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Australia: The Land Where Time Began |
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Devonian – Fish
Faunas of the Late Devonian The
Devonian has often
been referred to as the age of Fishes. Detailed information has been
found in many locations dating to the Late Devonian about fish faunas in
a variety of different habitats. One of the most intensively studied is
the Gogo Formation
in Western Australia, the site of a reef that dates to the Late
Devonian. Though this reef on land has been known of since the 1940s, in
recent years excavations at this site have uncovered many fishes with
minute detail of their internal soft tissue that has been preserved. The
reproductive structures of placoderms and their embryos have been
described (Long et al. 2009;
Ahlberg et al., 2009), and
among the amazing finds is evidence of internal fertilisation in these
fishes. Long (2006) has described the fossil fish of the Gogo formation
in Swimming in Stone [Also
The Rise of Fishes, 1995,
2011]. In the Late Devonian this locality was a marine tropical reef. A
tetrapod-related animal has been recovered from this locality,
Gogonasus, the closest
known form related to a tetrapod in this deposit, though to date none of
the complement of genera that are usually also present in the same
deposits as those that yield tetrapods have been found, and according to
Clack1 this suggests that the Gogo deposit may have been a
deposit that was rather different from those previously known to contain
tetrapods. The Miguasha or Escuminac Bay locality in eastern
Canada is one of the richest localities for Devonian fossil fish,
providing a picture of the kinds of fish that lived in the same habitats
as the elpisostegalian fish and their environments. As recognition of
the importance which has been attributed to this fossil fauna it is now
a World Heritage Centre. The sequence in this locality has been dated to
the early Frasnian and recent work has provided a detailed picture of
this locality and its fauna (Schultze & Cloutier, 1996; Cloutier et
al., 1996). Previously this
locality had been interpreted as representing a freshwater environment
(Chidiac, 1996), it has been shown by geochemical and faunal studies to
have been a coastal brackish to marine environment. Schultze and
Cloutier edited a compendium volume in 1996 that described the flora and
fauna. In Miguasha the chondrichthyans were the only
vertebrate group that was not represented, and the
anaspids and the
osteostracans
represented the jawless (agnathan)
fishes that are the primitive fishes related to lampreys of the present.
The anaspids, which are elongated, narrow-bodied fish with a downturned
tail and no paired fins are believed to have been filter feeders. The
osteostracans, such as
Alaspis, had flat,
semicircular head shields that were composed of bony plates up to 300 mm
across, and paired eyes that were close together on the top of the
shield, and a single nostril. The head shields of some osteostracans had
fields of what has been believed may have been electric organs. They had
a pair of pectoral appendages that were flaplike attached to the back of
the head shield; with the rest of the body being covered by scales, and
they are believed to possibly have been detritus feeders on the sea
floor. The heavily armoured placoderms,
antiarchs and
arthrodires, were
common at Miguasha. It has been suggested recently that the placoderms
as a whole may not have been a natural group (Brazeau, 2009). The head
shields of both forms were heavily armoured and there was a joint
between the head amour proper and the shoulder region.
Bothriolepis, one of the
best known of the antiarchs was a moderately sized form about 200-250 mm
in total length. This genus had a worldwide distribution, being found in
comparable sites around the world, and it is the most common fish at
Miguasha. Compared to the shoulder shield the head shield of antiarchs
was short, and there were a pair of appendages near the junction that
articulated at a complex shoulder joint. These appendages were armoured
externally and jointed in the middle, which gave them the appearance of
a crab’s claws. Though they had jaws they had no true teeth and have
been suggested to have been scavengers. They have been suggested to
probably have had lungs, based on some that were preserved at Miguasha,
though Clack1 says this is debatable (Denison, 1941). Unlike antiarchs, arthrodires were major predatory
forms. Some reached quite large size, having heads that were almost a
metre long and with bodies that were possibly a further 2 m long. They
had enormous jaws that had built-in toothlike pincers or scissors. Their
pectoral appendages and pelvic girdles were unarmoured, though their
pelvic fins are not well known. Much of the body doesn’t appear to have
had scales. The arthrodire present at Miguasha was
Pleurdosteus, which
measured up to 250 mm in length.
Placoderms are
gnathostomes, (jawed fishes), though their relationships with other
gnathostomes is not certain. Placoderms and another group of early
gnathostomes, the acanthodians (the spiny sharks), are all extinct.
There were several genera of placoderms at Miguasha, some of which were
larger, being up to 159 mm long, long-bodied, small-finned forms, and
some with shorter bodies and longer fins, up to 80 mm long. The most
notable feature of
acanthodians was the fins. A bony spine supported the midline and
paired fins along the leading edge. There were more than 2 pairs of
paired fins ventrally on some acanthodians. The
tetrapods are the only one of the true osteichthyans groups not to
be represented at Miguasha.
Cheirolepis, a primitive
form, that grew up to 500 mm long, represented the ray-finned fishes,
and the lobe-fins,
lungfishes,
coelacanths,
porolepiforms, and
osteolepiforms are
all represented there. The lungfishes were short-bodied forms that had
elongated second dorsal fins.
Fleurantia, that was
rather rare, and occurred only in the lower half of the sequence
(Cloutier et al., 1996) was
420 mm in maximum length, had a long snout and a denticulated palate,
while
Scaumenacia, the 4th
most abundant genus at Miguasha, occurred throughout the sequence, grew
to a maximum length of 645 mm, had a short snout and tooth plates on the
palate, which indicated the 2 genera had different habits and diets.
Miguashaia was a
primitive coelacanth that grew to 450 mm, did not have the
characteristically modified tail, dorsal and anal fins that are present
in all later coelacanths.
Holoptychius is the
porolepiforms that is best known from this locality, grew up to 470 mm,
and
Eusthenopteron, which is
the most common tristichopterid, occurred throughout the sequence, is
one of the most famous and best-known fossil fish in the world. It is
known from individuals that cover a wide range of sizes, growing to
about 500 mm in length. With regard to tetrapod origins and
relationships
Elpistostege, the closest
to a tetrapod in the Escuminac Bay fauna, is one of the most significant
fishes in the fauna, though only 3 incomplete specimens have been
recovered. The skull of
Elpistostege, at about
210 mm, is slightly smaller than that of
Panderichthys. It has
been found only near the top of the sequence (see Schultze & Cloutier,
1996). This overview of the Miguasha fauna gives an idea
of the range of vertebrates known from the Middle to Late Devonian,
against which the record of the earliest tetrapods can be viewed.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |