Paranaichthys, Dias, Eliseu Vieira, 2012

Dias, Eliseu Vieira, 2012, A new deep-bodied fossil fish (Actinopterygii) from the Rio do Rasto Formation, Paraná Basin, Brazil, Zootaxa 3192, pp. 1-23 : 3-15

Paranaichthys gen. nov.

Type species. Paranaichthys longianalis sp. nov.

Etymology. “ Parana ” in reference to the Paraná sedimentary Basin in the Paraná State where the fossil was found; from the Greek “ ichthys ” meaning fish.

Distribution and age. Rio do Rasto Formation in the northeast of Paraná state; Permian, Guadalupian (Wordian-Captanian).

Generic diagnosis. As for species, by monotypy.

Paranaichthys longianalis sp. nov.

( Figs. 3–11 )

Holotype. MCP –3371–PV, a fossil fish of around 20 cm in length and 14.5 cm in height, deposited in the collection of Museu de Ciências e Tecnologia da Pontifícia Universidade Católica do Rio Grande do Sul.

Type locality. Outcrop at the town of Santo Antônio da Platina, in the northeast of Paraná State, BR–153 highway, Km 42 ( Fig. 1 ), geographic coordinates 23° 17' 34'' S; 50° 4' 42'' W. The specimen was collected in the late 1980’s and, unfortunately, a new track of the highway constructed in the 1996 partially destroyed the outcrop.

Stratigraphic horizon. Yellowish carbonate siltstone from the top of the Serrinha Member, Rio do Rasto Formation, Passa Dois Group, Paraná Basin.

Etymology. The specific name “ longianalis ” refers to the remarkably elongated anal fin.

Diagnosis. Deep-bodied actinopterygian fish, 1.5 times deeper than long; lateral flank scales three times higher than long; head is short and posteriorly high, around 0.18 of total length; crushing palatal dentition; dentary narrow and long; no evidence of marginal teeth; snout blunt; numerous small conical odontodes occur in the anterior portion of the rostro-premaxilla and the dentary; operculum 1.7 times higher than long and larger than the suboperculum; frontal elongated and almost rectangular; anal fin base wide and extends posteriorly until the base of the caudal peduncle; anal fin remarkably elongated; caudal fin heterocercal, high and boomerang-shaped; pelvic fin wide-based and different in shape compared to the pectoral fin; fringing fulcra present in pelvic and anal fins and on the ventral lobe of caudal fin; three fulcral scales anterior to anal fin; two fulcral scales forward of ventral lobe of caudal fin; at least nine dorsal fulcral scales forward of caudal peduncle; seven long fulcra supporting the chordal lobe at the level of the caudal peduncle, and gradually smaller fulcra covering the chordal lobe posteriorly.

Remarks. The prominent conical odontodes on the lower jaw and snout, associated to the elongated anal fin, could be due to sexual dimorphism but impossible to confirm based on a single specimen.

Description. Due to poor preservation of the holotype, the known postcranial features are considered more secure to identify the new taxon than cranial features, which would change only with other specimens that can provide a more precise description of the skull bones. Therefore, gaps in the reconstruction represent areas where the morphological interpretation is inaccurate. The proposed reconstruction of the skull must be treated as a hypothesis, and the discovery of further specimens may provide new data to improve it.

Body shape and dimensions. The body shape in Paranaichthys longianalis is deep and laterally compressed with a rounded ventral outline, with an angle around 130º at the level of the anal fin. The dorsal outline of the specimen MCP–3371–PV is incomplete due to a straight broken line from the back of the head to the posteriormost rays of the dorsal fin, which is almost totally missing. Observing the head bone dimensions and scale superposition on the specimen, especially near the cleithrum and anal fin region, the probable original body form was robust with moderate lateral compression. The head is relatively small when compared with the body proportions and the caudal and anal fins are prominent ( Figs. 3 , 4 and 6 ). The measures were taken on the reconstruction as shown in Figure 2. The total length of the fish was estimated around 215 mm. The standard length is 170 mm, the preserved height is 129 mm, and the total height is estimated to be around 141 mm. This indicates that the fish is around 1.5 times longer than deep. The body length is 131 mm and the length of the trunk is 93 mm, which represents 0.7 times the body length. The total length of the caudal region is 83 mm, 0.6 times the body length and almost 0.4 times the total length. The post-cranium length including the caudal fin is about 175 mm ( Table 1).

