Tuesday 15 February 2022

Mambawakale ruhuhu: A new species of Pseudosuchian Archosaur from the Middle Triassic Manda Beds of Tanzania.

The Archosaurs, the group which includes modern Birds and Crocodiles, as well as the extinct non-Avian Dinosaurs and Pterosaurs, appeared in the Early Triassic and underwent a major evolutionary radiation in the Middle Triassic, becoming the dominant group of terrestrial Vertebrates. One of the most important sources of Middle Triassic Archosaur fossils is the Manda Beds of the Ruhuhu Basin, in southwest Tanzania, which is thought to be Anisian in age (247.2-242 million years old), and therefore documents the early stages of the Archosaur radiation.

The first recorded collections of material from the Manda Beds were made in the 1930s by Gordon Murray Stockley of the Tanganyika Geological Survey and Francis Rex Parrington of the University of Cambridge, followed later in the same decade by the Austrian geologist Ernst Nowack, who were all involved in surveying the Ruhuhu Basin for mineral resources, with the fossils they collected being sent to museums in South Africa, the UK, and Germany. Most of the material collected by Parrington was brought back to the UK, where it was described by Alan Charig as part of his PhD dissertation in the 1950s, but this dissertation was not published, and no descriptions of this material was published until after Charig's death in 1997. 

Tanganyika gained independence from Britain in 1961, and in 1964 merged with another former colony, Zanzibar, to form the modern Republic of Tanzania. In 1963 Charig took place in another expedition to the region, organised by the British Museum (Natural History) (now the Natural History Museum) and including researchers from South Africa, Uganda, and the UK, as well as large numbers of unnamed Tanzanian and Zambian workers, who identified many of the sites from which fossils were extracted, as well as excavating the fossils, and building the roads upon which they were transported away, with the fossils once again being brought back to the UK and deposited in the collection of the Natural History Museum.

This expedition collected a number of important specimens, including the material from which the Pseudosuchian Archosaur Hypselorhachis mirabilis was described, and another specimen which Charig referred to as Pallisteria angustimentum in a number of publications, but appears never to have described; he referred to the specimen being described as being ‘in press’ at the journal Palaeontology, but no record of this paper appears to exist, suggesting it was never submitted, making the name invalid under the terms of the International Code of Zoological Nomenclature.

In a paper published in the journal Royal Society Open Science on 9 February 2022, Richard Butler of the School of Geography, Earth and Environmental Sciences at the University of Birmingham, Vincent Fernandez of the Imaging and Analysis Centre at the Natural History Museum, Sterling Nesbitt of the  Department of Geosciences at Virginia Tech, João Vasco Leite of the Department of Earth Sciences at the Natural History Museum, and David Gower of the Department of Life Sciences at the Natural History Museum, formally describe the material identified as Pallisteria angustimentum by Alan Charig under the name Mambawakale ruhuhu.

Renaming a specimen to which a name has previously been applied is an unusual step even where the name is not now considered valid; it would have been possible for Butler et al. to simply describe the specimen under Charig's nominated name, thereby legitimising it, but they chose to rename it in Kiswahili, in honour of the unnamed Tanzanian workers, who located, extracted, and transported the specimen, and who would have spoken Kiswahili. Charig's original name, Pallisteria angustimentum, comprised a genus name, Pallisteria, intended to honour John Weaver Pallister, who was Commissioner of Mineral Resources of Tanganyika at the time of the 1963 expedition, and provided some support for it, although he was in no way directly involved in the collection of the specimen, and a specific name, angustimentum, which means 'narrow chin' in Latin. The new name proposed by Butler et al. comprises the genus name Mambawakale, meaning 'ancient Crocodile' in Kiswahili, and the specific name ruhuhu, in reference to the Ruhuhu Basin, where the specimen was collected. As none of the team working on the project came from Tanzania or spoke Kiswahili, the name was chosen with the help of Tanzanian herpetologist John Lyakurwa of the University of Dar es Salaam. An audio recording of the pronunciation of this name is provided on the Natural History Museum's Data Portal.

Photographs showing the collection of NHMUK R36620, holotype of Mambawakale ruhuhu, in 1963. Alan Charig is sat in the bottom right of the frame in the top left image, and is accompanied by Alfred ‘Fuzz’ Crompton. The top left and bottom right images also show Tanzanians (names unfortunately not recorded in archival material) who were employed by the 1963 expedition team and were critical to its success, discovering many of the fossil sites, constructing roads and carrying excavated fossils out of the field. Butler et al. (2022).

