Acanthodian
The Acanthodian shark was considered a transitional species between sharks and bony fish like tuna. These creatures were one of the first animals to have a jaw, and just like modern sharks, they had a skeleton made entirely of cartilage. Its fins were supported by bony spines, like the modern-day ray-finned fish.
Known as “spiny sharks,” they rarely grew more prominent than 30 centimeters, but since they lived more than 360 million years ago, they were still considered one of the largest filter-feeding animals of its time. Filter feeders trap food particles through their gills.