[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.
DOI: 10.1038/srep20478
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Abstract
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
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Figure 1: Skeletal silhouette of FPDM-V8461 and the stratigraphic section of the Lower Cretaceous Kitadani Formation in the Kitadani Dinosaur Quarry.
DOI: 10.1038/srep20478
|
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
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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
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