One of the plesiosaurs of Jurassic period of Mesozoic era is a simolestes – 6 m marine predator with 1 m skull and pack of teeth on ends of jaws. Earlier it was thought, that animal hunted on large prey and teared off large chunks of meat by rotating, like today’s crocodiles do; but modern studies show, that simolestes’ jaws were not so strong and they ate mostly mollusks, fishes and other small reptiles. They had 2 pairs of paws-fins, with which they made simultaneous or alternating swings, like other plesiosaurs did; modern simolestes’ bones studies show, that they belonged to true plesiosaurs and not to the earlier primitive forms.
Simolestes had short neck, broad jaws and short tail, which make them typical pliosaurs, belonging to plesiosaurs. During life their teeth were positioned much more vertically, then fossilized skull of almost complete skeleton of reptile has; their rotation to outer sides is caused by crushing and deformations of skull during millions of years in Earth‘s crust. Fossils of animals were found in England and France, and also India. For the first time they were described in 1877 year by Richard Lydekker.
Among long-necked plesiosaurus there were animals with diamond-shaped tail fin, similar to one, which flying pterosaursrhamphorhynchus had, for example, seeleyosaurus. They lived on Earth in the beginning of Jurassic period of Mesozoic era; animals had large paws-fins, with which were able to perform alternating or simultaneous swings, achieving high acceleration and maneuverability. Seeleyosaurus ate fish, birds and other small marine reptiles, wherein they themselves were vulnerable to their short-necked relatives pliosaurus with larger jaws, teeth and mass; they had to show agility, intelligence and maneuverability to not become their breakfast. Currently there is no precise data, but maybe many plesiosaurus had the same diamond-shaped tail fin, there’ll be ability to say this for sure, when more fossils with tail skin impressions will be found. At these days seeleyosaurus are known only from 1 almost complete skeleton, which was found in Germany, it contains impressions of this steering adaptation of the animals.
In Triassic period of Mesozoic era lakes, seas and oceans were already inhabited by large reptiles, for example, ceresiosaurus were among them – ancestors of plesiosaurus, which appeared in a later Jurassic period. They reached in length 3 m and already had much of the features of a plesiosaurus : long neck, small head and limbs, almost transformed into the fins. They were fast swimmers and propelled in the water like modern penguins do, chasing small and fast prey. Species name means “Lizard from Ceresio” – this is a name of lake on the south of Switzerland, near which fossils of these reptiles were found for the first time.
Ceresiosaurus lived on Earth in the middle Triassic period 230 – 220 millions of years ago and inhabited small coastal lagoons, living alongside with other marine and landreptiles. Form of their limbs tells, that they were able to move on the ground, but not as effective, as other nothosaurus ( animals species, to which ceresiosaurus belong ) did, and already moved to completely marine lifestyle. For the first time fossils of this animals species were found and described in 1931 year by Bernhard Peyer, later people found other fossils of ceresiosaurus in this area.
In Jurassic seas of Mesozoic era different animals lived : among them there were liopleurodons – reptiles belonging to plesiosaurs, which had teeth like of modern orcas. Their scientific name is translated from Ancient Greek as “having smooth sided teeth“, they grew up to 7 m in length, had mass around 1 – 1.7 tons and were one of the largest marine predators of their times. Modern studies show, that their mouth closing strength was not so big, as modern crocodiles have or replaced then in Cretaceous periodmosasaurs had, but they compensated this with gigantic size and sharp teeth; they moved in water by flapping alternately ( for maneuverability ) or together ( for acceleration ) with both pairs of their flippers-like limbs and flew, like birds, in depths of Mesozoic waters. Such kind of propulsion is not the most energy-efficient, but provides high acceleration and maneuverability, which is needed for ambush predator.
Liopleurodons lived approximately 160 – 150 millions of years ago; their skull length reached 1/5th of their overall length and was 1.2 m maximum in absolute value; they had short neck, which is characteristic for pliosaurus family ( which belongs to plesiosaurus ), this differentiated them from long-necked members of this species and allowed them to hunt on larger preys. Up to current time not much fossils of liopleurodons were found : 1 almost complete skeleton and small amount of fragments, including teeth. Mainly they were found in Europe : in England and France, there are also findings in Russia. For the first time animals of this species were studied and described in 1873 year by Henri Émile Sauvage; later this marine reptiles species was examined again by other paleontologists, performed its reclassification.
