The development of life in the Oceans during Silurian and Devonian Periods (443-354 MY) Silurian and Devonian development of life in the Ocean • A rapid spread of jawless fish, and the appear- ances of both the first known freshwater fish as well as the first fish with jaws. Fishes and ammo- nites develop very rapidly during the Devonian. • Coral reefs appear and expand; a widespread radiation of crinoids (sæliljur) and a continuation of the expansion of the brachiopods (armfætlur). Aquatic recovery after the Ordovician mass extinction

Most of the marine taxa re-diversified quickly after the Ordovician mass ex- tinction, with the exception of the trilobates. They never recovered fully, and drifted slowly towards extinction. Great expansion of tropical reefs

Biodiversity is commonly compared between different environments. This diagram shows that Silurian reefs contain more species than Silurian non- reef environments http://www.mpm.edu/reef/intro.html The reefs as ecosystems Reefs are built in warm shallow seawater in the tropics and subtropics. Reefs occur only in waters that are relatively free of suspended, land-derived sediment, which allows sunlight to penetrate to the reef surface, permitting photosynthetic organisms to live. Both geological and living reefs are characterised by three main groups of animals and plants:

Framework builders build the resistant framework of calcium carbonate. In living coral reefs, corals and sponges are most important. In earlier geological periods, stromatoporoids (strýtuþörungar), bryozoans, and corals have functioned as framework builders. The reef as ecosystem Reef bafflers (rifstilkir) have upright fronds or stick-like growth forms that interfere with currents and trap sediment on the reef surface

Reef binders, like white-colored algae, grow over and around loose sediment and skeletons of reef organisms and literally bind them together.

Reef dwellers consist of a variety of species that live in and among the constructors and binders, but they do not directly build the reef framework. The reef as ecosystem

Reef destroyers bore into or scrape away parts of the reef surface, converting hard reef framework into loose particles of sediment. These photos show holes in a coral produced by the bivalve at the far left.

The reef community is characterized by complex interactions among these types of organisms. Corals, which are common reef constructors, form a rigid framework that offers habitats for reef- dwelling bivalves, which may cement to coral heads or nestle in cavities between them. Certain sponges, worms and other bivalves act as reef-destroyers by boring into the coral framework and producing loose particles of broken coral. Baffling organisms may concentrate these sediment particles on particular parts of the reef, where organisms such as calcareous algae can bind them to create a new type of rigid surface. Silurian-Devonian reefscape

A Silurian-Devonian sea floor, with numerous corals, large Stromatoporoids (strýtuþörundur), the "sunflower coral" (possibly a green algae). Crawling among the coral and stromatoporoids are trilobates and gastropods. In the background are crinoids and orthocerid (réttskélja) nautiloids (kuggar). Devonian reefs were enormous

During the Devonian, reefs grew to proportions never experienced before. The largest reefs grew near the Equator, in the tropical seas close to Gondwana as well as in Canada Geology of reefs Within and around the huge Devonian Reefs, the remains of economical of various plants and animals importance... thatlivedintheseaoronthe reef accumulated layer upon layer. As these layers accumu- lated, the organic materials transformed into oil and natural gas due to pressure, heat and other geological factors. These layers managed to seal the corals (due to the porous rock's tiny holes) into A Devonian reef in Canada; large pockets of pressurized >800 m long oil and natural gas waiting to be tapped. Numerous reef extinctions Reefs are particularily sensitive to sea level changes Ammonites develop rapidly during the Devonian

Ammonites radiated and developed rapidly during the upper Paleozoic; they were prominent until the K/T boundary, when the went extinct. Silurian fishes

This fauna is dominated by jawless fishes, but jawed fishes are evolving (10). All Silurian fishes lived in a marine environment. The development of the fishes

The Devonian has been called the "Age of the Fishes" because of the tremendous diversity of fish groups that evolved during this period of geologic time. Small, jawless, and finless ostracoderms (brynfiskar) were among the earliest vertebrates. They were filter feeders. Although extant jawless fishes lack protection, many early jawless fishes had large defensive head shields. Development of fishes Shield: a defense against other predators

Sea scorpions were common in Silurian and Devonian time The • The eurypterids were among the largest marine predators of Eurypterids the Paleozoic, some reached more than 2 m in length. • They arose in the Ordovician, and went extinct in the Permian. Most have been found in rocks that were laid down in brackish water or freshwater; the earliest groups may have lived in the sea, and some eurypterids may have spent at least short intervals on land. • The eurypterids have been called sea-scorpions. And in fact they are closely related to scorpions. The agnaths (kjálkalausir fiskar; Agnatha= vankjálkar) Early jawed fishes

The evolution of jaws is an example of evolutionary modi- fication of existing structures to perform new functions. Jaws are modified gill arches, and allowed the exploitation of new roles in the habitats: predators with powerful jaws. The Plachoderms (brynháfar)

The first jawed fish were the Placoderms, an extinct group of Devonian-aged jawed fishes. Placoderms were armored with heavy plates and had strong jaws and paired pectoral and pelvic fins. Paired fins allow fish to balance and to maneuver well in water, which facilitate both predation and escape. The Plachoderms...

