But cycads and other plants were more diverse in the Jurassic period than today and they would have had their predators and associated animals. Ecosystems in the Jurassic period were probably just as complex as those of today, with different animals and plants. The problem is that our record of terrestrial ecosystems is extremely patchy. We have good fossil evidence for the complexity of marine ecosystems because sediment accumulates in the ocean basins and this is where fossils are formed. On land most environments are areas of erosion, albeit only slight in many places, rather than areas of sediment accumulation.
Exceptions are some of the larger inland lakes and river systems.
The deposits commonly found in river systems are often gravels and sands that are not condusive to the exceptional preservation of delicate organisms such as insects. There are exceptions of course. In the north east of Brazil there is a deposit that yields hundreds of different types of fossil insects and it shows that dragonfly diversity here was as high, if not higher, in the Cretaceous period than it is today. The fossil record on the whole is extremely bad, but it has some windows to the past that are really rather clear. These deposits show that some ecosystems were very complex.