Scleropages (Asian Arowanas) are superstars in an aquarium for their ornate colouration in some variants. Fish enthusiasts from Thailand, China (Taiwan and Hong Kong), and Japan believe that the Arowanas bring them good luck and fortune. The Arowana Fish exhibit a trans-marine distribution, living on both sides of the Wallace’s line. Among the four extant species of the genus, Scleropages formosus is distributed in Sumatra, Kalimantan, Peninsular Malaysia, Thailand, and Cambodia, Scleropages inscriptus only in Myanmar, Scleropages jardinii and Scleropages leichardti in Australia and New Guinea. This distribution of Arowanas has attracted the interests of some molecular biologists who reconstructed the biogeography based on a molecular phylogeny and molecular-clock. Fossil remains of scales, otoliths and fragments of bones of Scleropages have long been known from the Maastrichtian of India, the Maastrichtian/Late Paleocene of Africa, the Palaeocene of Europe, the Eocene of Sumatra, the Oligocene of Australia. It has been proposed that a fragment of a toothed jaw and some squamules of an undetermined osteoglossid from the continental Campanian (Late Cretaceous) of Champ-Garimond in southern France may represent as the earliest fossil record of the genus Scleropages. Recently, a complete fossil Scleropages was described from the Lower Eocene strata in Xiangxiang, Hunan Province and Songzi, Hubei Province, China that made it possible to carry out a detailed comparative analysis between the fossil and extant species for the first time. Here the author reports on the second fossil species of Scleropages from the Lower Eocene Huayong Formation in the Sanshui Basin of Guangdong Province, China.
In a paper published in the journal Vertebrata PalAsiatica on 20 April 2020, Zhang Jiang-Yong of the Key Laboratory of Vertebrate Evolution and Human Origins at the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, and the Center for Excellence in Life and Paleoenvironment, describes a new species of Scleropages from the the Eocene of Guangdong, China.
The Sanshui Basin is located in the Northwest of the Pearl River Delta. Most of the area belongs to Sanshui, Nanhai, Chancheng and Shunde Districts of Foshan City, covering an area of about 3300 km². The large regions of the surface of the Sanshui Basin are covered by Neogene loose sediments and vegetation. Paleogene strata are only sporadically exposed and are divided into four formations from bottom to top: Xinzhuangcun, Buxin, Baoyue and Huayong Formations. The lithology of the lower part of the Huayong Formation is the interbedding of volcanic and sedimentary rocks, different from the underlying the Baoyue Formation that has a large suit of volcanic rock as its marker. The upper part of the Huayong Formation is composed of conglomerate, sandy gravel with brownish red siltstone, fine sandstone, dark gray silty mudstone and scattered purple gray basalt, with a thickness of nearly 1000 m.
The Palaeogene Fish fossils in the Sanshui Basin are abundant and widely distributed. Fish were mainly collected from the Buxin Formation. Only one genus, Tungtingichthys, is from individual drill holes of the Xinzhuangcun Formation and scales and fragments of bones are from the Baoyue and Huayong Formations. A 1981 study described three families, 8 genera, 15 species, including 10 new species from the Buxin Formation except two indeterminate members of the Leuciscinae (True Minnows) from the Huayong Formation. However, most of these specimens are not well preserved and need to be reidentified. Two new fish faunas have been discovered since 2006; the Tungtingichthys fauna can be seen from the upper part of the Xinzhuangcun Formation to the bottom of the Buxin Formation. The fossil Fish are rich in a set of hard oil shales, which are well-preserved but poor in diversity. This fauna is characterised by the occurrence of a large number of Tungtingichthys, a type of extinc Perch, and occasionally Cyprinids and Bagrids. The Osteoglossid fauna can be seen in the black tuffaceous shale of the Huayong Formation in the central basin. The fossil Fish from the second fauna are rich in both individual number and taxonomy, including Osteoglossids, Jianghanichthys, Aoria lacus, Tungtingichthys and Amiiforms. Some members of this fauna are also seen in the sandstone of upper part of the Huayong Formation including Scleropages. In addition to Fish, other fossil vertebrates were also found in the Huayong Formation recently, including the Toad Frog, Sanshuibatrachus sinensis, the Stork, Sanshuiornis zhangi, the Mammal, Meridiolophus expansus, as well as Turtles, Crocodiles and Lizards. Ostracodes, Gastropods, spores and pollen, Charophytes and plants were also seen in this formation.