Unlike all other jawed vertebrates, plachoderms never had teeth, and did not descend from toothed ancestors. Instead, bony plates associated with the jaws performed the function of teeth, sometimes forming razor-like, literally self-sharpening edges (Dunkleosteus above). A Dunkleosteus

Dunkleosteus was the biggest member of the family Dinichthyidae ("terrible fishes"). It was a heavily armored primitive fish, up to 8-10 m (!) long, from the Late Devonian period, living about 370 million years ago. A monster for our imagination A remarkable fish... Dunkleosteus looked like the violent brute it was: power- fully built and armour-plated round its head. Pigment cells suggest Dunkleosteus had dark colours on its back and was silvery on its belly.

Dunkleostus ate fish, sharks and even its own kind. And it seems that Dunkleosteus suffered from indigestion as a result: its fossils are often associated with regurgitated, semi-digested remains of fish. Dunkleosteus may have been one of the earliest animals to exist as male or female meaning that pairs of fish had to mate More on the Plachoderms

In 1997, a plachoderm fossil from Antarctica was found to contain preserved pigment cells: iridescent silver on the ventral side (belly) and red on the dorsal side (back). Placoderms are the oldest vertebrates for which we know something about their color in life. This further implies that placoderms may have had color vision. The Plachoderms lasted for 50 MY • The Plachoderms were a highly successful and diverse taxon, but they lasted only about 50 MY. Contrast this with the history of sharks, which appeared at about the same time as placoderms - but which have survived for over 400 million years! In a sense, plachoderms repre- sent "early experiment" in the evolution of jawed fish. • A number of Devonian plachoderms have been found in freshwater habitats: placoderms included some of the first vertebrates to colonize fresh water. They also included the earliest vertebrates to colonize the open ocean. • Plachoderms survived until the very end of the Devonian, and their extinction appears to have been quite sudden, but its causes are still unknown. The spiny fishes (háfiskar, gadduggar)

Acanthodians (háfiskar) are among the earliest jawed vertebrates known. Frag-mentary remains have been recovered from as early as the Upper Ordovician of North America and Lower Silurian of China. Their record extends about 160 million years to the Lower Permian. Diversity is greatest from the Upper Silurian through the Devonian. The Acanthodians (háfiskar)

Acanthodians are a poorly understood group of extinct jawed fishes that are distinguished by the bony spines projecting in front of their fins. Most of them also have relatively large eyes set near the front of their short blunt heads. Like most early fishes, acanthodians had heterocercal caudal fins (tail fins with the top longer that the bottom). A view of a spiny fish...

Side and bottom views of Gyracanthides murrayi . They ranged in size between 0.2-1.2 m Evolution of the sharks...

Tracing the evolution of sharks is frustrated by the nature of the beast. Due to their characteristic cartilaginous skeleton, ancient sharks have left behind precious few clues to enable us to figure out what they were like. The oldest undisputed shark remains are about 420 MY scales, from early Silurian deposits in Siberia. The earliest complete Cladoselache fossil shark, from the mid-Devonian. It had 3- pointed teeth with a large central point and smaller cusps flanking it. It was 0.5-2 m long and had large spines in front of each of the 2 dorsal fins. It had neither an anal fin nor claspers. There were 5 gill slits on each side of the head. Fossils found in the Cleveland shales including fossilized stomach contents reveal that it ate fish. The Cladoselache

The early shark Cladoselache hunted in the warm seas of North America 375 million years ago. It had a markedly weaker jaw joint than present-day sharks, but it compensated with impressive jaw-closing muscles that are frequently fossilized. Caldoselache had a deep, forked tail and was probably a fast swimmer, which is just as well since it may have been preyed upon by the giant 3.5 m Dunkleosteus. Some ancient sharks...

