alb21-3

A TETRAPOD-BASED TRIASSIC TIMESCALE

Spencer G. Lucas

Eight, temporally successive assemblage zones of tetrapod (amphibian and reptile) fossils provide the basis for dividing Triassic time into eight land-vertebrate faunachrons (LVF). The beginning of each LVF is defined by the first appearance datum (FAD) of a widespread tetrapod genus. These LVFs, the taxa whose FADs define their beginnings, and their approximate correlation to the standard global chronostratigraphic scale (SGCS) are (ascending order): 1. Lootsbergian, FAD Lystrosaurus = late Dorashamian-Induan; 2. Nonesian, FAD Cynognathus = Olenekian; 3. Perovkan, FAD Shansiodon = Anisian; 4. Berdyankian, FAD Mastodonsaurus = Ladinian-early Carnian; 5. Otischalkian, FAD Paleorhinus = late early-early late Carnian; 6. Adamanian, FAD Rutiodon = latest Carnian; 7. Revueltian, FAD Pseudopalatus = Norian; 8. Apachean, FAD Redondasaurus = Rhaetian. These Triassic LVFs provide a framework for the correlation of Triassic nonmarine deposits with a temporal resolution comparable to the seven Triassic Stages/Ages of the SGCS.

Introduction
Triassic biochronology is dominated by the ammonoid paleontologist and marine micropaleontologist. The fossils they study provide the basis upon which most of the Triassic SGCS has been built. However, ammonoids and marine microfossils are largely irrelevant to the correlation of the nonmarine strata deposited across the vast expanse of Triassic Pangaea. Tetrapod fossils are key to these correlations, and their organization into a global biochronology provides a time scale applicable to the nonmarine Triassic.

Fossil tetrapods are widespread (Fig. 1) and have long been used to correlate nonmarine Triassic strata (e.g., Ochev & Shishkin, 1989; Lucas, 1990, 1997, 1998). Lucas (1998) presented a comprehensive tetrapod biostratigraphy and biochronology for the Triassic strata of Pangaea by defining, or redefining, Triassic LVFs, which are eight time intervals identified by Triassic tetrapod fossils. Here, I present a concise summary of this work.
Previous vertebrate-based subdivisions of Triassic time
Although tetrapods have long been used to correlate nonmarine Triassic strata, relatively few efforts have been made to establish a formal tetrapod biostratigraphy or biochronology of the global Triassic (Fig. 2). Broom (1906, 1907, 1909) introduced the earliest, and perhaps most influential, Triassic tetrapod biostratigraphy for the Lower Triassic of the Karoo basin in South Africa. He identified three successive biostratigraphic intervals, the Lystrosaurus, Procolophonand Cynognathus "beds." Watson (1914a, b) later termed these "zones," and, since Kitching (1970), the Lystrosaurus and Procolophon zones have been combined into a single, Lystrosaurus zone. Identification of the Lystrosaurus and/or Cynognathus "beds" or "zones" has long been possible in Antarctica, South America, India, China and Russia because of the cosmopolitanism of Early Triassic tetrapods, especially the genera Lystrosaurus and Cynognathus.

Romer (1975; also see Cox, 1973) presented the first global Triassic tetrapod biochronology when he identified three successive types of land-vertebrate "faunas:" A (Early Triassic), B (Middle Triassic) and C (Late Triassic) (Fig. 2). Cosgriff (1984) divided Romer's division A into A1 (= Lystrosaurus biochron) and A2 (Cynognathus biochron). Ochev & Shishkin (1989; also see Anderson & Cruickshank, 1978) recognized the same intervals as Romer, but chose to name them the proterosuchian epoch (= A), kannemeyerioidean epoch (= B) and dinosaurian epoch (= C).

Cooper (1982) proposed a more detailed global tetrapod biostratigraphy of the Triassic than did Romer (Fig. 2). He recognized a succession of six Triassic zones based largely on a perceived stratigraphic succession of dicynodonts (see Lucas & Wild, 1995 for a revised dicynodont bio zonation; also, note that Cooper [1982] considered the Lystrosaurus zone to be Permian). However, Cooper's zonation has not been used by subsequent workers. The status of global Triassic tetrapod biostratigraphy and biochronology thus has not progressed much beyond that of Romer (1975), as a recently published synthesis (Ochev & Shishkin, 1989) indicates.

