Silurian and Devonian

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Important Silurian and Devonian Biology

Sea Scorpions: Eurypterids (Arthropod)

• Top predator of the Silurian are Eurypterids (sea scorpions).

• Marine, lagoon, freshwater, and swamp environments.

• Lived through Paleozoic, heyday was the Silurian.

Age of the Fish

• The first vertebrates were jawless fish found in rocks of early Paleozoic; by Ordovician, these jawless fish diversify.

• Late Silurian fish begin to diversify.

• The Age of the Fish was the Devonian

  • Fish were common in marine and freshwater environments.

Jawless Fish

• Known from Late Cambrian to Ordovician.

• Become more complex

• Develop better armor (head shields/body armor)

• Lack of jaws implies they were bottom feeders and filter feeders.

• Cartilage skeletons did not preserve because it was too soft, only armor was preserved.

• Eventually, fish with jaws evolved.

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Acanthodians

• Appear in the Silurian, although most abundant during Devonian.

• These were small fish with large spines, paired fins, scales covering much of the body, jaws, and teeth.

Vertebrate Jaw Evolution

• Studies suggest jaw evolved from gill arches (gills are soft, but arches are made of bone or cartilage).

  • Gill arches 1 become modified to form upper and lower hinged jaws.

  • Gill arches 2 became modified to form hyoid arch.

• Evolution of arches allowed for mouths to open wider, increasing food consumption.

First True Shark

• First true sharks may have evolved from Devonian acanthodians.

• Skeleton composed of cartilage.

• Serrated teeth

• Very broad pectoral fins and long, flat skull

Placoderms

• One of the other first fish with jaws.

• Evolved in Silurian; peak in Devonian.

• Skeleton composed of cartilage.

• Heavily armored

• Flat bellies, likely fed on the ocean bottom.

Dunkleosteus

• Largest predator of the Devonian sea (Placoderm).

• Largest predator of the Devonian.

• Heavily armored head and shoulder region.

• Razor sharp bony teeth.

• Must have terrorized sharks.

Bony Fish

• Played key role in tetrapod evolution

• Most numerous, varied, and successful of all aquatic vertebrates.

• Two categories:

  • Ray-fin

  • Lobe-fin

Ray-Fin Fish

• Type of bony fish

• Lacked muscular base to their paired fins; lacked paired nasal passages that open into the throat, had to breath through gills.

• Evolved in the Late Devonian in freshwater lakes and streams, quickly expanded to marine.

• 99% of living fish are in this group.

Lobe-Fin Fish

• Lobe-finned fish: Group of fish that gave rise to humans.

• Sturdy, muscular fins and a pair of opening in roof of mouth that lead to external nostrils allowing them to rise to surface and take in air, which was passed on to functional lungs.

• Club-shaped fins supported by stout bones

  • Homologous to our arm bones

  • Give rise to humans.

Nautiloids

• Two orders of cephalopods evolve during Paleozoic Era.

• The first were the Nautiloids

  • Top predator of the Ordovician (+ sea star)

  • Most nautiloids had straight shells; although more became curved (particularly during Silurian). The curved shell lessened the weight it carried and helped it move faster.

  • Though remained important predators in the Silurian.

Ammonoids

• Ammonoids: Devonian cephalopod with more complex sutures compared to the Nautiloids; has coiled, spiral shells.

• The second order of cephalopod.

• Animals which bear their young in an egg that is independent of water.

• 1st ammonoid cephalopod appeared in Devonian.

• Complex sutures of ammonites versus nautiloids provided extra strength to protect against predation.

• Based on the suture patterns, ammonoids are divided into goniatites, ceratites, and ammonites.

Silurian Land Plants

• First land colonizers were likely microbiotic crusts.

  • Communities of fungi, bacteria, and algae.

  • Evidence going back to Ordovician

• First evolved in marine environment, moved to freshwater, then onto land.

• Plants definitely colonize land by the Silurian.

  • Leafless stalks; none taller than half a meter.

  • Simple vascular tubes to transport nutrients and provide structure.

Devonian Land Plants

• Devonian saw evolution of more complex vascular systems for nutrient exchange and the evolution of roots.

  • Roots perform nutrient acquisition as well as support.

• Late Devonian saw evolution of lycopsids (ex: club mosses)

  • Lived near water ways

  • Reached up to a meter or more in height.

More Late Devonian Land Plants

• Second important group → Sphenopsids (ancestors of the modern Horsetails)

• Third group to evolve in the Late Devonian is the groups which includes the modern ferns.

  • Glossopteris: woody gymnosperm - plant with seeds unprotected by an ovary fruit.

    • Gymnosperm = Conifers and ginkos

First Bugs on Land

• With foliage fully intact, the first animals to take advantage are the Arthropods (bugs).

• Millipedes likely made their way onto land by Early Silurian (trace fossils found in Scotland).

