Sponges or poriferans

Acritarchs (Late Precambrian - Quaternary)

They are marine, hollow, organic-walled unicellular vesicles. Their biological affinities are unclear. They are placed together with other palynomorphs due to their resistant cell wall.  Most of them are 0.02-0.150 mm across.

The term Acritarchs was  coined by Evitt in 1963, to refer to microfossils "of uncertain origin", they are an artificial group. The group includes any small (most are between 20-150 microns across), organic-walled microfossil which cannot be assigned to a natural group. They are characterised by varied sculpture, some being spiny and others smooth. They are believed to have algal affinities, probably the cysts of planktonic eukaryotic algae. They are valuable Proterozoic and Palaeozoic biostratigraphic and palaeoenvironmental tools.

Acritarchs are extant but little is known about the organism which produces them. It is generally accepted that they are probably the resting cysts of marine phytoplankton and therefore represent only one stage of a multi-stage lifecycle. The gross morphology of acritarchs, a single organic sac or hollow vesicle, and their size, generally between 15 to 80 microns, suggests derivation from a unicellular organism. Many show simple excystment structures which may be divided into two groups: linear sutures which vary in position and shape, and pylomes which are complete small circular holes in the vesicle which may have a plug which fits in the hole called an operculum. These features are strongly suggestive of a dinoflagellate-like host organism. The fact that they are only found in rocks of marine origin, their great age and other preservational associations allows us to be almost sure they are wholly marine.

The earliest palynomorphs which we now classify as acritarchs are probably those reported by White in 1862 from the Ordovician to Devonian of New York. In the early 1930's Eisenack, working on material from the Baltic, re-assigned many species to the acritarch group and came to regard them (as we do now) as being derived from phytoplankton. It was Downie, Evitt and Serjeant however who tidied up the classification, providing a usable system, even though we are still uncertain of the groups true affinities.

The oldest known Acritarchs are recorded from shales of Palaeoproterozoic (1900-1600 Ma) age in the former Soviet Union. They are stratigraphically useful in the Upper Proterozoic through to the Permian. From Devonian times onwards the abundance of acritarchs appears to have declined, whether this is a reflection of their true abundance or the volume of scientific research is difficult to tell.

Natural classifications of acritarchs have not often been attempted nor accepted. The artificial scheme introduced by Downie, Evitt and Serjeant has been widely accepted and serves the purposes of most palynologists so there is little incentive to change it. Acritarchs are therefore divided into the following groups (most common first) based on their morphology:

Acanthomorphs have spherical bodies with spines which usually open into the body.

Polygonomorphs have a body-shape defined by the number and position of spines, they are often triangular or square in outline.

Netromorphs have a fusiform body with one or more spines.

Diacromorphs have spherical to ellipsoidal bodies with ornament confined to the poles.

Prismatomorphs have prismatic to polygonal bodies the edges of which form a flange or crest which may be serrated.

Oomorphs have an egg shaped body with ornament confined to one pole.

Herkomorphs have a roughly spherical body divided into polygonal fields, rather like a football.

Pteromorphs have a roughly spherical central zone often compressed, surrounded by a flange or wing lamella which may be sustained by radial folds or processes, they resemble under the light microscope a fried egg!

Sphaeromorphs have simple spherical morphology.

In summary: Acritarchs are spherical microfossils that look very much like the resting stage cysts of dinoflagellates and other algae. Some have spines; some are completely bare; but all lack some of the characteristic features of dinoflagellate cysts. These enigmatic fossils may belong to many different groups. Acritarchs first appeared in the fossil record in the Precambrian about 1.8 billion years ago, and they are still around today.

Acritarchs are organic-walled cysts of unicellular protists that cannot be assigned to any known group of organisms. Most acritarchs are probably the resting cysts of marine eukaryotic phytoplankton. Some acritarchs are thought to be dinoflagellate cysts but lack the requisite morphology to make a positive attribution. Others, however, can be confidently assigned to the chlorophytes (green algae), but for convenience, are still commonly included in the acritarchs. Thus, acritarchs are a heterogeneous, polyphyletic collection of organic-walled microfossils of unknown or uncertain origin. Acritarchs vary in size from < 10 microns to more than 1 mm, but the majority of species range from 15 to 80 microns. Because of their small size, abundance and diversity, as well as widespread distribution, acritarchs are very useful in biostratigraphic correlation, as well as paleobiogeographic and paleoenvironmental studies. Acritarchs are found throughout the geologic column but were most common during the Late Proterozoic and Paleozoic. Because they represent the fossil record of the base of the marine food chain during the Proterozoic and Paleozoic, acritarchs played an important role in the evolution of the global marine ecosystem.

