Specializations of the mandibular anatomy and dentition ofSegnosaurus galbinensis(Theropoda: Therizinosauria)

Abstract

Definitive therizinosaurid cranial materials are exceptionally rare, represented solely by an isolated braincase and tooth in the North American taxon Nothronychus mckinleyi, the remarkably complete skull of the Asian taxon Erlikosaurus andrewsi, and the lower hemimandibles of Segnosaurus galbinensis. To date, comprehensive descriptions of the former taxa are published; however, the mandibular materials of S. galbinensis have remained largely understudied since their initial description in 1979. Here we provide a comprehensive description of the well-preserved hemimandibles and dentition of S. galbinensis (MPC-D 100/80), from the Upper Cretaceous Bayanshiree Formation, Gobi Desert, Mongolia. The subrectangular and ventrally displaced caudal hemimandible, extreme ventral deflection of the rostral dentary, and edentulism of the caudal dentary of S. galbinensis are currently apomorphic among therizinosaurians. Unique, unreported dental traits including lingually folded mesial carinae, development of a denticulated triangular facet on the distal carinae near the cervix, and extracarinal accessory denticles, suggest a highly specialized feeding strategy in S. galbinensis. The presence of triple carinae on the distalmost lateral tooth crowns is also unique, although may represent an abnormality. Contrasted with the simplistic dentition of the contemporaneous therizinosaurid E. andrewsi, the dentition of S. galbinensis is indicative of niche partitioning in food acquisition, processing, or resources among known therizinosaurids inhabiting Asian ecosystems in the Late Cretaceous. Although not quantitatively correlated with diet, this suite of specializations is otherwise unique among theropod dinosaurs and supports derived inferences of facultative or obligate herbivory in therizinosaurids, ultimately adding novel information to our understanding of ecomorphology in theropods.Subjects Paleontology, Taxonomy

The distribution of dental features in non-avian theropod dinosaurs: Taxonomic potential, degree of homoplasy, and major evolutionary trends

“…Some dentary teeth of Segnosaurus also show the peculiarity of having a sporadic variation of denticle size along the mesial carina, a split distal carina, extracarinal mesial denticles, and a third denticulated carina centrally positioned on the lingual surface of the crown (Zanno et al, 2016). The combination of these autapomorphic dental features in Segnosaurus was inter-preted by Zanno et al (2016) as indicative of increased shredding capabilities and a higher degree of oral processing than in other therizinosaurs. A bifurcated distal carina, triple carinae, and extracarinal denticles have not been observed in the dentition of other therizinosaurs or theropods, and a sporadic variation of denticle size only occurs in the conidont teeth with minute denticles of baryonychine theropods.…”

“…Lateral dentition. In the oviraptorosaurs Incisivosaurus (IVPP V13326; Osmólska et al, 2004), Protarchaeopteryx (Ji et al, 1998), and Similicaudipteryx (Li et al, 2018, figure S3f), as well as the therizinosaurs Erlikosaurus (Zanno et al, 2016) and Falcarius (Zanno, 2010a), only the lateral dentition is constricted between crown and root, the mesial dentition of these taxa being conidont or subconical/incisiform (Zanno, 2010a;Zanno and Makovicky, 2011). Likewise, the juvenile megalosaurid Sciurumimus (BMMS BK 11, Rdt4, Rdt6) and the carcharodontosaurids Carcharodontosaurus (SGM Din 1) and Giganotosaurus (MUCPV-CH-1) clearly show a ziphodont dentition but a few lateral teeth are weakly constricted at the cervix.…”

“…A twisted mesial carina does not occur in the lateral dentition of Carcharodontosauria, non-avetheropod theropods and Metriacanthosauridae. In non-eudromaeosaurian neocoelurosaurs, this condition has only been observed in a few lateral teeth of the basal and derived therizinosaurs Falcarius (UMNH VP 14528; Button et al, 2017, supplemental figure S2) and Segnosaurus (the 'folded carina' sensu Zanno et al, 2016), respectively. Remarks and synapomorphy.…”

“…Unlike Falcarius, the spiraling mesial carina of Segnosaurus is denticulated along most of the carina and, in some cases, bears apicobasally tall and vertically oriented denticles creating the 'lingual folding of the mesial carina' noted by Zanno et al (2016). Some dentary teeth of Segnosaurus also show the peculiarity of having a sporadic variation of denticle size along the mesial carina, a split distal carina, extracarinal mesial denticles, and a third denticulated carina centrally positioned on the lingual surface of the crown (Zanno et al, 2016). The combination of these autapomorphic dental features in Segnosaurus was inter-preted by Zanno et al (2016) as indicative of increased shredding capabilities and a higher degree of oral processing than in other therizinosaurs.…”

“…In maniraptoriforms, the dromaeosaurid Dromaeosaurus albertensis from the Upper Cretaceous Prince Creek Formation of Alaska (Fiorillo and Gangloff, 2001) and an indeterminate dromaeosaurid from the Barremian of Uña in Spain (Rauhut, 2002) also show a bifurcated mesial carina. The therizinosaur Segnosaurus galbinensis from the Upper Cretaceous Bayanshiree Formation of Mongolia (Zanno et al, 2016) has a split distal carina occurring at the crown base delimiting a flattened triangular facet. This is present from the second to the twelfth dentary tooth on both left and right dentaries (Zanno et al, 2016) and appears to be unique to this taxon.…”