Ichthyosaur

Convergent evolution is a unique concept in biology which refers to the animals having similar revolutionary niches. An Ichthyosaur is the best example to explain more about this concept. They also have certain similar evolutionary natures which match with the dolphins and Blue Fins present today. The ichthyosaur is a Greek word that means ' fish lizard'. These are considered giant aquatic reptiles. Let's have a detailed knowledge of them.

Origin of Ichthyosaur

Ichthyosaurs were observed 200 million years ago just before finding out the dinosaurs. They also became extinct a few years before the extinction of the dinosaurs. Usually, they can be found from the middle of the Triassic period to the late Cretaceous period. After ichthyosaurs only, Pteranodons, dinosaurs, etc., were observed by scientists. In the early stage of the Triassic period, these are treated as unidentified land reptiles but move to water at times. But later, it is clear that they are giant aquatic reptiles.

Trends in Evolution

The golden age of ichthyosaurs is 200 - 175 million years ago. During this time, several species were observed, like ichthyosaur, which is more important. Some of them are - Mixosaurus refers to mixed lizards, Stenopterygius has solid ear bones, Ophthalmosaurus of four inches, etc. Platypterygius is the ichthyosaurus Jurassic world as the remaining species were extinct by the mid-Cretaceous period. Later on, different reasons were explained for the extension of the ichthyosaurus.

Initially, they were found in 1811 by Mary Anning in Lyme Regis. Later in 1905, John C. Merriam, a scientist at the University of California, found 25 species in central Nevada. Shonisaurus was found in 1977 in the state fossil of Nevada. Then, in 1992, Dr. Elizabeth Nicholls, a Canadian ichthyologist discovered the largest fossil specimen. 

Description of Ichthyosaurs

Ichthyosaurs can go to 4 m in length. All the ketosis may not be seen. Some may grow larger, and some may remain small. They have a head and a toothed snout. They use this snout to increase their speed. This similarity can be adapted from modern tuna. They can swim at a speed of 40 km per hour. Whereas some ichthyosaurs are deep divers, who have shared similarity from modern whales. The reproductive system may not lay eggs similar to the other reptiles because of their speed. Instead, they directly give birth to the live young ones, which other species have proved.

According to Ryosuke Motani, different weights were recorded by different ichthyosaur sizes. For instance, Stenopterygius is around 163 - 167 kgs with a height of 8ft. On the other hand, the Ophthalmosaurus is around 930 - 950 kgs with 13ft height. It is almost a ton. Thus the 

Ichthyosaur size may vary from one to another. It is not constant. Also, Ichthyosaur doesn't have a hard skeleton structure similar to that of the dorsal fin. Instead, Ichthyosaurs had fin-like limbs, which helps to maintain stability and to control direction. The Ichthyosaurs have lungs to breathe air instead of gills like fishes. They have a small tail like a shark with two lobes. They appear like dolphins. All these similarities can give a crystal clear idea that these are the best examples of convergent evolution. 

The Ichthyosaurs have large eyes which help to swim to the in-depth seas as they live in oceans. They have strong teeth and jaws to eat the flesh of some swimming animals like - fish, octopus, etc. They are carnivores, and they won't come and stay on land. So they gave birth in the sea itself. They have four crescent-shaped fins. The ichthyosaur skull has two holes in the rear upper part and two holes behind the eyes.

Generally, they are found in Europe, northern and southern parts of America. 

Structure in Detail

As the ichthyosaurus is known as the top aquatic predator, its body parts help to get that position. So let us explore more about each part.

Eyes-  The ichthyosaurs have large eyes. A bony ring is there to protect those eyes. They have flat and eyeballs. Some studies explained that these bony rings help to maintain the sphere shape of the eyes properly. It reflects that they don't have spherical eyes. As they have large eyes, they can hunt even at night. 

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Another reason behind the eyes of ichthyosaurs is, usually, the fish-shaped body animals need to swim in the water by combining both pushing and pulling activities in parallel. That means somebody's parts may push the water, and somebody's parts may pull the water. Here, in this case, the front body part needs to push the water, which is near to the eyes. The back body part helps to pull the water. So eyes should be safe. The anterior part of the eyeball helps to push the water whereas the posterior part of the eyeball helps to pull the water. The average eye size of an Ophthalmosaurus is around 23 centimeters. And the average eye size of the largest ichthyosaurus is 264mm.