The sponges or poriferans: Po·rif·e·ra (n. pl.)
[NL., fr. L. porus pore + ferre to bear.]

A grand division of the Invertebrata, including the sponges; -- called also Spongiæ, Spongida, and Spongiozoa. The principal divisions are Calcispongiæ, Keratosa or Fibrospongiæ, and Silicea.

The sponges or poriferans (from Latin porus "pore" and ferre "to bear") are animals of the phylum Porifera. They are primitive, sessile, mostly marine, water dwelling filter feeders that pump water through their bodies to filter out particles of food matter.

Sponges represent the simplest of animals. With no true tissues (parazoa), they lack muscles, nerves, and internal organs. Sponges have cellular-level organization, meaning that that their cells are specialized so that different cells perform different functions, but similar cells are not organized into tissues and bodies are a sort of loose aggregation of different kinds of cells. This is the simplest kind of cellular organization found among parazoans.

Their similarity to colonial choanoflagellates shows the probable evolutionary jump from unicellular to multicellular organisms. There are over 5,000 modern species of sponges known, and they can be found attached to surfaces anywhere from the intertidal zone to as deep as 8,500 m (29,000 feet) or further.

Though the fossil record of sponges dates back to the Neoproterozoic Era, new species are still commonly discovered.

Sponges are a diverse group of sometimes common types, with about 5000 species known across the world. Sponges are primarily marine, but around 150 species live in fresh water.


One of the divisions of phyla of the animal kingdom containing snails, slugs, octopuses, squids, clams, mussels, and oysters; characterized by a shell-secreting organ, the mantle, and a radula, a food-rasping organ located in the forward area of the mouth.

Brachiopods are marine animals that, upon first glance, look like clams. They are actually quite different from clams in their anatomy, and they are not closely related to the molluscs. They are lophophorates, and so are related to the Bryozoa and Phoronida.
Although they seem rare in today's seas, they are actually fairly common. However, they often make their homes in very cold water, either in polar regions or at great depths in the ocean, and thus are not often encountered. There are about 300 living species of brachiopods.
The brachiopods are a large group of solitary and exclusively marine organisms with a very good geologic history throughout most of the Phanerozoic and are among the most successful benthic macroinvertebrates of the Paleozoic. They are typified by two mineralized valves which enclose most of the animal. Like the bryozoans, brachiopods are filter feeders which collect food particles on a ciliated organ called the lophophore. An excellent example of a brachiopod lophophore can be seen in the Recent terebratulid.  Brachiopods differ in many ways from bryozoans (in both soft and hard-part morphology), and are thus considered by most workers as a separate but closely related phylum. However, one of the most distinguishing features of brachiopods is the presence of a pedicle, a fleshy stalk-like structure that aids the animal in burrowing and maintaining stability. The pedicle can be seen in the Recent Lingula.  Currently, brachiopods are divided into two or three major groups. We depart from your text in considering two major groups: Class Inarticulata (including lingulids), and Class Articulata based on the presence or absence of hinge teeth and sockets.

Brachiopods superficially resemble bivalve mollusks in that the animal secretes a bivalved (two-part) shell of calcium carbonate or a combination of calcium phosphate and chitinous organic substance.  However, Bivalve mollusks generally have shells that are equal in size and shape (although mirror images of each other), whereas the two shells of brachiopods are of unequal size (the technical term is inequalvalved).  The valve (shell) that has the attachment for the pedicle is the pedicle valve which is usually the lower and larger valve.  This valve includes the pedicle opening.

Ecology and physiology
A. Marine, mostly within the shelf, but some forms are abyssal.
B. Benthic : sessile or burrowing. Substrates: rock, shells, algal stems, soft sediment.
C. Not colonial but tend to aggregate intraspecifically and interspecifically, which seems to depend on the larval settlement.
D. Inhabit largely the cold waters, but are present in all latitudes. Usually highly endemic.
E. Feeding biology
1. Feed on fine phytoplankton (diatoms), and dissolved and colloidal material.
2. Adjustable ciliary current created by the lophophore ciliation. Distinct inhalant and exhalant apertures. Particles are trapped on the filaments and converge in the groove down to the mouth.  There is mucus, but it seems not to play a major role here.
3. In burrowing species, the mantle setae prevent fouling by the sediment.  Mucus is secreted by the gland zone of the mantle lobes.
4. Some species can reverse the water current when particles accumulate within the lophophore.
5. Ingestion is controlled by peristaltic movements of the esophagus and stomach.
F. Predators : fish, men.
G. Parasites : gregarines.

