Does anyone know why there are ammonite and trilobite fossils on Io? I know the Traveler has a habit of terraforming the worlds it touches, but fossilization takes a great deal of time – far, far longer than has elapsed since the arrival of the Traveler in the Sol system, much less it’s departure, since Io is supposed to be the final place it touched.
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In one of Byf’s videos on the bones on Io, he included an audio recording of Jesse Van Dyke from Bungie talking about how much they take science into account when designing the worlds of Destiny (skip to 3:32). He said that they focus more on if something is fun to play than if it is scientifically accurate. They are inspired by science, but they’re not constrained by it. For example, if the world of Titan were scientifically accurate, then Sloane should not be able to walk around on the surface of Titan with her helmet off and her arms uncovered, since the temperature required for the methane seas to be in a liquid form is -161.49 degrees Celsius or -258.68 degrees Farenheit (from Destinypedia), far too cold for a human to survive in without any protection. Van Dyke said that if anyone questions them about something not being scientifically accurate, they pass it off as being possible due to “space magic”.
I suspect the presence of the ammonite and trilobite fossils similarly falls under the heading of “they’re there because space magic made it possible”. As you said, it seems very unlikely that they could have existed on Io enough years ago that those fossils could have naturally formed, since this would have required them to be living there long before the Traveler arrived. Io in its natural, unterraformed state has the least amount of water of any astronomical object in the solar system, an atmosphere that is mainly sulphur dioxide, and receives high amounts of radiation from Jupiter (source). Not exactly great conditions for life to exist in.
It has been hypothesised that there may have been a lot of water on Io shortly after its formation but the radiation from Jupiter would have led to the water, at least on the surface, disappearing, probably within 10 million years. Microbial life could have arisen in that time frame, but the chances of more complex organisms, such as ammonites and trilobites, evolving that quickly seems highly unlikely. Life first formed on Earth 3.8 billion years ago, and it took over a billion years after that for the first multicellular organisms to evolve. More complex multicellular organisms, such as arthropods like trilobites, only appeared 570 million years ago. Even if the conditions on Io in its early stages were more favourable to the formation of life than on Earth, it would still take a very long time for microbial, single-celled organisms to undergo all the necessary evolutionary changes that could result in the formation of complex organisms such as ammonites or trilobites.
Given that it’s highly unlikely that the presence of ammonite and trilobite fossils on Io is scientifically accurate, I think that they were probably put there for aesthetic reasons, or possibly to get players to ask questions about why they are there. Earlier in Byf’s video (skip to 0:35), there is an audio recording of Van Dyke talking about the bones on Io. He said that they deliberately put in things that “evoke more questions than they answer”. Although this was referring to the bones on Io, it could also pertain to the ammonite and trilobite fossils. Or perhaps they were put in just because they look cool 
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Adding on to the idea of aesthetics > science, Bungie has played fast and loose with the idea of ‘fossils’ before- namely through the Ahamkara. Despite being a “post-Traveler neolife,” their bones are referred to as fossils both in D1 and D2.
I had a broken vertebrae in my pocket that I’d borrowed from - yes, borrowed, I was going to put it back - what do you think you are, my conscience? It was a fossil, that means mineral replacement, a rock, basically.
Bearers of a particular species of bone do claim—and mind that I’m not recommending these bones—that the fossils grant them unusual powers.
So, while scientifically impossible, space magic has seemingly done similar things in the past.
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