As an educator at the American Museum of Natural History, I knew enough about paleontology to tell people that there were no known aquatic dinosaurs. To many peoples’ surprise plesiosaurs, icthyosaurs, mosasaurs (and all the other fossil marine reptiles so often depicted in books and TV shows) – none of them were actually dinosaurs. They were very different types of reptiles, extinct sauropterygians and icthyopterygians (mosasaurs were actually a kind of lizard) that lived in the seas during the Mesozoic Era while dinosaurs ruled the land.
Jurassic Park 3 came out in 2001, which featured a Spinosaurus that swam like a crocodile, and all the people were like, “See! Dinosaurs COULD swim!” But I never said dinosaurs couldn’t swim. There has long been fossil evidence – in the form of trackways that resemble scraped claws against river floors – that dinosaurs probably were pretty good swimmers, and waterways wouldn’t have presented much of a dispersal barrier, just like caribou and other large modern migrating animals who eventually encounter water at some point in their life and have to cross it. Still, there were no fossil remains ever described as belonging to a dinosaur that actually lived in the water.
Since it’s discovery over 100 years ago, Spinosaurus has been known as one of the largest theropods. Theropods are a large group of dinosaurs which includes not only all birds but all the familiar carnivorous species, including Allosaurus and Velociraptor. Spinosaurus was bigger than even the biggest Tyrannosaurus. This week, the journal Science published the paper “Semiaquatic Adaptations in a Giant Predatory Dinosaur“, in which a group of scientists make the case for a long list of adaptations supporting the idea that this huge meat-eating dinosaur with a sail on its back was also well adapted to life in the water.
Finally, a dinosaur that lived in the water and swam and ate fish like a crocodile. I have been waiting my whole life for there to be one, and here it was under our noses the whole time. Good ol’ Spinosaurus. Museum educators will have to adjust their curriculum accordingly.
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.
Archaeopteryx Not the First Bird (Duh)…
Xu X, You H, Du K Han F (2011) An Archaeopteryx-like theropod from China and the origin of Avialae. Nature 475, 465-470.
This paper made a big splash this week because many news organizations took it and ran with it. Some of the headlines read “Fossil May Evict Archaeopteryx From Avian Family Tree” and “Archaeopteryx Knocked From Roost as Original Bird“. Archaeopteryx is a famous fossil that for about 150 years now has been referred to in popular culture as “the first bird”, and often used as an example of an evolutionary transitional form demonstrating how birds evolved from reptiles. It’s not surprising that creationists at best misunderstood and at worst distorted the findings from this paper because they hope to finally vanquish a long-hated opponent in their quest to delegitimatize evolutionary theory. What they are not reporting on is that there is no problem. Phylogenetic systematics, which is the biological sub-field concerned with reconstructing the branching pattern of species diversification over time, is constantly reworking evolutionary trees when new data comes in. The take-home message from this week’s paper in Nature is that other fossils that have been found in the past hundred years or so have illuminated the fact that Archaeopteryx was not the ACTUAL ANCESTOR of every bird that appeared after it. Rather, it represents more of a cousin of the first birds.
Paleontologists already knew this, but exciting new fossil discoveries from China of bird-like dinosaurs, especially one new to science named Xiaotingia, have demonstrated that early on in bird evolution there were many traits we now attribute to birds that can be found in many of these bird-like dinos that are so close to that bird ancestor (ie feathers, avian respiratory systems, endothermy). We are not ever going to find the one true bird ancestor, however we are now getting enough fossils that allow us to more accurately imagine what it may have looked like.
Which is why I think this paper is cool, and not because it BLASTS ARCHAEOPTERYX FROM ITS POST AS THE FIRST BIRD, as many sensational news outlets would prefer to report. Archaeopteryx turns out to be closer to some other types of bird-like dinosaurs than to other types of extinct creatures that eventually gave rise to the lineage leading to modern birds. Again, this is not completely surprising because Archaeopteryx has many traits that make it more dino-like than bird like. Without getting into detailed morphological analyses (that is out of my area of expertise), these traits include teeth, lack of a beak, and a bony tail.
Remember, evolution looks like a bush, not a ladder. It’s too bad that while systematics (and the tree-type thinking that accompanies it) is an essential field in biology, it is probably the least understood by the general public. I hope to be able to help change that someday, as an educator and as a scientist.
PS: PZ Meyers is going ape on the creationists over their exploitation of this article. Keep it up, PZ!