Updated 11/22/2015 with high rez data sent by Dr. Zhou. A new analysis nests the embryo with the holotype Archaeopteryx lithographica, the London specimen, a basal enantiornithine bird.
Zhou and Zhang (2004)
described a small, precocial, final stage bird embryo from the Liaoning Province (Early Cretaceous, 121mya, IVPP V14238). Strangely, no eggshell was preserved (Fig. 1), but the tucked shape of the embryo indicated that it had not yet hatched. Northern China was a forested landscape dominated by active volcanoes and sprinkled with lakes and streams at the time. No adults were closely associated, but enantiornithine birds are common in that formation.
The Zhou and Zhang Abstract
“An embryo of an enantiornithine bird has been recovered from the Lower Cretaceous rocks of Liaoning, in northeast China. The bird has a nearly complete articulated skeleton with feather sheet impressions and is enclosed in egg-shaped confines. The tucking posture of the skeleton suggests that the embryo had attained the final stage of development. The presence of well-developed wing and tail feather sheets indicates a precocial developmental mode, supporting the hypothesis that precocial birds appeared before altricial birds.”
Zhou and Zhang
did not create a reconstruction (Fig.1) nor attempt to untuck the embryo. Bird embryos shift into a tuck position before hatching as they begin to occupy most of the egg. No egg tooth is present on this specimen.
Zhou and Zhang report [with my observations in brackets]:
“The embryo has several enantiornithine apomorphies such as a strutlike coracoid with a convex lateral margin [yes], a V-shaped furcula [maybe], metacarpal III extending well past metacarpal II distally [no], and metatarsal IV being more slender than metatarsals II or III [no].“ My observations were improved with a high resolution image (Fig. 2). The Liaoning embryo nests with the holotype Archaeopteryx (London specimen), which nests at the base of the Enantiornithes.
This is the first
Cretaceous avian embryo preserved with feathers, sheathed, not open vanes. These indicate the embryo was precocial, able to move and feed independently shortly after hatching. This specimen demonstrates that the genus Archaeopteryx survived into the Early Cretaceous.
Compare this bird embryo to a precocial pterosaur embryo or three
like Pterodaustro, the IVPP embryo or the JZMP embryo. Embryo pterosaurs have the proportions of an adult. They grow isometrically. Hatchling birds, like the Liaoning embryo, had juvenile proportions with a large head, short tibia and short metatarsus. They grew allometrically, but not as allometric as living altricial (helpless) bird hatchlings.
“Several previously known theropod embryos and the late Cretaceous avian embryos all seem to be preocial animals, judged purely from skeletal evidence,” Zhou said.
reported, “Zhou said several other enantiornithine species are known from the deposit where the latest fossil was found, but that it was difficult to link the embryo to a specific genus or species.” Unfortunately Zhou and Zhang eyeballed the embyro. They did not attempt a phylogenetic analysis (Fig. 4).
quoted in NatGeoOnline noted that half of the fossil’s characteristics are not exclusive to enantiornithines. He added that characteristics that would identify the fossil an enantiornithine are “either dubious or not well preserved on the specimen. But then, what else could it be?” Padian asked. I agree, but then neither of us has seen the fossil first hand.
Others have warned me
that juveniles and embryo reptiles, like pterosaurs and tritosaurs, cannot be added to phylogenetic analyses because they tend to nest with other adults*. Actually I’d like to see that happen. At present I’m a skeptic. This was a test of that hypothesis, but it was done with a precocial embryo with a relatively larger head, shorter neck and shorter limbs. I don’t see the problem with adding this embryo (Fig. 1) or precocial pterosaur embryos to analyses. But I’m willing to listen to good arguments with valid data.
*Bennett (2006) considered small adult pterosaurs as juveniles of larger germanodactylids based on long bone lengths rather than phylogenetic analysis. Eyeballing, charts and clouds of data points are no replacements for reconstructions and phylogenetic analysis. Hope you agree…
If this is an enantiornithine
which one is it most like? Archaeopteryx lithographica.
If this is an archaeopterygid
we now have some more ontogenetic clues and patterns to work with. You can see (Fig. 1) which body parts get larger and which get smaller during maturation.
Actually it’s both!
Bennett SC 2006. Juvenile specimens of the pterosaur Germanodactylus cristatus, with a review of the genus. Journal of Vertebrate Paleontology 26:872–878.
Zhou Z and Zhang F-C 2004. A Precocial Avian Embryo from the Lower Cretaceous of China. BREVIA Science 22 October 2004: 306 no. 5696 p. 653. DOI: 10.1126/science.1100000. online abstract here