This story was at Dr Wile's blog a way back. At https://phys.org/print405956660.html ... dinosaurs are mostly known from fossilised bone but last year palaeontologists claimed they have now found collagen in a rib from 195 million years ago - which caused a bit of a stir as proteins were not supposed to survive for that long a time. An article in Nature Communications (Feb 2017) re-examines the issues involved, particularly the survival of soft tissue.
At https://phys.org/print405934592.html ... well preserved remains of sponges have been found in China, dating back over 400 million years. Sponges are soft tissue animals so this is remarkable in itself. They are found close to the mass extinction event at the end of the Ordovician period, when 85 per cent of species are estimated to have met a catastrophic fate. A sudden ice age is part of the scenario (a rapid cooling of the earth) but the theme is that many species at the boundary event wee quickly buried in water formed sediments.
A new fossil fauna has been uncovered by the Chinese, thought to date from just after the extinction event. A narrow band of mudstone was exposed in bamboo forest in China that included a varied fauna - but a remarkable variety of sponges. Over 100 species have been recorded so far - not all of them sponges as nautiloids and sea scorpions are also mentioned. The problem here is that life in the seas was thriving - just after the big event. Why did sponges proliferate?
In the Cretaceous period sponges were also common, found inside round flint balls that can be split open. The flint would have been viscuous at the time and accumulated around the sponges (these varieties were round) and have survived as one of the few fossils within the upper chalk layers (apart from the coccolith shells that actually form the chalk). Going back to the Ordovician boundary event, sponges were obviously thriving. The study authors speculate that conditions suited them at the time - high temperature ocean water and low levels of oxygen. There would also have been a lot of organic material to feed on as a result of the die-off event. An interesting discovery.