Early in 2009 a team of paleontologists led by Philip Gingerich announced the discovery of a baby archaeocete (early whale) embedded inside the skeleton of an adult of the same species. Since these fossils represented a new species of fossil whale to boot the story was immediately picked up by news outlets, but less well-publicized was another discovery made later the same year. In the pages of Comptes Rendus Palevol paleontologists Alfredo Zurita, Angel Mino-Boilinia, Esteban Soibelzon, Gustavo Scillato-Yane, German Gasparini, and Freddy Paredes-RÃos described an unborn specimen of a very different kind of mammal; an armored relative of living armadillos called Glyptodon.
Even though actual papers describing them are few and far between, paleontologists have found a number of fetal-to-juvenile specimens of large mammals from the Pleistocene (~2.5 million years ago to 12,000 years ago). Of the few that have been reported most belong to different genera of ground sloths, but up until this new study no one had described a fetal glyptodont before. Evolutionary cousins of the ground sloths, glyptodonts can be thought of as extensively armored ground sloths with hard bowl-shaped shells and club-like tails (sometimes covered in knobs of bone). Given that the shell was among the hardest parts of the glyptodont body they are among the most frequently recovered parts of the skeleton, and while many are broken up into fragments a few have made it to the present day more-or-less intact.
Such was the case with part of a shell found in the Tarija Valley in the southern part of Bolivia. It seemed to be relatively well-preserved, with no sign that it had been transported after death, and inside that shell the researchers found the bones of a tiny glyptodont. The details of the adult shell allowed the researchers to narrow down its identification to the genus Glyptodon, and given its state of preservation and association of the remains it was reasonable to propose that the small bones inside were those of an unborn baby. (Glyptodon was herbivorous, so it is unlikely that the small bones represent a last meal.)
Unfortunately, however, the bones of the unborn Glyptodon were not exceptionally preserved. The bones consisted of two parts of the upper jaw/cheek, left half of the lower jaw, parts of the right and left shoulderblades, and the shafts of the right of left femurs (see image to the left). This state of preservation is not all that surprising. Portions of the little Glyptodon skeleton would have still been made up of cartilage and would have less of a chance of becoming preserved. This is most immediately apparent when looking at the femur shafts. The parts of the femurs that would articulate with the hips and lower leg bones are missing. This is because those parts of the femur were not completely ossified (or the cartilage had not fully been replaced by bone yet) and so they decayed away.
The kind of descriptive work the authors of this study engaged in might not make the New York Times, but it is still very important. It is vital to get specimens like this into the record so that comparisons can be made and more synthetic studies can be undertaken. As the paper notes there are plenty of similar specimens out there waiting to be described, fossils that could help us understand how these strange mammals grew during the early parts of their lives, and I hope that an effort is made to get more of them into the literature.
Zurita, A., MiÃ±o-Boilini, A., Soibelzon, E., Scillato-YanÃ©, G., Gasparini, G., & Paredes-RÃos, F. (2009). First record and description of an exceptional unborn specimen of Cingulata Glyptodontidae: Glyptodon Owen (Xenarthra) Comptes Rendus Palevol, 8 (6), 573-578 DOI: 10.1016/j.crpv.2009.04.003
As you certainly know, Brian, but as some of your readers may not, there is a fossil late-term fetal mammal on public display at the AMNH in New York: in the Perissodactyl alcove of the fossil mammal hall on the 4th floor there is a slab of a Pliocene (I think-- been a while since I was there) mare of some three-toed (again: I think) horse species with the skeleton of a breech-presentation foal in situ: mare probably having died of obstetrical complications.