The ones for which we do have a good idea of their parasites/symbionts, are the species where we have found their coprolites since many gut parasites have eggs/propagules that exit the host via the faecal route. An example can be found here: journals.plos.org/plosone/arti...

A Megafauna’s Microfauna: Gastrointestinal Parasites of New Zealand’s Extinct Moa (Aves: Dinornithiformes)

We perform the first multidisciplinary study of parasites from an extinct megafaunal clade using coprolites from the New Zealand moa (Aves: Dinornithiformes). Ancient DNA and microscopic analyses of 8...

There's an okay record of parasitic traces in mollusk shells (mostly from trematodes and spionid polychaetes) but it's all really ambiguous. Today, gymnophallid trematode larvae make rimmed pits in clam shells and have fish or seabirds as their definitive hosts. We see similar pits long before the

For more info on extinct parasites, I've written two publications on the subject of fossil parasites here: onlinelibrary.wiley.com/doi/full/10....books.google.com.au/books?hl=en&...

Fossils of parasites: what can the fossil record tell us about the evolution of parasitism?

Parasites are common in many ecosystems, yet because of their nature, they do not fossilise readily and are very rare in the geological record. This makes it challenging to study the evolutionary tra...