Athena Review Vol. 5, no. 1


Records of Life: Fossils as Original Sources


The Permian-Triassic Mass Extinction

           The Triassic, as the first period of the Mesozoic era, lasted for 51 million years (251.2 - 201.3 mya) (table 1). It represents a severe reduction in species, following the mass extinction at the end of the Permian Period. Only about 4 % of the world's marine species, and an estimated 30% of terrestial species,  survived the Permian-Triassic Mass Extinction (Benton 2005; Sahney and Benton 2008). Also occurring was the only known mass extinction of insects, with 57% of all insect families and 83% of all insect genera becoming extinct (Labandeira  and Sepkoski 1993).

            A majority of dominant land plant species also disappeared. Glossopterids were one of the major casualties of the end- of-Permian, mass extinction event. The Glossopterid family, which arose in the Southern Hemisphere by the Early Permian period, became the dominant elements of the southern flora through the rest of the Permian. Glossopteris was a woody, seed-bearing tree, growing as high as 30 meters.  They are interpreted to have thrived in very wet soil conditions. Fossils of Glossopteris (fig.1) have been found in all of the southern continents, with over 70 fossil species recognized in India alone, and other species known from Africa, Madagascar, Australia, Antarctica, and South America

Fig.1: Fossilized leaves and stem from Glossopteris, a Permian tree.

            The  Glossopteris distribution across several, now detached, landmasses led Austrian geologist Eduard Suess to infer that these areas had once been connected by a land bridge. He named this large land mass Gondwanaland, after the district in India where the plant Glossopteris was found. These same observations would also lend support to Alfred Wegener's Continental drift theory, now embodied in the mega-continent Pangea, which formed in the Permian from the two supercontinents of Laurasia and Gondwanaland, and which lasted through most of the Triassic period.

        Direct effects of the Permian-Triassic extinction lasted at least through the Early Triassic, a period of five million years. The mass extinction was probably caused by a combination of volcanic activity, global warming, and increasingly arid climate in the mega-continent Pangea (fig.2).  At the the start of the Early Triassic, during the Induan stage (252-251 mya), all continental plates were joined into Pangea except China, Indonesia, and the Cimmerian zone including Thailand and parts of Turkey, the latter areas being islands in the Tethys Sea.
            Regarding volcanism as a likely cause of radical atmospheric changes, much attention has been paid to evidence provided by the extensive lava fields known as the 
Siberian Trapss, a vast area in Russian Siberia extending for about 3000 km. The Siberian Traps indicate massive volcanism which occurred between 252.6 and 250.3 mya, starting at the very end of the Permian, and lasting through the initial Induan Stage and part of the Olenikian stage of the Early Triassic  (Kump et al. 2005;  Knoll et al. 2007; Ogdena and Sleep 2011).

Fig.2: Pangaea at the start of the Early Triassic (Induan Stage).

        The Siberian Traps consist primarily of basalt and tuff formations. The term "traps" is derived from "trappa", the Swedish word for stairs, referring to the step-like hills of the region, which were created by massive outpourings of lava that hardened into basalt. The major eruptive events which formed the traps, the largest known over about 500 million years since the Cambrian period, are thought to have resulted not only in the extensive outpourings of lava forming the basalt traps,  but also frequent explosions and coal/gas fires with significant impact on the atmosphere.  In combination, these released enough acid and carbon dioxide over an extended period, to have critically damaged both the marine environment and the Earth's atmosphere and its vegetation. 

            Some evidence suggests that equatorial temperatures exceeded the thermal tolerance for many marine vertebrates over a long period, spanning at least two thermal maxima , whereas terrestrial equatorial temperatures were sufficiently severe to suppress plant and animal abundance during most of the Early Triassic  (Sun et al 2013). Consequences included atmospheric and oceanic warming by 5–10° C, anoxia in the oceans, and acid rain leading to massive killing of forests. Most damaging to marine life forms were acidification and anoxia  (Knoll et al., 2007); while on land, most harm was caused by acid rain and an increasingly arid climate (Benton and Newell, in press; Benton et al. 2013)  

Table 1: Time chart of the Triassic Period, showing stages.

