Origin of the Tetrapods

The following article reproduces the article “Origin of the Tetrapods – not a smooth transition after all” by Dr. Reinhard Junker, which appeared in the “Studium Integrale Journal” (26th year ┃ Issue 2; October 2019). Quotations without further source references originate directly from the mentioned original article.

Introduction: So far, some genera from the Upper Devonian have been interpreted as evolutionary links between fish and tetrapods, as they combine both fish-typical and tetrapod-typical features. However, newer findings call this interpretation into question and significantly increase the distance between fish and tetrapods.

In 2006, the genus Tiktaalik was discovered in the Upper Devonian and interpreted as evidence of an evolutionary transition from fish to tetrapod (four-legged animals).

Image source

Public Domain

Tiktaalik possesses both fish-typical and tetrapod-typical features. Fish-typical are:

1. the large gill cavities,

2. the pronounced fin rays,

3. the scaled skin,

4. the lower jaw,

5. and the palate.

On the other hand, the following point to tetrapods:

1. the shortened skull roof,

2. the flat skull with upward-directed eyes,

3. the ear region,

4. the mobility of the neck region,

5. the absence of opercula (gill covers),

6. the relatively strong ribcage with overlapping ribs,

7. the flat body structure,

8. and the possession of a wrist.

Tiktaalik was grouped together with the fish-like genus Panderichthys and the genus Elpistostege into the group of Elpistostegids; meanwhile, it is used to group these three genera together with the tetrapods. Accordingly, the Elpistostegids of the Upper Devonian, which do not belong to the tetrapods, are interpreted as their precursor group.

Yet it was already pointed out in 2006 that some features do not really fit into a transitional position (Ahlberg & Clack 2006). See also the Studium Integrale article from 2006 "Tiktaalik – ein erstklassiges Bindeglied?" ("Tiktaalik – a first-class link?")

Ahlberg and Clack end their comment with the observation "that almost nothing is known about the step between Tiktaalik and the earliest tetrapods." In this transition, the body structure is said to have undergone the "most drastic changes" (Ahlberg & Clack 2006). Expressed clearly, this means as much as: "we do not know how such extreme changes could have occurred within the framework of a slow and small-step macroevolution in a relatively short time."

In 2010, the image of the perfect transitional form changed. A variety of tetrapod tracks were found in the lower Middle Devonian in Zachelmie/Poland. Likewise, tetrapod tracks were found on Valentia Island in Ireland in the upper Middle Devonian. Consequently, according to evolutionary theory, the tetrapods must have lived before their assumed ancestors.

Niedzwiedzki and colleagues conclude from this a feigning of the evolution from Elpistostegids to tetrapods through the fossil record (Niedzwiedzki et al. 2010). Instead of a short transitional stage, the body structure of the Elpistostegids is now a stable and adapted rank. According to Niedzwiedzki and colleagues, their features must therefore be "evaluated as an adaptation to certain ecological conditions" and are not suitable as evidence for an evolutionary transitional stage.

In a work from 2018, Qvarnström and colleagues present a new interpretation of the tracks found in Zachelmie (Qvarnström et al. 2018). According to this, the habitat of the tetrapods in question was a sequence of short-lived lakes near the coast. Not, as previously assumed, a coastal zone of the sea. This supported the interpretation that the creators of the tracks were capable of moving on land, which also agreed with the tail shape.

This interpretation is supported by Ahlberg in an overview article from 2018 (Ahlberg 2018). For him, the ancestral evolutionary hypothesis is a "misinterpretation." According to Ahlberg, the tracks from the Middle Devonian show that tetrapods were capable of shore excursions before the end of the Middle Devonian. He also notes that "the fossil record of the early tetrapods and related forms is very incomplete."

Tetrapods and Elpistostegids are now said to have lived at the same time and undergone simultaneous, independent radiations (speciation). Ahlberg (2018, p. 17) summarizes:

It is tempting to view the Elpistostegids simply as an intermediate step in a directed evolutionary development, but of course they were nothing of the sort; like all organisms, they were adaptively optimized for their own way of life and were not "on the way" to anything.

Ahlberg sees Tiktaalik and Elpistostege as the last survivors of the radiation of the Elpistostegids. Their habitats are said to have been taken over after their extinction by tetrapods, which had a way of life no longer found today by inhabiting the shore area.

This also fits with an extensive character comparison (baraminological analysis) by Garner & Asher (2018), which resulted in a clear distinguishability between tetrapods of the Devonian and Carboniferous and the Elpistostegids. "The derived features of the body structure of Elpistostegids and Devonian tetrapods would be in connection with movement on the ground, supporting weight, and seeing in the air, thus being an expression" of the ability to live on land.

The retained lateral line canals, gills, and fin rays are meant to show meanwhile that they were simultaneously "closely connected to the water." Ahlberg thus presents a new hypothesis:

Some genera from the Devonian and a series of tetrapods from the Cambrian are said to have evolved repeatedly from water-dwelling forms and not, as previously assumed, from land-dwelling forms. To this he also includes the genus Acanthostega, which was also often interpreted as a link. This genus did have eight fingers on its fins, but was completely water-dwelling.

If one agrees with Ahlberg and Niedzwiedzki with their interpretation, a whole series of alleged links is lost, whereby the gap between fish and land animals becomes even larger. He concludes from the fossil situation, based on evolutionary theory, that Elpistostegids and land-dwelling tetrapods must have lived at least at the lower boundary of the Middle Devonian, but are hardly preserved there as fossils.

Thus, another important hypothetical evolutionary transition is not documented by fossils. The fossil find is in agreement with the biblical separation between aquatic creatures and land animals (Genesis 1:20-25).

Literature:

• Ahlberg PE & Clack JA (2006) A firm step from water to land. Nature 440, 747-749. ↩︎

• Ahlberg PE (2018) Early Vertebrate Evolution. Follow the footprints and mind the gaps: a new look at the origin of tetrapods. Earth Environ. Sci. Trans. R. Soc. Edinb., doi: 10.1017/S1755691018000695 ↩︎

• Garner PA & Asher J (2018) Baraminological analysis of Devonian an Carboniferous tetrapodomorphs. In: Whitmore JH (ed) Proc. Int. Conf. Creat., 8, 458-471. Pittsburgh, Pennsylvania: Creation Sience Fellowship, doi: 10.15385/jpicc.2018.8.1.36 ↩︎

• Niedzwiedzki G, Szrek P, Narkiewicz K, Narkiewicz M & Ahlberg PE (2010) Tetrapod trackways from the early Middle Devonian period of Poland. Nature 463, 43-48. ↩︎

• Qvarnström M, Szrek P, Ahlberg PE & Niedzwiedzki G (2018) Non-marine palaeoeninvironment associated to the earliest tetrapod tracks. Sci. Rep. 8:1074. ↩︎