Teaching about Earth's history is a challenge for all teachers. Time factors of millions and billions of years is difficult even for adults to comprehend. However, "relative" dating or time can be an easy concept for students to learn. Once they are able to manipulate the cards into the correct sequence, they are asked to do a similar sequencing activity using fossil pictures printed on "rock layer" cards. Sequencing the rock layers will show students how paleontologists use fossils to give relative dates to rock strata. Once students begin to grasp "relative" dating, they can extend their knowledge of geologic time by exploring radiometric dating and developing a timeline of Earth's history.
This is called relative dating. Relative dating tells scientists if a rock layer is "older" or "younger" than another. This would also mean that fossils found in the deepest layer of rocks in an area would represent the oldest forms of life in that particular rock formation.
In reading earth history, these layers would be "read" from bottom to top or oldest to most recent. If certain fossils are typically found only in a particular rock unit and are found in many places worldwide, they may be useful as index or guide fossils in determining the age of undated strata.
By using this information from rock formations in various parts of the world and correlating the studies, scientists have been able to establish the geologic time scale. This relative time scale divides the vast amount of earth history into various sections based on geological events sea encroachments, mountain-building, and depositional events , and notable biological events appearance, relative abundance, or extinction of certain life forms.
When you complete this activity, you will be able to: The first card in the sequence has "Card 1, Set A" in the lower left-hand corner and represents the bottom of the sequence.
If the letters "T" and "C" represent fossils in the oldest rock layer, they are the oldest fossils, or the first fossils formed in the past for this sequence of rock layers. Now, look for a card that has either a "T" or "C" written on it. Since this card has a common letter with the first card, it must go on top of the "TC" card. The fossils represented by the letters on this card are "younger" than the "T" or "C" fossils on the "TC" card which represents fossils in the oldest rock layer.
Sequence the remaining cards by using the same process. When you finish, you should have a vertical stack of cards with the top card representing the youngest fossils of this rock sequence and the "TC" card at the bottom of the stack representing the oldest fossils. Starting with the top card, the letters should be in order from youngest to oldest. Return to top Procedure Set B: Each card represents a particular rock layer with a collection of fossils that are found in that particular rock stratum.
All of the fossils represented would be found in sedimentary rocks of marine origin. Figure 2-A gives some background information on the individual fossils.
The letters on the other cards have no significance to the sequencing procedure and should be ignored at this time. Find a rock layer that has at least one of the fossils you found in the oldest rock layer. This rock layer would be younger as indicated by the appearance of new fossils in the rock stratum.
Keep in mind that extinction is forever.Relative Dating of Rock Layers
Once an organism disappears from the sequence it cannot reappear later. Use this information to sequence the cards in a vertical stack of fossils in rock strata. Arrange them from oldest to youngest with the oldest layer on the bottom and the youngest on top. This will enable your teacher to quickly check whether you have the correct sequence. Three-lobed body; burrowing, crawling, and swimming forms; extinct NAME: Many were large a few rare species were 5 feet in length ; crawling and swimming forms; extinct NAME: Primitive form of chordate; floating form with branched stalks; extinct NAME: Jellyfish relative with stony Cnidaria calcareous exoskeleton found in reef environments; extinct NAME: Multibranched relative of starfish; lives attached to the ocean bottom; some living species "sea lilies" NAME: Primitive armored fish; extinct NAME: Shelled, amoeba-like organism NAME: Snails and relatives; many living species NAME: Clams and oysters; many living species NAME: The study and comparison of exposed rock layers or strata in various parts of the earth led scientists in the early 19th century to propose that the rock layers could be correlated from place to place.
The fossil terminal taxa have the uncertainty intervals of their FADs plotted with a dashed line, none of which overlap in time. Note that the different assignments change the raw values of these metrics but the tree on the left always has a better fit than the tree on the right.
The terminal taxa have the uncertainty intervals of their FADs plotted with a dashed line. The uncertainty intervals of the FADs of some terminal taxa overlap.
Evolution Jeopardy Geologic Time Fossil Record Relative Dating
Note that the different age assignments change both the raw values of these metrics and the relative fit of these two trees. These examples show that it is not possible to calculate precise measures of stratigraphic fit when the age of the FADs of terminal taxa are imprecise.
Because there is no rational justification for choosing any particular set of age assignments, it is necessary to consider a range of possible values for measures of stratigraphic fit, instead of having a precise but arbitrary and in some cases biased metric value for each phylogenetic tree.
Fortunately, the temporal uncertainties of FADs of fossil taxa can be translated into a range of possible values for these two measures of stratigraphic fit. The possible range of stratigraphic fit values i. The randomization approach proposed here consists of performing multiple replicates, where in each replicate a precise age of first appearance is assigned to each terminal taxon taken randomly from the uncertainty interval associated to the age of each taxon's FAD—which is the imprecision of age caused by error or the duration of the referred unit of geologic time.
In each replicate, the ages assigned to the terminal taxa are used to calculate the stratigraphic fit e. Since the randomized assignments of FAD ages are replicated a given number of times e. Outcome of the randomization procedure replicates applied to the data and trees shown in Figures 2 and 3. This procedure was implemented in the scripting language of TNT Goloboff et al.
