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Australia: The Land Where Time Began |
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Jawless Fish
- Agnathans
Agnathans have a fossil record spanning almost 500 million years. The
earliest recognisable bony shield from an agnath came from a 450 million
years old Early
Ordovician shallow marine sandstone in Central
Australia. The oldest almost complete specimen of a related genus is
from Bolivia, South America, another part of Gondwana. The name agnathan
of the jawless fish is now seen as the basal level of fish evolution
instead of referring to a specific group of fish, they are thought of as
a paraphyletic group not being characterised by any features that are
unique.
Among these fish can be found the break-throughs in design that became
standard characteristics for vertebrate design on to the present, the
development of cellular bone, paired limbs, sensory-line systems,
dentine-like tissue, complex eye muscle patterns, the inner ear with 2
semicircular canals.
The primitive
Ordovician forms, including the
Australian forms,
Arandaspis and related species,
share the primitive feature of a number of paired openings for the
gills. The number of paired openings for the gills was reduced in all
known subsequent agnathans except some anaspids.
The high point of jawless vertebrates was during the
Ordovician to Early
Silurian. There were 2 major groups of jawless
fish that existed during the
Devonian,
the Pteraspidomorphi (or Diplorhina) and the
Cephalaspidomorphi (or Monorhina). There is still controversy
about how the
placoderms
should be divided between the 2 groups. It seems only the
Pteraspidomorphi had much of a presence in Australia, and possibly in
Gondwana, including the heterostracans
and
thelodonts. The Cephalaspidomorphi were
essentially restricted to the continents in the Northern Hemisphere,
included the
osteostracans and the anaspids.
A number of characteristics differentiate the pteraspidomorphs from the
Cephalaspidomorphs. An important distinguishing characteristic is the
position and number of the nasal sacs. The pteraspidomorphs had paired
nasal sacs, 1 on either side of the midline (called diplorhinans). The
Cephalaspidomorphi had a centrally located single nasal sac. In this
feature the pteraspidomorphs are similar to that in the
Arandaspis
and Sacambaspis, so are thought to be very primitive fish.
Heterostracan Pteraspidomorphs had a fairly solid cover over their head
and throat (pharynx), with a single outlet for the water passing over
their gills, they lacked any internal ossification of their internal
Skelton and their bony external shield was acellular, and none developed
paired fins.
There is a macrofossil and microfossil record of the pteraspidomorphs.
The heterostracans have left a reasonable macrofossil record, the
thelodonts, first appearing in the earliest Silurian, are mostly known
as microfossils. The bodies of the thelodonts seem to have been covered
by thousands of tiny, rhombic, non-overlapping scales. They are useful
for dating rocks of Silurian-Devonian age because of these microscopic
remains.
The Agnatha, including both groups, diversified greatly in the Late
Silurian to Early to Middle
Devonian, throughout the world. It was a
time of explosive adaptive radiation for the placoderms and for fish in
general.
There is a break in the known fossil record of the Agnathans until the
Devonian. When the agnathan record resumes it is
mostly thelodonts. The Australian forms seem to follow the trends in
diversity of the rest of the world. By the Middle Devonian they are
almost totally eclipsed by the extremely successful
placoderms. They, in turn, are replaced by the
end of the
Devonian,
by the evolution of more advanced fish. The agnathans were dominant for
almost 100 million years, a few even surviving until the present in the
form of the lampreys and hagfish.
The placoderms coexisted with the agnathans for 10-20 million years and
when the more advanced fish evolved they persisted for at least another
15 million years, competing with "spiny sharks" and the early bony fish.
The agnathans never developed both paired pectoral fins and pelvic fins,
so their speed and manoeuvrability was limited, which is probably why
they were unable to survive competition from the bony fishes in the long
run.
A variety of body shapes developed, flattened, fusiform, or cylindrical,
but they were mostly restricted to filter feeding by the lack of jaws,
with the exception of the lampreys, successfully evolving as parasites,
and the similar hag fish that specialised as scavengers.
When
Arandaspis and Porophoraspis
lived in the warm shallow seas of the
Ordovician,
Australia was still part of Gondwana and was situated near the equator.
It was on the north-west sector of Gondwana and was divided into 2 large
sections. Part of present-day Central Australia formed one block and to
the south another island. The 2 parts of Australia were separated by the
Larapinta Sea, a large tropical sea that formed
when the water invaded much of the continent. To the south was the
ocean.
Most of the agnathan families were extinct by the Late Devonian. Among
the few survivors were 4 species osteostracans and lamprey-like
anaspids, Euphanerops, that have been found in the
Escuminac Formation in Canada, 1 possible, though indeterminate, species
of
galeaspid
from Ningxia, northern China, 1 thelodont genus, Australolepis,
from Australia, and from Europe a few species of psammosteid
heterostracans that were large and flattened. Long suggests that the
steep decline in agnathan diversity at this time may be the result of
the rapid increase in the diversity of the gnathostomes that also
occurred at this time that may have either displaced them or preyed upon
the agnathans, their larvae or eggs.
Both in diversity and biomass the agnathans were the dominant fish type
in the Silurian, as the gnathostomes were comparatively rare at this
time. All major groups of gnathostomes were present in the oceans by the
start of the Devonian, some, such as the stem gnathostomes,
acanthodians, reaching high levels of diversity in the latest part of
the Early Devonian. Many of the gnathostome groups, such as placoderms,
porolepiforms, tetrapodomorph fish and dipnoans reached a peak of
diversity by the Middle and Late Devonian.
Long suggests that some gnathostomes could possibly have taken over the
niches of agnathans that had been eliminated by increasing predation
pressure because they had similar body shapes. The phyllolepid
placoderms appeared immediately after the flattened psammosteid
agnathans, the transition being seen in the same succession in the east
Baltic region. It is suggested that the early placoderms with long
shields outcompeted the heterostracans with long shields, possibly
because they had better defences or because they were faster swimmers. A
suggestion is that the many new forms of bottom-feeding placoderms, such
as antiarchs, may have displaced the detrital bottom-feeding agnathans.
The lampreys and hagfishes, forms of agnathans that lacked armour, were
the only surviving agnathans by 355 Ma at the close of the Devonian.
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Fish |
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |