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Triassic land vertebrate faunachrons
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 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 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.
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 (LMA) system already in use for Cenozoic 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 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, 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 and the FAD (estimated speciation) of Cynognathus. Some taxa which are index fossils for one stage may persist into a later stage.
LVFs of the Triassic Period from youngest to oldest:
Several paleontologists have independently questioned the validity of Lucas’s system, criticizing its inconsistent and often contradictory approach to taxonomy and faunal correlations.
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” (Machaeroprosopus), Redondasaurus, and Redondasuchus, among others. For the Berdyankian LVF, very few species are shared between the index assemblage (the Bukobay Formation of Russia) and other correlated assemblages. Direct relationships between Russian, German, and South American dicynodonts are conjectural and based on undiagnostic European fragments.
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Triassic land vertebrate faunachrons
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 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 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.
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 (LMA) system already in use for Cenozoic 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 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, 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 and the FAD (estimated speciation) of Cynognathus. Some taxa which are index fossils for one stage may persist into a later stage.
LVFs of the Triassic Period from youngest to oldest:
Several paleontologists have independently questioned the validity of Lucas’s system, criticizing its inconsistent and often contradictory approach to taxonomy and faunal correlations.
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” (Machaeroprosopus), Redondasaurus, and Redondasuchus, among others. For the Berdyankian LVF, very few species are shared between the index assemblage (the Bukobay Formation of Russia) and other correlated assemblages. Direct relationships between Russian, German, and South American dicynodonts are conjectural and based on undiagnostic European fragments.