Visit Your Local PBS Station PBS Home PBS Home Programs A-Z TV Schedules Watch Video Donate Shop PBS Search PBS
  High-Bandwidth Version
Search Evolution  
 
Click to return to the Evolution Home Page
darwin change extinction survival sex humans religion
Darwin    
   
Deep Time

Intro | Precambrian Eon | Paleozoic Era | Mesozoic Era | Cenozoic Era

Paleozoic Era: (543-248 mya)

Cambrian | Ordovician | Silurian | Devonian | Carboniferous | Permian

border

border

Devonian Period (417-354 mya)

Intense reef building activity in shallow-water habitats indicates that the Devonian climate is, on the whole, warm and stable. Early relatives of squids called ammonoids first appear. Jawless fishes remain dominant among marine vertebrates, though bony fishes and sharks diversify.

An explosion of plant life occurs during the Devonian. Early in the period, land plants range in size from a few centimeters to no more than a meter tall. By the end of the Devonian, complex branch and root systems produce trees 30 feet in height. Arthropods, which now include early wingless insects, radiate. Before long, the first four-legged animals emerge from the waters and join them on land.

The two prominent landmasses, Gondwana and Laurussia (sometimes called Euramerica), are drawn together as a subduction zone forms where the tectonic plates in which they ride meet. (Their collision won't happen until the early Permian.) The Devonian period ends with a cataclysmic extinction event, particularly devastating to warm-water marine communities. Nearly 70-80 percent of marine invertebrate species are wiped out over two extinction pulses.

400 mya: Oxygen nears present-day levels

Oxygen nears present-day levels of 21 percent by volume of the atmosphere.

375 mya: Land vertebrates

Amphibians are the first four-legged animals, or tetrapods, on land. With, in most cases, well-defined limbs and feet, a supportive rib cage, and a neck that enables its skull to rotate, amphibians are far better suited to moving around and resting their bodies on land than their ancestors, fleshy-finned bony fish called rhipidistians. Despite their modified bone structure, amphibians maintain a strong connection to the water, as adults feed on fishes in shallower waters. They also spawn in water, laying a number of smaller eggs that hatch into swimming larvae. Early amphibians are large animals that do not much resemble living frogs or salamanders.

border

Read more

From water to land: Fish out of water (375 mya)

As it was for the first land plants and arthropods, several key adaptations make the vertebrates' transition to life on land successful. Amphibians evolve skeletal and muscular features that support body weight, enable walking, and keep their heads up off the ground. But they are not the first vertebrates to venture onto land; scientists think their ancestors, the rhipidistian fish, used fleshy, lobed fins to shuffle ashore.

Many unmistakable physical similarities exist between rhipidistians, which resemble modern lungfish, and the first true land vertebrates, the amphibians. From fossil evidence, skulls, teeth, and vertebrae are nearly identical between some "lobefins" like the rhipidistians and the earliest amphibians. It's also clear that fleshy ventral fins, attached directly to lobefin skeletons near the tail-end of the fishes, have moveable bones and muscles. Remarkably, the bones within these fins are matched, one to one, with those in the legs of early amphibians. Another shared trait is the ability to breathe air. Rhipidistians, along with several other groups of early fishes, had nostrils and lungs.

It's important to understand that these physical adaptations, while later recruited for use on land, originally evolved in the water. Lobefins would have used ventral fins to improve swimming and steering, and to scuttle through very shallow waters and stir up the silty bottom as they searched for food. Rhipidistians probably evolved the capacity to breathe air to cope with environments where oxygen levels in water were seasonally low, like in shallow lagoons.

border


Late Devonian extinction

Date:

364 mya

Intensity:

2

Affected:

Between 50-55 percent of marine invertebrate genera, and 70-80 percent of marine invertebrate species go extinct

Hypotheses:

Meteor impact, volcanism, changes in ocean chemistry, oxygen depletion, glaciation

Summary:

During the Devonian period, survivors from the late Ordovician extinction steadily recover. As the period nears its end, however, two new extinction pulses occur, mainly affecting marine populations. The first of these, at 364 mya, is more severe. Rugose corals and stromatoporoids, the primary reef-builders of the period, are nearly wiped out. Among the other marine invertebrate victims are brachiopods and trilobites. As for the marine vertebrates, the enigmatic conodont animals, known only for their widely scattered toothlike fossils, suffer; the jawless fishes are almost entirely eliminated; and the jawed and heavily armored placoderms go completely extinct. Interestingly, terrestrial plants and animals escape largely untouched. While some populations may have been under stress from changes in ocean chemistry and rapid cooling, a meteor strike and volcanism have been suggested as possible triggers.



border

Read more

Selective effect (364 mya)

The late Devonian extinction affects marine life far more than life on land. Looking closely at the death toll, nearly all the jawless fish, as well as every last placoderm, dies. Unlike these bottom-feeders, many open-water swimmers, like bony fish and sharks, survive the extinction. Knowing whether mass extinctions have random consequences, or whether risk varies from species to species or group to group, would greatly enhance our understanding of both extinction and evolution.

Selectivity is the idea that certain factors, such as body size, geographic range, or feeding behavior, determine whether species, groups, or entire families are more or less likely to die in a mass extinction. Data from some extinction events seem to reinforce this idea. For example, many shallow-water and reef-dwelling species probably died off in the Devonian because they (or their habitats) were more sensitive to changes in ocean chemistry or temperature than surviving animals that lived in deeper waters.

border

360 mya: Seed plants

The earliest seed-bearing plants, or gymnosperms, are the seed ferns. Unlike spore-producing plants, which need standing water nearby in which swimming sperm can fertilize eggs, seed plants evolve pollen, which can be dispersed by wind or animals from the male to the female reproductive organ. Hence, seed plants can reproduce away from water and expand their range into drier habitats.

-> Go to the Carboniferous Period

Intro | Precambrian Eon | Paleozoic Era | Mesozoic Era | Cenozoic Era

  related web activities  
   
Origins of Humankind
See the humanlike species that came before us.
 
 
A Modern Mass Extinction?
Are we in the midst of one? And if so, did we trigger it?
 
 
Life's Grand Design
Are nature's complex forms evidence of "intelligent design?"
 
   
  related topics  
   
  Deep Time/History of Life  
   
  Evolution of Diversity  
   
  Evidence for Evolution  
 
 
       
         
 
Videos Web Activities Site Guide About the Project FAQ Glossary Site Map Feedback Help Shop