The First Vertebrates

Agnathans are fish or fish-like jawless vertebrates, which range in size from just a few centimeters up to 2 meters. Agnathans were the first vertebrates to evolve and range from the Cambrian to recent times. Many extinct agnathans had heavy armor plates around their head and thick protective scales covering their tails. Paleozoic agnathans underwent an adaptive radiation during the Ordovician, reaching their peak in the Late Silurian and Devonian (Pinna, 1990, p. 187). Agnathans were widespread during this time inhabiting both marine and freshwater environments. Only two extant jawless vertebrates, the hagfish and lamprey, represent the great diversity of the Paleozoic agathans. Except for the characteristic of being jawless these present day fish have little in common with past jawless craniates. Extant jawless fish have eel-like bodies with no scales or paired fins.

Hagfish

Hagfish or “slime eels” (Subclass Myxinoidea) range from the Late Carboniferous to recent times. Hagfish are primitive jawless marine fish that possess a scaleless eel-shaped body with multiple gill slits, and unpaired fins. Hagfish have vestigial eyes with no lenses or no eyes at all. A cartilaginous skull and a notochord support the hagfish body (hagfish lack vertebrae). Hagfish live on the continental shelf feeding on dead or dying fish and polychaete worms. Hagfish attach themselves to dead or dying fish with their barbell adorned, suction cup-like mouth and use a toothed tongue to scrape a hole into the fish or burrow into the body cavity. Sometimes they tie themselves into a knot and work the knot downward, using it to leverage themselves against their food item to tear off flesh. This knot tying behavior combined with the production of large amounts of slime can also help hagfish escape from predators (Prothero, 1998, p. 339).

Lampreys

Lampreys (Subclass Petromyzontida) range from Devonian to recent times. Lampreys are primitive jawless marine fish that possess a scaleless eel-shaped body with multiple gill slits, unpaired fins, and an internal skeleton made of cartilage. Lampreys have two large eyes with a single nostril on top of their head. Lampreys are parasitic on other fish. Lampreys have a circular jawless mouth equipped with hundreds of teeth that they use to attach to other fish. They use a rasping tongue to scrape away flesh after which they suck up tissues and fluids. Lampreys live in both marine and fresh-water environments. Lampreys swim up streams to bread in freshwater rivers or lakes. Lampreys have become established in the Great Lakes after connecting canals were built. The cost of protecting native fish from this invasive species has been great (Prothero, 1998, 339).

The First Jawless Vertebrates

The first evidence of jawless fish are found in the Early Cambrian aged Chengjiang Lagerstatten in Yunnan Province, south-west China. The eel-like fish Haikouichthys and Myllokunmingia are two possible agnathans found in the Maotianshan shales (Benton, 2005, pp. 9-10, 39-40).

Conodonts

Conodonts (Subclass Conodota “cone tooth”) have been known as microscopic tooth-shaped objects found in Paleozoic to Triassic marine sedimentary rock since 1856. The organism to which these tooth-shaped structures belonged remained a mystery for over one hundred years. Even so, their rapid evolution and abundance have made them important biostratigraphic index fossils for the Paleozoic. The color of these calcium phosphate structures changes with heat and can aid in determining if rock contains hydrocarbons that have properly “baked” into oil, thus making them economically valuable (Protheros, 1998, p. 342). A complete conodont specimen Clydagnathus was reported from the Lower Carboniferous of Edinburgh in 1983. Conodonts were lamprey-like organisms. The tooth-like structures were embedded in the pharyngeal region and acted to seize and chop prey (Benton, 2005, pp 45-47).

Armored Jawless Vertebrates

Agnathans in the Subclass Pteraspidomorphi range from Ordovician to the Devonian. In general, these armored agnathans had a tadpole-shaped body with large bony plates covering the head and a scaly tail. Evidence suggests that they lived in shallow marine environments as bottom feeders. These primitive armored fish are grouped into four orders: Astraspids, Arandaspids, Heterostraci, & Eryptichiida. Heterostrachans (order Heterostraci “different scales”) range from the Silurian to the Devonian and are the best-known Pteraspidomophs. In general heterostrachans had a dorsal and ventral plate protecting the head region. Heterostrachans did not have paired fins; however, some had wing-like extensions on their head shield. Scales covered their tail. Astraspids (order Astraspida “star shield”) and Arandaspids (order Arandaspida) are known from the Ordovician. These small fish, 20 cm, had massive head shields that covered most of their body. The tail was mobile and covered with protruding pointed plates. Fossil specimens show the presence of a lateral line system that is characteristic of all fish except the hagfish. The lateral line is made of open pores that form a line along the sides of a fish, which can detect pressure changes from disturbances in the water (Benton, 2005, p. 47).

Jawless Vertebrates with Limited Armor

Two orders of extinct agnathans are of unknown affinity. Members of the order Anaspida (without shields”) range from the Silurian to the Devonian. Anaspids had bodies covered with rows of heavy scales. Members of the order Thelodonti (“nipple teeth”) range from the Ordovician to the Devonian. Thelodonts possessed a tube-like body covered with scales.

Armored Jawless Vertebrates & Paired Fins

Three clades of jawless fish are united by the possession of a massive head shield. These armored fish are sometimes grouped as Cephalaspidomorphs. Cephalaspidomorphs were the most advanced jawless vertebrates. The subclass for these three orders (Osteostraci, Galeaspida, Pituraispida) is unnamed (Benton, 2005, p. 390). Osteostracans (order Osteostraci “bony shield”) are the best known of this group and range from the Ordovician to the Devonian. The heavily armored head shield resembles the toe of a boot. In some species the head shield is fused into a single bony plate. Two eyes and a pineal gland (“third eye”) pointed upwards. Osteostracans had a pair of pectoral flaps just behind the head shield, making them the first vertebrates with paired fin-like structures. These were not true fins, but acted to stabilize the fish as it swam (Prothero, 1998, p. 340). Galeaspids (order Galeaspida “helmet shield”) range from the Silurian to the Devonian. Galeaspids possessed more gill pouches (up to 45) than any other vertebrate group. Many galeaspids had long spines growing from their head shield that may have acted to stabilize them when swimming. Pituriaspids (order Pituraispida “Pituri shield”) are known from two Early Devonian aged species of Australia. Pituriaspids had a long nose-like rostrum. Pituri is a hallucinogenic drug and was used to name these unusual fish because its discoverer could not believe what he was seeing.