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Great Barrier Reef - Australia
Living in association with the Great Barrier Reef is a multitude of higher animals which
include shelled animals of the phylum Mollusca (clams, cowries, mussels and sea-snails),
radially symmetrical creatures of the phylum Echinodermata and includes sea urchins and
starfish, elongated animals with segmented bodies occurring in the phylums Annelida and
Arthropoda which includes bristle worms, shrimps and crabs as well as the vertebrates
(phylum Chordata) which includes cartilaginous and bony fishes and marine mammals
such as dolphins and seals.
The giant clam Tridacna gigas and a Parrot Fish. The Giant Clam is the largest
living bivalve - mollusc. One of a number of large clam species native to the
shallow coral-reefs of the South Pacific and Indian oceans, they can weigh more
than 400 pounds and measure as much as 1.5 meters across. Sessile in
adulthood, the creature's mantle tissues act as a habitat for the symbiotic singlecelled dinoflagellate-algae from which it gets it nutrition. By day, the clam
spreads out its mantle tissue so that the algae receive the sunlight they need to
photosynthesize.
Parrot Fish are mostly tropical, perciform (perch-like) marine fish of the family
Scaridae. Abundant on shallow reefs of the Atlantic, Indian and Pacific Oceans,
the Parrot Fish family contains nine genera and about 80 species. Parrot Fish are
named for their oral-dentition: their numerous teeth are arranged in a tightly
packed mosaic on the external surface of the jaw bones, forming a parrot-like
beak which is used to rasp algae from coral and other rocky substrates. Many
species are also brightly coloured in shades of blue, green, red and yellow.
Although they are considered to be herbivores, Parrot Fish eat a wide variety of
organisms that live on coral reefs.
Image Source
http://www.richard-seaman.com/Underwater/Australia/Coral/
Tridacna gigas
Tridacna gigas
Giant Clam
Conservation status: Vulnerable
Scientific classification
Kingdom: Animalia
Phylum: Mollusca
Class: Bivalvia
Order: Veneroida
Family: Tridacnidae
Genus: Tridacna
Species: gigas
Binomial name
Tridacna gigas
Linnaeus, 1758
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Parrotfish
Parrotfish
Parrotfish
Midnight parrotfish (Scarus coelestinus)
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
Order: Perciformes
Family: Scaridae
Genera
Bolbometopon
Calotomus
Cetoscarus
Chlorurus
Cryptotomus
Hipposcarus
Leptoscarus
Nicholsina
Scarus
Sparisoma
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Fossil History of Marine Invertebrates
To trace the invertebrate lines we must also look for fossils where animals were deposited
continuously and the fossil remains to have survived in a relatively undistorted condition
such as has occurred in the Atlas Mountains of Morocco and Chengjiang, China and
numerous other places including Burgess deposit in Yoho National Park, Canada.
Chengjiang has the Maotianshan shale and is a late pre-Cambrian rock
formation, of ca 522 Mya, now lying exposed in the Yunnan Province of China. The
deposit consists of nearly 50 meters of mudstone sedimentary strata are exposed,
revealing many excellently-preserved soft-bodied fossilized organisms, which form a
major Lagerstätte, "probably the most significant exceptional preservation above the
Precambrian-Cambrian. The Maotianshan shales provide even stronger evidence than
does the Burgess shale for a Cambrian Explosion wherein a large number of very
different animal body plans seem to have appeared in a disconcertingly short time
interval. Click here for a taxonomic list for the Maotianshan shale fossils.
A Perfectly Preserved Cambrian Worm. The soft body of this worm from the
Cambrian Period was perfectly preserved with pyrite, otherwise known as fool's
gold, over a half billion years ago.
