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Description

The Silurian period began after the Ordovician 445 mya and ended before the Devonian 420 mya. Life had recovered after the Ordovician extinction event and continued to flourish in the warm seas. Large orthocones(straight-shelled nautiloid cephalopods) and giant eurypterids or "sea scorpions" served as apex predators as they did in the Ordovician. Some eurypterids grew up to 8 feet in length; the largest arthropods of all time. Trilobites were still abundant along with other invertebrates. The early jawless fish of the Ordovician grew in diversity during the Silurian, many of them evolving heads and bodies covered in armor plating for protection. Fish also developed paired fins at this time, and most important of all, jaws appeared in some groups. Likely formed from the foremost gill arches connecting to the skull, jaws allowed fish to become efficient predators. The oldest known bony ray-finned fish, sharks, and lobe-finned fish appeared along with the now extinct acanthodii, or "spiny sharks". Dry land was still a harsh environment, with carbon dioxide levels much higher than today, oxygen levels low, and no protection from the harmful radiation of the Sun until the ozone layer became well established in the atmosphere during this period. Nonetheless it was during the Silurian that multicellular life first began to really emerge from the oceans. The first terrestrial arthropods, including millipedes, started to wander the land. Most crucial of all were the first vascular land plants growing near streams and ponds. For now these first land-dwellers were restricted to the waters edge, but they would soon spread, creating a viable early terrestrial ecosystem by the start of the Devonian.

†Mixopterus kiaeri/ †Pterygotus anglicus: eurypterids, or sea scorpions, were an abundant group of arthropods that were common predators in marine and freshwater ecosystems throughout the Paleozoic. Despite their common name, they were not scorpions but were most closely related to living horseshoe crabs. Most species had long arms with claws for slashing prey, pairs of jointed legs for walking, a pair of paddle-like appendages for swimming, and a long tail with either a pointed or paddle-shaped end. The legs were strong enough for eurypterids to walking along the sea-bed, and some may have been able to walk on land to scavenge on the shore. Pterygotus grew up to six feet long, one of the biggest of its kind, but at least one other species grew up to eight feet long.

†Procephalaspis oeselensis: part of a group of jawless fish called Cephalaspidomorphi, close relatives of living lampreys, this fish and its relatives were peaceful grazers of algae and small animals on the beds of estuaries, rivers and shallow seas. They were among the first fish to evolve paired fins that helped increase maneuverability. They also had a tail fin. The biggest feature of fish like Procephalaspis is their bizarre head shield shaped like the head of a shovel. This would have been good protection from predators but also made them very slow swimmers. Fossils of the head-shields show patches of sensory pits connected to the brain, likely used to detect vibrations of both predators and food.

†Climatius reticulatus: Climatius is a genus of fish in the class Acanthodii, one of the first groups of fish to evolve jaws and teeth. Acanthodians are commonly called "spiny sharks" but are not true sharks, although they did have a skeleton made of cartilage like sharks. Many of their fins were supported by a bony spine, and their skin was covered in tiny rhomboid scales, which are often the most common fossils of this group. Climatius was a 3 inch long marine predators, but other acanthodians inhabited freshwater ecosystems too.

†Phacops rana: this was a very abundant species of trilobite that lived from the Ordovician to the Devonian. It is a common fossil in eastern United States and is often found preserved in a rolled-up state. These trilobites were able to roll into a defensive ball like modern roly-polies.

†Furcacauda heintzae: a species of jawless fish from the group Thelodonti, this fish had a well developed caudal fin and what looks like a small dorsal fin. Thelodonts differed from other jawless fish in being covered in small spiny scales.

†Pneumodesmus newmani: this small arthropod found in Scotland is the earliest known millipede and the earliest known animal able to breath on land.

†Guiyu oneiros: Guiyu is the earliest known bony fish, a fish with a skeleton made of bone instead of cartilage. The structure of its fins indicate it was coser to the lobe-finned fish; fish with fins supported by bones with muscle rather than just rays with skin as in the ray-finned fish.

†Cooksonia sp.: Cooksonia is the first known vascular land plant; the first plants with tissues that carried water and nutrients. It grew only a few inches tall but already showed the structure that would come to establish all the worlds great forests, stems that shoot upwards to capture light from the sun. Cooksonia lacked leaves, flowers and roots, and likely reproduced with spores as primitve plants like ferns do today. Patches of Cooksonia probably never grew away from the edges of streams and ponds.

