia, 2001). They share with other Teleosteomorpha the presence of a unique supramaxillary bone lying on the last dorsal third of the maxilla, two suborbital bones, a complex pectoral ray (for complex ray definition see Arratia, 2008, 88–91), two principal rays forming the dorsal and ventral margins of the caudal fin, and the main leading ray (the longest segmented-and-branched ray) corresponding to the second principal ray in the dorsal and ventral margins of the caudal fin, as well as in the dorsal and anal fins, besides other characters (Arratia, 2008, 2013; Giordano, 2015).
Detailed observations of the macro- and microstructure of the scales have been made mostly on complete or almost complete specimens, but also on disarticulated ones, as well as on isolated scales from different regions of the body (see list of samples below).
The material revised for this research is held in the following Argentinian institutions: MIC, Museo Interactivo de Ciencias in Universidad Nacional de San Luis (UNSL), San Luis; CORD, Museo de Paleontología in Universidad Nacional de Córdoba, Córdoba; and MLP, Museo de La Plata in Universidad Nacional de La Plata, Buenos Aires.
Specimens of Teleosteomorpha n. sp. 1 (after Giordano, 2015) are listed below:
Complete or almost complete specimens: MIC-V46 b; MIC-V52; MIC-V519 a; MIC-V523; MIC-V535 a, b; MIC-V568; MIC-V621 a, b; MIC-V635; MIC-V560; MIC-V568; MIC-V644; MIC-V659 a, b; MIC-V660 a, b; MIC-V661 a, b; MIC-V662; MIC-V695 a; MIC-V701 a, b; MIC-V702; MIC-V703 a, b; CORD-PZ 2027, 2029; CORD-PZ 2028, 2033; CORD-PZ 2087; CORD-PZ 2088; CORD-PZ 2092; CORD-PZ 2030; MLP 85-IV-15-1; MLP 85-IV-15-2 -4; and MLP 85-IV-15-6 -7.
Isolated scales and disarticulated-fragmented specimens (most of them not catalogued and bearing the field name): MIC-V634; MIC-V699; MIC-V700; MIC-V706 a, b; “T-5.5”, “T-99”; “T-123”; and “8”, “C1”, “C3”, “P2008”, and “5.5-AM”.
Teleosteomorpha n. sp. 1.
The senior author mechanically prepared the material, and some specimens were also prepared for histological and scanning electron microscopy (SEM) studies in the laboratories of the UNSL.
Specimens were studied by direct observation and interpretative drawings were made using a binocular microscope with an attached camera lucida (Leica M80) at UNSL, following Schultze (1966, 1996) for interpretation of scales. The surfaces of scales were observed under SEM, and histological sections were analyzed under a petrographic microscope (Leica DM750P) using regular and polarized light, following Gayet and Meunier (1986). Photographs were taken with digital cameras (Sony Cyber-shot DSC-HX1 and Canon EOS T3i).
Sample of pre-anal ganoid scales with well-developed
peg-and-socket articulation of Teleosteomorpha n. sp. 1.
Post-anal rhombic scales of Teleosteomorpha n. sp. 1. from
the La Cantera Formation.
Inner view of middle flank scales of Teleosteomorpha n. sp. 1,
showing main elements of peg-and-socket articulation, the keel, the
groove, and the peg.
Teleosteomorpha n. sp. 1. Rounded ventral pre-anal scales in specimen MIC-V621b. Note the growth lines; ant, anteriad.
Squamation of the flank of Teleosteomorpha n. sp. 1 from the
La Cantera Formation.
Postcranial region of Teleosteomorpha n. sp. 1 (MIC-V701a). Detail in photograph and drawing of uppermost scale row in lateral view. It shows the second row also, as well as the first row scale from the left side of the specimen; ant, anteriad.
