A bizarre theropod from the Early Cretaceous of Japan highlighting mosaic evolution among coelurosaurians

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.

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.

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.

Figure 3. Premaxillary and maxillary teeth of FPDM-V8461.

(a) Isolated anteriormost? premaxillary tooth in lateral (left) and posterior (right) views. (b) Enlarged view of the crown in (a) lateral (left), anterior (middle), and posterior (right) views. (c) Crown of an isolated left premaxillary tooth in distal (left), lateral (middle), mesial (right) views with an enlarged view of the mesial “ridge” (red box). (d) Intact left maxillary teeth in lateral view arranged as those in Fig. 2b. Scale bar = 10 mm for (a,d), 7.5 mm for (b), and 12.5 mm for (c).

Figure 4. Braincase of FPDM-V8461 with interpretive drawings.

Images in right lateral (left), posterior (middle) and ventral (right) views. Scale bar = 20 mm. Abbreviations: bo, basioccipital; fm, foramen magnum; ls, laterosphenoid; mef, medial eustachian foramen; oc, occipital condyle; pp, paroccipital process; ps, parasphenoid; so, supraoccipital. Roman numerals indicate positions of corresponding cranial nerves.

Figure 5. Comparisons of the inner ears of Fukuivenator and other selected animals.

Inner ears of (a) Fukuivenator paradoxus, (b) Allosaurus fragilis, (c) Tyrannosaurus rex, (d) Gorgosaurus libratus, (e) Struthiomimus altus, (f) Falcarius utahensis, (g) Deinonychus antirrhopus, (h) Archaeopteryx lithographica, (i) Gambelia wislizenii, and (j) Aythya fuligula. Note that the caudal semicircular canal runs behind the lateral semicircular canal in (a) (red arrow). Plots: comparative proportions of the inner ears of Fukuivenator (red squares) against those of selected modern birds (blue circles), Archaeopteryx (purple circle), non-avian archosaurs (green triangles) and non-archosaur reptiles (brown diamonds), modified from Domínguez et al. 2004 (ref. 69). Scale bar = 20 mm for (a) and (e), 10 mm for (b–d) and (f,g), and 30 mm for (h–j). Abbreviations: % caudal canal height, height from the base of the caudal canal to the plane of the lateral canal/height of the caudal canal; c, cochlear duct; crc, crus communis; csc, caudal semicircular canal; lsc, lateral semicircular canal; rsc, rostral semicircular canal. References: (a) this paper; (b–e,g,h) Witmer & Ridgely, 2009 (ref. 37); (f) Lautenschlager et al., 2012 (ref. 70); (i,j) Walsh et al., 2009 (ref. 68).

Figure 6. Postcranial axial skeletal morphology of FPDM-V8461.

Anterior (a), middle (b), and posterior (c) cervical vertebrae in lateral (top), anterior (middle), and dorsal (bottom) views. Anterior (d), middle (e), and posterior (f) dorsal vertebrae in lateral (top), anterior (middle), and dorsal (bottom) views. (g) Sacral vertebrae in lateral (top), dorsal (middle), and ventral (bottom) views. Anterior (h), middle (i), and posterior (j) caudal vertebrae in lateral (top), anterior (middle), and dorsal (bottom) views. Scale bar = 25 mm for (a–f,h), 50 mm for (g), and 20 mm for (i,j). Note that the anterior cervical vertebra (a) is reconstructed based on CT-scanned images because it is severely fused to the braincase beyond preparation. Abbreviations: dp, diapophysis; ep, epipophysis; in, axis intercentrum; li, attachment scar of interspinal ligament; pl, pleurocoel; po, postzygapophysis; pp, parapophysis; pr, prezygapophysis; prr, prezygapophyseal rod.

Figure 7. Appendicular skeletal morphology of FPDM-V8461.

(a) Left coracoid (left) and scapula (right) in lateral view. (b) Left humerus in lateral (left) and posterior (right) view. (c) Left ulna in lateral (left) and posterior (right) view. (d) Left radius in lateral (left) and posterior (right) view (red dotted lines indicates the boundary between the radius and manual ungual? fused to each other). (e) Right manual elements in dorsal (left) and lateral (right) views. (f) Right femur in lateral (left) and posterior (right) view. (g) Left tibia in lateral (left) and anterior (right) views. (h) Right pedal elements in dorsal (left) and lateral (right) views. Scale bar = 50 mm. Note that the right metetarsals in (h) are reconstructed based on CT-scanned images because they are severely fused to each other. Abbreviations: acr, acromial process; cf, coracoid foramen; g, glenoid; mtV, metatarsal V.

Figure 8. Suggested systematic position of Fukuivenator paradoxus within the Coelurosauria.