Permineralization is a process of fossilization that occurs when an organism is buried. The empty spaces within an organism spaces filled with liquid or gas during life become filled with mineral-rich groundwater. Minerals precipitate from the groundwater, occupying the empty spaces. This process can occur in very small spaces, such as within the cell wall of a plant cell. Small-scale permineralization can produce very detailed fossils.
For permineralization to occur, the organism must be covered by sediment soon after death, or soon after the initial decay process. The degree to which the remains are decayed when covered determines the later details of the fossil. Fossils usually consist of the portion of the organisms that was partially mineralized during life, such as the bones and teeth of vertebrates or the chitinous or calcareous exoskeletons of invertebrates.
Interestingly, Heath et al. This is perhaps not surprising given that the same fossil data were used in both the computation of the confidence intervals and in the FBD analysis, which takes into account the average preservation rate based on the fossils incorporated into the analysis.
Nonetheless, it is heartening that there is broad agreement between a purely paleontological approach the paleontological parameterization of the lognormal distribution and an approach that incorporates fossil and DNA data, as well as an explicit branching model as part of its inference engine the FBD process. As a methodological aside, note that modifications of the FBD model can accommodate variation in diversification and fossil recovery rates [e.
Figure 5 Broad agreement in the estimated uncertainties in the divergence times of selected bear and outgroup lineages based on the FBD fossilized birth—death process gray bars Heath et al. Pink vertical lines correspond to the FAD s. The pink range extensions have been added to a reproduction of Figure 4 from Heath et al.
Moreover, 4 the rock and fossil records are spotty both temporally and geographically [e. Below is a simple example that illustrates the impact of the incompleteness of the rock record has in computing confidence intervals, and then I discuss ways in which the decrease in preservation potential can be accommodated.
As part of my Ph. Multiple confounding difficulties made this difficult, which are exemplified here by my analysis of the time of origin of the genus Mellita. However, while the rock record always appears complete in outcrop, it is typically riddled with temporal gaps Sadler, ; Holland, ; Patzkowsky and Holland, ; Holland, Further, even when rocks are present in a given time interval, they might not represent suitable environment for the taxa of interest.
For example, Mellita only lives on sandy substrates, and thus, its fossils are only found in sandstones—the St. Figure 6 Idealized representation of the rock formations on the Atlantic Coastal Plain region of the USA Lindberg, , with the fossil record and minimum number of localities n for each formation for the fossil sand dollar Mellita silhouette.
But there are no fossil Mellita or Leodia known south of Caribbean, and so, it is quite possible that these genera had their origins in geographic region from which there has been very little paleontological effort exerted, the Atlantic coast of South America; the fossil record might be giving us a record of when Mellita and Leodia migrated into Caribbean and then into the Pacific , not when they originated.
Using the fossil record of these now-extinct basal members of the clade as taphonomic controls see Section 3. In Figure 1 , I have emphasized the fact that the probability of fossil recovery generally drops the further back we go in the history of a clade. Marshall developed a method for accommodating decreasing probabilities of fossil recovery with time or in fact any non-random distribution of fossil recovery potential. However, we do not yet have standard methods for developing the required empirical non-random fossilization potential curves Marshall, They parameterized and tested the efficacy of their model with the Lissamphibian fossil record 1, localities across the global history of the group to establish confidence intervals on its time of the origin using the generalized confidence interval approach of Marshall There are also simpler analytic methods for accommodating trends of decreasing fossil recovery within the known stratigraphic range to approximate the assumed further decrease in fossil recovery beyond the known stratigraphic range.
The first methods used the Weibull distribution, which assumes a decreasing rate of preservation Roberts and Solow, ; Solow and Roberts, However, these methods tend to overestimate the true temporal endpoint Rivadeneira et al. The most recent and best performing method is the flexible beta method of Wang et al. Thus—for example, from the divergence of our own species from chimpanzees to about 4 million years ago, our own evolutionary branch may well have consisted of just one lineage.
This yields a stratigraphic range R of 2. This is in good agreement with the taphonomic control group approach see Section 3. The difficulty in quantifying the fossil recovery potential of a taxon beyond its known stratigraphic range has led many to rely on a more qualitative approach, the age of taphonomic control groups found beyond the FAD of the focal taxon as a maximum estimate for the time of origin.
Taphonomy is the study of how organisms decay and become fossilized Behrensmeyer et al. To control for geographic incompleteness see section 3. The clypeasteroid echinoids, the sand dollars, and sea biscuits e. Ali documents species in the fossil record known from localities so on average, each species is known from about two localities. With this quality of fossil record, we have reasonable confidence that the genus had its origin in the equatorial Tethys Sea Table 2 , now seen in the rock record around the Mediterranean and in the Middle East.
