Monday, March 25, 2019

What is Lingyuanosaurus?

A quick one here.  I've had the flu for the past week, so haven't felt up to doing any professional work, but we did get the description of a new fragmentary theropod taxon- Lingyuanosaurus sihedangensis (Yao et al., 2019).  It's from the same beds as Iteravis, which I think are Jiufotang because Ikrandraco is known from both there and another Jiufotang locality (Lamadong).  The holotype is four mostly partial vertebrae, some ribs, a proximal and distal humerus, two manual unguals, ilium, possible proximal ischium, femur, distal tibia and astragalus exposed proximally.  With a femoral length of 200 mm, it's about the same size as Jianchangosaurus (206 mm) and was run by its authors in a version of Zanno's (2010b) TWiG analysis.  They recovered it as a therizinosaur closer to therizinosaurids than Falcarius, Jianchangosaurus and Beipiaosaurus but less than Alxasaurus.

While my first lumping thought was "are we sure this isn't just a fragmentary Beipiaosaurus?", the more I scored it for the Lori matrix's 700 characters, the weirder it seemed.  Unlike any therizinosaur, there's only a "longitudinal depression, which may represent a pneumatic fossa" in the cervical (scored as unknown for presacral pneumatization by Yao et al.), the dorsal's parapophysis is centrally placed, the proximal caudal's complete transverse process is distally tapered (contra Yao et al., this is not true in the illustrated specimen of Falcarius- Zanno, 2010a: Fig. 9E), the humeral bicipital crest is barely projected (also in Erlikosaurus; scored unknown by Yao et al.), the distal humerus is expanded less than twice of shaft width, the ilial cuppedicus fossa is overhung laterally, the proximal femur has a lateral longitudinal ridge (scored unknown by Yao et al.), and the femoral ectocondylar tuber is medially positioned (not used by Yao et al.).  The dorsal parapophyseal position is rare outside enantiornithines so is near certainly an autapomorphy, and the cervical pneumatization may be present but difficult to see externally as in some ornithomimosaurs, but the other characters would be odd for a therizinosaur, especially the undeveloped and narrow humerus.

Lingyuanosaurus sihedangensis holotype humeri (IVPP V23589)- left proximal humerus in medial view (a), right distal humerus in posterior (b) and anterior (c) views. Scale = 30 mm. After Yao et al., 2019.

So I ran it in the Lori matrix, and it emerged... as an oviraptorosaur.  Makes more sense of the distal humerus, cuppedicus fossa and femur, and the manual ungual and ilial shape still work out.  Constraining it as a therizinosaur is only one step longer though, so maybe it really is one.  But note that of the supposedly therizinosaurian characters listed by Yao et al. (2019:9), Zanno's "dorsal vertebrae with a complex laminar structure" refers to mid-dorsals ("anterior dorsals" in Zanno's nomenclature) which are not preserved in Lingyuanosaurus and nor was that character (their 274) scored for the taxon, and oviraptorosaurs can have both "laterally flattened manual unguals with dorsally positioned collateral grooves" (e.g. Currie and Russell, 1988: Fig. 4b, mu I) and "a highly modified ilium with a deep preacetabular process, a reduced postacetabular process, a preacetabular process whose ventral margin is dorsally displaced relative to the acetabulum" (e.g. Funston et al., 2017: Fig. 9C).  The slender pubic peduncle is therizinosaur-like, however.

Interestingly, only four steps are needed to place it in Ornithomimosauria, where it emerges by the Jehol Hexing which also has a reduced humerus and highly curved manual unguals.  A further thought was that the proximal humerus and manual unguals were rather like the mysterious Yixianosaurus, but the two are otherwise incomparable.  Enforcing them to be synonymous leads to trees eight steps longer (the pair emerge as oviraptorosaurs), which isn't bad either but not as parsimonious as letting Yixianosaurus' complete pectoral girdle and arms go elsewhere.  Still, we know from compartmentalization studies that analyzing different body areas leads to different cladograms, so maybe Yixianosaurus/Lingyuanosaurus falls within the expected homoplasy range of an oviraptorosaur with arms a bit more similar to another clade?

In any case, Yao et al. modified several of Zanno's TWiG characters, but four of these are done in a way that create composite characters that hide potential homology.  For the classic character on hyposphene morphology, in addition to the original two states "0: abutting one another above neural canal, opposite hyposphenes meet to form lamina" and "1: placed lateral to neural canal and separated by groove for interspinous ligaments, hyposphenes separated", the authors add a third state for Lingyuanosaurus' supposedly autapomorphic condition "2: abutting one another above neural canal, opposite hyposphenes meet ventrally and form a transversely expanded intumescence."  But that's just another form of state 0, as the character's variable condition is abutting versus separated.  Ditto for character 158 ("Postacetabular ala of ilium in lateral view 0: squared 1: acuminate 2: reduced, iliac blade terminates at rectangular end just posterior to level of acetabulum") where new state 2 is still squared like state 0, regardless of how small the process is, and 209 ("Neural spines on caudal dorsal vertebrae in lateral view 0: rectangular or square 1: fan-shaped, with craniocaudally expanded dorsal ends 2: dorsal borders curved, but not craniocaudally expanded") where new state 2 is still unexpanded like state 0 and there should be another character for curvature.  The final example is their 310 ("Pubic peduncle of ilium 0: straight 1: anterior margin straight, posterior margin curved, articular surface ventrally directed 2: anterior margin straight, posterior margin curved, articular surface caudoventrally directed 3: anterior and posterior margins both curved, articular surface caudoventrally directed").  See if you can figure out the issue(s) with that one.

For now, I'm considering Lingyuanosaurus one of several maniraptoriform taxa that belong to a non-pennaraptoran clade but can't be securely placed given the analyzed data.

References- Currie and Russell, 1988. Osteology and relationships of Chirostenotes pergracilis (Saurischia, Theropoda) from the Judith River (Oldman) Formation of Alberta, Canada. Canadian Journal of Earth Sciences. 25(7), 972-986. DOI: 10.1139/e88-097

Zanno, 2010a. Osteology of Falcarius utahensis: Characterizing the anatomy of basal therizinosaurs. Zoological Journal of the Linnaean Society. 158, 196-230. DOI: 10.1371/journal.pone.0198155

Zanno, 2010b. A taxonomic and phylogenetic re-evaluation of Therizinosauria (Dinosauria: Maniraptora). Journal of Systematic Palaeontology. 8(4), 503-543. DOI: 10.1080/14772019.2010.488045

Funston, Mendonca, Currie and Barsbold, 2017. Oviraptorosaur anatomy, diversity and ecology in the Nemegt Basin. Palaeogeography, Palaeoclimatology, Palaeoecology. 494, 101-120. DOI: 10.1016/j.palaeo.2017.10.023

Yao, Liao, Sullivan and Xu, 2019. A new transitional therizinosaurian theropod from the Early Cretaceous Jehol Biota of China. Scientific Reports. 9:5026. DOI: 10.1038/s41598-019-41560-z