The aim of the project presented in this website is to present the transitional fossils that record the evolutionary changes between any group of animals or plants and their ancestors. There are several ways of finding information on any group of interest:
- From the menu bar above (under the "Evolution of Life" tab or from the "Navigation" tab)
- From highlighted links in individual pages
- Using the search box at the top of every page.
Presuppositions
Before going further, I consider it necessary to declare my opinion with respect to the theory of evolution. But before that, I need to clarify what I mean by the “theory of evolution”. I will do that by following Ernst Mayr (1991), who stated (on pages 36 and 37 of his book) that the Darwin’s theory is really a paradigm that comprises the following five theories:
This last point provides the basis for the approach followed in this website. If many groups of organisms are descended from common ancestors, it seems reasonable to hold as a null hypothesis that published phylogenetic trees (equivalent to family trees), which display relationships between ancestor and descendant fossil organisms, are generally correct. If there is any clear inconsistency between any tree and the fossil evidence, we can question common ancestry in that particular case. However, this is not to say that phylogenetic trees are not a useful tool for understanding the history of development of fossil life.
Given that I believe that parts of the theory of evolution, as outlined above, are valid, I use the term “evolution” freely throughout this website.
- Evolution as such: This is the theory that the world is not constant nor recently created nor perpetually cycling but rather is steadily changing and that organisms are transformed in time.
- Common descent: This is the theory that every group of organisms descended from a common ancestor, and that all groups of organisms, including animals, plants, and microorganisms, ultimately go back to a single origin of life on earth.
- Multiplication of species: This theory explains the origin of the enormous organic diversity. It postulates that species multiply, either by splitting into daughter species or by ''budding,'' that is by the establishment of geographically isolated founder populations that evolve into new species.
- Gradualism: According to this theory, evolutionary change takes place through the gradual change of populations and not by the sudden (saltational) production of new individuals that represent a new type.
- Natural selection: According to this theory, evolutionary change comes about through the abundant production of genetic variation in every generation. The relatively few individuals who survive, owing to a particularly well-adapted combination of inheritable characters, give rise to the next generation.
This last point provides the basis for the approach followed in this website. If many groups of organisms are descended from common ancestors, it seems reasonable to hold as a null hypothesis that published phylogenetic trees (equivalent to family trees), which display relationships between ancestor and descendant fossil organisms, are generally correct. If there is any clear inconsistency between any tree and the fossil evidence, we can question common ancestry in that particular case. However, this is not to say that phylogenetic trees are not a useful tool for understanding the history of development of fossil life.
Given that I believe that parts of the theory of evolution, as outlined above, are valid, I use the term “evolution” freely throughout this website.
Background
The use of phylogenetic trees allows us to define exactly what we mean by the term “transitional fossil”. However, we first need to clear up a common misunderstanding. The common concept of evolution is that of a single chain representing a linear process; this has given rise to the idea of “missing links” or “transitional fossils”, the absence of which could call into question the validity of the theory of evolution. The chain concept has also engendered objections such as “If humans descended from monkeys, why are there still monkeys?”
These objections are based on a misunderstanding of the evolutionary process. Rather than a single “chain of life”, evolution has generated a tree with a huge number of branches. Thus the evolutionary concept is that of a “tree of life’ in which ancestors give rise to descendants and those descendants in turn generate further descendants, just as in a family tree. According to this view, transitional or intermediate forms only exist as extinct branches of the tree:
These objections are based on a misunderstanding of the evolutionary process. Rather than a single “chain of life”, evolution has generated a tree with a huge number of branches. Thus the evolutionary concept is that of a “tree of life’ in which ancestors give rise to descendants and those descendants in turn generate further descendants, just as in a family tree. According to this view, transitional or intermediate forms only exist as extinct branches of the tree:
Ancestors represent nodes of the tree and are shown as black dots. However, we cannot identify fossils that we know are ancestors in the fossil record; we can only find the fossils that are descended from ancestors. Fossils a and b are extinct descendants of ancestors A and B respectively, while living forms c1 and c2 are descendants of ancestor C. Ancestor B is transitional between ancestor A and ancestor C, but its closest representation in the fossil record is fossil b. Thus fossil b is only an approximate representation of the intermediate ancestor B. If we apply the above diagram to relationships within the archosaurs (crocodiles, dinosaurs and birds), living forms c1 and c2 might represent birds, fossil b could represent dinosaurs, and fossil a could represent extinct crocodiles. However, dinosaurs are only transitional between crocodiles and birds in the sense that they have an ancestor that is intermediate between the ancestors of the crocodiles and the birds. All three groups share a common ancestor, that of all archosaurs.
