Ichthyosaur Page

Introduction

 

Figure 1. The Ichthyosaur (Enchanted Learning).

Introduction

The word ichthyosaur comes from two Greek roots. “Ichthyos” means fish and “sauros” means lizards or reptiles. Ichthyosaur was a marine carnivorous reptile that lived mostly during the Mesozoic era. Ichthyosaur fossils were found all over the world, a sign that they migrated extensively, just as whales do today (Motani, 2000). These animals had a fish, reptile-like shape and were thought to have been powerful pursuit predators, evolving to chase small animals. Ichthyosaur was an air breathing animal that still had to come to the surface to fill their lungs with air (Motani, 2000). Early ichthyosaurs were more slender and looked much like a reptile. As it evolved it changed many key physical features but, in general it changed from a lizard-shaped body plan to fish-shaped. Eventually the ichthyosaurs evolved into a successful, tuna-like animal. Even though they look like fishes they are actually reptiles which themselves originated from a fish.

General Information

Ichthyosaurs ranged from 15 centimeters to about 70 centimeters in size. The larger ichthyosaur group, Shonisaurus Popularis , was found in the Late Triassic. The medium sized group was the Olphamosaurus, while the smaller group was the Chaohusaurus Geishanesis . They had long jaws with small, numerous, sharp teeth. Some of the most distinctive features that late ichthyosaurs shared were large eyes, dorsal fin, and tail fluke, which are shown in Figure 2 .

Figure 2. The Ichthyosaur Page (Motani, 2002)

The size and structure compared to other living animals can give a hint of what an animal's diet consists of, but some fossils of the ichthyosaur gave much more information. Some fossilized ichthyosaurs have been found with stomach contents preserved with fossilized hooks of cephalods. A fossil from the Lower Cretaceous period also contained turtle and bird remains. Poor swimming ability may have made hatchling turtles easy prey that could have been either swallowed whole or processed by shake feeding (Kear, Boles, Smith, 2003).

Ichthyosaurs first appeared slightly earlier than dinosaurs around 150 million years ago during the early Triassic period. They disappeared earlier than the dinosaurs around 90 million years ago. During this period the climate was warm, but for the most part dry. The continents were grouped together in a landmass called Pangaea where the climate was very arid in the middle. By the early Triassic the Tethys Sea formed between the two continents Laurasia and Gondwanaland. This Seaway became the home of many unique marine reptiles; mostly coastal and shallow water dwellers, which include the ichthyosaur and placadont (Kazlav, 2002). Figure 3 below shows the Tethys Sea location.

Figure 3. Tethys Sea (Kazlav, 2002)

Classification

Ichthyosaurs adapted so well to their marine environment that researchers lacked concrete evidence on their origin. These extreme adaptation to living in water meant that most of them had lost key features- such as particular wrist bones and ankle-bones- that would have made it possible to recognize their distant cousins on land (Motani, 2000). For many years the ichthyosaurs were grouped with a wide range of animals such as lizards, crocodiles, amphibians, and even mammals.

Researchers knew that ichthyosaurs were not a fish but rather a reptile because of several reasons. The first and most important feature is that they did not have gills like fish do, but rather had lungs and needed to go to the surface to breathe air. Additionally they had two pairs of limbs, which were not present in fishes. These features indicated that ichthyosaurs were tetrapods, which means “four limbs”, even though they resembled fish and had many fish like features.

One of the tetrapod lineages that developed during the late Carboniferous period (roughly 350 million years ago) was the Diapsids . The word Diapsid means “two arches”; they are a group of tetrapod animals that developed two holes, temporal fenestra and anorbital fenestra in each side of their skulls as shown in Figure 4.

Figure 4. Late Triassic Web Page (University of Bristol).

The key question that many researchers had was when the ichthyosaur branched from the rest of the diapsids. In 1982 researchers found fossils of early ichthyosaurs and found that they still retained some key features of their land-dwelling ancestors, such as the configurations of the skull and limbs. Motani suggests that based on the configuration of the skull and limbs, ichthyosaurs branched off from the rest of the diapsids near the separation of the two major groups of living reptiles lepidosaurs and archosaurs. The diagram below shows the branching of the diapsid groups:

Figure 5. Rulers of the Jurassic Seas (Motani, 2000)

 

Evolution

Ichthyosaurs diversified very quickly once they appeared (Motani, 2000). They went through many dramatic changes during their evolution, but they generally changed from a more lizard like, slender, body to a larger, tuna shaped body.

  • Early ichthyosaurs were the Charohusaurus ; they appeared in the early Triassic about 245 million years ago and were about 0.5 to 0.7 meters in length. They have no dorsal or caudal fin and were not very large.
  • Mixosaurus appeared about 235 million years ago in the middle Triassic and were about 0.5 to 1 meter in length. This species had an elongated preorbital skull and had a dorsal fin present, but no caudal fin.
  • The largest and most fish like species was the Opthalmosaurus . They appeared during the middle to late Jurassic and lived around 165 to 150 million years ago. They were large and had a tuna shaped body with the fully developed caudal fin or tail fluke.

Figure 6 below shows the general body transition.