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The Early Triassic fossil record

            Overall, Early Triassic vertebrates are best represented in the Beaufort Group of South Africa's Karoo Basin; and secondly, in the Vetluga series in the Buskunchyak of Russia. Both regions have continuous successions of Permian to Early Triassic therapsids. In South Africa, postdating the Late Permian fossil deposits of the Balfour formation, are extensive Early Ttriassic deposits, including the Lystrosaurus Assemblage Zone in the Katberg Formation, and the Cynognathus Assemblage zone in the Burgersdorp formation.

         Other components of the Early Triassic fossil record occur in China, in the Heshanggou and Livijagou formations of Shansxi and Inner Mongolia, and the Shaofanggou and Jiucaiynan Formations of Xinjiang province in the northwest.

        Early Triassic vertebrates are also recorded in the Puesto Viejo formation of Argentina, equjivalent to the Mendoza formation in Brazil Other, comparable Early Triassic vertebrate fossils from the Gondwana conjunction of southern continents have been found in the Arcadia Formation of Australia, and the Fremouw Formation of Antartic.

        During the Early Triassic, Induan and Ikenikian stages (252-247 mya; table 1) some entirely new groups of large aquatic vertebrates appeared, as evidenced by findings in the Yunnan and Ghouzou deposits in south China and elsewhere (Benton et al. 2013).  This included first major radiation of marine reptiles, including ichthyosaurs, sauropterygians, and thalattosaurs. In the Permian, there had been only a short-lived expansion of marine reptiles around the southern shores of Gondwanaland. These late Paleozoic marine reptiles are collectively called mesosaurs, a mixed grouping which includes various unrelated taxa (McGowan and Motani, 2003). 

            While many families of fish disappeared in the massive marine extinction, during the Early Triassic there was a large radiation of bony fish, especially the actinopterygians or spiny-fin groups.  Marine invertebrates also saw major changes. The most common Paleozoic groups such as brachiopods, trilobites, and rugose and tabulate corals all became extinct, to be replaced during the Early and Middle Triassic (252-235 mya) by echinoderms, mollusks (both bivalves and gastropids), arthropods, ammonites, and scleractinian corals (Benton et al. 2013). 

            

Lystrosaurus zones

           On land, many groups of tetrapods disappeared during the end-of-Permian extinction, to be replaced by new forms. For Early Triassic land vertebrates, two main patterns occurred throughout the continental land mass called Pangea (Colbert 1973; Sun 1980). One consisted mainly of labyrinthodont amphibians, with little evidence of reptiles except occasional trace fossils (i.e., footprints) of smaller reptiles such as Chilotherium. This condition existed in a portion of Laurasia from Greenland to Spitzbergen to North America, and Madagascar, which was then joined to North America.  

            The second pattern, much more widely distributed in the southern Pangean region of Gondwanaland, comprising South Africa, IndiaChina, Australia, Antartica, and Russia, included both amphibians and a standard assemblage of reptiles and synapsids known as the Lystrosaurus fauna, after the Early Triassic faunal zone of that name in the South African Karoo Basin. Lystrosaurus (fig.3) was an herbivorous synapsid of the widespread dicynodonts. With only two upper canines, their name meaning "two dog-teeth" derives from di- ("two"), -cyno, ("dog" or "canine"), and -dont ("tooth"). Lystrosaurus fossil species are very similar  throughout the regions where they migrated. The associated environments for this small, terrestial plant-eater are wooded, well-watered, riverine settings. In Gondwana, this pattern may have carried over from the Late Permian, when the distribution of dicynodonts accords well with that of forest vegetation, as shown by Glossopteris distribution.

            The Early Triassic strata in the Beaufort Group of South Africa have been separated into two sequential fossils zones: the lower Lystrosaurus Zone, correponding to the earliest part of the Early Triassic, and the upper Cynognathus Zone, at the end of the Early Triassic. Originally, there was a third intermediate zone, named the Procolophonid zone for common small anapsid  reptiles. This, however, was later merged with the earlier Lystrosaurus zone, as the two type taxa, Procolophon and Lystrosaurus, proved to be largely contemporary (Broom; Kitching J.W.; Sun 1980). 

Fig.3: Lystrosaurus, a small herbivorous dicynodont common throughout Pangea in the Early Triassic.

