Say when a scientist discovers a new skull fragment from a T-Rex, how do they know it’s a skull fragment from a T-Rex and not like a fragment of some unknown species of dinosaur?
In: Biology
It’s very hard to tell with just a fragment. Even when you have most of a skeleton there can be questions. Considerable debate exists today on whether micro-tyrannosaurus is a different species or just the juvenile T-Rex form.
This can get contentious, and the dinosaur family tree is constantly being revised as new fossils are found and old fossils are reclassified.
With a single incomplete skeleton that’s similar but not the same as a known species it can be very difficult to say exactly what you have.
Is this a juvenile of a species we only know from adults?
Is it a female of a species we only know from males?
Is it a new species entirely? A regional variation? An abnormal individual that’s not representative of the species? Diseased? Hybrid?
With modern animals we can observe hundreds of individuals at different life stages to get a complete picture of what they look like but that’s not possible with dinosaurs.
It’s still hotly debated whether small tryrannosaurid and triceratops-like fossils are separate species or just juveniles of known species.
Even with species alive today we sometime discover that what we thought was 1 species was in fact 2 or that what we thought was 2 species is in fact 1 species
And that is with thing alive today, because nature is a very stubborn thing that refuse to neatly to fit into or human definition
So now you can imagine how complicated it is to do with millenias old bone shaped rock
TL:DR, it’s very difficult and we get it wrong a lot and constantly have to correct our old assumptions when new evidence are found
Comparison with known parts.
When a new dinosaur is described, they describe the traits that it has and doesn’t have that make it unique. These can then be compared. For example one part of the skull might be curved in one dinosaur, but angular in another.
However, sometimes we just don’t know . *Irritator* was a relative of *Spinosaurus* that lived in South America, and either lived with another spinosaur called *Angaturama* or Angaturama is actually a junior synonym (another name for an already existing species, and so is invalid). There’s no overlap between the known material of the two so we don’t know for sure if they’re the same or not.
Question on top of your question, how do they know it’s a dinosaur bone in the first place? Why not a new species of extinct mammals?
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