There are probably some people somewhere who have not seen a dinosaur book at some point in the last fifty years and not spotted that it beings by explaining the difference between an ornithischian and saurischian pelvis and what this means for dinosaur taxonomy. In case anyone has missed it, I’ll go over it here with my lovely assistants, Tyrannosaurus and Nanyangosaurus.
So here they are then the pelves of each of the two major dinosaur clades. The saurischian is to the left and the ornithischian to the right. In each case the animal would be facing to the left with the tail on the right. The three bones of the pelvis are indicated with the ilium in blue (top), the pubis in red (lower left) and the ischium in yellow (lower right).
In the modern field of palaeontology the pelvis alone does not form the defining characteristic of the ornithischians and saurischians but they are still a pretty fundamental part of how we separate and distinguish between the two clades. The names themselves relate to the morphology of the pelvis and how they appear either bird-like (ornithischian) or reptile-like (saurischian). As you can see here the critical part is the pubis, pointing to the front in the saurischian Tyrannosaurus and having been reversed to point to the rear in Nanyangosaurus. Even if you can’t tell which way is to the front on an isolated skeleton or partial picture like this you can still tell them apart with the pubis lying close to the ischium in the ornithischians and being well separated in the saurischian version.
Life however is a delightful continuum and the sheer variation in biology and it’s fantastic and persistent efforts to flout even the most basic of rules taxonomy that we as scientists try to impose on it make it a very difficult subject and thus inevitably there are some exceptions to this rule.
First off, the easier ornithischians. Plenty of taxa have a pubis that while it does extend posteriorly and lie alongside the ischium, they also have a process or even a kind of plate extending forwards from it that looks superficially like the pubis of a saurischian (smaller green arrow, middle left). You can still tell the two apart pretty easily, but it might take a second glance.
Onto the saurischians and a point that has been lost on at least a few people over the years – birds evolved from saurischian dinosaurs. Birds (and with them their bird-like hips) evolved from the dinosaurs with reptile-like hips. There are then, as you might have already guessed, a number of saurischians with bird-like hips and it will probably come as no surprise (to those who don’t already know) are those saurischians closest to the birds – the maniraptoran theropods.
While dromaeosaurs like Velociraptor are perhaps the most obvious and well known ‘bird hipped’ saurischians (given their close ties to birds) there are a number of others. The theirizinosaurs also have a rather reversed pubis as do the troodontids (another close bird relative). All three are still rather different to the ornithischians, since the pubis is not so much retroverted (facing backwards) as simply pointing downwards, or only slightly to the rear with an only mildly rear facing ischium. Indeed in this aspect they look much live very early birds (like Archaeopteryx) rather than the far greater reversal of the pubis in many modern birds which is where the ornithischian term came from. One can therefore fairly readily distinguish between these taxa and ornithischians, and of course when laid together one can see the progression of an increasingly rear facing pubis from a classic saurischian baupla to that of modern birds and demonstrate that birds are indeed saurischians.
Right, I’ve finally dealt with that one, now onto some more esoteric archosaurian musings, though of course it’s always worth covering old ground as well as possible for new audiences.