Nyasasaurus

Nyasasaurus Temporal range: Middle Triassic, 243 Ma PreЄ Є O S D C P T J K Pg N ↓
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
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauriformes
Genus:	†Nyasasaurus Nesbitt et al., 2013
Type species
†Nyasasaurus parringtoni Nesbitt et al., 2013

Nyasasaurus (meaning "Lake Nyasa lizard") is an extinct genus of dinosauriform reptile from the Middle Triassic Manda Formation of Tanzania that appears to be the earliest known dinosaur. The type species Nyasasaurus parringtoni was first described in 1956 in the doctoral thesis of English paleontologist Alan J. Charig, but it was not formally described until 2013. Previously, the oldest record of dinosaurs was from Argentina and dated back to the late Carnian stage, about 231.4 million years ago. Nyasasaurus comes from a deposit that dates back to the Anisian stage, meaning that it predates other early dinosaurs by about 12 million years.^[1]

Description

The type specimen, NHMUK R6856, is a partial skeleton belonging to an individual estimated to have been two to three metres in length. It consists of a right humerus, three partial sacral vertebrae and three presacral vertebrae. A second specimen, SAM-PK-K10654 consisting of three cervical 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 R6856 and SAM-PK-K10654, including a connection between two bony projections called the hyposphene and hypantrum, are also found in other Triassic archosaurs. Since these characteristics are not unique to the two species they do not by themselves provide sufficient evidence for grouping NHMUK R6856 and SAM-PK-K10654 under the same species. The 2013 description of Nyasasaurus by Sterling Nesbitt, Paul Barrett, Sarah Werning and Christian 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 of Nyasasaurus parringtoni and those of the enigmatic avemetatarsalian archosaur, Teleocrater rhadinus (a nomen nudum from Charig's thesis, based on the holotype specimen, NHMUK PV R6795, from the Lifua Member of the Manda Formation). Additionally, the anterior cervical vertebra attributed to NHMUK PV R6795 is extremely elongated relative to that of the middle dorsal vertebrae with a low centrum to neural arch ratio and a significant displacement between the two sides of the articular facet of the centrum. However, it is probable that the limb bones and other elements included in NHMUK PV R6795 do not belong to the same individual. Therefore, it is possible that the vertebrae of Teleocrater rhadinus are also referable to Nyasasaurus parringtoni.^[1]

An analysis of the interior structure of the humerus indicates that bone growth was rapid, with interwoven bone fibers, many channels for blood vessels that radiate in all directions, and few lines of arrested growth. This structure more closely matches that of the early dinosaur Coelophysis than it does of dinosaur ancestors, suggesting that Nyasasaurus was closer to the ancestry of dinosaurs than other archosaurs at the time.^[1]

History of study

In the 1930s, the holotype of Nyasasaurus was collected in Parrington’s locality B36 from the Lifua Member of the Manda Formation, Ruhuhu Basin near Lake Nyasa in southern Tanzania by Francis Rex Parrington. Other fossils from the same locality included those of cynodonts, dicynodonts, and rhynchosaurs. Most, including those of Nyasasaurus, 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".^[2] In 1967 Charig used the name Nyasasaurus parringtoni, in a review of Archosauria, but without any description, so it was commonly considered a nomen nudum; the dissertation was also never published.^[3] The generic name referred to Lake Nyasa and the specific name honouring Parrington. 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 name Nyasasaurus parringtoni.^[1] The generic name is occasionally misspelled as "Nyasaurus",^[1] as by Theodore Elmer White in 1973.^[4]

The referred specimen of Nyasasaurus, 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.^[5] 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 black carbonate). The consistent sizes of the remains indicate that they 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, produced cynodonts, lungfish, amphibians, and a shark. Dicynodonts, cynodonts and archosaurs such as Asilisaurus were also found nearby in the Lifua Member.^[1] The name Thecodonotosaurus alophos was coined for this specimen by Haughton (1932).^[6] 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 of Thecodontosaurus alophos was made only with Coelophysis longicollis. Since then, the species has been largely ignored by all subsequent vertebrate workers and no formal diagnosis of the specimen was ever provided. Nesbitt et al. (2013) found the specimen to not be diagnostic because it does not have any autapomorphic features or an unique combination of characteristics. Therefore, they suggested to abandon the name Thecodonotosaurus alophos and to refer its specimen to Nyasasaurus parringtoni.^[1]

