Tiktaalik

Skull showing otic notches above the eyes

The skull of Tiktaalik was low and flattened, more similar in shape to that of a crocodile than most fish. The rear of the skull was excavated by a pair of indentations known as otic notches. These notches likely housed spiracles on the top of the head, which suggest the creature had primitive lungs as well as gills. Tiktaalik also lacked a characteristic that most fishes have—bony plates in the gill area that restrict lateral head movement. This makes Tiktaalik the earliest known fish to have a neck, with the pectoral girdle separate from the skull. This would give the creature more freedom in hunting prey on land or in the shallows.[4]

Unlike many previous, more fishlike transitional fossils, the "fins" of Tiktaalik have basic wrist bones and simple rays reminiscent of fingers. The homology of distal elements is uncertain: exactly how did the bones of the lower limbs evolve into the feet and toes of vertebrates? There have been suggestions that the fin rays of Tiktaalik are homologous to digits, although this is incompatible with the digital arch developmental model, a hypothesis describing how the two parallel bones of lower limbs evolved to produce feet. According to this model, digits are supposed to be postaxial structures, but only three of the (reconstructed) eight rays of Tiktaalik are postaxial.[5]

However, the proximal series (bones of the upper limb) can be directly compared to the ulnare and intermedium of tetrapods. The fin was clearly weight bearing, being attached to a massive shoulder with expanded scapular and coracoid elements and attached to the body armor, large muscular scars on the ventral surface of the humerus, and highly mobile distal joints. The bones of the forefins show large muscle facets, suggesting that the fin was both muscular and had the ability to flex like a wrist joint. These wrist-like features would have helped anchor the creature to the bottom in a fast current.[6][4]

Restoration

Strong lungs (as supported by the plausible presence of a spiracle) may have led to the evolution of a more robust ribcage, a key evolutionary trait of land-living creatures.[7] The more robust ribcage of Tiktaalik would have helped support the animal's body any time it ventured outside a fully aquatic habitat.[4]

Tiktaalik is sometimes compared to gars (esp. Atractosteus spatula, the alligator gar) of the family Lepisosteidae, with whom it shares a number of characteristics:[8]

• Diamond-shaped scale patterns common to the Crossopterygii class (in both species scales are rhombic, overlapping and tuberculated);
• Teeth structured in two rows;
• Both internal and external nostrils;
• Tubular and streamlined body;
• Absence of anterior dorsal fin;
• Broad, dorsoventrally compressed skull;
• Paired frontal bones;
• Marginal nares;
• Subterminal mouth;
• Lung-like organ.

Life restoration of Tiktaalik

The phylogenetic analysis of Daeschler et al. (2006) placed Tiktaalik as a sister taxon to Elpistostege and directly above Panderichthys, which was preceded by Eusthenopteron. Tiktaalik was thus inserted below Acanthostega and Ichthyostega, acting as a transitional form between limbless fish and limbed vertebrates ("tetrapods").[1] Some press coverage also used the term "missing link", implying that Tiktaalik filled an evolutionary gap between fish and tetrapods.[20] Nevertheless, Tiktaalik has never been claimed to be a direct ancestor to tetrapods; rather, its fossils help to illuminate evolutionary trends and approximate the hypothetical true ancestor to the tetrapod lineage, which would have been similar in form and ecology.

In its original description, Tiktaalik was described as a member of Elpistostegalia, a name previously used to refer to particularly tetrapod-like fish such as Elpistostege and Panderichthys. Daeschler et al. (2006) recognized that this common usage referred to a paraphyletic grade, since elpistostegalian fish have few unique traits not also inherited by later tetrapods. In response, they redefined Elpisostegalia as a clade, including all vertebrates descended from the common ancestor of Panderichthys, Elpistostege, and tetrapods. Nevertheless, they still retained the phrase "elpistostegalian fish" to refer to the grade of early elpisostegalians which had not acquired limbs, digits, or other specializations which define tetrapods. In this sense, Tiktaalik is an elpistostegalian fish.[1] Later papers also use the term "elpisostegid" for the same category of Devonian fish.[21][22]

Such order of the phylogenetic tree was initially adopted by other experts, most notably by Per Ahlberg and Jennifer Clack.[23] However, it was questioned in a 2008 paper by Boisvert et al., who noted that Panderichthys, due to its more derived distal forelimb structure, might be closer to tetrapods than Tiktaalik or even that it was convergent with tetrapods.[24] Ahlberg, co-author of the study, considered the possibility of Tiktaalik's fin having been "an evolutionary return to a more primitive form."[25]

Zachełmie trackmakers predate not only ichthyostegids and elpistostegids (including Tiktaalik) but also a number of tetrapodomorph fish which until 2010 were unanimously considered ancestors of tetrapods.

The proposed origin of tetrapods among elpistostegalian fish was called into question by a discovery made in the Holy Cross Mountains of Poland. In January 2010, a group of paleontologists (including Ahlberg) published on a series of trackways from the Eifelian stage of the Middle Devonian, about 12 million years older than Tiktaalik.[21][26] These trackways, discovered at the Zachełmie quarry, appear to have been created by fully terrestrial tetrapods with a quadrupedal gait.[21]

Tiktaalik's discoverers were skeptical about the Zachelmie trackways. Daeschler said that trace evidence was not enough for him to modify the theory of tetrapod evolution,[27] while Shubin argued that Tiktaalik could have produced very similar footprints.[28] 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.[29] 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.[21][30] Instead, the authors of the publication suggested that "ichthyostegalian"-grade tetrapods were the responsible trackmakers, based on available pes morphology of those animals.[21]

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",[31] a notion partially shared by Philippe Janvier.[32] To resolve the questions posed by the Zachelmie trackways, several hypotheses have been suggested. One approach maintains that the first pulse of elpistostegalian and tetrapod evolution occurring in the Middle Devonian, a time when body fossils showing this trend are too rare to be preserved. This maintains the elpistostegalian-tetrapod ancestor-descendant relationship apparent in fossils, but also introduces long ghost lineages required to explain the apparent delay in fossil appearances.[21] Another approach is that elpistostegalian and tetrapod similarities are a case of convergent evolution. In this interpretation, tetrapods would originate in the Middle Devonian while elpisostegalians originate independently in the Late Devonian, before going extinct near the end of the period.[33][34][35][36]

Estimates published after the discovery of Zachelmie tracks suggested that digited tetrapods may have appeared as early as 427.4 Ma ago and questioned attempts to read absolute timing of evolutionary events in early tetrapod evolution from stratigraphy.[34]

However, a reanalysis in 2015 of the Zachelmie trackways find that it fails the criteria for it being identified as Devonian tetrapod trackways and were instead reinterpreted as fish nests or feeding traces.[37]

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