Extraordinarily thick-boned fish linked to the aridification of the Qaidam Basin (northern Tibetan Plateau)

Abstract

Scattered with numerous salt lakes and ≈2,700–3,200 m above sea level, the giant Qaidam inland basin on the northern Tibetan Plateau has experienced continuing aridification since the beginning of the Late Cenozoic as a result of the India–Asia plate collision and associated uplift of the Tibetan Plateau. Previous evidence of aridification comes mainly from evaporite deposits and salinity-tolerant invertebrate fossils. Vertebrate fossils were rare until recent discoveries of abundant fish. Here, we report an unusual cyprinid fish, Hsianwenia wui, gen. et sp. nov., from Pliocene lake deposits of the Qaidam Basin, characterized by an extraordinarily thick skeleton that occupied almost the entire body. Such enormous skeletal thickening, apparently leaving little room for muscles, is unknown among extant fish. However, an almost identical condition occurs in the much smaller cyprinodontid Aphanius crassicaudus (Cyprinodonyiformes), collected from evaporites exposed along the northern margins of the Mediterranean Sea during the Messinian desiccation period. H. wui and A. crassicaudus both occur in similar deposits rich in carbonates (CaCO₃) and sulfates (CaSO₄), indicating that both were adapted to the extreme conditions resulting from the aridification in the two areas. The overall skeletal thickening was most likely formed through deposition of the oversaturated calcium and was apparently a normal feature of the biology and growth of these fish.

Increasing evidence ascribes a profound climatic shift, such as Asian monsoon systems and widespread aridification, to the uplift of the Tibetan Plateau caused by the India–Asia plate collision during the Late Cenozoic. The cyprinid fish reported here (Fig. 1) are from the lacustrine deposits of the Pliocene Shizigou Formation in the northeastern wing of Yahu Anticline, Qaidam Basin, northern Tibetan Plateau (1) [Fig. 2A and for more details see supporting information (SI) Fig. S1], collected from 20 localities in a 220-m-thick sequence of siltstones, fine sandstones, and marls under a marker bed, Horizon K (2) (Fig. 2B). The deposits are rich in carbonates (CaCO3) and sulfates (CaSO4). The unusually thick-boned fish represents an adaptive mode unknown in any extant fish and extremely rare in fossils. Based on this fossil fish, our morphological description and cladistic analysis shed light on schizothoracin phylogeny. We also suggest that the overall skeletal thickening was formed through deposition of the oversaturated calcium and reflects the fish's dramatic physiological adjustment to severe environmental distress. The fish thus witnessed the process of aridification and provided a convincing link between environmental changes on the Tibetan Plateau and biological responses of its inhabitants.

Fig. 1.

H. wui gen. et sp. nov. (A) Body without head and tail (IVPP V 15012) in right view. (B) The same specimen in left view, revealed by CTScan, flipped horizontally. (C) Gymnocypris przewalskii Kessler, radiograph of body in right view. (D) box area in C in higher magnification. (E) A. crassicaudus (Agassiz), middle portion of body in left view, after Gaudant (25). (Scale bars: 30 mm in A–C); 3 mm in E.) an, anal fin; ds, dorsal fin spine; epn, epineurals; epp, epipleurals; hs, haemal arches and spines; ns, neural arches and spines; ptr, pterygiophores; ri, ribs; ver, vertebrae.

Fig. 2.

Localities and section. (A) The studied area, Yahu and Eboliang in Qaidam Basin. (B) Yahu section, numbers to the left of the column, thickness of the section; numbers to the right of the column, the numbers of the relevant localities. CD, Chaidam (Qaidam), in the numbering system used by Wang et al. (2). (Scale bar: 1,000 km.)

Systematic Paleontology

Ostariophysi Sagemehl, 1885.

Cypriniformes Bleeker, 1860; Cyprinidae Bonaparte, 1837; Schizothoracinae Berg, 1912; Hsianwenia wui, gen. et sp. nov.

Holotype.

Nearly complete skeleton, only with posterior part of the caudal fin and a small anterodorsal portion of the body missing, IVPP V 15244, from locality CD0649, Fig. 3 A–C.

Fig. 3.

