The infinite variety: the beginning of life
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- Fossil History of Marine Invertebrates
- Platyhelminthes: the building block for other invertebrates
- Platyhelminthes: a surprisingly diverse group
- Annelids: the first segmented animals
- Brachiopods: developing a bivalve shell
- The Molluscs diversified
- Molluscs: Feeding mechanisms
- Molluscs: Evolving and keeping the shell
- Molluscs: Secondary loss of the shell
- Echinoderms: Penta-symmetrical creatures of the oceans
- Echinoderms: A hydrostatic structure
- Echinoderms diversity: variations on a theme
- Arthropoda: the most successful animal phylum
- Arthropoda: Segmentation the successful formula
- Early Arthropods: The fossil record
Genera Bolbometopon Calotomus Cetoscarus Chlorurus Cryptotomus Hipposcarus Leptoscarus Nicholsina Scarus Sparisoma 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. 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. Platyhelminthes: the building block for other invertebrates Simpler animals than those first found in the fossil records 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. Their 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. Platyhelminthes: a surprisingly diverse group There are some 3000 species 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 Download 2.01 Mb. Share with your friends:
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