Skull. Due to the poor preservation of the anterior part of the fossil many bones are missing or incomplete, rendering difficult the interpretation of cranial morphology ( Figs. 3–5 ). Apart from the opercular bone and skull roof, where it was possible to identify greater pieces of the frontal and dermopterotic, all head bones described were tentatively identified from fragments. For example, the opercular and subopercular bones were damaged during fossilization but it was still possible to estimate their probable original shape. However, the fossilization of other parts of the head was worse, especially the anterior portion, which limited the skull reconstruction. Even so, a tentative reconstruction is proposed ( Figs. 6–7 ). The head is 1.6 times higher than long and relatively small, measuring 62 mm height and 39 mm length, representing 0.23 times the standard length.

Measurements (mm) Total length (a) 215 Standard length (b) 170 Body length (c) 131 Trunk length (d) 93

Head length (e) 39

Caudal peduncle length (f) 13

Caudal fin length (g) 45

Total length of the caudal region (h) 83

Caudal fin height (i) 147 Caudal peduncle height (j) 31

Estimated body height (k) 141 Head height (l) 62

Anterior anal fin known length (n) 30

Anal fin base length (o) 53

Estimated posterior anal fin length (m) 69

Pelvic fin height (p) 32

Pelvic fin base length (q) 26

Pectoral fin width (r) 20

Pectoral fin length (s) 23

Frontal bone length (t) 15

Post-cranial length (d+h) 175

Skull roof. In the holotype of Paranaichthys longianalis , the frontal, dermopterotic, parietal and extrascapula of the left side are preserved. The dermosphenotic is incomplete and located antero-ventrally to the dermopterotic, and ventrally to the frontal ( Figs. 5 and 7 ). The frontals are slightly elongated antero-posteriorly, almost rectangular in shape with rounded anterior borders. Frontal length is 1.3 times the dermopterotic length. The frontal presents small irregular bean-shaped odontodes, ordered in curved lines that occur over the entire bone surface. The dermopterotic is a sub-rectangular bone as wide as the frontal, around 5 mm wide, with a similar ornamentation. The dermosphenotic is incomplete. Only the dorsal sutures of the left dermosphenotic are preserved; these are positioned dorsally with the frontal, dorso-posteriorly with the dermopterotic, and dorso-anteriorly with the posterior portion of the postrostral. The boundaries between the dermosphenotic are unclear and its anterior border was also not preserved. Due to bad preservation, a small triangular piece of bone that could be a dermohyal is present. Even though the dermohyal is uncommon in platysomids, it is present, for instance, in Paramesolepis tuberculata Traquair ( Moy-Thomas & Dyne, 1938) and Cheirodopsis geikiei Traquair ( Moy-Thomas & Dyne, 1938) , both taxa listed in the Platysomus Group of Gardiner & Schaeffer (1989) and also included within platysomids in the cladogram of Mickle et al. (2009). Therefore the dermohyal is considered to be present in Paranaichthys longianalis . The parietal is posterior to the dermopterotic and is rectangular, almost 2.4 times deeper than long, with 12 mm in height. The extrascapula is the most posterior bone of the skull roof and is medially incomplete.

