| Nyasasaurus | |
|---|---|
 | |
| Approximation of animal based on partial skeleton shown in black (first specimen, six vertebrae and a humerus) and blue (second specimen, three cervical vertebrae) | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Archosauria |
| Clade: | Avemetatarsalia |
| Clade: | Ornithodira |
| Clade: | Dinosauromorpha (?) |
| Genus: | †Nyasasaurus Nesbittet al.,2013 |
| Type species | |
| †Nyasasaurus parringtoni Nesbittet al., 2013 | |
| Synonyms | |
| |
Nyasasaurus (meaning "Lake Nyasa lizard") is anextinctgenus ofavemetatarsalianarchosaur from the putativelyMiddle TriassicManda Formation ofTanzania that may be the earliest knowndinosaur. Thetype speciesNyasasaurus parringtoni was first described in 1956 in the doctoral thesis of English paleontologistAlan J. Charig, but it was not formally described until 2013.
Previously, the oldest record of dinosaurs was fromBrazil andArgentina and dated back to the mid-lateCarnian stage, about 233.23 to 231.4 million years ago.Nyasasaurus comes from a deposit conventionally consideredAnisian in age, meaning that it would predate other early dinosaurs by about 12 million years.[1] Some studies cast doubt on this age,[2] suggesting that the deposits may actually beCarnian in age,[3][4] which would considerably reduce this temporal gap. However, this claim has been heavily disputed, and more accurate dating methods are needed to resolve the debate.
In the 1930s, the holotype ofNyasasaurus was collected in Parrington's locality B36 from the Lifua Member of the Manda Formation, Ruhuhu Basin nearLake Nyasa in southern Tanzania byFrancis Rex Parrington. Other fossils from the same locality included those ofcynodonts,dicynodonts, andrhynchosaurs. Most, including those ofNyasasaurus, consist only of fragments of bone. The remains were first described in English paleontologist Alan J. Charig's 1956 doctoral thesis and referred to as "Specimen 50b".[5] In 1967 Charig used the name "Nyasasaurus cromptoni", in a review of Archosauria, but without any description, so it was commonly considered anomen nudum; the dissertation was also never published.[6] The generic name referred to Lake Nyasa and thespecific name honouring Alfred Crompton. In 2013 a new description was published by Sterling Nesbitt, Paul Barrett, Sarah Werning and Christian Sidor, including the late Charig as posthumous co-author, ensuring the validity of the nameNyasasaurus, though the specific name was changed toparringtoni, in honour of Parrington.[1] The generic name is occasionally misspelled as "Nyasaurus",[1] as by Theodore Elmer White in 1973.[7]
The referred specimen ofNyasasaurus, SAM-PK-K10654, was collected by G. M. Stockley in the early 1930s in the western portion of the Manda Formation at Stockley's locality B27.[8] This locality is listed as a locality from the "Upper Bone Bed" of the Manda Formation (currently understood to be from the Lifua Member) by Haughton (1932). The specimen was collected under a single field number, S507, presumably from a small area. The specimen was probably associated as evidenced by the bone quality, color and surrounding matrix (dark gray to blackcarbonate). The consistent sizes of the remains indicate that they probably represent a single individual.
