Ostracoderms

Filter feeding Ostracoderms with head shields

Ostracoderms are among the earliest vertebrate fossils going back over 500 million years to the late Cambrian period.  They are a grouping of convenience and do not represent a natural evolutionary assemblage.  For the most part the term Ostracoderm describes several extinct groups of armored, but jawless, fish.  They are generally small, ranging up to only a few centimeters, with some exceptions.  They have no paired fins so their swimming ability would be somewhat hampered by a lack of stability.  Much like Archaeopteryx represented a very early effort at winged flight despite real limitations the earliest swimming vertebrates worked with similar structural handicaps.  Despite this, these animals succeeded.  The reason being is that they were the pioneers of a new style of life and they had no other competition during their initial radiation throughout the watery environment. 

Pteraspids displaying dorsal spines on armor

The bony scales of this class of animal gave them the name Ostracoderm, which means “shell skins”.  Besides scales many of these animals also had heavy armor, particularly about the head.  This armor was made up of three principle layers – an outer layer which consisted of dentine with possible enamel projections; a middle layer of bone that was riddled with channels inhabited by blood vessels and, possibly, sensory pits; the innermost layer also bone, but with few vascular channels, and lamellar, layered like an onion.  The most primitive vertebrates would be generally more heavily armored but, over the millions of years, their descendants would optimize the virtue of strength with the burden of weight. 

While the armor of ostracoderms contained elements of bone the animal’s internal skeleton was all cartilage much as are the skeletons of modern sharks.   Armor would protect the animal’s brain and well as its delicate gills.  The gills were not slits like most fish today but existed as filaments arranged over the surface of a pouchlike chamber that exited to the body’s surface through a pore.  These gill chambers could be highly variable in number, depending on the type of ostracoderm, ranging from five to fifteen on each side of the animal. 

Anaspids were without head armor

The earliest ostracoderms were undoubtedly filter feeders, as their mouth was very small and they had virtually no oral cavity.  There was a large pharynx, though, and mucus would cover the gill bars, trapping the microscopic food particles.  Cilia would move the food-enriched mucus to the intestine, completing the feeding process.  Over time most ostracoderm variations developed larger mouths and oral cavities that came with a rasping organ that served much like a tongue.  Despite obtaining these features ostracoderms, such as anaspids and cephalaspids, were limited to nibbling on smaller animals because they lacked jaws and, presumably, anything we might consider teeth. 

Earliest Placoderms had jaws but no paired appendages

The ostracoderms flourished for more than 100 million years before their extinction in the late Devonian period.  By that time numerous forms of jawed fish were beginning to dominate the environments of both fresh and marine waters.  Jaws, and the teeth that eventually came with them, opened up the diet possibilities of animals.  Jawed fish would be able to successfully radiate into all available niches and eventually outcompete the more ancient body-plan of ostracoderm.  The appearance of paired lateral appendages in the form of fins added immensely to the fishes’ ability to swim and maneuver.  Given the fact that placoderms, or jawed fish, were around at least fifty million years prior to the extinction of ostracoderms makes the length of reign of these jawless vertebrates all the more remarkable.