One of the most commonly cited cases of convergent evolution -- the evolution of similar traits in unrelated organisms -- concerns ichthyosaurs and dolphins. Both families consist of stream-lined, fish-shaped, air-breathing predators that evolved from animals on land and gave birth to live young.
However, a comparison can be made between ichthyosaurs and all whales, given the evolution, adaptations, and diversity of the former during their 150-million-year existence. The features shared between the two groups is even more surprising when one considers that roughly 40 million years separate the last ichthyosaur from the first whale.
TREADING THE SAME WATER
Ichthyosaurs are diapsid reptiles like lizards, snakes, crocodiles, and non-avian dinosaurs. Whales, on the other hand, are cousins of the hippopotamus and are the mammals best adapted for marine living.
The origins of ichthyosaurs and their transition from land to sea is murky, as the earliest known members of the family all seem to have been completely aquatic already. In contrast, the whale's move to the water is one of the best-documented cases of evolution in the fossil record, with wolf-like, hoofed predators like Pakicetus developing into increasingly aquatic forms and eventually leaving land for good by the Late Eocene.
Despite these differences, several crucial developments are present in the evolution of both ichthyosaurs and whales:
- Undulating prototypes- Both the earliest known ichthyosaurs and the first fully-aquatic whales had very long, eel-like bodies. Though these animals quickly gained a global distribution, this design provided limited maneuverability, especially for larger animals.
- Retirement through extinction- Both of these sets of prototypes were annihilated by extinction events, allowing only the more advanced, fish-shaped genera to survive into the next period. In the case of the ichthyosaurs, this crucial event (or events) marked the end of the Triassic and guaranteed the dominion of the dinosaurs on land. The most primitive marine whales, meanwhile, seem to have been the victims of the radical climate change which closed out the Eocene epoch (c. 34 million years ago) and killed many creatures on land as well.
- Shrinking rear appendages- All known ichthyosaurs had a pair of front and rear paddles, but the latter were dramatically smaller in the more advanced species of the Jurassic and Cretaceous. These animals relied more on their flukes for propulsion. Rear limbs are absent altogether in modern whales. Even in Basilosaurus, a monstrous yet primitive genus from the Late Eocene, they were so small that they played no role in locomotion.
- Dorsal fin development- Another feature found only in the more advanced ichthyosaurs, the dorsal fin allowed them to be more maneuverable as well as more stable swimmers than their undulating Triassic brethren. The majority of living baleen and toothed whales have dorsal fins as well. They are generally smaller and much more curved than those ichthyosaurs, with the exception of the high, sail-shaped fins sported by male orcas.
MIRRORED SHAPES AND NICHES
The classic ichthyosaur mold is almost half-dolphin and half-shark, with a long, thin "beak", prominent front lateral fins, and a vertical, crescent-shaped fluke. They are often depicted behaving like dolphins, leaping, frolicking, and chasing schools of fish together. However, this shape and lifestyle was only one of several among the ichthyosaurs. These marine reptiles filled more than one ecological niche, some of which were later assumed by both living and now-extinct forms of whales.
- Serpentine superpredators- Surprisingly, the some of the earliest-known ichthyosaurs and fully-marine whales seized the role of top predator rather quickly (in evolutionary terms). Both the Early Triassic ichthyosaur Thalattoarchon and the aforementioned Basilosaurus had long, snake-like bodies and were equipped with jaws suited for tearing into the flesh of large animals, rather than just gutting small fish and squid.
- Toothless giants- The largest whales today are all baleen whales. In place of teeth, these giants' baleen plates serve to trap thousands of minuscule crustaceans in their mouths with each gulp. The largest ichthyosaurs -- the fifty-foot Shonisaurus and the seventy-foot Shastasaurus -- were intermediary forms from the Late Triassic. Though neither them seem to have had anything like baleen plates (keratinous structures tend not to fossilize anyway), both of them were toothless by the time they reached adulthood and gulped down tons of tiny, soft-bodied invertebrates. Also like modern baleen whales, Shonisaurus may have lived and traveled in pods: The remains of over forty specimens were uncovered in Nevada at the Berlin-Ichthyosaur State Park alone, over a distance of only two and half square miles.
