Triassic land vertebrate faunachrons

Tetrapod biostratigraphy has been used for the Triassic of South Africa since 1906 and Argentina since 1966, and eastern and western 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. List of Triassic LVFs LVFs of the Triassic Period from youngest to oldest: == Criticism ==

Several paleontologists have independently questioned the validity of Lucas’s system, criticizing its inconsistent and often contradictory approach to taxonomy and faunal correlations. 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. Conversely, other "Adamanian" strata, such as fossiliferous layers in the lower Ischigualasto Formation 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 Exaeretodon, as well as the aetosaur Aetosauroides and herrerasaurid dinosaurs. Taxonomic uncertainty and dubious correlations '', a purported index fossil of the Berdyankian LVF 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 of China is correlated with the Moenkopi Formation of the United States via a tenuous (and likely unjustifiable) comparison between proposed erythrosuchid fossils. The primary index fossil of the Perovkan LVF, Eocyclotosaurus, is absent from China. One particularly contradictory index fossil is Mastodonsaurus, 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 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. 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. 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, an Early Jurassic crocodylomorph, as with Lucas's system. 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 (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. Alternatively, the Adamanian-Revueltian turnover may be a consequence of the gradual aridification of western Pangea as it drifted north into arid latitudes. == References ==