More detailed subdivision of Triassic time has been made in provincial biochronologies proposed for Argentina, North America and China. Bonaparte (1966, 1967, 1982) introduced a set of "provincial ages" for the Triassic of Argentina, but he never defined these terms (Fig. 2). (Lucas & Harris [1996] did define the Chanarian as a LVF). Lucas (1993) proposed a succession of LVFs for the Chinese Early-Middle Triassic tetrapod record. Lucas & Hunt (1993) proposed Late Triassic LVFs based on the Chinle Group tetrapod record from the western United States, and Huber et al. (1993; Lucas and Huber, 1998) proposed Middle-Late Triassic LVFs based on the Newark Supergroup record of eastern North America (Fig. 2).

Lucas & Huber (1998) reviewed global Late Triassic tetrapod biochronology and demonstrated the broad utility of the Chinle Group tetrapod biochronology proposed by Lucas & Hunt (1993; also see Lucas, 1997). The status of "provincial" tetrapod biochronology of the Triassic is that schemes exist for the Argentinian and Chinese record and for the Middle-Late Triassic record from North America. Although I advocate a global tetrapod biochronology, this does not obviate the need for some provincial biochronologies, whose utility as a secondary standard (Cope, 1996) is noted below.

Lootsbergian LVF
Lootsbergian time begins with the FAD of the dicynodont Lystrosaurus. The end of the Lootsbergian is equivalent to the beginning of the Nonesian, which is defined by the FAD of the cynodont Cynognathus. The Lootsbergian LVF is characterized by the Lystrosaurus Assemblage Zone in the Balfour (Palingkloof Member), Katberg and Burgersdorp (lower part) formations of the Karoo basin of South Africa (Groenewald and Kitching, 1995). The following tetrapod genera are restricted to Lootsbergian time and are widespread and/or common enough to be useful as index fossils: Wetlugasaurus, Tupilakosaurus, Luzocephalus, Lydekkerina, Procolophon, Lystrosaurus, Scaloposaurus, Thrinaxodon, Proterosuchus (= Chasmatosaurus) and Prolacerta.

The terms Lystrosaurus zone, beds or fauna have been applied to a wide geographic range of strata/fossils of Lootsbergian age. The most significant vertebrate fossil assemblages of Lootsbergian age are from the upper Guodikeng and lower Jiucaiyuan formations, Junggur basin, China; Heshanggou Formation, Ordos basin, China; lower part of Fremouw Formation, Antarctica; Panchet Formation, India; Vokhmian horizon of Vetluga Series, Russian Urals; and Wordy Creek Formation, eastern Greenland.

Direct cross-correlation of the Lootsbergian to part of the Induan is provided by the occurrence of characteristic Lootsbergian temnospondyls in ammonite-bearing Induan strata of the Wordy Creek Formation in eastern Greenland (Trümpy, 1961). Apparently, the beginning of the Lootsbergian does not correspond to the beginning of the Triassic. Indeed, the FAD of Lystrosaurus has long been assumed to equate to the beginning of the Triassic, but a close correlation has not been documented. Whether or not the end of the Lootsbergian correlates to the end of the Induan also is uncertain.

Nonesian LVF
Nonesian time begins with the FAD of the cynodont Cynognathus. The end of the Nonesian is the beginning of the Perovkan LVF, which is defined by the FAD of the dicynodont Shansiodon.The Nonesian LVF is characterized by the Cynognathus Assemblage Zone from the upper two- thirds of the Burgersdorp Formation in the Karoo basin of South Africa (Kitching, 1995). The following tetrapod taxa are restricted to Nonesian time and are widespread and/or common enough to be considered index fossils: Parotosuchus, Trematosuchus, Erythrosuchus, Cynognathus, Diademodon, Trirachodon, Kannemeyeria cristarhynchus.

Kitching (1977) reviewed the Cynognathus Assemblage Zone localities, and Kitching (1995) provided a recent synopsis of the stratigraphic ranges of the genera. Watson (1942) and Kitching (1977) subdivided the Cynognathus Assemblage Zone into two subzones. Hancox and Rubidge (1994), Hancox et al. (1995) and Shishkin et al. (1995a) divided the Cynognathus Assemblage Zone into three successive zones: (1) Kestrosaurus acme zone; (2) "Parotosuchus" africanus acme zone; and (3) advanced capitosauroid zone. Kitching (1995), however, made no attempt at subdivision, and no formal subdivisions are advocated here, pending publication of research in progress by P.J. Hancox and his co-workers.

