Land vertebrate faunachrons (LVFs) 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 Spencer George Lucas is an American paleontologist and stratigrapher, and curator of paleontology at the New Mexico Museum of Natural History and Science. His main areas of study are late Paleozoic, Mesozoic and early Cenozoic vertebrate fossils ...

in 1998, LVFs are primarily used within the Triassic Period (252 - 201 Ma), though Lucas later designated LVFs for other periods as well. Eight worldwide LVFs are defined for the Triassic. The first two of these LVFs, the Lootsbergian and Nonesian, are based on

South African __NOTOC__ South African may relate to: * The nation of South Africa * South African Airways * South African English * South African people * Languages of South Africa * Southern Africa Southern Africa is the southernmost subregion of the Afric ...

synapsids Synapsids + (, 'arch') > () "having a fused arch"; synonymous with ''theropsids'' (Greek, "beast-face") are one of the two major groups of animals that evolved from basal amniotes, the other being the Sauropsida, sauropsids, the group that inc ...

and faunal assemblage zones estimated to correspond to the Early Triassic. These are followed by the Perovkan and Berdyankian, based on

temnospondyl Temnospondyli (from Greek language, Greek τέμνειν, ''temnein'' 'to cut' and σπόνδυλος, ''spondylos'' 'vertebra') is a diverse order (biology), order of small to giant tetrapods—often considered Labyrinthodontia, primitive amphi ...

amphibians and Russian assemblages estimated to be from the Middle Triassic. The last four 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 Taxonomy is the practice and science of categorization or classification. A taxonomy (or taxonomical classification) is a scheme of classification, especially a hierarchical classification, in which things are organized into groups or types. ...

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

and his colleagues began to define tetrapod biostratigraphy intervals in the Triassic of

China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's most populous country, with a population exceeding 1.4 billion, slightly ahead of India. China spans the equivalent of five time zones and ...

and eastern and western

North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Car ...

. These named biostratigraphic intervals were inspired by the Land Mammal Age (LMA) system already in use for

Cenozoic The Cenozoic ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterised by the dominance of mammals, birds and flowering plants, a cooling and drying climate, and the current configura ...

faunal assemblages. Triassic tetrapod biozones, under the term "land vertebrate faunachrons" (LVFs) were formalized on a global level by Lucas in 1998. They were diagnosed by a primary index fossil (a particular genus of widespread time-constrained tetrapod) and characterized by a faunal

type Type may refer to: Science and technology Computing * Typing, producing text via a keyboard, typewriter, etc. * Data type, collection of values used for computations. * File type * TYPE (DOS command), a command to display contents of a file. * Ty ...

assemblage (distinguishing collection of taxa) 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 The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonifero ...

, 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 (and strata) between two paleobiological events: the first appearance datum (FAD) 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 (estimated speciation) of ''

Lystrosaurus ''Lystrosaurus'' (; 'shovel lizard'; proper Greek is λίστρον ''lístron'' ‘tool for leveling or smoothing, shovel, spade, hoe’) is an extinct genus of herbivorous dicynodont therapsids from the late Permian and Early Triassic epochs (a ...

'' and the FAD (estimated speciation) of '' Cynognathus''. Some taxa which are index fossils for one stage may persist into a later stage.

List of Triassic LVFs

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 ''Typothorax'' is an extinct genus of typothoracine aetosaur that lived in the Late Triassic. Its remains have been found in North America. Two species are known: ''T. coccinarum'', the type species, and ''T. antiquum''. Description ''Typothor ...

'', “'' Pseudopalatus''” (''

Machaeroprosopus ''Machaeroprosopus'' is an extinct genus of mystriosuchin leptosuchomorph phytosaur from the Late Triassic of the southwestern United States. ''M. validus'', once thought to be the type species of ''Machaeroprosopus'', was named in 1916 on the b ...

''), '' Redondasaurus'', and ''

Redondasuchus ''Redondasuchus'' is an extinct genus of aetosaur. It may be a junior synonym of '' Typothorax coccinarum'', another aetosaur. ''Redondasuchus'' is a member of the clade Typothoracisinae within the subfamily Aetosaurinae, and lived during the mi ...

