Patterns in the fossil record, part 1

Fossils are remains of organisms or traces of their activity preserved in the rock record. The scientific significance of fossils is truly remarkable, because they represent the only available archive of past forms of life. Through fossils, not only can we reconstruct the morphology of extinct creatures but also infer aspects of their ecology and environment. Fossils are also very relevant in discussions about the origin of the varieties of life forms seen on Earth today.

This blog is divided in two parts. In the first, we will review some of the patterns that emerge from a general look at the fossil record. In the second, we will discuss the patterns in the light of naturalistic and biblical models of origins.

 

Fossils and the geologic column

In order to extract general patterns related to the distribution of fossils, it is first necessary to establish a spatial relationship between the rocks that contain them. Geologists have created a framework, called the geologic column, to serve the purpose of ordering rocks from various locations in an idealized succession of units and events. Imagine the geologic column as a layered cake, so that rocks at any given locality can be readily assigned to a specific layer of the column and their relationship with others rock units become self-evident. The geologic column is the outcome of scores of local observations, measurements and correlations between outcrops from all over the world. It is continually being refined, but its basic subdivisions are well established.[1]

In spite of skepticism from some creationists about the reliability of the geologic column as a valid construct, many endorse it as an effective tool to organize spatial information in the rock record.[2] In this paper, most of the patterns discussed assume the validity of the geologic column as a necessary foundation for their reconstruction.

In standard geologic practice, the geologic column is tied to a chronology of hundreds of millions of years. However, creationists generally do not agree with such long chronology for the formation of the many intervals of the column. Therefore, creationists who accept the geologic column keep the stratigraphic[3] data (i.e., order and correlation of rock units) separate from the absolute chronology (i.e., the numerical age) assigned to the rock units.

 

Review of some patterns in the fossil record 

Ordered distribution

One of the most prominent features of the fossil record is that every fossil species consistently occurs within a specific interval of the geologic column. Some taxa[4] may have a very restricted distribution, whereas others may be found in larger portions of the column. The interval of distribution for a taxon is sometimes called its stratigraphic range. For example, the stratigraphic range of human fossils corresponds to the Quaternary (the uppermost portion of the geologic column) whereas the stratigraphic range of dinosaurs is restricted to the Mesozoic (lower in the geologic column). Therefore, the distribution of fossils of these two groups does not overlap and they are never found together at the same site.

The stratigraphic range of fossil taxa is obtained through collection of samples from many localities. If numerous specimens are available (such as with microfossils, which can be collected by the thousands per sample) the stratigraphic range can be very precisely defined. On the contrary, when only a few specimens of a taxon are known, the upper and lower boundaries of its stratigraphic range can be expanded by new discoveries.

From exclusively marine to marine and terrestrial

Determining the mode of life of extinct organisms is not always straightforward, but it is usually possible to at least infer whether a fossil lived in a marine or terrestrial habitat. A remarkable feature of the fossil record is that all organisms found fossilized in the lower part of the geologic column (up to the Silurian) have generally been interpreted to be adapted for life in fully marine conditions. This implies a lack of terrestrial organisms among the millions of fossils of the lower Paleozoic. The Silurian-Devonian represents the first interval where fossils of terrestrial affinity begin to appear. What is particularly notable is that very different groups such as plants (Edwards & Burgess, 1990), invertebrates (Selden, 1990) and vertebrates (Milner, 1990) all present their first terrestrial representatives in this interval of the geologic column. Some of these forms, such as the lowermost occurring tetrapods[5] (Carroll, 2005), seem to have been adapted for life in coastal or riparian environments, at the interface between water and land. From the Devonian upwards, marine and terrestrial organisms are both widely represented in the fossil record.

Increasing modernity

Many of the fossils preserved in the rock record pertain to groups of organisms that are now extinct. Some fossil groups however, have representatives still living in modern times. One can estimate what percentage of the fossil taxa found in a specific interval of the geologic column is still living today. This exercise could be done at the species level or at a higher taxonomic category (such as genus or family). This analysis of similarity between present and fossil faunas and/or floras shows that as we move lower in the geologic column the similarity decreases (or, said in other words, fossils in the upper part of the geologic column are more similar to living organisms than those in the lower part of the column).

Disparity

Disparity is a measure of the morphological difference between two organisms. When examining the fossil record, we could look for the degree of disparity between forms found in a specific interval of the geologic column. It is particularly interesting to estimate disparity at the appearance of new groups, because we want to see if they are relatively homogeneous or already differentiated when they first occur as fossil. The general trend emerging from the fossil record is one of high disparity right from the first appearance of a new group. A classic example of high initial disparity is the Cambrian “explosion” of metazoans[6], where very disparate organisms belonging to essentially all the animal phyla appear for the first time in Cambrian strata (Marshall, 2006).

The fossil record of many groups begins with few but morphologically very distinct organisms and is followed, in overlying strata, by an increase in diversity but as variations of already established themes.

