Fins to Feet

The Pelvis
July 14, 2010, 1:07 am
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Taking Wing: The Dinosaur-Bird Connection

Part 3: The Pelvis.

“Resemblances to the structures found in some birds had already been noted by Prof. Owen in the sacrum of the Dinosauria; but these specially ornithic peculiarities of the pelvic girdle had not been indicated by any anatomist, and opened up a very interesting field of inquiry.”
Thomas Huxley, 1870.

Imagine yourself in the dusky storage facility of an old Natural History Museum, placing the replica pelvises of an Alligator, a Theropod dinosaur, and a Kiwi on a dusty table-top. You pause to adjust your plaid Armchair Anatomist hat before proceeding (unless, of course, you are in possession of a Real Anatomist™ hat – in which case you really shouldn’t be perusing this blog). What similarities might we expect to see in the hip bones of these disparate forms?

And disparate they are: Alligators are sprawling, semi-aquatic creatures. Tyrannosaurus rex was a giant, predaceous biped. The Kiwi is a flightless New Zealand bird that feeds on seeds and small invertebrates.

Meet the fockers. From left to right: The Kiwi, Tyrannosaurus rex and the Alligator.

How related are they really?

These three species, alongside all the other Bird, Dinosaur and Crocodillian species known to science, belong to a taxonomic group called Archosauria, which means “Ruling reptiles”.  Birds and Crocodillians are the last living representatives of this large and diverse panoply of swimming (crocodiles, phytosaurs etc.), flying (birds, Pterosaurs) and walking (Dinosaurs, Raiusuchians etc.) beasts.

There is a certain romanticism to the idea that the robotic, mud-mired Crocodillians could be next of kin to those flamboyant aerial acrobats, the Aves. Yet it must also be noted that over 220 million years of divergent evolution separate these two lineages. They are widely separated twigs on the tree of Archosaur evolution. It really is a shame, though, that the rest of their mighty clade now languishes in various degrees of deadness – Alas, the best we can do, as this impressively nerdy T-shirt suggests, is to “Never forget”!

I will leave the question of Archosaur origins and taxonomy for another post/series. I will, however, link to David one’s capable introduction to Archosaur evolution here – check it out if you want to learn more.

And here are the pelves/pelvises. In a complete skeleton, the frontal portion of the body would be positioned to the right of each pelvis, whilst the tail would be located to the left. In other words, <– would be the direction when travelling from head to tail.

Side-view of the pelvis - from left to right: Alligator, Tyrannosaurus rex, Kiwi. Click to enlarge.

I am not wordsmith enough to hammer out a description of the Dinosaur/Crocodile/Bird hip that could enthrall anybody but the most insufferable Dinophile. I will, nonetheless, try to construct a cursory overview of some of the major changes that the primitive Archosaur hip underwent on the path to the birds. This will, as usual, entail a fair amount of anatomical rambling.

How is the hip structured?

In all three cases, the pelvic girdle is composed of three paired bones – the Pubes, the Ischia and the Ilia. The illium is the dorsal-most bone whilst the Pubis and Ischium are situated beneath it. The proportions, shapes and relative spatial orientations of said bones are pretty variable across the Archosauria (as one might be able to infer from the diagram).

A cavity/socket (called the Acetabulum) which receives the head of the femur is present at the junction of the Ilium, Pubis and Ischium.

And now for some housekeeping: I didn’t discuss this in the earlier sections, but the femoral head in Dinosaurs and Birds is at right angles to the femoral shaft/body. This is an adaptation for erect walking. Refer to the diagram. Dinosaurs are distinguished from other Archosaurs by the presence of an open acetabulum – that is, the acetabulum doesn’t have an inner bony wall – and a fowardly (preacetabular) expanded Ilium. The acetabulum was subsequently closed among the Birds.  

Some features of the dinosaur hip. Adapted from skepticwiki's article on Dinosaurs.

In birds and in several Dinosaur species, the vertebrae near the pelvis are fused into a skeletal structure called the synsacrum. In many cases, the pelvic girdle is fused to the synsacrum.

What is the function of the pelvis?

– The pelvis connects the hind limbs to the trunk of the body (through the acetabulum). In Dinosaurs, a robust ridge present above the acetabulum on the ilium helps transmit the weight of the animal to the hind limb.