Figure 6. Rulers of the Jurassic Seas (Motani, 2000)

To be successful in the water animals need an efficient way of swimming to enable them to catch prey. One of the most important changes that the ichthyosaurs exhibit during their evolution is the change from limbs to flippers. Having all the toes enclosed would have enhanced the rigidity of the flippers, as it does in living whales, dolphins, seals and sea turtles (Motani, 2002). Motani states that the process was long and complex as the following figure shows:

Figure 7. The Ichthyosaur Page (Motani, 2002)

Motani describes the following changes of the limb;

(1) The lower arm bones became shorter and shorter along the family tree, although there are exceptions.

(2) The finger bones also became shorter and shorter, and eventually became disk-shaped.

(3) The number of finger bone increased early in the evolution.

(4) The thumb disappeared at one point, and then additional digits appeared on both sides of the remaining digits.

Every animal has a swimming style that is primarily dictated by its body structure. This can also indicate what marine environment that animal is in. Fossils indicated that the structure of the vertebrae of the ichthyosaurs changed generally from a thin long shape to a thicker shaped vertebrate. Researchers did not know the cause of this until they looked at the swimming styles of sharks.

Early ichthyosaurs were slender, lacked a caudal fin, and had vertebrae that are quite normal in being elongated and cylindrical (Motani, 2002). Cat sharks are also slender and lack the same caudal fin. Motani concluded that the early ichthyosaur swam in the same undulatory style as the cat shark does today. Undulatory swimming means that the animal swims by moving its entire body instead of a tail fin.


As the body of the ichthyosaurs evolved the caudal fin became more and more important. In the early ichthyosaurs locomotion was largely by the aid of the limbs; in the later ones almost exclusively by the aid of the caudal fin (Williston, 2000). The caudal fin was progressively starting to appear as shown in Figure 8 . By the late stage the caudal fin was present and caused a dramatic change in the ichthyosaurs swimming style.

Figure 8. The evolution of the caudal fin.

In later ichthyosaurs the body thickened together with the vertebral column (Motani, 2002). Their vertebrae became disk shaped or "hockey-puck" shaped. According to Motani hockey-puck vertebrae are known only in thick-bodied sharks like great sharks and mackerel sharks. Fish-shaped ichthyosaurs had a caudal fin, and their thick body profile implies that they probably swam like mackerel sharks (Motani, 2002). Instead of moving their whole bodies to swim they swim by moving only their caudal fin. Figure 9 shows the vertebrate changes during the evolution of the ichthyosaurs and Figure 10 shows a hockey-puck shaped vertebrae.

Figure 9. Vertebral changes (Motani, 2002). Figure 10. Hockey puck shaped vertebrae. (Motani, 2002)

 

Ichthyosaur fossils contained sclerotic rings that revealed that they had a very large eye. In fact the largest eye of an ichthyosaur that was measured was about 264 mm across. Sclerotic rings (Figure 11.) are found in animals whose eyes were not spherical. Motani (2000) suggests that the ichthyosaur probably had them to help keep the eyes in shape.

Figure 11. Sclerotic (Motani, 2002).

The larger the size of the eye the more light-sensitive cells and let in more light. A measurement called the f-number is used as an index to rate camera lenses for brightness sensing ability. The lower the f-number is the better the ability seeing in low light settings. Some researchers have estimated that the ichthyosaur had an f-number that was about f/1.1 to f/1.3. Motani and other researchers believe that this large eye may have enabled at least some species of ichthyosaurs were deep divers.

 

Final Thoughts

Even though ichthyosaurs were a very successful animal that lived for more than 155 million years they became extinct before the dinosaurs became extinct. They adapted very successfully to their marine environment by developing flippers, a caudal fin, and perfected a swimming style that allowed them to explore the open ocean, but this still was not enough to enable them to survive. There are many theories as to why they went extinct, but their extinction is still a mystery. The strongest supported theory is that they were just not able to compete with other types of speedy pursuit predators since advanced sharks and other fast swimming predators started to appear at around the same time that the ichthyosaurs disappeared.

 

References

Ellist, R. (2003). Sea Dragons, Predators of the Prehistoric Oceans.

University Press of Kansas , Kawrence , Kansas .

Kazlev, A. (1998). Tethys Sea [Online]. Available:

http://www.palaeos.com/Earth/Geography/Tethys.htm

Kear, B. , Boles, W. (2003). Unusual gut contents in a Cretaceous ichthyosaur.

The Royal Society, 0962-8452 (Paper) 1471-2954 (Online), p. 206 - 208 (10.1098/rsbl.2003.0050) [Online]. Available: http://www.pubs.royalsoc.ac.uk/biologyletters.shtml

Kerr, Tim. (2000). Late Triassic Period, University of Bristol . [Online]. Available:

http://palaeo.gly.bris.ac.uk/Palaeofiles/Triassic/glossary.htm#Diapsid

Motani, R. (2002). The Ichthyosaur Web Page [Online]. Available:

http://www.ucmp.berkeley.edu/people/motani/ichthyo/index.html

Motani, R. (2000). Rulers of the Jurassic Seas . Scientific American

Williston, Samuel (1914). Water Reptiles of the Past and Present.

University of Chicago Press.

Wikipedia (2003). Tetrapods [Online] Available: http://www.wikipedia.com/tetrapods