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.
One of my favorite evolutionary phenomena to talk (and write) about is convergent evolution. Convergence has occurred whenever phylogenetically distant species, in either or both disparate geographic regions or geological epochs, have evolved very similar phenotypes. This is usually due to similar ecological conditions across the times and places in which the species reside, and in response to similar selective pressures, the different species have in common parallel yet independently evolved adaptations (this concept is illustrated in Figure 1). These similarities are not due to common ancestry and are therefore not homologous. It seems that this would be rare pattern, however the tree of life is full of amazing examples of convergence, and the prevalence of convergence throughout Earth’s history suggests there is at least some degree of determinism in the evolutionary process. I want to write a series of posts which highlight some of my favorite examples of convergent evolution. These posts will try to frame each case in evolutionary, biogeographical, anatomical, and environmental terms.
My first case study in convergent evolution will be the wonderful story of two predatory mammals, which lived at approximately the same time but in separate corners of the globe. One of these is extinct and the other… well it’s complicated. During the Miocene Era which lasted between ~23,000,000 and ~5,200,000 years ago, an environmental change occurred which allowed vast grasslands to open across Eurasia, and with that, the development of a diverse community of crown-toothed herbivorous perissodactyls (“odd-toed” ungulates which include modern-day horses and rhinos) and artiodactyls (“even-toed” ungulates which include modern day cows, sheep and camels) which grazed and browsed across the new prairies. Not surprisingly, there were carnivores which became adapted to chasing down this quick quarry, including canids (the dog family, which includes foxes, jackals and wolves). Most folks are familiar with wolves. Modern wolves (Canis lupus) likely appeared during the early to middle Pleistocene (between ~2,500,000 and 1,000,000 years ago), and are slender, powerful predators adapted to pursuing similarly-sized prey over long distances. They have deep chests and long legs, reminiscent of distance runners, and bone-crunching jaws with pointy canines well-suited for holding onto prey (say, for instance, a fleeing deer), as well as scissor-like carnassial teeth for meat-slicing. No wonder wolves have inhabited a special place in the human psyche – with results that were often bad for wolves. I love wolves, and as a teenager I was mesmerized by reading about their natural history as well as accounts of human-wolf interactions (which often bordered on the fictional), such as “Lobo the King of Currumpaw” by Ernest Thompson Seton in Wild Animals I Have Known (1898) (Figure 2), and especially that of “Thinking Like a Mountain” by Aldo Leopold in A Sand County Almanac (1949), where Leopold recalls the moment he changed from wolf-hater/hunter to one of the pioneers of the environmental and conservation movements. If you ask me, a better appeal for whole-ecosystem conservation has never been written:
We reached the old wolf in time to watch a fierce green fire dying in her eyes. I realized then, and have known ever since, that there was something new to me in those eyes – something known only to her and to the mountain. I was young then, and full of trigger-itch; I thought that because fewer wolves meant more deer, that no wolves would mean hunters’ paradise. But after seeing the green fire die, I sensed that neither the wolf nor the mountain agreed with such a view.
Let’s turn back the clock and take a trip to the the land down under, where after the breakup of the supercontinent Gondwana during the Jurassic Period ~200,000,000 years ago, the resident life forms were stranded at sea on an island landmass known as Australia. Placental mammals had not yet dispersed globally, and the only members of the furred kind in Australia at the time of the continental breakup were the pouched marsupials – and it remained that way until very recently (when humans brought their rats and sheep). During the Miocene, as in Eurasia, grasslands began to open up across Australia, although many regions were still heavily forested. This created many new and unique niches that allowed the resident marsupials to adapt and radiate into a wide variety of forms, such as diprotodonts (which include arboreal possums, familiar and bounding kangaroos, and the extinct large lumbering herbivore Diporotodon), and dasyurids which include the Tasmanian devil and Tasmanian wolf (Figure 3). Wolf? Yes, there is a marsupial wolf. How can that be? The answer is: CONVERGENCE. The scientific name of the Tasmanian wolf is Thylacinus cynocephalus, which means “pouched animal with the head of a dog”. Another more unique and accurate common name is the thylacine.
Thylacines appeared in Australia during the Miocene, and the modern species was present in both Australia and Tasmania by the Pleistocene and into the Holocene, although it was just about absent from mainland Australia by the 1800s and restricted to Tasmania by the 20th century. Just as wolves across Eurasia and North America, thylacines were looked upon as threats to ranchers and were accused of killing chickens and sheep. As a result, they were hunted to extinction and the last one died in a zoo in the Hobart Zoo in Tasmania in 1938. The thylacine was a dog-sized predator, and while its method of prey acquisition is still under debate (whether it was a pursuit predator like wolves or more of an ambush predator like a cougar), together with canids it represents an exceptional case of convergent evolution, which is most evident in a comparison of the cranial anatomy with C. lupus (Figure 4). Both species have an elongated snout with pointed canines and scissor-like molars which are well suited for snapping up, killing and dismembering prey. You’d be hard-pressed to pick one or the other out of a line-up. One of the few cranial traits that can be used as a clue is that the thylacine has more teeth overall, including triangular-shaped molars, thus betraying its marsupial ancestry. But you’d have to be an expert to notice that. While they lived on opposite ends of the world, placental canids and marsupial thylacines developed into similar ecological roles and responded with a convergent evolution toward extremely similar phenotypes.
As mentioned above, while thylacines and wolves evolved into similar forms, they also were met with similar antagonism by humans. Alas, the thylacine died out before the conservation movement took hold. With their much wider distribution across the Northern Hemisphere, placental wolves were able to hold in many areas and were more recently aided by human efforts to bolster their recovery. As Aldo Leopold predicted, the reintroduction of wolves into Yellowstone National Park was a major triumph for conservation, as it returned a keystone species to an ecosystem which was out of equilibrium due to its absence. The return of the thylacine would require either a seance or de-extinctioning through DNA cloning. The probabilities of either of those producing a living thylacine anytime soon are close to zero.