The objection to the importance and/or existence of transitional fossils is based on a fundamental misunderstanding of the definition, implications, and usage of the term in the scientific literature. This post is therefore an attempt to accurately describe the significance of transitional fossils and their correct application as is evident in science articles. Popular science and media publications are in a sense obligated to use hyperbole when describing new fossil finds, and I personally consider them as on-the-go summaries of larger, more important bodies of work. While not always the case, such news reports often construe and overemphasize the results published by the paper in question. Granted, they are writing to the general public and must use language palatable to the lay person, although the use of certain terms in a news article may not accurately reflect their use within a scientific setting. That being the case, news publications are not and should never be viewed as a viable source or taken as such over the primary literature, even when said literature is not available to the public. When possible (unfortunately often not the case), always read the paper itself.
An example of these misconceptions at work is evident in any discussion with creationists regarding the elpistostegid fish Tiktaalik roseae and the discovery of an older set of Polish footprints, rendering Tiktaalik wholly “un-transitional”. To define the term, a transitional fossil is one that demonstrates characteristics in common both with its ancestral and descendent groups. As such, it may serve as a suitable representative of how morphological changes proceeded over time, and may have been closely related in some way to a true direct ancestor. The definition does not imply or assume that any such fossil represents direct relation or ancestry to any other, although it is implied that this is how it is viewed by some creationists who state that no evidence exists showing one organisms evolving into another. This misconception is intrinsically linked to the notion that evolution posits progression in a linear fashion. This view may or may not result from misinterpretation of diagrams that evidently depict primitive forms proceeding linearly into advanced forms in a series of strictly anagenetic events, but it must be noted that such graphical representations are often directed at non-scientists and simplified accordingly under the assumption that the viewer can place the image in the context of the correct version of evolution, which postulates the arrangement of organisms in a branching, tree-like structure. Transitional fossils do not imply a linear sequence of evolution and are instead imposed upon and considered within the context of branching lineages typical of phylogenetics (see figure below). The notion of one organism progressing into another also fails in that it makes an incorrect and rather tenuous assumption that any given form must die out and be succeeded by different, more advanced organisms. Recalling that transitional fossils are to be viewed in the context of lineages branching into consequent subsets, members of the branching lineages in question can coexist both in space and time, as each progresses following separate and independent trajectories. Any accumulation of changes would be similarly isolated. As a result, derived or advanced forms do not necessarily replace those that are basal or primitive, which can themselves persist unconstrained through time. This fact is directly observable in modern taxa and is both applicable to and demonstrable in fossil assemblages. Lastly and also crucial to understanding transitional fossils, a particular specimen such as Tiktaalik, is not considered to have been in the process of evolving itself, considering that evolution occurs at the population level, and not directly on individuals. Instead, the lineage containing Tiktaalik may show evolution, as it represents a collection of breeding populations progressing over a given length of time.
This and other arguments along a similar line tend to be incorrectly labeled by creationists as ad hoc rationalizations that feebly attempt to explain away major discrepancies in the fossil record. The description provided above is not however an after-the-fact justification, as it is the correct interpretation of transitional fossils that has always been applied in the scientific literature. The creationist argument that the finding of the older Zachelmie tracks necessarily implies Tiktaalik is out of stratigraphical order would only be true if it was considered to be ancestral to later forms. The distinct possibility that Tiktaalik represents a member of a perserverant primitive lineage alone invalidates this view, and even more so given that transitional fossils are not presupposed to fall perfectly into a neat, simple progression of morphological forms over time, or fit into an ordered, designated time-slot in geological history. As such, finding fossils seemingly out of temporal order in no way disagrees with the definition of a transitional fossil, or the theory of evolution as a whole. In agreement with the notion of branching lineages, Tiktaalik was first described as the sister taxon to Tetrapoda (represented in the article by Acanthostega and Ichthyostega) and therefore not directly ancestral to later, more derived forms (see figure below) (Daeschler et al., 2006). Tiktaalik remained as a sister group to its derived relatives in spite of the age of the Zachelmie tracks, and as such the particular pattern of branching and progression of morphological traits within this specific group was likewise unaffected (Niedzwiedzki et al., 2010). This serves to demonstrate that the discovery of new specimens cannot and do not diminish or alter the intermediate status of other fossils, in spite of what the popular media may proclaim. The changes that do occur result from the necessary revision of previous hypotheses in accordance with the discovery of new lines of evidence.
A phylogenetic tree showing Tiktaalik as the sister group to Tetrapoda and its relative position based on a set of morphological characteristics. Note that Glyptolepis is representing the sister group to Tetrapodamorpha. The addition of Zachelmie prints did not affect the topology (branching pattern) of this tree.
Modified from “A Devonian tetrapod-like fish and the evolution of the tetrapod body plan”, by E.B. Daeschler, N.H. Shubin, F.A. Jenkins, 2006, Nature 440: 757-763.