The new species described is named Scleropages sanshuiensis, meaning 'from Sanshui', in reference to the Sanshui Basin where the specimens were found. The species is described from 33 specimens; specimens V 26533.1–23 are from Niuweigang, Zidong, Nanzhuang Town, Chancheng District, Foshan City; Lower part of the Huayong Formaion, Lower Eocene. Specimens V 26534.1–10 are from Shicheng Middle School, Shishan Town, Nanhai District, Foshan City; upper part of the Huayong Formaion, Lower Eocene.
Scleropages sanshuiensis is a fossil species of Scleropages, different from Scleropages sinensis and extant species in: sensory canal of nasal in a groove on the anterior half but in a tube of the posterior half, third infraorbital slightly smaller than fourth, posteroventral margin of opercle concave slightly, sensory canal opening in a groove along the whole dermopterotic, last branchiostegal ray is much wider than the penultimate, pectoral fin extending just to the beginning of pelvic fin, distal ends of neural arches on abdominal centra not fused, parapophyses much longer than in Scleropages sinensis but not as long as in extant species, hypural 2 unconnected with ural centrum 1.
The body of the fish is fusiform. The standard length of the holotype is 116 mm, 3.5 times of the length of the head. The standard length of the largest specimen (V 26534.1) is 182 mm, 3.3 times of the head length.
The nasals are large and sutured in the midline along the anterior two thirds of their length, but are separated by the tapered frontals posteriorly as in Scleropages sinensis. There is a notch at the anterior medial corner of the bone probably to receive the dermethmoid. This notch can be seen in Scleropages sinensis (relatively small) but not seen in the extant species (based upon dried skeletons at the Institute of Vertebrate Paleontology and Paleoanthropology). The nasals are ornamented like the condition in extant species (again based upon dried skeletons at the Institute of Vertebrate Paleontology and Paleoanthropology) but unlike that in Scleropages sinensis. The sensory canal is quite different from other species of the genus. A groove is exposed in the anterior half of the nasal and goes into a tube in the posterior half of the bone.
The canal is exposed in a groove along the whole length of the nasal in Scleropages sinensis and Scleropages leichardti, but is presented as a tube in the nasal in Scleropages formosus.
The frontal is similar in shape and ornamentation to other species of Scleropages. It is long and subrectangular, with an anterior embayment for the reception of the nasal, and a posterior sinuous suture with its opposite member. The sensory canal is enclosed in bone for the middle third of its length, while it is exposed in grooves for the anterior and posterior thirds of its length in the frontal, similar to that in Scleropages sinensis and Scleropages leichardti but unlike in Scleropages formosus, in which the canal is enclosed in bone to, or almost to, its entry into the nasal.
The parietal is subrectangular and sutures with its opposite at the midline and with the pterotic laterally. The bone is sculptured except for the posterior quarter of the lower level. The bone is slightly wider than it is long, similar to that of Scleropages formosus but unlike the ratio seen in Scleropages leichardti.
The dermopterotic s a little larger and thicker than it is in extant species of Scleropages but smaller than in Scleropages sinensis. The bone seems not sculptured and bears the temporal sensory canal in an open groove, different from that in other species of the genus where only the anterior half of the canal is in an open groove (the holotype of Scleropages sinensis, a dried skeleton at the Institute of Vertebrate Paleontology and Paleoanthropology).
The extrascapular is a tubular bone. The epiotic, supratemporal and supraoccipital are not visible in the available specimens.
The orbital portion of the parasphenoid is toothless, moderately broad, and parallel-sided (V 26533.8). The parasphenoid is nearly complete in V 26533.14, with strong basipterygoid processes and a small tooth patch at the base of the process. The bone joins with the basioccipital posteriorly. The vomer is seen in V 26533.14 in lateral view, with four teeth, one much larger than the other three.
The circumorbital series is composed of six bones: an antorbital, four infraorbitals and a dermosphenotic. The supraorbital is absent. The antorbital, infraorbitals and the dermosphenotic are all prominently sculptured.