Snout. The sensory line canals of the snout, as well as other parts of the skull, were not preserved, thus bone identification is problematic, making hard to compare the snout of P. longianalis to the snout patterns presented by Gardiner (1963). Three bone fragments are present in the snout region: the rostro-premaxilla, the posterior part of the postrostral ( Figs. 5 and 7 ), and a third incomplete oval bone, preserved ventrally to the rostro-premaxilla, remains unidentified ( Fig. 5 ). It could be a lacrimal or a displaced nasal but it is difficult to confirm these possibilities. The bone tentatively identified as rostro-premaxilla is relatively thick and wide, with a concave ventral edge and a small lateral notch, which probably forms the opening of the anterior nostril (nasal opening). It is preserved in internal view but removing part of the bone, the external surface was exposed showing that it is covered by small conical and pointed odontodes that resembles the ornamentation of the rostral region of Redfieldiiformes ( Schaeffer 1967, 1984; Schaeffer & McDonald 1978). The bone tentatively identified as the postrostral is also incomplete; its posterior portion is as wide as the frontal, presenting elongated odontodes. Its anterior portion is not preserved. In the proposed reconstruction ( Fig. 6 ) the length of this bone was estimated.

Jaws. A squared bone fragment of the maxilla is preserved ( Fig. 5 ) and is ornamented by elongated odontodes that radiated from the center. There is no evidence of marginal teeth, but in this case it could be due to bad preservation. Numerous crushing teeth preserved below this bone fragment could belong to the palatal bones. The rostropremaxilla is also edentulous. The dentary represents the preserved portion of the lower jaw, and is incomplete in its proximal portion. It presents at least 18 mm in length and is also edentulous. Dentary ornamentation is visible only in the anteriormost portion and is composed of small pointed or conical odontodes that resembles the ornamentation of the rostro-premaxilla ( Fig. 7 ). The coronoid and splenial bones could not be distinguished.

Opercular series. The operculum is the biggest preserved bone of the dermatocranium and probably is in its original anatomical position ( Figs. 5 and 7 ). It is irregular in shape, but almost trapezoid, 1.7 times higher than long. The small ornamented portion remaining is composed of elongated odontodes, arranged converging the ventral part of the operculum, near the suboperculum suture. The suboperculum is heavily damaged, and is smaller than the operculum, almost square in shape and without preserved ornamentation. Anteriorly to the suboperculum and posteriorly to the fragment of the maxilla, there is a bone whose borders could not be determined. It is similar in position to the preoperculum3 of Ecrinesomus dixoni Lehman 1956 , to the suborbital of Ebenaqua ritchiei Campbell & Le Duy Phuoch 1983 , and to the quadratojugal of Platysomus schultzei Zidek 1992 , which was identified as the “platysomid quadratojugal” by Mickle & Bader (2009). In the reconstruction of Paranaichthys longianalis it is tentatively indicated as a preoperculum(?). Ventrally to the suboperculum several pieces of damaged bones are present, but due to uncertainty they were not included in the description and reconstruction.

Palate. The parasphenoid is the only bone identified at the base of the cranium. It is incomplete; however, it is an elongated bone with several rounded ventral teeth ( Figs. 5 and 7–8A ). The other palatal and pharyngeal bones are not preserved but these were probably also covered by crushing dentition, as indicated by the presence of this kind of teeth in profusion in the orobranchial chamber.

Teeth. No clear marginal teeth were recognized. The dentition of Paranaichthys longianalis comprises cylindrical-based teeth and conical teeth. Measuring around 0.25 mm in diameter and 1.2 mm in length, these cylindrical-based teeth present rounded tips and are inserted on the palate, at least in the parasphenoid ( Fig. 8A ). The conical teeth are shorter but similar to the cylindrical ones in basal diameter. The presence of numerous teeth in the oral region suggests that they had a wide distribution over the bones of the oral cavity and were probably a specialization for crushing. The high number of teeth present also suggests that they could be arranged in groups, making up a tooth plate like those described for platysomids ( Johnson & Zidek 1981), the amphicentrid Amphicentrum granulosum ( Zidek, 1992) , and Bobasatrania ceresensis Burgin 1992 . However, poor preservation precludes the identification of tooth plates in P. longianalis .