The whole body flank, including the bases of the unpaired fins, is covered by ganoid scales (Fig. 2b). Most scales have a well-developed peg-and-socket articulation (Figs. 3 and 4), except the postanal ones. The anterior margin of all scales is devoid of anterodorsal or anteroventral processes. The posterior margin of most scales is straight; nevertheless, some flank scales have one serration (Fig. 3a–c). The ventral border is sinuous (Fig. 3b and c). The inner surface of each scale bears the typical keel of ganoid scales that serves as attachment for Sharpey's fibers, which connect with the keel of adjacent scales in the next vertical row (Figs. 3c and 5a). Most scales, as well as the surfaces of head bones, have a smooth ganoine surface. The scales of the most ventral scale row present a subtle concentric pattern of growth lines in the prepelvic margin (Fig. 6). The same concentric pattern can be seen in a few caudal scales (Fig. 4b and c). At the level of the dorsal fin, scales are arranged in five to seven (in exceptional cases eight) horizontal rows along the flank, demarcating a shallow body, which is about 23 % of standard length.
Shapes and sizes of the scales vary according to body regions. The scales are rectangular and deeper than long between the head and pre-anal region, and they become smaller and rhomboidal with no peg caudally, conferring more flexibility to this region (Fig. 7). The scales of each horizontal scale row, especially those in the pre-anal region, are different in shape.
The most dorsal scale row consists of a series of deep scales devoid of spines or processes. Such a scale row forms the dorsal margin of the body on each side (Figs. 8 and 9). These scales are peculiar, forming the dorsal margin in pairs; there is a row on the left side and another on the right side at the dorsal margin (Figs. 9 and 10). This pattern begins behind the skull in some specimens (Figs. 9 and 10), but it starts in about the 10th vertical scale row in others (Fig. 8a). In specimen MIC-V701, the dorsal scales have a caudad-directed point in the opposite direction to the rest of the flank scales (Figs. 2a and 10).
The second to fourth horizontal scale rows (from dorsal to ventral) carry the lateral line so that the lateral line has a middle to dorsal position on the flank. There are some exceptions where the lateral line is placed in the fifth horizontal scale row. The lateral line extends over 32 or 33 scales along the body. The lateral line scales change their shape and size, depending on the regions of the flank, as in the other scales of the body. Changes in the scales along the lateral line are gradual, but it is possible to differentiate between four morphologies: (1) the lateral line scales of the pre-pelvic region are deeper than long and present a notch on their posterior margin (Figs. 3a and 7a). (2) The scales decrease in depth in the pelvic fin region, becoming almost square, and are marked by a small notch at their posterior margin. (3) Lateral line scales above the anal fin are square (Fig. 11). (4) The lateral line scales of the caudal peduncle are rhombic, and in some cases have, in addition to the posterior opening, a pore in a nearly central position (Fig. 7b).
The lateral line ends in the last scale of the basal axial lobe of the caudal fin with the inversion of the scale rows (Fig. 12). No accessory lateral line has been observed in any specimen; however, there are some scales with additional pores in branches of the lateral line canal (Fig. 11b).
Vertical scale rows above the lateral line have the same pattern as vertical rows below it, so there is no intercalation of scale rows into the dorsal vertical rows. Three or four horizontal scales rows are arranged below the lateral line. Those of the pre-anal region are deeper than long, except the scales of the most ventral horizontal row, which are rounded to square and show concentric growth pattern (Fig. 6). All ventral scales become rhomboidal in the caudal region (Figs. 4 and 7).
In the upper lobe of the caudal fin, there are four or five scale rows with reversed direction to that of the body (Fig. 12). The most posterior scale row forms the posterior margin of the dorsal lobe with three to five scales that start just above the last lateral line scale. The row continues with a line of two to five scales that cover the base of the first principal caudal ray (Fig. 12). Both dorsal and ventral margins of the caudal fin are preceded by elongate, oval scutes. The dorsal scute is slightly broader than the ventral one; nevertheless, both scutes are almost of the same size (Figs. 12 and 13a–b). Also, large, slightly oval or round scutes precede the pelvic, dorsal, and anal fins. All mentioned scutes are covered by unornamented ganoine. Specimen MIC-V519a shows three anal scutes of different shapes and size just in front of the anal fin (Fig. 13c).