– The Illium of the pelvis bears ridges and processes (like the Brevis fossae) which serve as attachment surfaces for various important muscle groups that play roles in locomotion, respiration and support. The Pubes and Ishcia also serve as anchoring sites for a number of muscles.

– The pelvic girdle also offers protection to various soft tissues and organs. This is especially true of birds, where the pelvis is the only bony structure in the abdomen.

Wait, why is the Pubis of the Alligator dissociated from the rest of the pelvis in the diagram?

In crocodilians, the pubis is excluded from the Acetabulum and is attached to the Ischium through movable cartilaginous joints. Interestingly, this bone is mobile and actually plays a role in crocodilian respiration. During inhalation, the pubis rotates downwards, increasing the volume of the abdominal cavity and contributing to the inward suction of air. Pretty neat adaptation!

What broad changes does the pelvis undergo on the way to the birds?

The Theropod pelvis primitively has forwardly directed pubes (as seen in Tyrannosaurus rex). This sort of pubic orientation is called “propuby”. In some species, like T. rex, the pubes are expanded into backward-pointing “boots” at their far/distal ends.

In modern Birds, however, the pubes point backwards. This is known as opisthopuby. It is observed in Archeopteryx.

But what of those switchblade-claw bearing, bird-like Theropods – the Deinonychosaurians? Well, it depends. In Velociraptor mongolensis, the pubis points backwards. Reconstructions of other species have the pubis pointing downwards – a sort of middle-ground between the Tyrannosaur and Avian condition. We may call it Mesopuby

Pelvis of Archeopteryx and Velociraptor. They are both opisthopubic. And LOL, I misspelled Ilium.

The Therizinosaurs – a group of strange herbivorous Maniraptoran Theropods – also seem to have bird-like pelvises. But this trait was probably acquired independently of the bird-deinonychosaurian line.

 The “retroversion” (backwards orientation) of the pubis is one of the major themes of Theropod evolution on the line to modern birds. What possible functional significance could this change have had? Well, I’m not sure. This is one of those rare instances when Google Scholar, my 800+ page long digital copy of Padian & Currie’s “Encyclopedia of Dinosaurs” and my University Library databases seem to come up short.

Carrier and Farmer (2000) suggest that changes in pubic orientation may have altered the muscular mechanics associated with respiration.

It almost certainly had broad implications for hindlimb posture and motion – although, again, I’m not aware of any of the particulars. Seriously, if any professional/amateur Paleontologists are still reading this article and aren’t wholly incensed by my penchant for oversimplification (and error no less), please do E-mail me at with an answer. Thanks.  

Bottom view of an albatross pelvis

The purpose of the diagram to the left is to demonstrate this: The pubes of modern birds are slender and unfused at their far ends. Among more “primitive” Theropods the pubes are rod-like structures that fuse with one another distally. In early birds like Confuciosornis and Archeopteryx, the tips of the pubes fuse to form a bony ring. Gary Kaiser, author of The Inner Bird (just so you know that you’re getting this information third-hand), notes that this severely limits the size of the egg that a bird can lay and makes some interesting extrapolations.

The argument flows thus: Fused pubic ends => Bony ring with small opening => Small Egg size => Large brood size => Limited parental care per hatchling (as in Crocodiles and possibly many Dinosaurian groups).

He further posits that the unfused pubes found in modern birds do not place such an anatomical constraint on the size of the Egg. And thus: Unfused pubic ends => Large opening => Large Egg size (generally speaking) => Smaller brood size => Intensive parental care (as in modern Birds).

 It’s an elegant argument for sure!

And for some less exciting changes on the road to birdhood:

         The Ischia also end up unfused.

         The Ilium expands as more vertebrae are incorporated into the synsacrum.

         Shifts in the attachment sites for various muscles on the Ilium are apparent. Many of the “ridges” on the Illium either move or disappear.

         The pelvic bones fuse into a single unit called the Innominate bone.

That’s all for now folks. Thanks for reading! I will be trying something somewhat more exciting for my next big post.


5 Comments so far
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Comment by dietformula

The pelvis of Archaeopteryxis is wrong drawed.

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