Histology of crushing dentition. The crushing teeth present a large pulp cavity and the dentine (orthodentine) has elongated tubules with lateral short ramifications ( Figs. 8 B–C). The apical region of some crushing teeth presents a small rounded portion of enameloid, forming an incomplete cap of acrodin ( Fig. 8 B–D, v acr), very similar to the verruciform acrodin of Colobodus Meyer and Nephrotus de Alessandri (∅rvig 1978b). A thin enamel layer of about 1.4Μm thickness covers the teeth, which can be referred to as collar ganoine following Richter & Smith (1995). Over the verruciform acrodin, the collar ganoine becomes more slender or locally absent ( Fig. 8 D). The histological structure seems to differ from the phyllodont tooth plates described by Johnson & Zidek (1981), as neither a superimposed series of teeth nor reduced pulp cavity with a basilar foramen were identified in P. longianalis .

Dermal skull bone histology. A small piece of the maxilla presenting elongated odontodes was sectioned for histological observations. This dermal bone cross section shows some dome-like odontodes with small pulp cavities in transverse view; in longitudinal view the odontodes are plateau-like structures with elongated pulp cavities ( Figs. 9A –B). Some odontodes formed over an older odontode, indicating that not only lateral accretion but also superpositional growth (sensu ∅rvig 1978a) is present in the new taxon. In cross section, the ganoine seems to cover the whole surface of the bone.

Shoulder girdle. The shoulder girdle is ventrally and dorsally incomplete and the clavicle is not preserved. The supracleithrum is present, but the post-cleithrum seems to be absent. The supracleithrum is narrow and 0.2 times the height of the shoulder girdle. The post-temporal is dorsally incomplete and is around 0.3 times of the shoulder girdle height. The preserved portion of the cleithrum is very narrow and high, becoming wider at its ventral portion; there is no evidence of ornamentation. The estimated height of the cleithrum is around half of the total shoul- der girdle height ( Figs. 6 and 7 ).

Fins. The pectoral fin is incomplete and displaced from its original position, at least 20 mm in width and 23 mm in length. The pelvic fin is nearly triangular, 32 mm in height with a wide base of 26 mm, but only four basal rays measuring 1.3mm width each are preserved; fringing fulcra are present. Although incomplete distally, the anal fin is remarkably long and wide-based ( Figs. 3–4 and 6 ). The anal fin base measures around 53 mm ( Table 1) and presents at least 55 rays. The preserved portion of the anal fin measures around 30 mm on the anterior margin ( Fig. 2 ) probably with 16 to 22 lepidotrichia, which measure from 3 mm at the base to 1 mm distally. The posterior margin is obscured by the caudal fin, precluding precise measurements. The estimated posterior anal fin length measured around 69 mm.

The caudal fin is also incompletely preserved, lacking the most distal part of the chordal lobe, and its ventral lobe is displaced over the distal part of the anal fin. Despite the incompleteness, it is reconstructed as heterocercal and almost equilobate, at least twice higher than long, and with a well-developed chordal lobe. The ventral lobe is better preserved and it seems to be as long as, or even longer than the chordal lobe, forming a boomerang shaped caudal fin with an angle of about 120 degrees. Its height is about 147 mm, approximately as high as the body. The caudal peduncle is well-developed and taller than long ( Figs. 3–4 and 6 ).

Although the dorsal fin is not preserved, its position in P. longianalis is indicated by a small set of lepidotrichia on the dorso-posterior portion of the body, which indicates the position of its posteriormost rays as somewhat anterior to the first dorsal fulcra of the caudal region, resulting in an asymmetric position in relation to the anal fin that is not commonly found in platysomids ( Figs. 4 and 6 ).

Scale morphology. Most of the scales of P. longianalis are not very well preserved and some parts were omitted in the reconstruction ( Fig. 6 ). Due to the taphonomic compression, the scales overlap each other more than usual, creating many fractures, mainly in their posterior borders ( Fig. 10A ).