              Procolophonoids (fig.4) also have a very wide Early Triassic distribution, their fossils having been found in North America, Europe, China, Russia, Africa, and Antarctica. Their affinities have long been debated. Some consider them to be closely related to turtles, as shown by the presence of a large posttemporal fenestra or opening in the back of the skull for jaw muscle attachment (Reisz and Lauren 1991). All known Late Permian and some Early Triassic procolophonoids had sharp teeth, and were either insectivorous or carnivorous. Later forms from the Middle and Late Triassic, however, have broad, bulbous teeth that indicate a herbivorous diet. Upper Triassic procolophonoids include Hypsognathus, which has a much enlarged eye orbit on the posterior end. The presence of  a slender stapes (a sound-conducting middle ear ossicle) in a groove of the quadrate bone near the lower jaw joint indicates that procolophonoids had a tympanum or ear drum  and could hear some high frequency airborne sounds. Primitive traits shared by all  procolophonoids include a pineal foramen, a large hole between the orbits now lost in extant anapsid reptiles.

   Fig.4: Procolophon, a small anapsid reptile that was widespread in Pangea during the Early Triassic.        

            Findings of the anapsid Procolophonis in the Early Triassic Fremouw Formation of Antarctica has helped establish faunal distribution in Gondwanaland, the southern half of Pangea. The Antarctic fossils were all identified as Procolophon trigoniceps Owen, typically also found in the Lystrosaurus Zone in South Africa. The presence of this particular species in both Africa and Antarctica provides further evidence of widespread distribution in Gondwanaland of Lystrosaurus, and also indicates close links between the two continents at the beginning of the Mesozoic (Colbert and Kitching 1975).   

           Another successful Early triassic reptile group were the archosaurs, diapsids 1-3 meters in length who were present in modest numbers in the latest Permian, then expanded considerably in the Early Triassic. Protorosaurus ("first lizard"), a lizard-like reptile about two meters long who had lived in the area of Germany during the Late Permian, is the earliest known archosauromorph. 

            One of the most common archosaurs in the Early Triassic, and one of the largest,  was Proterosuchus,  associated with the Lystrosaurus faunal complex in both South Africa and China.  The type species is Proterosuchus fergusi,  first described  by Robert Broom in 1903 from the Beaufort Group in South Africa (Broom 1903). It was about the same size as a Komodo Dragon (2-3 meters long), and somewhat resembled a crocodile both in appearance, and in its living, hunting, and eating habits. 

            A distinctive trait of  P. fergosi is its an expanded premaxilla with sharp, conical teeth, which extends downward like a scoop, and effectively extends reach of the upper jaw. Proterosuchus spent most of its time in rivers or estuaries, but could also walk on land. At first it was considered to have been a direct ancestor of modern crocodiles, but it is now understood to be more basal (i.e., it is ancestral to a series of archosaurs, that eventually led to crocodiles, dinosaurs, and birds). 
           
A Chinese variant of Proterosuchus is named Chasmatosaurus, which is also linked with the Lystrosaurus fauna in the Early Triassic Heshankou formation in Shanxi Province, in northeast China (Sun 1980).  The Triassic sequences in Shanxi, Xianjiag, and Yunnan provinces will be described below.  

            Another related archosaur is Tasmaniasaurs triassicus, first discovered in 1960 in Hobart, Tasmania on the southeast coast of Australia, and recently identified as a probable member of the Proterosuchidae family, with similarities in its premaxilla and dentition to P. fergosi (Ezcurra 2014). Recovered were skeletal parts and a partial skull, which has provided a partial cranial endocast, the first for Proterosuchidae. This revealed structures related to the olfactory bulb, the brain section associated with the nasal passages and the sense of smell.

 Triassic synapsids and archosaurs in northern China    

            China has three zones of exposed Triassic formations containing vertebrate fossils (fig.5).  One of these regions is in the south, located around the boundary of Yunnan and Gouzhou provinces. These are deposits of ancient marine or estuarine formations with an abundance of Triassic marine reptiles, fish, and invertebrates, but few terrestial animals (Benton et al 2013).

            The other two regions are in the north, in the Ordos and Jungghar Basins. These contain a variety of fossils of synapsids and reptiles, as well as bony fish, with a much scarcer representation of labyrinthodont amphibians. In the northeast, the provinces of Shanxi and Shaanxi straddle the Yellow River, along whose valley many fossil outcrops are exposed (fig.5). In northwestern China, significant Triassic exposures occur in the Jungghar basin in Xinjiang province (Young 1946; Sun 1980). Together, these two regions contain virtually all of the known land vertebrates or tetrapod fossils from the Triassic. Lucas (1993) and Sun (1980) summarize the Triassic tetrapod fauna in northern China, while Benton et al. (2013) review findings from southern China.