The antorbital is well-preserved in V 26533.8 and is polygonal, making contact with the dermopterotic posterodorsally, the nasal and frontal dorsomedially, and the first infraorbital ventrally. The concave anterior and orbital margins are free. Dorsally the circumorbital sensory canal enters the antorbital via a short and small groove, and then passes through the bone in a tube, entering the first infraorbital where a pore communicates with the exterior as in Scleropages sinensis. In the extant species of the genus (based upon dried skeletons at the Institute of Vertebrate Paleontology and Paleoanthropology) and Osteoglossum (IVPP OV 2712), the canal is completely enclosed in bone. The antorbital of the two fossil species is relatively larger than in the extant species of Scleropages and in Osteoglossum.
The first and the second infraorbitals are narrow and tubular. The first is slightly expanded, longer, and more ornamented than the second.
The two posterior infraorbitals (third and fourth) are very large as in Scleropages sinensis but do not quite reach the size of those in extant Scleropages and Osteoglossum, in which they extend posteriorly to the articulation of the opercle, completely concealing the dorsal end of the preopercle. In both fossil species of Scleropages there is a narrow gap through which the dorsal limb of the preopercle may be seen. The third infraorbital is slightly smaller than the fourth while it is slightly larger than the fourth in other species of the genus including Scleropages sinensis. In Osteoglossum the lower one is much larger than the upper. The infraorbital sensory canal is carried in a tube near the orbital margin of both posterior infraorbitals. Near the anterodorsal corner of the fourth infraorbital where the sensory canal enters the dermosphenotic, there is a short backward groove. This groove is also seen in other species of Scleropages and Osteoglossum.
The dermosphenotic is a nearly triangular bone, large, thick, and sculptured. A pore of the sensory canal is seen at the posteroventral corner of the bone. This bone is larger in Scleropages sanshuiensis and Scleropages sinensis than in the two extant species of the genus and in Osteoglossum.
The premaxilla is small and sculptured. There is a small ascending process in its anterior half. Seven teeth are seen on the specimens V 26533.1 and V 26533.6, but eight on specimen V 26533.10. The anterior four teeth are much larger than the posterior nes while the number of the large teeth on the bone in Scleropages sinensis is three. The number of teeth on the premaxilla of Scleropages sinensis is seven, but in extant Scleropages the number has been counted differently; 4–5 by Walter Ridewood (1905), 3–5 by Diana Kershaw (1976), and 11 by Louis Taverne (1977). In the Institute of Vertebrate Paleontology and Paleoanthropology's dried specimens of extant Scleropages this number is 4–8.
The maxilla is long and slender, and takes an angle of about 45° with the long axis of the Fish when the mouth is closed as it is in Scleropages sinensis. It extends posteriorly nearly to the level of the mandibular articulation and ends well behind the posterior margin of the orbit. Ornament is most prominent at the anterior and posterior ends. There is no supramaxilla.
The maxilla bears 40 conical teeth in the holotype and V 26533.6, a resemblance in number with Scleropages sinensis and Scleropages formosus and a difference from Scleropages leichardti, in which the tooth number is about 35 (based upon dried skeletons at the Institute of Vertebrate Paleontology and Paleoanthropology). The teeth decrease in size steadily from anterior to posterior, with 4–5 teeth much larger than the others.
The mandible is long and lacks a distinct coronoid process, consisting of dentary, angulo-articular, and retroarticular. The dentary forms the great majority of the length of the mandible. Anteriorly, the dentary curves inwards to form the shallow symphysis. 33 teeth are counted on the oral margin of the dentary in V 26533.19A, with the anterior five or six much larger than the posterior ones. Dense long ridges are ornamented on the lower part of the lateral surface of the dentary.
The angulo-articular is short, and articulates with the quadrate as seen in lateral view. The anterior part of the angulo-articular is thin and pointed overlapping the recess on the internal surface of the dentary. Posteriorly the bone is thick and deeply sculptured. The articular facet for the quadrate is formed by the bone alone and lies in the upper part of the posterior end of the bone. The facet clearly showing in V 26534.5 is a little deeply excavated. The coronomeckelian can be seen on the medial surface of the angulo-articular in V 26533.19A. The coronomeckelian is also visible on the medial surface of the mandible of Scleropages sinensis (V 26536.2).
The retroarticular is very small and applies to the postero-ventral surface of the angulo-articular.
