Edit: just now I re-read the question and saw that I missed the part about the sauropods. What I describe below can apply to any species with a brain or some other form of neural network, whether intelligent or not.
Fungi and plants can communicate amongst themselves through chemicals.
Now let's handwave
a bit a lot. Let us imagine a fungus that communicates through electromagnetic waves as well. Let it be in the form of microwaves. Perhaps all lifeforms on their planet evolved to communicate thus at some level. This would be to their advantage - they can react much faster to many situations if they can communicate through radio signals. This would require evolution in their planet to have taken a turn different than our own at some point.
Let's say these fungi were bioluminescent in the past, and somehow shifted the frequency of their EM radiation from visible to microwave.
This is quite possible. One reason we can't see into the microwave spectrum is because, in order to see/sense an electromagnetic wave, we need a structure that is about the size of the wavelength involved. Visible light has its wavelengths around the nanometers, and we detect it with molecules within our retinal cells that are nanometers long. In order to "tune" into the microwave spectrum, we would need much larger structures - microwaves have their wavelength between milimeters and meters, units which are a million to a billion times greater than nanometers.
That probbly wouldn't do for our cone and rod cells, but a fungus might be able to evolve such a structure.
Now let's say that some intrepid humans have landed on this planet. Someone is bound to try to eat the fungus at some time. But the fungus isn't like anything which ever evolved on Earth. Most of it will be digested... But its microscopic spores may pierce through the bowels of the mad gourmet. If this doesn't cause the human to bleed to death, then the spores will travel through the bloodstream and eventually settle on some tissue, probably destroying it. Muscles may be destroyed, limbs may be amputated, but if the spores grow in the heart of lungs that's it.
Say that our glutton of a friend was lucky enough that the fungus didn't reach the heart or lungs. It landed on the brain instead. Now the bastard will die because the fungus will grow inside his head, causing the internal pressure of the brain to go way up. Vessels may burst. Dude may go blind or deaf depending on where the fungus is growing.
Ok, so he still didn't die of that, didn't lose any motor skills or sensory capacities, just gut a little stupider than he or she already was. The body's typical immunological response to that is calcifying the strange body that has appeared in the brain.
But that didn't happen. Instead, the fungus stopped growing, and now the neurons of the brain are starting to interface with the cells of the fungus.
Such interface is possible as well. There are people who implant magnets in their fingers so that the nerves of the finger grow around the magnet and interface with it, giving the biohacker a magnetic sense at the fingertips. If neural tissue can do that with non-organic matter, it should be able to do so with a fungus as well.
Congratulations to our crazy xenogastronomer, he or she can now probably sense and/or emit microwaves.
Now all they need is to find someone else with the same combination of genes and blind luck who can eat the fungus and go through the same process, so that they can talk radiotelepathically. I say combinations of genes because there are so many immunological responses to be bypassed, and so much phisiological and physical damage to be survived, that this is truly a dangerous experiment.
At some point people can develop technology to implant a fungal structure in the brain in a safe way. Until then, telepathy will be a gift from mother nature, and the people with the genes that allow one to survive the ordeal of eating the fungus may eventually be selected into a new human species.