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Before understanding what and who modern dinosaurs are, we first have to understand where the term dinosaur comes from and what qualifies some creatures to be called so. Apart from birds, who are the actual dinosaurs today since the beginning of time, we have other animals today that are dinosaurs, and since we are in a modern time, that is why they are called “modern dinosaurs.”
Before scientists ever existed, many people around the world would come across some artifacts from the past, such as drawings in ancient caves or bones that did not resemble the morphology of the animals present. Nobody could tell what they were or even what they meant. Even when naturalist William Buckland named the first dinosaur, the Megalosaurus, in 1824, they didn’t have a grip on who or what these creatures were; at the time, they were just seen as large reptiles.
What is a dinosaur?
It wasn’t until British paleontologist and biologist Richard Owen entered the scene that these findings started to make sense. As many dinosaur fossils were discovered in the 19th century, Owen started to look into the discoveries of his peers. He discovered that the skeletons found had similarities in the hip area. His observations showed that these creatures were not reptiles as many scientists thought because they had five fused vertebrae in a part of the hip called a sacrum, which reptiles didn’t have. In his 1842 report, Owen wrote that this was “sufficient ground for establishing a distinct tribe or sub-order of Saurian Reptiles, for which I propose the name Dinosauria.”
And today, we have two general groups of dinosaurs, the first being the Saurischia, who have lizard-like hips; this group consisted of sauropods and theropod dinosaurs. And the second group is the Ornithischia, who are dinosaurs with bird-like hips; this group consisted of horned dinosaurs, armored dinosaurs, and duck-billed dinosaurs. In both cases, these hip and thigh structures gave dinosaurs an upright posture, balanced by their huge legs and tails for some species.
Even though these creatures laid eggs like reptiles, they weren’t. Another point that qualifies one of being a dinosaur is common ancestors. If you take a Tyrannosaurus and hummingbird and trace their lineage back to their first ancestors, they will be found in the dinosaur family and have common traits.
Another point that characterizes one being a dinosaur is time. We all think that all prehistoric animals are called dinosaurs, but this is not true. For example, the Dimetrodon was considered a dinosaur because it is a prehistoric creature, but, this is far from the truth. The Dimetrodon existed 60 million years before the first dinosaurs appeared. When we take a look at its lineage, mammals, including us humans, are more related to this ancient beast than he is to dinosaurs or reptiles. Another case is the Pterosaurs, a cousin of the dinosaurs, whose ancestry departed from the dinosaur lineage by an earlier ancestor.
As more fossil discoveries continue, paleontologists find it more difficult to know who is a dinosaur and who isn’t because these discoveries bring along more peculiar but awesome creatures. To date, paleontologists have found over 1,000 non-avian species.
In an ever-evolving world, theories evolve too. In early 2017 paleontologist, Matthew Baron and his colleagues made some interesting discoveries on Richard Owen’s Dinosauria classification. They found evidence that theropod dinosaurs in the Saurischia (lizard-shaped hips) are closely related to the Ornithisians (bird-like hips). Another point of discovery was that sauropods, which were also classed under the Saurischian group, may be related to Herrerasaurids, a group of ancient carnivorous dinosaurs. As a result of this evidence, Baron and his colleagues named this Ornithiscian-theropod group Orthinoscelida, a term that 19th-century natural Thomas Henry Huxley first used.
Discoveries like this may bring some disarray among fellow paleontologists and geologists but still make for more in-depth analysis, leading to more general information on dinosaurs.
Modern dinosaurs
Crocodiles
Crocodiles share part of their lineage with dinosaurs. They come from a dinosaur group called “Archosaurs” that existed during the early Triassic period. The first crocodilian evolved in the late Cretaceous period, 95 million years ago. The crocodiles we see today are descendants of Deinosuchus, a prehistoric alligator; it had a long snout, a jaw filled with teeth, and a strong tail, the only archosaurs who have survived up to today are birds. So it’s safe to say that the crocodile is closely related to a chicken.