Fig.5: Locations of Chinese provinces with Triassic outcrops.

            As late as 1991, no Late Triassic tetrapod fossils were known in China, although plant fossils definitely existed from that period across northern China (Lucas 1993).  This apparent gap in the Triassic vertebrate fossil record was resolved in 1992, when the  medium sized archosaur Yonghesuchus sangiensis was found in Upper Triassic strata in Shanxi province (Shao-Chun et al. 2001), as discussed below.

           The earliest Mesozoic land vertebrate fauna in China are in western Xinjiang, in the Early Triassic Guodikeng Formation. The faunal assemblage there at the Permian-Triassic boundary is called the Jimsarian fauna (Yang et al. 1992; Spencer 1993). This is equivalent to the Lystrosaurus fauna from South Africa, as discussed by Sun (1980). Synapsids in the Jimsarian fauna include two dicynodonts,  Lystrosaurus, and the contemporary Dicynodon (called Jimsusaria) ; and the therocephalian ("beast-headed") carnivore Urumchia (Young 1952).  Therocephalians are advanced synapsids who survived from the Late Permian into the Early Triassic (ca. 265-245 mya). These were numerous in South Africa, and spread to Antarctica as well as China, Russia, and Eastern Europe. Reptiles in the Jimsarian fauna include the procolophon Santisaurus (Koh 1940); the proterosuchus Chasmatosaurus (Young 1937); and the protosaur Prolacertoides (Young 1973). All are typical Early Triassic fauna.  

           The equivalent  Early Triassic faunal assemblage from Shanxi and Shaanxi provinces (fig.6) is called the Fugguan fauna, named for the town of Fugu which lies amid the Heshangou Formation (Lucas 1993; Sun 1980). Present are various reptilian taxa, including procolophonids (two species of Eumatabolodon); the protosucherian Xilosuchus, and Fugusuchus, a large Erythrosuchid ("red crocodile"), the largest archosaur predators in the Cygnognathus zone in South Africa. Also from the Fugguan fauna were a few unidentfied remains of labyrinthodont amphibians; and the lungfish Ceratodus ("horned tooth"), similar to examples found in the Bashunchank Formation in Russia

 Fig.6: Late Permian and Triassic vertebrate sites in Shanxi and Shaanxi provinces (after Lucas 1993, fig.3).       

            Synapsid remains found in Fugguan fauna in Shanxi included the large therocephalian Hazhenia, and smaller therocephalians named Scapulosaurus, also typical of  the earliest Triassic of South Africa (Sun 1980; Huttenlocker 2014). Table 2 gives a brief comparison of Early Triassic animal types found in both Xinjiang and Shanxi/Shaanxi.

Table 2: Comparative Early Triassic fauna in northern China
Region:
Xinjiang Shanxi/Shaanxi
Period: Early Triassic Early Triassic
Formation: Guodikeng Heshangou
Faunal assemblage: Jiimsarian Fugguan
Class: Order: Group: Genus: Genus:
Synapsid Therapsid Dicynodont
Lystrosaurus
Synapsid Therapsid Dicynodont Jimsusaria
Synapsid Therapsid Therocephalian Urumchia Hazhenia
Synapsid Therapsid Therocephalian Scapulosaurus
Reptile Anapsid Procolophon Santisaurus Eumatabolodon
Reptile Diapsid Protosaurian Prolacertoides
Reptile Diapsid Proterosuchian Chasmatosaurus Xilosuchus
Reptile Diapsid Erythrosuchid Fugusuchus
Amphibian Labyrinthodont Capitosaurid (unidentified) (unidentified)
Amphibian Labyrinthodont Benthosuchidae (unidentified)
Fish: Ostyichthyes Sarcoptygerian Dipnoi (lungish) Cerotodus

            Conspicuous by its absence in the Shanxi/Shaanxi fauna is the small synapsid dicynodont Lystrosaurus, otherwise found in Xinjiang, and throughout most of Pangea in the Early Triassic. This discrepancy was reviewed by Sun (1980), who provides a useful summary of the distribution of Lystrosaurus-related fauna in Pangea.  Differences in the faunal assemblages between  Shanxi and Xinjiang fossil deposits may be linked to geological processes such as variations in micro-plate formation and the physical linkage of different regions of Pangea, something which has been documented for these provinces (Xiong and Coney 1985). These geological processes may have caused differing environments, as well as relative isolation of some areas

Fig.7: Skull of Parakannemeyeria: A) lateral view, showing position of tusks in maxilla; B) basal view, indicating stapes (after Sun 1960, figs. 2 and 4).