- Sea javelins- The similarly named Eurhinosaurus (from the Early Jurassic) and Eurhinodelphis (from the mid-Miocene) were both shaped like modern swordfish, with an extended, pointy upper jaw. Like modern sword- and sailfish, they may have used this adaptation to maim their prey. Unlike these modern predators, however, the upper jaws of both Eurhinosaurus and Eurhinodelphis were lined with dozens of sharp tooth, which would have served to draw more blood from their victims.
- Deep-sea leviathans- Growing to nearly seventy feet in length and the weight of several elephants, the sperm whale is the largest living toothed whale. While its great size enables it to tackle giant squid, this may initially have been an adaptation for storing greater quantities of oxygen in order to hunt deeper and for longer. This may also have been the case with Temnodontosaurus, a T. rex-sized deep-sea ichthyosaur from the Early Jurassic. While this monster likely relied on different senses to apprehend its prey (more on this later), like the sperm whale, it fed on large cephalopods, using its powerful jaws to crack open the tough shells of ammonites.
For all these similarities, the comparison between ichthyosaurs and cetaceans does not extend to one crucial aspect of their behavior: The sense most vital for their survival.
Both toothed and baleen whales are extremely sonic animals. They are not only endowed with a good sense of hearing, but also possess a vast repertoire of sounds for a wide variety of purposes.
However, cetacean sight on the whole is far less developed. While orcas have been found to have very keen eyesight both in and out of water, the color vision of whales as a whole is extremely limited. Some freshwater dolphins are almost, if not completely, blind. Even sperm whales, the deepest-dwelling of cetaceans, have tiny eyes for such large animals, yet even blind individuals have been known to still catch prey successfully.
While much less is known about their hearing and vocal capabilities, sight appears to have been as crucial to ichthyosaurs as sound is to whales. Even the most primitive known genera possessed eyes reinforced with sclerotic rings to protect them from the crushing pressure of ocean water. In addition, ichthyosaur eyes as a whole were enormous: Temnodontosaurus had the largest eyes of any known animal, measuring ten inches across, while the slightly smaller eyes of Ophthalmosaurus ("eye lizard") are the biggest relative to body size.
Like their origins, the demise of ichthyosaurs is also shrouded in mystery. These predators thrived throughout the Jurassic and the Early Cretaceous, competing with another successful family of marine reptiles, the plesiosaurs. But by around 95 million years ago, they had all vanished into extinction (along with their probable predators, the pliosaurs).
Following the catastrophe that ended the Eocene, whales diversified and continued to go strong, outlasting, among other forces, the gigantic Carcharias megalodon (the largest shark in history) and the Pleistocene ice age. Among their ranks today are the blue whale, the largest animal known to science, and the bottlenose dolphin, one of the most intelligent creatures on the planet.
Even so, many species of whale are classified as vulnerable or endangered by the IUCN (International Union for the Conservation of Nature), and it's no mystery why: Pollution and hunting from humans, along with climate change across the globe, put the survival of these creatures in jeopardy. While many things about ichthyosaurs will continue to elude us--their beginnings, their growth patterns, their intelligence and social behavior--insights about whales will remain open to us, so long as we value and protect the source of those insights.
http://www.karencarr.com/News/motani/1200motani.html (Article on ichthyosaurs from The Scientific American by expert Ryosuke Motani)
http://parks.nv.gov/parks/bi/ (Berlin-Ichthyosaur Park website)
http://www.ucmp.berkeley.edu/people/motani/ichthyo/eyes.html (early article by Motani on ichthyosaur eyes)
http://www.afsc.noaa.gov/nmml/education/cetaceans/sperm.php#see (on sperm whale vision and other features)
Burnie, David and Don E. Wilson et al. Animal: The Definitive Visual Guide; Dorling Kindersley, New York, NY, 2001 (2011 Smithsonian edition)
Uhlenbroek, Charlotte et al. Animal Life: The Definitive Visual Guide; Dorling Kindersley, New York, NY, 2008 (American Museum of Natural History edition)