• Early Devonian deposits in Scotland of early forest also preserve scorpions, spiders, mites, millipedes, and wingless insects.

• By Late Devonian, forests were filled with a variety of crawling, burrowing, and flying insects.

Vertebrates Make Grand Entrance

• Although vertebrates appear to have radiated in the late Ordovician, earliest vertebrates during Cambrian explosion.

• In Late Devonian, vertebrates can now reproduce without water as the medium.

• Limbs strong enough to support weight out of water.

• Skin to prevent drying out.

Tiktaalik

• Also called fishapod

• Missing link fossil for the fish to first tetrapod transition; has features of both fish (Lobe-fin) and early land animals (tetrapods).

• Seeing the link between fish and tetrapods

• Found in the Canadian Arctic

• ~375 Ma

• Similarities of the lobe-finned fish (early Devonian) and the tetrapod Ichthyostega (Late Devonian).

⭐️ Devonian Extinction Event ⭐️

• 70% of all marine invertebrates go extinct.

• 15% of brachiopods survive.

• Trilobites and gastropods take a hit.

• Tabulate-Rugosid-Stromatoporid reefs devastated and never rebound.

• Armored jawless fish and placoderms go extinct.

Similar Mass Extinction to Ordovician

• Warm water taxa hardest hit.

• Polar S. American taxa show almost no sign of the event.

• Glass sponges take over reef environments.

• Extinction likely another global cooling event.

Carboniferous and Permian

Important Carboniferous and Permian Biology

Tethys Sea

• A sea way which separated the northern continents of Laurasia from the southern continents of Gondwanaland.

• Sea that formed when Pangea formed.

• Pac-Man’s mouth

• Warm equatorial sea

• High productivity

• High biodiversity

Petroleum Source Rocks

• Tethys Sea was shallow with high productivity.

  • Deposits become petroleum source rocks.

• It is Tethyan petroleum which they find in such vast amounts in Middle East.

Source Rock

• Organic material deposition: Dead plants and microorganisms settle with sediments like mud and clay.

• Burial and compaction: Sediments are buried, compacted, and exposed to heat and pressure, forming kerogen.

• Maturation: Kerogen breaks down into hydrocarbons, creating a source rock rich in oil and gas.

Tethyan Fauna

• Warm water equatorial fauna separated from cold water faunas of polar regions.

• Relicts of Tethys Sea include Caspian, Black, and Mediterranean.

• Overall Pangean diversity was low and widespread.

• Mesozoic break-up of Pangea, biodiversity increases again.

Pangean Biography

• During Permian, life covering vast stretches of Pangea were homogenous.

• Land reptile diversity was markedly low.

• Mammal-like reptiles, or synapsids (e.g., Lystrosaurus), were common.

• Example of disjunct fauna.

  • Disjunct: Describes something separated by some kind of geographic boundary.

Disjunct Flora and Fauna

• Two or more groups that are related but considerably separated from each other geographically.

Late Paleozoic Life (Early Pennsylvanian)

• Continental seas far less expansive during Pennsylvanian, thus leading to more land for terrestrial organisms to colonize.

• Marks increase in terrestrial forests as well as continued evolution of marine communities.

Late Paleozoic Marine Life

• Late Devonian extinction strongly affected reef communities and caused widespread extinctions of brachiopods and ammonoids.

• Most phyla bounce back from this.

• Important new organisms to appear in Pennsylvanian is a special group foraminifera known Fusilinids.

  • Single-celled plankton.

  • Tightly packed, secreted micro-granular calcite.

  • Best index fossil for marine rocks of Pennsylvanian-Permian.

Pennsylvanian Coal and Permian Pines

• Late Devonian saw 1st forests (thigh high) arise from swamps with diverse arthropods and amphibians.

• Late Paleozoic → terrestrial diversified

• Plants become so abundant in Late Paleozoic coal is the hallmark rock.

• Coal swamps were vast during the time

  • Coal beds were largely responsible for the industrial revolution.

Late Paleozoic

• Mississippian, pulse of epeiric seas limited terrestrial habitat (Kaskaskia transgression).

• Plants not all that dissimilar from those of the Devonian.

• Drop in sea level into Pennsylvanian, swampy habitats spread across the continent on a scale not known today.

  • Plants diversified

  • Club moss trees attained heights of 30 m.

Ushering out of the Club Mosses

• Mid-Permian global climate continued to cool.

• Continents continued to emerge becoming drier.

• Club mosses and other water dependent plants began to decline.

Insects (Late Paleozoic)

• Late Carboniferous saw rapid diversification of insects.

• “Beaked” insects.

Amphibians

• Date back to Devonian; evolved from lobe-finned fish.

• Carboniferous amphibians were diverse and large.

Amniotes

• Animals which bear their young in an egg that is independent of water.

• Amphibian eggs require water, thus they are tied to bodies of water.

• Late Mississippian, evolution of amniote egg freed tetrapods from constraint of nearby water bodies.