'', among others. For the Berdyankian LVF, very few species are shared between the index assemblage (the

Bukobay Formation The Bukobay Svita (also anglicized as Bukobay or Bukobai Formation) is a Middle Triassic geological unit in Russia.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 ''Angistorhinus'' (meaning "narrow snout" or "hook snout") is an extinct genus of phytosaur known from the Late Triassic period of Texas and Wyoming, United States. It was first named by Mehl in 1913 and the type species is ''Angistorhinus grand ...

'', '' Hyperodapedon'', ''

Paleorhinus ''Paleorhinus'' (Greek: ''"Old Nose"'') is an extinct genus of widespread basal (phylogenetics), basal phytosaur known from the Late Triassic (late Carnian stage). The genus was named in 1904 based on the type species ''Paleorhinus bransoni'', wh ...

''/''

Parasuchus ''Parasuchus'' is an extinct genus of basal phytosaur known from the Late Triassic (late Carnian to early Norian stage) of Andhra Pradesh and Madhya Pradesh, India. At its most restricted definition, ''Parasuchus'' contains a single species, ' ...

'' (all Otischalkian index taxa) range into the Adamanian, fossils referred to ''

Rutiodon ''Rutiodon'' ("Wrinkle tooth") is an extinct genus of phytosaur belonging to the family Parasuchidae. It lived during the Late Triassic period, and was about in length. ''Rutiodon'' is known from the eastern United States (North Carolina, New J ...

'' (an Adamanian index taxon) range into the Revueltian, and ''

Metoposaurus ''Metoposaurus'' meaning "front lizard" is an extinct genus of stereospondyl temnospondyl amphibian, known from the Late Triassic of Germany, Italy, Poland, and Portugal. https://www.app.pan.pl/archive/published/app47/app47-535.pdf This mostly a ...

'' 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 (Carnian, Norian, and Revueltian) 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 (the Blue Mesa Member and equivalent units) is Carnian (>220 Ma) 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 (see below). 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 (<228 Ma). 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 The Ischigualasto Formation is a Late Triassic fossiliferous formation and Lagerstätte in the Ischigualasto-Villa Unión Basin of the southwestern La Rioja Province and northeastern San Juan Province in northwestern Argentina. The formation da ...

of Argentina, can be assigned to the late Carnian (~231 Ma). 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 (AZs) 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 (1966) established the Chanarian (named after the Chañares Formation) and the Ischigualastian (named after the Ischigualasto Formation). 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 The cynodonts () (clade Cynodontia) are a clade of eutheriodont therapsids that first appeared in the Late Permian (approximately 260 mya), and extensively diversified after the Permian–Triassic extinction event. Cynodonts had a wide variety ...

'' Exaeretodon,'' as well as the aetosaur '' Aetosauroides'' and

herrerasaurid Herrerasauridae is a family of carnivorous dinosaurs, possibly basal to either theropods or even all of saurischians, or even their own branching from dracohors, separate from dinosauria altogether. They are among the oldest known dinosaurs, fi ...

dinosaurs Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...

Taxonomic uncertainty and dubious correlations

Some correlations are based on connections between fragmentary or poorly-constrained taxa rather than direct correlations between type assemblages or LVF-defining index taxa. For example, the

Ermaying Formation The Ermaying Formation is a sedimentary succession of Anisian (Middle Triassic) age. It is found in the Shaanxi Province of China. It is composed of an up to 600 m thick sequence of mudstone and sandstone. It is famous for its fossils of tetrapods. ...

of China is correlated with the

Moenkopi Formation The Moenkopi Formation is a geological formation that is spread across the U.S. states of New Mexico, northern Arizona, Nevada, southeastern California, eastern Utah and western Colorado. This unit is considered to be a group in Arizona. Part ...

of the United States via a tenuous (and likely unjustifiable) comparison between proposed

erythrosuchid Erythrosuchidae (meaning "red crocodiles" in Greek) are a family of large basal archosauriform carnivores that lived from the later Early Triassic (Olenekian) to the early Middle Triassic (Anisian). Naming The family Erythrosuchidae was named b ...

fossils. The primary index fossil of the Perovkan LVF, ''

Eocyclotosaurus ''Eocyclotosaurus'' is an extinct genus of mastodonsauroid temnospondyl from the Middle Triassic (Anisian In the geologic timescale, the Anisian is the lower stage or earliest age of the Middle Triassic series or epoch and lasted from millio ...

