2010 - now Tiktaalik

restoration

in january 2010, group of paleontologists (including ahlberg) published paper accompanied extensive supplementary material (discussed in nature documentary) showed first tetrapods appeared long before tiktaalik , other elpistostegids. conclusions based on numerous trackways (esp. muz. pgi 1728.ii.16) , individual footprints (esp. muz. pgi 1728.ii.1) discovered @ zachełmie quarry in holy cross mountains (poland). tetrapod origin of tracks suggested based on:

distinct digits , limb morphology;
trackways reflecting quadrupedal gait , diagonal walk;
no body or tail drag marks;
very wide stride in relation body length (much beyond of tiktaalik or other fish);
various size footprints unusually big (up 26 cm wide) indicating body lengths of on 2.5 m.

track-bearing layers assigned lower-middle eifelian based on conodont index fossil samples (costatus zone) , previous biostratigraphic data obtained underlying , overlying strata subsequent studies confirming dating.

zachełmie trackmakers predate not ichthyostegids , elpistostegids (including tiktaalik) number of tetrapodomorph fish until 2010 unanimously considered ancestors of tetrapods.

both tiktaalik s discoverers skeptical zachelmie trackways. edward daeschler said trace evidence not enough him modify theory of tetrapod evolution, while neil shubin argued tiktaalik have produced similar footprints (in later study shubin expressed modified opinion of zachelmie footprints, lacked digits, may have been made walking fish). however, ahlberg insisted tracks not have possibly been formed either natural processes or transitional species such tiktaalik or panderichthys. instead, authors of publication suggested ichthyostegalians trackmakers, based on available pes morphology of animals. however, paper published in 2015 undertook critical review of devonian tetrapod footprints called question designation of zachelmie marks , instead suggested origin fish nests/feeding traces. earlier study in 2012 indicated zachelmie trackmakers more advanced ichthyostega in terms of quadrupedalism. grzegorz niedźwiedzki s reconstruction of 1 of trackmakers identical of tulerpeton.

prof. narkiewicz, co-author of article on zachelmie trackways, claimed polish discovery has disproved theory elpistostegids ancestors of tetrapods , notion partially shared philippe janvier. there have been number of new hypotheses suggested possible origin , phylogenetic position of elpistostegids (including tiktaalik):

their phylogenetic position remains unchanged , footprints found in holy cross mountains attributed tetrapods result there @ least 6 long ghost lineages separating zachelmie trackmakers various elpistostegalian , ichthyostegalian species;
they late-surviving relics rather direct transitional forms ;


they evolutionary dead-end ;


they result of convergent or parallel evolution apomorphies , striking anatomical similarities found in both digited tetrapods , elpistostegalians evolved @ least twice.

it should noted convergency considered responsible uniquely tetrapod features found in other non-elpistostegalian fish period sauripterus (finger-like jointed distal radial bones) or tarrasius (tetrapod-like spine 5 axial regions).

estimates published after discovery of zachelmie tracks suggested digited tetrapods may have appeared 427.4 ma ago , questioned attempts read absolute timing of evolutionary events in tetrapod evolution stratigraphy.

until more data become available, phylogenetic position of tiktaalik , other elpistostegids remains uncertain.