Tiktaalik

Discovery

The discovery team, led by Drs. Ted Daeschler (then of the Philadelphia Academy of Natural Sciences), Neil Shubin (University of Chicago) and the late Farish Jenkins (Harvard), was interested in the evolution of animals with hands and feet, called tetrapods, from the lobe-finned fish that were common in the Devonian. The first archaic tetrapods lived during the late Devonian Period (Famennian Stage; 372.2–358.9 million years ago) in East Greenland and other Northern European localities. Their closest fish relatives, the elpistostegids including Panderichthys from Latvia and Elpistostege from Quebec’s Gaspé Peninsula, are found earlier in the Devonian, from the latest Givetian Stage (387.7–382.7 million years ago) to the earliest Frasnian Stage (382.7–372.2 million years ago).

However, these fish lack many of the features present in the earliest tetrapods, so the team set out to find a fossil showcasing a species that better represents a step towards tetrapods. They targeted rocks from an area that preserved the right kind of fossil environment (aquatic freshwater), the right age (early Frasnian), in the right place (northern hemisphere) but not yet explored by paleontologists. This led them to the Fram Formation of Southeastern Ellesmere Island. After two unsuccessful field seasons, finding only already well-known fossil fish, they planned one final “do or die” field trip since fieldwork in the high Arctic is logistically challenging and expensive during the limited “summer” season. This last field trip in 2004 fortunately proved wildly successful.

Name

Tiktaalik comes from a word in Inuktitut that refers to “large, fresh-water fish seen in the shallows.” The name was suggested by the Nunavut Council of Elders (Inuit Qaujimajatuqangit Katimajiit) and was chosen among the two offered. The specific epithet, roseae, is named in honour of an anonymous benefactor of Devonian paleontology. The fossils are currently held in Ottawa at the Canadian Museum of Nature until research facilities are developed in Nunavut.

Description

The type specimen of Tiktaalik includes a complete skull and the front part of the skeleton; the remaining skeleton is known from additional specimens. Bony scales completely cover the body. The skull is large, triangular from above and covered by a fine, ridged texture. The eyes are located on the top of the skull, unlike most fish, which have them on the sides of the head. The skull has an outside row of many small, evenly-sized teeth. The inside row and roof of the mouth (palate) have several large fangs, a pattern common to lobe-finned fish and early tetrapods. The bony connection between the back of the head and the shoulder girdle present in most fish has been lost, creating the first vertebrate neck known from fossil records.

Within their fleshy lobes, the front fins contain the same (homologous) bones as in the arms and wrists of tetrapods, suggesting a shared ancestry. Unique to Tiktaalik, those bones show evidence of having had mobile joints between them.

The hip bone, known as the ilium, is enlarged compared with other closely related fish but lacks a bony connection to the vertebrae. The hip bone called the pubis is similarly expanded and, together with the ilium, forms a socket for the hind fin. However, the third hip bone, the ischium, remains unossified.

The vertebrae, like in most fish, are made of multiple separate bones that wrap around the notochord, providing reinforcing strength. No fins are found above the vertebrae, unlike most fish. Ribs articulate with each vertebral segment and overlap with one another to help resist body torsion. There is some evidence for regional differences along the vertebral column, and the ribs near the pelvis were expanded, forming a connection via soft tissues. However, the specialization of the first two vertebrae for articulating with the back of the skull, common in tetrapods, is absent in Tiktaalik.

Habitat and Diet

The teeth demonstrate a carnivorous diet. The lack of dorsal fins and the eyes on top of the head suggest Tiktaalik might have lurked along the water–air horizon, perhaps near the water’s edge, as an ambush predator.

Recent work using digital models derived from Computed Tomographic (CT) scanning suggests Tiktaalik had a feeding style like the living alligator gar and crocodiles, called lateral (side) snapping. When feeding in water, most fish have mobile connections between skill bones that help expand the oral cavity to suck food into the mouth. In Tiktaalik and alligator gar, most mobile (kinetic) joints fuse firmly to one another, focusing on a strong bite between the upper and lower jaws. Mobility is retained in the part of the skull related to the gill chamber, meaning that Tiktaalik exclusively breathed underwater.

Water-to-Land Transition

The very first tetrapods known from the late Devonian were still obligatorily aquatic and remained fish-like in their anatomy. However, older footprint fossils suggest there may still be a missing part of the fossil record left to uncover. In many aspects, Tiktaalik represents an intermediary between these archaic tetrapods and a fully “fish-like” body plan. A neck allows for independent motion of the head. Expanding the hip bones and developing a connection to the vertebrae are essential steps toward locomotion on land. The development of mobile joints in the fore fin is also necessary for interacting with the ground.