Nundasuchus is an extinct genus of crurotarsan, possibly a suchian archosaur related to Paracrocodylomorpha. Remains of this genus are known from the Middle Triassic Manda beds of southwestern Tanzania. It contains a single species, Nundasuchus songeaensis, known from a single partially complete skeleton, including vertebrae, limb elements, osteoderms, and skull fragments.
Nundasuchus lived in what is now Tanzania, Africa around 240 million years ago. Members of this genus were likely carnivores, with ziphodont (steak knife-like) teeth and rows of bony plates (osteoderms) along their back. Phylogenetic analyses consistently place this genus within the group Crurotarsi based on features of the ankle. Most studies also consider it a pseudosuchian, meaning that it was more closely related to modern crocodilians than it was to dinosaurs. However, Nundasuchus had an upright stance, with legs situated directly underneath the body, as with various other early pseudosuchians (such as "rauisuchians" and aetosaurs) but unlike modern crocodilians.
The classification of Nundasuchus relative to other pseudosuchians is somewhat controversial. Some phylogenetic analyses place it near or at the base of the group, sometimes along with phytosaurs, based on certain plesiomorphic (primitive) features such as teeth on the palate, a short pubis, and characteristics of the calcaneum (heel bone). Another hypothesis, supported by its original 2014 description, considers it to be somewhat more "advanced" than those groups, instead being closer to Ticinosuchus and paracrocodylomorphs (the group containing "rauisuchians" and the ancestors of modern crocodilians). This classification scheme is justified by the presence of "staggered" osteoderms, heart-shaped "spine tables", and a groove on the femoral head. Regardless of these hypotheses, it is clear that Nundasuchus represents a previously unknown group of reptiles with a mixture of features both plesiomorphic and derived with respect to suchian archosaurs.
Discovery
Nundasuchus is known solely from the holotype NMT RB48, a partially complete disassociated and mostly disarticulated individual housed at the National Museum of Tanzania, Dar es Salaam of Tanzania. The holotype consists of a partial right pterygoid, nearly complete right dentary, the right splenial, the right surangular, and isolated teeth, as well as the following postcranial bones: a partial atlas, two articulated mid-neck vertebrae, two articulated mid-back vertebrae, the last back vertebrae, the sacrum with sacral ribs, front-most tail vertebrae, dorsal ribs, gastralia, articulated and isolated paramedian osteoderms, partial shoulder girdle including interclavicle, parts of both clavicles, complete left and right scapulae, and the right coracoid, the left humerus, both pubes, both femora, the left fibula, inner and outer ends of the left tibia, as well as the left astragalus, left calcaneum, left fourth tarsal and all but fourth metatarsals, and outer ends of right third-to-fifth metatarsals, numerous isolated phalanges, a partial ungual and many other fragments.
NMTRB48 was discovered by Dr. Sterling Nesbitt in 2007 at an isolated outcrop approximately 100 square meters in area known as locality Z41, along with remains of other archosaurs and rhynchosaurs. Another locality (Z42) occurs in the immediate vicinity, and yielded four cynodonts including two unnamed forms, Scalenodon attridgei and Mandagomphodon hirschsoni, four dicynodonts including Tetragonias njalilus, Sangusaurus parringtonii, Angonisaurus cruickshanki, and Rechnisaurus cristarhynchus, and the archosauromorphs Stenaulorhynchus stockleyi and Asilisaurus kongwe. These localities, located between the Ndatira and Njalila rivers, belong to the fluviolacustrine mudstone-sandstone sequence in the middle of the Lifua Member of the Manda beds of Ruhuhu Basin in Tanzania. Based on comparison with the better studied tetrapod fauna of Subzone C of the Cynognathus Assemblage Zone of South Africa, this member is considered to date to the Anisian stage of the early Middle Triassic.
Description
Skull Skull material is very limited for Nundasuchus; only a right lower jaw and a right pterygoid bone (which formed part of the roof of the mouth) were preserved in the holotype. However, even these fragments are informative. The pterygoid is convex when seen from above and concave from below. This ventral (lower) concavity manifests in the form of a deep depression bounded by posterior (rear) and medial (inner) ridges. The portion of the pterygoid which would have been positioned along the midline of the palate is unusual compared to other archosaurs due to possessing teeth. These teeth were small and rounded, placed in three rows parallel to the midline of the mouth. Pterygoid teeth are typically absent in archosaurs, although a few avemetatarsalians (Eudimorphodon, Eoraptor, Eodromaeus) and another likely pseudosuchian (Turfanosuchus) are known to have possessed them.
The jaw is typical compared to that of most carnivorous archosaurs, being low and long, with serrated, knife-like teeth set in deep sockets. At least 14 teeth were present, with the teeth at the middle of the jaw (around tooth nine) being smaller than those at the front and rear of the jaw. The lower edge of the lateral (outer) side of the jaw was covered in small longitudinal grooves, a feature seemingly unique to Nundasuchus among early archosaurs. Less unusual is the presence of several rows of pits on the lateral surface and large grooves on the medial surface. The surangular bone at the back of the jaw forms a deeply concave jaw joint preceded by a longitudinal ridge (unique to the genus) and followed by a large, vertically-oriented plate of bone.
