Gnathostomes (Infraphylum
Gnathostomata) include all of the jawed vertebrates. Jaws
opened up many adaptive pathways including predation. Jaws
allow prey to be secured, manipulated and eaten. Evidence
suggests that jaws formed from modified gill arches. Placoderms
(class Placodermi "tablet or plate skin") were
heavily armored jawed fish that range from the Silurian
to the
Devonian.
Armored Jawed Fish
Placoderms reached
their greatest diversity during the Devonian, often referred
to as the "Age of Fish", and were in fact the dominant fish
group during this time (Prothero, 1998, p. 345). Placoderms
possessed articulated bony plates over their head and
shoulder regions. A neck joint allowed placoderms to
lift the anterior portion of their head shield. A cartilaginous
skeleton supported the placoderm body. Placoderms had
scales or skin covering the rest of their body and
tail. Placoderms had a long dorsal fin and a heterocercal
tail (an asymmetrical tail in which the dorsal lobe is
extended due to an upward flexion of the spine). Placoderms
had paired pectoral fins and were the first vertebrates
to evolve paired pelvic fins (Benton,
2005,
p. 55). The majority of placoderms did not have true
teeth, their head shield and mandibular shield were modified
into sharp-edged cutting surfaces. Most placoderms were
small fish reaching lengths of 15 cm;
however, a
few species reached lengths of 4 to 10 meters. Placoderms
lived in both marine and freshwater environments. Currently,
Placodermi includes seven clades.
Arthrodira
Arthrodires (order
Arthrodira "jointed neck") make up more than
half of all known placoderms and reached their greatest
diversity in the Devonian. Although sometimes heavily
armored these fish diversified into most marine ecological
niches. Arthrodires had a ball-and-socket joint between
their head and neck armor. This joint allowed the upper
jaw
to swing upwards, while the lower jaw dropped creating
a wide mouth opening. Arthrodires had paired pectoral
and pelvic fins. The edges of their bony plates acted
as a cutting surface, although there is evidence that
some had teeth inside their mouth. Arthrodires have
a ring of bony plates surrounding their eyes. Some
Arthrodires reached lengths of up to 10 meters. Dunkleosteus was
the largest predator known from the Devonian seas (Benton,
2005, pp. 55-57). Some arthrodire plates have puncture
wounds from other arthrodires (Prothero, 1998, p. 346).
Acanthothoraci
Acanthothoracids (order
Acanthothoraci "spiny chests") are basal
placoderms that resemble early arthrodires. As adults
their armor plates fused together. Like typical placoderms
they had beak-like tooth projections and bone enhanced
eye sockets.
Rhenanida
Rhenanids (order
Rhenanida "Rhine Fish") had scales and tubercles covering the head
and neck region instead of heavy armor. Rhenanids looked like modern day skates
orrays as their body was flattened and possessed broad pectoral fins. This
is a good example of convergent evolution as rhenanids were placoderms not
chondrichthyans.
Antiarchi
Antiarchs (order
Antiarchi "opposite anus") were mostly small,
freshwater forms and retained heavy armor. The thorasic
shield covered almost half of their
body. They had a flattened
head
shield
with eyes pointing
upward and lived along the bottom swallowing sediment
and extracting organic
matter.
Their pectoral
fin was
enclosed in bone and was jointed. This arthropod-like
limb may have been used to crawl along the bottom and
cover the fish with sediment. Antiarchs
were the second most successful group of placoderms.
Petalichthyida
Petalichthyids (order
Petalichthyida) were small bottom-dwelling forms. They
had splayed fins and numerous tubercles decorating the
plates and scales of their armor.
Phyllolepida
Phyllolepids (order
Phyllolepida "leaf scales") are flattened bottom
dwelling freshwater placoderms. The armor of phyllolepids
were made of whole plates. Phyllolepids had vestigial
eyes and were most likely blind.
Ptyctodontida
Ptyctodonts (order
Ptyctodontida "beak teeth") are small (20 cm)
placoderms with reduced armor plating. They had large heads
and eyes with long whip-like tails. The anterior dorsal
spine was supported by long spine. The posterior dorsal
fin was long. Some ptyctodonts had claspers associated
with their pelvic fins that may have been used for internal
fertilization (Benton, 2005, p. 58). Ptyctodonts had thick
tooth plates for crushing molluscs and looked superficially
like the modern day chimaera or ratfish.
Placoderms
were one of the first groups of fish to evolve jaws and
reached their greatest diversity during the Devonian. Placoderms
did
not survive
the extinction
event
at the
end of the
Devonian.
Science Olympiad Fossil Event
The 2016 Science Olympiad Fossil List includes the class Placodermi (Armored Fish).
Two genera are included within this category Dunkleosteus and Bothriolepis. |