The oldest fossils are in Middle Eocene. Other irregular echinoids are found in the region in the Lower Eocene and in the older Paleocene [see Souto et al. Thus, a reasonable maximum estimate for the time of origin of Clypeaster was by beginning of the Eocene, and we can be even more certain that it had its time of origin somewhere in the interval bracketed by its Middle Eocene FAD and the beginning of the Paleocene.
Confidence intervals on stratigraphic ranges have long been used to assess likely times of extinctions, especially mass extinctions Marshall, a ; Marshall and Ward, ; Jin et al. The most powerful approach for mass extinction victims is to combine all the data, effectively collapsing all the species into a single super-taxon Wang et al. The same logic has been applied in reverse, using all the FAD s to form a super-taxon, to estimate the time of origin of a clade, as well as the internal branches, where the relative positions of the FAD s are adjusted by the relative length of their branches on an un-calibrated ultrametric tree timetree Marshall, Figure 7.
This approach has the advantage of not requiring estimates of maximum divergence times but has the disadvantage of the potentially unrealistic assumptions about the fossilization process although see Marshall for discussion.
It differs from most approaches for constructing time trees in that it is sequential in nature—an ultrametric tree is constructed first in the absence of any absolute time constraints, and then the scaling of that tree is established using the super-taxon paleontological approach.
Figure 7 Schematic for the super-taxon approach for using multiple FAD s to constrain the time of origin of a clade. A Hypothetical ultrametric tree dashed lines with the FAD s for each lineage. See Marshall for further explanation. Adapted from Figure 1 in Marshall Several analyses of turtle divergence times Joyce et al. All three studies used the same fossil calibrations, updated from Near et al. Marshall also used the Near et al. When the super-taxon approach is adjusted by eliminating the three FAD s identified as being questionable by Joyce et al.
Table 3 Taphonomic control group and super-taxon confidence interval approaches to estimating maximum and minimum age constraints on turtle divergence times are broadly congruent when they all use the same fossil FAD s. As one examines successively older rocks focal taxa disappear with only successively more plesiomorphic sister groups being found from the point of view of the focal group.
Thus, using a taphonomic control group type reasoning, the presence of these plesiomorphic taxa without taxa from the focal group gives the sense that the focal group had not yet evolved, providing a maximum age estimate for the focal taxon. Following this logic—for example, Gustafsson et al. The method is developed in a Bayesian framework and is implemented in R Lloyd et al.
It can been adjusted for groups that violate this requirement by leaving out inconsistent groups Friedman and Brazeau, ; Friedman et al. Typically, only a small proportion of all species that have ever existed are found in the fossil record. Martin noted that when the proportion of species preserved is small, and especially for clades that have been steadily expanding, the oldest fossil species found in the group might be several species durations younger than the very first species.
This paleontological method yielded a mean estimate of More recently, Wilkinson et al. Foote et al. The reason for the discrepancy has not been determined, but while it is not unreasonable that a relatively poorly preserved group of mammals, crown group primates, for example, might have a very deep time of origin; it is harder to believe that all the other better preserved mammalian orders, which all diverged from each other at about the same time as primates, also had a similarly deep time of origin.
Note, however, that Wilkinson et al. Primates continue to be a test case for combined DNA-paleontological timetree construction. For example, Reis et al. They also find that primary sources of uncertainty in the analysis are associated with fossil calibration uncertainty.
Newer paleontological approaches have been developed to estimate the amount of time missing history prior to FAD s that explicitly use the fossil record to calculate average speciation, extinction, and preservation rates from the fossil record Bapst, ; Nowak et al. Most recently, Wagner has developed a method for estimating branch durations and stratigraphic gaps in phylogenies when rates of speciation, extinction, and fossil sampling vary with time. The first two methods confidence intervals and taphonomic control groups make use of multiple calibrations across the tree Table 1.
This has the advantage that the process of calibration is not so dependent on difficulties that might be associated with any one specific lineage e. The use of multiple calibrations also allows for the possibility of cross validation, that is, the search for consistency between the temporal calibrations of one calibration to the next.
The initial idea of cross validation was to see if various subsets of calibration points FAD s yielded similar absolute ages for the timetree, with the goal of eliminating calibrations that yielded anomalously young or old divergence time estimates Near and Sanderson, ; Near et al.
However, given that FAD s all underestimate the divergence times they are being used to estimate, simple cross-validation on FAD s will provide divergence time estimates that are too young, eliminating the best calibrations Marshall, , as well as the worst.
