While paleontologists have a wealth of vertebrate fossils at their disposal, their knowledge of the ecology of ancient extinct species, particularly regarding their relationship with invertebrate species, is relatively poor. As bones and hard shells fossilize much better than soft tissues and cartilage, scientists are limited in their ability to infer the presence of parasitic or symbiotic organisms living in or on these ancient vertebrates. As a result, relatively little is known about the evolutionary relationships between these ancient clades and their modern descendants.
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Source: Phys.org