H. wui gen. et sp. nov. (A) Holotype (IVPP V 15244) in right view. (B) Caudal skeleton of holotype in right view. (C) Head of holotype in right view. (D) Ossified gill rakers (IVPP V 15012). (E) Pharyngeal bone with teeth in ventromedial view (IVPP 15245.1). (F) Four pharyngeal teeth, top view, showing grinding surface (left) and posterior view (right) (from left to right: IVPP V 15307.2, 7, 4, 14). (G) Left parapophysis of 4th vertebra in ventromedial (left) and dorsolateral (right) view (IVPP V 15306. 33). (H) Two disarticulated epineurals (left, IVPP V 15306.20; right, IVPP V 15306.18). (I) Disarticulated epipleural (IVPP V 15306.19). (Scale bars: 20 mm in A and C; 10 mm in B, E, G, H, and I; 5 mm in D and F.) an, anal fin; d, dentary; ds, dorsal fin spine; ect, ectopterygoid; ent, entopterygoid; ep, epural; epn, epineurals; epp, epipleurals; fr, frontal; hs, haemal arches and spines; hy, hyomandibular; iop, interopercle; io, infraorbitals; mpt, metapterygoid; mx, maxilla; ns, neural arches and spines; op, opercle; pa, parietal; pop, preopercle; pmx, premaxilla; ptr, pterygiophores; q, quadrate; ri, ribs; un, uroneural; ver, vertebrae.

Included material.

V 15012, nearly complete skeleton with the tail, a short portion between the body and head, and the snout missing (Figs. 1 A and B and 3D) from locality CD0507; V 15245.1 and V 15245.2, both are middle part of the fish skeleton, from locality CD0649; V 15306.1–37, detached bones of head and body, V 15307.1–50, detached pharyngeal teeth, from localities CD0506 and CD0507.

Etymology.

The genus and species name is dedicated to the late Prof. Hsianwen Wu, who made great contributions to the study of the Chinese cyprinids.

Horizon and localities.

Localities CD0504–0507, CD0641–0653, CD0663–0666, upper part of the Shizigou Formation, Yahu Anticline, Qaidam Basin, Qinghai Province, China, N37°44′38″–46′22″, E93°36′55″–38′38″; Pliocene.

Diagnosis.

Schizothoracine with a unique combination of characters shared with barbines and primitive schizothoracines on one hand and specialized schizothoracines on the other: body elongated, nearly cylindrical; head relatively long; orbit small, situated anteriorly; anterior margin of lower jaw shovel-like; pharyngeal bone stout, triangle-shaped, with three rows of cylindrical teeth, new teeth with rounded top, worn teeth with truncated, flat grinding surface; distal end of transverse process of 4th vertebra blunt and slightly expanded; fork in pelvic bone shallow, both branches comparatively broad; dorsal fin with three unbranched and seven branched rays, the longest unbranched ray with robust serrations; proximal portion of epural expanded.

Description and remarks.

The body (Figs. 1 A and B and 3A) is elongated, more or less rounded anteriorly and slightly compressed posteriorly. The standard length in the holotype V 15244 is 5.4 times that of the body depth (78 mm). The head is relatively long (110 mm), 1.7 times its depth (65 mm), and nearly a quarter of the standard length. All specimens, including the disarticulated bones, are comparatively large. The holotype V 15244 has a standard length (length from the tip of snout to base of caudal fin) of 425 mm, whereas the estimated standard length of V 15012 is ≈445 mm. Body length with the caudal fin is estimated as 450–500 mm. Judged from the preserved parts of the body, V 15245.1 and V 15245.2 must have been even larger. The growth rate of recent schizothoracines is very slow. Those with a body length 400 mm typically have an age of at least 9–10 years (3). Thus, most of these schizothoracines were probably 10 (or even 15) years old.

The structure of the skull roof, the opercular system, and the palate is similar to that of other cyprinids. Judged from the position of the infraorbital bones, the orbit is quite small and situated rather anteriorly. The postorbital distance (distance between posterior margin of opercle and posterior margin of orbit) is long, more than four times that of the antorbital distance (distance between the anterior margin of the orbit and tip of snout) (Fig. 3C). Among recent schizothoracines, those living in broad rivers or lakes often use the caves beneath the sod-held bank edges as refuge during their winter hibernation. In summer, they also hide in the caves during the day, and only come out at dusk to search for food (3). It is therefore likely that their eyes became reduced and their sight weakened, like some cave-living barbines today.

The mouth is subterminal, with the upper jaw protruding beyond the lower (Fig. 3C). A shovel-like anterior margin of the dentary was apparently covered with a horny sheath when the fish was alive and is related to the habit of feeding on algae attached to the bottom. As in most cyprinids, the anterior process of premaxilla and coronoid process of the dentary are well developed. The quadrate has a very shallow notch for articulation of symplectic and is in contact with the metapterygoid, leaving no opening between them (Fig. 3C). The ectopterygoid is deep and narrow. The pharyngeal bone is stout and triangle-shaped (Fig. 3E). There are three rows of teeth on each bone, with a pattern of 2.3.5–5.3.2. The teeth are cylindrical in shape; new teeth have rounded top, whereas the worn teeth are truncated with flat grinding surface (Fig. 3F).

