H is for Hip

Trex takes a step to the ri-i-i-ight. Photo from Youtube.

We’ve got to get into the hips. Hips are what the dinos are all about, which is why I’ve spent the last half hour trying to create a picture of a dinosaur doing the Time Warp. You know, put your hands on your hips? Apparently, pelvic thrust is considered NSFW. And, when I asked several AI picture generators, they took forever, asked for a lot of info I didn’t want to give them, and then gave me a very lame picture of a dinosaur dancing, which I could have just got off Clip Art anyway. Which I did — so there’s our buddy dino doing the Time Warp.

Did you know there’s a whole wiki showing you how to do the Time Warp? Because you can learn anything on the Internet!

Along with the extra hole in the head–remember letter “A”?–dinosaurs are dinosaurs because of the way their hips were designed. It’s what helps distinguish dinosaurs from the crocodiles and other non-bipedal lizards. And it separates the two dinosaur divisions from each other. Although that’s becoming a controversy, as we shall see.

The Hip Bone’s Connected to the …

So let’s get into the ischia of it all. (Ischion was the Greek root word for the hip joint.) First off, just a brief discussion of hip anatomy.

You may be more familiar with human anatomy, so that is a better starting point. Animals generally have a pelvis with somewhat symmetric sides, a top, and a bottom. The torso, vertebrae etc attach from the top, and the legs and tail …well, for humans that’s our coccyx… attach to the bottom. In humans (and dinosaurs), the femurs attach on either side of the hip through a ball-and-socket joint that snugs into a hole. The femur juts out of the hole, then quickly descends, allowing humans to stand upright. By fitting into a socket which is a hole, the leg can bear more weight, which allows the angle of standing.

That hole is surrounded by two other bones, the pubis in front and the ischium in back. In humans, those two bones come together. You sit on the ischium, the hip bones, but they are still fairly connected to the pubis in the front.

Crocodiles Crawl, Don’t Run

Non-dinosaur reptiles, both back in the day and now, have similar bits and pieces: the pelvis, the ischium, the pubis, and the femurs. The pubis juts out in front and the ischium in back. The division of those into two pieces, different from mammals, doesn’t separate them from dinosaurs. What separates them is that the femur fits into a depression, not a hole. The leg then extends out at almost a 90 degree angle to the pelvis, which is why they squat or crawl.

Crocodile pelvis from Michelle & Brad, Biology of the Crocodile, Added Pubis

This locomotion makes them relatively slow on land, although their tail action helps speed the process. Crocs are much faster in the water. In the water, long femurs sticking downward would encounter water resistance and slow them down. I don’t recommend trying to outswim a crocodile.

Crocs on the left, dinos in the middle, and non dino-reptiles on the right.

There were some other ancient reptiles called rauisuchiaians which stood erect in a manner different from dinosaurs. But these rauisuchaians were too hard to pronounce, and they did not know how to do the Time Warp.

Dinosaur Hipsters

The difference between dinos and crocs are the angle of the legs. Dinosaurs have two types of hip designs, but both of them have a hole with the ball-and-socket joint. Early in the evolutionary time scale, 200 million years ago (the Triassic), dinosaurs developed as reptiles with these firmer hip sockets, which allowed them to move upright.

There are two types of dinosaur hips. These are called saurischians and ornithischians. Saurischian relies on the Greek roots for lizard and hip, so it means lizard-hipped. Ornithischian relies on the roots for bird and hip, so it means bird-hipped. The guy who used those names distinguished them between whether the pubis (the “front” part of the structure) was pointing forward or backward.

The two branches, based on hip. Graphic from pinterest.

If you say Ornithischian and Saurischian to a dinosaur nerd, they will nod sagely, yep know all about that. But get ready for this.

The Ornithischians, Named for Birds, Did Not Include…

Ornithischians were so-named because the pubis pointed backward, like that of modern birds. The ornithischians with minor exceptions included all vegetarians (herbivores).

Note pubis and ischium facing backward. Graphic from Didactalia.net.
Stegosaurus was bird-hipped (pubis points back, to the left), so part of the ornithischian clade. Photo by kajmeister.

Saurischian–lizard-hipped–were named because those dinosaurs had hips with the pubis forward, like our crocodile friend. However, this group included both those big giants, the sauropods, and the theropods, the carnivores. And way down below the theropod clade are the birds.

TRex pelvis, with the “lizard-hipped” pubis pointing forward. Photo by kajmeister.

Yep. The lizard-hipped-named group eventually included birds, which didn’t have lizard hips. And the bird-hipped group didn’t include the birds. Also, yep, that’s how tall the T rex was compared to us puny humans.

It’s a problem with naming things based on one primary physical feature. A century later, with lots more data and computers and databases, the scientists argue that what grouped the birds and theropods together was not their hips. There has been a lot of sciency chat in the last few years that suggests theropods should not be classified as a saurischian at all, but as a third group.

Time to change all the paleontology text books!

2 Replies to “H is for Hip”

    1. Probably blathering about hip bones to my children, who just wanted some ice cream. Thanks for being a receptive reader!

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