@nebulousflynn is right, in that it largely depends on what your contract says on the subject of copyright ownership. It also depends on the applicable laws in your country.
Generally speaking, in the EU, ownership of copyright vests in the individual that created the works. So, in the absence of an agreement stating otherwise, you will own the copyright.
Don’t forget, however, that a contract doesn’t have to be a formal document. A contract can be implied by correspondence (e.g. email) or even by verbal conversation. Contracts can arise without you realising it.
Obviously, if a contract has been formed by emails and verbal conversations, proving the terms of that contract will be difficult - for both parties. That’s why it’s preferable to have a written agreement clearly setting out all the agreed terms, so you don’t have to argue about it later.
From what you have said, I suspect that here you will own the copyright, but you will have granted a licence to the museum to use the work through your conversations with them and your conduct (which will create an implied contract).
- You will argue that it was clear from all conversations that the contract was for a model dinosaur to be used for research and non-commercial purposes by them alone. You will argue that the license you granted them to use your copyrighted work did not extend to them reselling such work to third parties;
- They will argue that it was a contract for a model dinosaur to be used by them however they wished. They will argue that you granted them a wide licence to use your works however they wanted, including reselling it.
Who is right? It all depends what evidence either of you can produce to support your position. Go back over your emails and other communications. Look for anything that supports your case that you only ever intended to license this to them for their own, non-commercial use. Find that evidence!
Note: I am a lawyer but this is not legal advice. I am also not based in your region (the Netherlands?) and so I am speaking from a general understanding of EU law only. Your local laws regarding contract formation and IP ownership may be different from the general EU position. I don’t honestly know