Choristoderes and the freshwater assemblagesof Laurasia/ Coristoderos y las asociaciones de agua dulce de Laurasia.
Choristodera was erected by Cope (1876) to encompass Champsosaurus, a new fresh water reptile from the Upper Cretaceous Judith River beds of Montana, North America. A year later, Gervais (1877) described Simoedosaurus from the Upper Paleocene at Cernay, near Rheims, France. These two genera were the only representatives of Choristodera for more than a century, but over the last three decades, new discoveries have extended the temporal and geographical range of the group, and provided increased understanding of its diversity. Choristoderes have been found across Laurasia, from Japan in the east to Alberta in the west (e.g. Evans and Hecht 1993; Gao and Fox, 1998; Matsumoto et al., 2007, 2009) in deposits ranging in age from Middle Jurassic to Miocene. However, despite the improvement in the record, the position of choristoderes within Diapsida remains uncertain. Some analyses have suggested that choristoderes are basal Archosauromorpha (e.g. Evans, 1988; Gauthier et al., 1988), but others have placed them alternatively as the sister group of Archosauromorpha (e.g. DeBraga and Rieppel, 1997), of Archosauromorpha +Lepidosauromorpha (e.g. Evans, 1988; Gao and Fox, 1998; Dilkes, 1998), or of Euryapsida within an expanded Lepidosauromorpha (e.g. Muller, 2004). Resolution of their phylogenetic position requires a better understanding of the early history of the group, but each of the above hypotheses predicts that choristoderes had separated from their (undetermined) sister group by the end of the Permian.
Eleven choristoderan genera are currently considered valid. Of these, Champsosaurus and Simoedosaurus (Late Cretaceous-earliest Eocene, Euramerica), Ikechosaurus and Tchoiria (Cretaceous, China and Mongolia, Efimov, 1975; Sigogneau-Russell, 1981) are medium-sized (2-5 m total length, skulls up to 700mm from rostral tip to occiput) (Fig. 1), gavialiform reptiles with cordiform skulls, long rostra, and short necks. All recent analyses support the placement of these four genera in a clade, Neochoristodera (Evans and Manabe, 1999; Gao and Fox, 1998; Matsumoto et al., 2007, 2009; Skutchas, 2008). The more basal 'non-neochoristoderes' are smaller (0.3-1.0m) and more diverse in their morphology. Cteniogenys (Middle-Late Jurassic, Euramerica, Evans 1990), Lazarussuchus (Oligocene-Miocene, Europe, Hecht, 1992; Evans and Klembara, 2005), Monjurosuchus (Early Cretaceous, China and Japan, Gao et al., 2000; Matsumoto et al., 2007), and Philydrosaurus (Early Cretaceous, China, Gao and Fox, 2005) are essentially lizard-like choristoderes with short necks and either open (Cteniogenys) or closed (the other three) lower temporal fenestrae, whereas the Early Cretaceous Shokawa (Japan, Evans and Manabe, 1999) and Hyphalosaurus (China, Gao et al., 1999), the Hyphalosauridae of Gao et al. (2008), are nothosauriform with elongated necks. Khurendukhosaurus (Early Cretaceous, Mongolia, Sigogneau-Russell and Efimov, 1984, Skutchas 2008; Matsumoto et al., 2009) is of similar/ or slightly larger size to Hyphalosaurus (holotype, IVPP V11075) (similar in dorsal centrum length but femur twice as long: Matsumoto pers. obs. 2008; Matsumoto et al., 2009), but is known only from fragmentary material and its skull and body shape (long or short necked) are not yet known. The Early Cretaceous Mongolian Irenosaurus (Efimov, 1979) is based on isolated postcranial bones that lack sufficient diagnostic characters to differentiate them from Tchoiria (Evans and Hecht, 1993; Efimov and Storrs, 2000) and the choristoderan status of the European Triassic Pachystropheus (Storrs and Gower, 1993; Storrs et al., 1996) is equivocal. Most recently, the genus Liaoxisaurus was named on the basis of a new specimen from the Jiufotang Formation of China (Gao et al. , 2005), but it is closely similar to Ikechosaurus pijiagouensis from the same horizon and its validity as a distinct taxon is in doubt.
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Our concept of Choristodera has thus changed substantially in recent years. As it currently stands, the fossil record of Choristodera suggests there was a gradual Jurassic/ Cretaceous increase in maximum size, coupled with an increase in morphological disparity, particularly with respect to the neck and skull. This resulted in several distinct morphotypes (e.g. small short-necked brevirostrine; small long-necked brevirostrine; large short-necked longirostrine), some of which paralleled those of contemporaneous crocodiles.
Choristoderes are typically found in association with a diverse freshwater assemblage, most members of which, including choristoderes, survived the end-Cretaceous extinction. However, their presence and their level of morphological diversity in different localities shows considerable variation, that may be indicative of underlying biological or ecological signals. Moreover, on current evidence, choristoderes became extinct in the Miocene while other members of the assemblage survived. Here we review choristoderan distribution and diversity from the Jurassic to the Miocene, and offer a preliminary analysis of the possible ecological relationship between choristoderes and other aquatic taxa, notably crocodiles with which they are most often compared.
1.1. Institutional abbreviations
GUI, Guimarota collections, Freie Universitat, Berlin, Germany; IVPP V, Institute of Vertebrate Paleontology and Paleoanthropology, Beijing, China; SMNS Staatliches Museum fur Naturkunde, Stuttgart, Germany; SMM, The Science Museum Minnesota, St. Paul, Minnesota, USA; UALVP, Laboratory for Vertebrate Paleontology, Department of Biological Science, University of Alberta, Edmonton, Canada.
2. Temporal and geographical distribution
Although the Triassic genus Pachystropheus (England, Germany) has been referred to the Choristodera (e.g. Storrs and Gower, 1993; Storrs et al., 1996), it is too poorly known to be attributed with any confidence. The most diagnostic features of choristoderes are in their skulls, and there is no confirmed skull material for Pachystropheus.