The trackway does bring into question the relative timing of tetrapod divergence, which was previously thought to occur at some point in the Late Devonian. Incorporating this new data with known body fossils pushed this divergence prior to the Eifelian age and into the Early Devonian. This resulted in the designation of Tiktaalik and other close relatives as ghost lineages, as phylogenetic analysis implied their existence in spite of no fossil specimens. In effect, the relative lengths of the branches signifying time scale changed in accordance, recalling however that the branch arrangement remains the same. This may be viewed as an excuse to explain away an obvious discrepancy, the above explanations as to why this is untrue notwithstanding. However, other studies have corroborated an older divergence date for Tetrapoda and therefore support the ghost lineage label. An earlier publication described a fragmentary Middle Devonian stem-tetrapod, Livoniana, predating Tiktaalik and coeval with Panderichthys, but more derived than both based on discernible morphological characteristics (Ahlberg et al., 2003; Niedzwiedzki et al., 2010). In contrast to what most fossil evidence at the time suggested, the authors predicted that the divergence of Tetrapoda occurred prior to the Late Devonian. As such, the discovery of the Zachelmie tracks simply confirmed their earlier findings. In addition, one phylogenetics paper using Bayesian credible and maximum-likelihood confidence intervals found tetrapod divergence spanned the Early Devonian and potentially extended into the very Late Silurian, a possibility briefly acknowledged by Niedzwiedski et al., (2010), the authors reporting on the Polish footprints (Friedman and Brazeau, 2010). The recent discovery of the fossil fish Tungsenia (409 MYA), now the oldest and most basal tetrapodomorph known, lends further support (Lu et al., 2012). This finding likewise pushed the divergence of early stem tetrapods back roughly 10 million years into Pragian of the mid-Early Devonian, and resulted in an Eifelian time frame for divergence of Tetrapoda; a later date than previous predictions but nevertheless close to the estimates given by Niedzwiedzki et al., (2010) and Friedman and Brazeau (2010). As it is, ghost lineages do not always remain as such, and new fossil material may effectively fill in the gaps over time.
While the dissenting viewpoint is all too eager to tout the trackway as proof against the theory or evolution as a whole, this standpoint foregoes the fact that the authors describing the Zachelmie prints cautioned against drawing hasty conclusions without a definitive body fossil, and that the implications of this finding on the timing of the appearance of tetrapods is suggestive, but nevertheless unresolved. Consideration must also be given to the general paucity of fossil record and other problematic factors such as the geographical sampling bias resulting from collection focused within more developed nations. These points taken together, the creationist’s incriminating position on the implications of the Polish trackways is duplicitous at best. As more relevant fossil specimens are uncovered, including trackways, other factors must be considered. A recent study by King et al., (2011) studying fin movements in the African lungfish (Subclass: Dipnoi, a separate but closely related lineage to Tetrapodamorpha (see figure: Glyptolepis is a Porolepiform and closely related to the Dipnoi)), P. annectans, found that a tetrapod-like gait potentially originated in primitive and fully aquatic sarcopterygian fish prior to the split of Tetrapodomorpha, and as a result those fossil footprints and trackways lacking definitive digit impressions may in fact not belong to terrestrial tetrapods at all, but to primitive stem-tetrapods or related sarcopterygian fish. As per usual, more evidence is required, and new fossil discoveries will no doubt shed further light on the issue.
My hopes are that this post offers a brief but comprehensive review of the literature reporting on Tiktaalik and tetrapods as a whole, as well as describes clearly what a transitional is and isn’t, and what it means, and what it doesn’t.
Ahlberg, P.E., Lukševičs, E., Mark-Kurik, E. 2003. A near-tetrapod from the Baltic Middle Devonian. Palaeontology 43 (3): 533–548.
Daeschler, E.B., Shubin, N.H., Jenkins, F.A. 2006. A Devonian tetrapod-like fish and the evolution of the tetrapod body plan. Nature 440: 757-763.
Friedman, M., Brazeau, M.D. 2000. Sequences, Stratigraphy, and Scenarios: what can we say about the fossil record of early tetrapods? Proc Bio Sci B 278 (1704): 432-439.
King, H.M., Shubin N.H., Coates, M.I., Hale, M.E. 2011. Behavioral evidence for the evolution of walking and bounding before terrestriality in sarcopterygian fishes. Proc Natl Acad Sci USA 108(52): 21146–21151.
Lu, J., Zhu, M., Long, J. A., Zhao, W., Senden, T. J., Jia, L., Qiao, T. 2012. The earliest known stem-tetrapod from the Lower Devonian of China. Nature Communications
Niedzwiedzki, G., Szrek, P., Narkiewicz, K., Narkiewicz, M., Ahlberg, P.E. 2010. Tetrapod trackways from the early Middle Devonian period of Poland. Nature 463(7277): 43-48.