The trunk scales are deep, rectangular and 3.5 times higher than long, measuring almost 14 mm in height and 4 mm in length. These scales present a peg-and-socket articulation and have their surface ornamentation composed of ganoine with some punctuations, ridges and furrows that sometimes bifurcate. In the dorsal half of each scale the furrows are diagonal, however, in the ventral half, they are shallower and parallel to the body axis, and continuous with fewer dichotomies. As seen with the scanning electron microscope (SEM), some furrows are contiguous to the punctuations and distally interrupted ( Fig. 10 B). The ganoine surface present openings of variable shape, from a small circular punctuation of 10 Μm to furrows of less than 10 m in width that may be elongated towards the posterior end of the scale ( Fig. 10A –B). Ridges occur between the furrows, which can also be elongated at the posterior end of the scale. In the bottom of the furrows, exposed dentine can be seen under SEM and the tubules appear as small openings ( Fig. 10 B– C). The ganoine surface is ornamented with aligned, elongated microtubercules of 10 Μm in length ( Fig. 10 C).

In the region of the anal fin base, the scales measures 6 mm in height and 4 mm in length, thus they are less deep. The ornamentation is more delicate with slightly shallow and narrow furrows.

In the caudal peduncle, the scales are lozenge-shaped and longer then tall, measuring 5 mm in length and 2 mm in height. The ganoine cover is very thick and its ornamentation is composed of about 10 furrows parallel to the body axis, with few interruptions and without bifurcations. The chordal lobe is laterally covered by small elongate lozenge-shaped scales that are non-imbricating and without preserved ornamentation in which the peg-and-socket articulation seems to be absent, a similar configuration to that of Amphicentrum jurgenai Zidek 1992 .

Fulcral scales are present anterior to the anal fin and ventral and dorsal lobes of the caudal fin. There are three enlarged semicircular fulcral scales preceding the anal fin base, and two fulcral scales anterior to the ventral lobe of the caudal fin. Little ornamentation is preserved on them. The dorsal fulcral scales of the caudal region are composed of a series of nine anterior fulcral scales covering dorsally the anterior portion of the caudal region, and increasing in size postero-ventrally. Their ornamentation shows five convergent ridges and each one corresponds to a pointed termination at the posterior border of the scale. The dorsal surface of the caudal peduncle presents seven long and narrow spike-like fulcral scales that decrease in size toward the chordal lobe. These large-sized elongated fulcra in the caudal peduncle gave support to the base of the chordal lobe. The fulcra covering the chordal lobe itself diminish gradually. The most distal portion of the chordal lobe is damaged but some isolated smaller fulcral scales were preserved and these probably cover the entire chordal lobe roof.

Scale histology. As typical for primitive actinopterygian fishes, the scales of P. longianalis are three layered: a cellular bone (isopedine), dentine, and an external cover of ganoine. The lamelar isopedine is basal, comprising almost 0.7 times the scale thickness with evidence of osteocite lacunae. Under SEM the dentine appears to be composed of pine tree-like tubules with many secondary lateral ramifications of smaller diameter, which was considered to be orthodentine. The thickness of the ganoine layer is variable, depending on the region of the scale and it position on the body. The fulcral scales of the caudal region present a thick ganoine cover ( Fig. 11A ). In cross section the punctuation and the furrows were not distinguished and were identified as punctuations for simplification ( Figs. 11 B–C). The scales show a growth pattern by lateral accretion in which they grow in area, defined as areal growth (∅rvig 1978a), resulting in the superposition of ganoine ( Figs. 11A and C, zs).

Scale counts. Since the majority of scale numbers were estimated, a squamation formula is not presented. The trunk scales of the lateral line system have at least 18 preserved scales, but it is estimated that originally there were about 26 to 30 scales. From the ventral portion of the cleithrum to the pelvic fin origin there are at least 10 scale rows, although they are badly preserved. From the same portion of the cleithrum to the middle of the anal fin, the estimated number is 30 scales. There are at least 12 scales counting from the most dorsal preserved scale to the pelvic fin base. In the caudal peduncle there are 8 dorsoventral scales, without counting the fulcra.

Figures

FIGURE 1. Map of the type locality of Paranaichthys longianalis gen. et sp. nov. (Brazilian states: RS, Rio Grande do Sul; SC, Santa Catarina; PR, Paraná, SP, São Paulo; MG, Minas Gerais; MS, Mato Grosso do Sul; MT, Mato Grosso; GO, Goiás; DF, Distrito Federal; RJ, Rio de Janeiro; ES, Espírito Santo; BA, Bahia).

