Wonderful fossil dinosaur embryo shows birdlike “tucked” posture before hatching

December 24, 2021 • 11:00 am

This is one the most stunning fossils I’ve seen in a long time. It’s an almost perfectly preserved dinosaur embryo that somehow died in the egg during the Late Cretaceous (100 mya-66mya). It’s not just amazing for its preservation, but also for the posture of the unhatched embryo, which resembles the posture that modern bird embryos (an also early birds themselves) assume soon before hatching. The inference is that the behaviors that precede hatching in birds, and help them through the tough process of getting out of the egg, actually evolved from their reptilian ancestors—the theropod dinosaurs, of which this specimen is one.

The paper appears in iScience and is free; click on the screenshot below or get the pdf here.

I’ve really conveyed the gist of the paper in the first paragraph above, but you need to see this embryo! Click to enlarge; all the photos are high-resolution

(from paper): Figure 1. Oviraptorid embryo inside an elongatoolithid egg (YLSNHM01266) Abbreviations: cev, cervical vertebra; cv, caudal vertebra; dv, dorsal vertebra; f, femur; fi, fibula; II-1, pedal phalanx II-1; il, ilium; is, ischium; m, mandible; mt-I, metatarsal I; mt-III, metatarsal III; mx, maxilla; p, pubis; pm, premaxilla; r, radius; s, scapula; t, tibia; ul, ulna. Scale is 1 cm.


The specimen is given the number YLSNHM01266, and is described as a “new non-avian theropod dinosaur embryo. . . from the Late Cretaceous Hekou Formation of southern China.” No species name is given because without a fossil of an adult in the vicinity, we have no idea. We can tell, however, that it is a theropod dinosaur, and an “oviraptorid oviraptorosaur“.

Oviraptors constitute is a group of theropod dinosaurs of varying sizes, which lived in what is now North America and Asia. Fossils show that they had feathers, parrot-like beak mandibles, sometimes bony crests on the head, and walked on their hind legs. Paleontological analysis combined with phylogeny shows, as Wikipedia notes, that they are “close to the ancestry of birds.” (The ancestor of birds is thought by most but not all paleontologists to be theropod dinosaurs.)

Here’s a group of diverse ovoraptors from Wikipedia. You can see that their skeletons are more birdlike than those of other dinosaurs. Some scientists, indeed, group them with birds! Four species have been found with feather impressions, so it’s likely that the group (including the baby above) had feathers, but couldn’t fly. Maybe one of the species below is the adult that would have developed from the juvenile above!

Back to the fossil.  Here’s part of a later figure that helps you make sense of what’s what in the photos above. The air cell, also present in modern bird eggs, is to the right between the embryo and the shell.

If you want the technical description of the posture, here it is from the paper. I’ve bolded the important parts.

The articulated embryonic skeleton is preserved curled inside its egg (YLSNHM01266), with the skull positioned ventral to the body (Figure 1). The egg is elongate ovoid in shape with dimensions of 16.7 cm long by 7.6 cm wide, and has characteristics typical of the egg family Elongatoolithidae (see STAR Methods for eggshell analysis). The skeleton is almost complete, without much apparent postmortem disruption. The anterior surface of the skull faces toward the pointed pole and is situated about egg mid-length at the level of the ilium in-between the flexed hindlimbs, with a pes [foot] on either side. The anterior cervical vertebrae are in line with the long axis of the skull. The presacral vertebral column is strongly bent in an angular manner, so that the upper back of the embryo faces the blunt pole of the egg (similar flexion of the vertebral column is found in modern in ovo skeletons, e.g. Balanoff and Rowe, 2007: Figure 4, Day 18, and is not likely to be a taphonomic artifact). The skeleton is ∼23.5 cm in total length, measured from the anterior tip of the skull to the last preserved caudal vertebra, and occupies nearly the entire width of the egg and most of the length, with the exception of a ∼1.9 cm space between the dorsal vertebrae and the blunt pole of the egg. This space may represent the air cell, a space usually found between the back of the embryo and the blunt pole of bird eggs (e.g., Rahn et al., 1979). However, this inference is tentative and awaits further evidence. The posterodorsal, sacral and caudal vertebrae almost form a straight line along the long axis of the egg. Although the precise developmental stage of the embryo is unclear, it is likely to represent a late-stage embryo because the skeleton is well ossified and is large in size relative to the space inside the egg, as inferred in MPC 100/971 (Norell et al., 2001).