'', is absent from China. One particularly contradictory index fossil is ''

Mastodonsaurus ''Mastodonsaurus'' (meaning "teat tooth lizard") is an extinct genus of temnospondyl amphibian from the Middle Triassic of Europe. It belongs to a Triassic group of temnospondyls called Capitosauria, characterized by their large body size and pre ...

'', the defining index fossil of the Berdyankian LVF. Fossils referable to this genus can be found across Ladinian-age Europe, but the proposed Russian species (''M. torvus'') may be unrelated to the endemic German type species (''M. giganteus''). Moreover, if one approaches ''Mastodonsaurus'' from a broader taxonomic perspective (as expected if ''M. torvus'' is included), they must also incorporate Anisian and Carnian material referred to the genus, including the small species '' “Heptasaurus” cappelensis''. This precludes any reason to use ''Mastodonsaurus'' as a time-constrained index taxon. Some LVFs are based on evolutionary grades as index taxa. This ignores the potential for high diversity and long temporal ranges within a given grade, and may lead to arbitrary and subjective inclusion or exclusion of descendant taxa. “'' Stagonolepis''”, in its broadest form, is a wastebasket taxon of

basal Basal or basilar is a term meaning ''base'', ''bottom'', or ''minimum''. Science * Basal (anatomy), an anatomical term of location for features associated with the base of an organism or structure * Basal (medicine), a minimal level that is nec ...

aetosaurs ranging through the Otischalkian and Apachean. Lucas’s usage of ''Stagonolepis'' lumps in many genera separated by other authors, such as '' Aetosauroides'' and '' Calyptosuchus''. A similar situation occurs in ''Paleorhinus/Parasuchus'', which has historically been used as a persistent grade of early phytosaurs. On the other hand, the characteristic phytosaur (''Redondasaurus'') and aetosaur (''Redondasaurus'') genera of the Apachean LVF are very similar to, and perhaps synonymous with, index taxa of the underlying Revueltian LVF: “''Pseudopalatus''” (''Machaeroprosopus'') and ''Typothorax'', respectively.

Martz & Parker (2017) revision

Although the utility of a global LVF system is questionable, LVF-derived biostratigraphy may be useful in limited circumstances. Phytosaurs in particular have played a large role in the tetrapod biostratigraphy of the Chinle and Dockum Group of the southwest United States. A revision of the LVF system in this narrow context was undertaken by Jeff Martz and Bill Parker (2017), retaining several names and concepts previously used by Lucas and colleagues. Martz and Parker argued that the term "faunachron" was misleading and redundant, as each "faunachron" is bound by a single taxon rather than an assemblage ( fauna) of multiple taxa. They preferred using a specific type of interval biozone known as a teilzone, referring to a local interval of strata equivalent to an interval of time. The base of each teilzone was marked by the Lowest known Occurrence (LOk) of a particular category of phytosaur, i.e. the oldest layer where fossils of that category are found in the study area. LOks are local points in time and stratigraphy, disregarding occurrences in other regions or the estimated time of

speciation Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution within ...

. For the Otischalkian, Adamanian, and Revueltian, the top of each teilzone is marked by the LOk of a more exclusive subgroup of phytosaurs. The top of the Apachean is marked by the LOk of ''

Protosuchus ''Protosuchus'' is an extinct genus of carnivorous crocodylomorph from the Early Jurassic. The name ''Protosuchus'' means "first crocodile", and is among the earliest animals that resemble crocodilians. ''Protosuchus'' was about in length and ...

'', an Early Jurassic

crocodylomorph Crocodylomorpha is a group of pseudosuchian archosaurs that includes the crocodilians and their extinct relatives. They were the only members of Pseudosuchia to survive the end-Triassic extinction. During Mesozoic and early Cenozoic times, cro ...

, as with Lucas's system. "Faunachrons" could also be defined beyond the constraints of teilzones; other biozonation categories include holochronozones (a stratigraphic interval, involving multiple study areas) and holochrons (an estimated time interval, involving the time of speciation or immigration into the region). Each phytosaur-based "faunachron" could be considered a teilzone (in local biostratigraphy), an estimated holochronozone (in regional chronostratigraphy), or an estimated holochron (in regional biochronology). One complication in defining biozones based on phytosaurs is instability in phytosaur systematics. Many proposed phytosaur taxa are dubious,

paraphyletic In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...