Image Source
http://dsc.discovery.com/news/briefs/20041025/goldfossil_zoom0.html
Maotianshan
Fossil Species of the Maotianshan
Shales
Arthropods
* Acanthomeridion
* Anomalocaris
* Canadaspis
* Chengjiangocaris
* Chuandianella
* Fortiforceps
* Fuxianhuia
* Kuamaia
* Kuanyangia, a trilobite
* Leanchoilia
* Naraoia
* Retifacies
* Saperion
* Sinoburius
* Squamacula
* Xandarella
* Yunnanocephalus, a trilobite
Worms and relatives
* Hallucigenia
* Microdictyon
* Palaeoscolex
Chordates
* Cathaymyrus
* Haikouella
* Haikouichthys
* Myllokummingia
* Yunnanozooan
Others
* Dinomischus
* Eldonia
* Maotianshania
* Opabinia
* Xidazoon
* Extinct
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Flatworm Fossils of Inyo Mountains, California
Whitey Hagadorn looks for tiny tracks in the rocks, evidence he believes, of some
of the earliest animals capable of moving in a forward direction -- animals akin to today's
flatworm. These traces are from the seafloor of 540 million years ago, but is now located
in California's Inyo Mountains, between Kings Canyon and Death Valley National Parks.
"I get a clue here, a clue there, and I try to piece them together and create a story
to find out who the culprit was. About 565 million years ago, there was sort of a
revolution in animal body plans. Organisms for the first time became able to
move on their own accord in a directed fashion. We know this because we can
look at their trace fossils. By looking at this trail, we can certainly say that
whatever made it had the ability to move sediment. Prior to this time, there
weren't many things that could do that."
Fossil trails of a flatworm living on the seafloor 540 million years ago. It seem the
organism responsible likely possessed an obvious front end or head, was equipped
with senses arranged on its head facilitating forward movement, and most likely
featured the first primitive brain.
Cambrian explosion
From this fossil record and from other sites scattered around the world there appears a
clear dichotomy in the history of earth where fossils are found and they are not found.
This period of transition corresponds with about 600 million years and records the first
annuals which are characterized by the presence of shells. It is conceivable that before
this period the animals were soft bodied and did not fossilize. It has also been suggested
that seas were not at the right temperature and or chemical composition to favour
deposition of lime from which most marine shells and skeletons are constructed.
Ediacaran Fossils of Canada
Image Source
http://learning.berkeley.edu
The Cambrian explosion refers to the geologically sudden
appearance of complex multi-cellular macroscopic organisms between roughly
542 and 530 million years ago (mya). This period marks a sharp transition in the
fossil record with the appearance of the earliest members of many phyla of
metazoans (multicelluar animals). The "explosive" appearance of this adaptive
radiation results both from rapid evolutionary change and the limits of previous
technology to appreciate microfossils which formed the foundation of the fossil
record before this time.
With time, advanced microscopy has gradually revealed the range of earlier
microfossils. Prior to the discovery in 1909 of the Burgess Shale—incompletely
published at the time and largely forced into existing categories as "precursors"—
no fossilizations of early soft-bodied organisms had been published, and the vast
reach of undiscovered earlier life was consigned to an enormous space of time—
the "Pre-Cambrian" of old-fashioned schoolbooks.
More recent microfossil finds have showed "Pre-Cambrian" life consisting of
more than single- celled organisms or simple diploblastic fauna (two-layers of
cells in mats and sheets, allowing every cell to be in contact with its watery
mineral-rich environment). In 1994, triploblastic animals (organisms with more
than two layers, and who therefore rely on internal organs and systems for their
cells' supplies of food and waste disposal), were discovered preserved as
phosphatized embryos in rocks from southern China [Xiao et al. 1998]. These
fossils were estimated to be 570 million years of age and thus were even older
than the Ediacaran fauna found in strata about 10 million years younger.
Fossils
This period of evolution is source to some of the most unusual fossils ever
recovered. A single formation, the Burgess shale, has provided some of the best
insights into this period of dramatic evolutionary change and experimentation that
laid the foundation for most major modern animal body plans. Also appearing at
this time are a wide variety of enigmatic and exotic configurations that appear to
be unrelated to any modern animals.
Before the explosion, the fossil record is dominated by single-celled organisms
with only the rare soft-bodied Ediacaran fauna and certain microfossils showing
that multi-cellular life forms had arisen roughly 30 million years earlier (Xiao et al.