The mandibular sensory canal extends the length of the dentary and angulo-articular within a canal, with five pores located near the ventral margin of the dentary.
The palato-ectopterygoids are seen in V 26533.10, in which there are small teeth on the lateral margins and a patch of much smaller teeth behind the margin.
Two detached entopterygoids are preserved in specimens V 26533.17–18, one in lateral view and the other in medial. The bone is triangular, with a row of 20 large conical teeth existing on the medial edge, and fine denticles covering the remainder of the surface. The metapterygoid is not visible. The whole quadrate can be seen in V 26533.19A, B. It is a small fan-shaped bone, with the postero-ventral process a little short and the incision for the symplectic somewhat shallow. A short, rod-like symplectic is visible in V 26533.19A, which inserts into the incision of the quadrate.
The hyomandibula (V 26534.5) is vertical and articulates with the cranium by one head. The opercular process is strong and lies in the upper third of the bone. Anteriorly, the bone has a broad wing. Two foramina opened in the external surface of the bone, one at the upper third and the other near the lower end.
The anterior ceratohyal is seen in V 26533.19A and V 26534.4. Its proximal part is rod-like while the distal part is nearly rectangular. The hypohyal and the basihyal are not visible. A small urohyal can be seen in V 26534.4. There are 10 slender, acinaciform branchiostegal rays in V 26533.19A, with the last three gradually broadened. The last branchiostegal ray is much wider than the penultimate, differing from what is seen in other species of the genus.
The preopercle is similar to that in Scleropages sinensis and the extant species of Scleropages. The upper limb is not completely covered by the posterior infraorbitals as it is in the extant species of the genus. The dorsal limb is slightly more than twice the length of the ventral limb. The latter is bluntly rounded anteroventrally. The posteroventral angle of the preopercle is not produced posteriorly to a point. In the description of the original specimen, the posteroventral angle of Scleropages sinensis is produced posteriorly to a point just like that in Chauliopareion and Singida. On the newly found specimens (V 26536.1–2) of Scleropages sinensis from Songzi, the type locality of the species, the posteroventral angle of the preopercle is complete and does not show this feature.
The preopercular sensory canal is similar to that of Scleropages sinensis and is open ventrally beneath a long, horizontal shelf. The surface ventral to the shelf is smooth but with some granules near the ventral margin for the preopercle (V 26533.19A). The sensory canal is enclosed in bone on the anterior edge, but opens ventrally with five pores beneath the shelf and posteriorly with a large pore at about the lower third height of the preopercle as that in Scleropages sinensis and the extant species of the genus.
The opercle (V 26533.10, V 26533.19A) is nearly semicircular. The ratio depth/width of the bone is 1.89 in V 26533.10 and 1.98 in V 26533.19A. The ratio of that in Scleropages sinensis is 1.80 in the holotype and 1.91 in V 26536.2. The ratio of extant species of the genus is of two opposing extremes, 2.25 in Scleropages formosus and 1.72 in Scleropages leichardti. The posteroventral margin of the opercle is concave slightly, whereas it is concave sharply in Scleropages sinensis. In extant species of genus, by contrast, the margin is not concave. The opercle is prominently sculptured with radial ornamentation on the lateral surface except for its anterior margin and dorsal extremity. The medial surface is smooth (V 26533.19A). The subopercle is only partially shown in V 26533.1 and V 26533.8. The interopercle is not visible.
The upper part of the pectoral girdle is seen in V 26533.10 and V 26534.5. The posttemporal is not visible. The supracleithrum is long and thickened along the anterior margin. The anterior margin is a little straight but the posterior margin is rounded. A small postcleithrum is present and lies medial to the junction between the supracleithrum and cleithrum. The cleithrum shows only its upper part, terminating dorsally in a long, rod-like process. The lower part of the girdle can be seen in V 26533.11, but the coracoid, scapula, and mesocoracoid have not been seen. Three proximal pectoral radials are visible in V 26533.6, one large and the rest two small.
The pectoral fin (V 26533.2, 11) is very long and extends just to the beginning of the pelvic fin, as compared with that in Scleropages sinensis in which it extends well behind the beginning of the pelvic fin. In the extant species of Scleropages it does not reach the beginning of the pelvic fin. There are seven pectoral fin rays (V 26533.1, 11). This number is also seven in Scleropages sinensis and Scleropages formosus but eight in Scleropages leichardti (based upon dried skeletons at the Institute of Vertebrate Paleontology and Paleoanthropology). All rays are branched and segmented except the first one which is exceptionally thick and unbranched, though segmented. A claw-shaped bone is showing in V 26534.2. The same bone is also found adjacent to the base of the smallest ray of Scleropages sinensis.
