1. Silurian survivors of the Late Ordovician mass extinction were mostly cold-adapted animals from high latitudes or deep waters.

2. Figure 12.3: The following invertebrates continued into the middle Paleozoic, but mostly as new families and orders:

(a) crinoids

(b) brachiopods

(c) bryozoans

(d) trilobites

(e) corals

(f) graptolites

3. Silurian brachiopods included pentamerids (teardrop-shaped shells) which lived in dense comminities within reefs. Pentamerids declined in abundance during the Devonian.

4. Spirifers dominated the Devonian sea floor and resembled a pair of wings. Spirifers continued into the late Paleozoic and survived until the Jurassic.

5. Trilobites were relatively scarce during the Silurian.

6. Devonian trilobites included the unusual Phacops which had huge compound eyes and could roll up into a ball.

7. Figure 12.4: The top predators during the Silurian were the eurypterids , or sea scorpions, that were arthropods. Most eurypterids were 13-50 cm in length, but could grow as long as 2.5 m (9 feet). Eurypterids began as marine animals during the Early Paleozoic but eventually spread into brackish lagoons, fresh waters and even swamps. Eurypterids became extinct in the late Paleozoic.

8. The Nautiloids declined in number during the Silurian and were largely replaced during the Devonian by their descendants, the ammonoids. Unlike Nautaloids that were mostly straight shelled, ammonoids grew a shell that curled into a tight spiral. As the cephalopod grew, it vacated old chambers that were closed off through secretion of a wall or septum. Closed, hollow chambers provided flotation. The intersection of the septum with the wall produced suture patterns.

9. Graptolites, which were nearly wiped out during the Ordovician extinction, again flourished during the Silurian.

10. By late Silurian time, however, free-floating graptolites declined in abundance such that only Monograptus remained at the Silurian-Devonian boundary. Planktonic graptolites became extinct in the Early Devonian, leaving only primitive attached, bushy types to continue until the Pennsylvanian.

1. Figure 12.28: The Silurian was characterized by abundant reef complexes that reached 10 meters above the seafloor and stretched up to 3 kilometers long.

2. Silurian reefs were mostly built by corals, stromatoporoids and a variety of unusual sponges.

Figure 12.7: The middle Paleozoic is known as the Age of Fishes. Two major groups of fishes were present by the middle Paleozoic, the Jawless Fish and the Jawed Fish.

1. Jawless fish (also known as agnaths) first appeared during the Late Cambrian and continued to be abundant in the Middle Paleozoic. They are survived today by their descendents, the Hagfishes and Lampreys.

2. Jawless fish of the early Paleozoic are collectively called ostracoderms. These early jawless fish had a simple, ring-like opening for a mouth and probably filtered food either out of the mud on the seafloor or directly from seawater.

3. Figures 11-9 & 11-10: Some of the early jawless fish were armored only with isolated bony scales and plates. During the Silurian, however, jawless fish developed armored head shields and body armor. Armored jawless fish became extinct by the end of the Devonian.

1. Figure 12.6: The acanthodians were the first and oldest fish with jaws. Acanthodians first appeared during the Silurian and survived until Permian time.

2. Acanthodians resembled spiny sharks and had large eyes and a row of fins supported by spines along the side of the body.

3. Acanthodians became extinct in the Permian.

4. The first true sharks appeared during the Devonian and had skeletons made of cartilage. These early true sharks were typically less than 4 feet in length.

5. Figure 12.1: Placoderms, probably the largest predator in the Late Devonian seas, appeared in the Devonian but became extinct by the Carboniferous. Placoderms had an internal skeleton made of cartilage and extensive bony armor plating on the head and trunk. Some were heavily armored down to their fines while others were only lightly armored.

Fish with Bony Skeletons.

1. Fish with skeletons made of bones also appeared during the Devonian and have abundant descendants living today.

2. Figure 12.7: One group of bony fish called the Ray-finned fish. Ray-finned fish lack a muscular base to their paired fins. Instead, the fins are thin structures supported by bony spines that radiate from the body.

3. The ray-fins began their evolution in Devonian lakes and streams and quickly expanded into the marine realm. They became the dominant fishes of the modern world.

4. Lobe-finned fish were characterized by club-shaped fins that were supported by stout bones (later to become legs on the first amphibians). Lobe-finned fish also developed internal nostrils and eventually lungs that allowed them to breath out of water.