To overcome this shortfall, rather than cross-validating on the FAD s, cross-validation may be performed using the temporal ranges between the minima FAD s and soft maxima Clarke et al. However, cross-validation is not generally recommended because the sequential use of single calibrations does not have the same effect as the simultaneous use of all the calibrations Warnock et al. So far, we have ignored the second step in the estimation of maximum age constraints on divergence times, the fact that the fossil record calibration methods discussed above give estimates of the true time of origin of the first diagnosable fossilizable morphological feature of a lineage, not the divergence time from its sister group Figure 1A.
The reason is that the fossil record can only be used to constrain the time of origin of taxa where those taxa can be morphologically recognized as belonging to that lineage.
Below, I examine situations that span this range see Table 4 for a synopsis. I begin with the expectation with a complete fossil record. Generally speaking, the appearance of a new fossilizable autapomorphy results in the recognition of a new paleontological species.
Probably the richest fossil record is the marine skeletonized single-celled eukaryotic microplantkon. In some geographic regions, marine macroinvertebrates are also well represented in the fossil record.
Didier et al. As discussed in section 3. Even if primates diverged from their nearest relatives at the upper limit of Wilkinson et al. The pervasiveness of extinction has left large lacunae in the record of cladogenic events that can be accessed via the living biota. Those lacunae, unbroken branches on molecular phylogenies, can be very long and typically represent stem groups diagrammed in Figure 1A.
For example, the last common ancestor of all living birds, the base of the crown group, dates to the late Cretaceous, perhaps 66—87 million years ago Benton et al.
Similarly, for angiosperms, where the fossil record is more difficult to work with Coiro et al. While fossil mammals were abundant before the end-Cretaceous mass extinction Luo, , it appears that there was an increase in the importance of mammals in terrestrial ecosystems after the mass extinction, accompanied by the relatively rapid evolution of new morphologies Alroy, For example, it appears that the last common ancestor of the well-skeletonized animal phyla was un-skeletonized—the first representatives of the animal phyla were probably not readily diagnosable in the fossil record e.
Thus, it is difficult to use the fossil record to assay how much before skeletonization the actual divergences between the phyla really were. Another group whose preservation potential appears to have changed dramatically during its history are the Scleractinian corals. Based on molecular clock data, it appears that their crown group extends in the Carboniferous, perhaps some million years ago, well before the oldest fossils in the Middle Triassic, some million years ago—the inference is that there was a substantial history where they were unskeletonized and therefore invisible in the fossil record Romano and Palumbi, , with more than one independent skeletonization event much later in the Triassic Stanley, The entire discussion above on using the fossil record to constrain the absolute divergence times between lineages is predicated upon the assumption that the focal clade and, for some of the methods, the outgroups are known from at least several well diagnosed and dated fossils.
However, for many groups, there is virtually no fossil record, or no fossil record at all. In these cases, well constrained calibrated timetrees are obviously difficult to obtain.
Nonetheless, I want to make the case that hypothesis testing is still possible, especially if the minimum age estimate for a divergence time leads to a timetree that yields older dates than that proposed by the hypothesis—sometimes testing hypotheses is much less demanding of the data than trying to reconstruct the actual history of a group. Paleobiologists typically work with groups with tens to tens of thousands of fossil species e. However, some groups are known from just a few species.
With such an awful fossil record, it is difficult to estimate reasonable maxima for the divergences within the orchids, or for the group as a whole but see Section 3. This result was obtained by simply using the fossil to date one node in an ultrametric tree, a result further supported in a Bayesian analysis using all three fossils Gustafsson et al.
Particularly at lower taxonomic ranks, many groups have no fossil record, neither do their immediate outgroups. Nonetheless, despite the lack of direct temporal data, average rates of molecular evolution estimated for closely related groups can sometimes provide valuable temporal data. Almost all clades, at least within animals and plants, lie within more inclusive clades where minimum and maximum age constraints are available e.
Thus, at some level, temporal constraints can always be found for most groups, even if the dating precision might be low within the unfossiliferous ingroup. The quality of temporal calibration is highly variable, depending on the group and the fossil record available.
If care is taken with the paleontological calibrations themselves, and with judicious analysis of data with multiple approaches, robust timetrees are well within our grasp for many taxa. Table 5 Relative magnitude of major factors that challenge our ability to estimate robust soft maxima on divergence times. The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Ali, M. The paleogeographic distribution of Clypeaster Echinoidea during the Cenozoic Era. Neues Jahrb. Google Scholar. Alroy, J. The fossil record of North American mammals: evidence for a Paleocene evolutionary radiation. Antoine, P. A rhinocerotid skull cooked-to-death in a 9. PLoS One 7, 1— Bapst, D.