This script reads the dataset containing the age character with their associated step matrix , the topologies to compare, and the minimum and maximum ages of each taxon's FAD i. This implementation includes two options for constraining the age assignments of FADs according to additional stratigraphic information on the relative order of FADs. The first option allows forcing the age assignment of some terminal taxa to be equal during each replicate rather than assign them independently at random.
This is necessary if, for example, the FADs of two or more terminal taxa are recorded at the same horizon, or if there is a high degree of certainty on the correlation of the sediments bearing these FADs. The second option allows constraining the age assignments of some terminal taxa to be necessarily older or younger than those of other taxa.
This is useful when, for example, there is no doubt on the relative age of the FADs of two fossil taxa e. This option is potentially useful because the relative ages of fossil taxa are usually more precisely known than their absolute ages. The randomization procedure was applied to measure the stratigraphic fit of competing hypotheses for two dinosaur groups. The first case contrasts the stratigraphic fit of two trees based on recently published hypotheses on the early radiation of Sauropodomorpha Yates, ; Galton and Upchurch, , one of the major groups of Dinosauria.
These two trees differ markedly in their topology and evolutionary implications Fig.
Relative dating fossil records
The competing tree depicts Prosauropoda as a paraphyletic arrangement of taxa relative to the clade Sauropoda Fig. Their relative ordering in stratigraphic fit, however, is strongly dependent on the age assigned to the FADs of terminal taxa and, therefore, in absence of further evidence, they must be viewed as having a similar degree of agreement with the temporal information of the fossil record Fig.
Outcome of the randomization procedure applied to two competing hypotheses of the phylogeny of basal sauropodomorph dinosaurs. The original trees of both studies were modified taxa not present in both analyses pruned from trees. The tree shown in B was randomly chosen from the set of most parsimonious trees other trees produced similar results. The second test case is an analysis of trees depicting the long-standing debate surrounding the origin of birds Fig.
The data analyzed here were taken from Brochu and Norell , who made a similar comparison but used the midpoint value of the uncertainty interval associated to the age of each taxon's FAD. These authors concluded that the hypothesis depicting the dinosaurian origin of Aves has a higher stratigraphic fit than any of the proposed alternative hypotheses.
Our application of the methods described here demonstrates that this conclusion is robust to the incorporation of age uncertainty in the evaluation of stratigraphic fit to phylogeny Fig. Outcome of the randomization procedure applied to the competing hypotheses on the evolutionary origins of birds. A Tree depicting the phylogenetic position of Avialae within Diapsida, the tree shows the bird lineage circled in gray deeply nested within Dinosauria.
Previous approaches to calculating the stratigraphic fit of phylogenetic trees based on ghost lineages were derived assigning an exact age for the FAD of each terminal taxon, irrespective of the actual precision of the chronostratigraphic information on this datum.
The stratigraphic fit of phylogenetic trees should be viewed as a comparison between the temporal content of two independently derived hypotheses—one of toplogy and one of age. It could be argued there is more certainty on the age estimate of a fossil taxon's FAD than on its phylogenetic placement, although this issue depends on the taxon, the method of age estimation, and several other factors.
This, however, is a distinction on the precision and degree of support of these hypotheses, not on their status as observations or hypotheses. We show above that taking rough approximations of chronostratigraphic information, treating this data as precise by rounding to the mean can produce misleading results regarding the relative stratigraphic fit of alternative phylogenetic trees.
These misleading conclusions will affect most applications of these indices, especially if they are considered as auxiliary optimality criteria. We have focused here on the effects of age uncertainty in these two measures because they are the most sensitive to differences in temporal data.
Other metrics that are solely based on the relative ordering of FADs, not measuring the extent of the mismatches implied by a tree i. In particular, measures such as SCI Huelsenbeck, , SRC Norell and Novacek, , or stratocladistic approaches Fisher, could only be affected if there is overlap between uncertainty intervals associated to the FADs of terminal taxa.
Further refinements of the method proposed here could incorporate more complex models in the age assignment process, instead of the randomized equiprobable age assignments based on the uncertainty intervals of the FADs. For instance, some age assignments could be drawn from a specified probability distribution of a particular direct dating estimate, or using probabilistic models that incorporate extensions to the observed temporal range of a fossil taxon based on confidence intervals stemming from the distribution of its occurrences in the stratigraphic record see above.
Although the methods of calculating phylogenetic trees are well understood, these and other modifications of the current implementation need to be further explored in order to integrate all the available chronostratigraphic information into better measures of stratigraphic fit to phylogenies.
We would like to thank P. Siddall for useful discussions on this subject. Christie-Blick provided useful comments on earlier versions of this manuscript. Page provided insightful suggestions that significantly improved the quality and clarity of this manuscript. Part of this contribution was based upon work supported by the National Science Foundation under Agreement No.
Oxford University Press is a department of the University of Oxford. It furthers the University's objective of excellence in research, scholarship, and education by publishing worldwide. Sign In or Create an Account. Close mobile search navigation Article navigation. Age Uncertainty of FADs. Effect of Age Uncertainty. Fontana , Trelew , Chubut, Argentina E-mail: Abstract The ages of first appearance of fossil taxa in the stratigraphic record are inherently associated to an interval of error or uncertainty, rather than being precise point estimates.
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