The first definitive records of choristoderes are from the Middle Jurassic of Europe and Asia. Cteniogenys is a small, probably amphibious, choristodere from the Bathonian of England (e.g. Kirtlington Mammal Bed, Oxfordshire, England, Evans, 1990) and Scotland (Skye, Evans and Waldman, 1996). It is also recorded from the Kimmeridgian of Portugal (Guimarota, Seiffert, 1973; Evans, 1989) and North America (Brushy Basin Member, Morrison Formation, South Dakota, Wyoming and Utah: Gilmore, 1928; Evans, 1990; Chure and Evans, 1998; Foster and Trujillo, 2000). There is unnamed (but broadly similar) material from the Callovian Balanbansai Svita, Fergana Basin, of Kyrgyzstan (Averianov et al., 2006). Plotted on a paleoclimate map (Scotese, 2000), most of these records are in relatively warm temperate areas (Fig. 2-a). Although the Morrison Formation falls broadly into the arid climate zone, it covered a huge area and encompassed a range of palaeoenvironments, including wetlands suitable for small aquatic reptiles (Turner and Peterson, 2004). Interestingly, Cteniogenys is not evenly distributed through the Morrison (Brushy Basin Member) microvertebrate assemblages (Chure and Evans, 1998). It seems to be most common in the northern part of the formation (most notably at Quarry 9, Como Bluff), representing coastal swamp environments, but rare (Dinosaur National Monument) or absent (Fruita Paleontological area) on the Colorado Plateau (interior continental water bodies) (Chure and Evans, 1998). This may be significant because the British Bathonian localities and Guimarota both represent coastal freshwater-brackish lagoonal environments (e.g., Schudack, 2000), and the Balabansai Svita of the Fergana Depression has been described as transitional from terrestrial to marginal marine (Averianov et al. , 2005).
2.3 Lower Cretaceous
Fossil deposits in the Lower Cretaceous of Asia (Eastern Russia, Inner Mongolia, China, Japan) have yielded the greatest known choristodere diversity, with at least seven recorded genera representing all known choristoderan morphotypes (Fig. 2-b): the long-necked Japanese Shokawa (Evans and Manabe, 1999) and Chinese Hyphalosaurus (Gao et al. , 1999; Ji et al., 2004; Gao and Ksepka, 2008); the longirostrine gavial-like neochoristoderes Tchoiria (Mongolia, Efimov, 1979) and Ikechosaurus (Mongolia, China, Brinkman and Dong, 1993; Liu, 2004); the small lizard-like Monjurosuchus (China, Japan, Gao et al., 2000; Matsumoto et al., 2007) and Philydrosaurus (China, Gao and Fox, 2005; Gao et al., 2007); and the problematic Khurendukhosaurus (Mongolia, Sigogneau-Russell and Efimov, 1984; Skutschas, 2008; Matsumoto et al., 2009). At this time, Eastern Eurasia was divided into two paleophytogeographic provinces: a northern Siberian-Canadian Region (Vakhrameev, 1978), with Tetori-type floras at its southern margin (Kimura, 1979) and a temperate to humid climate; and a southern Euro-Sinian Region (Vakhrameev, 1978) with Ryosekitype floras (Kimura, 1979) and a suggested subtropical to tropical climate, with an annual dry season. All major choristodere localities occur in the northern province, in continental wetland (e.g. Kuwajima and Okurodani formations), fluvial (e.g. Khuren Dukh) or lacustrine (e.g. Yixian Formation) environments; they have not been reported from the southern province (e.g. the semi-arid fluvial ecosystem of the Khorat Group in Thailand [Meesook, 2000]), although several semi-aquatic freshwater crocodiles (goniopholidids Sunosuchus, Goniopholis, and Siamosuchus, and the neosuchian Khoratosuchus) are known from the area (e.g. Lauprasert et al., 2009).
The Lower Cretaceous record for the rest of Laurasia is restricted to a partial femur, possibly neochoristoderan, from the Barremian Yellow Cat Member of the Cedar Mountain Formation of Utah (Britt et al., 2006). Palaeoenvironmental reconstructions for the Yellow Cat Formation (Kirkland et al., 1999) suggest a semiarid monsoonal environment, but one with enough standing water to support a stable freshwater community. The overlying Mussentuchit Formation contains a more diverse tetrapod assemblage, but there is no record of choristoderes.
2.4 Upper Cretaceous
In sharp contrast to the Lower Cretaceous, there is currently no record of choristoderes in the Upper Cretaceous of East Asia. This may reflect a lack of suitable depositional environments. Most Late Cretaceous fossil reptiles from China and Mongolia come from arid terrestrial environments. For choristoderes, the focus shifts to North America where there are abundant records of the gavial-like neochoristodere Champsosaurus in Utah, Wyoming, Montana, North Dakota, Alberta, Saskatchewan and the Canadian Arctic (Fig. 2-c). These localities, including those of the Arctic (Tarduno et al., 1998; Vandermark et al., 2007), were mostly located in low-lying river channels and deltaic deposits bordering the Western Interior Seaway; the climate was warm. This distribution may represent an ecological preference, or simply a prevalence of this type of environment within the Late Cretaceous rock record (e.g. Markwick, 1998). Putative records of non-neochoristoderes are limited to the Oldman and the Dinosaur Park formations, Alberta (Irvine locality and Dinosaur Provincial Park). These small specimens were assigned to Cteniogenys (Gao and Fox, 1998), but the material (incomplete dentaries and maxillae) is not sufficiently complete or diagnostic for generic identification (Matsumoto et al., 2009), especially with such a large age gap.
A single vertebra from the Lower Campanian of Austria (Gosau Beds, Muthmannsdorf; Buffetaut, 1989) demonstrates the presence of choristoderes in the Late Cretaceous of Europe. The fossil bearing horizon has been interpreted as a brackish estuarine environment (Sachs and Hornhung, 2006), with a humid, paratropical climate (Herman and Kvacek, 2007). Given the rarity of the choristodere material in the deposit, it may have been transported from further upstream.
2.5 Paleocene to Miocene
Champsosaurus survived the end-Cretaceous extinction and in the Paleocene was joined by another neochoristodere, Simoedosaurus. Conditions were temperate and neochoristoderes continued to be well represented in North America (Fig. 3-a), with limited European and Asian records. In Europe, Champsosaurus and Simoedosaurus have been recorded from littoral and infralittoral deposits at Erquelinnes, Belgium (Sigogneau-Russell and de Heinzelin, 1979) and from Mont Berru, France (Sigogneau-Russell and Russell, 1978; Sigogneau-Russell, 1985), respectively, and there is undescribed material of Champsosaurus from the latter locality (Matsumoto pers. obs. 2008). Postcranial elements of Simoedosaurus have also been reported from Upper Paleocene-Early Eocene coastal deposits (Dzhylga 1a-1b) in southern Kazakhstan (Averianov, 2005), where conditions were warmer and more humid.
