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:

References:

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.

Traces First #2: On dry ground!

This is one of my favourite. I think this event is special for several instances. First of all, it must have been ONE event. Although quite more important, lifes complexity itself and the rise of metazoans as well as the Cambrian radiation are transitions. These progresses have many mothers and fathers and all are the result of (r)evolutionary teamwork of biotic and abiotic factors. The early terrestrialisation, however, had a chosen one that did it first.

The earliest convincing record of animals on land is described by MacNaughton et al. (2002) from the Cambro-Ordovician Nepean Formation of southeastern Ontario, Canada. They found arthropod trackways on bedding planes of sandstones that represent aolian desert deposits. Evidence for the sub-aerial origin is given by the presence of large scale high-angle trough cross-bedding, and moreover, adhesion marks and wind ripples.

The trackways itself are composed of a series of imprints (fig.1). There are some tracks possessing a continous midline interpreted as dragmark from posterior appendages. Interesting features of these walking traces are little bumps behind individual imprints. This suggests that the arthropod pushed the sediment backwards during walking which is virtually not likely to happen at a sediment surface that is covered with water. I think there is no doubt about the sub-aerial origin of these traces.

Fig. 1: Trackways of early terrestrial Arthropods and their assumed producer. link

The next question is who did it? The authors discussed several possibilities and favoured a group called Euthycarcinoidea which are placed within the crustaceans but are considered to be closely related to the earliest myriapods (fig.2).

Fig. 2: Euthycarcinoid Arthropods. link

A little bit of discussion is necessary here though. The Euthycarcinoids are probably traceable to the upper Cambrian (Vacarri et al. 2004). Accordingly, the trace fossils described by MacNaughton et al. (2002) may be slightly younger, which implies that the designated trace makers were present before the trackway reported by MacNaughton et al. (2002) was produced. However, there is no terrestrial body fossil described as early as upper Ordovician to Silurian. Furthermore, the alleged producer was not observed in situ with their traces up to now. It cannot be told if they truly did it. Except of smoking gun reports with in situ associations, I tend to be highly incredulous concerning trace-trace maker suggestions. Furthermore, the first terrestrial traces were most likely produced by amphibious arthropods that strolled around in back-beach areas or that were otherwise able to migrate into plains by swimming upstream. They were still predominantly aquatic organisms. However, they were (at least temporarily) able to deal with harsh conditions on land like high temperatures, increased gravity, the problem of respiration and higher impact of UV-radiation and so on.

References

Vacarri, N.E., Edgecombe G.D. and Escudero C., 2004: Cambrian origins and affinities of an enigmatic fossil group of arthropods, Nature 430, 554 – 557.

MacNaughton, R.B., Cole, J.M., Dalrymple, R.W., Braddy, S.J., Briggs, D.E.G., Lukie, T.D., 2002: First steps on land: Arthropod trackways in Cambrian-Ordovician eolian sandstone, southeastern Ontario, Canada. Geology 30/5, 391 – 394.

My PhD project!

Whoo hoo!

On Tuesday, I gave a presentation and had an interview on a PhD position at the Palaeontological Institute and Museum of the Zürich University. I got the the job and I proudly announce that I will work on the recovery of benthic marine communities after the Permian-Triassic mass extinction event. I will (and will have to) learn alot. The project will besupervised by Dr. Michael Hautmann and Prof. Hugo Bucher. The invitation letter outlining the project is posted on aragosaurus.blogspot.com. A research abstract is available here (pdf).

My upcoming home: The world of the Lower Triassic (from scotese.com)

I highly DO NOT recommend...

...the following book:

Elewa, Ashraf M. T. (ed.), 2008: Mass extinction. Springer

I like mass extinction events. Accordingly, this book catched my eye as I browsed through the shelves of our library. Back home, I started flipping some pages and reading here and there - awfull. Except for the guest authors articles, which are well done, this "book" appears to be cheap joke.
All of the editors articles (9 of 15) are short, badly written and not sufficiantly referenced. For example, the author refers constantly to wikipedia articles or cites for every occasion standard palaeo textbooks that are suited for undergraduate education. The chapters are filled with fuzzy standard statements and do not really help the reader to get into the topic. In fact, most wikipedia articles provide a more profound picture on certain aspects. For instance the chapter about the Permo-Triassic extinction event beggars all description. It barely covers 1.5 pages and provides very blurry general information without presenting any evidence or serious references. A fourth of the article is wasted for comparision of the PT-event with Agatha Christie's "Murder on the orient express".

Just one example from page 129:"One of the mysteries of the history of the Earth is the layer of clay that was deposited around the entire globe approximately 65.5 million years ago. It is also known as the K-T extinction event [...]."

Don't get me wrong - I really don't want to tease the editor, but this is just a very bad book that costs the unhappy purchaser 135 € (170 $). I am still wondering how this book came through the review at Springer. Perhaps they hoped to make some money because most library order books unseen.

Long story short: Please do not buy this book!