A new study on the internal anatomy of the skull of the extraordinary fish, Tiktaalik roseae, which lived 375 million years ago, provides more evidence of how vertebrate life transitioned from water to land. The head showed changes from more primitive fish that helped adapt to the new feeding and breathing conditions presented by a terrestrial environment, scientists said.
Image: Ted Daeschler, Academy of Natural Sciences [larger view].
A paper was just published that documents another study of the remarkable transitional fossil, Tiktaalik roseae, detailing how this new “fishapod” or “fishibian” (tetrapodomorph fish) continues to provide evolutionary insights to scientists. In this case, the cranium of T. roseae, a fossil sarcopterygian fish that lived 375 million years ago, provides evidence for the intermediate steps by which terrestrial animals evolved from aquatic fishes.
Meticulous studies of the internal structure of the cranium from several fossil fishapods, T. roseae, reveal the step-wise process that morphological changes followed as terrestriality evolved in tetrapods.
T. roseae was originally discovered on Canada’s Ellesmere Island, 600 miles above the Arctic Circle, in 2004. It was a large aquatic predator, measuring between 3 and 9 feet long, with sharp teeth and a flattened head that resembles that of a crocodile. It is thought that this animal inhabited the mudflats of freshwater flood plains in a subtropical environment.
At the time of its discovery, the underside of Tiktaalik’s skull was fully encased in rock. But several fossil preparators cleaned the rock away using a needle over a period of several years, revealing the internal structure of the skull (figure 1);
“The braincase, palate and gill arch skeleton of Tiktaalik have been revealed in great detail,” reports Jason Downs, a research fellow at the Academy of Natural Sciences who is the lead author of the study. “By revealing new details of the pattern of change in this part of the skeleton, we see that cranial features once associated with land-living animals were first adaptations for life in shallow water.”
According to Downs and his colleagues, the cranium of Tiktaalik has features that are intermediary between the primitive transitional fish, Panderichthys rhombolepis, and the transitional tetrapod, Acanthostega gunnari.
Even though these morphological features reveal Tiktaalik to be more fish-like than amphibian-like, this fossil animal fits into the morphological gap between more primitive and more derived fishapods. One of the most intriguing findings was the reduction in size of the hyomandibular bone that, in fish, joins the braincase, palate and gills and is associated with underwater feeding and respiration. In more primitive fish, the hyomandibula is large and shaped like a boomerang, but in Tiktaalik, this bone is very small, no bigger than a human thumb.
“This could indicate that these animals, in shallow-water settings, were already beginning to rely less on gill respiration,” Downs said.
In the transition from water to land, the hyomandibula gradually lost its original functions and later gained a role in hearing. In humans and other animals that posess a middle ear, the hyomandibula, or stapes, is one of the tiny bones involved in sound conduction.
But Tiktaalik also shares several structural similarities with its close relatives, P. rhombolepis and A. gunnari. For example, the entopterygoid bone in T. roseae is horizontal and shallow as it is in the other fishapods (figure 2);
Another interesting morphological change was documented for the neck bones, which suggests that the neck was more flexible in the fishapods than in fishes (figure 3);
This increased flexibility gave Tiktaalik an advantage over less-flexible species because they could move their heads without reorienting the whole front end of their bodies, as fishes must do.
“When feeding, fish orient themselves by swimming, which is fine in deep water, but not for an animal whose body is relatively fixed, as on the bottom of shallow water or on land,” reports Neil Shubin, an evolutionary biologist at the University of Chicago and the Field Museum and co-author of the paper. “Then a flexible neck is important.”
Combined with its sturdier front limbs, this flexible neck could have allowed Tiktaalik to leave the water for short periods of time.
“It’s not to say that Tiktaalik itself is a terrestrial animal. It spent most of its time in water, for sure,” Downs pointed out.
Earlier work on Tiktaalik also indicated that its head wasn’t fused to its shoulders. In fact, Tiktaalik has the first known flexible neck.
Based on this study, these morphological data indicate that significant changes to the braincase occurred relatively late in the evolutionary transition to the tetrapods and further, the general cranial dimensions of tetrapods were first attained in the fishapods. Additionally, the observed changes in the shape of the head appear to be related to the reduction and reorientation of the hyomandibular bone. With the loss of the gill elements, head mobility increased in T. roseae relative to other fishapods. The evolutionary relationships between these animals based on cranial morphology are summarized in a cladogram (figure 5);
So these data provide a clearer picture of the sequence of events that occurred as primitive fishes gave rise to terrestrial tetrapods.
“The new study reminds us that the gradual transition from aquatic to terrestrial lifestyles required much more than the evolution of limbs,” concluded Edward Daeschler, a co-author on the paper.
Jason P. Downs, Edward B. Daeschler, Farish A. Jenkins, Neil H. Shubin (2008). The cranial endoskeleton of Tiktaalik roseae Nature, 455 (7215), 925-929 DOI: 10.1038/nature07189.
Official Tiktaalik website.