Why Dinosaurs are not just big lizards, Part 3

Part 1, Part 2

So last time I talked about the stance of dinosaurs and how that differs from that of lizards. Well I am sure that people were saying how do we know this, or humans just set them up to look this way. That leads to the next point:

Hip

So when we look at mammals, another group of animals that have their legs positioned under their body seen below in a specimen of a florida spectacled bear (Tremarctos floridanus) from the Florida Museum of Natural History (see story about my visit here).

photo by author

So this is a typical ball and socket joint that mammals evolved that allows us to position our legs under our bodies but it also allows some side to side (abduction and adduction) of our legs as well. This trait probably evolved to help us move over less than smooth terrain.

So I kind of implied that mammal hip bones differed from those of the dinosaurs and we will see why here in a minute but first lets look at the hip of a lizard:

Modified from here

So you can see that there is some similarity but the way the femur is formed it prevents the legs of the lizard from being able to sit directly under the body giving it a more sprawling posture (note to self: take more pictures of lizard skeletons).

So finally what did the dinosaur hips look like:

Modified from Holtz and Brett-Surman (1997)

So what is the first thing you probably notice about the dinosaur hips, hint there is a red arrow pointing to it? That's right there is a hole in the center of each one, called the acetabulum and in dinosaurs we say it is perforated. This takes a femur that has been modified from that of the earlier reptiles and actually puts the head of it inside of the hip itself. This characteristic allows them to position the legs directly under the body but unlike the way that mammals developed a more upright posture dinosaurs had less flexibility when it comes to the ability of them to move their legs side to side over uneven terrain.

Again but those aren't fossils alright lets look at some actual fossils:




And just to point out something not entirely unrelated check out the hip structure on this bird skeleton he clearly shares something with dinosaurs:


Part 1, Part 2

Source

Holtz, T.R., Jr., and M.K. Brett-Surman. 1997. The osteology of the dinosaurs. In J.O. Farlow and M.K. Brett-Surman (eds.), The Complete Dinosaur, pp. 78-106. Bloomington: Indiana University Press.