|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 single-celled 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.
Conservation status: Vulnerable
Midnight parrotfish (Scarus coelestinus)
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.
Fossil Species of the Maotianshan Shales
* Kuanyangia, a trilobite
* Yunnanocephalus, a trilobite
Worms and relatives
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.
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
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.
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.
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
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 soft-bodied 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 gas-filled 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 paper-thin 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 tube-feet. 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, leg-like 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 co-ordinates 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 Cambrian-aged 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.
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.
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