Vertebrae and osteoderms The two incompletely preserved cervical (neck) vertebrae were amphicoelous (concave from both the front and the rear), with a large lateral pit near the base of the neural arch. The concave pleurapophyses (lower rib facets) were near the lower front edge of the vertebrae, while the convex diapophyses (upper rib facets) were positioned higher up. The lower edge of each vertebra possessed a large keel which was deepest towards the rear. Isolated cervical neural spines were similar to those of Batrachotomus, being expanded towards their upper front tips into heart-shaped structures when seen from above.
Most of the dorsal (back) vertebrae were tall and amphicoelous, and possessed a pair of ventral keels adjacent to the midline, separated by a shallow groove. These ventral keels are similar to those of the unusual aquatic archosauriform Vancleavea, although not as pronounced. Considering that Nundasuchus is not closely related to Vancleavea, its ventral keels were probably an example of convergent evolution, and they can be considered a unique trait compared to other archosaurs. The rib facets were short, positioned high on the vertebrae at the base of the neural arches. Three ridges radiate outwards from each diapophysis; one connects to the parapophysis, another connects to the main body of the vertebra, and the last one reaches the postzygapophyses (rear vertebral joints). One notable feature of the vertebrae of Nundasuchus is the possible presence of hyposphene-hypantrum articulations, as reported by the original description. However, a 2018 study doubted this interpretation, arguing that the supposed articulations were actually misidentified components of the zygapophyses. As with the cervicals, the neural spines of the dorsal vertebrae expand outwards into flat structures sometimes called "spine tables". Some dorsals near the hip lose the characteristic ventral keel, instead acquiring additional pits near the rib facets.
The sacrum was probably formed by two unfused vertebrae, connected to massive sacral ribs. The rib facets are huge in conjunction with the sacral ribs, but not large enough to annihilate the pits at the base of the neural arch. Each rib facet of the first sacral vertebra had a distinct upper and lower connection to the trumpet-shaped first sacral rib. The second sacral rib is flatter, and tilts diagonally so that its tapering rear edge is positioned higher than its front edge. A groove is present along the outer edge of this rib, roughly where it would connect to the ilium. The sacrum in general is similar to that of the rauisuchid Stagonosuchus, except for having thinner neural spines.
The caudals (tail vertebrae) near the hip were tall and generally similar to the dorsals, except that their zygapophyses were inclined upwards at a higher angle. Further down the tail, they acquire chevrons and fuse to the caudal ribs, as well as becoming smaller and simpler in general. The dorsal ribs were two-headed, with concave and convex joints corresponding to the convex and concave rib facets of the vertebrae. They were flattened near the vertebrae, and at their tips they had small facets for attachment to the densely packed gastralia (belly ribs).
Several groups of osteoderms (plate-like bony scutes) were also preserved in Nundasuchus. Individual osteoderms were lightly ornamented and heart- or leaf-shaped, with rounded edges and tapering front tips. They are similar to those of most other pseudosuchians as well as archosaur relatives such as Euparkeria, but are less elongated than those of some of these taxa, instead being about as wide as they are long. Most of the osteoderms referred to Nundasuchus are thin bones, with sharply pointed front tips, a thin longitudinal ridge, and a fair amount of overlap. A rare "thick" morphotype is also present, with less overlap, a small and mound-like ridge, and more rounded front tips. At least the "thin"-type osteoderms formed several longitudinal rows stretching along the backbone of the animal. When seen from above, these rows would not have been symmetrical on each side of the animal. Instead, they were "staggered", with the left and right rows slightly offset from each other. An estimated five osteoderms were present per every two vertebrae.
Pectoral girdle and forelimbs The scapula (shoulder blade) was paddle-like and somewhat small. The lower portion of the scapula is where the glenoid (shoulder socket) is located, pointing backwards from the rear edge of the bone. This general area is the widest part of the bone when seen head-on, and underlies the thinnest part when seen from the side. Directly above the glenoid lies a small pit, likely the origin point of the triceps muscle. Meanwhile, the front edge of the scapula has a distinct ridge, probably the acromion process. The scapula of Nundasuchus differs from advanced pseudosuchians due to its small triceps pit, but also from phytosaurs due to its distinct acromion. The lower edge of the scapula connects to the semicircular coracoid bone. The rear portion of the coracoid extends back and curves upwards to form the lower half of the glenoid, but is not offset from the socket by a distinct notch. However, a wide groove is present below the glenoid when seen from the side, similar to the case in aetosaurs and Postosuchus. In front of the glenoid, a hole known as the coracoid foramen pierces the coracoid, and, in a feature unique to Nundasuchus, this hole is surrounded by several knob-like structures. Other fragments of the pectoral girdle, such as an interclavicle and clavicles, are also preserved. They are covered in grooves, with thin, blade-like edges. The interclavicle is spoon-shaped, concave from above and convex from below, although its true shape is unknown due to the rear portion being broken off. Overall it is more similar to those of paracrocodylomorphs rather than phytosaurs.