The pelvic girdle and fins are very small (V 26533.11). Each pelvic fin originates in about the middle length between the pectoral and anal fins, different from that in Scleropages sinensis in which the pelvic fin originates slightly closer to the anal fin than to pectoral fin. The pelvic girdle (V 26533.11) is short and flat. There are six pelvic fin rays, all branched but the first, a condition agreeing with that of S. sinensis and Scleropages leichardti but differing from Scleropages formosus, which has five fin rays.
The dorsal and anal fins are rounded in outline and located posteriorly (V 26533.4, 5). The dorsal fin is small and originates posterior to the origin of anal fin, opposite the middle of the anal fin. There are three short procurrent dorsal rays, one long segemented but not branched ray and 11 branched rays, for a total of 12 principal rays in V 26533.4–5. The dorsal pterygiophores can be counted only in V 26533.4, about 14 in number. The anal fin is much larger than the dorsal fin, with three small, unsegmented procurrent rays and 22 principal rays. The anal pterygiophores cannot be counted.
There are 48 vertebrae in V 26533.5, of which about 22 are abdominal and 26 are caudal including the two ural centra. This number is much less than in extant osteoglossids and agrees with that of Scleropages sinensis and early Osteoglossomorphs such as Kuntulunia and Xixiaichthys. The first three centra are covered by the opercle. The centra are slightly deeper than long. The neural spines are paired on abdominal centra (V 26533.2), in which some paired spines may fuse at the distal end, the case different from that of S. sinensis in which only the first four neural spines are fused.
The parapophyses (V 26534.5) are much longer than in Scleropages sinensis and most early Osteoglossomorphs.
There are 22 pairs of pleural ribs (V 26533.2), which extend to the ventral margin of the trunk, except for the last pair, which is only about half the length of the more anterior ones.
The epineurals are not visible. Some long, slender supraneurals are seen in specimen V 26533.2.
The caudal skeletons are very similar to those of Scleropages sinensis and the extant species of Scleropages with certain disparities. These are best preserved in the specimen V 26534.1. Three haemal spines in Scleropages sanshuiensis are lengthened to support the caudal fin rays. The first preural centrum bears two complete neural spines as in Scleropages leichardti, contrasting single in Scleropages sinensis and Scleropages formosus.
The first ural centrum (U1) has a complete neural spine. The second ural centrum (U2) is fused with the proximal ends of hypurals 3–5 (H3–5). There are six hypurals. H1 is very deep and joints with U1 proximally. H2 is less than half the width of H1 and is probably unconnected with U1. Hypurals 3 through 5 are fused proximally and fit tightly together distally. A free rod-like bone dorsal to hypurals 3–5 is probably the sixth hypural. Just above this bone, a similarly shaped bone is interpreted here as an uroneural. A greatly enlarged first hypural is otherwise present only in Scleropages sinensis among this genus. This hypural is as deep as the first two (of three lower) hypurals in Scleropages leichardti.
The caudal fin has a rounded posterior profile. There are 16 principal rays, as in Scleropages sinensis and Scleropages leichardti, versus 14 in Scleropages formosus. The first and the last principal rays are unbranched and almost as long as the remaining rays just as in Scleropages sinensis, whereas in living species of Scleropages and Osteoglossum, the upper and lower rays are only half the length of the innermost ones. One or two procurrent rays are present anterior to the principal rays.
The scales are large, cycloid or oval, and exhibit the reticulate pattern, involving small units called squamules, typical of osteoglossids. The external surface of the scale shows circuli in the basal portion and granular ornamentation in the apical area. The squamules are rhombic, polygonal, or irregular in shape, usually larger in basal portion than in apical. The mesial surface of squamules in the apical area bears rounded, raised tubercles, each of which has a minute transversal-pore at its center while that in the basal portion is smooth, but with large pores in several squamules.
