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.  

Ammocoete and Primitive Vertebrates

Ammocoete with gill-like pores and hooded mouth

Ammocoetes are the long-lived larva of the jawless lamprey, a parasitic vertebrate in its adult form.  The larval lamprey lives as a mostly sedentary filter-feeder in the soft sediment of freshwater streams.  It can live in this larval stage for up to seven years before its metamorphosis into a marine or freshwater adult.  The adult lamprey usually lives one, possibly two, years before it reproduces.  In the case of the North American brook lamprey the animal breeds in the spring following its metamorphosis and never feeds, so it is not parasitic.  In any event the female and male adult animals die once the eggs are laid and fertilized.

Adult Lamprey with gill-like pores and toothed mouth

North America has 41 known species of lamprey of which 19 are strictly freshwater.  It has the appearance of a primitive looking eel.  The lamprey has no jaws, no scales, no paired fins like other fish and it has no bone.  It does have teeth on its tongue that it uses to rasp a hole in the side of large fish to which it clings and feeds.  The lamprey has all the features needed to classify it as a vertebrate and it belongs in the class of animals known as Agnatha.  Except for lamprey and hagfish all the many representatives of this jawless class of vertebrates have been extinct for more than 200 million years.  These ancient agnathans are believed to be the first known vertebrates to evolve.

Lamprey has no paired appendages

Biologists concerned with how these first vertebrates lived have focused their interest on the ammocoete, the larval form of the lamprey.  Even though the adult lamprey retains most of the characteristics of its larval stage, it is the ammocoete that is believed to most closely resemble the body plan of the earliest agnathans.  The ammocoete has a long, slender body with an oral hood that surrounds its mouth.  The animal feeds on micro-organisms in the water it takes in by creating a current through a muscular pumping action that is much like modern fish.  The base of its pharynx has what is called an endostyle, a glandular groove that produces mucus – trapping the food particles and passing them directly to the intestine through the patterned beating of cilia. 

Lamprey's mouth is a suction cup with teeth

There is a rodlike structure, the notochord, which extends the length of the body and serves as the skeletal axis from which muscles are attached.  The skeleton of lamprey is cartilaginous.  There is a dorsal nerve cord above the digestive tract from which nerves extend to enervate the animal’s segmented muscles enabling it to propel itself through the water using an undulating motion.  The anterior portion of the nerve cord is enlarged to create the animal’s brain.  Two eyes are present but they lie beneath layers of skin and will not emerge until the adult stage.  There is one median nostril, auditory vesicles as well as both the thyroid and pituitary glands.  There are also seven pair of pharyngeal pouches with pharyngeal bars separating these perforated pouches and act in the manner of gills on modern fish. 

Lamprey cling to fish like giant leeches 

Ammocoete has kidneys and a liver with a gall bladder.  It has pancreatic tissue but no distinct pancreatic gland.  There is also a closed circulatory system with a two-chambered heart – containing an atrium and a ventricle.  The atrium receives blood from the veins while the ventricle pumps blood into the arteries. 

The hypothesis that the earliest vertebrates were likely to have a body plan such as this has to have been drawn from inductive reasoning as the fossil record gives little evidence as to the nature of an animal’s soft, internal tissue.  Comparative anatomy of contemporary species is one source and DNA mapping increases in its influence as genetic coding becomes better known and understood.