Principal vertebrate fossil assemblages of Nonesian age come from the upper Fremouw Formation, Antarctica; Petropavlovsk Formation ("Yarenskiy horizon") in the Russian Urals; Wupatki and Torrey formations of the Moenkopi Group, Arizona-Utah, USA; Puesto Viejo andRio Mendoza formations, Argentina; lower part of Ermaying Formation, Ordos basin, China; Omingonde Formation, Namibia; lower N'tawere Formation, Zambia; K7 horizon of the Kingori Sandstone, Tanzania; and Sticky Keep Formation of Svalbard.

The occurrence of Parotosuchus in marine Olenekian strata of the Mangyshlak Peninsula in western Kazakstan (Lozovsky & Shishkin, 1974) is the most direct cross-correlation of the Nonesian to the SGCS. Aphanerama or Parotosuchus records in Svalbard, Germany and/or North America also support correlation of the Nonesian with at least part of the Olenekian.

Perovkan LVF
The beginning of the Perovkan is defined by the FAD of the dicynodont Shansiodon. The end of the Perovkan is the beginning of the Berdyankian, which is defined by the FAD of the temno spondyl Mastodonsaurus. The Perovkan LVF is characterized by the vertebrate fossil assemblage of the Donguz "svita" (= Formation) in the Russian Urals (Shishkin et al., 1995b). The following tetrapod taxa are common and/or widespread enough to be useful index taxa of the Perovkan: Eryosuchus, Eocyclotosaurus, Shansiodon, Scalenodon, Parakannemeyeria, Sinokannemeyeria, Kannemeyeria simocephalus. Principal vertebrate assemblages of Perovkan age are from the upper Ermaying Formation, Ordos Basin, China; lower Kelamayi Formation, Junggur basin, Xinjiang, China; Holbrook and Anton Chico members of the Moenkopi Formation, western USA; Upper Buntsandstein (Röt), Germany; Lower Zarzaitine Formation, Algeria; lower Manda Formation, Tanzania; and the Otter Sandstone of the United Kingdom.

Direct correlation can be made of the Perovkan to the SGCS because marine facies of the lower Ršt contain early Anisian conodonts, which justifies assigning the Perovkan age tetrapods from the Ršt an early Anisian age (Wild, 1980). Perovkan tetrapod assemblages are best known in Russia and China where they contain numerous dicynodonts. Correlatives are either dicynodont dominated (Manda, Omingonde) or amphibian dominated (Moenkopi, Upper Buntsandstein). It seems likely that no Perovkan vertebrate assemblage is younger than early Anisian. Although Perovkan time is the interval up to the beginning of the Berdyankian, the latter part of the Perovkan may lack vertebrate representation. This indicates the possibility that there is a need for another LVF between the Berdyankian and Perovkan, though I cannot define and characterize one at present because of inadequate data.
Berdyankian LVF
The beginning of the Berdyankian is defined by the FAD of the temnospondyl Mastodonsaurus, whereas the end of the Berdyankian is the beginning of the Otischalkian, which is defined by the FAD of the phytosaur Paleorhinus. The Berdyankian LVF is characterized by the vertebrate fossil assemblage of the Bukobay Formation ("svita") in the Russian Urals. The type Berdyankian vertebrate assemblage is directly superposed on the type Perovkan assemblage. The following tetrapod genera are common and/or widespread enough to be index fossils of the Berdyankian: Dinodontosaurus, Mastodonsaurus, Exaeretodon, Massetognathus, Stahleckeria. The vertebrate fauna of the Lettenkeuper in Germany and the Chanarian LVF localities in Argentina and Brazil are the principal correlatives of the Berdyankian type assemblage.
Berdyankian is correlated to the Ladinian based mostly on the age of the Lettenkeuper. The Chanarian tetrapods from Argentina are older than late Carnian, which is the age of the overlying tetrapod assemblage from the Ischigualasto Formation. The Chanarian has long been considered Ladinian (e.g., Bonaparte, 1966), but without a strong basis.

