Friday, January 8, 2010

Traces First #3: The tetrapod terrestrialisation

This post actually just represents one of those reviews that swamp the internet right now. However, it just fits so perfectly my Traces First-label that I simply can't refuse to put this one on nologic.

Recently, Niedźwiedzki and co-workers (2010) discovered vertebrate trackways in the early Middle Devonian of Poland. These traces, which represent imprints of walking tetrapods, demonstrate that backboned animals walked the earth approximately 20 Million Years earlier than has been previoulsy inferred from body fossils like Tiktaalik and Panderichthys for instance. Tiktaalik from the late Devonian of the Canadian arctic is suggested to represent a sarcopterygian fish, which is closely related to early land-based tetrapods like no other (primary) aquatic animal of the fossil record (Daeschler et al. 2006)

The recent finding from Poland is remarkable for two reasons:

(1) The morphology of the imprints suggests that they were produced by real limbs. If a creature would use stump-like fins as seen in Tiktaalik for extra-aquatic locomotion, the trace fossil would appear more like drag way, not a track way (haha). The traces from Poland show discrete imprints with no drag marks or something. The implication of this feature is that free moving limbs must have evolved much earlier.

(2) The traces were found in a completely unexpected environment. The terrestrialisation of vertrebrates has always been suggested to have happend somewhere in swamp-like landscapes or along river channels. The sedimentary rocks of the trace fossil bearing level are interpreted as strata deposited on a tidal flat. This offers two possibilties: Either first vertrebrates emerged from the marine (which to my knowledge has not been considered so far) or they came from somewhere else. Togehter with point (1), the latter opportunity pushes the terrestrialisation even further back in time.

With the contribution of Niedźwiedzki et al. (2010), another macro-evolutionary step is well predated by means of traces fossils.

As neat video from nature is available on youtube:


Niedźwiedzki, G., Szrek, P., Narkiewicz, K., Narkiewicz, M., Ahlberg, P.E., 2010: Tetrapod trackways from the early Middle Devonian period of Poland , nature, v. 463, p. 43-48 , doi:10.1038/nature08623.

Daeschler, E.B., Shubin, N.H., Jenkins, F.A.Jr, 2006: A Devonian tetrapod-like fish and the evolution of the tetrapod body plan, Nature, v. 440, p. 757-763, doi: 10.1038/nature04639.

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