Following all of the giant-rauisuchian based excitement of last week’s TOTW, I’ve decided to calm it down a bit by looking at an unassuming fish. This particular unassuming fish has been in the news recently, and illustrates a transition without which there would be no Postosuchus, no mammals and (perhaps worst of all) no ‘Dinosirs’. The fish of which I speak is, of course, the Devonian tetrapodomorph Tiktaalik roseae, but before we look at it closely, let’s have a look at the bigger evolutionary picture into which it fits.
Vertebrates that live on land are known as tetrapods, and all possess (or at least their ancestors possessed) four limbs with digits: amphibians, lizards, crocodiles, birds and mammals are all modern tetrapods. Despite their many obvious differences from what you’d typically think of as a ‘fish’, tetrapods are actually the largest group of lobe-finned fish, or sarcopterygians, named for their fins’ fleshy bases. The only other modern groups of sarcopterygian are lungfish and the famous ‘living fossil’ coelacanth.
The most obvious difference between tetrapods and other fish is that the latter are aquatic and the former mainly terrestrial (with the exception of groups that went back into water, like whales and icthyosaurs). At some point during the evolution of tetrapods they switched from an aquatic lifestyle to a terrestrial one, and this pretty drastic transition is of great interest to palaeontologists. Originally pictured as being a case of fish struggling onto land and then evolving limbs, in recent(ish) years very early tetrapods from the Late Devonian (~365 Mya), such as Acanthostega and Ichthyostega, have changed this view. These fossils have four limbs with digits, but appear to be fully aquatic, suggesting that the tetrapod body plan evolved in water first, only later proving handy (pun obviously intended) on land.
Various fish-like animals have been described that have body plans somewhere between these early aquatic tetrapods and the ancestral fishy form, and which are more closely related to tetrapods than the next closest group of sarcopterygians, the lungfish. These animals are known as ‘tetrapodomorphs’, and show an evolutionary trajectory towards air breathing and a four-finned body plan, while still retaining fish-like characters such as gills and fin rays. It has been suggested that this was as a result of them living in shallow rivers during the oxygen-poor Late Devonian, where being able to breathe air as well as oxygen dissolved in water would have been advantageous, as would being able to navigate shallow, relatively predator free, water.
Enter Tiktaalik. Tiktaalik is something of a celebrity amongst tetrapodomorph fish. Described in 2006 from fossils found on Ellesmere Island in Nunavut, Canada, its name means ‘Burbot’ in Inuktitut. It was subsequently popularised by one of its discoverers, Neil Shubin, in his book ‘Your Inner Fish’ (recommended), and is even the star (or possibly victim) of its own song on Youtube. This stardom is well deserved, as Tiktaalik gives us a good deal of interesting information on the water-land transition.
The original fossil material of Tiktaalik consisted of a pretty spectacularly articulated front half of the animal, possessing a number of interesting features. The animal was pretty ‘fish-like’, but with some tetrapod-like characters, such as large shoulders bones and pectoral fins with wrist-like joints, a neck, and a robust ribcage. These qualities, together with the shallow river environment in which it appears to have lived, led to the suggestion that it might have used its robust fore-fins to prop it up in ‘press-ups’, lifting its head (with the help of its neck) above water to breathe. While there was no knowledge of the back half of Tiktaalik itself, the small pelves (plural of pelvis) of other tetrapodomorph fossils like Panderichthys, led to a ‘front-wheel drive’ picture of tetrapodomorphs moving largely with the fore-fins, with ‘four-wheel drive’ locomotion, with all four fins (or limbs), being a tetrapod innovation.
Earlier this week however, the back half of Tiktaalik (or at least some of it) was described, and it transpires that this picture of exclusively ‘front wheel drive’ tetrapodomorphs is incorrect. Tiktaalik’s pelvis and parts of the hind limb were recovered, and it turns out that its hind-fins were as large as its fore-fins, with a fairly good range of movement. While still compatible with their function as ‘props’ mentioned above, this also suggests that it would have been able to do more in terms of fin-based movement, with the possibility of underwater gaits, like the ‘walking’ seen in African lungfish. It also changes our picture of the evolution of limbs, suggesting that tetrapodomorphs became ‘four-wheel drive’ before the evolution of tetrapods.
While this recent information has changed our picture of Tiktaalik, it just adds to its importance as a source of information on the water land-transition. This unassuming Devonian fish, along with various other fishy friends, helps illustrate that many of the changes that we associate with living on land, such as breathing air and having limbs, actually evolved as adaptations to aquatic life. This transition also acts as a prime example of the mosaic nature of evolution: tetrapods didn’t evolve gradually from a fish-like form to a tetrapod-like form, but instead evolved tetrapod-like characters piecemeal while retaining ‘primitive’ ones. This mosaic theme is one that comes up time and time again in evolution, and is one that we’ll be discussing in future blog posts.