Everyone thinks they have a sternum.
But it’s not the same sternum that lizards have, or birds have or frogs have. Let’s take a closer look.
In the large reptile tree an ossified sternum appears about seven times:
- Rana the frog
- Palaeagama, Jesairosaurus and the rib gliders + Megachirella and Pleurosaurus + Tritosauria + Squamata (sans Eichstaettisaurus – snakes) (sans Shinisaurus – Ophisaurus)
- Sphenodon and Kallimodon
- Petrolacosaurus + Araeoscelis
- Hovasaurus + Tangasaurus + Thadeosaurus
- LImusaurus through birds
- Haya and Heterodontosaurus
Note there are no synapsids
(including mammals) on this list. Note also the sternum is not present in basal tetrapods and basal amniotes. The sternum in fenestrasaurs, including pterosaurs is actually the sternal complex (clavicles + interclavicle + sternum). And finally, there does not appear to be a sternum in the mesosaur, Stereosternum.
what we call a sternum is actually a novel set of bones forming a ventral anchor for the ribs (as the sternum does in most tetrapods). The interclavicle is retained in basalmost mammals, but it too disappears in higher forms only to be replaced by these novel rib anchors.
I had no idea about this
until I found the Luo et al. 2007 reference. Thought I’d share it with you, especially if you need to get up to speed, like I did.
Luo Z-X, Ji Q and Yuan C-X 2007. Convergent dental adaptations in pseudo-tribosphenic and tribosphenic mammals. Nature 450, 93-97. doi:10.1038/nature06221