A redescription of the Triassic kannemeyeriiform dicynodont <i>Sangusaurus</i> (Therapsida, Anomodontia), with an analysis of its feeding system

<p>The Triassic dicynodont <i>Sangusaurus</i> is poorly known because the holotype of the first species to be described (<i>S. edentatus</i> from Zambia) is fragmentary, and the description of the second species (<i>S. parringtonii</i> from Tanzania) is extremely brief. Nevertheless, the genus has played a role in Triassic biostratigraphy by linking the Ntawere Formation and the Manda Beds. Here, we redescribe <i>Sangusaurus</i>, including a new, nearly complete skull from Tanzania. The incomplete holotype of <i>S. edentatus</i> results in few characters diagnosing the genus. The skull of <i>S. parringtonii</i>, however, is highly autapomorphic. The snout is telescoped, with only a thin strip of frontals separating the nasals and the parietals. The external surfaces of the premaxilla, the maxillae, the nasals, and the prefrontals are extremely rugose; a sharp median crest extends from the premaxilla to the frontals, terminating in a thickened boss. The occiput is broader than the skull is long, and the adductor fossa on the squamosal is nearly vertical. Our phylogenetic analysis shows that <i>Sangusaurus</i> is not a kannemeyeriid but instead is a stahleckeriid. The feeding system of <i>Sangusaurus</i> emphasizes an orthal jaw motion. The articular surfaces of the jaw joint form a single posteroventrally sloping surface; translating the quadrate and the articular results in a primarily orthal movement of the jaw symphysis. The internal and external jaw adductors provide a strong, dorsally-directed component to the power stroke. Adductor mechanical advantage is similar to <i>Stahleckeria</i>, but <i>Sangusaurus</i> likely had a stronger transverse component of jaw movement because of the very lateral placement of M. adductor mandibulae externus lateralis.</p> <p>SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVP</p> <p>Citation for this article: Angielczyk, K. D., P. J. Hancox, and A. Nabavizadeh. 2018. A redescription of the Triassic kannemeyeriiform dicynodont <i>Sangusaurus</i> (Therapsida, Anomodontia), with an analysis of its feeding system; pp. 189–227 in C. A. Sidor and S. J. Nesbitt (eds.), Vertebrate and Climatic Evolution in the Triassic Rift Basins of Tanzania and Zambia. Society of Vertebrate Paleontology Memoir 17. Journal of Vertebrate Paleontology 37(6, Supplement).</p

External mould of a fragment of Bivalvia indet. cf. Unionoida (BP/1/7048).

<p><b>A.</b> SEM image. <b>B.</b> Reconstruction. Scale bar equals 1 mm.</p

Possible ultrastructural impressions of nacre on (BP/1/7047).

<p>Close up of the anterior subumbonal region of the internal mould of the right valve showing poorly-preserved sheet-like structures. Inset shows the position of the close-up. Scale bar equals 200 µm.</p

Stratigraphic column of the Driefontein site, showing the position of the bivalve fossils.

<p>Stratigraphic column of the Driefontein site, showing the position of the bivalve fossils.</p

‘<i>Unio</i>’ <i>karooensis</i>, Manda Formation, Tanzania.

<p><b>A.</b> Hinge plate of an isolated left valve (FMNH Z.48). <b>B.</b> Interpretative drawing of A. <b>C.</b> Conjoined pair of valves (FMNH Z.31) in left lateral view. <b>D.</b> FMNH Z.31 in dorsal view. <b>Abbreviations: aas</b>, anterior adductor scar; <b>act</b>, anterior cardinal tooth; <b>e</b>, escutcheon; <b>ln</b>, ligamental nymph; <b>mct</b>, median cardinal tooth; <b>pplt</b>, posterior pseudolateral tooth; <b>s</b>, socket; <b>u</b>, umbo. Scale bar equals 20 mm.</p

SEM images of Bivalvia indet. cf. Unionoida (BP/1/7047).

<p><b>A.</b> Internal mould of right valve in lateral view. <b>B.</b> Interpretation of the preserved features of the right valve. <b>C.</b> Close-up of the umbonal region of the right valve. <b>D.</b> Dorsolateral view of the internal mould of the right valve. <b>E.</b> Silcone rubber cast of the external mould of the left valve in dorsal view. <b>F.</b> Close-up of the umbonal region of the left valve. <b>Abbreviations: aas</b>, anterior adductor scar; <b>hp</b>, hinge plate; <b>pas</b>, posterior adductor scar; <b>u</b>, umbo. Scale bars in <b>A</b>, <b>B</b>, <b>D</b> and <b>E</b> equal 4 mm, those in <b>C</b> and <b>F</b> equal 1 mm.</p

Reconstruction of the right valve of Bivalvia indet. cf. Unionoida (BP/1/7047).

<p><b>A.</b> External view. <b>B.</b> Internal view. <b>Abbreviations: aas</b>, anterior adductor scar; <b>hp</b>, hinge plate; <b>pas</b>, posterior adductor scar; <b>u</b>, umbo. Scale bar equals 4 mm.</p

Measurements of the Driefontein bivalves.

<p>Shell thickness is measured as the distance between the internal and external mould at the posterior end of the right valve of BP/1/7047.</p

Quantifiable characters for the <i>Garjainia madiba</i> sp. nov. elements thin sectioned in this study.

<p>“MW”  =  Midshaft Width; “TBA”  =  Total Bone Area; “Vasc”  =  vascularization; “AvPOD”  =  Average Primary Osteon Diameter; “K”  =  ratio between the internal and external diameter of the bone (cortical thickness) <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0111154#pone.0111154-Currey1" target="_blank">[38]</a> measured for limb bones with a preserved midshaft region; “C”  =  bone compactness <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0111154#pone.0111154-Girondot1" target="_blank">[40]</a>.</p><p>Quantifiable characters for the <i>Garjainia madiba</i> sp. nov. elements thin sectioned in this study.</p

Humerus of <i>Garjainia madiba</i> sp. nov.

<p>A–D, BP/1/7336, right humerus in dorsal (A), proximal (B), ventral (C) and distal (D) views. E–G, cast of distal end of right humerus of NMQR 3051 in dorsal (E), distal (F) and ventral (G) views. Abbreviations: dpc, deltopectoral crest; ect, ectepicondylar groove; supr, supinator ridge.</p