The specimen from which Mambawakale ruhuhu is described comprises a partial skull including the premaxillae, maxillae, vomers, palatines, pterygoids, ectopterygoids and fragments of the jugals and basipterygoid, with associated hemimandibles, hyoids and isolated maxillary or dentary teeth, found in association with an incomplete postcranium, including an atlantal intercentrum, partial axis and partial third cervical vertebra, a mostly complete left manus, and additional poorly preserved fragments.

Photographs of the skull of NHMUK R36620, holotype of Mambawakale ruhuhu, in right lateral (a) and left lateral (b) views, with a close-up of the left premaxilla in lateral view (c). Butler et al. (2022).

The skull was found exposed on an eroded surface. It was in occlusion with the mandibles when found, but apparently separated from them during preparation, during which process several of the teeth were also broken. Both premaxila are present, as are the maxila; the join between these bones is hard to detect visually, but can be seen in CT scans. The region of this join is toothless, creating a gap between the premaxilary and maxilary teeth. Only the lower part of the external naris (nose bone) is preserved, attached to the upper part of the premaxila and surviving part of the maxila, though this is enough to suggest that it was a large structure, extending backwards over at least the first two maxilary teeth; something seen in other Pseudosuchian Archosaurs.  

Segmentation based on CT data of the skull of NHMUK R36620, holotype of Mambawakale ruhuhu, in right lateral (a), left lateral (b), anterior (c) and posterior (d ) views, with a medial view of the left side of the skull (e). Abbreviations: afos, antorbital fossa; apmx, ascending process of the maxilla; cut, small part of the fossil that was cut off and not included in the CT scan; en, external naris; fill, area of artificial repair; for, foramen; gr, groove on the lateral surface of the posterodorsal process of the premaxilla; lbpt, left basipterygoid process of the basisphenoid; ljg-lect, left jugal and left ectopterygoid; lmx, left maxilla; lpal, left palatine; lpmx, left premaxilla; lpt, left pterygoid; lv, left vomer; nf, narial fossa; pdp, posterodorsal process of the premaxilla; rbpt, right basipterygoid process of the basisphenoid; rjg-rect, right jugal and right ectopterygoid; rmx, right maxilla; rpmx, right premaxilla; rpt, right pterygoid. Butler et al. (2022).

Small parts of the jugals are preserved, although these are hard to define, even in CT images.The palate is fairly complete, with substantial parts of the vomers, palatines, pterygoids and ectopterygoids all preserved.

Photographs of the skull of NHMUK R36620, holotype of Mambawakale ruhuhu, in dorsal (a) and ventral (b) views. Butler et al. (2022).

Both hemimandibles (separately articulated halves of the lower jaw) are preserved, but poorly, and their articulating surfaces are damaged. The dentaries (forward parts of the lower jaw, which bear the teeth) is largely intact. There are fifteen or sixteen teeth on each dentary (the exact count is unclear because of damage to the posterior ends of the tooth rows), although only the roots and a few broken crowns of the teeth are present. 

Photographs of the left and right hemimandibles of NHMUK R36620, holotype of Mambawakale ruhuhu. (a)–(d) The left hemimandible and (e)–(h) the right hemimandible in lateral (a), (e), medial (b), (f), dorsal (c), (g) and ventral (d), (h) views. Butler et al. (2022).

The post cranial remains comprise two parts of the hyoid apparatus, three partial cervical vertebrae, and a fairly complete left manus (hand), comprising metacarpals I to IV, the proximal portion of metacarpal V, phalanges of the first digit including an ungual, the first two phalanges of digits II and III, and phalanges 1–4 of digit IV as well as a potential ungual.

Left manus of NHMUK R36620, holotype of Mambawakale ruhuhu. Metacarpals in articulation in proximal view (a), dorsal surface directed to the bottom of the image. Manus in dorsal view (b). Digits ordered from I (left) to V (right) in both (a) and (b). Butler et al. (2022).

Resolving the phylogenetic position of Mambawakale ruhuhu proved to be difficult, due to the fragmentary nature of the remains from which the specimen is described, but it appears to be a basal member of the Paracrocodylomorpha. Mambawakale ruhuhu is the largest Archosaur from the Manda Beds described on the basis of cranial remains; a number of similar-sized species have been described from post-cranial remains, but only one Paracrocodylomorph, Mandasuchus tanyauchen, and that is a much smaller Animal, leaving Butler et al. confident that the remains assigned to Mambawakale ruhuhu do indeed represent a new species.

Life reconstruction of Mambawakale ruhuhu. Only the skull, mandible and a few postcranial elements are known for Mambawakale ruhuhu, so the rest of the body, tail and limbs are reconstructed based on the anatomy of hypothesised close relatives of similar size. Gabriel Ugueto in Butler et al. (2022).

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