Last week, I was distracted somewhat by a palaeontology article in Nature: Reisz and colleagues reported their discovery of some fossilised dinosaur embryos. Not exactly relevant to my research, but very cool nonetheless…
The remains that they unearthed in southern China are from the early Jurassic period, almost 200 million years old, and are thought to belong to a Lufengosaurus species. This was a sauropodomorph dinosaur: a group distinguished by their large size, with a very long neck and tail and a small head. The most famous of the sauropodomorphs were probably the Diplodocus species.
These fossils are so unusual, and so informative, because they include embryos at a range of developmental stages. The majority of fossilised dinosaur embryos discovered to date have been single clutches of eggs, all synchronised in their development, which provides only a snapshot of development in that particular species. Finding a whole collection of samples from the same species, but at different stages, gives a rare insight into the dynamics of development in an extinct animal.
The authors focused on the growth of the thigh bone, analysing 24 femurs that ranged in length from 12 to 22 mm. Using sectioning and histological techniques, they showed that these bones were highly vascularised at all stages, so they think that these giant dinosaurs began life with rapid embryonic growth.
They also observed that the dinosaur femurs became thicker on one side as they grew larger, and developed a prominent fourth trochanter (an outgrowth to which the main thigh muscle attaches). In living tetrapods, asymmetrical bone thickening and the growth of skeletal features at muscle attachment sites depends on the muscles being active during embryonic development. This suggests that these ancient embryos also used their muscles to move around inside their eggs, and that these movements were an important part of their development too.
I was really amazed by how much information could be gleaned from these tiny fossilised remains. Geology rocks! In evo-devo, we use observations from extant species to make inferences about their common ancestors, but if palaeontology can provide insights into the embryonic development of extinct animals, it might help us to think about the evolution of some developmental processes from a different, and very interesting perspective.
Reisz, R.R. et al (2013) Embryology of Early Jurassic dinosaur from China with evidence of preserved organic remains, Nature 496: 210-214.