SEM studies reveal that even though flank scales seem to have a smooth
surface, at a magnification of 3000
Histological slides show a lepisosteoid-type scale where ganoine layers directly overlie the surface of the basal bony plate, and dentine is lacking between both tissues (Fig. 15a). Ganoine, as in most actinopterygians, is composed of multiple layers (Sire et al., 2009), as can be seen in photographs under normal and polarized light (Fig. 15b and c). Besides the ganoine layers, the histological microstructure shows the basal bony plate with all the lepisosteoid features (Schultze, 1966; Fig. 15d herein): stratified lamellar bone, osteocyte spaces, canaliculi of Williamson, Sharpey's fibers, and vascular canals. Sharpey's fibers are connective fibers that attach keels horizontally in neighboring vertical scale rows. Vertically, scales are connected by peg-and-socket articulation (Schultze, 1996).
The macromorphology of the scales described shows features found in other neopterygians, including most Teleosteomorpha or stem-group teleosts, with particular characters unique to Teleosteomorpha n. sp. 1.
Scales of Teleosteomorpha n. sp. 1.
Picture and camera lucida drawing of caudal fin squamation of Teleosteomorpha n. sp. 1 (MIC V523). Dorsal lobe shows the base of the first principal ray covered by scales. Note the dorsal scute preceding caudal fin; ant, anteriad; des, dorsal caudal scute; llls, last lateral line scale; 1st cr, first caudal ray; 1st r s, first ray scales; 1st pms, first posterior margin scale.
Scutes of Teleosteomorpha n. sp. 1.
Histological slides of Teleosteomorpha n. sp. 1 (specimen
MIC-V523) observed under petrographic microscope.
The clade Teleosteomorpha comprises the large group of Teleostei plus their
stem groups (Arratia, 2001: Fig. 3). According to Arratia's (2013)
phylogenetic hypothesis, the Aspidorhynchiformes, the Pachycormiformes, and
the Middle Triassic fish
Two principal types of scales are present in the clade Teleosteomorpha.
Elasmoid scales of cycloid type are the most common. These are restricted to
most Teleostei from
Scales of Teleosteomorpha n. sp. 1 from the La Cantera Formation are of the
lepisosteoid type with multiple ganoine layers, canaliculi of Williamson
(Schultze, 1966, 2015; Sire et al., 2009) in the basal bony layer, and no
dentine. The surface of the flank scales is smooth, and microscopically it
is bare of tubercles or denticles. This condition is shared with certain
teleosts, such as
The presence of ganoid scales with peg-and-socket articulation is also a
common feature of holostean neopterygians, such as semionotiforms,
lepisosteiforms, and macrosemiiforms (Schultze, 1966, 1996, 2015). Ganoid
scales of Teleosteomorpha n. sp. 1 of the La Cantera Formation present a
well-developed dorsal peg, but they lack an anterodorsal process. The same
pattern occurs, for example, in the Macrosemiiformes (e.g.,
Teleosteomorpha n. sp. 1 presents deeper than long flank scales in their preanal region, as in other Teleosteomorpha with ganoid scales (see Schultze, 1966; Brito, 1997; Brito and Ebert, 2009; Bartholomai, 2004; Arratia, 2000, 2013; Gouiric-Cavalli, 2015). Even though most of the scales studied have straight margins, some have a sinuous posterior margin on the same specimen. In this sense, these scales resemble the scales of some species of the Triassic family Pholidophoridae (Zambelli, 1977; Arratia, 2013). Unique to the fishes from the La Cantera Formation is the undulating ventral margin of the scales (Fig. 3).