Note that the specimen is 23.5 cm, or a bit more than nine inches long: as long as a dollar bill and half of another one (American dollar bills are almost exactly 6 inches long, and can be used for emergency measurements).

When modern birds hatch, they assume this position as the first of three stages prior to hatching: “pre-tucking”, “tucking” and “posttucking” (we know this clearly because, sadly, many pre-hatched birds have been dissected from the egg). I won’t go through the complicated description of the changes in posture, but here’s how it happens in a chicken, with the fetal dinosaur placed between “pretucking” and “tucking”. “Membrane penetration” is when the bird uses its bill to get out of the membrane in which the embryo is enclosed, and “pipping” is when it begins to peck through the shell (often a long process).

Apparently birds always tuck their heads below their right wing, not their left, before pipping. How they know left from right (genetically) is beyond me; but somehow this asymmetry is coded in the DNA:

And here are three examples of embryonic oviraptors compared to a modern bird (chicken) at the assumed similar stages:

(from paper): Figure 3. In-ovo late-stage embryos of non-avian and avian dinosaurs (A) Oviraptorid specimens (MPC 100/971, YLSNHM01266 & IVPP V20183), which potentially correspond to various tucking stages. (B) Domestic fowl Gallus ontogenetic series (day 16-20) (modified from Rowe (2003)). Not to scale. Silhouettes modified from PhyloPic.

Now the authors are very careful not to overinterpret a single fossil, but I do think it’s likely that the oviraptor fossils show that their pre-hatching positions and behavior was passed on to birds, as oviraptors are phylogenetically close to the ancestor of birds (though we don’t know whether the ancestor of birds was an oviraptor).

The only question remaining is: do all dinosaur embryos—not just those closely related to the ancestor of modern birds—show similar embryonic behavior? The answer is, as usual, we just don’t know. There’s a severe shortage of well-preserved dinosaur embryos, as you might imagine One specimen of a sauropod, a distant relative, seems to show a different fetal posture than the ones above.

I hope we can find more fossil embryos, because, although behavior doesn’t fossilize, the correlates of behavior—represented by the posture of embryos—do. In that sense the way modern birds hatch might what some systematists call a synapomorphy: a character shared by two species (or groups) because it was present in an ancestor—in this case the common ancestor of the ovoraptors and modern birds. And it’s surely an adaptive synapomorphy, because birds that can’t get out of the shell don’t leave any genes behind.


Xing, L. et al. 2021.  An exquisitely preserved in-ovo theropod dinosaur embryo sheds light on avian-like prehatching posturesiScience, in press.

18 thoughts on “Wonderful fossil dinosaur embryo shows birdlike “tucked” posture before hatching

  1. If an ancestral character is retained relatively unchanged for a long time (like this embryo posture), is it really an “atavism”? Pentadactyly in primates like ourselves is an ancestral character relative to all mammals, but would we call it an atavism? I always thought that an atavism refers to a “throwback” or reversal to ancestral character, like a human or other ape born with a post-anal tail, or a horse born with side toes, or a whale with an external hindlimbs. It is useful to distinguish a throwback vs. a character that has simply persisted for a long time with little change? I don’t know; perhaps this is not a useful distinction.

  2. I was amazed by this fossil when you showed it in the Hili dialogue yesterday. Thanks for the deeper analysis. One question I have: why is the fossil pink? I’m sure it’s the mineral composition, but wondering what that might be. I’ve never seen a pink fossil and it adds to the beauty.

    1. It does have a slightly different colour cast to most haematite-stained sandstones. I’d suspect there is a percent or several (of the colourant species, so absolute levels of a few hundredths of a percent) of manganese oxide (psilomelane – https://en.wikipedia.org/wiki/Psilomelane – which, as it’s name suggests, is a pretty dark colour. Manganese has a “trans-” nature in regard of it’s valance (oxidation state) and likes to be mobilised by oxidation and reduction, as you often get around the organic matter in fossils.
      If you happen to have a high-resolution scan of an Archaeopteryx specimen, you’re likely to find “dendritic” growths of psilomelane near joints in the rock. Very similar processes, but concentrated into mm-size granules instead of nanometre clusters of atoms mixed in with iron hydroxy-oxides and carbonates to form the “cement” holding the sand grains in place.

