To survive on land, the first vertebrates to emerge from the water didn’t just need legs instead of fins and lungs instead of gills. The way they eat had to change too. Fossils of the prehistoric fish Tiktaalik now show that a feeding strategy suitable for rural life has already developed in the water. Instead of just sucking in the prey like most water dwellers – a procedure that no longer works on land – Tiktaalik could also make bite and snap movements. This was ensured by a special skull and jaw anatomy, which is still found in alligator pike today.
The transition from aquatic to terrestrial vertebrates required numerous adaptations that enabled the animals to live in the new, terrestrial conditions. But at what point in evolution did these adaptations come about? An important transition form to the first tetrapods is the prehistoric fish Tiktaalik rosae, which was first described in 2004. Although it was still equipped with fins and gills, it already had primitive lungs and a robust pelvic belt, which was the prerequisite for the four-legged locomotion that developed in its descendants.
Basis for life on land
Researchers led by Justin Lemberg from the University of Chicago have now shown that Tiktaalik had another feature that was crucial for the later development of the terrestrial vertebrates: the jaw of the fish was designed so that it could snap and bite. Most other vertebrates in the water suck in their prey by creating a vacuum with their mouths that washes the victim into it. “On land, sucking food intake is ineffective because it no longer works remotely and it is difficult to generate enough pressure to suck something in,” says Lemberg. “So terrestrial vertebrates had to resort to other methods of preying on food.”
The researchers found evidence of Tiktaalik’s nutritional strategy by examining the skulls of four fossil specimens with the help of micro-computed tomography and reconstructing them in three dimensions on the computer. They found that the skull was on the one hand very articulated, which suggests that Tiktaalik could create suction with its mouth. The individual parts of the skull are shifted in this way of catching prey in such a way that the interior space is greatly expanded and water can flow in. On the other hand, the CT images showed that the cranium, the roof of the skull and parts of the upper jaw had grown together to form a rigid, inflexible unit – an anatomy that is more reminiscent of crocodiles. The structure of the row of teeth also indicated that Tiktaalik could bite.
Sucking and snapping combined
“Most studies so far assumed that animals could either suckle or bite, functionally exchanging one for the other,” the researchers write. “We argue that understanding nutritional strategies at the transition from water to land requires a more differentiated perspective.” In her opinion, the results suggest that Tiktaalik did not replace sucking with snapping, but was able to combine both strategies. In 2019, Lemberg and colleagues showed that this is basically possible with alligator pike (Atractosteus spatula) living today. The large freshwater fish from North America have special sliding joints between the skull bones that allow them to create suction while snapping. The researchers have now also been able to identify such sliding joints at Tiktaalik.
The adaptation of the hunting strategy, which was an important prerequisite for going ashore, probably developed long before the time when vertebrates first colonized the land. Lviv colleague Neil Shubin says, “What really amazes me is that every innovation used by vertebrates on land originally appeared in fish in some form, including lungs, limbs and now diet.”
Source: Justin Lemberg (University of Chicago) et al., Proceedings of the National Academy of Sciences, doi: 10.1073 / pnas.2016421118