In North America, the faunal transition from the Paleocene to the Eocene (Clarkforkian-Watsachian) has been well studied in continuous sections such as those of the Bighorn Basin, Wyoming (Bartels, 1983; Gunnel et al., 1993). Choristodere material is abundant in the Tiffanian and early Clarkforkian horizons in the Fort Union Formation of Wyoming, but is quite rare in the younger (early Eocene, Watsachian) Willwood Formation. Bartels (1983) attributed this decline to local effects (a reduction in the number and size of the streams), but this was also an interval of major faunal change associated with elevated temperatures. These are also the last records of neochoristoderes, and may mark the extinction of the clade (Gingerich, 2000; Gunnell et al., 1993; Bartels, 1983). However, non-neochoristoderes, having effectively disappeared from the record in the Early Cretaceous, unexpectedly reappeared in the Paleogene of Europe (France, Germany, Czech Republic) in the form of the small, lizard-like Lazarussuchus (Hecht, 1992; Evans and Klembara, 2005; Bohme, 2008; Evans et al., unpublished). These small reptiles were in continental lake environments. Their last record in Europe is from the Lower Miocene of the Czech Republic, in a warm (subtropical) wet environment (Fig. 3-b).
In summary, Choristodera were distributed throughout Laurasia from the Jurassic to the Miocene in warm temperate-paratropical localities. However, the group is notably absent or poorly represented in some localities and time periods, for example the Lower Cretaceous of Europe and the Upper Cretaceous of Asia. As so often with the fossil record, the challenge is to determine whether these absences are simply an artifact of sampling, preservation and taphonomy, or are real absences that require explanation and may provide an insight into the biology, lifestyle, or ecological interactions of the group.
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3. Faunal associations and ecological relationships with crocodiles
3.1. Assemblage composition
A comparison of well-known Mesozoic to Cenozoic assemblages (Fig. 4) demonstrates that choristoderes occur in mesic depositional environments that are rich in vertebrates, especially fresh water and amphibious animals such as fish, frogs, salamanders, turtles and crocodiles (Fig. 4-a). Typically, these assemblages contain a greater diversity of crocodiles than choristoderes (Fig 5; Table 2-4) although they did not necessarily all live together. For example, the North Horn Formation (Upper Cretaceous-Lower Paleocene, Utah, USA) has reportedly yielded five crocodile taxa (possibly an overestimate as the count is based on isolated teeth) and one choristodere. However, representatives of the two groups are found in different localities (Cifelli et al., 1999a) and this may reflect differences in habitat preference/tolerance or, possibly, some form of competitive exclusion. Similarly, six crocodiles have been recorded from the Morrison Formation (Fig 5; Table 2), with one small choristodere (Cteniogenys), but three of the crocodiles (the 'Fruita crocodile', Hallopus, and Hoplosuchus) are basal forms that were mainly or fully terrestrial and come from localities on the Colorado Plateau where Cteniogenys is rare or absent. In addition, several very productive horizons/localities in the Cretaceous and Eocene of Europe yield crocodiles but no choristoderes (Fig. 4-b) despite an otherwise generally similar associated freshwater fauna. These include: the Purbeck Limestone Group (Berriasian) of England (three valid crocodile genera, e.g. Salisbury, 2002); the limestones of the La Huergina Formation (Barremian) at Las Hoyas, Spain (at least five crocodile genera, Ortega et al., 1999, Buscalioni and Fregenal-Martinez, 2006); and the Wessex Formation (Barremian) of England (eight crocodile genera, Sweetman, 2009, but based mainly on teeth); as well as the Middle Eocene beds of Messel, Germany (seven crocodile genera, Morlo et al., 2004).
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By contrast, in those Lower Cretaceous Asian horizons that contain the greatest diversity of choristoderes (e.g. Yixian Formation, China; Kuwajima and Okurodani formations, Japan; Fig. 5; Table 4), crocodiles are conspicuously absent. Whether this is an ecological or temporal phenomenon is not clear, as possible goniopholid crocodile remains have been reported (Azuma, 2003), from the somewhat drier (Yabe et al., 2003) late Hauterivian to late Aptian Kitadani Formation (e.g. Kusuhashi, 2008; Tsubamoto, 2004), which overlies the choristodere bearing Kuwajima and Okurodani formations in Japan (e.g. Fujita, 2003; Kusuhashi, 2008). To date, only one Asian horizon (the Barremian Laohongdong Formation, Inner Mongolia) is known to contain both choristoderes (Tchoiria) and crocodiles. Sha et al. (2006) report that crocodiles were also found in the Khuren Dukh Formation (Mongolia), but no further data is available and Averianov and Skutchas (2000) do not include crocodiles in their faunal list for this formation. Significantly, Buscalioni et al. (2003) have reported that the Cretaceous crocodile fauna of Asia was dominated by basal groups (protosuchids, gobisuchids, ziphosuchians) that were largely terrestrial in their habits.
3.2 Comparison of size and rostral morphology in crocodiles and choristoderes
Based on Table 1, Tables 2-4 present comparisons of body size for crocodiles and choristoderes in selected horizons where they co-occur; Fig. 6 compares rostral morphologies in the same horizons. Body size estimates (total length: tip of snout to tail tip) are taken mainly from the literature, as listed. However, where body size data is lacking, size has been estimated for crocodiles based on skull/ body length ratios for extant Crocodylus (Wermuth, 1964; Bellairs, 1969), and for choristoderes based on the proportions of a complete articulated skeleton of the neochoristodere Ikechosaurus pijiagouensis (IVPP V13283, Liu, 2004) with a head:body ratio of 1:6.2 (Table 4). As skull morphology and size change ontogenetically in crocodiles (Dodson, 1975), the skulls used in these estimates were restricted to mature specimens.
