Pachytheca.

Pachytheca was a very primitive sphere-shaped plant that lived from the late Silurian to the early Devonian. Pachytheca is related to Prototaxites, so my hypothesis is that it is a fungus. It was up to 6mm long. 


This organism had a prominent outer layer. It's possible that in juvenile specimens this layer was thicker, and on the adults it was thinner. The structure of Pachytheca was made up of tubes. Thicker, stronger tubes on the outer layer, and thinner, more brittle ones on the inner layer. 

Some of the fossils look like marbles, while others look like tiny geodes. At first the organism was thought to have been a piece of a bigger plant or the tooth of a fish. Then it was incorrectly classified as an alga. 

Pachytheca and Prototaxites are now classified as Nematophytes, enigmatic organisms that were either plants or fungi. Pachytheca has been found mostly in western Europe, but they also lived in places such as Canada and Australia. 

References:

http://steurh.home.xs4all.nl/engpach/epachy.html

http://www.chertnews.de/Pachytheca.html

http://de.wikipedia.org/wiki/Pachytheca

Conodonta.

Conodonts were bizarre, fish-like probable chordates that may have resembled modern lampreys. They first evolved in the Cambrian, or possibly even the Precambrian, and died out in the Triassic-Jurassic extinction.

Conodonts were eel-shaped in form and most had large eyes, at least in comparison to the body. They had various toothy blades in the mouth to form what is known as "the conodont apparatus," which vaguely resembles the radula of a snail or slug.

Conodonts were probably capable of maintaining a cruising speed, but could not perform bursts of speed because their eel-like form would probably get them all tangled up. They would then be easy prey for any kind of predator trying to eat them. They probably swam in about the same style as an eel or loach. Although they had sharp teeth, they probably were not predators. Instead, they supposedly used "the conodont apparatus" as a sort of baleen to filter plankton from the water.

The largest conodont that has been found so far is Promissum, which reached lengths of 16 inches. Specimens of Promissum can be found in the Soom Shale of South Africa. Unlike most conodonts, Promissum had smaller eyes relative to body size. Promissum was about as long as an average house cat's body, without the head or tail.

The fist conodont specimens found were its individual toothy bars known as "conodont elements."

References:

http://www.ucl.ac.uk/GeolSci/micropal/conodont.html

http://en.wikipedia.org/wiki/Conodont

http://oceans1.csusb.edu/cdont_art.htm

Graptolithinia.

Graptolithinia is a class of shelled hemichordates that lived from the Cambrian to the Carboniferous. One of the first graptolites was called Chaunograptus, from the Burgess Shale. Chaunograptus made a living by hitching onto other animals like sponges and arthropods. Later graptolites, like Monograptus, had pelagic lifestyles and drifted with the ocean currents in the open sea. But one group of Graptolites, the dendroids, retained a benthic or a parasitic lifestyle.

The name graptolite means "writing on the rocks" in Greek, which refers to the fact that most graptolite fossils look like hieroglyphics. In life, some graptolites, like Monograptus, may have resembled hacksaw blades, where others, like Didymograptus, resembled pinking shears. There were hundreds, probably even thousands, of different forms of graptolites. The morphology of graptolites was very diverse.

The Ordovician graptolite Didymograptus.

Some graptolites were benthic, some were parasitic, some were pelagic, and there were many forms of graptolites living each of those lifestyles. Graptolites were hemichordates. They were not chordates, but they were very important in chordate evolution leading up to humans.

The Ordovician graptolite Phyllograptus. 

Graptolites are index fossils for the Ordovician and Silurian. Some graptolites are very common, like ammonites, which are also good index fossils. Most good index fossils are common, widely distributed, and from a limited time span. This helps scientists date rocks.

The Devonian graptolite Spirograptus. 

References:

http://www.ucmp.berkeley.edu/chordata/hemichordata.html

http://www.asoldasthehills.org/Graptolites.html

http://en.wikipedia.org/wiki/Graptolithinia

http://paleo.cortland.edu/tutorial/misc%20fossils/miscfossils.htm

Scyphocrinites.

Scyphocrinites (sky-foe-cry-NITE-ees) is a genus of crinoid that lived from the late Silurian to the early Devonian. Its fossils have been found in Asia, North America, Europe, and Africa.

