User:Sceptic view/sandbox

2006 - 2010
The phylogenetic analysis by Daeschler at al. placed Tiktaalik as a sister taxon to Elpistostege and directly below Panderichthys preceded by Eusthenopteron. Tiktaalik was thus inserted above Acanthostega and Ichthyostega as a transitional fossil and a true "missing link".

Such order of the phylogenetic tree was initially adopted by other experts, most notably by Per Ahlberg and Jennifer Clack. However, it was questioned in a 2008 paper by Boisvert at al. who noted that Panderichthys, due to its more derived distal portion, might be closer to tetrapods than Tiktaalik or even that it was convergent with tetrapods. Ahlberg, co-author of the study, considered the possibility of Tiktaalik's fin having been "an evolutionary return to a more primitive form."

2010 - now
In January 2010 a group of paleontologists (including Ahlberg) published a paper accompanied by extensive supplementary material (discussed also in a Nature documentary ) which showed that first tetrapods appeared long before Tiktaalik and other elpistostegids. Their conclusions were based on numerous trackways (esp. Muz. PGI 1728.II.16) and individual footprints (esp. Muz. PGI 1728.II.1) discovered at the Zachełmie quarry in the Holy Cross Mountains (Poland). A tetrapod origin of those tracks was established based on:
 * distinct digits and limb morphology;
 * trackways reflecting quadrupedal gait and diagonal walk;
 * no body or tail drag marks;
 * very wide stride in relation to body length (much beyond that of Tiktaalik or any other fish);
 * various size footprints with some unusually big (up to 26 cm wide) indicating body lenghts of over 2.5 m.

Track-bearing layers were assigned to the lower-middle Eifelian based on conodont index fossil samples (costatus Zone) and "previous biostratigraphic data obtained from the underlying and overlying strata" with subsequent studies confirming this dating.

Both Titaalik's discoverers were skeptical about the Zachelmie trackways. Edward Daeschler said that trace evidence was not enough for him to modify the theory of tetrapod evolution, while Neil Shubin argued that Tiktaalik could have produced very similar footprints (in a later study Shubin expressed a significantly modified opinion that some of the Zachelmie footprints, those which lacked digits, may have been made by walking fish ). However, Ahlberg insisted that those tracks could not have possibly been formed either by natural processes or by transitional species such as Tiktaalik or Panderichthys. Instead, the authors of the publication suggested ichthyostegalians as trackmakers, based on available pes morphology of those animals. However, a 2012 study indicated that Zachelmie trackmakers were even more advanced than Ichthyostega in terms of quadrupedalism. Grzegorz Niedźwiedzki's reconstruction of one of the trackmakers was identical to that of Tulerpeton.

Prof. Narkiewicz, co-author of the article on the Zachelmie trackways, claimed that the Polish "discovery has disproved the theory that elpistostegids were the ancestors of tetrapods", a notion partially shared by Philippe Janvier. There have been a number of new hypotheses suggested as to a possible origin and phylogenetic position of the elpistostegids (including Tiktaalik):


 * their phylogenetic position remains unchanged and the footprints found in the Holy Cross Mountains are attributed to tetrapods but as a result there are at least six long ghost lineages separating Zachelmie trackmakers from various elpistostegalian and ichthyostegalian species ;


 * they were "late-surviving relics rather than direct transitional forms" ;


 * they were "an evolutionary dead-end" ;


 * they were a result of convergent or parallel evolution so that apomorphies and striking anatomical similarities found in both digited tetrapods and elpistostegalians evolved at least twice.

It should be noted that convergency is considered responsible for uniquely tetrapod features found also in other non-elpistostegalian fish from the period like Sauripterus (finger-like jointed distal radial bones) or Tarrasius (tetrapod-like spine with 5 axial regions).

Estimates published after the discovery of Zachelmie tracks suggested that digited tetrapods may have appeared as early as 427.4 Myr ago and questioned attempts to read absolute timing of evolutionary events in early tetrapod evolution from stratigraphy.

Until more data becomes available the phylogenetic position of Tiktaalik and other elpistostegids remains uncertain.

Comparison of with tetrapod-like features
A number of fish, both extant and prehistoric, have featured some characteristics typical of tetrapods.