(such as ''

Leptosuchus ''Leptosuchus'' is an extinct genus of leptosuchomorph phytosaur with a complex taxonomical history. Fossils have been found from the Dockum Group and lower Chinle Formation outcropping in Texas, New Mexico, and Arizona, USA, and date back to th ...

'' and ''

)'' or have unclear relationships to each other. Nevertheless, a series of nested clades is apparent in most recent overviews. Rather than relying on a single index taxon per biozone, Martz and Parker allowed multiple representatives per a given stage of phytosaur evolution. These representatives were chosen based on their occurrence in the southwest United States, commonness, and relatively stable phylogenetic position despite paraphyly in some circumstances.

Adamanian-Revueltian turnover

Although most LVFs or equivalent concepts are not marked by major biotic changes, one exception is apparent in the southwest United States. The boundary between the Adamanian and Revueltian zones is marked by a faunal turnover, an event where several tetrapod species quickly disappear from the fossil record as others appear for the first time. At

Petrified Forest National Park Petrified Forest National Park is an American national park in Navajo County, Arizona, Navajo and Apache County, Arizona, Apache counties in northeastern Arizona. Named for its large deposits of petrified wood, the park covers about , encompassin ...

, the event occurs in the Jim Camp Wash beds. This sediment layer is positioned in the middle of the Chinle Formation's Sonsela Member, and would have been deposited around 215 million years ago. '' Trilophosaurus'', '' Poposaurus'', '' Desmatosuchus'', dicynodonts, and non-

mystriosuchin Mystriosuchini, historically known as Pseudopalatinae, is an extinct tribe (formerly subfamily) of derived phytosaurs in the clade Leptosuchomorpha. As with all other phytosaurs, mystriosuchins lived during Late Triassic. The name is derived from ...

phytosaurs are extirpated from the area around this time, while

metoposaurs Metoposauridae is an extinct family of trematosaurian temnospondyls. The family is known from the Triassic period. Most members are large, approximately long and could reach 3 m long.Brusatte, S. L., Butler R. J., Mateus O., & Steyer S. J. (201 ...

and

allokotosaurs Allokotosauria is a clade of early archosauromorph reptiles from the Middle Triassic, Middle to Late Triassic known from Asia, Africa, North America and Europe. Allokotosauria was first described and named when a new monophyletic grouping of spec ...

as a whole decline in abundance. New species of aetosaurs and phytosaurs replaced losses across the purported boundary event. Palynomorph assemblages overturn to more dry adapted species, and a higher concentration of

pedogenic Soil formation, also known as pedogenesis, is the process of soil genesis as regulated by the effects of place, environment, and history. Biogeochemical processes act to both create and destroy order (anisotropy) within soils. These alterations l ...

carbonate nodules may also support increasing aridity. The cause and relevance of this turnover is debatable, as it may indicate only a small localized extinction. The Manicouagan Impact, the second-largest bolide impact of the

Mesozoic Era The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last Era (geology), era of Earth's Geologic time scale, geological history, lasting from about , comprising ...

(besides the Chicxulub Impact which caused the K-Pg Mass Extinction at 66 Ma), is dated to around 215.4 Ma. While certainly large enough to momentarily devastate areas near the impact point in Quebec, broader environmental effects of the Manicouagan impact are mostly conjectural. Besides the Adamanian-Revueltian turnover, the impact has also been linked to a minor marine extinction in eastern Panthalassa. Alternatively, the Adamanian-Revueltian turnover may be a consequence of the gradual aridification of western Pangea as it drifted north into arid latitudes. Comparative estimates of extinction rates and occurrences find little support for a synchronized Adamanian-Revueltian turnover, and instead support a model where extinctions are stretched out over several million years. For most species, extinction probabilities are "decoupled" in time from other species, as well as geological or climatological drivers. The only plausible correlation is between the Manicouagan Impact and palynomorph turnover, and even then the probability of synchronicity is only about 34%.

References

{{reflist Biochronology Regional geologic time scales Triassic geochronology Triassic life