• Amniotes egg is in essence a miniature egg (bypasses the need for a larval stage in water).

  • Amnion - liquid filled sac

  • Yolk - Food supply

  • Allantois - Waste sac

Amniotes Split

• Evolutionary split in Pennsylvanian.

• Based on temporal fenestrae

• Post-orbital opening in that allow muscles to expand and lengthen → stronger support in biting.

• Gave rise to new lineages: mammals, turtles, lizards, dinosaurs, and bird.

Anapsids, Synapsids, and Diapsids

• Anapsid

  • No temporal opening

  • Turtles (only anapsids living today)

• Synapsids

  • Single temporal opening

  • Mammal-like reptiles

• Diapsids

  • Double temporal opening

  • All reptiles, crocodiles, and birds

Synapsids

• Before the Age of Dinosaurs, Permian life on land included these mammal-like reptiles.

• These, are not considered reptiles; closer to mammals.

• Became very diverse.

  • Large bodied herbivores

  • Large bodied carnivores

• Example: Pelycosaurs

Pelycosaurs

• Earliest, most primitive synapsids.

• Finback reptiles

• The fin → sexual display, form of protection, a way to make them look more ferocious, or a thermoregulatory structure.

• E.g., Dimetrodon, dominated Permian

Lystrosaurus

• Disjunct fauna

• Wide-spread during late Permian

• Stout and designed to be a burrower

• Survives P-T extinction and becomes dominate land animal in the early Triassic.

Diapsids

• Will be relatively small in the Permian, go nearly extinct at PTr extinction.

• Then rise to dominance in the mid-Triassic.

• Group responsible for modern reptiles, turtles, and birds as well as dinosaurs and pterosaurs.

The Great Dying

• Permian-Triassic (PTr) Mass Extinction

• Trilobites were gone before the end, Tabulate and Rugose Corals die out, Productid brachiopods are gone, Fusulinids die out, same with most Bryozoans, Crinoids almost go out entirely.

• 75% of vertebrates go extinct (Amphibians, Synapsids, and Primitive diapsids).

• 90%-95% of all marine animals die out.

• Rather than a split second in geologic time the PTr event was a drawn out saga (>5 million years).

What Caused It?

• We don’t fully known.

• Global glaciation in middle Permian followed by green house conditions.

• Loss of continental shelves with the formation of Pangea.

• Stagnant oceans as a result of Pangea

• Siberian Traps: Largest flood basalts in Earth history.

• Maybe all of the above

End of an Era

• Life would take 10 million years to recover from what happened at the end of the Paleozoic.

• Set the stage for our modern flora and marine biota.

• Set the stage for a fight for terrestrial dominance between the descendants of the synapsids and diapsids which the synapsids lose out and are stuck in the background for the next 145 million years.

Review Questions

• Were there general names for specific periods and/or eras (e.g., Age of Fish)?

  • Age of Fish: Devonian

• How did life change from Precambrian into (and through) the Paleozoic?

  • Evolution of jawless fish (Late Cambrian to Ordovician).

  • First land colonizers were likely microbiotic crusts (Silurian)

  • Devonian saw evolution of more complex vascular systems for nutrient exchange and the evolution of roots.

  • Late Devonian, forests were filled with a variety insects and vertebrates could now reproduce without water as the medium.

  • Increase in terrestrial forests as well as continued evolution of marine communities (Early Pennsylvanian).

• What were dominant predators, new organisms, and index fossils for specific Periods?

  • Silurian:

    • Apex predator: Eurypterids (sea scorpions), nautiloids

  • Devonian:

    • Apex predator: Dunkleosteus

  • Pennsylvanian-Permian

    • Index fossil: Fusilinids

• What was the evolutionary path of life from marine to land?

  • Silurian plant first evolved in marine environment, moved to freshwater, then onto land.

  • Continental seas far less expansive during Pennsylvanian, thus leading to more land for terrestrial organisms to colonize.

• What were the mass extinction event(s)? When did they occur? What caused them?

  • Devonian Extinction Event ️

    • 70% of all marine invertebrates go extinct, 15% of brachiopods survive. trilobites and gastropods take a hit, Tabulate-Rugosid-Stromatoporid reefs devastated and never rebound, armored jawless fish and placoderms go extinct.

    • Cause: most likely global cooling event

  • The Great Dying (Permian-Triassic)

    • Trilobites, Tabulate and Rugose Corals, Productid brachiopods, Fusulinids, and most Bryozoans go extinct. Crinoids nearly vanish.

    • 75% of vertebrates, including Amphibians, Synapsids, and Primitive diapsids, die out.

    • 90%-95% of marine life perishes.

    • Causes:

      • Global glaciation followed by greenhouse conditions.

      • Loss of continental shelves with Pangea's formation.

      • Stagnant oceans due to Pangea.

      • Siberian Traps: Largest flood basalts in Earth's history.

      • Possibly a combination of these factors.