The total number of vertebrae (≈46) is greater than in primitive barbines. The dorsal fin has three unbranched and seven branched rays. The posterior edge of the longest unbranched ray is equipped with gross serrations (Figs. 1A and 3A). The anal fin has three unbranched and five branched rays. The posterior edge of the longest unbranched ray is smooth. The pelvic fin is situated more posteriorly than in primitive barbines. Its origin is under the 5th dorsal fin ray. The fork in the anterior part of the pelvic bone is less than half of the main part of the bone, and both branches are broad. The structure of the caudal skeleton (Fig. 3B) resembles that of most cyprinids. The usually slender, singular epural of cyprinids is widely expanded in this fish (Fig. 3B).

The most striking aspect of the fish specimens from the Yahu Anticline is that almost all bones from the skull, paired fin girdles, vertebrae with their arches and spines, pterygiophores, and even all intermuscular bones and gill rakers (Figs. 1 A and B and 3 A–I), are extremely thickened. On specimen V 15012, the ribs and epineurals in anterior portion of the body grow so close to each other that the vertebrae in this region are completely covered by them and cannot be seen from the lateral side (Fig. 1 A and B). The parapophysis of the 4th vertebra is very robust (Fig. 3G), whereas parapophyses of other vertebrae are also very thick and often preserved in disarticulated state. This makes many isolated bones look odd and difficult to recognize at first glance (Fig. 3 G–I). The bones in V 15244 (the smallest among the available specimens) are obviously thinner than those in three larger specimens V 15012, V 15245.1, and V 15245.2 (compare Fig. 1 A and B, Fig. 3A, and Fig. S2). This indicates that the thickness of the bone appears to increase with age.

Results and Discussion

Phylogenetic Position of H. wui.

H. wui resembles barbines and primitive schizothoracines (Cyprinidae) in many respects and, at the same time, shares a few characters with some specialized schizothoracines. The schizothoracines [Schizothoracin lineage = Oreininae, Cyprininae (4), Tribe Oreinini, Cyprininae (5), Cyprinidae] are endemic to the aquatic systems of the Tibetan Plateau and surrounding areas (including southwestern China, Kazakhstan, Kyrgyzstan, Tajikistan, Afghanistan, Pakistan, India, Nepal, and Bhutan) (5–8). The Cyprinidae are the largest and most widespread group of freshwater fish and are of considerable significance in fisheries. They comprise 2,420 species (9) distributed on all continents except Australia, South America, and Antarctica. Although the group has been studied for nearly two centuries, views about its subdivision still diverge. Schizothoracines are regarded as either a subfamily of the Cyprinidae (6, 10–12) or a monophyletic lineage within the subfamily Cyprininae (sensu lato, including the Barbinae) (4, 5) or Barbinae (sensu lato) (9, 13). The origin and diversification of schizothoracines is thought to be closely related to the uplift of the Tibetan Plateau. The recent schizothoracines have been divided into three categories: primitive, specialized, and highly specialized, based on the extent of modifications of their scales, barbells, and rows of pharyngeal teeth and their distribution at three successive altitudes at which water temperature and precipitation decrease, and solar radiation and evaporation increase (8). Phylogenetic studies of the Schizothoracinae using cladistic method have arrived at similar conclusions (14, 15).

Fossil schizothoracines are extremely rare. Relatively complete specimens from the Neogene Dingqing Formation of Lunbola Basin, northern Tibet, that are similar to schizothoracines were described as a new genus and species Plesioschizothorax macrocephalus and referred to the Cyprinidae (16). Incomplete pharyngeal bones and disarticulated pharyngeal teeth have also been reported from early Oligocene to early Pliocene deposits of East Kazakhstan, the mountainous region of Altai, and Mongolia (17). Information about these fossils remains sketchy. They indicate the presence of possibly related fish in these areas during earlier time intervals. The Yahu schizothoracines are the best preserved to date. The localities where they occur and the Eboliang locality, where unidentified disarticulated fish bones were found, are just outside the distribution of the extant schizothoracines.