FIGURE 2. Measurements taken on the reconstruction of Paranaichthys longianalis gen. et sp. nov. a, total length; b, standard length; c, body length; d, trunk length; e, head length; f, caudal peduncle length; g, caudal fin length; h, total length of the caudal region; i, caudal fin height; j, caudal peduncle height; k, estimated body height; l, head height; m, posterior anal fin length; n, anterior anal fin length; o, anal fin base length; p, pelvic fin height; q, pelvic fin base length; r, pectoral fin width; s, pectoral fin length; t, frontal bone length.

FIGURE 3. Photograph of the holotype (MCP – 3371 – PV) of Paranaichthys longianalis gen. et sp. nov.; Scale bar = cm.

FIGURE 4. Camera lucida drawing of specimen MCP – 3371 – PV of Paranaichthys longianalis gen. et sp. nov.

FIGURE 5. Detail of the skull of Paranaichthys longianalis gen. et sp. nov. (MCP – 3371 – PV); question marks indicate unidentified or dubious bone fragments. Abbreviations: cd, crushing dentition; cl, cleithrum; d, dentary; dh, dermohyal; dpt, dermopterotic; dsph, dermosphenotic; ex, extrascapula; f, frontal; mx, maxilla; nr, nostril; od, odontodes; op, operculum; pa, parietal; pop?, preoperculum?; pr, postrostral; psph, parasphenoid; pt, post-temporal; r-pmx, rostro-premaxilla; scl, supracleithrum; sop, suboperculum.

FIGURE 6. Proposed reconstruction of Paranaichthys longianalis gen. et sp. nov. Illustration by Eduardo Kickhöfel.

FIGURE 7. Skull bones tentatively identified and positioned in the reconstruction of Paranaichthys longianalis gen. et sp. nov. Abbreviations: cd, crushing dentition; cl, cleithrum; d, dentary; dh, dermohyal; dpt, dermopterotic; dsph, dermosphenotic; ex, extrascapula; f, frontal; mx, maxilla; nr, nostril; od, odontodes; op, operculum; pa, parietal; pop?, preoperculum?; pr, postrostral; psph, parasphenoid; pt, post-temporal; r-pmx, rostro-premaxilla; scl, supracleithrum; sop, suboperculum.

FIGURE 8. A, Photograph of the crushing dentition of Paranaichthys longianalis in the parasphenoid. B, Tooth cross-section. C, Interpretative drawing of the tooth. D, Detail of verruciform acrodin. Abbreviations: cd, crushing dentition; de, dentine; dt, dentine tubules; e, enamel; pc, pulp cavity; psph, parasphenoid; vacr, verruciform acrodin.

FIGURE 9. A, Photograph of the cross-section of a skull dermal bone (maxilla) under light microscopy. B, Interpretative illustration of the cross-section. Odontodes on transverse section on the right, with evidence of superpositional growth and a more longitudinal section on the left side. Abbreviations: de, dentine; dt, dentine tubules; g, ganoine; is, isopedine; od, odontodes; os, osteocite lacune; pc, pulp cavity; vc, vascular canal.

FIGURE 10. A, Stereomicroscopic photograph of trunk scales coated with ammonium chloride. B – C, Scanning Electron microscopic images of a fulcral scale of the caudal peduncle. Abbreviations: fw, furrows on the ganoine; g, ganoine; mt, microtubercules on the ganoine surface; p, punctuations on the ganoine cover (pores in cross section); ri, ridges in the ganoine cover.

FIGURE 11. A, Scanning Electron Microscopic image of a fulcral scale of the caudal peduncle in cross-section. B – C, Light microscope photographs of trunk scales in cross-section. Abbreviations: de, dentine; g, ganoine; is, isopedine; od, odontodes; p, pores on the ganoine (punctuations of the surface); vc, vascular canal; zs, zone of superposition of the ganoine.