1998).
With the Cambrian explosion came the evolution of shells and other hard body
parts. As shells are more easily preserved in sediment than soft body parts, this
makes life forms of this and subsequent periods much easier to study in the fossil
record than their Precambrian counterparts. This also contributes to the
perception of an abrupt change in the fossil record.
Causes of the Cambrian Explosion
The Cambrian explosion may have been precipitated by several environmental
changes occurring in and just before this period. First the Varangian glaciation
gave rise to a snowball Earth in which all, or nearly all, of the oceans are covered
entirely with ice. This was followed by a deglaciation and rapid global warming
just before the beginning of the explosion itself.
In modern Arctic environments, single-celled organisms often form mats on the
underside of ice sheets in order to maximize their exposure to sunlight. It is
possible that adaptations useful to the maintenance of such colonies also
assisted in the formation of the first triploblastic animals (organisms with more
than two layers of cells) estimated to be 570 million years of age (Xiao et al.
1998). In addition, the snowball Earth environment would have given rise to
relatively few ecological niches, so the subsequent deglaciation and global
warming may have provided to impetus for rapid evolution to fill many new
environments.
Diversification
Of the 20 metazoan phyla with extensive fossil records, at least 11 first appeared
in the Cambrian. Of the remainder, 1 is known to Precambrian and the other 8
first appear more recently (Collins 1994). An additional 12 soft-bodied phyla have
poorly defined fossil records, but it is speculated that a significant number of
these may also be Cambrian in origin.
Though this period is definitely of special significance in terms of rapid
diversification and the emergence of new forms, some of that significance is
likely to be overstated by the focus on macroscopic forms in the ways phyla are
observed and defined. Molecular evidence suggests that at least six animal phyla
had established themselves as distinct evolutionary paths during the
Precambrian (Wang et al. 1999).
The sheer variety of forms found in the Burgess shale and other sites, has made
some skeptical that single period of ~10-15 million years could have been long
enough to give rise to such diversity. An emerging view is that the Cambrian
explosion is the macroscopic conclusion to a prolonged period of evolution begun
~30 million years earlier with the innovation of multi-cellular organisms.
To get the latest views on this subject down the PDF from the link below
http://www.mcz.harvard.edu/Departments/InvertZoo/pdf_files/Giribet%202002b4.pdf
Platyhelminthes: the building block for other invertebrates
.
Simpler animals than those first found in the fossil records of the Cambrian Explosion
still inhabit the earth and its oceans and their ancestors may have represented the
predecessors for the shelled invertebrates that are found in the fossil records. These softbodied animals belong to the phylum Platyhelminthes.
The most basic of these animals is the flatworm, a flat-leaf shaped worm which like
jellyfish have a single opening to their gut through which food is ingested and waste is
ejected. Flatworms are bilaterally symmetrical with a defined head and tail region and a
centralized nervous system containing a brain and nerve cords. Clusters of light-sensitive
cells make up what are called eyespots. The head region of the flatworm also contains
other paired sense organs, which are connected to the flatworm's simple brainTheir
bodies have differentiated into three layers, the ectoderm, mesoderm and endoderm.
Cells with a different structure and function have aggregated to form a primitive system
(eg nervous system which consists of a network of nerve fibres). Nevertheless, they
have no breathing system with oxygen diffusing directly through the skin. Their
undersides are covered with cilia which, by beating, permits them to glide over surfaces.
Their front end has a mouth on the under-surface and a few light sensitive spots above.
Flatworms are hermaphroditic and capable of sexual and asexual reproduction. Their
bodies have only a single opening, which serves as both a mouth and an anus.
Platyhelminthes: a surprisingly diverse group
There are more than 20,000 species of Platyhelminthes. They range from brilliantly
colored creatures that swim in the ocean to parasitic flatworms that live inside the bodies
of an estimated 200 million humans around the world. They varying in size from
microscopic to 600 mm, and although most are marine some species have managed to
inhabit humid terrestrial environments and move on a bed of mucus. Many species in
this phylum have become parasitic and live on the surface and inside bodies of other
animals including man. Some of these parasitic forms such as liver flukes still resemble a
basic flatworm form whereas others such as the tape worm have a highly modified
morphology with hooks on their heads and an ability to detach egg-bearing sections of
their posterior body parts.