  3. My guess is, there will be some extra holds on the launch and then a cancel due to technical issues or weather. Weather, I heard, was a bit iffy, and if every reading is not totally nominal, they won’t risk the launch. I could be wrong.

    1. “Weather, I heard, was a bit iffy, and if every reading is not totally nominal, they won’t risk the launch.” – with $10 billion of equipment at stake I’d err on the side of caution too! Fingers crossed for a safe launch.

  4. In the illustrations of embryonic head-tucking, I’m seeing the head turned to the right before tucking under the wing occurs. This got me thinking. In mammals, the heart develops from a single tubular structure of mesoderm derived from what will become the throat. It undergoes a complex process of looping, rotation, and folding to form a four-chambered heart that has to be able to shift over from fetal circulation (umbilical blood, bypass the lungs) to post-natal (opposite). Crucially, the initial turn is to the embryo’s right and is of sufficient magnitude in the confined space of the primordial “chest” to deviate the “head” of the embryo to the right. This is of course well before the “heartbeat” appears at 6 weeks so you can imagine how tiny a space we are talking about. If this also occurs in a chick embryo — bird hearts are pretty much the same as ours and the ontogeny must surely be similar — it could well be that the path of least resistance is just to leave the head deviated as the embryo develops and enlarges. When it comes time to tuck, you just tuck under the nearer wing.

    Of course, how the DNA knows to turn the primordial heart loop to the right is still mysterious; it just pushes the mystery further back. But this symmetry-breaking is fundamental to embryological development. The GI tract folds, loops, and rotates in an analogous fashion. And even before left-right symmetry breaks, the embryo from being a ball of cells develops a ventral and a dorsal surface, and soon after a head and a tail end. Consider at the biochemical level that all but the smallest non-planar biomolecules are chiral, that is they are not superimposable on their mirror images, and it is well established that enzymes will recognize only the D- or the L- stereoisomers. For practical purposes, all eukaryotic life uses (and makes) only D-glucose and only L-amino acids, for example. Consequently, anywhere that helices appear in life forms (proteins, nucleic acids), for the same polymer they will twist only one way even though you could in principle assemble their mirror images artificially from their stereoisomers.


    https://www.youtube.com/watch?v=vJA1A0v6Aa4 This is a neat little video animation of the rotation of the GI tract, included because it comes from the Pritzker Medical School at our host’s university.

    https://en.wikipedia.org/wiki/Symmetry_breaking_and_cortical_rotation in frogs, mostly.

  5. Hmmm. Thought provoking.
    Bird hearts are four-chambered like mammalian hearts and unlike squamate (lizard) hearts. Whether they are four-chambered in the same way, I don’t know. Needs a “reptile-ologist” of some stripe.
    The question of whether snakes (or all snakes) also have the same form of four-chambered heart, or two chambered heart would also remain open, as does the question for Rhynchocephalian “lizards” (tuatara, and yes, we do have an outstanding need to know what “mātauranga Māori” has to say about their position in the phylogenetic mosh pit that “reptiles” is – breath not being held on that one), and the nature of hearts in pterosaurs, ichthyosaurs and mammal-like reptiles.
    I’m not a “herp” person myself, but the fact that there are still disagreements about the topology of splitting of various populations of early-reptiles, early dinosaurs, early not-dinosaurs, early mammal-like reptiles and early mammals suggests that either the embryology of hearts, or the fossil record of the embryology of hearts isn’t well-enough known to help, or is uninformative on the question.
    It’s a mess down there, between the development of the amnion and the development of birds. Still nobody knows if endothermy developed once or twice, and has been lost zero, one or two times. But yeah, a bit of developmental history has the potential to cast light into shadows.

  6. I do not mean to bother you, Dr. Coyne. I just wanted to state that the proper term for these dinosaurs are “oviraptorosaurs,” not “oviraptors.”

Leave a Reply