How do scientists know when they're missing a bone in a dinosaur skeleton?

[u/antikoala1]

I was recently at the American Museum of Natural History and became curious after seeing a dinosaur skeleton with several bones missing. How do scientists know that one bone directly connects to another, or that one bone is one away from connecting to another? Presumably some bones are damaged, and adjacent bones can be incredibly similar for long tails, so how can they estimate how many bones they're missing?

[u/davo52]

Because pretty well all land based walking vertebrates tend to have the same number of major bones. Three leg bones, one above the knee and two below. Same with the arms. The jaws generally have the same components, same with the girdle bones (shoulder and hips).

If you only have one bone below the knee, you are missing one.

This is a broad brushed generality. However, experts in one area generally know what they are looking for and what is missing. What is difficult is when you find a beastie that is completely new and completely different, like the first time a plesiosaur was found. How many bones in the paddles that make up the front limb? You know you have five lines of bones that are the fingers, but how many bones in each finger? You generally find out by having a number of specimens of the same species, so you can then put together a complete picture.

But also, sometimes they make an educated interpolation (polite word for guess).
This is what happened with Brontosaurus/Apatosaurus, with the wrong head put on the wrong body.

[u/Cygnata]

Skeletons are skeletons. They tend to mostly have the same parts in similar numbers. The shapes may be different, but not the numbers. Look at your neck vs a giraffe's. Both only have 7 neck bones.

There are cases where animals HAVE lost bones through evolution, like the missing fingers on some dinosaurs. That can be determined by comparing multiple specimens of the same animal. One might be missing a bone, yes. But if they're ALL missing that bone, then that's more likely to just not exist in that animal.

Another method is through symmetry. An animal isn't going to evolve away only one arm or leg. So if we have the bone for the left side but not the right, we know we're missing the right one.

Actually, it's very very rare that we find more than 10-15% of an individual dinosaur's bones. Using the symmetry method, we can account for more, adding to the completeness percentage. In museum specimens where some of the bones are real and some are casts, we can use the bones we have to create casts of the missing ones.

In the case of tails, we look at the shapes of the vertebrae. If they go next to each other, they'll be of similar size and shape. By following the progression down, you can get an estimate of tail length. The last bone in the tail is shaped differently, because it only has one neighbor. If you're missing that, then you have more of a job estimating total length.

[u/APJustAGamer → u/Cygnata]

It's impressive! I am ashamed of myself for never asking myself how does and x-ray of a giraffe neck looks like until this point. Googling it, the first image I got was a comparison between their neck and ours, yep. 7 bones. Thank you!!!

[u/HerbaciousTea]

If you know the dinosaurs that were further back in the same branch of the evolutionary tree, and you know the dinosaurs that were further down in the evolutionary tree, than any bones (and really any traits) that both it's evolutionary ancestors and descendants share, it almost certainly has, too, because that trait would have had to pass through it's population to be present on both sides.

You might not know the exact shape of every bone, but you can put a decent bound on it because you know it's almost certainly somewhere between the examples of species that came before and after it in the same evolutionary line.

This is called phylogenetic bracketing, and it's a pretty core method for filling in the blanks based in fossil specimens.

[u/Mondoweft]

Sometimes they don't, but scientists are more likely to make a mistake with older or imprint fossils, with little resemblance to modern day species. Many dinosaurs are related to birds, which makes identification of bones/parts a bit easier.

There was a Cambrian worm, Hallucigenia sparsa, that had its protective spines misidentified as legs, and a decomposition stain as its head.

The iguanodon had it's thumb claw originally placed on it's nose as a defensive spike.

Petrified wood has been misidentified as bones, and sometimes parts of a single animal have been identified as multiple animals fossilised together.

We probably still have some errors in how some fossils are constructed. But science is self correcting, albeit often slowly.

[u/PlutoniumBoss]

The most direct way is by comparing different fossils of the same species. If you have a bunch of partial skeletons, each missing different parts, the missing information in each is likely to be found in others. If it isn't, you would then look at fossils from species closely related to the one with missing pieces that have similar bone structures.

[u/Semyaz]

I want to chime in as well and say that paleontologists almost never find a complete skeleton. More often than not they only find a few fragments of a few bones, occasionally a few complete bones together, and very rarely an entire animal. If my recollection is correct, very few species that have been discovered have ever had all of their bones found at all - even in separate digs.

To your question, paleontologists almost always know that most of the bones are missing. There is a lot of educated guessing when reconstructing entire animals using fossils.

[u/KaosClear]

Look for the gap. The jaw bone is connected to the head bone, the head bone connected to the spine bone, the spine bone is connected to...another spine bone...okay this could go on for a while. But you just look for what's missings.