Annelids: the first segmented animals
It is hypothesized that the period between 600 and 1000 million years considerable
erosion of the continents was producing great expanses of mud and sand adjacent to the
continental shelf. This environment may have contained abundant quantities of organic
material. However, in order to give protection and concealment in this environment
burrowing would be a pre-requisite, and more tubular body plan would become
necessary. It is possible that under such conditions the segmented worms evolved. Some
of these animals became active burrowers who tunnelled through mud in search of food,
whereas others lay half-buried, with their mouthparts filtering food above the sediment.
Brachiopods: developing a bivalve shell
Some of these animals lived in secreted protective tubes, whereas others evolved two flat,
protective shells which represented the first Brachiopods descendants which exist belong
to the genus Lingula. Brachiopods had great variations in their design, including heavy
lime shells, and large tentacles contained inside, whereas others developed a hole at the
hinge end of one of the valves through which a stalk emerged and fastened the animal
onto the ground.
The first Molluscs
Other kinds of annelids also developed in which the animal did not attach itself to the sea
floor but continued to crawl and secreted a small conical tent under which it could escape
from predators and probably represented the prototype for the Mollusc group, with a
primitive representative being Neopilina. Today there are at least 60 000 different
species of mollusc. Anatomically these animals usually possess a foot which may be
used for locomotion, a shell, a mantle composed of thin sheets of body tissue that covers
the internal organs, and an internal cavity that coats the central part of the body, in which
most species have gills which extract oxygen from water.
The Molluscs diversified
The shell is secreted by the upper surface of the mantle, with limpets producing shell at
equal rates along the edge of the mantle, in other animals the front end of the mantle
secretes at a faster level than the rear end and produces a flat spiral. The maximum
secretion may be to one side and develops twisted or turreted-shaped shells, or in the case
of cowries the secretion is concentrated along the sides of the mantle creating a shell
resembles a clenched fist. Molluscs may have either single shells (limpets), two shells or
bivalves (mussels) or a number of shell plates (chitons). In some molluscs the shell has
become reduced and totally internal (cuttlefish) whereas in others it is total absent
(octopuses).
Molluscs: Feeding mechanisms
Molluscs have a variety of different feeding mechanisms. The bivalve molluscs can
filter-feed fine particles form the water. Some of the single-shelled molluscs (limpets)
possess a ribbon-shaped tongue or radula, covered with rasping teeth, which enables the
animal to scrape algae from the rock. Whelks have a radula on a stalk that can extend
beyond the shell and be used to bore into the shells of other molluscs. Through these
holes that they have bored they poke the tip of the radula and suck out the flesh of the
victim. The cone-shells also have a stalked radula which is modified into type of harpoon
with which they secure their prey before injecting it with poison. In still more active
carnivores the heavy shell is reduced in size and may even be lost as has occurred in the
sea-slugs which have an upper surface covered with tentacles. One species of sea-slug
actively hunts jelly fish and ingests these animals stinging cells which it then
concentrates in the tentacles and uses them for protection.
Molluscs: Evolving and keeping the shell
An early group of molluscs retained the protection of a shell yet were still able to
maintain a high degree of mobility. This was achieved through the development of a gasfilled floatation tanks. The prototype forms had a flat-coiled shell with an end walled-off
to form a gas chamber. As the animal grew it added buoyancy with the development of
new chambers. Such animals survive today and are known as nautiluses. A tube runs
from the body chamber of the nautilus to the floatation tanks in the shell. The nautilus is
an active carnivore eating animals such as crabs and moves in a form of jet-propulsion
where water is squirted through a siphon. In this animal the original muscular foot is
divided into long grasping tentacles with which it secures its prey. The mouthparts are
modified to form a horny beak with which the nautilus is able to crack shells of other
animals. Variations on the float chamber theme gave rise to the enormously successful
group of animals called the ammonites whose circular shells were up to 2 meter in size.