The lateral line (V 26533.22,) runs just below the vertebral column, but the number of scales along the lateral line cannot be counted. The size of the scales in Scleropages sanshuiensis is about equal to that of Scleropages sinensis and therefore, the same number of lateral line scales of 24 is expected.
The materials from the Sanshui Basin, Guangdong province of China very much resemble Scleropages in skull bones, caudal skeleton, the shape and position of fins, and reticulate scales. Therefore, it is attributed to the genus.
Among the four extant and one fossil species of Scleropages, the new Fish is very similar to the fossil one, Scleropages sinensis in: there is a notch at the anterior medial corner of the nasal, the teeth on anterior oral margin are much larger than those on the posterior; the upper limb of the preopercle is not completely covered by posterior infraorbitals; the posteroventral margin of the opercle is concave; the pectoral fin is very long; a claw-shaped bone is present adjacent to the base of the smallest ray of the pectoral fin; the ray number of all the fins is equal respectively; vertebrae is same in number; hypural 1 is very deep; the number of lateral line scales is probably same.
Although Scleropages sanshuiensis shares some similarities with Scleropages sinensis, it is different from the latter in many aspects: the nasal is ornamented; the sensory canal of the nasal is exposed in a groove on the anterior half of the bone but in a tube in the posterior half unlike in the groove of whole length of the nasal of Scleropages sinensis; the temporal sensory canal of the dermopterotic is in an open groove, different from that in other species of the genus where only the anterior half of the canal is in an open groove; dorsally the circumorbital sensory canal enters the antorbital via a small groove rather than a broad groove in Scleropages sinensis; the third infraorbital is slightly smaller than the fourth while it is slightly larger than the fourth in other species of the genus including Scleropages sinensis; the posteroventral margin of the opercle is concave slightly vs concave sharply in Scleropages sinensis; the pectoral fin extends just to the beginning of the pelvic fin rather than extending well behind the beginning of the pelvic fin in Scleropages sinensis; the distal ends of the neural arches on abdominal centra are not fused unlike the condition in Scleropages sinensis that only the anterior four neural arches are not fused; the parapophyses are much longer than in Scleropages sinensis; H2 is probably unconnected with U1 vs either articulates with or fused to U1 in Scleropages sinensis and extant species of Scleropages.
In 2007 Louis Taverne, Dirk Nolf, and Annelise Folie described Scleropages sp. based on some bony remains, otoliths and squamules from the continental Paleocene of Hainin (Mons Basin, Belgium). The bone remains include a right premaxilla, an incomplete right maxilla, a fragment of a right entopterygoid, a fragment of a left palato-ectopterygoid and an abdominal vertebra. The otoliths are not seen in Scleropages sanshuiensis, and the squamules are nearly the same as in Osteoglossids. The bone remains are difficult to compare with those of Scleropages sanshuiensis except for the premaxilla on which the anterior teeth of the bone are very large in Scleropages sanshuiensis and Scleropages sinensis but small in Scleropages sp. of Hainin.
Based on these comparisons, a new species, Scleropages sanshuiensis is established.
Sharing many characters, the two fossil species of Scleropages are clearly more closely related to each other than either of them to the extant species. As to the relationship between the two fossil species and the extant ones, Scleropages sanshuiensis shares two characters with living species (ornamented nasal and long parapophyses) and Scleropages sinensis shares two characters with living species (the third infraorbital is slightly larger than the fourth and the distal ends of the neural arches on most vertebrae fused). Therefore, it is not clear which one of the two fossil Fish is more closely related to the living Fish. The two fossils are probably in the same evolutionary level.
Sexual dimorphism may exist in Scleropages sinensis, with males having a slimmer and shallower body depth while females have a more rounded body. This is not seen in the available specimens of Scleropages sanshuiensis.
The biogeographic history of the Osteoglossomorpha was undoubtedly complex. The remarkable diversity of basal Osteoglossomorphs and primitive Osteoglossiforms in southeast Asia and the centre of modern biodiversity in Africa are two phenomena that, on present knowledge, seem inexplicable by simple hypotheses of centres of origin, or of Pangaean or Gondwanan continental breakup. Biogeography of Scleropages seems simple, but in fact, it is not.
In 2000 Yoshinori Kumazawa and Mutsumi Nishida first tested biogeographical hypotheses for Scleropages using a molecular phylogenetic time-scale and they concluded that the divergence time between Asian Arowana (Scleropages formosus) and Australia Arowana (Scleropages leichardti and Scleropages jardinii) is about 138 million years, which is close to or slightly older than the probable time of the India-Madagascan separation from Gondwanaland (120–130 million years ago). They consequently argued that the Asian Arowana originated on a part of Gondwanaland and was carried to Eurasia by the Indian subcontinent. However, in 2015 Sébastien Lavoué reported an age credibility interval of Scleropages ranging from 79.9 to 101.4 million years, which is significantly younger than the age (138 ± 18 million years) inferred by Kumazawa and Nishida and even younger than 115.0 million years (latest possible age for a direct connection of the Indian subcontinent to Australia–Antarctica). Therefore, Lavoué’s result rejects the Gondwanan origin hypothesis to explain the distribution of Scleropages. More recently, a team of researchers led by Marcelo de Bello Cioffi reported that a time-calibrated phylogenetic tree revealed that Osteoglossum and Scleropages divergence occurred approximately 50 million years ago, at the time of the final separation of Australia and South America (with Antarctica). Asian Scleropages formosus and Australian Scleropages diverged about 35.5 million years ago, substantially after the latest terrestrial connection between Australia and Southeast Asia through the Indian plate movement.
Since a vicariant event is not available to explain the transmarine distribution of Scleropages, a marine dispersal between Australia and Asian across Wallace’s Line was suggested. Although several authors have supported the marine dispersal hypothesis, none of them pointed out a direction for the dispersal. Is it from south to north or just the opposite? Traditionally, it was supposed that the ancestor of Scleropages lived in Australia during the Oligocene and then moved to East Asia by a marine dispersal event. Eocene Scleropages sanshuiensis and Scleropages sinensis from China predate this hypothetical dispersal and therefore, the direction of the dispersal need to be reconsidered.
Early Osteoglossomorphs have mostly been recovered from China, including a Late Jurassic Paralycoptera, one of the oldest Osteoglossoids, and consequently Peter Greenwood in 1970 and Zhang Miman and Zhou Jiajian in 1976 supposed that East Asia might be the ancestral of Osteoglossomorphs. In addition to these Mesozoic species, the Eocene Osteoglossomorphs were also found from China, such as Sinoglossus lushanensis, Eohiodon shuyangensis, Phareodus songziensis, Scleropages sinensis, and Scleropages sanshuiensis. It is reasonable to suppose that the common ancestor of Scleropages and Osteoglossum lived before the Eocene in East Asia and then dispersed to Australia and other places in the world.
To firmly support the marine dispersal hypothesis of Scleropages, a marine fossil of the genus may well be needed. The Sanshui Basin, where Scleropages sanshuiensis was found, was probably connected with the sea at the time when the Palaeocene Buxin Formation was deposited. It may therefore be difficult but not impossible to find marine fossil Scleropages in the Sanshui Basin.
A unified view has not been formed about the geological age of the Huayong Formation in the Sanshui Basin. It has been considered to be Late Eocene by the majority of researchers, but Early Eocene or Oligocene by some others. An isotopic age of the volcanic rocks in the Huayong Formation has beem established at 43–55 million year, but a potassium-argon age from trachyte has given an alternative age of 51.5 ± 2.6 million years.
Potasium-Argon dating relies on determining the ratio of radioactive Potasium⁴⁰ to Argon⁴⁰ within minerals from igneous or metamorphic rock to determine how long ago the mineral cooled sufficiently to crystallise. Potasium⁴⁰ is often incorporated into cooling volcanic rocks, whereas any inert Argon present will escape as a gas. No further Potasium⁴⁰ or Argon⁴⁰ will enter the mineral from this point, but Argon⁴⁰ is produced by the decay of radioactive Potassium⁴⁰ at a steady rate, enabling scientists to establish a precise date for the crystallisation of the minerals containing the two elements.
Scleropages sanshuiensis from the Huayong Formation of the Sanshui Basin and Scleropages sinensis from the Yangxi Formation of Songzi are very close to each other and they are probably in same evolutionary level. Therefore, the Huayong and Yangxi Formations are comparable and possibly deposited contemporaneously. At present, the Early Eocene age of the Yanxi Formation is not disputed. In view of this, Zhang's work supports the view that the Huayong Formation is of the Early Eocene.
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