Follow that Tuna: Case Studies in Convergent Evolution IIPosted: March 11, 2014
This is the second post in my series on my favorite examples of convergent evolution, a concept which I defined previously and demonstrated with marsupial and placental wolves. That post featured two modern terrestrial mammalian species with relatively recent evolutionary histories. For the current example, I would like to take us into the oceans and examine a much wider range of organisms as disparate as cartilaginous fish, bony fish, reptiles and mammals — spanning about 530,000,000 years of Earth’s history. Within each of these highly divergent lineages, a particular adaptation for high-speed underwater movement evolved, known as thunniform locomotion – which means literally “swimming like a tuna”.
The thunniform body plan consists of a suite of traits that makes an animal a streamlined swimming apparatus: a rigid anterior section (from the head to about the pelvic region) that becomes much more flexible posteriorly, with a crescent-shaped caudal fin (a.k.a tail fin or fluke). A thunniform swimmer need only to undulate its caudal region (meaning basically its tail) and the result is efficient propulsion through the watery medium with minimal head-wiggling (see Figure1). Thus, the swimmer can disperse over long oceanic distances at high speeds, and keep its eyes toward whatever its pursuing. That this has evolved in so many different types of top marine predators over the eons strongly suggests that the common environment of the ocean presents strong selective pressure for an optimal form of locomotion. In sharks and tuna, thunniform locomotion represents more of a parallel pattern of evolution, which is is when two separate lineages evolve into similar forms independently but had mostly similar starting points (the common ancestor of sharks and tuna was already aquatic). It is even more remarkable, in my opinion, that it developed in both reptiles and mammals, since the common ancestor of these groups lived on land. The convergent evolution of this highly derived body shape thus required totally independent exoduses of terrestrial animals back into the ocean.
Among the many kinds of reptiles that inhabited the world’s oceans during the Mesozoic Era, there were none so well-suited to marine life as the ichthyosaurs (“fish lizards”). The ichthyosaurs are the most highly derived group within the reptilian superorder Ichthyopterygia, which were ocean-going reptiles who had developed their characteristic “fish flippers” very early on in their history. It is generally accepted that the ancestors of the ichthyopterygians were land-dwelling amniotes, however the precise taxonomy of their relationships to other reptiles has been highly contentious and was hotly debated throughout the 20th century. This is because the fossil record is devoid of any transitional forms that could be seen as terrestrial precursors – the oldest ichthyopterygian fossils, which date to the Early Triassic ~250,000,000 years ago, are already so “ichthyosaurian” – and it is extremely difficult to retrace the evolutionary steps taken towards their aquatic lifestyle. However, most modern cladistic analyses indicate that ichthyopterygians were diapsid reptiles along with lepiodsaurs (snakes, lizards and tuatara) and archosaurs (crocodilians, pterosaurs, and avian and non-avian dinosaurs) (Motani et al. 1998).
Soon after reptiles started dominating terrestrial environments during the early Mesozoic, they began to conquer the sea as well. But by about 90,000,000 years ago, the last ichthyosaurs died out. Regardless of their ancestry and the length of time they have been extinct, the ichthyosaurs just might represent my favorite historical example of convergent evolution. Figure 2 shows a skeletal and a life restoration of the iconic Ichthyosaurus communis and demonstrates just how well-adapted these animals were to marine life. The overall shape of the body is reminiscent of a streamlined torpedo. The hands and fingers on its front limbs became pectoral fins; other fins included a dorsal fin that is more obvious in fossil impressions as well as a crecsent-shaped caudal fin that makes the tail look very shark-like. You can see how lateral (side-to-side) undulation of its rear end would thrust the animal forward through the water with minimal effort. I included a dolphin skeleton in the figure to highlight how precise the convergence between these two highly divergent vertebrates was. They do undulate on different axes; dolphins thrust their tails up-and-down, not side-to-side, but that seems to be a strongly conserved trait among mammalian swimmers and I think it doesn’t take away from the overall similarities. The evolution of whales and dolphins from their terrestrial ancestors that began ~55 million years ago is another amazing evolutionary story that I won’t get into here. What I wanted to highlight in this post is how closely these air-breathing vertebrates came to resemble one another.
A very good overview of the various forms of undulatory locomotion, including thunniform, can be found here.