The development of life in theOceans 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 aswell as the first fish with jaws. Fishes and ammo-nites develop very rapidly during the Devonian.

• Coral reefs appear and expand; a widespreadradiation of crinoids (sæliljur) and a continuation of the expansion of the brachiopods (armfætlur).

Aquatic recoveryafter the Ordovician

mass extinction

Most of the marine taxare-diversified quickly afterthe Ordovician mass ex-tinction, with the exception of the trilobates. They never recovered fully, and drifted slowly towards extinction.

Greatexpansionof tropical

reefsBiodiversity iscommonly comparedbetween differentenvironments. Thisdiagram shows that Silurian reefs contain more species than Silurian non-reef environments

http://www.mpm.edu/reef/intro.html

The reefs as ecosystemsReefs are built in warm shallow seawater in the tropicsand subtropics. Reefs occur only in waters that arerelatively 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 resistantframework of calcium carbonate. Inliving coral reefs, corals and spongesare most important. In earlier geological periods, stromatoporoids (strýtuþörungar), bryozoans, and corals have functioned as framework builders.

Reef bafflers (rifstilkir) have uprightfronds or stick-like growth forms that interfere with currents and trap sediment on the reef surface

The reef as ecosystem

Reef binders, like white-coloredalgae, grow over and around loosesediment and skeletons of reef organisms and literally bind them together.

Reef dwellers consist of a variety of speciesthat live in and among the constructors and binders, but they do not directly build the reef framework.

The reef as ecosystemReef destroyers bore into or scrape awayparts of the reef surface, converting hardreef 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 amongthese types of organisms. Corals, which are common reefconstructors, form a rigid framework that offers habitats for reef-dwelling bivalves, which may cement to coral heads or nestle incavities 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 newtype of rigid surface.

Silurian-Devonian reefscape

A Silurian-Devonian sea floor, with numerous corals, largeStromatoporoids (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 enormousDuring the Devonian, reefs grew to proportionsnever experienced before. The largest reefsgrew near the Equator, in the tropical seas close to Gondwana as well as in Canada

Geology of reefsof economical importance...

Within and around the hugeDevonian Reefs, the remainsof various plants and animalsthat lived in the sea or on thereef accumulated layer uponlayer. As these layers accumu-lated, the organic materialstransformed 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 large pockets of pressurized oil and natural gas waiting to be tapped.

A Devonian reef in Canada; >800 m long

Numerous reefextinctions

Reefs are particularily sensitive to sea level changes

Ammonites developrapidly during the

Devonian

Ammonites radiatedand developedrapidly during theupper Paleozoic; they were prominent until the K/T boundary, when the went extinct.

Silurian fishes

This fauna is dominated by jawless fishes, but jawedfishes 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 thetremendous diversity of fish groupsthat evolved during this period of geologic time. Small, jawless, andfinless 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 werecommon in Silurian and Devonian time

TheEurypterids

• The eurypterids were amongthe largest marine predators of 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 rocksthat were laid down in brackishwater 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 beencalled 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 extinctgroup of Devonian-aged jawed fishes. Placoderms werearmored 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 hadteeth, 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 familyDinichthyidae ("terrible fishes"). It was a heavilyarmored 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...

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

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.

More on the Plachoderms

In 1997, a plachoderm fossil from Antarctica was foundto contain preserved pigment cells: iridescent silver onthe 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 diversetaxon, but they lasted only about 50 MY. Contrast thiswith the history of sharks, which appeared at about thesame 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 infreshwater habitats: placoderms included some of thefirst vertebrates to colonize fresh water. They also included the earliest vertebrates to colonize the open ocean.

• Plachoderms survived until the very end of theDevonian, 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 vertebratesknown. Frag-mentary remains have been recovered from as early asthe 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 jawedfishes that are distinguished by the bony spines projecting infront 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 natureof the beast. Due to their characteristic cartilaginousskeleton, 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 completefossil shark, from themid-Devonian. It had 3-pointed teeth with a large central point andsmaller cusps flankingit. It was 0.5-2 m longand had large spines infront 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.

Cladoselache

The CladoselacheThe early shark Cladoselachehunted in the warm seas of North America 375 millionyears ago. It had a markedlyweaker jaw joint thanpresent-day sharks, but itcompensated with impressivejaw-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 appearedabout 400 MY, and becameextinct 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 appearedin the late Silurian or earlyDevonian, about 395 MY ago. Theearly forms were freshwaterfishes, for no fossil remains of modern bony fishes have beenfound in marine deposits older thanTriassic time, about 230 MY ago. The ray-finned fishes became and have remained the dominant group of fishes throughout the world. It was not the ray-finned fishes, however, that led to the evolution of the land vertebrates...

There are >20.000 species of ray-finned fish in the World.

The lobe-finnedfishes (skúfuggar)

Lobe-fins, withmonaxial fin support, were very successfulduring the PaleozoicEra. They weretypically the top predators in many of the marine and freshwater habitats they occupied.

Moreover, one group of lobe-fins gave rise to thetetrapods, which have become other most successfulgroup of vertebrates. Strictly speaking, since tetrapods evolved from lobe-fins, all tetrapods –including us— are also lobe-fins.

Lobe-fins senso stricto not verysuccessful in the long run...

Latimeria chalumnaeLobe-fins are currently repre-sented only by the coelacanth(bláfiskur- Latimeria chalumnae), 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-leggedvertebrates most closely related to fishes are the amphibians. They still return to water to lay their eggs.

Ichthyostega, uppermost Devonian rocks in East Greenland

• Ichthyostega was a rela-tively large (1.5 m) early tetrapod with a stout body.

• Sedimentologicalinformation suggest that it lived in streams.

• Ichthyostega may havehauled itself onto the shore, but probably spent most of its time in water.

• Its spine was notochordal(hryggstrengur), rather thanbased on a series of flexibly jointed but interlocking vertebrae.

The Ichthyostega

Late Devonian mass extinction(s)

The Late Devonian mass extinction was either a prolongedmarine 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 particularlysevere for benthic marine organisms that lived inshallow 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 Devonianmass extinctions. These include asteroid impacts, globalanoxia (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