Paleontologists have uncovered yet another specimen in the lineage leading to modern tetrapods, creating more gaps that will need to be filled. It’s a Sisyphean job, working as an evolutionist.
This creature is called Tiktaalik roseae, and it was discovered in a project that was specifically launched to find a predicted intermediate form between a distinctly fish-like organism, Panderichthys, and the distinctly tetrapod-like organisms, Acanthostega and Ichthyostega. From the review article by Ahlberg and Clack, we get this summary of Tiktaalik‘s importance:
First, it demonstrates the predictive capacity of palaeontology. The Nunavut field project had the express aim of finding an intermediate between Panderichthys and tetrapods, by searching in sediments from the most probable environment (rivers) and time (early Late Devonian). Second, Tiktaalik adds enormously to our understanding of the fish-tetrapod transition because of its position on the tree and the combination of characters it displays.
Here’s the beautiful beastie as preserved:
, Left lateral view; b
, dorsal view with enlargement of scales; and c
, ventral view with enlargement of anterior ribs. See Fig. 3 for labelled drawing of skull in dorsal view. Abbreviations: an, anocleithrum; bb, basibranchial; co, coracoid; clav, clavicle; clth, cleithrum; cbr, ceratobranchial; ent, entopterygoid; hu, humerus; lep, lepidotrichia; mand, mandible; nar, naris; or, orbit; psp, parasphenoid; ra, radius; suc, supracleithrum; ul, ulna; uln, ulnare. Scale bar equals 5 cm.
The analysis of the fossil clearly positions it as an intermediate: it has a more mobile skull/neck than a fish, and although its limbs are clearly fin-like, they also have features that presage the digits of tetrapods.
The lineage leading to modern tetrapods includes several fossil animals that form a morphological bridge between fishes and tetrapods. Five of the most completely known are the osteolepiform Eusthenopteron
; the transitional forms Panderichthys
; and the primitive tetrapods Acanthostega
. The vertebral column of Panderichthys
is poorly known and not shown. The skull roofs (left) show the loss of the gill cover (blue), reduction in size of the postparietal bones (green) and gradual reshaping of the skull. The transitional zone (red) bounded by Panderichthys
can now be characterized in detail. These drawings are not to scale, but all animals are between 75 cm and 1.5 m in length. They are all Middle?Late Devonian in age, ranging from 385 million years (Panderichthys
) to 365 million years (Acanthostega
). The Devonian?Carboniferous boundary is dated to 359 million years ago.
The limbs alone have a whole paper dedicated to them. Tiktaalik‘s limb is third from the right in the diagram below, and you can see how it still has the fin rays of its predecessor, Panderichthys, but also has a robust bony axis with smaller bones branching off of it—it’s not quite the clear digits of Acanthostega, but it’s a step in that direction.
Unlike other tetrapodomorph fishes (1), Tiktaalik
has reduced the unjointed lepidotrichia, expanded the radials to a proximal, intermediate and distal series, and established multiple transverse joints in the distal fin. The fin also retains a mosaic of features seen in basal taxa. The central axis of enlarged endochondral bones is a pattern found in basal sarcopterygians and accords with hypotheses that a primitive fin axis is homologous to autopodial bones of the tetrapod limb. In some features, Tiktaalik
is similar to rhizodontids such as Sauripterus
. These similarities, which are probably homoplastic, include the shape and number of radial articulations on the ulnare, the presence of extensive and branched endochondral radials, and the retention of unjointed lepidotrichia. Figures redrawn and modified from Glyptolepis
Those limbs tell us something about the evolution of limbs. Tiktaalik was definitely not a terrestrial animal, but had developed muscular, bony limbs and a strong pectoral girdle that had helped it prop itself up on the substrate, perhaps even holding itself partly out of the water. Those jointed digits were capable of extension and flexion, splaying out when they were pressed against the ground. That simple function, of spreading out to increase the surface area of limb contact, could be the precursor to the flexibility we now have in our hands.
, Anterolateral view. c
, Ventral view. a
, Resting posture with the fin partially flexed at the antebrachium. In this position the radius is slightly more flexed than the ulna. b
, Resistant contact with a firm substrate entails flexion at proximal joints and extension at distal ones. The shoulder joint is flexed by ventral muscles, including the trans-coracoid muscle. The elbow is flexed (d
, arrow 1), with slight pronation of the radius (d
, arrow 2) and rotation of the ulna (d
, arrow 3). The transverse joints distal to the ulnare and intermedium are extended (d
, arrows 4).
Lancelet has more.
As does Carl Zimmer, of course.
Ahlberg PE, Clack JA (2006) A firm step from water to land. Nature 440:747-749.
Daeschler EB, Shubin NH, Jenkins FA (2006) A Devonian tetrapod-like fish and the evolution of the tetrapod body plan. Nature 440:757-763.
Shubin NH, Daeschler EB, Jenkins FA (2006) The pectoral fin of Tiktaalik roseae and the origin of the tetrapod limb. Nature 440:764-771.