A stochastic rate-calibrated method for time-scaling phylogenies of fossil taxa. Methods Ecol. Barba-montoya, J. Constraining uncertainty in the timescale of angiosperm evolution and the veracity of a cretaceous terrestrial revolution.
The British anatomist Richard Owen was the first scientist to recognize the fundamental difference between analogies and homologies. Bat and pterosaur wings are an example of analogous structures, while the bat wing is homologous to human and other mammal forearms, sharing an ancestral state despite serving different functions.
The opposite of convergent evolution is divergent evolution, whereby related species evolve different traits. On a molecular level, this can happen due to random mutation unrelated to adaptive changes.
Some organisms possess structures with no apparent function which appear to be residual parts from a past ancestor. For example, some snakes have pelvic bones despite having no legs because they descended from reptiles that did have legs.
Another example of a structure with no function is the human vermiform appendix. These unused structures without function are called vestigial structures. Other examples of vestigial structures are wings which may have other functions on flightless birds like the ostrich, leaves on some cacti, traces of pelvic bones in whales, and the sightless eyes of cave animals. Vestigial appendix : In humans the vermiform appendix is a vestigial structure; it has lost much of its ancestral function.
There are also several reflexes and behaviors that are considered to be vestigial. The arrector pili muscle, which is a band of smooth muscle that connects the hair follicle to connective tissue, contracts and creates the goose bumps on skin.
Vestigial structures are often homologous to structures that function normally in other species. Therefore, vestigial structures can be considered evidence for evolution, the process by which beneficial heritable traits arise in populations over an extended period of time.
The existence of vestigial traits can be attributed to changes in the environment and behavior patterns of the organism in question. In some cases the structure becomes detrimental to the organism. Whale Skeleton : The pelvic bones in whales are also a good example of vestigial evolution whales evolved from four-legged land mammals and secondarily lost their hind legs. Letter c in the picture indicates the undeveloped hind legs of a baleen whale. If there are no selection pressures actively lowering the fitness of the individual, the trait will persist in future generations unless the trait is eliminated through genetic drift or other random events.
Although in many cases the vestigial structure is of no direct harm, all structures require extra energy in terms of development, maintenance, and weight and are also a risk in terms of disease e. The vestigial versions of a structure can be compared to the original version of the structure in other species in order to determine the homology of the structure. Homologous structures indicate common ancestry with those organisms that have a functional version of the structure.
Vestigial traits can still be considered adaptations because an adaptation is often defined as a trait that has been favored by natural selection. Adaptations, therefore, need not be adaptive, as long as they were at some point. The biological distribution of species is based on the movement of tectonic plates over a period of time. Biogeography is the study of the geographic distribution of living things and the abiotic factors that affect their distribution.
Abiotic factors, such as temperature and rainfall, vary based on latitude and elevation, primarily. As these abiotic factors change, the composition of plant and animal communities also changes. Ecologists who study biogeography examine patterns of species distribution. No species exists everywhere; for example, the Venus flytrap is endemic to a small area in North and South Carolina. An endemic species is one which is naturally found only in a specific geographic area that is usually restricted in size.
Other species are generalists: species which live in a wide variety of geographic areas; the raccoon, for example, is native to most of North and Central America.
Since species distribution patterns are based on biotic and abiotic factors and their influences during the very long periods of time required for species evolution, early studies of biogeography were closely linked to the emergence of evolutionary thinking in the eighteenth century.
Some of the most distinctive assemblages of plants and animals occur in regions that have been physically separated for millions of years by geographic barriers. Biologists estimate that Australia, for example, has between , and , species of plants and animals. Australia : Australia is home to many endemic species. The a wallaby Wallabia bicolor , a medium-sized member of the kangaroo family, is a pouched mammal, or marsupial.
The b echidna Tachyglossus aculeatus is an egg-laying mammal. The geographic distribution of organisms on the planet follows patterns that are best explained by evolution in conjunction with the movement of tectonic plates over geological time. Broad groups that evolved before the breakup of the supercontinent Pangaea about million years ago are distributed worldwide.
Groups that evolved since the breakup appear uniquely in regions of the planet, such as the unique flora and fauna of northern continents that formed from the supercontinent Laurasia and of the southern continents that formed from the supercontinent Gondwana.
Biogeography : The Proteacea family of plants evolved before the supercontinent Gondwana broke up. Today, members of this plant family are found throughout the southern hemisphere shown in red. Privacy Policy. Skip to main content.