The tree configuration illustrated above is further developed in the concept of crown and stem groups, ideas critical to an understanding of how evolution is represented in the fossil record. Let us first consider the crown group. In the following figure, the letters A through E represent species that are related to one another; A and E are living at the present day, while B†, C† and D† are extinct. All these species share a last common ancestor (LCA), represented by the black dot:
The tree configuration illustrated above is further developed in the concept of crown and stem groups, ideas critical to an understanding of how evolution is represented in the fossil record. Let us first consider the crown group. In the following figure, the letters A through E represent species that are related to one another; A and E are living at the present day, while B†, C† and D† are extinct. All these species share a last common ancestor (LCA), represented by the black dot:
All of species descended from that LCA, whether extinct or not, comprise the crown group of the living species. Thus the crown group of any taxon, or group of related organisms, is defined as the group that contains the following elements:
With respect to the stem group, we can illustrate the relationships as shown below:
- All living members of the taxon
- The LCA of all those members
- All extinct descendants of the LCA
With respect to the stem group, we can illustrate the relationships as shown below:
The above figure depicts two crown groups A and B, with crown nodes 1A and 1B, respectively. The two crown groups are related and hence share an LCA, represented by node 1. This node is then the crown node of the crown group A + B. Crown groups are thus nested within one another in a hierarchical fashion.
Now the lines that connect crown nodes 1A and 1B to node 1 in the above diagram represent stem lineages. The extinct taxa that branch off those lineages represent the stem groups to crown groups A and B respectively. The stem group corresponding to a particular crown group thus comprises those fossils which are more closely related to that crown group than to any other crown group, but do not fall within the crown group to which the stem group corresponds. The crown and stem groups taken together are known as the total group.
In order to relate this discussion to the idea of transitional fossils, we can say that fossils a1† and a2† are transitional fossils between Node 1 and Node 1A, while fossils b1† and b2† are transitional fossils between Node 1 and Node 1B. In other words, the stem group comprises transitional fossils.
Now the lines that connect crown nodes 1A and 1B to node 1 in the above diagram represent stem lineages. The extinct taxa that branch off those lineages represent the stem groups to crown groups A and B respectively. The stem group corresponding to a particular crown group thus comprises those fossils which are more closely related to that crown group than to any other crown group, but do not fall within the crown group to which the stem group corresponds. The crown and stem groups taken together are known as the total group.
In order to relate this discussion to the idea of transitional fossils, we can say that fossils a1† and a2† are transitional fossils between Node 1 and Node 1A, while fossils b1† and b2† are transitional fossils between Node 1 and Node 1B. In other words, the stem group comprises transitional fossils.
Approach
The objectives of the website are as follows:
- To identify and illustrate as many transitional fossils (i.e. stem-group members) as possible for the evolution of the vertebrate animals and the land plants.
- To document the history of appearance of these transitional fossils through geological time, with particular focus on identifying episodes of particularly slow or fast evolutionary change.
- For clades for which no transitional fossils appear to exist, to determine the range of geological age in which transitional fossils might be discovered in the future.
Time scale
The ages of rocks, in millions of years, quoted in this website correspond to the geological time scale of version 2015/01 of the International Chronostratigraphic Chart produced by the International Commission on Stratigraphy. A summary of this chart is shown below:
Assumptions
- Evolutionary relationships for which there is no unresolved disagreement in the literature represent the best available scientific consensus. Note that the website contains no independent assessment of evolutionary relationships between or within taxa.
- With respect to the use of Linnaean hierarchical classification of organisms, the system used throughout this website is generally that of Ruggiero et al (2015). Their hierarchy is summarized as follows:
- In general, this website will consider evolutionary changes only at hierarchical levels higher than the order. Only in exceptional cases will evolution within an order be discussed.
Additional information
- Files containing detailed fossil data can be found here or in the site navigation menu under "Data" .
- Attributions, with source and license information, for all images used on each page of this website are given at the bottom of the page in question.
References
Hermsen, E. J., & Hendricks, J. R. (2008). W (h) ither fossils? Studying morphological character evolution in the age of molecular sequences. Annals of the Missouri Botanical Garden, 95(1), 72-100.
Hillis, D. M. (1987). Molecular versus morphological approaches to systematics. Annual review of Ecology and Systematics, 18(1), 23-42.
Mayr, E. (1991). One long argument: Charles Darwin and the genesis of modern evolutionary thought (Vol. 2). Harvard University Press.
Ruggiero, M. A., Gordon, D. P., Orrell, T. M., Bailly, N., Bourgoin, T., Brusca, R. C., ... & Kirk, P. M. (2015). A higher level classification of all living organisms. PloS one, 10(4), e0119248.
Hillis, D. M. (1987). Molecular versus morphological approaches to systematics. Annual review of Ecology and Systematics, 18(1), 23-42.
Mayr, E. (1991). One long argument: Charles Darwin and the genesis of modern evolutionary thought (Vol. 2). Harvard University Press.
Ruggiero, M. A., Gordon, D. P., Orrell, T. M., Bailly, N., Bourgoin, T., Brusca, R. C., ... & Kirk, P. M. (2015). A higher level classification of all living organisms. PloS one, 10(4), e0119248.
Photo credit
Header: By Wolfgang Sauber (Own work) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
Header: By Wolfgang Sauber (Own work) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons