More on those fascinating Middle Devonian tetrapod tracks

Surprisingly,
Middle Devonian tetrapod tracks (Fig. 1; Niedźwiedzki et al. 2010)  precede fossil taxa that could have made those tracks by tens of millions of years.

Wide-gauge 385 million year old tracks from Valentia
could only have been made by a tetrapod with laterally extended limbs found in 360 million year old strata, 25 million years later.

Figure 1. From Niedźwiedzki et al. 2010 showing the Valentia track (above), the Zalchemia track (below) and possible trackmakers (middle). Pink lines link corresponding forelimb and hind limb in the Zalchemia track.

Figure 1. From Niedźwiedzki et al. 2010 showing the Valentia track (above), the Zalchemia track (below) and possible trackmakers (middle). Pink lines link corresponding forelimb and hind limb in the Zalchemia track. Note the wide gauge of the Valentia track versus the narrow gauge of the earlier Zalchemie track.

Narrow-gauge older tracks from Zalchemie
(387 million years ago) also had a shorter stride on a longer torso, matching tetrapods without long lateral limbs, but with short stubs or limbs, like Tiktaalik appearing 12 million years later.

Figure 2. Chronology of Devonian stem tetrapod taxa and trackways. Frame one shows traditional tree without tracks. Frame two extends ghost lineages to consider the tracks as evidence of undiscovered fossils. Fossils represent rare discoveries typically long after major radiations to millions of individuals, increasing the odds of their being found.

Figure 2. Chronology of Devonian stem tetrapod taxa and trackways. Frame one shows traditional tree without tracks. Frame two extends ghost lineages to consider the tracks as evidence of undiscovered fossils. Fossils represent rare discoveries typically long after major radiations to millions of individuals, increasing the odds of their being found.

The problem is
the wider tracks come from an era in which Tiktaalik-like taxa are known as fossils, some 25 million years too soon based on fossil taxa like Ichthyostega, (Fig. 3).

Figure 3. Best Devonian Valentia track with various overlays.

Figure 3. Best Devonian Valentia track with various overlays.

The solution is
fossils of all sorts can be discovered close to the genesis of a clade, but are more likely to be discovered close to the maximum radiation (in terms of numbers of individuals), increasing the odds for preservation and discovery. Applying logic here, the skeletons must be appearing near the maximum radiation while the ichnites must be appearing near the genesis of the clade. But wait, there’s more:

Figure 5. Various stem amniotes (reptiles) that precede Tulerpeton in the LRT. So these taxa likely radiated in the Late Devonian. And taxa like Acanthostega and Ichthyostega are late-survivors of earlier radiations documented by the earlier trackways.

Figure 5. Various stem amniotes (reptiles) that precede Tulerpeton in the LRT. So these taxa likely radiated in the Late Devonian. And taxa like Acanthostega and Ichthyostega are late-survivors of earlier radiations documented by the earlier trackways.

The taxa listed above
(Fig. 5) all precede Latest Devonian Tulerpeton in the large reptile tree (LRT, 1027 taxa), though their first appearance in the fossil record occurs much later in every case. That must mean the genesis of the various radiations that produced these taxa must have occurred in the Late Devonian. Currently that’s heresy. But that’s where the current evidence leads us. At present these clues tell us where to look in the geological column and what to look for.

And for all you future paleontologists:
there’s a great paper waiting for the next person or team to find these pre-Tulerpeton taxa in Late Devonian strata. Based on the stress to living things that occurred during the Latest Devonian extinction event, perhaps these taxa radiated quickly and widely.

References
Niedźwiedzki G, Szrek P, Narkiewicz K, Narkiewicz M and Ahlberg PE 2010. Tetrapod trackways from the early Middle Devonian period of Poland Nature 463, 43-48. doi:10.1038/nature08623

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