Molluscs: Secondary loss of the shell
One of these group of molluscs took the same path as the sea slugs and disposed of its
shell entirely (octopuses and squids) whereas relict of the ancestral shell persist as the
cuttlebone found in the cuttlefishe. One species of octopus (Argonauta) secretes a paperthin replica of the nautilus shell, the chambers of which are used to lays its eggs.
Both squids and octopuses have reduced the number of tentacles (10 and 8 respectively),
but squids have become more mobile with the development of undulating lateral fins. The
brains and eyes of these animals is the most advanced of any invertebrate, eyes greater
than 400 mm in size have been recorded for squid. Squids, in particular can reach
immense sizes with one individual 21 m long (found in New Zealand in 1933).
Echinoderms: Penta-symmetrical creatures of the oceans
Another group of animals that had diverged from early stage and also reached immense
sizes are the crinoids or sea lilies which belong to the phylum Echinodermata. These
animals have an architecture plan that is based on a five-fold symmetry and possess large
lime plates that occur just below the skin. Fossil crinoids were up to 20 m long, although
their present day counterparts are considerably reduced in both size and species diversity.
Echinoderms: A hydrostatic structure
The bodies of all members work on a unique hydrostatic principle. The hydrostatic
skeleton is closed fluid-filled system that terminates as a series of blind tubes called tubefeet. Each tube feet ends in a sucker. Changing the local pressure within the tube feet
allows to be extended and contracted. Extensions and contractions of these tube feet
occur as waves down the length of the arms (or ray) and this allows the animal to move
itself and to move particulate matter down the arm. The water from this system circulates
separately from that in the body cavity. It is drawn through a pore into a canal
surrounding the mouth and circulated throughout the body into the myriads of tube feet.
When suspended particles of food touches an arm, the tube feet fasten on to it and pass it
from one to another until it reaches the groove that runs down the upper surface of the
arm to the central mouth. Although stalked, sessile sea-lilies were the most abundant
crinoids in the fossil records, the most common form today is the stalkless feather stars.
Echinoderms diversity: variations on a theme
Five-fold symmetry and hydrostatically operated tube feet also occur in the starfish and
the brittle stars, however their body plan has become inverted and the mouth is on the
undersides. Yet in another group of echinoderms the five-fold symmetry is less
conspicuous and the body plan is elongated with a mouth and anus at the two ends. At
the mouth the tube feet have become modified into tentacles which filter fine food
particles. The five-fold symmetry and hydrostatic mechanisms did not develop further
and the group is generally considered to be an evolutionary cul-de-sac.
Arthropoda: the most successful animal phylum
The third major line in the evolution of invertebrates was the development of the
segmented bodies (Arthropoda) which evolved at a very early stage and are contemporary
with the jellyfish fossil patterns found in Flinders, Australia. This group of animals
shares one important feature with the molluscs, and that is a spherical larvae possessing a
belt of cilia, whereas the echinoderm larvae have a twisted morphology with winding
bands of cilia. This suggests that molluscs and arthropods evolved from flatworms
(Platyhelminthes), with the echinoderms having an independent evolutionary line.
Arthropoda: Segmentation the successful formula
Segmentation may have increased the efficiency for burrowing in mud. A line of
separate limbs that are repeated down the length of the body seems to have been the most
primitive form. Each segment is equipped with its own set of organs - on either side, leglike projections sometimes accompanied by bristles and feathery appendages through
which oxygen could be absorbed, and within the body wall, a pair of tubes opening to
the exterior from which waste is secreted. A gut, a large blood vessel and a nerve cord
run through all segments from the anterior to the posterior end of the organism and coordinates the segmentation. a great variety of these segmented animals have been almost
perfectly fossilized in the Burgees shale of the Rocky Mountains in British Columbia,
Canada.
