Dinosaurs, crocodiles, turtles, snakes, and lizards are all members of the robust and ancient class of creatures known as reptiles. With about 12,000 distinct species and a significant presence in the fossil record, this is a very diversified group. Once the dominant land vertebrates in the world, reptiles still occupy just about every single environment outside of the extreme north and south.
Reptiles are primarily an evolutionary categorization. All the species in this class have a common ancestor who lived over 300 million years ago. However, it also has several traits in common. Fundamentally, all reptiles are four-legged animals, or they are descended from four-legged animals (including snakes, which still seem to contain some of the genes needed to make legs). They are also vertebrates that have a spinal cord housed in their backbone.
Reptiles are primarily an evolutionary categorization. All the species in this class have a common ancestor who lived over 300 million years ago. However, it also has several traits in common. Fundamentally, all reptiles are four-legged animals, or they are descended from four-legged animals (including snakes, which still seem to contain some of the genes needed to make legs). They are also vertebrates that have a spinal cord housed in their backbone. Furthermore, the majority of reptiles have the following traits in common:
Except for a few noteworthy exceptions, oviparous, or egg-laying, reproduction is the most common method of reproduction among reptiles. Boas are among the 20% of lizards and snakes that really lay live offspring rather than eggs. These viviparous reptiles pass nutrients from the mother to the young and waste from the mother to the young via a non-mammalian placenta or another mechanism. The primary benefit of viviparous birth is that it shields the eggs in a dangerous environment from predators. However, there is a cost to this delivery procedure because it is demanding on the mother.
The yellow-bellied three-toed skink of Australia is one of just three reptile species that really uses both live birthing techniques and eggs. Its rarity indicates that evolution likely does not favor this intermediate stage. Like any other reptile, the skink lays eggs to hatch into young critters. However, as the embryo grows, the egg thins out until, at delivery, only a little membrane remains. The primary issue with this approach is that there is insufficient calcium in the thin egg shells to support the developing hatchlings.
All known turtle species belong to this order, which is the only one in the subclass Anapsida. The hard shell made of cartilage that protrudes from the ribs and serves as a protective barrier is what makes it unique. The names “turtle,” “tortoise,” and “terrapin” are derived from local vernacular and do not denote any particular biological or taxonomic distinctions.
One of the biggest carnivorous predators on the earth is the crocodile order, which contains all extant alligators, crocodiles, caimans, and gharials. The Crocodilia species, which have a long tail, strong skin, a flattened snout, and rows of big teeth, spend a considerable portion of their life in or near water. The nearest extant cousin of the bird class is this order. This is due to the close ancestry of the Crocodilia forebears with the dinosaurs.
This order was originally quite diversified, having originated between 200 and 250 million years ago during the Triassic Period. But now, it is reduced to a single extant genus, symbolized by the two-species tuatara of New Zealand, which resemble lizards. As a sister group to Squamata, despite their lizard-like look, this order shares basic morphological characteristics with dinosaurs and crocodiles.
All known turtle species are members of the single-order Testudines, which is part of the subclass Anapsida. Its main characteristic is a thick shell made of cartilage that protrudes from the ribs and acts as a barrier of defense. The names “turtle,” “tortoise,” and “terrapin” are affected by geography and do not denote any unique biological or taxonomic distinctions.
When four-legged vertebrates first emerged on land, some 400 million years ago, semi-aquatic amphibious animals underwent the first stages of evolution. Their appearance and behavior were molded by this semi-aquatic existence. However, the first reptiles were able to move out of the water thanks to major improvements in their lungs, bones, and egg composition, which allowed them to spread into a number of unexplored ecological niches around the globe.
It is estimated that reptiles first appeared clearly in the fossil record between 310 and 320 million years ago when wetlands made up a large portion of their environment. The anapsids, distinguished by the lack of a hole in the rear of the skull, were among the first developing reptiles. Diapsids, which include almost all extant reptiles as well as those from the last 250 million years, are distinguished by having two holes close to the rear of the head. Even though they are the only animals currently classed as anapsids, turtles are not strictly speaking “true” anapsids since their solid back skulls most likely evolved later in their evolutionary history.
Over millions of years, the reptile class underwent significant development and change. More than 200 million years ago, in the Late Triassic Period, dinosaurs, crocodiles, and birds—known as archosauromorphs—came into being. They flourished most during the Jurassic Period. Proto-turtles emerged in the same period and have preserved their body form since. True snakes first appeared about 100 million years ago, but squamata most likely began to evolve in the Middle Jurassic, between 160 and 170 million years ago.