Finding: Using "fossil snapshots" found in rocks more than 500 million years old, three University of Kansas researchers have described the oldest definitive jellyfish ever found. This is significant because the fossil record is biased against soft-bodied life forms such as jellyfish, because they leave little behind when they die. That means that scientists are still working to solve the evolutionary development of many soft-bodied animals. But now they have a clear picture of what the soft-body form looked like when it comes to Jelly-Fish.
Here is what happened: researchers describe four types of cnidarian fossils preserving traits that allow them to be related to modern orders and families of jellyfish. The specimens are about 200 million years older than the oldest previously discovered jellyfish fossils.
Rapid Species diversification
Research jellyfish the group describes, found in Utah, offer insights into the puzzle of rapid species diversification and development that occurred during the Cambrian radiation, a time when most animal groups appear in the fossil record, beginning roughly 540 million years ago. The fossil record reveals much less about the origin and early evolution of animals such as jellyfish than it does about animals with hard shells or bones.
The Problem resolution:
The fossil record is full of circular shaped blobs, some of which are jellyfish. That's one of the reasons the fossils we describe are so interesting, because you can see a distinct bell-shape, tentacles, muscle scars and possibly even the gonads. The jellyfish left behind a film in fine sediment that resembles a picture of the animal. Most jellyfish do not leave such a clear impression behind because they are often preserved in coarse sand.
With the discovery of the four different types of jellyfish in the Cambrian, however, provide enough detail to assert that the types can be related to the modern orders and families of jellyfish. The specimens show the same complexity. That means that either the complexity of modern jellyfish developed rapidly roughly 500 million years ago, or that the group is even older and existed long before then.
The jellyfish described in the article are also unique because they push the known occurrence of definitive jellyfish back from 300 million to 505 million years, a huge jump, and show more detail than anything previously described that is younger.
Showing posts with label Cambrian Period. Show all posts
Showing posts with label Cambrian Period. Show all posts
Friday, November 2, 2007
Sunday, August 12, 2007
Trilobite variation declined after the Cambrian Explosion
From an evolutionary perspective, the more variable a species is, the more raw material natural selection has to operate on. So a highly variable species will evolve more rapidly than others.
Is that statement true?
Paleontologists for decades have suspected that highly variable species evolved more rapidly than others, and several studies have approached questions pertaining to it--but this is the first to convincingly document it in any group.
Most studies have focused on the variability that occurs between species rather than within them, but one recent study analyzed 982 species of trilobites, ancient relatives of spiders and horseshoe crabs.
When did Trilobites live?
Trilobites have been extinct for over 250 million years. They were once the most common creatures in the world's oceans. They ranged in size from nearly microscopic to more than a foot long, though most of the 17,000 known species measured from one to four inches. They were very diverse.
Trilobites were among the creatures that emerged 500 million years ago, during what paleontologists call "the Cambrian explosion," or "the Cambrian radiation." Before this time, life on Earth was limited mostly to bacteria, algae, single-celled organisms and only the simplest animal groups. But during the Cambrian Period, more complex creatures with skeletons, eyes and limbs emerged with amazing suddenness.
What does the research show?
So the question is what fueled the Cambrian radiation, and why was that event so singular? The answer: It appears that organisms displayed "rampant" within-species variation in the 'warm afterglow' of the Cambrian explosion, but not later.
A study focused on actively evolving characteristics during the Cambrian time. The trilobite head alone displayed many different characteristics. There were differences in ornamentation, number and placement of spines, and the shape of head segments. Overall, approximately 35 percent of the 982 trilobite species exhibited some variation in some aspect of their appearance that was evolving. But more than 70 percent of early and middle Cambrian species exhibited variation, while only 13 percent of later trilobite species did so.
Conclusion: There's hardly any variation in the post-Cambrian. Even the presence or absence or the kind of ornamentation on the head shield varies within these Cambrian trilobites and doesn't vary in the post-Cambrian trilobites.
Why does variation withing a species decline through time?
Paleontologists have proposed two ideas to account for why variation within species declined through time.
1)Ecological. In the very early Cambrian seas, fewer organisms existed than today, which meant that they faced less competition for food. You didn't really have to be tightly specialized to make a living in the Cambrian. But as evolution gave rise to more varieties of organisms, ecological communities became more diverse. You had to be very fine-tuned to your particular niche to make a living and to beat out competitors for a limited resource. More organizms in the ocean meant that there must be more genetic variation in order to survive.
2) The genomic hypothesis offers a second explanation for the decline of within-species variation over time. According to this idea, internal processes in the organism were the key factors. Various developmental processes interact with one another to control the growth and formation of body parts as any organism progresses from egg to adult.
It's been suggested that early on in evolutionary history, in the Cambrian Period, the degree to which these different developmental processes interacted with each other within the organism was a lot less. As a result, the constraints on what the final organism looked like were relatively low.
Is that statement true?
Paleontologists for decades have suspected that highly variable species evolved more rapidly than others, and several studies have approached questions pertaining to it--but this is the first to convincingly document it in any group.
Most studies have focused on the variability that occurs between species rather than within them, but one recent study analyzed 982 species of trilobites, ancient relatives of spiders and horseshoe crabs.
When did Trilobites live?
Trilobites have been extinct for over 250 million years. They were once the most common creatures in the world's oceans. They ranged in size from nearly microscopic to more than a foot long, though most of the 17,000 known species measured from one to four inches. They were very diverse.
Trilobites were among the creatures that emerged 500 million years ago, during what paleontologists call "the Cambrian explosion," or "the Cambrian radiation." Before this time, life on Earth was limited mostly to bacteria, algae, single-celled organisms and only the simplest animal groups. But during the Cambrian Period, more complex creatures with skeletons, eyes and limbs emerged with amazing suddenness.
What does the research show?
So the question is what fueled the Cambrian radiation, and why was that event so singular? The answer: It appears that organisms displayed "rampant" within-species variation in the 'warm afterglow' of the Cambrian explosion, but not later.
A study focused on actively evolving characteristics during the Cambrian time. The trilobite head alone displayed many different characteristics. There were differences in ornamentation, number and placement of spines, and the shape of head segments. Overall, approximately 35 percent of the 982 trilobite species exhibited some variation in some aspect of their appearance that was evolving. But more than 70 percent of early and middle Cambrian species exhibited variation, while only 13 percent of later trilobite species did so.
Conclusion: There's hardly any variation in the post-Cambrian. Even the presence or absence or the kind of ornamentation on the head shield varies within these Cambrian trilobites and doesn't vary in the post-Cambrian trilobites.
Why does variation withing a species decline through time?
Paleontologists have proposed two ideas to account for why variation within species declined through time.
1)Ecological. In the very early Cambrian seas, fewer organisms existed than today, which meant that they faced less competition for food. You didn't really have to be tightly specialized to make a living in the Cambrian. But as evolution gave rise to more varieties of organisms, ecological communities became more diverse. You had to be very fine-tuned to your particular niche to make a living and to beat out competitors for a limited resource. More organizms in the ocean meant that there must be more genetic variation in order to survive.
2) The genomic hypothesis offers a second explanation for the decline of within-species variation over time. According to this idea, internal processes in the organism were the key factors. Various developmental processes interact with one another to control the growth and formation of body parts as any organism progresses from egg to adult.
It's been suggested that early on in evolutionary history, in the Cambrian Period, the degree to which these different developmental processes interacted with each other within the organism was a lot less. As a result, the constraints on what the final organism looked like were relatively low.
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