To examine the phylogenetic position of H. wui, we compiled a dataset with 12 taxa and 32 characters (see Table S1). The 12 taxa include 7 extant schizothoracines and 2 fossil forms, plus 3 cyprinin and barbinin genera serving as outgroups. Characters were taken mainly from the literature (14, 15), augmented with our own observations of both extant and fossil forms. The phylogenetic analysis using PAUP Version 4.0b 10 (18) resolves Plesioschizothorax macrocephalus and H. wui among the early branching schizothoracines, in a position slightly more derived than Schizothorax. The remaining schizothoracines retain positions indicated by previous authors (Fig. 4; see also SI Text). In this case, the two fossil forms, H. wui and Plesioschizothorax macrocephalus, are not the most ancient members of the group; they are somewhat in between the primitive members of the group, e.g., the extant genus Schizothorax and more derived members. The history of the group would go back to a time earlier than the Neogene. Our analysis shows that schizothoracines form a monophyletic group within the Barbinae (sensu lato) (11, 13) indicating that the Barbinae sensu stricto (11, 12, 19) is paraphyletic.

Fig. 4.

Cladogram showing the phylogenetic position of H. wui gen. et sp. nov. among schizothoracines and related taxa.

Interpretations of the Skeletal Thickening.

Swollen or partially swollen bones are not uncommon in recent tropical and subtropical marine teleosts and are observed in taxonomically diverse groups (20). The phenomenon is referred to as hyperostosis (21), that is, the usually thin bones or parts of bones expand into rounded, bulky structures. The swollen parts only occur in a few bones in the same fish (see Fig. S3), and although their location varies from species to species, they always occur on the same bones in a given species (20, 21). Similar structures have also been reported in fossil fish from Europe, North America, and Africa, known as “Tilly Bones” (21–23), and are presumed to have been caused by disease or aging (21) or by abnormal hydrochemical conditions, such as high content of fluorine in the water (24).

The overall thickening of the skeleton in the Yahu schizothoracine H. wui is not comparable with that in the recent marine fishes nor is it comparable with the “Tilly bones” of other fossils. A similarly thickened skeleton has been observed only in the cyprinodont fossil Aphanius crassicaudus (Fig. 1E) recovered from the late Miocene Messinian evaporites exposed along the northern coast of the Mediterranean (southeastern Spain, Sicily, and Crete) (25–27). A. crassicaudus was first described as a fossil species of the extant genus Lebias and later reassigned to the fossil genus Pachylebias (25). It is now referred to as the genus Aphanius (27), an extant multispecies genus that has a circum-Mediterranean distribution and is adapted to varying degrees of salinities (25). The authors referred to the bone hypertrophy as pachyostosis.

Although the two known pachyostotic species, H. wui and A. crassicaudus, are of different age (Pliocene and late Miocene respectively) and are from different environments (lacustrine versus marine or lagoonal), they share many features in common. Both show an overall thickening of skeleton, which is not known in any other fish. This thickening apparently had little adverse influence on the life of the fish, and all individuals seem to have lived a relatively long, and normal, life (growing to more than half a meter long in the case of H. wui) even with increasingly thickened skeleton. In both cases, thickening of bones seems to appear when the fish reached a certain size (or age) and increases with increasing size (or age). The timing of bone thickening in A. crassicaudus varies from locality to locality (26). Although we do not have specimens of H. wui smaller than 445 mm in length (V 15244), we did notice that the larger the fish the more pronounced the degree of the bone thickening. Hence, it is conceivable that the bones in H. wui might not have been thickened at all when the fish was below a certain size.

The fish fauna associated with both species at most localities is dominated by one species, suggesting unfavorable conditions for a more diversified fish fauna to survive. Perhaps restricted movement resulting from the thickened bones may not have been disadvantageous because of the lack of predators in the environment. It may also be significant that the recent relatives of both H. wui and A. crassicaudus, various species of Aphanius and some schizothoracines (e.g., Gymnocypris przewalskii), live in varied salinities (euryhaline), from water with high salinity to brackish or even completely fresh water.

The geological occurrence of H. wui and A. crassicaudus is also similar. Both were collected from deposits rich in marl (CaCo₃) and gypsum (CaSo₄), indicating similar hydrochemical conditions with a high calcium content. A. crassicaudus occurs in marl–gypsum along the coastal region of the Mediterranean deposited during the Messinian Salinity Crisis (28), where the less soluble carbonates and sulfates precipitated first, and the more soluble sodium chloride and other minerals deposited later in more central part of the basin. In the deposits of the Yahu Anticline, carbonate ooliths and crystals of gypsum are often seen in the rock samples, also documenting a high concentration of calcium. Geological evidence also suggests that both taxa survived for a significant period. The ≈220-m-thick fish-bearing sediments from the Yahu Anticline indicate that Hsianwenia existed for a period of up to 200,000 years (2) (Fig. 2B) before a phase of general desiccation, although the Mediterranean area was later flooded by the Atlantic marine water, whereas the Qaidam Basin went on to develop into an arid badland.