Early choristoderes were small. In most Jurassic horizons where choristoderes and crocodiles are found together (e.g. Middle Jurassic of UK and Kyrgyzstan; Upper Jurassic of Portugal, North America), the adult crocodiles (mainly goniopholids but also atoposaurids) are up to ten times larger (~2.5-3.0 m) than the choristoderes (Cteniogenys: ~0.3 -0.5 m). Recently discovered choristodere vertebral centra from the Balanbasai Svita, Fergana (Middle Jurassic), Kyrgyzstan (Averianov et al., 2006), are of a similar size and morphology to those of Cteniogenys from the Morrison Formation, North America (Foster and Trujillo, 2000). Although, the body proportions of this Asian choristodere are still unknown, the total length would have been less than 1 m even if it was long-necked (by comparison with the largest specimens of the Lower Cretaceous Hyphalosaurus IVPP V11075). Two crocodiles are known from the Balanbasai Svita (Averianov et al., 2006). The first, the neosuchian Sunosuchus, has a total length of about 2.5 m. The other is an undetermined thalattosuchian of unknown length. At Guimarota, Portugal, the Kimmeridgian lignites have yielded several crocodiles (Martin and Krebs, 2000). The largest is the longirostrine Machimosaurus, about 30 times the size of Cteniogenys, but this is an open sea crocodile (Krebs, 1967). Three crocodiles are of similar length to Cteniogenys: the poorly known Lusitanisuchus (0.4 m) (Schwarz and Fechner, 2008), Lisboasaurus estesi and the atoposaurid Theriosuchus (Schwarz and Salisbury, 2005: 0.5 m). These three are relatively brevirostrine crocodiles (rostral length/ basal skull length ~ 50%), not dissimilar in overall skull shape to Cteniogenys (Evans, 1990), but they are primarily terrestrial. The Morrison Formation is similar, with both large aquatic crocodiles (up to 3 m: Eutretauranosuchus, Goniopholis spp., Macelognathus) and smaller terrestrial ones (0.2-1.0 m: Hallopus victor, Hoplosuchus kayi, and the 'Fruita crocodile') (Clark, 1994; Chure et al., 1998; Gohlich et al., 2005; Hups et al., 2005).
Crocodile teeth and osteoderms are recorded from the Yellow Cat Member of the Cedar Mountain Formation, in conjunction with the choristodere femur recorded above (Britt et al., 2006). A more diverse crocodile assemblage is known from the overlying Mussentuchit Formation (Cifelli et al., 1999b), but without choristoderes. The Barremian Laohongdong Formation, Inner Mongolia, is currently the only Lower Cretaceous deposit in Asia yielding both choristoderes (the large gavial-like Ikechosaurus sunailinae) and crocodiles (the protosuchian Shantungosuchus, 0.3 m: Wu et al., 1994; and the atoposaurid cf. Theriosuchus sp., 0.5 m: Wu et al., 1996, known also from Guimarota [see above]). Isolated larger (mature) specimens of I. sunailinae (e.g. IVPP V 1596.5) from the Laolonghuoze locality (Brinkman and Dong, 1993) are similar in size (centrum length 18.4 mm at cervical [C]3) to the adult of Ikechosaurus pijiagouensis (Liu, 2004) from the Jiufotang Formation of China (centrum length 17-18 mm through C2-C9; 1.7 m total body length). Thus, in the one Asian locality where crocodiles and choristoderes are known to co-occur, the choristoderes appear to have been larger. Moreover, as protosuchids and atoposaurids are considered to be primarily terrestrial (Brinkman, 1989; Schwarz and Salisbury, 2005; Wu et al., 1994), the two groups had different niches.
The isolated choristoderan vertebra from the Gosau Group of Muthmannsdorf (Lower Austria) is about 15 mm in length. This equates to a body length of about 2 m, assuming neochoristoderan proportions (e.g. Ikechosaurus pijiagouensis: presacral centrum length 17-20 mm, total length ~2.0 m). The Gosau bone bed has also yielded two crocodiles, the ziphosuchian Doratodon (Company et al., 2005) and Alligatoridae indet. (Buffetaut, 1979). At Muthmannsdorf, Doratodon is known only from cranial elements but there have been recent discoveries of the same genus from other localities and formations (Grigorescu et al., 1999; Company et al., 2005). It is a small, probably terrestrial ziphosuchian, possibly similar to the genus Bergisuchus in having a tall short-snouted skull (Company et al., 2005).
In North America, Champsosaurus is common at many localities and is typically found with more aquatic crown-group crocodilians, most usually the similar-sized Leidyosuchus (some species of which have been placed in the genus Borealosuchus, Brochu, 1997). The two differ, however, in rostral morphology and thus, presumably, feeding habit. Champsosaurus has a long slender rostrum whereas that of Leidyosuchus-Borealosuchus is wider and has been described as 'generalised' (Brochu, 2001). In the Hell Creek Formation (Maastrichtian) of southwestern North Dakota and northwestern South Dakota, crocodiles with a wider range of rostral shapes are found with Champsosaurus (Pearson et al., 2002). The closest match is the longirostrine gavialoid Thoracosaurus (e.g. Leidy, 1852), which is slightly smaller than Champsosaurus (Champsosaurus 3-4 m: Thoracosaurus 2.5 m). However, although the remains of Thoracosaurus occur in the same layer as Champsosaurus, its bones are much rarer than those of other crocodiles (Pearson et al., 2002), suggesting they may be allochthonous. In support of this, Buscalioni et al. (2003) suggested Thoracosaurus might be a coastal dweller like other Gavialoidea (including Eosuchus, see below), although juveniles may have lived in freshwater environments as in the extant saltwater crocodile, Crocodylus porosus (Jouve et al., 2008).
Small incomplete choristoderan jaw elements have been reported from the Oldman and the Dinosaur Park formations of Alberta, Canada, in association with Champsosaurus and Leidyosuchus (Brinkman, 1990; Gao and Brinkman, 2005; Gao and Fox, 1998) The affinities of these specimens (e.g. UALVP29794; UALVP29795) are uncertain, but the reptiles from which they came, if adult, would have been of similar size to Portuguese (Guimarota) specimens of Cteniogenys (e.g. GUI. A.33, Seiffert, 1973).
