To tame a Rhyniognatha, first dispatch one, collecting the Rhyniognatha Pheromone it drops. Transfer this pheromone to a larger dinosaur's inventory and allow it to ingest it. Next, seek out a female Rhyniognatha, reducing her health to under 10%. This will lead her to impregnate your dinosaur. During the gestation period, provide your dino with its desired foods. After a while, your new Rhyniognatha will emerge.