Caihong: the iridescent Jurassic troodontid

The preservation in situ is spectacular,
(Figs. 1, 2), but probably pales in comparison to the in vivo appearance of early Late Jurassic Caihong juju (PMoL-B00175 (Paleontological Museum of Liaoning, 161 mya), a new troodontid theropod dinosaur, which includes iridescent feathers.

Figure 1. Skull of Caihong from Hu et al. 2018.

Figure 1. Skull of Caihong from Hu et al. 2018. Arrow points to bony lacrimal crest/protuberance. At a screen resolution of 72 dpi this image of a 6cm long skull is about twice life size.

Caihong differs from other theropods

  1. Accessory fenestra posteroventral to promaxillary fenestra
  2. Lacrimal with prominent dorsolaterally oriented crests
  3. Robust dentary with anterior tip dorsoventrally deeper than its midsection
  4. Short ilium (<50% of the femoral length, compared to considerably >50% in other theropods).

Furthermore,
Caihong shows the earliest asymmetrical feathers and proportionally long forearms in the theropod fossil record. But the coracoids remained short discs. So it was not flapping those long feathered arms. It had extensively feathered toes. (Remember, chicken leg scales are former feathers and otherwise birds are naked beneath their feathers.)

About that unique lacrimal crest…
Note that the parietal has taphonomically moved anterior to the frontal. That’s odd, but it sets up another possibility for that elliptical crest bone. Look how it would precisely fit into the space created by the posterior parietal in dorsal view (Fig. 1). More precise, higher resolution data might provide some insight into this possibility.

Figure 2. Caihong overall in situ. This taxon nests better with Buitraptor, not Xiaotingia.

Figure 2. Caihong overall in situ. This taxon nests better with Buitraptor, not Xiaotingia.

Hu et al. nested Caihong
as a basal deinonyychosaur with the coeval Xiaotingia outside of the Troodontidae, but inside of the clade that includes two Solnhofen birds (only Archaeopteryx and Wellnhoferia). Microraptor, Dromaeosaurus and Rahonavis and others. The cladogram nests long-snouted Buitreraptor with Rahonavis and Unenlagia in an unresolved sister clade to the Xiaotingia/Caihong clade. Only a few nodes had Bootstrap scores higher than 50 and the nodes proximal to Caihong are not among them.

By contrast
the large reptile tree (LRT, 1153 taxa) nests long-snouted Caihong with even longer-snouted Buitreraptor in the troodontid clade that includes Anchiornis and Aurornis, basal to more derived troodontids and ‘Later’ Jurassic Solnhofen birds. Rahonavis and Microraptor nest with therizinosaurs and ornitholestids respectively.

Figure 1. Buitreraptor skull with bones and missing bones colorized.

Figure 3. Buitreraptor skull with bones and missing bones colorized. This skull is over 3x the size of Caihong.

Aurornis (Fig. 4) was basal, Caihong was transitional and Buitreraptor was derived in this clade of small troodontids with increasingly longer rostra.

Figure 1. Eosinopteryx and kin, including Xiaotingia, Aurornis and Archaeopteryx (Thermopolis).

Figure 4. Eosinopteryx and kin, including Xiaotingia, Aurornis and Archaeopteryx (Thermopolis).

Caihong may share these ‘unique’ traits
which are damaged in Buitreraptor. 

  1. Accessory fenestra posteroventral to promaxillary fenestra
  2. Lacrimal with prominent dorsolaterally oriented crests
  3. Robust dentary with anterior tip dorsoventrally deeper than its midsection
  4. Short ilium (<50% of the femoral length, compared to considerably >50% in other theropods).

References
Hu et al. (9 co-authors) 2018. A bony-crested Jurassic dinosaur with evidence of iridescent plumage highlights complexity in early par avian evolution. Nature.com/Nature Communications, 12 pp.  DOI: 10.1038/s41467-017-02515-y

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