At some Paleocene localities, Champsosaurus (~4 m) and Simoedosaurus (~5 m) occur together (e.g. Mont-Berru, France, Sigogneau-Russell, 1985, Matsumoto pers. obs. 2008; Fort Union Formation, Wyoming, USA, Bartels, 1983; Ravenscrag Formation, Saskatchewan, Canada, Gao and Fox, 1998). In the Fort Union Formation (Wyoming, Bartels, 1983), these neochoristoderes are recorded with three genera of crocodiles. Of these, the basal eusuchian Leidyosuchus-Borealosuchus is similar in size to the choristoderes, but the blunt-snouted alligatorines Allognathosuchus and Ceratosuchus are smaller. In the Ravenscrag Formation (Russell, 1974) and at Mont Berru (Martin, 2008), Leidyosuchus-Borealosuchus again occurs with the neochoristoderes, in conjunction with the large (~4 m) crocodyloid Asiatosuchus at Mont Berru (Prasad and Lapparent de Broin, 2002). In each deposit, the crocodiles and choristoderes differ in rostral morphology. By contrast, in the Hannut Formation, Belgium, Champsosaurus co-occurs with the gavialoid crocodile Eosuchus, but although they were both recovered from the same shallow marine layer (Sigogneau-Russell and de Heinzelin, 1979), they probably did not live in the same environment. The bones of Champsosaurus are rounded, suggesting they were transported into the deposit from upriver, whereas Eosuchus may be salt water tolerant (Delfino et al., 2005), as evidenced by the occurrence of the North American species of Eosuchus (Marsh, 1870) in marine deposits.
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The Paleocene-Eocene Dzhylga 1a and 1b locality, Kazakhstan (Averianov, 2005), is the only Asian locality of this age to yield choristoderes. Crocodiles have not been recorded but the significance of this is limited, as the faunal assemblage as a whole is depauperate and lacks other freshwater components such as amphibians (Averianov, 2005).
The only recorded post-Eocene choristodere is the small European (France, Czech Republic, Germany) Lazarussuchus (0.3 m body length, Hecht, 1992). At the Oligocene-Miocene French locality of Armissan, the fauna is poorly known and the presence of crocodiles is uncertain (Hecht, 1992). However, at Oberleichtersbach, Germany (Late Oligocene, Bohme, 2008) and Merkur, Czech Republic (Lower Miocene, Evans and Klembara 2005), Lazarussuchus is found with the much larger (~x7) alligatoroid crocodile Diplocynodon (Fejfar and Schelich, 1994).
4.1 Choristoderes within freshwater assemblages
Choristoderes were distributed across Laurasia in the Mesozoic and early Paleogene, typically in relatively warm, temperate climates (occasionally sub-tropical areas, Fig. 5), and in association with a diverse mesic vertebrate assemblage including fish, frogs, salamanders, turtles, and crocodiles (Fig. 4-a). Typically, crocodiles are more diverse than choristoderes (up to 6:1, Fig. 5), although allowance must be made for the fact that many basal crocodiles were predominantly terrestrial. However, there are some well-sampled localities in which either crocodiles (e.g. Lower Cretaceous of China, Japan) or choristoderes (e.g. Lower Cretaceous Purbeck Limestone Group and Wessex Formation, UK) are completely absent. This suggests either differences in habitat preference or some level of competition (whereby choristodere diversity was limited by the presence of aquatic crocodiles). Generally, the associated freshwater assemblage is similar whether the assemblage contains crocodiles, choristoderes or both, but there does seem to be a tendency for choristoderes to be found in low lying freshwater environments close to coastal margins, at least in the Jurassic, Late Cretaceous and Paleocene (although this may partially reflect the prevalence of this type of deposit). Climate and other physical conditions are likely to have played an important role in choristoderan distribution and possibly extinction (as they do for crocodiles, Markwick, 1998), and this needs to be analyzed more thoroughly than we have space to do here.
Basal, non-neochoristoderan taxa were small and were unlikely to have been in serious competition with the adults of contemporaneous crocodiles, most of which were either larger or occupied a different, more terrestrial, niche. The choristoderes are more likely to have competed for resources with a range of other freshwater animals including predatory fish, turtles, juvenile crocodiles and aquatic lizards, as well as other small choristoderes. Nonetheless, a recent study in central Africa (Luiselli et al. , 1999) examined the ecological relationship between the small extant crocodile Osteolaemus and a similar-sized potential competitor, the predatory lizard Varanus. The two reptiles could co-exist as long as prey was abundant, and this is likely to have been the case for the choristoderes in relation to other small aquatic predators.
With the evolution of the neochoristoderes, there may have been a greater potential for competition with crocodiles, particularly as neosuchian crocodiles were themselves also radiating into more aquatic niches. Neochoristoderes are generally equal, or nearly equal, in size to contemporaneous crocodiles, the only exception being in the Lower Cretaceous Laohongdong Formation of Inner Mongolia, where the neochoristodere Ikechosaurus sunailinae is nearly twice the size of the associated crocodiles. However, this is a special case as contemporaneous East Asian crocodiles mainly belong to basal groups (protosuchians, gobisuchidans and ziphosuchians, Buscalioni et al., 2003) that were primarily terrestrial.
Where several crocodiles, extinct or extant, co-occur they tend to differ morphologically, most notably in rostral shape. Rostral shape in crocodiles is considered to reflect ecology and feeding habits (Busbey, 1994; Brochu, 2001) and it is reasonable to assume the same was true of choristoderes. Busbey (1994) characterised crocodile skulls as platyrostral (dorsoventrally compressed) or oreinirostral (mediolaterally compressed) according to rostrum width/depth indices, and as short, medium or long according to the length of the rostrum relative to overall skull length. However, the same metrics do not work well for choristoderes because their skulls and rostra are more dorsoventrally flattened and because the postorbital skull proportions of crocodiles and choristoderes are very different. Thus plotting choristoderes onto Busbey's (1994) graph of crocodilian skull morphotypes (Fig. 7) fails, for example, to identify neochoristoderes as longirostrine (despite widespread use of the gavial as a functional analogue for Champsosaurus: e.g. Erickson, 1972, Evans and Hecht, 1993), and the clearly longirostrine Simoedosaurus emerges within the brevirostrine crocodile cluster. Different parameters need to be identified for meaningful comparison. Nonetheless, if crocodiles and choristoderes were interacting ecologically, one would predict that, in any one locality, the choristodere rostral shape would not be replicated by any of the co-occurring crocodiles. A comparison of rostral shapes (Fig. 6) mainly confirms this prediction, although the comparisons are not statistical. There are two horizons (Upper Cretaceous Hell Creek Formation, Palaeocene Hannut Formation) where crocodiles and choristoderes with slender rostra are found together, but there is reasonable evidence to suggest they did not live together (see above). Indeed, longirostrine crocodilians were primarily in coastal and marine environments, leaving a vacant niche in freshwater ecosystems that neochoristoderes occupied.