Early Arthropods: The fossil record
An early segmented animal was the trilobite. These animals had a bony armour
composed of lime and a horny substance called chitin. The armour was not expandable
and therefore shed periodically. Many of these shed exoskeletons have been preserved as
fossils. Where the entire animal is preserved you can observe the jointed legs that are
attached to each segment of the body, the feathery gill next to each leg, two feelers at the
front of the head, the gut running the length of the body, and even muscle fibres along the
back which enabled the animal to roll itself into a ball. Comparatively high resolution
eyes composed of mosaics of separate cells and a crystalline calcite lens. The very thick
lens of some trilobites may have reflected their colonization of deeper water where light
is considerably reduced. However, the optimal properties of the calcite lens operating in
water would not have permitted a fine focus. This shortcoming was compensated by the
evolution of the two-part lens with a waved surface at the junction of the two lens
elements.
The trilobite Asaphiscus wheeleri preserved as a very clear fossil from Cambrianaged shale in Utah
Living descendents of the Trilobites
Although they radiated throughout the oceans, only one descendent of this group survives
today, the horse-shoe crab (Limulus). This animal is larger than its ancestral trilobites,
and segmentation of its armour have fused to form a large domed shield. These animals
generally live at great depths but each spring they migrate towards the coast and during
full moon and high tides they drag themselves onto the beach where they copulate.
Today the similarities between the horse-shoe crabs and the trilobites are only evident in
the larval stage where segmentation of the armour plates are clearly discernable in the
horse-shoe crab larvae.
Crustaceans: Arthropod success in the sea
Another group of armoured animals also evolved from the original segmented worms the
crustaceans which exist today in the form of some 35 000 species. They may prowl
around rocks and reefs as crabs, shrimps, prawns, lobsters and crayfish, they may become
sessile such as barnacles, or congregate and swim in vast shoals such as krill. The size of
the crustacean and the form of the exoskeleton varies considerably from the paper-thin
exoskeleton of the almost microscopic water flea (Daphnia) to the carapace of giant
Japanese spider crab (Macrocheira kaempferi) which measures 3 m from claw to claw.
In the crustaceans the paired legs have become modified for a variety of purposes. At the
anterior end they have become modified into pincers or claws, those in the middle are
paddles, or walking legs or tweezers. Some have feather branches acting as gills through
which oxygen can be absorbed. All limbs are jointed, tubular and operate by way of
muscles. Like the primitive trilobites for crustaceans to grow they need to dispose of
their calcareous carapace. As time approaches for moulting the animal absorbs as much
calcium carbonate from the carapace into the blood stream, and begins to secrete a new
soft wrinkled skin under the carapace. The outgrown armour splits and the crustacean
swells its body by absorbing water, and wrinkled new skin stretches and hardens into a
new carapace.
Arthropod Exoskeleton: Evolving to occupy land
This exoskeleton may work to advantage for animals to colonize land if a mechanism of
breathing in air as opposed to water can be secured. By developing almost closed air
chambers lined with folds of moist skin crustaceans are able to absorb oxygen from air.
In this way sand shrimps, beach hoppers and wood lice have been able to colonize land
that retains a moist environment. The most spectacular of land dwelling crustacean is the
big robber crab Birgus which exploits coconuts.
Other descendent of the invertebrates have left the sea for a terrestrial life style the first of
which were probably derived from segmented marine worms, but more recently included
the familiar snails and slugs. These changes started about 400 million years ago and gave
rise to the most numerous and diverse of land animals; the insects.
Assignments
IN YOUR OWN WORDS WRITE A ONE TO TWO PAGE ESSAY ON THE
FOLLOWING TOPICS
Discuss the variations in shell structure that have occurred in the phylum Mollusca.
Describe the water vascular system that characterizes animals that occur in the phylum
Echinodermata.
Describe the diversity of segmented marine invertebrates that have evolved.
PLEASE REVIEW THE FOLLOWING INTERNET MATERIAL
http://www.pbs.org/kcet/shapeoflife/episodes/hunt_explo1.html
http://www.pbs.org/kcet/shapeoflife/episodes/hunt_explo2.html