If there's one kind of animal that fascinates me, that'd be parasitoid wasps. How did they become endoparasitoids? There are as many extant intermediate steps, so to speak, as imaginable, and treatises that describe them. But then there's also the fossil record:
"Parasitoid biology preserved in mineralized fossils", by van de Kamp et al. 2018 https://www.nature.com/articles/s41467-018-05654-y
"About 50% of all animal species are considered parasites. The linkage of species diversity to a parasitic lifestyle is especially evident in the insect order Hymenoptera... using high-throughput synchrotron X-ray microtomography, we examine 1510 phosphatized fly pupae from the Paleogene of France and identify 55 parasitation events by four wasp species"
Evidence for a parasitic lifestyle in extinct species tends to be indirect. Here, the authors provide direct evidence through X-ray examination of approximately 30–40 million year old fossil fly pupae, revealing 55 parasitation events by four newly described wasp species.
NIOO-KNAW
Albert Cardona