In this section, we have focused on the possible ecological relationship between choristoderes and crocodiles because neochoristoderes are most frequently compared with gavialids in terms of lifestyle. However, understanding these potential interactions requires a fuller knowledge of choristoderan diet and lifestyle, which ongoing studies of the gut contents, dentition and skull morphology should help to elaborate.
[FIGURE 7 OMITTED]
4.2. Choristoderan evolution and palaeobiogeography
Although the phylogenetic position of choristoderes within Diapsida has yet to be resolved, all of the proposed positions (stem-neodiapsid, basal archosauromorph, sauropterygian relative etc) would predict an origin in the Late Permian or earliest Triassic. As yet, there is no record for that period and thus any discussion of early choristoderan evolution is speculative. Nonetheless, the earliest taxon that can be attributed to the group with confidence is the Middle-Late Jurassic Cteniogenys as well as similar, but currently indeterminate material from Central Asia. These early choristoderes are small and constitute part of a freshwater aquatic assemblage that continues almost unchanged (except for the representative genera) until the Miocene. In most cases, these small Jurassic choristoderes are found in swampy or lagoonal freshwater coastal environments. The absence of Permian or Triassic representatives almost certainly reflects the paucity of suitable depositional environments, as evidenced by the absence, or near absence, of Triassic frogs, salamanders, lizards, turtles, and albanerpetontids which typically occur with choristoderes in younger deposits. Choristoderes presumably evolved from small terrestrial-amphibious Permian diapsids within the northern part of Pangea. The Permo-Triassic arid belt may have represented a significant barrier as they did not reach southern Pangea (and have never been recorded from Gondwana).
Small choristoderes had spread across Laurasia by the end of the Jurassic but then disappeared from the Euramerican record (the enigmatic fragments from the Late Cretaceous of Alberta excepted). In Eastern Asia, isolated from the rest of Laurasia in the Middle Jurassic-Lower Cretaceous, the group radiated and diversified, giving rise to the larger neochoristoderes, as represented by Tchoiria and Ikechosaurus. The apparent absence of aquatic neosuchian crocodiles in these continental localities/ecosystems may well have been crucial to this morphological diversification and almost certainly facilitated the evolution of the large gavial-like choristoderan ecomorphs. In the Lower Cretaceous, Eastern Asia established a connection with western North America, and this may be the route through which neochoristoderes (first recorded in the Barremian of Utah, Britt et al., 2006) entered North America. Neochoristoderes then seem to have established themselves across Laurasia, successfully sharing freshwater ecosystems with eusuchian crocodiles as long as resources were sufficient and there were no direct longirostrine crocodilian competitors. The possible role of crocodiles in the ultimate extinction of neochoristoderes remains unknown, given the accompanying climatic changes at the Paleocene-Eocene transition, but may be clarified with a greater understanding of choristoderan functional anatomy and thus possible areas of niche overlap.
The reappearance of non-neochoristoderes in the Paleogene of Europe was surprising, as reflected by the name Lazarussuchus (Hecht, 1992). Whether this genus is a relict of the original Jurassic distribution, or was derived from an Asian lineage that reinvaded Euramerica when contact was reestablished, is currently uncertain, as the phylogenetic position of Lazarussuchus is not fully resolved (e.g. Matsumoto et al., 2007, 2009; Skutschas, 2008). Material of earlier choristoderes would shed light on this. Finally, although the last record of choristoderes is currently from the lower Miocene, the group may have survived longer at other localities. Albanerpetontid amphibians provide a parallel case. They were long thought to have become extinct in the Miocene (e.g. Estes, 1981) but have since been found in the Lower Pliocene of Hungary (Venczel and Gardner, 2005) and the Late Pliocene of Italy (Delfino and Sala, 2007). A dedicated reexamination of all suitable Mesozoic, Paleogene and Neogene small vertebrate assemblages throughout Laurasia is needed to provide more confident presence/absence data for these periods so that the history, palaeobiogeography, phylogeny, and ecology of this enigmatic reptilian group can be better understood.
Choristoderes are generally found in association with a diverse freshwater vertebrate assemblage, in warm temperate environments across Laurasia.
Allowing for preservational and collecting biases, much of the Jurassic choristodere record, and at least some of that for Late Cretaceous and Paleocene neochoristoderes, comes from low-lying depositional freshwater environments close to the coast. In the Lower Cretaceous of Asia and the Paleogene of Europe, they appear to have been in more continental water bodies.
Choristoderes typically co-occur with crocodiles, but were morphologically most diverse in Asian deposits where aquatic crocodiles were absent.
Neochoristoderes apparently evolved in an isolated Jurassic-Cretaceous Asia, within freshwater environments where the absence (or rarity) of crocodiles left a vacant large longirostrine piscivore niche to be exploited.
Where large neochoristoderes and crocodiles co-occur, the former are typically the only reptiles with strongly longirostrine skulls, suggesting a degree of niche partitioning, aided by the absence of longirostrine crocodiles from Mesozoic and Paleogene freshwater deposits.
We would like to thank Dr Alexander O. Averianov (Zoological Institute, Russian Academy of Sciences, St Peterburg), Ms Chamero Beatriz (Universidad Autonoma Madrid), Dr Marco Branadlise de Andrade (University of Bristol, UK), Dr Shin-ichi Fujiwara (University of Tokyo, Japan), Dr Pavel Skutchas (St Petersburg University, Russia), Dr Denise Sigogneau-Russell (Paris), Dr Komsorn Lauprasert (Mahasarakham University, Thailand), Dr Harufumi Nishida (Chuo University, Japan), and Dr Xiao-Chun Wu (Canadian Museum of Nature, Ottawa, Canada) for creative suggestions and information for this study; as well as Dr Bruce Erickson and Ms Jackie Hoff (Science Museum Minnesota), Drs Don Brinkman and Don Henderson (Royal Tyrrell Museum), Dr. Bernard Battail, (Museum National d'Histoire Naturelle), Dr Rainer Schoch (Staatliches Museum fur Naturkunde, Germany), and Dr Yuan Wang (Institute of Vertebrate Paleontology and Paleoanthropology, Beijing, China) for access to specimens. Our particular thanks to Drs Angela D. Buscalioni (Universidad Autonoma Madrid) and Eric Buffetaut (CNRS, France) for discussion of crocodilian phylogeny, palaeoecology and distribution. RM acknowledges funding from a 10th MTE Student Travel Scholarship; the Palaeontological Association, England; the University of London Central Research Fund; and a Synthesys grant (MNHM, Paris).
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R. Matsumoto, S. E. Evans
Department of Cell and Developmental Biology, University College London, Gower Street, London, WC1E 6BT, England
Received: 11/11/09 / Accepted: 30/06/10
Table 1.- Estimated total length (skull to tail tip) for choristoderes and crocodiles Tabla 1.- Longitud total estimada (craneo a cola) para coristoderos y cocodrilos. Region Choristoderes Skull Miocene-Upper EU Lazarussuchus 0.05 m Oligocnene Paleocene NA Champsosaurus gigas 0.49 m EU C. dolloi 0.65 m NA Simoedosaurus dakotensis 0.71 m EU Simoedosaurus sp. 0.52 m EU Diplocynodon NA Leidyosuchus NA Wannaganosuchus NA Ceratosuchus 0.23 m NA Allognathosuchus 0.24 m EU Eosuchus EU Asiatosuchus sp. Cretaceous Upper NA Champsosaurus natator 0.5 m C. laramiensis 0.41 m C. ambulator 0.4 m NA Brachychampsa montana 0.39 m NA Leidyosuchus sternbergi 0.34 m NA Thoracosaurus neocesariensis EU Doratodon carcharidens Lower AS Ikechosaurus pijiagoensis 0.27 m AS Tchoiria namsarai 0.3 m AS Khurendkuhosaurus ? AS Philydrosaurus proseilus 0.1 m AS Monjurosuchus splendens 0.06 m AS Hyphalosaurus 0.06 m lingyuanensis AS Shokawa ikoi ? AS Theriosuchus AS Shantungosuchus Jurassic Middle- EU Cteniogenys sp. 0.04 m Upper /NA EU Macelognathus EU Machimosaurus EU/NA Goniopholis EU Lusitanisuchus EU Theriosuchus EU Eutretauranosuchus NA Hoplosuchus kayi NA Hallopus victor NA "Fruitachamysa" AS Sunosuchus sp. 0.42 m Size References Miocene-Upper 0.3 m Hecht, 1992 Oligocnene Paleocene 3.5 m Erickson, 1985 4 m Sigogneau-Russell and Heinzelin, 1979 5 m Erickson, 1987 4 m SMNS 59026 0.3-1.5 m Piras and Buscalioni, 2006 4 m Erickson, 1982 1 m Erickson, 1982 2 m Bartels, 1984 2 m Simpson, 1930 3.5 m Delfino et al., 2005 > 4 m Russell et al., 1990 Cretaceous Upper 3.6 m Russell, 1956 3.0 m Brown, 1905 3 m Brown, 1905 3 m Norell et al., 1994 3 m Brochu, 1997 2.5 m Brandalise de Andrade, personal communication, August 2009 ? Lower 1.7 m Liu, 2004 2 m undescribed material 1 m? Matsumoto et al., 2009 < 1 m? Gao et al., 2007 ~0.5 m Gao and Li, 2006; Gao et al., 2000 1 m Holotype (IVPP V11705) > 0.6 m Evans and Manabe, 1999 0.5 m Schwarz and Salisbury, 2005 0.3 m Wu et al., 1994 Jurassic Middle- ~0.3 m Evans, 1990 Upper 3 m Hups et al., 2005 9.5 m Charig, 1976 3 m Schwarz and Salisbury, 2005 0.4 m Schwarz and Fechner, 2008 0.5 m Schwarz and Salisbury, 2005 3 m Hups et al., 2005 0.2 m 0.6 m Walker, 1970 1 m 3 m Schellhorm et al., 2009 Table 2.- List of choristoderan localities/horizons with crocodiles in North America Tabla 2.- Lista de localidades/horizontes de coristoderos con cocodrilos en Norte America. North Horizon No. Formations Skull America No. Paleocene NA-9 North Dakota Bullion Creek Fm. (Tiffanian) NA-8 Wyoming ch-10 Fort Union Fm. ch-9 (Upper Paleocene) NA-7 Utah North Horn Fm. (Paleocene) Cretaceous Upper NA-6 North Dakota ch-7 Hell Creek Fm. (Maastrichtian) NA-5 Alberta Dinosaur Park Fm (Campanian) NA-4 Wyoming Mesaverde Fm. Wind River Basin (Campanian) NA-3 Utah North Horn Fm. (Upper Cretaceous) Lower NA-2 Utah ch-5 Cedar Mountain Fm. Yellow Cat Member (Barremian) Jurassic Upper NA-1 Wyoming ch-3 Morrison Fm. (Kimmeridgian) North Choristoderes Size Skull America No. Paleocene Champsosaurus gigas 3.5 m Champsosaurus sp. 3-4 m Champsosaurus gigas 3.5 m cr-22 Simoedosaurus sp. 4-5 m cr-21 cr-20 cr-19 Champsosaurus 3-4 m Cretaceous Upper Champsosaurus sp. 3-4 m cr-17 cr-16 cr-15 Champsosaurus natator 3-4 m non-Neochoristodera ? Champsosaurus sp. 3-4 m Champsosaurus sp. 3-4 m Lower Neochoristodera indet. ? cr-12 Jurassic Upper Cteniogenys antiquus 0.3 m cr-8 cr-7 cr-6 cr-5 North Crocodiles Size References America Paleocene Leidyosuchus 3-4 m Erickson , 1999 formidabilis Wannaganosuchus 1 m brachymanus possibly juvenile of Allognathosuchus (Lucas and Estep, 2000) Allognathosuchus 2 m Bartels, 1983 (e.g. A. heterodon) Leidyosuchus 4 m formidabilis Ceratosuchus 2 m burdoshi (Diplocynodon 0.3-1.5 m sp.) Allognathosuchus 2 m Cifelli et al., 1999a Leidyosuchus 4 m Cretaceous Upper Brachychampsa 3 m Pearson et montana al., 2002 Leidyosuchus 3-4 m sternbergi (=Borealosuchus; Brochu, 1997) Thoracosaurus 2.5 m neocesariensis Leidyosuchus 3 m Brinkman, 1990 canadensis Albertochampsa Gao and langstoni Brinkman, 2005 Wu, 2005 Leidyosuchus sp. 3-4 m DeMar and Breithaupt, 2008 Alligatorinae ? ? Atoposauridae ? Cifelli et al., 1999a gen. and sp. Indet. gen. and sp. Indet. ? ? Pholidosauridae ? Leidyosuchus sp. 3-4 m Allognathosuchus 2 m Lower Crocodylia indet. ? Britt et al., 2009 Britt et al., 2006 Kirkland et al., 1999 Jurassic Upper Goniopholis sp. 3 m Clark, 1994 Hoplosuchus kayi 0.2 m Chure, et al., 1998 Hallopus victor 0.6 m Gohlich, et al., 2005 Macelognathus 3 m Hups, et al., vagans 2005 Eutretauranosuchus 2 m "Fruitachampsa" 1 m Table 3.