More About Brachiopods
Brachiopods (from Latin bracchium, arm + New Latin -poda, foot) may be divided into two types: inarticulate brachiopods are held together entirely by musculature, whereas articulate brachiopods have a hinge-like articulation between the shells. IThese two divisions correspond to the subdivision of the Phylum Brachiopoda into two classes: Articulata and Inarticulata

All brachiopods are marine and are found either attached to substrates by a structure called a pedicle or resting on muddy bottoms.

Brachiopods are suspension feeders with a distinctive feeding organ called a lophophore, which is found in two other animal phyla (Bryozoa and Phoronida). Modern brachiopods generally live in areas of cold water, either near the poles or in deep parts of the ocean. Modern brachiopods range in shell size from less than 5 mm (¼ in) to just over 8 cm (3 in). Fossil brachiopods generally fall within this size range, but some adult species have a shell of less than 1 mm across, and a few gigantic forms have been found measuring up to 38½ cm (15 in) in width.

Brachiopods  are a small phylum of benthic invertebrates. Also known as lamp shells (or lampshells), "brachs" or Brachiopoda, they are sessile, two-shelled, marine animals with an external morphology superficially resembling pelecypods (for instance, clams) of phylum Mollusca to which they are not closely related. It is estimated by paleobiologists that 99 percent of all documented lamp-shell species are both fossils and extinct.[1] Despite superficial similarities, bivalves and brachiopods differ markedly: Bivalves usually have a plane of symmetry between the shells, whereas most brachiopods have a plane of bilateral symmetry through the shells and perpendicular to the hinge. Both brachiopod shells are symmetrical as individual shells, but the shells differ in shape from one another. Whereas bivalves use adductor muscles to hold their two shells closed, and open them by means of an external or internal ligament once the adductor muscles are relaxed, brachiopods use muscle power (by internal diductor and adjustor muscles) to pull their two shells apart, and to close the two (by adductor muscles). A second major difference is that most brachiopods are attached to the substrate by means of a fleshy "stalk" or pedicle. In contrast, although some bivalves (pelecypods such as oysters, mussels and the extinct rudists) are fixed to the substrate, most are free-moving, usually by means of a muscular "foot". Furthermore, brachiopod shells may be either phosphatic or -- much more commonly -- calcitic, as mollusks generally are. Only rarely do brachiopods produce aragonitic shells, which are composed of a less-permanent form of calcium carbonate. Lastly, in contrast to most bivalves, some extinct lamp-shells exhibit elaborate flanges and spines. On July 16, 1986, the Kentucky State Legislature designated the brachiopod to be the Kentucky state fossil.

monoplacophorans (meaning “bearing one plate”) Class Monoplacophora, Phylum Mollusca
Prior to 1952, the monoplacophorans were known only from fossil shells from the Cambrian and Devonian. Then in that year the 'Galathea' expedition dredged up 10 living specimens of Neopilina from deep water off the Pacific coast of Mexico. These were named Neopilina galathea, but a few years later a second species N. ewingi was also discovered. These two species are the only known living representatives of this class.

Little is known about the monoplacophora. They have a single, flat, rounded bilateral shell that is often thin and fragile; it ranges in size from 3 to 30 millimetres. The apex of the shell is forward. The fossil shells exhibit a series of muscular attachment scars on the inner side, suggesting metamerism.



The opisthokonts (Greek: ??????- (opisth?-) = "rear, posterior" + ?????? (kontos) = "pole" i.e. flagellum) are a broad group of eukaryotes, including both the animal and fungus kingdoms, together with the phylum Choanozoa of the protist kingdom. Both genetic and ultrastructural studies strongly support that opisthokonts form a monophyletic group. One common characteristic is that flagellate cells, such as most animal sperm and chytrid spores, propel themselves with a single posterior flagellum. This gives the groups its name. In contrast, flagellate cells in other eukaryote groups propel themselves with one or more anterior flagella.

The name Anomalocaris (meaning "strange shrimp") originally referred to the detached arms (which were the first part to be named), and was later used for the whole animal because of the biological name priority rules. Curiously enough, when fully assembled, these animals do strongly resemble (in outside appearance) a gigantic brine shrimp with a pair of finger-like appendages near its mouth.