Instead of being rooted to the ground like most crinoids, Scyphocrinites had a lobolith, which is a floating sphere that keeps an animal afloat in water. Unlike other crinoids, Scyphocrinites hung upside down at the surface.

This is a fossil of a lobolith from Scyphocrinites. The lobolith would have allowed Scyphocrinites to move with the current, which benthic crinoids could not do. Scyphocrinites was probably the only crinoid with a lobolith, but there may have been others.

This fossil shows the calyx of Scyphocrinites. In zoology, a calyx is a cup-shaped structure. In botany, it's the sepals that protect a flower. The arms coming off of the calyx would have allowed it to snare tiny plankton to eat. We know that because living crinoids use their arms for the same purpose. Fossils of Scyphocrinites are sometimes found in huge clusters, similar to how the Cretaceous crinoid Unitacrinus is found.

References:

http://www.fossilmuseum.net/Fossil_Galleries/Crinoids/Scyphocrinites/Scyphocrinites.htm

http://www.mineraltown.com/Reports/crinoid_fossils/crinoids.php?idioma=2

http://www.mprinstitute.org/vaclav/Scyphocrinites.htm

http://dictionary.reference.com/browse/calyx

Deiphon.

Deiphon is a Silurian trilobite which had long, conical spines on its body. It had a very large, round glabella, or middle cheek. Some scientists believe the glabella was filled with fat or oil, to keep the trilobite buoyant. But others think Deiphon was predatory and that it stored prey inside the glabella. 

My interpretation of Deiphon on the Silurian sea floor. 

Deiphon's spines may have made it hard for larger predators to swallow it. Deiphon was either planktonic and ate phytoplankton, or was benthic and predatory.

Deiphon had long, curved spines along its thorax, and two spines that formed a v-shape on its pygidium, or tail. It also had two spines on its cephalon, one on each side. Deiphon's thorax was much skinnier than that of most other trilobites. All these odd features would have made Deiphon a very strange trilobite.

Click image to see more of Christian Richet's illustrations.

References:

http://www.britannica.com/EBchecked/topic/692300/Deiphon

http://en.wikipedia.org/wiki/Deiphon

Myriacantherpestes.

Myriacantherpestes (MEER-ee-uh-canth-er-PEST-eez) is a very spiny millipede that lived from the Silurian to the Carboniferous. Its many spines could have been used for protection against predators, or for identifying each other. I first heard about this creature in the book Paleo Bugs, by Timothy J. Bradley.

In the picture below, Myriacantherpestes is on the right and Archidesmus is on the left. Myriacantherpestes was three feet long, but Archidesmus was about 2.5 cm, as big as most extant North American millipedes. They both lived in the Silurian Period, along with the millipede that resembled a miniature Arthropleura, which was called Eoarthorpleura.

Myriacantherpestes was probably a descendant of the earliest known land animal, Pneumodesmus. In the Silurian, millipedes like Myriacantherpestes were the dominant animals on land.

Halysites.

Halysites, commonly called "chain coral," is a coral that lived from the Ordovician to the Silurian Period. It looked like a bunch of straws or tubes linked together. From above, Halysites looks like a mass of chains or a brain.

From the side it looks like a fence.

Each individual corallite, or cell, is 2 to 6 mm in diameter. In each individual tube there was a tiny sea anemone-like animal called a polyp, similar to what you find in modern corals.

Corals today prefer warm, shallow, clear seas, so tabulate corals like Halysites and their relatives probably did too. So when sea levels rose in the Devonian, they became rare and then died out two periods later, in the Permian mass extinction.

Phacops.

Phacops is a trilobite that lived from the Silurian to the Devonian Period. It had a large and bumpy glabella, or middle cheek.

Even though many other trilobites could roll up like pill bugs, Phacops could do it more efficiently than other trilobites. But sometimes when they were rolled up they could get buried in sediment. Many fossils of Phacops are found rolled up into balls.

Phacops was up to six inches long. Some people recognize it by its eyes, which resembled frog eyes.

Phacops has been found in the northeast United States and Morocco, and is the state fossil of Pennsylvania.

Phacops had a 360 degree view because it could turn its eyes to different angles. Phacops is one of the most common Devonian trilobites.

