Hi Thomas, thanks for your question!
Your question is a bit like asking if I am from Chicago, from Illinois, or from the United States. Chicago is one part of Illinois; if you are born in Chicago then you are automatically also born in Illinois. And Illinois is part of the USA, so if you are born in Chicago then you are automatically also born in the USA. If it turned out that I was actually born in a close suburb of Chicago, instead of within the city limits, it would not change that I was born in Illinois and born in the United States.
Similarly, Nyasasaurus is an archosaur whether it is a true dinosaur, or if it just a close cousin to dinosaurs. Archosauria is a category of related animals, made up of many smaller groups of related animals: crocodiles, dinosaurs (including birds), pterosaurs, and all of their their shared ancestors. So every species that is a dinosaur or a close relative of dinosaurs is also an archosaur. There are bigger groups that Nyasasaurus belongs to as well: archosaurs are just one type of reptile, so every archosaur is also a reptile. And all reptiles are animals.
The other part of the question is trickier: it is hard to tell if Nyasasaurus was a true dinosaur or not because we have found so little of its skeleton. We only have an upper arm bone (humerus) and eleven backbones (vertebrae). There are clues on each of these bones that let us know that Nyasasaurus is a close relative of dinosaurs, if not a dinosaur itself. One of these clues is a long ridge on the upper arm bone, in the place where the shoulder and chest muscles attach. Only dinosaurs (including birds), and Nyasasaurus have a ridge that looks like that. Also, the backbones in the hip region of Nyasasaurus are fused in exactly the same way they are fused in dinosaurs. A third clue is that the neck vertebrae have little pockets/dents on the fronts and sides, something we see that we see in theropod dinosaurs (the two-legged meat eaters). There are other similarities too.
If it were just one clue, we might think "OK, maybe Nyasasaurus evolved this ridge independently, and it's just a coincidence that dinosaurs have it, too". But every single bone we have has something in common with the same bone in dinosaurs. Outside of dinosaurs, we don't know of any other animal that has all three of those clues I mentioned above. The more unique features that Nyasasaurus and dinosaurs have in common, the less likely it is a coincidence. So we have good evidence that they are close relatives, at the very least.
By the way, you can see all the bones of Nyasasaurus known to science here.
However, it is sometimes hard to tell how closely related species (or people) are when you don't have all the information. The closer you are related, the more you have in common, so you need more parts of the body to tell very close relatives apart.
For example, I have blue eyes and I don't have freckles. My dad, all of my aunts, uncles, and cousins, my grandparents and my great-grandparents also have blue eyes and no freckles - these are features I share with all of my close ancestors. There are other features I share just with my dad, like my hair color and the shape of my nose. Then there are features that only I have, like the birthmark on my knee. If you were trying to figure out my closest relatives based on my eye color and that I had no freckles, you could probably tell which family I was in. But you would need to know more information about the rest of me (hair color, nose shape) to figure out if I was a close relative of my dad. Things only I have, like my birthmark, would tell you I was unique, but wouldn't help you figure out who my closest relatives were.
It's the same with different species; they have some things that set them apart from every other species, and some things in common with closely-related species. The problem with Nyasasaurus is that we have enough of its skeleton to know it's a close relative of dinosaurs, but not enough to know if it's just inside the group or just outside it. What would be really helpful would be to find a Nyasasaurus skull. There are a lot of bones in the skull, so you can look for subtle similarities and differences. Some of our friends and colleagues have been to Tanzania to look for more Nyasasaurus skeletons, but they haven't found them - yet.
Thank you! We really enjoyed this explanation and the link to the fossil images was fantastic!- Lindsey Nelson •
When: November 13, 2013 08:00 am PST