Saturday, 1 June 2024

Musankwa sanyatiensis: A new species of Massopodan Sauropodomorph from the Late Triassic of Zimbabwe.

The Upper Karoo Group deposits of South Africa and Lesotho have produced numerous Late Jurassic and Early Jurassic Dinosaur fossils, contributing considerably to our understanding of the early evolution of the group. Deposits of similar ages are known from across much of Southern and East Africa, although these have been less thoroughly explored by Palaeontologists. Such deposits are present in the Kalahari Karoo Basin of Botswana (where no identifiable Vertebrate remains have been found), Waterberg-Erongo region of Namibia (from which a partial skeleton attributed to the Prosauropod Massospondylus sp. has been described), and the Mana Pools Basin of Zambia (which have yielded a single partial Sauropodomorph). 

In Zimbabwe, Upper Karoo Group sediments are found in the Tuli, Mana Pools, Cabora Bassa, and Mid-Zambezi basins, all of which have yielded Dinosaur fossils. These deposits are capped off by flood basalts, of equivalent ag to the Drakensburg Group, which can be dated to between approximately 176 and 186 million years ago, giving a minimum age to any fossils found in the deposits below. As in other areas of Southern Africa, the most abundant Dinosaurs are Protosauropods, with most specimens assigned to Massospondylus sp., although none of the Zimbabwean material has been compared in any detail to the South African type material. This means that the presence of the taxon in Zimbabwe cannot actually be confirmed, which is important as it has been used as a major piece of evidence in the comparative dating of the two areas. Other Zimbabwean Protosauropods have been assigned to Euskelosaurus (though this is now considered dubious), Vulcanodon karibaensis, and Mbiresaurus raathi, both of which are only known from Zimbabwe. A number of Early Theropod skeletons have also been described from Zimbabwe, all of which have been assigned to the Coelophysoid 'Syntarsus' rhodesiensis.

In a paper published in the journal Acta Palaeontologica Polonica on 30 May 2024, Paul Barrett of the Fossil Reptiles, Amphibians and Birds Section at the Natural History Museum, and the Evolutionary Studies Institute at the University of the WitwatersrandKimberley Chapelle of the Department of Anatomical Sciences at Stony Brook University, and the  Evolutionary Studies Institute at the University of the Witwatersrand, Lara Sciscio of the Jurassica Museum, Timothy Broderick of Harare, Michel Zondo of the Natural History Museum of Zimbabwe, Darlington Munyikwa of National Museums and Monuments of Zimbabwe, and Jonah Choiniere, again of the Evolutionary Studies Institute at the University of the Witwatersrand, Johannesburg, describe a new species of Sauropodomorph Dinosaur from the Pebbly Arkose Formation of Spurwing Island on Lake Kariba, Zimbabwe.

The new species is named Musankwa sanyatiensis, where 'Musankwa' is the name of the houseboat used in two expeditions to Lake Kariba in 2017-18, and means 'boy close to marriage' in the Tchitonga language of the BaTonga people, who inhabited the land flooded by Lake Kariba following the construction of the Kariba Dam, and 'sanyatiensis' means 'from Sanyati' in reference to the Sanyati River Basin, which was flooded by the formation of Lake Kariba, which Spurwing Island was once part of.

(A) Map showing the geographic setting of the Mid-Zambezi Basin in northwest Zimbabwe. (B) Position of Spurwing Island relative to the Zimbabwean (southern) shoreline of Lake Kariba. (C) Spurwing Island; arrow indicates the fossil locality at the Spurwing East Palaeosol site. (D) =Sedimentology of the Spurwing East Palaeosol site. (E) Articulated hind limb of Musankwa sanyatiensis (NHMZ 2521) as discovered in situ. (F) Evidence of bioturbation in the form of invertebrate traces (e.g., Taenidium isp. with menisci highlighted by red mudstone). (G) Associated sediments of the fossil site: pedogenically-modified fines with desiccation cracks, carbonate nodules, and colour mottling. Abbreviations: f, fine; m, medium; vf, very fine. Barrett et al. (2024).

The species is described from a single partial right hind limb, consisting of a complete femur, tibia, and astragalus, with associated indeterminate bone fragments, discovered by Paul Barret during an expedition to the southern shore of Lake Kariba in March 2018. At the time of its discovery it was weathering out of the rock, with the preserved elements still in an articulated state. The whole specimen is heavily sun-cracked and weathered, with poor surface preservation that mightreflect a long period of surface exposure prior to collection,- as well as pre-burial damage. Following collection the limb was placed in the collection of the Natural History Museum of Zimbabwe, and given the specimen number NHMZ 2521.

Right hind limb of the sauropodomorph dinosaur Musankwa sanyatiensis (NHMZ 2521) from the Pebbly Arkose Formation (Norian, Upper Triassic) of Spurwing Island, Zimbabwe. (A) Right femur in posterior (A₁), lateral (A₂), anterior (A₃), medial (A₄), proximal (A₅), and distal (A₆) views. (B) Right tibia with conjoined astragalus in anterior (B₁), lateral (B₂), posterior (B₃), medial (B₄), and proximal (B₅) views. Barret et al. (2024).

Specimen NHMZ 2521 shows no unique features not seen in other Protosaurpods, which is unsurprising given the limited nature of the material available, but does show a sufficiently unique combination of features that Barrett et al. feel comfortable in describing it as a new species and genus. A phylogenetic analysis recovered Musankwa sanyatiensis as the earliest-branching member of the Massopoda, a paraphyletic 'grade' which gave rise to the Sauropods (the Massopoda is considered paraphyletic because all members of the group are descended from a single common ancestor, but not all descendents of that ancestor are considered to be members of that group). However, Barret et al. note that the phylogeny of the Massapoda is poorly resolved, and prone to changing each time a new specimen is found, due to the fragmentary nature of many of the specimens from which species assigned to the group have been described.

Time-scaled reduced strict consensus of over 10 000 maximum parsimony phylogenetic treess with lengths of 1669 steps resulting from inclusion of Musankwa sanyatiensis. Barrett et al. (2024).

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