Liaoningvenator: Bird-like troodontid? Or troodontid-like bird?

Shen et al. 2017 describe
a new troodontid, Liaoningvenator curriei (DNHM D3012; Dalian Natural History Museum; Figs. 1-2; Early Cretaceous), they nest Liaoningvenator outside of the Aves (birds).

Figure 1. Liaoningvenator has a long neck and short torso. It nests as a secondarily flightless bird in the LRT, rather than as a troodontid.

Figure 1. Liaoningvenator has a long neck and short torso. It nests as a secondarily flightless bird in the LRT, rather than as a troodontid.

From the abstract:
“A new troodontid, Liaoningvenator curriei gen. et sp. nov., is described based on a complete skeleton from the Early Cretaceous Yixian Formation of Beipiao City, Liaoning Province. It bears the following characteristics of Troodontidae: numerous and more closely appressed maxillary and dentary teeth; the teeth markedly constricted between the roots and crowns; the nutrient foramina in groove on the external surface of dentary; distal caudal vertebrae having a sulcus on the dorsal midline rather than a neural spine. Unlike other troodontids, Liaoningvenator exhibits a sub-triangular ischial boot in lateral view and slender ischial obturator process; transition point in caudal vertebrae starts from the seventh caudal vertebra. A phylogenetic analysis recovers Liaoningvenator and Eosinopteryx as sister taxa that belong to the same clade.”

Figure 2. Troodontid-like birds and bird-like troodontids shown together to scale.

Figure 2. Troodontid-like birds and bird-like troodontids shown together to scale. Note the robust hind limbs  in the secondarily flightless birds, Jianianhualong and Liaoningvenator.

By contrast,
the large reptile tree (LRT, 1011 taxa) nests Liaoningvenator with Jianianhualong as a large flightless basal sapeornithid bird—and all birds nest within the Troodontidae. Size-wise Liaoningvenator is midway between the smaller Archaeopteryx recurva (Fig. 2) and the larger Jianianhualong. So this might be a transitional taxon between the two.

Unrelated
Eosinopteryx (Fig. 2) continues to nest outside of Aves (birds). Distinct from Eosinopteryx, Liaoningvenator has a much shorter torso and much longer neck, as in other birds. Like Jianianhualong metarsal 4 is longer than 3 in Liaoningvenator, among many other traits (see below). Shen et al. did not mention Jianianhualong, probably because the two taxa were published within a few weeks of each other. You might remember earlier Xu et al. 2017 also nested Jianianhualong with the non-avian troodontids. Shen et al. included Sapeornis in their phylogenetic analysis. Not sure why they nested apart in the LRT.

A reconstruction of the Liaoningvenator skull
(Fig. 2) has a large openings and gracile bones. What Shen et al. identified as a maxillary foramen is identified here as the base of the naris. The in situ tail curls anteriorly and several caudal vertebrae are visible over the torso.

From the Shen et al. diagnosis:
“A new troodontid dinosaur bears the following unique combination of characters including autapomorphies indicated with an asterisk and new characters indicated with a double asterisk: prominent slender triradiate postorbital*; deltopectoral crest distinctly extended to the half of the humeral shaft*; no posterior process on the dorsodistal end of ischium**; slender obturator process of ischium**; manual phalanx I-1 longer than metacarpal II**, the length ratio of phalanx I-1 to metacarpal II about 1.49**; the width of metatarsus distally distinctly decrease**; transition point in caudal series starts from the seventh caudal vertebra**.

Troodontid or not?
The large flightless basal birds share a long list of traits in common with troodontids and a few that show they are distinct. Here is a list of the differences between bird-like troodontids, like Sinornithoides and Anchiornis, and the troodontid-like sapeornithid birds, like Jianianhualong and Liaoningvenator.

Liaoningvenator bird traits not shared with non-avian troodontids:

  1. Ventral aspect of premaxilla > 1/3 preorbit length
  2. Ascending process of premaxilla extends beyond naris and contacts frontals (nasal separated)
  3. Lacrimal deeper than maxilla
  4. Major axis of naris 30-90º
  5. Posterolateral premaxilla absent (also in Xiaotingia and Eosinopteryx)
  6. Nasals not longer than frontals (also in Xiaotingia and Eosinopteryx)
  7. Antorbital fenestra without fossa
  8. Manual mc2 and 3 do not align with joints on digit 1
  9. Metatarsal 5 not shorter than pedal digit 5

Shifting
Liaoningvenator and Jianianhualong to Sinornithoides adds 14 steps.

Paul 2002
considered the possibility of secondarily flightless (neoflightless) birds, unfortunately without the benefit of a phylogenetic analysis. Paul wrote: “Reversal normally associated with loss of flight is observed in ornithomimids, therizinosaurs and dromaeosaurs.” The LRT found possibly volant bird-like taxa associated with therizinosaurus (Rahonavis), Ornitholestes (microraptorids) and troodontids (birds), but not ornithomimids (related to Compsognathus) and dromaeosaurs (related to Shuvuuia).

Paul wrote:
“The less sharply flexed, broad coracoids of flightless birds recapitulate the dino-avepod condition. The loss of any sternal keel and shortening of the arms area also normal reversals for flightless birds. The semilunate carpal block and arm folding mechanism…are sometimes lost in flightless birds.”

References
Paul G 2002. Dinosaurs of the Air. Johns Hopkins Press
Shen C-Z, Zhao B, Gao C-L, Lü J-C and Kundrat 2017. A New Troodontid Dinosaur (Liaoningvenator curriei gen. et sp. nov.) from the Early Cretaceous Yixian Formation in Western Liaoning Province. Acta Geoscientica Sinica 38(3):359-371.
Xu X, Currie P, Pittman M, Xing L, Meng QW-J, Lü J-C, Hu D and Yu C-Y 2017. Mosaic evolution in an asymmetrically feathered troodontid dinosaur with transitional features. Nature Communications DOI: 10.1038/ncomms14972.

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