Sharks
Sharks existed way before dinosaurs appeared on the earth. They have witnessed and survived many periods of extinction ever since the earth’s waters were filled with the now-extinct Trilobites (they became extinct before the existence of the dinosaurs). The shark lineage is very, very long, so there were various shapes, sizes, and appearances of sharks. Ancestors of the sharks only started to look like the sharks we see today 450 million years ago, during the Silurian period. And today, we have many species of sharks, such as the hammerhead shark.
The most recognizable ancestor of the shark is the Megalodon.
Cassowaries
Cassowaries are the third largest bird species in the world and can be found in Northern Australia, Indonesia, and Papua New Guinea. They resemble dinosaurs closely because of their three huge, claw-like feet, and many believe they are direct descendants of Velociraptors.
Another physical trait that makes us think of “dinosaurs” is the impressive crest on their heads. This is the origin of their name, “kasu” means horned in Papuan and “weri” means head. This casque is the subject of debate by scientists as some believe it serves the purpose of reducing heat while others think it can emit a sound that is too low for the human ear to pick up.
Lizards
Although lizards fall specifically under the reptile category, if we were to climb up their family tree, we’ll find that lizards and dinosaurs once shared the same ancestry but got divided with time. One of the features that link dinosaurs and lizards was how the limbs protrude from their body and their ability to lay eggs; it’s no wonder Sir Richard Owen coined the term dinosaur literally means “terrible lizard” he did not stray from the origins.
Lizards are very adaptable beings; with time, they have developed different abilities due to circumstances and habitats. I believe this adaptability helped them survive numerous periods of extinction. Today we have lizards that can climb different surfaces, hang from our ceilings, lose and regrow a tail, swim, glide, and even walk on water.
Tuatara
The Tuatara is a perfect example of a living dinosaur in modern times. They are the sole survivor of a genetic lineage from the early Triassic period. They belong to a reptile group called Rhychocephalia that later gave way to other specifies, 240 million and 60 million years ago. Today according to statistics, the Tuatara is their species because, in the group of animals called amniote vertebrates, we’ll find 30,000 species divided into six big groups, which include birds (approx 15,845), lizards and snakes (10,078), mammals (5,416), turtles (341), crocodilians (25) and last but not least Tuatara (1).
Their main diet consists of spiders, snails, beetles, and small birds; they can live up to 100 years and handle cold weather. You can see them off the coast of the islands surrounding New Zealand.
Ostriches
Ostriches are a group of flightless birds called ratites, including kiwis, emus, and cassowaries. According to Ornithologist Peter Houde from the Smith-sonian Institute, small dinosaurs developed into small birds who then migrated into areas where their survival depended on keeping a low profile, so they stayed mostly on the ground.
His theory states that ratites progressed backwards and lost their ability to fly, or else how could you explain the rapid appearance of different islands. Houde’s theory on reverse evolution could also explain why a bird-like ostrich has a reptile-like jaw and thick feathers that may have become this size to defend itself from predators.
Chickens
Chickens share a similar molecular structure with the mighty Tyrannosaurus-Rex; this became evident thanks to research conducted by Jack Horner and Mary Sweitzer in 2003. The pair were struggling to pull out a fossilized T-Rex femur and were led to cut it in half to extract it from the dig. Inside the bone, they found molecules of the structural protein collagen. At the moment, they didn’t have any other dinosaur collagen to compare with; they used the collagen of modern animals such as mice, salmon, and humans. The closest match was with ostriches and chickens; the alligator came in third. These animals look very different but are all linked to a dinosaur genetically.
Sea turtles
Sea turtles belong to the Testudines group, a reptile group consisting of tortoises and terrapins found in salt and fresh water on all continents except for Antarctica. They appeared 230 million years ago and have a common ancestry with dinosaurs. Turtles have existed for quite some time and have shown to be quite resilient as they survived the extinction period at the end of the Triassic period. They saw a new breed of dinosaurs come to life and underwent another extinction period with the disastrous end of the Cretaceous period when dinosaurs became extinct.
After this moment, turtles species began to expand into various groups. And because of this diversification, many scientists are trying to decide whether turtles are closely related to Archosaurs or snakes and lizards. 
