I promised this blog would feature more fish this year, but thus far, all we have seen is the remarkable ocean sunfish…so today we travel way back in time to the oceans of the Paleozoic world to check out some spiny sharks. However these “sharks” are really different from what you are probably expecting! During the Late Silurian and Early Devonian the oceans were filled with Climatius reticulatus a fish which took up a niche analogous to great schools of anchovies & sardines which swim in today’s oceans. Climatius reticulatus grew to be only 7.5 centimetres (3 inches) long! Some of these remarkable illustrations are bigger than they were! I am calling them “sharks” because they are indeed commonly known as spiny sharks, but they are more properly acanthodians—an early order of jawed vertebrates which shared some features with both bony fish and cartilaginous fish. Climatius reticulatus did have a cartilaginous skeleton, so don’t go thinking I am completely misleading you with quotation marks and paleontological hokum.
Although this fish was tiny with a squishy skeleton, it was not defenseless: each little Climatius sported fifteen razor sharp spines. Presumably they also swam together in great schools which would dazzle and mislead predators of long ago just as shoals of fish do today. Speaking of which, the predators of 420 million years ago were most likely anomalocaridids (horrifying giant arthropods, which were on their way out) cephalopods, and scary new vertebrate predators like Dunkleosteus.
Climatius was itself a predator too. It had a powerful caudal fin and large complicated eyes in order to find and capture the little animals swimming in the plankton of the ancient seas. The first acanthodians had appeared in the ocean back during the Ordovician (the age of cephalopods). They predate sharks and bony fish and were probably related to a basal ancestor of both. By the early Devonian, however the bony fishes were coming into their own and fierce competition from these magnificent teleosts soon drove the thriving schools of Climatius (and other similar acanthodians) into oblivion.