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.
[1][3][4] 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.
[1] 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[10] and eastern[11] and western[12] North America.
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.
[1] LVFs of the Triassic Period from youngest to oldest: (but see below) (but see below) Several paleontologists have independently questioned the validity of Lucas’s system, criticizing its inconsistent and often contradictory approach to taxonomy and faunal correlations.
[8][21] 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.
[8] 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.
The Norian age of the lower Chinle Formation has been independently confirmed by U-Pb dating and magnetostratigraphic correlations to global time scales.
[8][27][28] Conversely, other "Adamanian" strata, such as fossiliferous layers in the lower Ischigualasto Formation of Argentina, can be assigned to the late Carnian (~231 Ma).
[6][29] 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.
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.
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.
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.
These representatives were chosen based on their occurrence in the southwest United States, commonness, and relatively stable phylogenetic position despite paraphyly in some circumstances.
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.
Palynomorph assemblages overturn to more dry adapted species, and a higher concentration of pedogenic carbonate nodules may also support increasing aridity.