Berdyankian is difficult to correlate globally, mostly because of a paucity of tetrapod assem blages of this age. Two clusters of localities (European and South American) are equated, largely on the basis of the Lettenkeuper dicynodont and the conclusion that "Elephantosaurus" is a stahleckeriid, possibly a synonym of Stahleckeria (Lucas and Wild, 1995). The South American Chanarian LVF thus is the provincial secondary standard correlative to the Berdyankian. Like the Perovkan, the Berdyankian is relatively long, at least as long as the Ladinian and part of the early Carnian. Nevertheless, Berdyankian vertebrate fossil assemblages probably only represent the earlier part of this time interval. Therefore, the potential exists to recognize another LVF between the Berdyankian and Otischalkian, although data to do this are insufficient at present.
Otischalkian LVF

The beginning of the Otischalkian is the FAD of the phytosaur Paleorhinus. The end of the Otischalkian is the beginning of the Adamanian, which is defined by the FAD of the phytosaur Rutiodon. The vertebrate fossil assemblage of the Colorado City Member of the Dockum Formation, Chinle Group near the defunct town of Otis Chalk, Howard County, Texas, USA characterizes the Otischalkian.

The following tetrapod genera are restricted to Otischalkian time and are widespread and/or common enough to be useful as index fossils: Paleorhinus, Angistorhinus, Longosuchus (= Lucasuchus), Metoposaurus, Doswellia. Besides Chinle Group correlatives, principal Otischalkian- age vertebrate assemblages are from the Sanfordian interval of the Newark Supergroup basins of eastern North America; Schilfsandstein, Kieselsandstein, Lehrbergschichten and Blasensandstein of the German Keuper; Irohalene Member (T4) of the Timesgadiouine Formation, Argana Group, Morocco; and basal part of Maleri Formation, Pranhita-Godavari Valley, India. The Otischalkian is of Carnian (late Julian-early Tuvalian) age on the SGCS based on Paleorhinus and Metoposaurusrecords in marine strata in Austria (Hunt and Lucas, 1991), palynostratigraphy (Litwin et al., 1991, 1993; Cornet, 1993) and magnetostratigraphy (Kent et al., 1995; Molina-Garza et al., 1996).

Adamanian LVF

The beginning of the Adamanian is defined as the FAD of the phytosaur Rutiodon. The end of the Adamanian is the beginning of the Revueltian, which is defined by the FAD of the phytosaur Pseudopalatus. The Adamanian LVF is characterized by the vertebrate fauna of the Blue Mesa Member of the Petrified Forest Formation in the Petrified Forest National Park, Arizona, USA. The following tetrapod taxa are restricted to Adamanian time and are widespread and/or common enough to be useful as index fossils: Scaphonyx, Stagonolepis and Rutiodon-grade phytosaurs, including Leptosuchus and Smilosuchus.

Besides the Chinle Group correlatives, major Adamanian-age vertebrate faunas are those of the Conewagian interval of the Newark Supergroup basins of eastern North America; Lossiemouth Sandstone Formation, Scotland; Ischigualasto Formation, Argentina; and upper Santa Maria Formation, Brazil.

The Adamanian is of latest Carnian age based mostly on palynostratigraphy and magnetostratigraphy (see references cited above under marine cross-correlation of the Otischalkian). In West Texas, Otischalkian and Adamanian tetrapod assemblages are stratigraphically superposed. Therefore, Adamanian time is a younger portion of the Tuvalian than the Otischalkian. Norian (Revueltian LVF) vertebrates are stratigraphically above Adamanian vertebrates in Arizona, New Mexico and Texas. Therefore, Adamanian vertebrates are the youngest Carnian vertebrates known. Like the Otischalkian, the Adamanian is relatively short, easily recognized over a broad area and relatively precisely correlated to the SGCS.

Revueltian LVF

Revueltian time begins with the FAD of the phytosaur Pseudopalatus. The end of the Revueltian is the beginning of the Apachean, which is defined by the FAD of the phytosaur Rendondasaurus The Revueltian LVF is characterized by the vertebrate fossil assemblage of the Bull Canyon Formation in east-central, New Mexico, USA. I term this the Pseudopalatus Assemblage Zone.The following tetrapod taxa are restricted to Revueltian time and are widespread and/or common enough to be useful as index fossils: Typothorax, Aetosaurus and Pseudopalatus-grade phytosaurs, including Nicrosaurus and Mystriosuchus.