Coordinated disappearance (mass extinction)

When no living representatives of a fossil taxon are known, the group is considered extinct. The great majority of fossil species is extinct, but the position of their highest occurrence in the geologic column varies greatly. However, some species disappear at the very same level (coordinated disappearance) and when the lost groups are numerous and belong to different categories of organisms the term “mass extinction” is applied. At least five distinct intervals of mass extinction have been recognized in the Phanerozoic (Sepkoski, 1986). The largest is the P-T (Permo-Triassic) extinction, where an estimated 54% of all marine families and 83% of all marine genera present in underlying strata disappear (Erwin, 1990). The most renowned is probably the K-Pg (Cretaceous-Paleogene) extinction because of its association with the disappearance of dinosaurs and its possible link with a high-energy meteorite impact (Alvarez et al., 1980). Beside these five major extinctions, there are several other examples of coordinated disappearances involving smaller numbers of taxa or geographically restricted provinces (Sepkoski, 1986).

Coordinated appearance (radiation)

The pattern of coordinated appearance (often referred to as radiation) basically represents the opposite phenomenon of coordinated disappearance. It relates to the occurrence in the same restricted portion of the geologic column of numerous taxa which were not present in underlying layers. Classic examples of radiations are the Cambrian “explosion” of metazoans (Marshall, 2006), the Ordovician diversification of marine faunas (Miller, 2001), the Cretaceous radiation of angiosperms (Friis et al., 2006), and the Eocene-Oligocene radiation of modern mammal orders and families (Bowen et al., 2002).

Stasis and gradual change

In a very influential paper, Eldredge and Gould (1972) proposed that fossil species show little morphological variation during the entire range of their stratigraphic

distribution. This hypothesis, called stasis, contrasted with the previously popular idea of a fossil record showing gradual and directional morphological change between species arranged in stratigraphic order. Several attempts have been made at clarifying which of the two patterns (stasis or gradual change) is truly represented in the fossil record. Some studies strongly suggest the reality of stasis (e.g., Cheetham, 2001) but others illustrate gradual morphological change (e.g., Arnold, 1983). A third modality, named “random walks,” is also possible and consists of appreciable change through the layers of the column but not in a definite direction. Statistical studies on the relative importance of these patterns of morphological change in the fossil record indicate that examples of gradual change are much less common than stasis or random walks (Hunt, 2007; Grey et al., 2008).

Intermediate forms between major groups

Since the publication of Darwin’s Origin of Species, one of the most sought after patterns in the fossil record is the presence of forms with transitional characteristics, arranged in sequential stratigraphic order. Of particular interest are morphological intermediates (or “links”) between major categories of living organisms (such as fish and amphibians, dinosaurs and birds, terrestrial and marine mammals). These connecting forms would represent the nodes between branches of an alleged evolutionary tree of life. However, such transitional forms are very scarce in the fossil record. Discussing the origin of higher taxa, Kemp (1999, p.246) states that “in virtually all cases a new taxon appears for the first time in the fossil record with most definitive features already present.”

It should be noted, however, that some forms with transitional characters or a mosaic of characteristics from different groups are indeed preserved in the fossil record. Representatives include Devonian tetrapod-like fish (Daeschler et al., 2006; Long et al. 2006; Ahlberg et al., 2008), upper Paleozoic-lower Mesozoic mammal-like reptiles (Kemp, 1999; Luo, 2007), Lower Cretaceous theropod dinosaurs with bird-like characters (Qiang et al., 1998; Xu et al., 2003), and Eocene terrestrial and amphibious cetaceans (Thewissen et al., 2001, 2007).

Other patterns

There clearly are numerous other patterns which could be investigated from the fossil record which are not discussed in this blog. These include trends in diversity, complexity, body size, specialization, preservation, trace fossils, and nature of embedding deposits. For a creationist commentary on most of these patterns the reader is referred to Gibson (1996).


Ronny Nalin

Geoscience Research Institute


Footnotes:

[1] For a standard version of a chart showing the subdivisions of the geologic column (some of which are mentioned later in this article) see http://www.stratigraphy.org/index.php/ics-chart-timescale.

[2] For an example of differing creationist views on the geologic column, see Reed & Oard, 2006.

[3] Stratigraphy is a branch of geology which aims at subdividing the rock record into discrete units characterized by specific combinations of observable parameters (such as rock type, fossil content, magnetic properties, isotopic composition, etc.).

[4] The nomenclatural scheme of biology has different hierarchical levels to rank organisms from the general to the specific (e.g., phylum, family, genus, species). Taxon (pl.: taxa) is the general word used to refer to any of these categories (the word taxon, for example, could equally be applied to a species or a family).

[5] The term tetrapod is used to indicate four-limbed vertebrates, a group including amphibians, reptiles, mammals and birds.

[6] Metazoan is a technical term used to refer to multicellular animals.

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