- List of choristoderan localities/horizons with crocodiles in Europe Tabla 3.- Lista de localidades/horizontes de coristoderos con cocodrilos en Europa. Europe Horizon Formations Skull No. No. Oligocene- EU-9 Czech Republic ch-14 Miocene Lower Orleanium, zone MN 3 Formation (Lower Miocene) France Armissan quarry (Upper Oligocene-Lower Miocne) EU-7 Germany ch-13 Oberleichtersbach (Upper Oligocene) Paleocene EU-6 Belgium ch-11 Hannut Fm. (Upper Paleocene) EU-5 France Mont Berru Cretaceous Upper EU-4 Lower Austria ch-8 Gosau Group Lower Campanina Jurassic Upper EU-3 Portugal ch-4 Guimarota Beds (Kimmeridgian) Middle EU-2 England ch-1 Kirtlington Mammal bed EU-1 Scotland Skye (Upper Bathonian) Europe Choristoderes Size Skull No. Oligocene- Lazarussuchus davoraki 0.3 m cr-25 Miocene Lazarussuchus inexpectatus 0.3 m Lazarussuchus sp. cr-24 Paleocene Champsosaurus dolloi 4 m cr-23 Simoedosaurus lemoinei 4 m Champsosaurus 3-4 m Cretaceous Upper Choristodera indet. ? cr-18 Jurassic Upper Cteniogenys sp. 0.3 m cr-11 cr-10 cr-9 Middle Cteniogenys sp. 0.3 m cr-2 cr-1 Cteniogenys sp. 0.3 m Europe Crocodiles Size References Oligocene- Diplocynodon sp. 0.3-1.5 m Evans and Klembara, 2005 Miocene ? Hecht, 1992 Diplocynodon sp. 0.3-1.5 m Bohme, 2008 Paleocene Eosuchus leichei 3.5 m Delfino et al., 2005 Asiatosuchus sp. >4 m Sigogneau- Russell, 1985 Borealosuchus sp. Prasad and F de Lapparent de Borin, 2002 Martin, 2008 Cretaceous Upper Doratodon ? Sachs and carcharidens Hornung, 2006 Alligatoridae ? Buffetaut, 1979 indet. Jurassic Upper Machimosaurus 9.5 m Evans and Chure, 1998 Goniopholis 3 m Martin and Krebs, 2000 Lusitanisuchus 0.4 m Theriosuchus 0.5 m Middle Goniopholididae ? Evans and (cf. Milner, 1994 Nannosuchus=? juvenile Goniopholis) Atoposauridae ? Atoposauridae ? Goniopholididae ? Evans and Waldman, 1996 Table 4.- List of choristoderan localities/horizons with crocodiles in Asia Tabla 4.- Lista de localidades/horizontes de coristoderos con cocodrilos en Asia. Asia Horizon Formations Skull No. No. Paleocene-Eocene AS-8 Kazakhstan 2 ch-1 Dzhylga 1a-1b Upper Paleocene- Early Eocene Cretaceous Lower AS-7 Inner Mongolia ch-6 Laohongdong Fm. (Barremian) AS-6 Inner Mongolia Khuren Dukh Fm. (Barremian- middle Aptian) Russia Murtoi Fm. (Upper Barremian- middle Aptian) AS-5 China Jiufotang Fm. (Aptian) AS-4 China Yixian Fm. (Barremian) AS-3 Japan Okurodani Fm. Barremian-Aptian AS-2 Japan Kuwajima Fm. Barremian-Aptian Jurassic Middle AS-1 Kyrgyzstan ch-2 Balabansai Svita (Callovian) Asia Choristoderes Size Skull No. Paleocene-Eocene Simoedosaurus sp. 4-5 m Cretaceous Lower Ikechosaurus sunailinae 1.7 m cr-14 cr-13 Tchoiria namsarai 2 m Khurendukhosaurus orlovi 1 m? Khurendukhosaurus sp. 1 m? Ikechosaurus pijiagoensis 1.7 m Philydrosaurus proseilus <1 m ? Hyphalosaurus baitaigouensis 1 m ? Liaoxisaurus chaoyngensis Monjurosuchus splendens 0.5 m Hyphalosaurus lingyuanensis 1 m Shokawa ikoi >0.6 m Monjurosuchus sp. Monjurosuchus sp. 0.5 m Jurassic Middle Choristodera indet. ? cr-4 cr-3 Asia Crocodiles Size References Paleocene-Eocene 4-5 m Averianov, 2005 Cretaceous Lower Theriosuchus 0.5 m Wu et al., 1996 Shantungosuchus 0.3 m Wu et al., 1994 Brinkman and Dong, 1993 Efimov, 1975 Sigogneau- Russell and Efimov, 1984 Averianov and Skutschas, 2000 Liu, 2004 Gao and Fox, 2005 Ji et al., 2004 Gao et al., 2005 Gao et al., 2000 Gao et al., 1999 Evans and Manabe, 1999 Matsumoto et al., 2007 Matsumoto et al., 2007 Jurassic Middle Thalattosuchia ? indet. Sunosuchus sp. 3 m Averianov et al., 2006
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|Author:||Matsumoto, R.; Evans, S.E.|
|Publication:||Journal of Iberian Geology|
|Date:||Jul 1, 2010|
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