Besides Chinle Group correlatives, the principal Revueltian-age tetrapod assemblages are those of the Newark Supergroup of eastern North America of Neshanician and Cliftonian (part) age; Ørsted Dal Member of the Fleming Fjord Formation, Greenland; Lower and Middle Stuben sandstein of the German Keuper; Zorzino Limestone and Forni Dolomite, northern Italy; and lower part of Dharmaran Formation, India.
Palynostratigraphy, magnetostratigraphy and sequence stratigraphy suggests the type Revueltian assemblage is of Norian age (Lucas, 1997). Based on stratigraphic position, magneto stratigraphy, and palynomorphs, the Neshanician LVF is of early to middle Norian age. Strati graphic position, magnetostratigraphy, and palynomorphs indicate the Cliftonian LVF is of late Norian-Rhaetian age. The Italian records of Aetosaurus provide direct evidence that at least part of the Revueltian = middle Norian (Alaunian). Revueltian correlates approximately with the entire Norian, which is consistent with the evidence cited above (Lucas, 1997). However, whether or not the beginning and end of the Revueltian and Norian are exact equivalents is unclear.

By any recent Triassic numerical timescale (e.g., Harland et al., 1990; Gradstein et al., 1995; Kent et al., 1995; Gradstein and Ogg, 1996), the duration of the Norian is at least 10 million years. This means the Revueltian is one of the longest Triassic LVFs recognized here. It merits subdivision, as Hunt and Lucas (1993) suggested, perhaps along the lines of the Cliftonian- Neshanician subdivision used in the Newark Supergroup, but no subdivision is attempted here.

Apachean LVF

Apachean time begins with the FAD of the phytosaur Redondasaurus. The end of Apachean time is the FAD of the crocodylomorph Protosuchus. The Apachean LVF is characterized by the vertebrate fossil assemblage of the Redonda Formation (Chinle Group) in east-central New Mexico, USA. The following tetrapod genera are restricted to Apachean time and are widespread and/or common enough to be useful as index fossils: Redondasaurus, Redondasuchus, Riojasaurus.

Principal vertebrate fossil assemblages of Apachean age are the Whitaker quarry in the Rock Point Formation of the Chinle Group at Ghost Ranch, New Mexico, the Cliftonian LVF assem blages, in part, of the Newark Supergroup and that of the Coloradan LVF of Argentina. Some of the fissure fill assemblages in the uppermost Mercia Mudstone Group and/or lowermost Penarth Group of the United Kingdom (Benton & Spencer, 1995) may be Apachean correlatives. However, their ages are problematic, in part, because they lack identifiable phytosaurs, aetosaurs or metoposaurs. Some of the so-called Rhaetian vertebrate sites in France, such as Saint-Nicolas-de-Port, may be Apachean correlatives as well (Lucas & Huber, 1998).

Correlation of the Apachean to the SGCS must be based on indirect lines of evidence. Apachean time is post-Revueltian (~ Norian) and pre-Jurassic, so I tentatively correlate it to the Rhaetian. However, whether or not it is in part of late Norian age is uncertain. Magnetostratigraphy of the uppermost Chinle Group in eastern New Mexico (Molina et al., 1996), correlated to the Newark Supergroup magnetostratigraphy (Kent et al., 1995), also suggests the Apachean is latest Triassic ("Norian-Rhaetian").

The Apachean is the most difficult Triassic LVF to correlate globally. This almost certainly reflects a provincialization of the global tetrapod fauna. Some of the apparent endemism of Apachean land vertebrate assemblages may also be due to facies, sampling and taphonomic biases. Rather than recognize a global Apachean LVF, it may be necessary to recognize two or more provincial LVFs during this time interval.
There is no evidence that the Apachean is in part of Jurassic age. The FAD of the crocodylo morph Protosuchus appears to correspond closely to the beginning of the Jurassic. Protosuchus occurs in the McCoy Brook Formation (Newark Supergroup), the upper Stormberg Group of South Africa and the Dinosaur Canyon Member of the Moenave Formation in Arizona (Colbert & Mook, 1951; Sues et al., 1996). The Moenave record of Protosuchus is stratigraphically super posed above Apachean-age body fossil assemblages of the uppermost Chinle Group (Lucas et al., 1997).

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