Relationships

Because it is based on incomplete remains, Nyasasaurus has been difficult to classify. It can be placed confidently within Archosauria, the group of reptiles represented today by crocodilians and birds, and within Dinosauriformes, the group that includes birds, non-avian dinosaurs, and several non-dinosaurian groups from the Triassic.^[1]

Nyasasaurus was suggested to have been a primitive prosauropod dinosaur in 1986,^[7] but this hypothesis was disputed. The 2013 analysis suggests that Nyasasaurus may be the earliest known dinosaur, dating to the late Anisian stage, about 243 million years ago,^[8] 10 to 15 million years older than any previously described dinosaur, such as Herrerasaurus.^[1][9] Dinosaur affinities of the holotype are supported by the long deltopectoral crest on the humerus, an unambiguously dinosaur character in all recent phylogenetic analyses, with a deflected top, another feature present only in dinosaurs. The humerus does not share any synapomorphies exclusively with any other Triassic archosaur clade. The possession of three sacral vertebrae instead of two could represent a dinosaur plesiomorphy, but has a complex distribution among dinosauriforms. The elongated neck vertebrae with hollowed-out sides of the referred specimen provides two characters that are exclusive to the derived silesaurid Silesaurus (but absent in the earlier and more basal silesaurid Asilisaurus), and to early theropod dinosaurs. These characters can be interpreted as possibly homologous with features that represent unambiguously skeletal pneumaticity in theropods.^[1]

Nyasasaurus is likely either a basal dinosaur or close to Dinosauria. Other phylogenetic analyses have indicated that the sister group to Dinosauria is a family of herbivorous dinosauriforms called Silesauridae. The fossil record of silesaurids dates back to the late Anisian (the silesaurid Asilisaurus is also known from the Manda Formation), suggesting that the first dinosaurs also appeared around this time.^[1] Before Nyasasaurus was recognized as a possible dinosaur, a long ghost lineage of approximately 16 million years existed between the earliest dinosaurs and earliest silesaurids.^[8]

Nesbitt et al. (2013) incorporated both specimens, NHMUK R6856 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 of Ornithischia (the group that includes most herbivorous Mesozoic dinosaurs), or a member of Theropoda (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 or fossae in its neck vertebrae, which are not found in NHMUK R6856 because of the limited overlap between the specimens.^[1] The following cladogram depicts these possibilities:

A large phylogenetic analysis of early dinosaurs and dinosauromorphs by Matthew Baron, David Norman and Paul Barrett (2017) found that Nyasasaurus may represent a derived member of Sauropodomorpha most closely related to massospondylids like Massospondylus and Lufengosaurus.^[10] If the hypothesis is proven correct, then the divergence times of all major clades within Dinosauria will be pushed back 10 to 15 million years. In the 2018 thesis on dinosaur interrelationships, Matthew Baron cast doubt on the referral of "Thecodontosaurus" alophos to Nyasasaurus, arguing that SAM-PK-K10654 instead represents a neotheropod due to the lack of skeletal pneumaticity seen in massospondylids, which would push back the divergence time of neotheropods 10-15 mya.^[11]

See also

• Eoraptor
• Alwalkeria
• Agnosphitys
• Dinosaur evolution

Contemporaries

• Stenaulorhynchus

References

• Lambert, David (1994). The Encyclopedia of Dinosaurs. Bloomsbury Books p 80.

External links

• Oldest Dinosaur Found from Discovery.com
• Discussion of "Nyasasaurus" from the Dinosaur Mailing List
• The oldest dinosaur: working from scraps - The Economist