Nature | Vol 579 | 12 March 2020 | 249The largely akinetic skull morphology of Oculudentavis suggests a bite
force that is relatively stronger than that of a neornithine of the same size^32 ,
which is supported by the unusual degree of cranial fusion^33 , the presence
of a well-developed coronoid process and the absence of an antorbital
fenestra. A distinct coronoid process is not developed in any other Meso-
zoic bird but it is well-developed in predatory birds of the Cenozoic era,
such as phorusrhacids^34. In archosaurs, this process provides attachment
for adductor muscles^35. Reduction in pneumatic features may suggest that,
in the Oculudentavis lineage, solid bone was favoured biomechanically
over opportunistic pneumatization^36. This reinforcement of the skull may
reflect the predatory lifestyle inferred for Oculudentavis. Alternatively, the
loss of the antorbital fenestra may be a consequence of miniaturization,
and provide additional space for the enlarged orbit^9.
HPG-15-3 preserves no features that unambiguously indicate its phy-
logenetic position, although the sum of the observed morphologies
suggests that the taxon falls outside Ornithuromorpha. The extensive
tooth row resembles noncoelurosaur theropods^26 and the ornithurine
Ichthyornis^12 , whereas the presence of premaxillary teeth (which are
absent in the ornithurines Hesperornis and Ichthyornis), complete
infratemporal and supratemporal fenestrae (which are typically absent
in Ornithothoraces)^37 , the quadrate with a broad orbital process (which
is typically narrow in Ornithuromorpha) and the absence of a preden-
tary (which is present in toothed Cretaceous ornithuromorphs)^38 all
suggest a more-basal position. However, nonornithothoracine avian
lineages (for example, Jeholornis and Sapeornis) are typically much
larger than Cretaceous ornithothoracines—particularly the enantio-
rnithines. Miniaturization is strongly associated with extreme homo-
plasy, which obfuscates phylogeny^9. Our cladistic analysis using a priori
weights produces a poorly resolved consensus tree in which all birds
more derived than Archaeopteryx form a polytomy (Extended Data
Fig. 10a). Approximately 90% of all equal-length trees resolve Oculuden-
tavis as more derived than Archaeopteryx but basal to Jeholornis; the
remaining 10% place it within Enantiornithes. Using implied weights
(which is recommended for taxa characterized by strong homoplasy,
including Aves^39 –^41 ), our analysis consistently resolves Oculudentavis as
intermediate between Archaeopteryx and Jeholornis (Fig. 4 , Extended
Data Fig. 10b, Supplementary Information). If this hypothesis were to be
supported by additional postcranial data, HPG-15-3 could possibly rep-
resent the first nonenantiornithine bird in the avifauna in the Hukawng
valley deposits. Given the unusual morphology of Oculudentavis (which
is clearly unlike that of any other bird), there is a strong potential for
new data to markedly alter our systematic conclusion. Regardless, this
discovery highlights the capacity of amber to document vertebrate
diversity that is unrecorded by other depositional environments.
Note added in proof: Since this paper was accepted, an eight specimen
that preserves the skeletal remains of an enantiornithine bird has been
described^42.
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maries, source data, extended data, supplementary information,
acknowledgements, peer review information; details of author con-
tributions and competing interests; and statements of data and code
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