Wednesday, February 24, 2016

[Paleontology • 2016] Fukuivenator paradoxus • A Bizarre Theropod from the Early Cretaceous of Japan Highlighting Mosaic Evolution among Coelurosaurians

Fukuivenator paradoxus
Azuma, Xu, Shibata, Kawabe, Miyata & Imai, 2016

Figure 1: Skeletal silhouette of FPDM-V8461 and the stratigraphic section of the Lower Cretaceous Kitadani Formation in the Kitadani Dinosaur Quarry.

Our understanding of coelurosaurian evolution, particularly of bird origins, has been greatly improved, mainly due to numerous recently discovered fossils worldwide. Nearly all these discoveries are referable to the previously known coelurosaurian subgroups. Here, we report a new theropod, Fukuivenator paradoxus, gen. et sp. nov., based on a nearly complete specimen from the Lower Cretaceous Kitadani Formation of the Tetori Group, Fukui, Japan. While Fukuivenator possesses a large number of morphological features unknown in any other theropod, it has a combination of primitive and derived features seen in different theropod subgroups, notably dromaeosaurid dinosaurs. Computed-tomography data indicate that Fukuivenator possesses inner ears whose morphology is intermediate between those of birds and non-avian dinosaurs. Our phylogenetic analysis recovers Fukuivenator as a basally branching maniraptoran theropod, yet is unable to refer it to any known coelurosaurian subgroups. The discovery of Fukuivenator considerably increases the morphological disparity of coelurosaurian dinosaurs and highlights the high levels of homoplasy in coelurosaurian evolution.

Systematic palaeontology

Dinosauria Owen, 1842
Theropoda Marsh, 1881
Maniraptora Gauthier, 1986

Fukuivenator paradoxus gen. et sp. nov.

Etymology: Fukui” refers to Fukui Prefecture in the central Japan, where the specimen was recovered; “venator”, a Latin word for hunter; the species name refers to the surprising combination of characters in this theropod dinosaur.

Figure 2: Cranial skeletal morphology of FPDM-V8461.
 (a) Partial right premaxilla in lateral view. (b) Partial left maxilla in lateral view. (c) Partial left lacrimal in lateral view. (d) Right frontal in dorsal view. (e) Right postorbital in lateral view. (f) Left squamosal in lateral view. (g) Partial right dentary in lateral view.
Scale bar = 10 mm. Abbreviations: aofe, antorbital fenestra; en, external naris; mxfe; maxillary fenestra, prfe; premaxillary fenestra.   DOI: 10.1038/srep20478

Figure 1: Skeletal silhouette of FPDM-V8461 and the stratigraphic section of the Lower Cretaceous Kitadani Formation in the Kitadani Dinosaur Quarry.

Holotype: FPDM-V8461 (Fukui Prefectural Dinosaur Museum), a disarticulated but closely associated skeleton found within a 50 by 50 cm area, preserved elements include: incomplete right premaxilla with two isolated premaxillary teeth, left maxilla with one isolated and four intact teeth, left lacrimal, right jugal, right postorbital, left squamosal, both frontals, braincase, possible left ectopterygoid, left pterygoid, right palatine, posterior part of right dentary with two intact dentary teeth, eight cervical vertebrae, 10 dorsal vertebrae, five sacral vertebrae, and 30 caudal vertebrae, several cervical ribs, dorsal ribs, gastralia and chevrons, nearly complete scapulas and coracoids, most of both forelimbs, portions of both pubes, partial left ischium, and nearly complete hindlimbs (Fig. 1).

Figure 1: Skeletal silhouette of FPDM-V8461 and the stratigraphic section of the Lower Cretaceous Kitadani Formation in the Kitadani Dinosaur Quarry.
Skeletal silhouette shows preserved bones in dark grey and missing bones in light grey. Positions of notable fossils including FPDM-V8461 are shown in the stratigraphic section.
 Scale bar = 50 mm. Abbreviations: cl, claystone; sl, siltstone; vfs, very fine sandstone; fs, fine sandstone; ms, medium sandstone; cs, coarse sandstone.   DOI: 10.1038/srep20478

Type locality and horizon: FPDM-V8461 was found in the Kitadani Dinosaur Quarry, which is on the Sugiyama River in the northern part of the city of Katsuyama, Fukui, Japan (36° 7′ 17.9″ N, 136° 32′ 41.4″ E) (see Supplementary Figs. S1 for quarry maps, and S2 for a field photograph). The quarry is locally referred to the Lower Cretaceous Kitadani Formation (Akaiwa Subgroup, Tetori Group). The age of the Kitadani Formation could be assigned to the Barremian to the Aptian on the basis of occurrences of the freshwater mollusk Nippononaia ryosekiana17 and charophytes18, and radioisotopic dates obtained from related sedimentary units (127–115 Ma?)19.

Diagnosis: A relatively small theropod with the following unique features: 1. unusually large external naris (slightly smaller than antorbital fenestra in dorsoventral height); 2. large premaxillary fenestra subequal in size to maxillary fenestra; 3. large oval lacrimal pneumatic recess posterodorsal to the maxillary fenestra on antorbital fossa medial wall; 4. lacrimal with a distinct groove on lateral surface of anterior process and a ridge on lateral surface of descending process; 5. postorbital frontal process with T-shaped-cross section and laterally-flanged squamosal process; 6. an elongate tubercle on posterior surface of basal tuber of the basicranial region; 7. highly heterodont dentition featuring robust unserrated teeth including small spatulate anterior teeth, large and posteriorly curved middle teeth, and small and nearly symmetrical posterior teeth; 8. cervical vertebrae with a complex lamina system surrounding the neural canal resulting in deep and wide grooves for interspinous ligaments and additional deep sockets; 9. anterior cervical vertebrae with interprezygapophyseal, postzygadiapophyseal, prezygadiapohyseal, and interpostzygapophyseal laminae connecting to each other to form an extensive platform; 10. anterior and middle cervical vertebrae with transversely bifid neural spines; 11. dorsal, sacral, and anterior caudal vertebrae with strongly laterally curved hyposphene and centropostzygapophyseal laminae that, together with the postzygapophyseal facet, form a socket-like structure for receiving the prezygapophysis; 12. dorsoventrally bifurcated sacral ribs; 13. caudal zygapophyseal facets expanded to be substantially wider than the zygapophyseal processes; and 14. middle caudal vertebrae with transversely and distally bifid prezygapophyses.

 Yoichi Azuma, Xing Xu, Masateru Shibata, Soichiro Kawabe, Kazunori Miyata and Takuya Imai. 2016.  A Bizarre Theropod from the Early Cretaceous of Japan Highlighting Mosaic Evolution among Coelurosaurians. Scientific Reports.  6(20478); DOI: 10.1038/srep20478