The thick, round scales in the ventral margin of the pre-anal region are
ganoid scales and not elasmoid scales, in contrast to the round amioid
scales in the anteroventral part of the body of Macrosemiiformes. There are
amioid scales in the gular region of some species of macrosemiiforms and, furthermore, there are ganoid scales on the flanks of the same individual
(e.g.,
The most dorsal scales of the flank are unique to Teleosteomorpha n. sp. 1 from the La Cantera Formation in being paired structures. Most basal neopterygians have a row of unpaired, bilaterally symmetrical ridge scales in the dorsal midline. For example, these dorsal ridge scales are characteristically elongated into a spine in the family Semionotidae (Olsen and McCune, 1991; López-Arbarello, 2012; Gibson, 2013). Paired scales like those present in Teleosteomorpha n. sp. 1 from the La Cantera Formation have not been reported in any other neopterygian. Indeed, this structure could be considered an autapomorphic character of this South American species.
Ganoine-covered scutes around the anus are not known among stem-group
teleosts such as pachycormiforms, aspidorhynchiforms,
Ganoine-covered scutes preceding the caudal fin are known within
Halecomorpha in general, for example
Rhombic, modified ganoid scales covering parts of the bases of the upper
epaxial caudal rays were defined as urodermals by Patterson (1968) and Arratia
and Schultze (1992). The base of the uppermost caudal principal fin ray in
specimens of Teleosteomorpha n. sp. 1 from the La Cantera Formation is
covered by a row of scales that cannot be considered as urodermal-like
elements. Urodermals have been described among teleosteomorphs for
The analysis of the morphological diversity of scales within Neopterygii could provide a set of characters to be used for interspecific differentiation and to show patterns of separation at higher taxonomic levels.
As in most other basal neopterygians, the fishes described herein show ganoid scales of the lepisosteoid type. The scales of Teleosteomorpha n. sp. 1 from the La Cantera Formation share many common features with other teleosteomorphs in their shape and arrangement. One of the most significant characters is the presence of a prominent scute preceding the dorsal margin of the caudal fin. Unique to the fishes studied here are deep paired dorsal scales with rounded dorsal margins and the lack of median dorsal ridge scales. Among features that Teleosteomorpha n. sp. 1 shares with holosteans are ganoine-covered scutes related to the anal fin.
P. G. Giordano and G. A. initiated and designed the manuscript. P. G. Giordano made the description of the scales and wrote the manuscript with the collaboration of G. Arratia and H.-P. Schultze. Careful corrections of content, as well as language and figures, were made by G. Arratia and H.-P. Schultze Illustrations were made by the first author.
The senior author would like to thank Andrea Arcucci (UNSL) and Laura Codorniú (CONICET-UNSL) for encouraging the publication of the present manuscript. The authors thank William E. Bemis (Cornell University) and an anomymous reviewer for their useful comments. We thank Mercedes Prámparo (CONICET-Mendoza) and Gabriela Castillo-Elías (UNSL) for their help with the geological background, professors and students of the paleontological program at UNSL, and all volunteers that provided fieldwork assistance to collect fishes from 2008 to the present. We are grateful to Esteban Crespo (CONICET-UNSL) for his help and assistance with SEM imagery, Federico Gianechini (CONICET-UNSL) for his help with photographs and figures, Daniel Codega (UNSL) for the preparation of petrographic sections, and Ignacio Cerda (CONICET-UNRN) for providing literature. Alejandra Mazzoni (Museo de Paleontología, UNC) and Alberto Cione (CONICET-Museo de La Plata, UNLP) kindly allowed us to study material from the La Cantera Formation held in their institutions. Terry J. Meehan (Rockhurst University, KC, MI) revised the English style of the manuscript. Funding was provided by project PROICO p-3-2-0114 from the Secretaría de Ciencia y Técnica, UNSL, and CONICET (Argentina). Part of this research was conducted at the Biodiversity Institute, University of Kansas, throughout two visits by the senior author. Edited by: F. Witzmann Reviewed by: W. Bemis and one anonymous referee