Fusulinida.

Fusulinids are extinct single-celled organisms called protists that lived from the Silurian to the Permian. Most fusulinids were about the size of a grain of rice, but some were up to two inches long. They had a hard wall that protected the cell inside.

Some fusulinids are so similar in shape that scientists have to use a cross section of the fossil to identify them.

Fusulinids probably lived in clear water and may have lived on reefs.

Fusulinids are very large and complex for single-celled life, which is usually microscopic. Fusulinids are marker fossils, which means by looking at the fusulinids in a rock formation, scientists can tell how old the rock is.

Note: Edited 8/31/11 to remove the word "animal" and replace it with "organism." Fusulinida was a protist, not an animal.

Mixopterus.

Mixopterus was a peculiar genus of eurypterid that lived in the late Silurian Period. Its spiny claws were similar to those of the Ordovician eurypterid Megalograptus.

Thanks to Wade Harrell for the cool illustration!

At the posterior end, Mixopterus has a stinger that it could have used to injure or kill its prey before eating it, like scorpions do today.

Mixopterus was a type of arthropod called a eurypterid. In the Silurian, eurypterids like Mixopterus were the top predators. It was about 75 centimeters long and was found in Norway.

Mixopterus had a very bumpy exoskeleton and spiny legs. It probably kept its stinger raised above its head like scorpions do. Eurypterids like Mixopterus are believed to be related to horseshoe crabs and scorpions.

Prototaxites.

Prototaxites (pro-toe-tax-eye-teez) is a mysterious genus of life form that lived from the Silurian to the Devonian Period. Prototaxites was first believed to be a conifer-like plant, but is now believed to be a giant fungus or huge liverwort. It was up to about 30 feet tall and could be three feet wide. Prototaxities was the biggest life form on land of its time.

Prototaxites looked like a giant tree trunk with no branches or leaves. It's possible that Prototaxities was so tall so its spores could move far away so that it would not have to compete with other Prototaxities for what it needed.

Prototaxites was worldwide and died out when vascular plants took over.

Furcacauda.

Furcacauda is a genus of thelodont from the Silurian to Devonian Periods. It had a large caudal fin which made it look very odd. Furcacauda was closely related to the Silurian thelodont Lanarkia.

Since the largest thelodont, Thelodus, was 30 cm, Furcacauda was smaller than that. Furcacauda and other thelodonts were jawless. Scientists have found a fossilized part of Furcacauda that looks like a stomach. So Furcacauda could have had the first stomach of any vertebrate.

This image shows a fossil of Furcacauda on the bottom, and the top fossil is Lanarkia. Although Furcacauda didn't look anything like Lanarkia, they were still both thelodonts. Thelodonts were covered in tooth-like scales rather than having armor on their head like most jawless fish did.

Eurypterus.

Eurypterus was the first ever eurypterid discovered. The first one that was discovered was Eurypterus remipes. The person who first discovered Eurypterus thought Eurypterus's front legs were barbels, and that Eurypterus was the first catfish. But later someone classified it as an arthropod, but instead of a eurypterid, he thought it was a branchiopod. Then someone described it as a eurypterid, and that person was right.

Eurypterus, like most other eurypterids, swam "breast stroke," the same way as frogs do today. One eurypterid that probably did not swim breast stroke was Hibbertopterus. It lived in swamps, rivers, and streams, but Hibbertopterus was more of a crawler than a swimmer. It may have crawled up onto land, which Eurypterus probably did not do.

Eurypterus was normally about 5 to 9" long. The largest one ever found was 4.3' long.

Eurypterus had a paddle at the end, with a large spine sticking out. The paddle was probably used for swimming, and the spine was probably used for defense against predators or for injuring its prey. Eurypterus means "wide wing" or "broad paddle."

Eurypterus had spiny legs which were probably used for grabbing prey, because the spines could badly injure the prey. Eurypterus was a generalist species, meaning it would rely on anything it could get for food. There are fifteen species of Eurypterus that are found so far. The reason that Eurypterus was so successful was that it was able to eat anything that contained any nutrients.

Cephalaspis and Lanarkia.

Lego Cephalaspis

Lego Lanarkia spinosa