Description

The Silurian period began after the Ordovician 445 mya and ended before the Devonian 420 mya. Life had recovered after the Ordovician extinction event and continued to flourish in the warm seas. Large orthocones(straight-shelled nautiloid cephalopods) and giant eurypterids or "sea scorpions" served as apex predators as they did in the Ordovician. Some eurypterids grew up to 8 feet in length; the largest arthropods of all time. Trilobites were still abundant along with other invertebrates. The early jawless fish of the Ordovician grew in diversity during the Silurian, many of them evolving heads and bodies covered in armor plating for protection. Fish also developed paired fins at this time, and most important of all, jaws appeared in some groups. Likely formed from the foremost gill arches connecting to the skull, jaws allowed fish to become efficient predators. The oldest known bony ray-finned fish, sharks, and lobe-finned fish appeared along with the now extinct acanthodii, or "spiny sharks". Dry land was still a harsh environment, with carbon dioxide levels much higher than today, oxygen levels low, and no protection from the harmful radiation of the Sun until the ozone layer became well established in the atmosphere during this period. Nonetheless it was during the Silurian that multicellular life first began to really emerge from the oceans. The first terrestrial arthropods, including millipedes, started to wander the land. Most crucial of all were the first vascular land plants growing near streams and ponds. For now these first land-dwellers were restricted to the waters edge, but they would soon spread, creating a viable early terrestrial ecosystem by the start of the Devonian.

†Mixopterus kiaeri/ †Pterygotus anglicus: eurypterids, or sea scorpions, were an abundant group of arthropods that were common predators in marine and freshwater ecosystems throughout the Paleozoic. Despite their common name, they were not scorpions but were most closely related to living horseshoe crabs. Most species had long arms with claws for slashing prey, pairs of jointed legs for walking, a pair of paddle-like appendages for swimming, and a long tail with either a pointed or paddle-shaped end. The legs were strong enough for eurypterids to walking along the sea-bed, and some may have been able to walk on land to scavenge on the shore. Pterygotus grew up to six feet long, one of the biggest of its kind, but at least one other species grew up to eight feet long.

†Procephalaspis oeselensis: part of a group of jawless fish called Cephalaspidomorphi, close relatives of living lampreys, this fish and its relatives were peaceful grazers of algae and small animals on the beds of estuaries, rivers and shallow seas. They were among the first fish to evolve paired fins that helped increase maneuverability. They also had a tail fin. The biggest feature of fish like Procephalaspis is their bizarre head shield shaped like the head of a shovel. This would have been good protection from predators but also made them very slow swimmers. Fossils of the head-shields show patches of sensory pits connected to the brain, likely used to detect vibrations of both predators and food.

†Climatius reticulatus: Climatius is a genus of fish in the class Acanthodii, one of the first groups of fish to evolve jaws and teeth. Acanthodians are commonly called "spiny sharks" but are not true sharks, although they did have a skeleton made of cartilage like sharks. Many of their fins were supported by a bony spine, and their skin was covered in tiny rhomboid scales, which are often the most common fossils of this group. Climatius was a 3 inch long marine predators, but other acanthodians inhabited freshwater ecosystems too.

†Phacops rana: this was a very abundant species of trilobite that lived from the Ordovician to the Devonian. It is a common fossil in eastern United States and is often found preserved in a rolled-up state. These trilobites were able to roll into a defensive ball like modern roly-polies.

†Furcacauda heintzae: a species of jawless fish from the group Thelodonti, this fish had a well developed caudal fin and what looks like a small dorsal fin. Thelodonts differed from other jawless fish in being covered in small spiny scales.

†Pneumodesmus newmani: this small arthropod found in Scotland is the earliest known millipede and the earliest known animal able to breath on land.

†Guiyu oneiros: Guiyu is the earliest known bony fish, a fish with a skeleton made of bone instead of cartilage. The structure of its fins indicate it was coser to the lobe-finned fish; fish with fins supported by bones with muscle rather than just rays with skin as in the ray-finned fish.

†Cooksonia sp.: Cooksonia is the first known vascular land plant; the first plants with tissues that carried water and nutrients. It grew only a few inches tall but already showed the structure that would come to establish all the worlds great forests, stems that shoot upwards to capture light from the sun. Cooksonia lacked leaves, flowers and roots, and likely reproduced with spores as primitve plants like ferns do today. Patches of Cooksonia probably never grew away from the edges of streams and ponds.