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 or “slime
Myxinoidea) range from the Late Carboniferous to recent times.
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
into the fish or burrow
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 (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).
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
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
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
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
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
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.