A new paravian dinosaur from the Late Jurassic of North America supports a late acquisition of avian flight

Abstract

The last two decades have seen a remarkable increase in the known diversity of basal avialans and their paravian relatives. The lack of resolution in the relationships of these groups combined with attributing the behavior of specialized taxa to the base of Paraves has clouded interpretations of the origin of avialan flight. Here, we describe Hesperornithoides miessleri gen. et sp. nov., a new paravian theropod from the Morrison Formation (Late Jurassic) of Wyoming, USA, represented by a single adult or subadult specimen comprising a partial, well-preserved skull and postcranial skeleton. Limb proportions firmly establish Hesperornithoides as occupying a terrestrial, non-volant lifestyle. Our phylogenetic analysis emphasizes extensive taxonomic sampling and robust character construction, recovering the new taxon most parsimoniously as a troodontid close to Daliansaurus, Xixiasaurus, and Sinusonasus. Multiple alternative paravian topologies have similar degrees of support, but proposals of basal paravian archaeopterygids, avialan microraptorians, and Rahonavis being closer to Pygostylia than archaeopterygids or unenlagiines are strongly rejected. All parsimonious results support the hypothesis that each early paravian clade was plesiomorphically flightless, raising the possibility that avian flight originated as late as the Late Jurassic or Early Cretaceous.

Keywords: Avialae; Dinosauria; Hesperornithoides miessleri; Late Jurassic; Morrison Formation; Origin of birds; Paraves.

Figure 1. Geographic relationship of the Jimbo Quarry and the majority of the Morrison Formation, Late Jurassic, USA.

Formation outcrop and map data based on paleobiodb.org.

Figure 2. Condensed stratigraphic sections demonstrating the lateral variability near the Jimbo Quarry.

“?” indicate loss of direct lateral correlation due to covered section. 1 = Jimbo Quarry; 2 = Lori locality; 3 = marginal wetland deposits.

Figure 3. Reconstructed quarry map of WYDICE-DML-001.

Association of skeletal elements assembled from 3D scans of specimen blocks prior to final mechanical preparation. Scale bar = 6 cm.

Figure 4. Primary blocks of WYDICE-DML-001.

“Left” (A) and “right” (B) sides of the blocks after final preparation (B). Scale bar = one cm. Images taken by Levi Shinkle, used with permission.

Figure 5. Rigorous skeletal reconstruction of WYDICE-DML-001.

Scale bar = 25 cm.

Figure 6. Skull block and interpretive drawing.

Skull block in left lateral (A and B) and right lateral (C and D) views. Abbreviations: An, angular; Aof, antorbital fenestra; Cd, mid caudal vertebrae; Cv, cervical vertebra; Den, dentary; Emf, external mandibular fenestra; Hyo, hyoids; Hum, humerus; Lju, left jugal; Llc, left lacrimal; Max, maxilla; Mc2, metacarpal II; Mxf, maxillary fenestra; Pal, palatine; Pmf, promaxillary fenestra; Qd, quadrate; Qj, quadratojugal; Raf, ridge under antorbital fossa; Rju, right jugal; Rlc, right lacrimal; Rn, right nasal; Sa, surangular. Scale bar = five cm. Photo credit Levi Shinkle, used with permission.

Figure 7. Segmented left maxilla of WYDICE-DML-001.

Shown in lateral (A) and medial (B) views.

Figure 8. Segmented right lacrimal of WYDICE-DML-001.

Shown in lateral (A), posterior (B), and medial (C) views.

Figure 9. Segmented left jugal of WYDICE-DML-001.

Shown in medial (A), lateral (B), and anterolateral oblique (C) views.

Figure 10. Paraquadrate foramen of WYDICE-DML-001.

Paraquadrate foramen and inset detail showing the contribution of the quadrate to the paraquadrate foramen. Abbreviations: Pqf, paraquadrate foramen; Qd, quadrate; Qj, quadratojugal.

Figure 11. Mandible and comparative tooth morphology of Koparion and Hesperornithoides.

Tooth of Koparion in “side” (A) and mesial (B) views, and a detail of the distal serrations (C). Hesperornithoides maxillary tooth in labial view (D), rendered from CT data in mesial view (E) and a detail of the distal serrations (F). Segmented CT data of fully erupted tooth (G) and anterior mandibular elements of Hesperornithoides in oblique view (H). Photo credit (A–C): Dan Chure.

Figure 12. Select axial elements of WYDICE-DML-001.

Cervical vertebra three in dorsal (A) and right lateral (B) views. (C) Mid-cervical vertebra in cross-section. Photo and schematic of association between axis and cervical three (D and E). (F) Cervicodorsal in right lateral view. Articulated middle (G) and distal (H) caudal vertebrae in right lateral views.

Figure 13. Segmented and interpretive drawing of distal caudal vertebrae of WYDICE-DML-001.

In dorsal (A), right lateral (B) and ventral (C) view. Anterior is to the right in all views. Abbreviations: Chv, chevron; Przg, prezygapophysis; Pszg, postzygapophysis. Scale bar = one cm.

Figure 14. Forelimb elements of WYDICE-DML-001.

Segmented left carpals and metacarpals in oblique (A) and extensor (B) views. (C) Distal portion of right MC III in lateral view. (D) Distal end of right humerus in anterior view. Proximal end of left humerus in proximal (E) and lateral views (F).

Figure 15. Unguals of WYDICE-DML-001.

(A) Left manus ungual I in medial view. (B) Left manus ungual II in medial view. (C) Left manus ungual III in medial view. (D) Pedal ungual II (mirrored for ease of comparison) with trenchant sickle morphology. Scale bar = one cm.

Figure 16. Select hindlimb elements of WYDICE-DML-001.

Proximal end of left metatarsal packet in posterior (A) and anterior (B) views. (C) Right metatarsal I in medial view. (D) Cross-section through left metatarsals from distal perspective, anterior is to the bottom. (E) Enlarged pedal ungual II: material outside the dotted line is inferred to be preserved sheath material. (F) Cross-section through the left tibia at mid-shaft.

Figure 17. Summary diagram of the findings from this phylogenetic analysis.

Strict consensus tree of maniraptoromorphs after a posteriori pruning with higher level taxa condensed (length = 12,123). The uncondensed tree and positions of pruned taxa can be seen in the Supplemental Data.

Figure 18. Partially expanded, time calibrated phylogenetic results.

Clades containing potentially volant taxa (red) are expanded to show their position nested within flightless taxa (black). Taxa exhibiting aerial locomotion directly connected to crown clade Aves are colored blue. Zhongornis is colored purple to reflect the uncertainty revolving around this juvenile specimen. Barred lineages indicate uncertainty in age (Xiaotingia) or referred taxa (Koparion to troodontids and non-Jehol taxa to Confuciusornithiformes).