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New Archaeorhynchus (basal ornithuromorph) adult specimen described

Ben Creisler

A new paper:

Min Wang & Zhonghe Zhou (2016)
A new adult specimen of the basalmost ornithuromorph bird
Archaeorhynchus spathula (Aves: Ornithuromorpha) and its implications
for early avian ontogeny.
Journal of Systematic Palaeontology (advance online publication)

Most living birds characteristically grow rapidly and reach adult size
within a year. Nevertheless, little is known about how such an
advanced developmental strategy evolved despite many discoveries of
early fossil birds. Here we assess the long-bone histology from a new
adult specimen of Archaeorhynchus spathula, the basalmost taxon of
Ornithuromorpha. Ornithuromorpha is the most inclusive clade
containing extant birds but not the Mesozoic Enantiornithes.
Histological analysis reveals that the cortex is composed of
parallel-fibred bone with three lines of arrested growth, indicative
of slow and annually interrupted growth for this taxon. Such bone
histology is significantly different from that of other known basal
ornithuromorphs, but resembles that of enantiornithines, which leads
us to suggest protracted slow growth in the common ancestor of
Ornithuromorpha and Enantiornithes. The fusion sequence of the
tarsometatarsus between Enantiornithes and Ornithuromorpha has long
been hypothesized to be different and regarded as indicative that
enantiornithines are not close relatives of ornithuromorphs. Due to a
lack of fossils recording early ontogenetic stages, little is known
about the development of the tarsometatarsus in basal ornithuromorphs,
making this hypothesis impossible to test. Here we show that the
fusion sequence of the tarsometatarsus in Archaeorhynchus is similar
to that of enantiornithines, and that the proximal-early fusion in
tarsometatarsus represents a plesiomorphic trait for basal birds. Our
findings also shed light on ontogenetic variation of sternal
morphology, which highlights the importance of ontogeny in the
taxonomic and phylogenetic study of early birds.