The palynological assemblage recovered from the fish-bearing deposits from the Yahu Anticline, mainly includes Picea, Abies, Pinus, Artemisia, Chenopodiaceae, and Gramineae, and documents a vegetation closely similar to that of arid area of northwestern China today. Pediastrum, an alga usually widespread in shallow freshwaters, is also frequently observed. In the deposits of Eboliang, there is a high percentage of Typha (82.9%), a plant often seen along the margin of lakes and rivers in northern China (see SI Text and Table S2). Ostracods from the fish-bearing strata are Ilyocypris bradyi, Eucypris sp., Leucocythere mirabilis, and Microlimnocythere reticulata. The first is a freshwater species, whereas the remaining species are typical brackish forms. Ilyocypris bradyi, certain species of Eucypris, and Leucocythere mirabilis inhabit the present day Qinghai Lake just to the east to our fossil fish localities. The present Lake Qinghai has a salinity of 14‰ and is rich in magnesium sulfate. The content of sodium chloride is the highest (66%), whereas MgSO₄ comes second (20%). Despite this relatively high salinity, but perhaps because of the moderate calcium content, the single schizothoracin species now living in the lake, G. przewalskii, has normal skeletons without any thickened bones (Fig. 1 C and D). Similarly, radiographs of a dozen specimens of the extant species A. iberus showed no abnormal bone thickening, although the specimens were collected from the drained surface of a salt lake Sebkha-el-Melah in the Sahara and preserved in salt (NaCl) (26). Thus, high salinity alone without a high concentration of calcium may not cause bone thickening.

Several interpretations have been presented for the skeletal thickening in A. crassicaudus (25, 26), including aging, regulation of equilibrium related to unstable salinity, high content of particular chemical elements in the water, or a combination of a number of these factors. Based on comparison with H. wui, high content of particular chemical elements in the water seems to be the most likely explanation. And because of growth of fish appears to have been unaffected, the chemical elements should at least meet two prerequisites: (i) they are harmless to the fish and (ii) they can form a component of the bone.

A high concentration of calcium in the water of the two areas where H. wui and A. crassicaudus lived was most probably the crucial chemical element causing the bone thickening. In both cases, concentrations of carbonate (CaCO₃) and sulfate (CaSO₄) were apparently high. Calcium is the main component of the bone, and it is not harmful, or at least not lethal, to fish in general. This is supported by the thickened cranial bones described for the cichlid Tilapia guinasana, endemic to Lake Guinas in northern Namibia. In this case, hyperostosis was attributed to the environmental induction rather than to genetic reasons. The lake water has very high calcium carbonate content (calcium hardness, as CaCO₃, 185 ppm). Observations on four specimens of T. guinasana originally derived from wild-caught parents but spawned and raised in laboratory tap water (calcium hardness, as CaCO₃, ≈45 ppm) show that their roofing bones are thinner than those of wild conspecifics (29).

Under natural condition, when the concentration of calcium is high, the amount of other minerals also tends to be higher than usual. Therefore, only euryhaline fishes are able to survive for a relatively extended period. Both H. wui and A. crassicaudus were the only species surviving high salinity, but they were probably also able to deposit the oversaturated calcium in their bones and thus form the overall thickened skeleton. The physiological mechanism by which the overdeposition of the calcium in the bones occurs remains to be established.

In the Upper Youshashan Formation (stratigraphically underlying the Shizigou Formation) in the Eboliang area, where Typha constitutes 82.9% of the total pollen count, indicating a freshwater or low-salinity environment (Table S2), the disarticulated fish bones are normal, not thickened at all. In the area of the Yahu Anticline, overlying the fish-bearing Shizigou Formation, is the Qigequan Formation, in which no remains of fish have been found. Instead, layers of pure gypsum are clearly visible. The transition from the Upper Youshashan Formation with normal-boned fossil fish, to the Shizigou Formation with thick-boned fish and then to the barren Qigequan Formation, appears to show progressive aridification in the area during the Late Cenozoic. This unusually thick-boned fish was apparently an eyewitness to, or perhaps a victim of, the drying of Qaidam Basin.

Materials and Methods

Most of the fish remains reported here are from the lacustrine deposits of the Pliocene Shizigou Formation in the northeastern wing of Yahu Anticline, Qaidam Basin. Most of the material consists of disarticulated bones and pharyngeal teeth, but two nearly complete and two partially preserved skeletons were also recovered (localities CD0507 and CD0649). A few disarticulated, nonthickened fish bones were also recovered from the latest Miocene to early Pliocene Upper Youshashan Formation exposed in the axis of Eboliang Anticline in the centrowestern area of the Basin (Fig. 2B; for more details see Fig. S1). Fish specimens were prepared mechanically.