Triassic land vertebrate faunachrons

Land vertebrate faunachrons are biochronological units used to correlate and date terrestrial sediments and fossils based on their tetrapod faunas. First formulated on a global scale by Spencer G. Lucas in 1998, LVFs are primarily used within the Triassic Period, though Lucas later designated LVFs for other periods as well. Eight worldwide LVFs are defined for the Triassic. The first two earliest Triassic LVFs, the Lootsbergian and Nonesian, are based on South African synapsids and faunal assemblage zones estimated to correspond to the Early Triassic. These are followed by the Perovkan and Berdyankian, based on temnospondyl amphibians and Russian assemblages estimated to be from the Middle Triassic. The youngest four Triassic LVFs, the Otischalkian, Adamanian, Revueltian, and Apachean, are based on aetosaur and phytosaur reptiles common in the Late Triassic of the southwestern United States.
The LVF system, though widely used, is also a controversial application of biostratigraphy, as many Triassic tetrapods are rife with complications which endanger their utility as index fossils. Limited occurrences, inaccurate age estimates, overlapping LVF faunas, or taxonomic disagreement may jeopardize global correlations between Triassic tetrapods. This could render some LVFs as misleading assessments of Triassic faunal change through time. Regardless, Late Triassic phytosaurs are considered to have strong biostratigraphic utility even among detractors of Lucas's system.

Lucas's LVFs

Tetrapod biostratigraphy has been used for the Triassic of South Africa since 1906 and Argentina since 1966, but without much connection to global faunas. Starting in 1993, New Mexico Museum of Natural History and Science paleontologist Spencer G. Lucas and his colleagues began to define tetrapod biostratigraphy intervals in the Triassic of China and eastern and western North America. These named biostratigraphic intervals were inspired by the Land Mammal Age system already in use for Cenozoic faunal assemblages.
Triassic tetrapod biozones, under the term "land vertebrate faunachrons" were formalized on a global level by Lucas in 1998. They were diagnosed by a primary index fossil and characterized by a faunal type assemblage from a fossiliferous geological formation. Together, the defining index fossil and assemblage could be used to correlate fossil assemblages worldwide. Updates to this system have been published continuously for Triassic LVFs, which remain a heavily-discussed topic in the study of Triassic chronology. Lucas has also defined LVFs for the Permian, Jurassic, and Carboniferous, though these are not as widely used as his Triassic LVFs.
Later authors characterized Lucas's LVFs as "interval eubiochrons". This means that they correspond to a segment of time between two paleobiological events: the first appearance datum of one index taxon and the FAD of another. A first appearance datum is a point in the geological record with the earliest known fossil of a given animal, which can estimate when that animal speciates or evolves into existence. As an example, the Lootsbergian LVF is defined as the period of time between the FAD of Lystrosaurus and the FAD of Cynognathus. Some taxa which are index fossils for one stage may persist into a later stage.

List of Triassic LVFs

LVFs of the Triassic Period from youngest to oldest:

LVF name	Namesake	Primary index fossil	Other index fossils	Proposed age estimate	Type assemblage	Other correlated assemblages
Apachean	Apache Canyon, New Mexico, USA	Redondasaurus	Redondasuchus, Riojasaurus	late Norian to Rhaetian	Redonda Formation, New Mexico, USA	Rock Point Formation, Wingate Sandstone, upper "Cliftonian" strata of the Newark Supergroup, Trossingen Formation, upper Arnstadt Formation, Los Colorados Formation?, Quebrada del Barro Formation?, Lower Elliot Formation?, upper Mercia Mudstone Group?, lower Penarth Group?, Rhaetian fissure fills?
Revueltian	Revuelto Creek, New Mexico, USA	Typothorax coccinarum	Revueltosaurus, ''Aetosaurus, Rioarribasuchus, Pseudopalatus-grade phytosaurs, Eudimorphodon	early to middle Norian	Bull Canyon Formation, New Mexico, USA	Painted Desert Member / Petrified Forest Formation, Owl Rock Member, "Neshanician" and lower "Cliftonian" strata of the Newark Supergroup, Fleming Fjord Formation, Stubensandstein, Lisowice, Calcare di Zorzino, Dolomia di Forni, lower Dharmaran Formation, Los Colorados Formation?, Quebrada del Barro Formation?, Lower Elliot Formation?
Adamanian	Adamana, Arizona, USA	Rutiodon-grade phytosaurs	Stagonolepis, Spinosuchus, Colognathus, Tecovasaurus, Crosbysaurus	late Carnian	Blue Mesa Member, Chinle Formation, Arizona, USA	Bluewater Creek Formation, Santa Rosa Formation, Garita Creek Formation, Tecovas Formation, "Conewagian" strata of the Newark Supergroup, Lossiemouth Sandstone, Krasiejów, Lehrberg Schichten / Blasensandstein / Kieselsandstein, DeGeerdalen Formation, Ischigualasto Formation, upper Santa Maria Formation, Caturrita Formation, upper Maleri Formation, Isalo II
Otischalkian	Otis Chalk, Texas, USA	Paleorhinus / Parasuchus	Metoposaurus, Placerias, Hyperodapedon, Doswellia, Angistorhinus, Longosuchus	middle Carnian	Colorado City Formation, Texas, USA	Popo Agie Formation, Salitral Formation, "Sanfordian" strata of the Newark Supergroup, Stuttgart Formation, Timezgadiouine Formation, lower Maleri Formation, Tiki Formation, Madygen Formation?
Berdyankian	Berdyanka River, Russia	Mastodonsaurus giganteus	Massetognathus, Dinodontosaurus, Stahleckeria	late Anisian to early Carnian	Bukobay Formation, Russia	Lower Keuper, Chañares Formation, lower Santa Maria Formation, upper Omingonde Formation
Perovkan	Perovka, Russia	Eocyclotosaurus	Eryosuchus, Paracyclotosaurus, Scalenodon, Shansiodon, Parakannemeyeria, Sinokannemeyeria, "Kannemeyeria cristarhynchus", Arizonasaurus	Anisian	Donguz Formation, Russia	Moenkopi Formation, lower Wolfville Formation, Otter Sandstone, Upper Buntsandstein, lower Kelamayi Formation, upper Ermaying Formation, Yerrapalli Formation, Cynognathus Assemblage Zone, Omingonde Formation, Manda Beds
Nonesian	Nonesi's Nek Pass, South Africa	Cynognathus	Parotosuchus, Odenwaldia, Trematosaurus, Trematosuchus, Diademodon, Trirachodon, Kannemeyeria simocephalus, Erythrosuchus	Olenekian	Cynognathus Assemblage Zone, South Africa	Moenkopi Formation, Sticky Keep Formation, Middle Buntsandstein, Yarenskian Gorizont, lower Ermaying Formation, Puesto Viejo Group, Rio Mendoza Formation, lower Zarzaïtine Formation, lower Ntawere Formation, Kingori Sandstone, upper Fremouw Formation
Lootsbergian	Lootsberg Pass, South Africa	Lystrosaurus	Wetlugasaurus, Tupilakosaurus, Luzocephalus, Lydekkerina, Scaloposaurus, Thrinaxodon, Procolophon, Prolacerta, Proterosuchus	latest Permian to Induan	Lystrosaurus'' Assemblage Zone, South Africa	upper Guodikeng Formation, lower Jiucaiyuan Formation, Heshanggou Formation, lower Fremouw Formation, Panchet Formation, Wordie Creek Formation, Vokhmian Gorizont, Sludkian Gorizont, Ustmylian Gorizont, Sanga do Cabral Formation, Rewan Formation, Arcadia Formation

Criticism

Several paleontologists have independently questioned the validity of Lucas’s system, criticizing its inconsistent and often contradictory approach to taxonomy and faunal correlations.

