George Arun A
03-03-23.
Dimetrodon is mistaken for a dinosaur more often than any other prehistoric reptile—but the fact is that this creature (technically a type of reptile known as a "pelycosaur") lived and went extinct tens of millions of years before the first dinosaurs had even evolved. Facts about dimetrodon are fascinating.
Mammal-like reptiles is a term used to describe the prehistoric animals that appear to be the reptilian ancestors of mammals. They were the dominant terrestrial animals by the Middle Permian period. The term "mammal-like reptiles" is most commonly used to describe the group Therapsida, although it can be also used more broadly to describe non-mammalian Synapsids. The Cynodonts were the most mammal-like of the Therapsids. Probably the most famous mammal-like reptile is Dimetrodon, which is often wrongly described as a dinosaur. While Dimetrodon is not thought to be a direct ancestor of mammals, it illustrates the differences between the mammal-like reptiles and mammals.
The term "mammal-like reptiles" is not considered a formal one by most experts; technically speaking, mammal-like reptiles were closer on the evolutionary branch to mammals than to reptiles as traditionally defined, as they possessed glandular skin that lacked scales. (Thus they can be better visualized as being "naked lizards", both furless and scaleless.) However, their overall character is more like a modern lizard than a modern mammal, and the distinguishing features are relatively fine ones of internal structure. It is currently unknown whether mammal-like reptiles possessed mammalian characteristics like body hair and mammary glands, as the only real evidence is provided by fossils that to date only suggest differences in skeletal structure.
Classification:
Most paleontologists hold fossilized jaw remains to be the distinguishing feature used to classify mammal-like reptiles and reptiles. The jaw transition is a good classification tool as most other fossilized features that make a chronological progression from reptilian to mammalian follow the progression of the jaw transition. The dentary, or lower jaw, consists of a single bone in mammals, where the lower jaw of modern and pre-historic reptiles consists of a conglomeration of smaller bones.
Mammalian jaw structures are also set apart by the dentary-squamosal jaw joint. In this form of jaw joint, the dentary forms a connection with a depression in the squamosal known as the glenoid cavity. In contrast, all other nonmammalian jawed vertebrates, including reptiles, possess a jaw joint in which one of the smaller bones of the lower jaw, the articular, makes a connection with a bone of the skull called the quadrate to form the articular-quadrate jaw joint. In transitional forms between mammals and reptiles, the jaw joint is composed of a large, lower jawbone (similar to the dentary found in mammals) that does not connect to the squamosal but connects to the quadrate with a receding articular bone.
Over time, mammal-like reptiles, as they became more mammalian and less reptilian, had a secondary palate, separating the mouth and nasal cavity, begin to form. In early mammal-like reptiles, a secondary palate began to form on the sides of the maxilla, still leaving the mouth and nostril connected.
Eventually, the two sides of the palate began to curve together, forming a U-shape instead of a C-shape. The palate also began to extend back toward the throat, securing the entire mouth and creating a full palatine bone. The maxilla is also closed completely. In fossils of one of the first mammal-like reptiles, Eutheriodont, the beginnings of a palate are clearly visible. The later Thrinaxodontid has a full and completely closed palate, forming a clear progression.
Facts:
Not Technically a Dinosaur
Named After Its Two Kinds of Teeth
Used its Sail as a Temperature-Regulation Device
A Close Relative of Edaphosaurus
Walked With a Splay-Legged Posture
Known by Various Names
Males Were Larger than Females.
Shared its Ecosystem with Giant Amphibians
There Are Over a Dozen Named Species
Lacked a Tail for Decades
The Dimetrodon is a genus of predatory carnivores that lived during the Permian period, 280 – 260 million years ago. Although they are sometimes grouped with dinosaurs, these animals were more closely related to mammals than to true reptiles and evolved long before the dinosaurs did. It is believed they went extinct some 40 million years before the dinosaurs.
While this creature looks like a dinosaur, the Dimetrodon is actually a reptile belonging to the order Pelycosauria. This group described mammal-like reptiles and the Dimetrodon had several mammal-like characteristics. The Dimetrodon is thought to have been well adapted to a wide range of habitats as its fossils have been found in Texas and Oklahoma in the US, Nova Scotia, Canada as well as all across Europe. The first fossil was discovered during the 19th century and was named by Edward Drinker Cope in 1878.
It is difficult for scientists to deduce the kind of habitat that Dimetrodons would have preferred as the Earth would have looked a lot different when Dimetrodons were alive to how it looks today. As fossils have been found across North America and Europe, scientists expect that these animals were able to adapt to a variety of habitats ranging from dry, arid regions to wetter areas that were like swamps, with lots of vegetation and water.
The Dimetrodon was cold-blooded like modern-day reptiles and walked on four legs. Its legs sprawled out to the side, and it had a large tail that made up a large proportion of its body length. It is thought that it moved much like crocodiles do today. Fossils suggest that these animals were sexually dimorphic, with the males being larger than the females.
While scientists can’t be sure what these creatures ate, they can be sure that Dimetrodon were meat-eaters. They had large, sharp teeth that were suitable for ripping and tearing at flesh.
It is thought that the Dimetrodon would have laid eggs, but it is near impossible for scientists to know the logistics of this animal’s reproduction.
The Dimetrodon wasn’t the only animal at this time that had a sail. A similar sail evolved independently in the Edaphosaurus, which were distant relatives of the Dimetrodon and both were Pelycosaurs.
Are Humans Descended from the Dimetrodon?
While these animals looked more like reptiles than mammals, it had certain characteristics that relate it to the earliest mammals and make it a distant relative of humans.
When scientists examined the skull of a Dimetrodon they found a single large hole there called the temporal fenestra, which is where some of the lower jaw muscles attach to the skull. What was significant about this finding is that animals that have a single hole in this area are called synapsids, which is the group of vertebrates to which modern mammals belong. In contrast, dinosaurs have two holes in this area and are called diapsids. This important finding showed that the Dimetrodon was not a dinosaur, but instead was a distant relative of modern mammals.
In terms of evolution, the lineages containing the synapsids, such as the Dimetrodon and modern mammals, and those containing the reptiles, such as dinosaurs, split over 324 million years ago from a common lizard-like ancestor. Many early synapsids looked like reptiles so information like this is important to help us decipher how things evolved.
What’s really interesting is that the Dimetrodon belongs to a group of synapsids called the sphenacodontians. This group also contains a distant relative of living mammals that would have looked very similar to Dimetrodon. This means that the Dimetrodon is some distant relative that lived long before the true mammals came about.
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