Okay kiddo, phylloceratina is a big word that describes a group of creatures that lived a long time ago. They were called cephalopods and they were related to squids and octopuses that you might see in the ocean today.
Phylloceratina had a hard outer shell kind of like a snail, but they also had tentacles that they used to catch food and move around. They lived in the water and they could be really big or really small.
But the most important thing about phylloceratina is that they lived a really long time ago - way before humans or even dinosaurs existed! Scientists study their fossils to learn more about what life was like on Earth a really long time ago.