 Lower Ermaying Formation:  Fossil vertebrates from this component, called the Ordonian fauna by Lucas (1993),  consists predominantly of  large dicynodonts. Among these is Parakannemeyeria dolichocephala (fig.7), found at the top of the section , together with a variety of Kannemeyeria. The latter animal was widespread in Pangea, found in South Africa, Tanzania, Zambia, Argentina, India, China, and Russia, always in strata at the boundary of the Early and Middle Triassic (Lucas 1993). Kannemeyeria's diagnostic role in this time period (ca. 247-240 mya) is reflected in its use as type species for the Middle Triassic fauna in Xingiangi (table 2). Kannemeyeria is characterized by an long, narrow, and highly projected parietal crest, as well as tusks of larger dimensions in the related Parakannemeyeria (Sun 1960, 1980).
           
The cynodont Ordosiodon is also represented in the Lower Ermaying Fauna, which is equated to the the Cynognathus Zone of South Africa.
          
Reptiles from the Lower Ermaying included a moderate sized procolophonid similar to Neoprocolophon, a later form which is larger than Procolophon.  Several thecodonts were also recovered from this level, including Fenhosuchus, a primitive ornithosuchid with armor plates above the neural arches (Sun 1980; Lucas 1993)

Fig.8: Skull of Shansiodon wuhsiangensis, showing large tusks that comprise the two teeth of this dicynodont (after Yuh 1961).

 Upper Ermaying Formation:

            The Ningwuan Fauna from this formation, named after the city of Ningwuan on the Sanggan River in the Ordos Valley,  corresponds to the Sinokannemeyeria fauna originally defined by C.C. Young. Young discovered the dicynodont synapsid Sinokannemeyeria pearsoni, the first identification of a Triassic tetrapod in China (Young 1937).   Subsequent work in the Ordos basin took place in the 1950s and 60s under the Chinese Institute of Vertebrate Paleontology and Paleoanthropology (IVPP). This revealed an assemblage of vertebrates that was named the Sinokannemeyeria fauna, thought to correlate with  the South African Cynognathus Zone.      

          Dicynodonts, representing therapsids who were non-mammalian synapsids, are the most numerous taxa in these late Early Triassic levels, consisting of the three genera Kannemeyeria, Parakannemeyeria, and Shansiodon. The former two are large individuals, while the latter, represented by three different species, is small. One of the Shansiodon species, S. wuhsiangensis, shows the large twin tusks characteristic of the genus (fig.8) . Another, S. wagni (Yuh 1959), showed good preservation of the stapes, a hearing ossicle attached to the quadrate bone in the occipital region at the lower rear of the skull (fig.9).

Fig.9: The skull of Shansiodon wagni viewed from behind, showing the stapes, a small tube-shaped bone used as an ossicle in hearing (after Yuh 1959).       

             Two Cynodonts (a Therapsid group ancestral to mammals) were also recovered from the Upper Ermaying deposits. One was diagnosed as Sinognathus (fig.10), and another identified as Traversodontoides , a bauriidae therapsid found most frequently in the Middle Triassic of South America (Young 1959,  l974; Sun 1980; Lucas 1993.   

            Sinognathus  belongs to the family Cynognathidae and is regarded as similar to Cynognathus from South Africa. Sun (1980), however, notes that, while the South African cynodont, Cynognathus is a relatively large individual with a skull that closely resembles a dog, displaying long and narrow post canine teeth, Sinognathus  appears dissimilar. It has an more complex maxilla or upper jaw area, and more closely resembles Traversodon or Belesodon.

