WHAT IS THE CLOSEST THING TO A MODERN-DAY DINOSAUR ?

Dinosaurs are prehistoric reptiles that started existence during the middle to the late Triassic Period of the Mesozoic era about 230 million years ago. They were a subclass of reptiles called Archosaurs; they included a group of birds and crocodiles. Although dinosaurs were wiped out of existence millions of years ago, currently, some animals have a great connection, and they are the closet thing to modern dinosaurs.

Mostly, birds like chicken are considered by scientists to be the closest thing to modern dinosaurs, although some reptiles are also considered to have a connection or common relatives. Some animals of different species are considered for counting dinosaurs as relatives or neighbors. These currently existing animals have a close relationship or have existed in Dinosaur-Era includes crabs, sharks, sea turtles, crocodiles, and birds/chickens.

Are Dinosaurs Reptiles or Birds?

The only animals living today that are thought to have descended from the dinosaurs are birds. Only recently has this theory been accepted by many scientists. About 65.5 million years ago, during what is called the Cretaceous-Tertiary extinction event, all of the dinosaurs went extinct, and it is thought that only the avian ancestors survived as well as many other reptiles and mammalian ancestors. 
Although birds are thought to be the only living dinosaur descendants, some other animals living today are related to the dinosaurs. Dinosaurs are in the reptile group, which includes turtles, crocodiles, birds, lizards, and snakes. Of these groups of animals, second to birds, crocodiles are probably the closest related to dinosaurs (though they are not considered modern dinosaurs themselves, unlike birds). Today, there are 23 known crocodile species and over 10,000 bird species in the world.

So Modern Birds are Dinosaurs? 

The scientific research on the origin of birds within the large groups of species birds could have evolved from is quite vital. Currently, scientists have reached a consensus that Modern birds are a group of maniraptoran theropod Dinosaurs that started their existence during the Mesozoic Era. In the 19th century, there was a proposal of a close relationship between ancient dinosaurs and birds after the discovery of the primitive bird in Germany referred to Archaeopteryx. Many skeletal traits are shared between birds and non-avian dinosaurs. Also, more than 30 fossils of species of non-avian dinosaurs show that the ancient animals had feathers since preserved feathers have been collected. 
There are even tiny dinosaurs, such as Microraptor and Anchiornis, which have long, vaned arm and leg feathers forming wings. The Jurassic basal avialan Pedopenna also shows these long foot feathers.

In 2009, Paleontologist Lawrence Witmer concluded that this evidence is ample to demonstrate that avian evolution went through a four-winged stage. Fossil indication also indicates that birds and dinosaurs shared features such as hollow, pneumatic bones, gastroliths in the digestive system, nest-building, and brooding behaviors. Although the origin of birds has historically been a disputative topic within evolutionary biology, only a few scientists still dispute the dinosaur close relationship and similarities with birds, suggesting descent from other types of archosaurian reptiles. Within the accord that supports dinosaurian ancestry, the exact ordering of evolutionary events that gave rise to the early birds within maniraptoran theropods is disputed.

Does That Mean Evolution has Turned T-rex to Chicken?

Well, maybe that's not too far from the truth. Recently, scientists have uncovered the modern closest relative to the Tyrannosaurus rex, which was the most feared and famous dinosaurs of all time. Researchers, for the first time, have managed to sequence proteins from the Tyrannosaurus rex. This led to the discovery that The protein sequence shows a remarkable similarity between the T-rex and the chicken. The first molecular evidence also shows that birds are the descendants of the modern-day dinosaurs. T-rex was the most fearsome animals that ever existed. However, this king of dinosaurs shares a remarkable DNA sequence with modern-day chickens, which are not terrifying at all. Birds are the most common thought animals to be close descendants of dinosaurs, according to scientists. 

What About Crocodiles?

Birds may be the only new close thing to a dinosaur. However, there are also other species of animals that share similar impressive connections to these ancient animals. Dinosaurs were mostly reptiles, and that group currently includes turtles, snakes, and crocodiles. Although the current group of reptiles has different characteristics compared to the dinosaurs, they share common ancestors. Modern alligators and crocodiles are considered to have unchanged from their ancient ancestors of the Cretaceous period that lived about 145 to 66 million years ago. Therefore, animals that looked like crocodiles also existed along with side dinosaurs millions of years ago. 

And Sea Turtles?

Sea turtles, snakes, and crocodiles are reptiles just like dinosaurs and are mostly referred to as dinosaur cousins. The sea turtles developed and evolved alongside dinosaurs, having emerged as a distinct type of turtle more than 110 years ago. Currently, there are around seven species of sea turtles, and all have an ancient origin. Archelon was the most impressive among these sea turtles that had existed more than 80 million years ago. This species of the sea turtle was more than 4 meters long and almost 5 meters in width. Currently, the Leatherback sea turtle is considered to be the closest living relative of this species.

The Story of Sharks and Dinosaurs

For a long time, sharks have been in existence; they are believed to have emerged more than 450 million years ago, with the modern type of sharks appearing about 1000 million years ago. The descendants of currents sharks swam alongside prehistoric time dinosaurs. Megalodon, which is the biggest predator that lived in water, lived just after the dinosaurs around 23 million years ago and was out of existence about 2.6 million years ago. This descendant of modern-day sharks could weigh up to 103 metric tones and could have been more than 20 meters long. 

And Crabs?

The emergence of crabs first happened during the Jurassic period about 200 to 146 million years ago. They flourished in the cretaceous period; this was just before dinosaurs went extinct. During this time of dinosaurs, the newest species of crabs was the Megaxantho Zogue, which was found in Mexico. This crab was more significant than modern crab and evolved a claw for breaking the shells of its prey. Modern crabs also have this claw, and that evolution was a significant step to the current crabs.