Evolution and the Origin of Species. Search for:. Evidence of Evolution. The Fossil Record as Evidence for Evolution Fossils tell us when organisms lived, as well as provide evidence for the progression and evolution of life on earth over millions of years.
Learning Objectives Synthesize the contributions of the fossil record to our understanding of evolution. Key Takeaways Key Points Fossils are the preserved remains or traces of animals, plants, and other organisms from the past.
Fossils are important evidence for evolution because they show that life on earth was once different from life found on earth today. Usually only a portion of an organism is preserved as a fossil, such as body fossils bones and exoskeletons , trace fossils feces and footprints , and chemofossils biochemical signals.
Paleontologists can determine the age of fossils using methods like radiometric dating and categorize them to determine the evolutionary relationships between organisms. Key Terms biomarker : A substance used as an indicator of a biological state, most commonly disease.
Fossil Formation Fossils can form under ideal conditions by preservation, permineralization, molding casting , replacement, or compression. Learning Objectives Predict the conditions suitable to fossil formation. Key Takeaways Key Points Preservation of remains in amber or other substances is the rarest from of fossilization; this mechanism allows scientists to study the skin, hair, and organs of ancient creatures.
Permineralization, where minerals like silica fill the empty spaces of shells, is the most common form of fossilization. Molds form when shells or bones dissolve, leaving behind an empty depression; a cast is then formed when the depression is filled by sediment. Replacement occurs when the original shell or bone dissolves away and is replaced by a different mineral; when this occurs with permineralization, it is called petrification.
In compression, the most common form of fossilization of leaves and ferns, a dark imprint of the fossil remains. Decay, chemical weathering, erosion, and predators are factors that deter fossilization.
Fossilization of soft body parts is rare, and hard parts are better preserved when buried. Key Terms amber : a hard, generally yellow to brown translucent fossil resin permineralization : form of fossilization in which minerals are deposited in the pores of bone and similar hard animal parts petrification : process by which organic material is converted into stone through the replacement of the original material and the filling of the original pore spaces with minerals.
Gaps in the Fossil Record Because not all animals have bodies which fossilize easily, the fossil record is considered incomplete. Learning Objectives Explain the gap in the fossil record. Because hard body parts are more easily preserved than soft body parts, there are more fossils of animals with hard body parts, such as vertebrates, echinoderms, brachiopods, and some groups of arthropods.
Key Terms transitional fossil : Fossilized remains of a life form that exhibits traits common to both an ancestral group and its derived descendant group.
Carbon Dating and Estimating Fossil Age The age of fossils can be determined using stratigraphy, biostratigraphy, and radiocarbon dating. Learning Objectives Summarize the available methods for dating fossils. Key Takeaways Key Points Determining the ages of fossils is an important step in mapping out how life evolved across geologic time. The study of stratigraphy enables scientists to determine the age of a fossil if they know the age of layers of rock that surround it.
For example, it is clear that the evolution of new functions for proteins commonly occurs after gene duplication events. These duplications are a kind of mutation in which an entire gene is added as an extra copy or many copies in the genome. These duplications allow the free modification of one copy by mutation, selection, and drift, while the second copy continues to produce a functional protein.
This allows the original function for the protein to be kept, while evolutionary forces tweak the copy until it functions in a new way. The evidence for evolution is found at all levels of organization in living things and in the extinct species we know about through fossils. Fossils provide evidence for the evolutionary change through now extinct forms that led to modern species. For example, there is a rich fossil record that shows the evolutionary transitions from horse ancestors to modern horses that document intermediate forms and a gradual adaptation o changing ecosystems.
The anatomy of species and the embryological development of that anatomy reveal common structures in divergent lineages that have been modified over time by evolution. The geographical distribution of living species reflects the origins of species in particular geographic locations and the history of continental movements.
The structures of molecules, like anatomical structures, reflect the relationships of living species and match patterns of similarity expected from descent with modification.
The fact that DNA sequences are more similar in more closely related organisms is evidence of what? A vestigial structure is an example of a homologous structure that has apparently been reduced through evolution to a non-functional state because its function is no longer utilized by the species exhibiting it; therefore, any mutations which might reduce its structure are not selected against. The fact that the species has vestiges of the structure rather than no structure at all is evidence that it was present in an ancestor and evolved to non-functionality through accumulation of random mutations.
Skip to content Chapter Evolution and Its Processes. Learning Objectives By the end of this section, you will be able to: Explain sources of evidence for evolution Define homologous and vestigial structures.
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