Fig.10: The skull of Sinognathus gracilis, showing its maxilla or upper jaw with a surmaxilla (after Young 1959 fig.3) 

            Fossils representing the primitive thecodont reptile, Shansisuchus are also abundant, as exemplified by massive fossiliferous block with the vast majority of elements belonging to this genus (Sun 1980). Other thecodont taxa include Fenhosuchus and Wangisuchus.  A single skull of the small, temporally diagnostic reptile Procolophonia is also represented in the Upper Ermaying fauna, in the form of  Neoprocolophon

 Kelemayi Formation in Xinjiang:

       The Middle Triassic Kelamayi Formation in Xinjiang contains the Kannemeyeria Fauna (Lucas 1993), which correlates with the entire upper section of the Ermaying Formation exposed along the Yellow River in Shanxi and Shanaxi provinces (Table 3). In more distant regions, according to Sun (1980), the Kelamayi Formation  may also be appropriately correlated to equivalent fauna in the Indian Yerrapalli and  Dongus System in Russia.  

Table 3: Comparative Early and Middle Triassic fauna in northern China
Region: Shanxi/Shaanxi Shanxi/Shaanxi Xinjiang
Period: Middle Triassic Late Early Triassic Middle Triassic
Formation: Upper Ermaying  Lower Ermaying Kelemayi
Faunal assemblage: Ningwuan Ordosian Kannemeyeria
Class: Order: Group: Genus: Genus:
Synapsid Therapsid Cynodont Sinognathus
Synapsid Therapsid Dicynodont
Parakannemeyeria (3 species) Parakannemeyeria Parakannemeyeria
Synapsid Therapsid Dicynodont Sinokannemayeria (2 species) Kannemayeria
Synapsid Therapsid Dicynodont Shansiodon (3 species)
Synapsid Therapsid Bauriid Traversodontoides
Synapsid Therapsid Therocephalian
Ordosiodon (3 species)
Synapsid Therapsid Therocephalian Yikezhaogia
Reptile Anapsid Procolophon Neoprocolophon
Paotaodon
Reptile Diapsid Proterosuchian
Guchengosuchus
Reptile Diapsid Euparkeriid Wangisuchus Halazhaisuchus, Turfanosuchus Turfanosuchus
Reptile Diapsid Erythysuchian Shansisuchus (2 species) Vjushkovia
Reptile Diapsid Ornithosuchid Fenhosuchus
Amphibian Labyrinthodont
(unidentified) (unidentified) (unidentified)
Fish: Osteichthyes
Semionotid
Sinosemionotus

  

A Late Triassic Archosaur from Shanxi Province

    In 1992 came the first record of Late Triassic tetrapod from China, when fossils of a medium-sized archosaur were found in the southeastern part of the Ordos Basin (Shao-Chun et al. 2001).  The fossils included two incomplete skulls (fig.11), with seven neck vertebra and several ribs.  After analyzing the fossils, Shao-Chun et al. (2001)  identified them as belonging to a new species of Archosaur, which they named Yonghesuchus sangiensis ("crocodilian from Yonghe [county]"). The frequently used "-suchus" designation in Triassic species often indicates crocodile-like dentition in diapsid reptiles who long predate crocodiles, but may be at least distantly ancestral to them.  Shao-Chun et al. (2001) describe Yonghesuchus as as the first Archosauriform (Diapsida) known from the Terrestial Upper Triassic of China. 

         The fossils, embedded in a sandstone block, were found on the north side of Sangbi Creek, about 1.5 km southwest of Sangbi Township, near Lishi in the southeast part of the Ordos Basin.  The find was within the Tongchuran Formation, which is higher stratigraphically in the Ordos Basin than the uppermost Ermaying Formation, which dates from the Middle Triassic (Lucas 1993; Sun 1980). 

Fig.11: Reconstructed view of the skull of Yonghesuchus sangiensis, with bones labelled (after Shao-Chun et al. 2001, fig.4).

            On the basis of stratigraphy, plus associated fossil fish taxa from the Late Triassic found in the same strata as Yonghesuchus, and some aspects of the more derived traits of Yonghesuchus compared to Middle Triassic archosaurs from China, Chao-Chun et al (2001) identified the fossils as dating from the earliest part of the Late Triassic period. This find thus represents the uppermost stratigraphic record of a tetrapod in the China Triassic geological formations. 

            Regarding its phylogeny, Y. sangiensis (Anapsida, Archosauromorpha, Archosauriformes) represents a new species of archosaur with some similarities to Turfanosuchus, found in the late Early Triassic, Ermaying formation in Shanxi province, and the Middle Triassic, Kelemayi Formation in Xianjing province (Table 2). Y. sangiensis, however,  is considerered more derived than both Turfanosuchus, and the early Late Triassic archosaur Proterochampsa.

     

 Triassic marine reptiles in southern China  

  

            A key zone of fossil-rich geologic formations spanning the Permian-Triassic boundary occurs  in the adjacent Yunnan and Ghozou provinces in southern China. This region contains the global standard sections for the Pemian-Triassic boundary, due to both excellect preservation and continuity of fossil assemblages.  The South China basin, measuring some 1500 by 3000 km in area, is the zone of rich fossil deposits in continuous sedimentary rock strata, serving to record a span  of over 70 million years from the  Middle Permian to the Late Triassic. 

            The South China Basin occupies a section of ancient lithosphere known as the South China Block. At the beginning of the Triassic, South China was an island in the Tethys Sea, located just southwest of  northern China, at the eastern end of Pangaea (fig.12).

            Chinese paleontologists including C.C. Young began intensive work in the Triassic deposits of South China in the 1930s to 70s. Within the past 30 years, several fossil-rich areas have been studied more intensively, with relatively precise chronology now obtained from the strata (Young 1946; Yin et al., 2001; Shen et al., 2011; Benton et al. 2013).

Fig.12: Location of South China in the Tethys sea in the Triassic (after Benton et al. 2013, fig.2A)

         Abundant fossil reptiles and fishes have now been excavated from the Triassic of Guizhou Province and neighbouring, eastern parts of Yunnan Province (fig.13). Two of these, the Panxian biota (Hao et al., 2006; Motani et al., 2008; Jiang et al., 2009),  and the Luoping biota, date from the Anisian stage of the early Middle Triassic (252-247 mya).  A third is the Guanling biota, dating from the from the Carnian stage of the Late Triassic, dated 235-228 mya (Wang et al., 2003; Jiang et al., 2005),

         Probably the most spectacular finds have come from the the Luoping biota of the early Middle Triassic. This represents an extensive array of both marine invertebrates and vertebrates. These shallow marine deposits show arthropods as their most abundant fossils, followed by fishes, marine reptiles, bivalves, gastropods, echinoderms, brachiopods, conodonts, foraminifers, and plants (Zhang et al., 2008; Hu et al., 2011).

Fig.13: Map of Triassic fossil deposits in southern China, showing Luoping, Panxian, and Guanling biotas (Benton et al. 2013, fig.2B)

         Nearly 20,000 individual macrofossils have so far been identified from the Luoping biota in its primary deposits (Benton et al 2013). Recent invertebrate finds include well-preserved belemnoids (cephalopods with bony parts, partly resembling small squids), hydromedusae, ammonoids, and lingulid brachiopods.   The dominant arthropods include crustaceans such as decapods (lobsters and shrimps), isopods, cycloids, and ostracods, tiny shrimp-like animals whose abundance and variation make them typically used as chronological markers (second only to conodonts in that regard). Other arthropods include millipedes and horseshoe crabs.  

            Reptiles from the Luoping biota include medium-sized (1-5 m -long) sea reptiles such as Mixosaurus, Dinocephalosaurus, and Sinosaurosphargis, plus unidentfied archosaurs.
 
          

Early Triassic vertebrate fossils in India 
        
         India contains regions with
a continuous succession of Early, Middle, and Late Triassic sediments, containing well preserved terrestial faunas/  In Eastern India, Early Triassic vertebrates, mainly tetrapods, have been identified from the Panchet Formation of the Raniganj coalfield of Damodar Valley. These include the following taxa (Bandyopadhyay 1999, pp.292-293):.

            Temnospondyl amphibians in the Panchet Formation include the trematosaurid  Gonioglyptus, a widespread taxa found from Antarctica to Russia. Very similar trematosaurids are in the Prionolobus beds of the Salt Range, of Dienarian age.  Four other temnospondyls are known. These include Indobrachiops, of the Indobrachyopid group, which is restricted to India and Australia Also found are Lydekkerina, the type genus of the widespread Lyderkkerinid group, found from Antarctica to Russia; Pachygonia of the Captitosaurids, another widespread group found from Antarctica to Russia; and Indobenthosuchus, which appears related to Lydekkerina.