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Flippers-  Several observations are made about the flippers of ichthyosaurs. They are like four things which are used for swimming. Even though the head and shoulders are helpful, these four fins are likely comfortable. The flipper's structure is different for different species of Ichthyosaurs. Some of them have bone in the thumb and some of them don't have a bone in their thumb. The major observations are summarized and listed below.

• The lower arm size may reduce gradually along with the family tree.

• The finger bones are present more in number at the beginning of the evolution.

• Later on, the finger bones, number and size became shorter and shorter.

• The finger bones became disciples gradually.

• After reaching a certain point, the thumb will completely disappear.

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Vertebrate- Several scientists said that the ichthyosaurs have a backbone in the shape of hockey pucks. But the diapsids are exceptional to this shape. As the backbone is typical for all the species, they need a flexible body to swim deeper and deeper. For this purpose, they use a crescent-shaped fin. It helps to improve closing efficiency and acts as an oscillating hydrofoil. Based on the number of vertebrate presents, the thickness of the body can be improved. The thickness may keep on changes and completely one and of times increases to give support to the volume of the body. It completely depends on the vertebrates, which help to create flexibility in the body.

Classification

The ichthyosaurs belong to Kingdom Animalia. Its Phylum and subphylum are Chordata and Vertebrata, respectively. They belong to a Superclass of Tetrapods because of their four limbs, and the class is Reptilia but not dinosaurs. Coming to their subclass, it is Diapsids which include all the reptiles and birds except turtles. 

The Order of Ichthyosauria or Ichthyopterygia had 5 families of marine-adapted reptiles. They are - 

1. Family Shastasaurus

Genus Cymbospondylus is 33 feet (10 m) long with one fin on the back and no fins on the tail. It had four paddle-shaped flippers. They also had sharp teeth in long jaws. They lived during the mid-Triassic period in Northern and Southern America.

Genus Shonisaurus : They are about 49 feet (15 m) long and have a huge body like a whale and four fin-like flippers like a dorsal fin. It also had a tail like fish, and teeth only in the front of the jaws. They lived during the late Triassic period in North America.

2. Family Mixosaurus

Genus Mixosaurus is just  3.25 feet (1 m) long with no fins on the entire body.  It had four paddle-shaped flippers. They lived during the mid-Triassic period in the Northern and Southern parts of America.

3. Family Ichthyosaurus

Genus Ophthalmosaurus is 11.5 feet (3.5 m) long with a bulky body. They have huge eyes, and the bone-lined eye sockets are 4 inches. They are from the late Jurassic period in England, Argentina, and western North America, France, etc.

Genus Ichthyosaurus is a dolphin-like reptile 6 feet (1.8 m) long. They can swim at 25 mph (40 kph). They were found in England, Greenland, and Alberta, Germany, Canada. They lived from the Early Jurassic to the early Cretaceous periods.

4. Family Taeniopterygidae

Genus Stenopterygius are 10 feet (3 m) long. It has similarities with that of dolphins.  It also had large eyes which helped to swim into the in-depth oceans. They lived from the early-mid Jurassic period only in Germany and England.

Genus Platypterygius - It refers to a flat wing. It is almost 23 feet (7 m) long and has a huge weight. They have large eyes. They are from the early Cretaceous period in Queensland, Australia.

5. Family Leptopterygius

Genus Temnodontosaurus is also known as Leptopterygius. They are 30 feet (9 m) in length with narrow flippers. They are unique with torpedo-shaped bodies.  They lived from the early Jurassic period in England and Germany, similar to the previous family species.

Genus Eurhinosaurus - Unlike other Ichthyosaurs, they are also special with an elongated upper jaw. They used it for acquiring food by poking into the sea bed. It was about 6.5 feet (2 m) long) comparatively small. They were found in Germany during the early Jurassic period.

Conclusion

Hence the Ichthyosaurs are aquatic reptiles that are not dinosaurs. They are the best examples for convergent evolution which refers to the phenomenon of sharing certain similarities from various other species. As they have several similarities from several organisms like lungs from turtles, tail from fish, etc. They eat various aquatic animals, and they can dive, swim into deep seas. The extinction of Ichthyosaurs took place in the late Cretaceous period.