5. Figure 12.7: There are three main groups of lobe-finned fishes.

(a) The lungfish first appeared during the Devonian and survive today in fresh water environments.

(b) The coelacanths lived in saltwater environments. The coelacanths were thought to have undergone extinction during the Late Cretaceous, but their survival to the present has been documented by several catches near Madagascar.

(c) A third lineage that led to the amphibians and other land vertebrates.

1. The first amphibians descended from the lobe-finned fish. Amphibians first crawled onto dry land at the end of the Devonian.

2. Figure 12.13: Ichthyostega was one of the first amphibians and a classic transitional form between lobed-finned fish and amphibians. Its legs were functional for locomotion on land yet were still basically similar to the bones of the lobed-finned fish.

3. Ichthyostega retained the fishlike vertebrae, bony gill covers and a tail fin of its ancestors.

4. Ichthyostega, however, also had features attributed to amphibians such as a long snout, functional legs, and reinforced ribs to support its lungs.

5. Amphibians lost their remaining fishlike features by Carboniferous time, although today they still return to water to lay eggs.

1. The first land animals were the arthropods. Fossilized burrows from Late Ordovician rocks suggest that a millipede-like animal was already terrestrial at that time.

2. Lower Devonian cherts in Scotland contain fossil scorpions, spiders, mites, millipedes and wingless insects. Devonian swamps were probably filled with a variety of crawling, burrowing and flying insects.

1. Figure 12.9: The middle Paleozoic was a time when land plants proliferated. Two major groups of land plants dominated the landscape by Carboniferous time, the Spore-bearing plants and Gymnosperms.

1. Prior to the Ordovician, land surfaces were largely barren. Any plant covering was probably sparse with soils held in place mainly by communities of fungi, algae and bacteria.

2. Figure 12.8: The emergence of more advanced land plants by late Ordovician time is evidenced by fossil spores found within Upper Ordovician rocks from Libya. Land plants, however, did not significantly evolve until the Silurian and Devonian.

3. The earliest land plants consisted of small, spore-bearing varieties. Spores require continuous moisture to survive and also require a water medium in order to reproduce. Spore-bearing plants therefore clustered along the edge of bodies of water such as swamps.

4. Figures 12,10: The first vascular plants emerged during the early Silurian. Vascular plants have tubes for transporting water and nutrients through the tissues. The Early Devonian Rhymia was built of simple leafless stalks.

5. Middle Silurian and Early Devonian vascular plants lacked roots or leaves and were confined to creeping along the ground.

6. By middle Devonian time, however, vascular plants developed a stronger stalk and evolved roots for support and for extracting nutrients from the soil and water.

7. Figure 12.11: By the Late Devonian, club mosses and ground pine (lycopsids) covered the landscape. Most were low, creeping forms but some reached 100 feet in height and had long, slender, simple leaves that issued directly from the trunk in a spiral arrangement.

8. The sphenopsids (joint-stemmed plants) were a second important group of spore-bearing plants in the middle Paleozoic (see figure 12.10E in book). Sphenopsids have a long, hollow stem that is jointed, with leaves and sporangia (spore-bearing organs) clustered at the joints

9. True Ferns also became prevalent spore-bearing plants in the middle Paleozoic.

1. Gymnosperms are seed plants. Unlike spores, seeds do not require continuous moisture to survive and do not require a water medium in order to reproduce. Modern seed plants include conifers such as pines, firs, cedars, spruces.

2. Figure 12.11: The first seed plants, Seed Ferns, emerged during the late Devonian and eventually developed into large trees up to 10 m (33 feet) tall.

3. Gymnosperms, however, didn't become important until the Carboniferous.

1. One of the most devastating mass extinctions of marine life in all of Phanerozic time took place near the end of the Devonian Period.

2. On land, vascular plants appear to have been unaffected by the Late Devonian crisis.

3. In the marine realm, however, brachiopods were hard hit with perhaps only 15 percent of the genera surviving the mass extinction event. In addition, many types of ammonoids, trilobites and gastropods disappeared.

4. Tropical marine animals were the most affected, leading to the demise of much of the middle Paleozoic reef communities. In contrast, polar communities of marine organisms were mostly unaffected by the Late Devonian mass extinction. This suggests that an episode of global cooling and glaciation may have led to the mass extinctions.

5. Evidence of Upper Devonian glacial deposits in northern South America, which was situated near the south pole during the late Devonian, supports this theory.