              Synapsids from the Panchet Formation include the widespread dicynodont Lystrosaurus, representing tetrapods of lowland habitats living in fluvial and lacustrine settings (Bandyopadhyay 1999, p.294). Also found is the cynodont Thrinaxodon,  associated with the Lystrosaurus Zone in South Africa, representing the initial phase of the Early Triassic. Table 4 lists Early and Middle Triassic vertebrate fauna from three regions of India.

 

Table 4: India: Early and Middle Triassic fauna
Region: Damodar Valley Pranhita-Godovari Basin Satpura Basin
Period: Early Triassic Mid Triassic- Mid Anisian Mid Triassic - Early Anisian
Formation: Panchet Yerrapalli Denwai
Faunal:  Panchet
Class: Order: Group: Genus: Genus: Genus:
Synapsid Therapsid Cynodont Thrinaxodon
Synapsid Therapsid Dicynodont; kannameyeriid
Wadiasaurus
most common taxa
Synapsid Therapsid Dicynodont
Lystrosaurus

Synapsid Therapsid Dicynodont : stahleckeriid Rechnisaurus
Synapsid Therapsid Dicynodon unid. genus- small
Synapsid Therapsid Dicynodon unid. genus- large
Reptile Diapsid Proterosuchian Chasmatosaurus

Reptile Diapsid rhychosaur Mesodapedon iunid. genus-
Reptile Diapsid archosaur unid. genus- small
Reptile Diapsid Prolacertid unid. genus
Reptile Diapsid rauisuchid Mondasuchnus -like
Reptile Diapsid erythrosuchid unid. genus
Amphibian Temnospondyl trematosaurid Gonioglyptus
unid. genus
Amphibian Temnospondyl Indobrachyopid Indobrachiops
Amphibian Temnospondyl Lyderkkerinid Lydekkerina
Amphibian Temnospondyl Captitosaurid Pachygonia Parotosuchus Parotosuchus (2 species)
Amphibian Temnospondyl  Benthosuchid  Indobenthosuchus
Amphibian Temnospondyl from Mangli beds of wardha valley
represets the upper Kamthi formation}  Brachyopid
Brachiops laticeps
unid. genus
Bony fish Sauropterygian Dipnoi
Ceratodus
Bony fish Chondrostei
Actinopterygian Saurichthys

 

A herd of Triassic herbivores 

            Wadiasaurus indicus, a small Middle Triassic dicynodont herbivore about 50 cm long, has been identifed as a kannemeyeriid, so far the only example of this family of therapsid grazers known in India. The Wadiasaurus type specimen is a skull found in an Anisian (247-242 mya) fluvial claystone deposit in the Yerrapalli Formation of the Pranhita- Godavari valley, about 2 km east-south-east of the town of Yerrapalli. The genus is named for the Indian geologist Darashaw Nosherwan Wadia (1883-1969).

            More recent findings of a larger sample of Wadiasaurus fossils, in a locality near that of the original type skull, indicate that some modification of the skull characters given in previous descriptions is necessary. Based on additional information provided by the new samples on the lower jaw and postcranial skeleton, meanwhile, the familial status of W. indicusas as a Kannemeyerid has been confirmed (Bandyopadhyay 1988).  A comparative study of Wadiasaurus and other kannemeyeriid genera indicates that it might have been most closely related to Kannemeyeria erithrea In Wadiasaurus, the parietal crest at the top of the skull is quite specialized, forming two long rounded lobe-like bars separated by a bay. Sexual dimorphism of the species is indicated by maxillary flanges, which in males are triangular, thick and swollen, with stout cylindrical tusks, while similar maxillary flanges of females lack tusks. 

            Interesting behavioral traits have been revealed by a taphonomic study of the bone assemblage found in the fluvial claystones near Yarapalli. These findings show that a herd of Wadiasaurus, including some juveniles and young animals, was trapped in the soft muds of a floodplain and buried in a small area. The herd wa composed only of females, with some juvenile members. Taphonomic and osteological studies tend to indicate that the female individuals of Wadiasaurus lived in herds, whereas the solitary males joined the herds only during the mating seasons (Bandyopadhyay 1999).

Fig.14: Skull of Rechnisaurus

            A second dicynodont from the Pranhita-Gondavari valley is Reichnisaurus (fig.14), with similarities to a South American taxa named Dinodontosaurus, from the Stahlekriid family (Colbert 1984; Bandyopadhyay 1988; Jain 1996).

   

 Glossary

.

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