Saturday, April 13, 2019

Imperobator the second named Antarctic Mesozoic theropod

Hi all.  Today we got the accepted manuscript version of Ely and Case's redescription of of the Naze dromaeosaur, initially briefly reported by Case et al. (2007).  The authors name it Imperobator and recover it as a paravian using Gianechini et al.'s (2018) version of Brusatte's TWiG analysis.

Despite the paper being titled "Phylogeny of a new gigantic paravian (Theropoda; Coelurosauria; Maniraptora) from the Upper Cretaceous of James Ross Island, Antarctica", the only hint at the size besides scale bars is "The approximate length of the pedal ungual of digit II in Utahraptor is 10 cm (approximate measurement made by one of the authors based on illustrations provided by Kirkland et al., 1993), and although the distal half of the pedal ungual in Imperobator is missing, an estimate for its total length would be 4-5 cm."  Their figure 7C suggests pedal phalanx IV-1 is 61 mm long, compared to 44.7 mm in Deinonychus specimen AMNH 3015, which would scale to 4.18 m using Paul's (1988) estimate.  For another comparison, the distal tibia is 60 mm wide according to figure 5A, while AMNH 3015's is 63.3 mm.  This would be 2.90 m if scaled to Paul's estimated length.  So Imperobator was ~3-4 meters long as far as I can tell.  Is that gigantic?

Reconstruction of the pes of Imperobator antarcticus (UCMP 276000) after Ely and Case (2019).

As regards the anatomy, I feel the authors missed an opportunity to fully figure this taxon known from multiple three dimensionally preserved fragments.  The initial paper just had a single photo of the reconstructed pes in anterior view, and this one has single perspectives for all but one of the fragments (distal mtIII is shown in anterior and posterior views).  Case et al. stated

"The dentition is poorly preserved except for two teeth that were preserved in a fragment of concretion. The teeth and the associated fragments all indicate long, narrow biconvex teeth. This shape suggests that both anterior and posterior carina were present; however, no serrations were noted on the carina, thus it is impossible to determine if serrations were present or absent. The teeth are incipiently laterally compressed, but retain a rounded outline, particularly anteriorly. The shape is indicative of teeth from the anterior region of the jaw."

... but here no teeth are even mentioned.  If they couldn't be definitely associated with the hindlimb, this should have at least been stated.  Their figure 4 showing the reconstructed pes is humorously bad as someone seemingly traced a photograph with no concern for theropod phalangeal proportions, so that digit III is shortest and phalanx IV-1 is three times longer than other phalanges in that digit.  I mean, if you have a decade to make a paper you can draw a realistic theropod foot.  If the figure's accurate, phalanx IV-1 is extremely robust, bringing to mind Austroraptor.  Contra the caption for figure 7 (which is wrong, as C is "Proximal phalanx of pedal digit IV in dorsal view" and E is "Distal articular surface of a phalanx possibly pertaining to digit III (?)"), no phalangeal part shown is likely to be from pedal digit I as all are about equal in width to digits III/IV.  This makes their statement

"Potential material from digit I may be present (Fig. 7D). It is distinguished by what may be a prominent flexor heel on the proximoventral surface, morphologically similar to that of the dromaeosaurid (avialan?) Balaur bondoc (Csiki et al. 2010)."

... confusing, as 7D seems to be the proximal half of a non-ungual phalanx positioned as III-1 in Case et al. (2007).  In the materials list, Ely and Case also state "material from metatarsal I, and even metatarsal V may be preserved", but we get no description or illustration of these.  Note contra line 428, fibular fusion is absent in Buitreraptor and Graciliraptor.

Holotype of Imperobator antarcticus (UCMP 276000) as photographed by Case et al. (2007).  Distal phalanges of digits III and IV not described by Ely and Case (2019), and distal metatarsal IV and phalanx IV-1 missing.

As regards the phylogeny, Ely and Case find Imperobator to fall out in Paraves in a polytomy with eudromaeosaurian and anchiornithine OTUs.  They reran this as a Bayesian analysis which favored placing it among anchiornithines, although exclusion from Dromaeosauridae was just barely supported at 50%.  While the authors seem to favor a basal position, as in their 2016 SVP abstract where it emerged as a basal deinonychosaur, their analyses suggests an anchiornithine troodontid or eudromaeosaurian dromaeosaurid placement are about equally supported. 

Imperobator was included in the Lori analysis (as the 'Naze dromaeosaur') and had a highly unstable position.  Rescoring it given the new paper and assuming the teeth cannot be assigned to the taxon, it now emerges as a halszkaraptorine.  Interesting.

References- Paul, 1988. Predatory Dinosaurs of the World. Simon and Schuster. 464 pp.

Case, Martin and Reguero, 2007. A dromaeosaur from the Maastrichtian of James Ross Island and the Late Cretaceous Antarctic dinosaur fauna. in Cooper and Raymond (eds). Antarctica: A Keystone in a Changing World – Online Proceedings of the 10th ISAES X. USGS Open-File Report 2007-1047, Short Research Paper 083, 4 pp. DOI: 10.3133/ofr20071047SRP083

Gianechini, Makovicky, Apesteguía and Cerda, 2018. Postcranial skeletal anatomy of the holotype and referred specimens of Buitreraptor gonzalezorum Makovicky, Apesteguía and Agnolín 2005 (Theropoda, Dromaeosauridae), from the Late Cretaceous of Patagonia. PeerJ. 6:e4558. DOI: 10.7717/peerj.4558

Ely and Case, 2019. Phylogeny of a new gigantic paravian (Theropoda; Coelurosauria; Maniraptora) from the Upper Cretaceous of James Ross Island, Antarctica. Cretaceous Research. DOI: 10.1016/j.cretres.2019.04.003

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