Stockley's locality B27 is located near the village of Gingama and it was probably the only specimen found at this locality, although a nearby locality B26, also listed as Gingama, producedcynodonts,lungfish,amphibians, and ashark.Dicynodonts, cynodonts and archosaurs such asAsilisaurus were also found nearby in the Lifua Member.[1]
The nameThecodontosaurus alophos was coined for this specimen by Haughton (1932).[9] Its holotype consists of three cervical vertebrae and two middle to posterior dorsal vertebrae that are poorly preserved as they are highly fractured and parts of the bone and bone surfaces are eroded. Originally, a comparison ofThecodontosaurus alophos was made only withCoelophysis longicollis. Since then, the species has been largely ignored by all subsequent vertebrate workers and no formal diagnosis of the specimen was ever provided. Nesbittet al. (2013) found the specimen to not be diagnostic because it does not have anyautapomorphic features or a unique combination of characteristics. Therefore, they suggested to abandon the nameThecodontosaurus alophos and to refer its specimen toNyasasaurusparringtoni.[1]
Thetype specimen,NHMUK PV R 6856, is a partial skeleton belonging to an individual estimated to have been two to three metres in length. It consists of a righthumerus, three partialsacral vertebrae and three presacral vertebrae. A second specimen, SAM-PK-K10654 consisting of threecervical vertebrae and two posterior presacral vertebrae, is also known. It was attributed to the same species as NHMUK R6856 because the dorsal or back vertebrae of the two specimens are nearly identical. However, the vertebral features that link NHMUK PV R 6856 and SAM-PK-K10654, including a connection between two bony projections called thehyposphene and hypantrum, are also found in other Triassicarchosaurs. Since these characteristics are not unique to the two species they do not by themselves provide sufficient evidence for grouping NHMUK PV R 6856 and SAM-PK-K10654 under the same species. The 2013 description ofNyasasaurus bySterling Nesbitt, Paul Barrett, Sarah Werning andChristian Sidor used a second line of evidence, the similar positions of the two specimens on the evolutionary tree, to justify their placement as the same species.[1]
The study also mentioned the similarity between the presacral vertebrae of both specimens ofNyasasaurus parringtoni and those of the enigmaticavemetatarsalian archosaur,Teleocrater rhadinus. Additionally, the anteriorcervical vertebra attributed to NHMUK PV R 6795 is extremely elongated relative to that of the middle dorsal vertebrae with a lowcentrum toneural arch ratio and a significant displacement between the two sides of thearticular facet of the centrum. However, it is probable that the limb bones and other elements included in NHMUK PV R 6795 do not belong to the same individual. Therefore, it is possible that the vertebrae ofTeleocrater rhadinus are also referable toNyasasaurus parringtoni.[1]
An analysis of the interior structure of the humerus indicates thatbone growth was rapid, with interwoven bone fibers, many channels forblood vessels that radiate in all directions, and fewlines of arrested growth. This structure more closely matches that of the early dinosaurCoelophysis than it does of dinosaur ancestors, suggesting thatNyasasaurus was closer to the ancestry of dinosaurs than otherarchosaurs at the time.[1]
Because it is based on incomplete remains,Nyasasaurus has been difficult to classify. It can be placed confidently withinArchosauria, the group ofreptiles represented today bycrocodilians andbirds,Dinosauria, the group of dinosaurs; and possibly withinDinosauriformes, the group that includes birds, dinosaurs, and several non-dinosaurian groups from the Triassic.[1][10]
Nyasasaurus was suggested to have been a primitiveprosauropoddinosaur in 1986,[11] but this hypothesis was disputed. The 2013 study suggests thatNyasasaurus may be the earliest known dinosaur, dating to the lateAnisian stage, about 243 million years ago,[12] 10 to 15 million years older than any previously described dinosaur, such asHerrerasaurus.[1][13] However, this age is being questioned by other papers.[2][3][4]