Mosasaurs is a group of ancient marine reptiles, that was flourished at the end of Cretaceous period. They occupied free ecological niches after extinction of ichthyosaurs and reduction of plesiosaurs species amount. The smallest of them were 1 m in length and largest grew up to 18 meters; they were the biggest marine predators of their age and hunted on fishes, ammonites ( shelled mollusks ) and other marine reptiles. Their fossils are found around the world, including Antarctic.
Mosasaurs looked like modern day varanus, but were more adapted for marine life and their body was much more streamlined. Skull and lower jaws of mosasaurs were similar to snakes’ ones : they were able to swallow as whole large animals, as snakes do this. Legs of most of them transformed into fins and their tails had upper and lower fins, as sharks and ichthyosaurs had. They swam by moving their tails from side to side and rest of their body remained almost still. Mosasaurs did not laid eggs, like other reptiles : they developed viviparity instead, as ichthyosaurs and modern day dolphins and whales did. They had no gills, so they had to swim to water surface to make air breath. Their hunting tactics was ambush : mosasaurs waited for prey in the hide and made rapid swims to it, when it swam closer to them. Mosasaurs skin scales had diamond form, like snakes have; currently there is not much data about color of their skin, but, presumably, it was dark at the top and light at the bottom to camouflage them in not deep seas waters. Closer to the end of Cretaceous period and whole giant reptiles epoch there were new unique mosasaur species appeared : round toothed forms, that specialized on mollusks shells crushing; fresh-water crocodile alike forms; and very streamlined forms, that moved from ambushes tactics to active and fast swimming in Mesozoic seas. But all mosasaurs went extinct at the end of Cretaceous period, like dinosaurs, ammonites and large amount of other species of animals and plants did.
In Silurian and Devonian periods Earth was inhabited by placodermi ( from Greek words, that mean “plate skinned” ) fishes, their heads and necks and sometimes tail part was covered by bony armor plates. Most of them had no teeth and had only sharpened bony plates near their mouth. Most of them were seas bottom-dwellers, but there were giant carnivores among them, for example, dunkleosteus, which grew up to 11 meters in length and 4 tons in weight; these fishes ate other placodermi, mollusks and other marine animals ( dunkleosteus is thought to be first super-predator on Earth – predator, which hunted on other predators ). There were also plate-skinned fishes, which ate plankton, like modern day whales. In Silurian and Devonian periods placodermi inhabited most of ecological niches and that was their age in Earth’s history.
Plate-skinned fishes are first know organisms on Earth, that developed viviparity, and they are among the first animals, that had jaws. Many of fishes had bone plates rings in eyes, that protected them from damage and from high pressure of water. Their armor was adapted for protection and some species were so heavily armored, that their head looked like armored box with eyes. This shield was very heavy and most of placodermi lived near seas bottom, but there were also fishes with light and streamlined armors, that were able to swim at high speeds in paleozoic seas. First members of this family appeared on Earth at the beginning of Silurian period; at the end of Devonian period all placodermi were went extinct without leaving ancestors. Earlier it was thought, that this happened due to strong competition from bony fishes and first sharks, but new studies show, that they went extinct, due to ecological and climate changes, that were happened on Earth at the end of Devonian and beginning of Carboniferorus periods on Earth.
