My courses as part of my batchelors degree contained little enough anatomy, and most of that was either avian or, more often, mammalian. There was therefore, quite a few surprises waiting for me when I got into the details of reptilian anatomy (and given my record with anatomy, probably quite a few still to come) as part of my PhD. One of these surprises was the gastralia.
These bones are sometimes called the belly-ribs, or collectively the gastral basket and with good reason – they are a series of bones that help support the abdominal wall and are found throughout the archosaurs (as well as plenty of more basal reptiles) until they were lost among early birds (sometime after the confuciusornithids arose). This is of course a useful function in big animals where there is a lot of stomach and other organs that must otherwise be supported by just the abdominal wall (think of some of the big sauropods) and this explains why it would be lost in birds where the massively expanded sternum would largely do the same job. There is also some evidence that gastralia in at least some archosaurs may have had a small role in breathing as well.
They are actually rarely found in fossils since they are typically very thin rods, but can be very robust and even form surprisingly complex shapes or collections. In tyrannosaurs for example they actually interlock with alternating bones from the left and right overlapping in the midline and little facets to hold them in place.
Something similar can be seen in this nice example – a troodontid on its back (white arrows point to the gastralia, including the overlap – for reference the tail is wrapped around the body and is marked in red).
That is pretty much about it for gastralia. I’d like to say more and while there are of course interesting bits and bobs about the subtlety of their morphology and distribution that will have to wait a fair while, I really just wanted to introduce them as a concept to the unsuspecting mammalian readers who have not heard of these little bones. Even some dinosaur enthusiasts I suspect will not know about them since they are rarely preserved, and rarely mounted on display skeletons.