Dinosaur affinities of the holotype are supported by the long deltopectoral crest on the humerus, an unambiguously dinosaur top, another feature present only in dinosaurs. The humerus does not share anysynapomorphies exclusively with any other Triassic archosaurclade. The supposed possession of three sacral vertebrae instead of two could represent a dinosaurplesiomorphy, but has a complex distribution amongdinosauriforms. The elongated neck vertebrae with hollowed-out sides of the referred specimen provides two characters that are exclusive to the derivedsilesauridSilesaurus (but absent in the earlier and morebasal silesauridAsilisaurus), and to earlytheropod dinosaurs. These characters can be interpreted as possibly homologous with features that represent unambiguouslyskeletal pneumaticity in theropods.[1]
Nesbittet al. (2013) incorporated both specimens, NHMUK PV R 6856 and SAM-PK-K10654, into a phylogenetic analysis. This analysis was based on data from a 2011 analysis by Sterling Nesbitt that included many Triassic archosaurs.[1] When NHMUK R6856 was added to the data set, several possible relationships were found. Various possible evolutionary trees place it as the sister taxon of Dinosauria, the most basal member ofOrnithischia (the group that includes most herbivorous Mesozoic dinosaurs), or a member ofTheropoda (the group that includes most carnivorous dinosaurs as well as birds). When SAM-PK-K10654 was added to the analysis, it was found to be a theropod. SAM-PK-K10654 possesses several theropod features, including deep pits orfossae in its neck vertebrae, which are not found in NHMUK PV R 6856 because of the limited overlap between the specimens.[1] The followingcladogram depicts these possibilities:
| Archosauria |
| ||||||||||||||||||||||||||||||||||||||||||||||||
A large phylogenetic analysis of early dinosaurs and dinosauromorphs by Matthew Baron,David B. Norman and Paul Barrett (2017) found thatNyasasaurus may represent a derived member ofSauropodomorpha most closely related tomassospondylids likeMassospondylus andLufengosaurus.[14] In his 2018 thesis on dinosaur interrelationships, Matthew Baron cast doubt on the referral of"Thecodontosaurus" alophos toNyasasaurus, arguing that SAM-PK-K10654 instead represents a neotheropod due to the lack of skeletal pneumaticity seen in massospondylids.[15]
In 2021,Fernando Novas and colleagues used the aforementioned characteristics shared byNyasasaurus andTeleocrater—and not by dinosauriforms such asAsilisaurus andSilesaurus—to suggest that a position forNyasasaurus in the Dinosauriformes is uncertain. However, they elected not to comment further onNyasasaurus' affinities given the fragmentary nature of the fossils.[10]
The age ofNyasasaurus, as well as theManda Formation it belongs to, is controversial.Biostratigraphy based on thesynapsid fauna (Cynognathus,Diademodon,Kannemeyeria) has correlated the formation with subzone B and C of theSouth AfricanCynognathus assemblage zone (CAZ). The fauna of the upper CAZ are typically considered to beAnisian in age. The presence oftraversodontids in the younger levels of the formation, which do not co-occur with the CAZ fauna in South Africa, suggests that these younger levels may be late Anisian.[16]
More recently,SHRIMPUranium-Lead dating obtained an age of 233.8-237.8 Ma from anignimbrite layer directly underlying theRío Seco de la Quebrada Formation, another putatively Anisian formation with CAZ fauna. These point to an unusually young (Carnian) age for the formation, up to 10 million years younger than biostratigraphy would indicate. This may imply that the CAZ fauna is not as old as traditional biostratigraphy has argued, or that the CAZ fauna persisted in some areas much longer than previously thought.[4] If the former is true, this would mean that the Manda Formation, includingNyasasaurus, is actually Carnian in age, drastically reducing the temporal gap betweenNyasasaurus and other putative early dinosaurs.[3]
Not all paleontologists are convinced by this, however. The estimated early Carnian age of the Río Seco de la Quebrada Formation is similar to that dated for theChañares Formation inLa Rioja Province, Argentina. However, the Chañares Formation lacks CAZ fauna, instead preserving more advanced species ofcynodonts,dicynodonts, andarchosauromorphs. This contradicts the widespread distribution of CAZ fauna and its close proximity to the Río Seco de la Quebrada Formation, implying a large temporal gap between the RSQ and Chañares formations.[17][18] The dating methods used for finding the age of the RSQ have also been criticized based on the argument thatSHRIMP dating is less accurate thanCA-TIMS dating.[19] More widespread radiometric dating is required to clarify the temporal extent of the CAZ fauna.[17] The upperErmaying Formation ofChina is correlated with subzone C of theCynognathus assemblage zone based on the presence ofShansiodon, and it is considered to be late Anisian based on CA-TIMS Uranium-Lead dating.[20]
Contemporaries
