- The gassy emissions from these giant dinosaurs may have been enough to warm the Earth, the researchers say.
The preposition from, which is modifying the noun emissions in the Original Poster's example, indicates the source, or origin of the noun. It is a very common usage of this preposition and is frequently observed. Consider the following examples:
- wines from France
- shouts of abuse from the audience
- posts from users
- letters from friends
- excerpts from the novel
- strange noises from the fridge
Here, the source of the wine is France, the abuse originates from the audience, so on and so forth. The gassy emissions, or farts as the Original Poster colloquially and accurately terms them, originated from the dinosaurs in question and so from the dinosaurs is a perfectly valid prepositional phrase with which to modify the Noun Phrase The emissions.
The Original Poster's question, however, also considers modification of The emissions by a prepositional phrase headed by the preposition by. Now, it is a pernicious myth peddled by English teachers and armchair grammarians, that verbs denote actions, nouns denote objects, adjectives describe nouns or objects, so on and so forth. In actual fact, of course, verbs often denote states, not actions; nouns can denote actions and not objects, adjectives can be used to describe states or actions and not objects - you get the picture.
For our purposes here, we need to consider nouns that describe actions and events. There are thousands if not tens of thousands of such words. Here are a few:
- theft, release, beating, baptism, massacre, emission
Now when we consider actions or events such as these, we are often able to perceive entities whom or which we consider to take the role of agent or patient in relation to the action concerned. The agent is the doer of the action, and the patient is the recipient of the action, the thing the action is done to. In the sentence, Bob punched me, therefore, Bob is the agent and I am the patient - the person who received the punch.
In relation to actions denoted by nouns, when we perceive both an agent and a patient, we commonly use by to modify the noun to show the agent, and we can also use of to show the patient:
- the massacre of the Daleks by the Vogons
- the baptism by the priests of the new members of the congregation
- the release of the baboons by the naturalists
- the theft by the cat burglar of the Crown Jewels
- the emission of green house gasses by the diplodocus
[Note that if the action is perceived as intransitive, then this pattern won't usually apply. We will use of to denote the agent. Consider: the arrival of the bus, the resignation of the managing director, the refusal of the authorities to...]
In the Original Poster's question, their alternative formulation emissions by dinosaurs would indicate the agent of the emitting action. It is of course perfectly grammatical and stylistically viable.
In terms of the structure of the Noun Phrase the emissions ..., both the Preposition Phrases from dinosaurs and by dinosaurs function as modifiers or adjuncts. They are not essential elements in the structure of the phrase. We could add either, or omit both, and the Noun Phrase would still be well-formed. In this particular instance, the two Preposition Phrases provide pretty much the same information, so little would be gained by using them both. The second would seem redundant if we did so:
- The emissions from and by dinosaurs may ...
In terms of which is stylistically best, in general terms they both seem fine. They're certainly both grammatically well-formed. However, the writer here seems to be trying to add a bit of colour to what might otherwise be a rather dry subject. They, for example, use the term gassy emissions, they don't say the methane released. The effect of emissions here is slightly comical, it mirrors the use of emissions we often see in the literature relating to factories and power stations, but also, because of the action-like nature of the noun, it clearly reminds us of farting. It therefore introduces a trace of very understated scatological humour in order to enliven the subject matter.
For these reasons, it seems to me that the faintly scatalogical undertone would be enhanced by the use of by, because it would increase the sense of the dinosaurs' agency in the farting process. Compare:
However, the last stylistic point is a matter of personal opinion. It only indirectly relates to the grammar. The upshot of this enquiry overall is that both from and by dinosaurs are perfectly grammatically correct.
Hope this helps