In the last two posts in this series, we examined the origins of vertebrates (chordates with a skull and a backbone). From their origin as jawless fish, vertebrates have diversified into a number of distinct body plans – body plans as diverse as jawed fish, whales, bats, snakes, and birds. The latter four groups are tetrapods – a remarkably successful monophyletic group of vertebrates that have four limbs as one of their defining characteristics (yes, snakes are tetrapods – we will discuss the loss of limbs in the snake lineage in a future post). The closest-living relatives of tetrapods are lungfish and coelacanths, both of which are groups of lobe-finned fish. Lobe-finned fish, or Sarcopterygii bear their fins on fleshy appendages (from the Greek sarx (flesh) and pteryx (fin)). Numerous lines of evidence support the inclusion of tetrapods within Sarcopterygii, not least of which is the recent whole-genome sequence analysis of coelacanths, and extensive genome comparisons between tetrapods, coelacanths, and lungfish. These lines of evidence place lungfish as the closest living relatives of crown-group tetrapods:
Stem tetrapods: the long road from fish to amphibian
Several years ago my family had the opportunity to visit a traveling fossil exhibit at a local science center. I recall the day well, as I was dragged from one spectacular dinosaur to another by a wide-eyed toddler who was experiencing them in person for the first time. Tucked in between the impressive giants my son was bent on seeing, however, was an odd little fish that suddenly Iwas very excited about – much to the amusement of everyone nearby at the time, who barely gave it a second glance. The object of my excitement was a member of the genus Eusthenopteron – a stem tetrapod close to the last common ancestral population of lungfish and tetrapods.
As we have seen in the last few posts, the key to understanding the transitions that a lineage makes over time is to look for stem groups that branch off the lineage leading to the crown group. While stem group species are not likely to be direct ancestors of the crown group it is possible to find ever-closer relatives of the crown group. These species – though not the direct, ancestral transitional forms leading to the crown group – nonetheless preserve the transitional features that the crown-group lineage passed through.
Perhaps those around me who were passing by the Eusthenopteron exhibit can be excused for their lack of excitement: after all, Eusthenopteron looks like a small, unimpressive fish. Its skeletal structure, however, is anything but dull to a paleontologist. Eusthenopteron had articulated bones in its front lobe fins – bones we recognize in modern-day tetrapods as the humerus, radius, and ulna. These are the long bones that make up the forelimb in crown-group tetrapods – but in Eusthenopteron, these bones are short, and serve as the support for fins, not limbs (in the tetrapod sense). The articulation of these three bones in the forelimb is a trait that Eustenopteron shares with lungfish, but not coelacanths, indicating that this characteristic was present in their last common ancestral population:
The various species of Eustenopteron lived around 385 million years ago, in the Devonian period – a period of earth’s history that was seminal for tetrapod origins, and a fruitful period to search for additional stem tetrapod species.
When you’re going out on a limb, it’s all in the wrist
Once researchers narrow down a timeframe during which a specific set of transitions took place, a concentrated effort to find numerous stem group species in that timeframe is possible. Early amphibian-like (stem-group) tetrapods such as Acanthostega and Ichthyostega appear in the fossil record in the mid-Devonian at about 365 million years ago. This placed the transition from lobe fins to tetrapod limbs within a range – from the branch point of the tetrapod lineage with Eustenopteron (perhaps around 400 - 390 million years ago) to the appearance of these amphibious tetrapods. Accordingly, in the last few decades, much effort has been expended to search for stem-group tetrapods from this time period to illuminate the transition to the crown-group tetrapod state.
One such find that generated significant interest when it was announced was Tiktaalik roseae, a stem-group tetrapod first discovered in the Canadian Arctic in 2004. Tiktaalik is a remarkable fossil in that it is a lobe-finned fish with strikingly tetrapod-like features, such as elbows, flexible wrists (including wrist bones that correspond to the wrist bones of crown-group tetrapods), a flexible neck, and forelimbs (forefins?) capable of supporting its weight, leading to speculation that it may have propped itself up out of its shallow water habitat from time to time.
Other discoveries have greatly filled in the space between the tetrapod and lungfish crown groups (see for example, Figure 6 in Lu et al., 2012, a paper that describes the oldest-known stem tetrapod found thus far, or Figure 4 in Swartz, 2012, a paper that describes a stem tetrapod that branches off the crown-group lineage in between Eustenopteron and Tiktaalik). Far from being a “problem” for evolution, we have an excellent series of stem-group tetrapods that reveal the step-wise transitions that the crown-group tetrapod lineage took from fin to limb.
Stem groups as “transitional forms”
Though we’ve repeatedly emphasized that stem-group species are not the direct ancestors of a crown group, it is by now hopefully clear that stem groups are in fact the “transitional forms” that the fossil record can be reasonably expected to show, given the infrequent nature of the fossilization process. The fact that a species like Tiktaalik is highly unlikely to be the direct ancestor of modern tetrapods is no objection to appreciating the combination of characteristics it possesses, and how (almost perfectly) intermediate it is in form between lobe-finned fish and tetrapods. To object that stem-group species are not direct ancestors – and thus not informative about modern-day species – is to miss the pattern that the fossil record presents to us: groups of species in nested sets, but sets whose boundaries are blurred wherever we have sufficient data to see it.
In upcoming posts in this series, we’ll trace the tetrapod lineage further into the future — and examine the diversification of birds and mammals.