The 350 MY old Echinochimaera

Orthacanthus appeared about 400 MY, and became extinct about 225 million years ago. Helicoprion lived about 250 MY (http://school.discovery.com/schooladventures/prehistoricsharks/gallery.html) The Ray finned (geislauggar) fishes The modern bony fishes appeared inthelateSilurianorearly Devonian, about 395 MY ago. The early forms were freshwater fishes, for no fossil remains of modern bony fishes have been found in marine deposits older than Triassic time, about 230 MY ago. The ray-finned fishes became and have remained the dominant group There are >20.000 of fishes throughout the world. It species of ray- was not the ray-finned fishes, finned fish in the however, that led to the evolution World. of the land vertebrates... The lobe-finned Lobe-fins, with monaxial fin support, fishes (skúfuggar) were very successful during the Paleozoic Era. They were typically the top predators in many of the marine and freshwater habitats they occupied.

Moreover, one group of lobe-fins gave rise to the tetrapods, which have become other most successful group of vertebrates. Strictly speaking, since tetrapods evolved from lobe-fins, all tetrapods –including us— are also lobe-fins. Lobe-fins senso stricto not very successful in the long run... ), repre-

halumnae c oelacanth c chalumnae the currently by

re Latimeria a - only

Latimeria Lobe-fins sented (bláfiskur and six species of lungfishes. The Bluefish can be 1.2-1.8 m long, and weigh up to 80 kg. Lungfishes (Dipnoi) – a group within the lobe-fins - • Have changed relatively little for the past 400 MY • Occur today in Australia, S America and Africa • Respiration using the lungs is critical for survival – they can drown if they cannot breath air • African and S American lungfish can survive the drying up of their native ponds The Lobe-finned fishes are our ancestors...

The lobe-finned fish are regarded as ancestors of early amphibians. Extinct crossopterygians had strong fins, lungs, and a streamlined body capable of swimming as well as traveling short distances out of water. Development of the legs... Animals move ashore... Anatomical evidence indicates that the four-legged vertebrates most closely related to fishes are the amphibians. They still return to water to lay their eggs.

Ichthyostega, uppermost Devonian rocks in East Greenland The Ichthyostega

• Ichthyostega was a rela- tively large (1.5 m) early tetrapod with a stout body. •Sedimentological information suggest that it lived in streams. • Ichthyostega may have hauled itself onto the shore, but probably spent most of its time in water. •Itsspinewasnotochordal (hryggstrengur), rather than based on a series of flexibly jointed but interlocking vertebrae. Late Devonian mass extinction(s)

The Late Devonian mass extinction was either a prolonged marine biotic crisis extending for 20-25 million years or a number (2-10) of extinction events. Which organisms were affected? • About 40% of all marine genera (ættkvísla) disappeared. Major victims included ammonites, benthic foraminifera, brachiopods, rugose and tablulate corals, jawless fishes, placoderms, stromatoporoid sponges and trilobites. • The Devonian extinctions were particularly severe for benthic marine organisms that lived in shallow tropical seas. In fact, many of the taxa that thrived during and after the extinctions were typically deep-water or high-latitude relatives of the decimated forms. The cause(s) of the Devonian mass extinction(s)

A variety of causes have been proposed for the Devonian mass extinctions. These include asteroid impacts, global anoxia (widespread dissolved oxygen shortages), reduced greenhouse gases in the atmosphere, sea level changes and growth of glaciers.

Expansion of forests in Late Devonian may have sub- stantially reduced atmospheric CO2 levels, and contri- buted to global cooling. A short, but intense episode of glaciation occurred at the very end of the Devonian in parts of Gondwanaland. A drop in sea level combined with an extended period of glacier growth and cooler temperatures may explain the mass extinction(s). References, some good web sites etc

• Stanley, Earth System History, chapter 14 • Fortey, R. : Life – a natural history of the first 4 billion years of life on Earth. New York, Vintage Books, 346 pp. • Fortey, R. Trilobate – Eyewitness to Evolution. London, Flamingo, 269 pp. • Tvær stórskemmtilegar vefsíður: Devonian Times (www.mdgekko.com/devonian/index.html) BBC Evolution Website (www.bbc.co.uk/education/darwin/index.shtml) • http://www.bbc.co.uk/science/seamonsters/factfiles/dunkleosteus.shtml?img4 • www.peripatus.gen.nz/Paleontology/Index.html • www.palaeos.com/Default.htm • www.elasmo-research.org/index.html • www.agiweb.org/news/evolution/index.html • http://www.ucmp.berkeley.edu/devonian/devonian.html • http://www.ucmp.berkeley.edu/silurian/silurian.html