Anomalocarids were flat free-swimming segmented animals, which in front of their mouths had two appendages that look like the bodies of shrimps. The mouth is a peculiar circular structure like a pineapple slice, but with a ring of hard sharp teeth in the central orifice. The mouth was more rectangular than round, and the teeth did not meet in the middle. This would still let it crack open shells of small arthropods and other like animals, such as trilobites. Indeed, many trilobites have been found with bite marks on them. Anomalocarids also had large eyes and a body half-flanked with a series of swimming lobes.

The anomalocarids thrived in the Early and Mid Cambrian and then apparently died out. It is possible that large Cephalopods replaced them as the dominant swimming predator at the end of the Cambrian.


Opabinia is a fossil animal found in Cambrian fossil deposits. Its sole species, Opabinia regalis, is known from the Middle Cambrian Burgess Shale of British Columbia. Fewer than twenty good specimens have been found. Opabinia was a soft-bodied animal of modest size, and its segmented body had lobes along the sides and a fan-shaped tail. The head shows unusual features: five compound eyes, a mouth under the head and facing backwards, and a proboscis that probably passed food to the mouth. Opabinia probably lived on the seafloor, using the proboscis to seek out small, soft food.

Priapulida (priapulid worms or penis worms, from Gr. ???????, pri?pos 'Priapus' + Lat. -ul-, diminutive)
are a phylum of marine worms with an extensible spiny proboscis.


Ottoia prolifica (a priapulid worm)
this creature lived within a U-shaped burrow that it constructed in the substrate. From that place of relative safety it extended its proboscis (the smaller protuberance topping the right side of the animal illustrated here) in search of prey. Ottoia is thought to have been an active burrower moving through the sediment after prey.

Ottoia is among the largest and most abundant worms found in the fossils of the Cambrian Burgess Shale formation of British Columbia. It is an early priapulid worm that averaged about 80mm in length. Typical of extant priapulids are the infaunal living habit and the spiny evertable proboscis.

Pikaia gracilens
originally found near Mount Pika in the Burgess Shale of British Columbia. It was discovered by Charles Walcott and who first described it in 1911. Based on the obvious and regular segmentation of the body, Walcott classified it as a Polychaete worm. During his re-examination of the Burgess Shale fauna in 1979, paleontologist Simon Conway Morris placed P. gracilens in the chordates, making it perhaps the oldest known ancestor of modern vertebrates, because it seemed to have a very primitive, proto-notochord.

Two species are know: (1) Waptia ovata The species is known mostly from the distinctive wrinkled carapace.
(2) Waptia fieldensis was a small, shrimp-like stem group crustacean. Many Cambrian crustaceomorphs such as Waptia lack the mouthparts to be classified as crown group crustaceans that lived during the Middle Cambrian about 510 million years ago .


was a bilaterally symmetrical animal. Viewed from the top the body was elliptical with no distinct head or tail, and from the front or rear it was almost rectangular. The animal was covered in small ribbed armor plates called sclerites.



The homeotic (Hox) genes can be viewed of as genetic switches that turn different programs of cellular differentiation on or off.
Hox Genes

The Hox genes, are a set of genes that are responsible for assigning specific regional identities on body parts (Ed Lewis won the Nobel for his work on them, for one thing).

General purpose control genes are important elements in building complicated organisms like flies. Some “control” genes are common to many organisms (they are homologous—inherited from our common ancestor). For example, Hox genes help lay out the basic body forms of many animals, including humans, flies, and worms. They set up the head-to-tail organization. All animals have Hox genes, which may be as few as 1, as in sponges, or as many as 38, as in humans and other mammals. Hox genes are clustered in the genome. Invertebrates have only one cluster with a variable number of genes, typically fewer than 13. The common ancestor of the chordates (which include the vertebrates) probably had only one cluster.

Hox Genes can be considered as directing instructions as an embryo develops: “Put the head here! Legs go over there!”genes active in the development of all animals that regulate other genes. Although they do different things in different organisms, they are very important in laying out the body plan of the organism. For example, they help organize the head to tail differentiation in both insects and vertebrates. Small changes in such powerful regulatory genes, or changes in the genes turned on by them, could represent a major source of evolutionary change.

The homeotic (Hox) complex governs both the obvious and non-overt segmentation that occurs in vertebrates. Homeotic genes specify the anterior-posterior axis and segment identity during early development of metazoan organisms. They are critical for the proper placement and number of embryonic segment structures (such as legs, antennae and eyes).

Homeotic gene: a gene which defines a region or position in the embryo. Mutations in homeotic genes lea d to transformations of one structure into another; the classic example is antennapedia, a mutation that turns the antenna of a fly into a leg.