In the Mesozoic Eracrocodilomorphs lived on Earth; they had lots of simillarities with modern crocodiles, but their body was very adapted for marine life and looked like fishes’ one. Their tails had fins, some of them had limbs, that were transformed into fins, and their body was far more gidrodynamical, than body of modern days land and freshwater living crocodiles. Most of crocodilomorphs was fish hunters, but there were big species, that hunted on other marine reptiles; dakosaurus ( from Greek words, that “biting lizard” ) was one of them. Their teeth was flat and serrated, such form is not adapted for small fishes hunting and was adapted for hunting on big reptiles, that has comparable sizes to dakosaurus themselve. Many of the crocodilomorphs had grooves in skulls, in which salt glands resided. Salt glands is organ, that helps marine animals to remove from themselves excess of salts, that accumulates in them due to constant contact with salt water. For now there are no direct proves, that crocodilomorphs had exactly salt glands in these grooves, but their presence in them is the most probable assumption. Modern day crocodiles are adapted for ambushes tactics on hunting, but crocodilomorphs of mesozoic seas was far more active hunters : their fast body says about this and they had need to swim to surface for air breaths and there is no possibility to do this without leaving cover.
In most of areas of mesozoic seas different species of crocodilomorphs lived on one territory and occupied different ecological niches. Their fossils are found in many countries of the world : in England, France, Switzerland, Germany, Polland, Russia, Argentine and Mexico. Crocodilomorphs was very adapted for living in seas and for now it is not clear whether they oviposited eggs on land or gave birth to already formed posterity. From one side : they were able to move on land on their fins, like turtles ( but their body made them vulnerable to big land carnivores and they were unable to hide in shell, like turtles ); from other side : they were able to develop viviparity, like their contemporaries ichthyosaurus and modern days whales and dolphins. So, untils nests and eggs of crocodilomorphs or evidences of viviparity of crocodilomorphs are found : this question is still open. Species was first described in 1856 year by paleontologist Friedrich August von Quenstedt.
Ichthyosaurus ( from Greek words fish and lizard ) are marine reptiles, that lived in Mesozoic Era, almost in all its time span 250 – 90 millions years ago. Ichthyosaurus reached in length 2 – 20 meters ( members of their family was the biggest marine reptiles, that lived on Earth to date ); their skin was scaleless; they were warm-blooded; and had very big eyes, this says, that ichthyosaurus hunted at night time or dived deep in Mesozoic seas. As all reptiles, they had lungs and had need to swim to surface to make air breathes. They were unable to lay eggs in seas and their body was not adapted for land walking, so they developed viviparity, like modern dolphins have. Ichthyosaurus are very similar to modern whales and dolphins in their shape and way of life, this is the result of convergent evolution, when animals of different species develop similar shapes and way of life in close living conditions. Ancestors of whales and dolphins made the same way, as ancestors of ichthyosaurus did 200 millions years before them.
For now no transitional forms between land reptiles and ichthyosaurus are found : the earliest of know already lived in seas. Previously it was thought, that they originated straight from amphibia, but later studies show, that they originated from reptiles, whose ancestors were amphibia. Ichthyosaurus was the most diverse in late Triassic Period, they occupied lots of ecological niches : from biggest marine predators to small fish hunters. In Jurassic their diversity was still high, but there were no such big species, like in previous period. Probably, this was caused by increased competition from sharks and first plesiosaurs. Previously it was thought, that in early Cretaceous Period amount of ichthyosaurus species abruptly decreased, but new studies show, that they were still diverse at that time. And only at the middle of Cretaceous they went extinct from Earth’s surface.
Ichthyosaurus had 4 limbs with big amount of phalanges, that were completely transformed into fins. Many species had back and tail fins, that were not present in any form in their ancestors – reptiles. Earlier scientists reconstructed ichthyosaurus without back and tail spins, because there are no bones, that support them. But later fossils were found, which contain prints of fins and animals were reconstructed from that time with these parts of bodies. Ichthyosaurus had circle of bone plates in each eye, that protected eyes from high pressure of deep waters. They had the biggest eyes from all vertebrates in both relative and absolute meanings. Their hearing was weakly developed, but smelling was advanced and, probably, they had some kind of bio electricity sensors on surface of their skin. Earlier it was thought, that their skin had colors, like modern dolphins have : dark on top and light at bottom to camouflage them closer to water surface; but new studies show, that they had dark pigment all around surface of their skin. This camouflaged them in deep waters and helped to perform thermoregulation. First fossilized bones of ichthyosaurus were found at the end of 18th century and during 19th and 20th centuries amount of found fossilized bones increased each year.