Endemic index taxa

Many index taxa are very rare or endemic to a single continent, and have no relevance to a global biostratigraphy system. These include Doswellia, Longosuchus, Typothorax, “Pseudopalatus”, Redondasaurus, and Redondasuchus, among others. For the Berdyankian LVF, very few species are shared between the index assemblage and other correlated assemblages. Direct relationships between Russian, German, and South American dicynodonts are conjectural and based on undiagnostic European fragments.

Imprecise or inaccurate time scales

Index taxa for a given LVF often range into the succeeding LVF, blurring the distinction between the two time periods. Angistorhinus, Hyperodapedon, Paleorhinus/''Parasuchus range into the Adamanian, fossils referred to Rutiodon range into the Revueltian, and Metoposaurus can be found throughout the Otischalkian, Apachian, and Revueltian LVFs.
Lucas's approach to correlating LVFs with global marine stages has been met with criticism. The Triassic timescale is under constant revision from a series of age dating methods, including magnetostratigraphy, cyclostratigraphy, radiometric dating, and biozones of marine invertebrates such as conodonts and ammonoids. However, there are only a few areas where fossils of Triassic land tetrapods and marine organisms overlap, mostly restricted to coastal sediments in central Europe. Palynomorph and conchostracan biozones can help correlate terrestrial strata to an extent. One complication is that Lucas's view of the Late Triassic time scale contradicts the consensus established by other biostratigraphers. Most paleontologists estimate that the three stages of the Late Triassic are strongly unequal in size, with the Norian far longer than the Carnian. Under this consensus "long-Norian" hypothesis, the Carnian-Norian boundary is close to 228 Ma. Lucas, on the other hand, prefers a "short-Norian" perspective, with a lengthier Carnian stage and a Carnian-Norian boundary at around 220 Ma.
For example, Lucas has maintained that the lower part of the Chinle Formation is Carnian in age. This was justified by the assumption that fossils of Stagonolepis, a European aetosaur, can be found in North and South America, allowing correlation between these regions. However, this proposed widespread occurrence of Stagonolepis is a debatable, as many species assigned to the genus may not be closely related.
According to the "short-Norian" interpretation, these lower Chinle Formation, and other strata of the Adamanian LVF, would be firmly pre-Norian in age, suggesting that any taxonomic change between the Adamanian and Revueltian represents a Carnian-Norian extinction event. However, the consensus "long-Norian" interpretation firmly places Adamanian strata of North America into the Norian stage. The Norian age of the lower Chinle Formation has been independently confirmed by U-Pb dating and magnetostratigraphic correlations to global time scales. Conversely, other "Adamanian" strata, such as fossiliferous layers in the lower Ischigualasto Formation of Argentina, can be assigned to the late Carnian. This supports the conclusion that LVFs such as the Adamanian are fraught with uncertain time estimates brought on by weak correlations on a global scale.
Some authors have elected to ignore LVFs in favor of older and more localized biostratigraphic units. Named tetrapod assemblages zones were well-established for the Triassic of Gondwana prior to the LVF, and recent updates have helped to constrain these units with greater clarity and agreement than global correlations. In Argentina, Bonaparte established the Chanarian and the Ischigualastian. Equivalents faunas are easily traced across Brazil, Africa, and India. These two biostratigraphic zones correlate with Lucas's Berdyankian, Otischalkian, and Apachean LVFs, but do not precisely overlap in time with those LVFs. Moreover, aetosaurs and phytosaurs, which are common in the Northern Hemisphere, are rarer and more scattered in the Southern Hemisphere. As a result, Gondwanan assemblage zones are defined by more common Southern taxa. For example, the Ischigualastian zone is defined by the rhynchosaur Hyperodapedon and the cynodont Exaeretodon, as well as the aetosaur Aetosauroides'' and herrerasaurid dinosaurs.