The Oldest Mammals Ever Found

Written by Emmanuel Kingsley
Published: January 9, 2023
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Although every animal class is interesting in its own way, mammals are among the most unique of the lot. To many people, mammals are simply animals that feed their young with milk from mammary glands, but this class of animals has more features than this. For instance, mammals are endothermic, meaning they can regulate their body temperature, unlike reptiles and other cold-blooded animals. Other exciting features include having a backbone and being able to give birth to their young alive in most cases.

Experts believe that mammals developed from members of the reptilian order Therapsida when they first appeared on Earth during the Triassic epoch, between 252 and 201 million years ago. Massive diversity in form and habit has resulted from the evolution of the mammalian class, with thousands of animals belonging to the class. As fascinating as extant species of mammals are, there are so many more extinct mammals with even more exciting features. Here are some of the oldest mammals ever discovered and random facts about them in no particular order:

1. Brasilodon Quadrangularis


Brasilodon had skulls ranging from 0.79 inches to 2.17 inches in length.

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The first animal on this list of ancient mammals is the Brasilodon quadrangularis, an extinct genus of small cynodont-looking animals. Since no complete fossil of this animal has ever been found, experts estimate that its total length was around 4.7 inches (12 centimeters). These animals are believed to have lived during the Norian period of the Late Triassic epoch, about 225.42 million years ago, and spent most of their existence in Brazil, based on the location of their fossils. The Brasilodon was named in 2003 by combining the area where it was found, Brazil, and the Greek word “odon,” which means tooth.

The study of the several tooth growth stages in each of the Brasilodon fossils demonstrated that they were the remnants of a mammal. Due primarily to age differences, the four Brasilodon specimens had significantly variable skull lengths, ranging from 0.79 to 2.17 inches (about 20 to 55 millimeters). Because there were no signs of mammary glands on the incomplete fossils, experts had to use hard tissue, bones, and primarily the teeth of these animals to determine whether or not they were actually mammals. These animals had heterodont teeth, which were divided into canines, incisors, and postcanines. Experts also examined the fossilized teeth of these creatures and found evidence of just one set of replacement teeth, or diphyodonty, a fundamental characteristic of mammals. Brasilodon likely lived in caves like modern shrews during the same period as the first known dinosaurs, which it coexisted with.

2. Morganucodon


Morganucodon was most likely nocturnal and spent the day in a burrow, just like modern mammals of the same size.

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The next animal on this list is the Morganucodon, which was once believed to be the oldest mammal before the discovery of the Brasilidon. This species belonged to an early mammal genus that lived around 205 million years ago, between the Late Triassic and the Middle Jurassic. In Wales, old limestone fissure fillings from the past were where the Morganucodon was initially found in 1949. When it was first discovered, it was the oldest creature known to have a jaw joint made up of the dentary and squamosal bones, a crucial skeletal element that acts as one trait that distinguishes mammals. Almost shaped slightly like larger rats, these animals had really long tails and toes and metatarsals that were flat on the floor when they walked. 

According to research, the skulls of these animals were about two to three centimeters long, and their whole bodies were about four inches, approximately 10 centimeters. Morganucodon was most likely nocturnal and spent the day in a burrow, just like modern mammals of the same size and supposed behavior. Another mammalian trait this animal had was its teeth. It had deciduous teeth that fell out and were replaced with more permanent ones that mostly lasted throughout the animal’s lifetime. Morganucodon likely nursed its young by lactation, given that their young grew rather quickly, and these young ones also did not have any teeth during their infancy. Also, many experts believe that these animals were among the first, if not the first, to raise their young by lactation.

Although the molars of this animal were not precisely shaped like present-day mammals, they still carried out the same functions. The Morganucodon is believed to have been carnivorous, feeding primarily on insects, like beetles and other smaller animals, that existed in its habitat. Another thing that differentiated it from modern mammals, apart from its teeth’ shape and placement, is that it had a slower metabolism and a lifespan like that of reptiles. The oldest species of this animal had a lifespan of around 14 years, and because of this, experts concluded that they were not as endothermic as modern mammals.

3. Monotremes

Weirdest Animal: Echidna

Echidnas fall into the extremely rare category of monotremes – they are quite different from any other mammal as they lay eggs and have no teats.

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Monotremes, belonging to the order Monotremata, are among the oldest mammals. The platypus and echidna are two examples of extremely specialized egg-laying predatory mammals known as monotremes. In addition to birthing their young by laying eggs, members of this order have primordial skeletal traits that have been lost in other living mammals, such as the shoulder girdle and skull characteristics. Because of the perplexing mix of primitive and specialized traits, sometimes known as mosaic evolution, monotremes’ relationships with other mammalian species have proven challenging to ascertain. Some skull characteristics seem to connect monotremes to the extinct early mammal families. Numerous other pieces of evidence, particularly genetic information, categorize the order Monotremata as more complex mammals.

Monotremes have a fast metabolic rate, are endothermic, and have hair on their body. They also generate milk through their mammary glands to feed their young. However, their dental structure is different from that of other mammals. Extant monotremes do not have teeth as adults, while extinct ones and the present young have tribosphenic molars. Their jaws are also constructed differently. According to the dominant school of thought, monotremes descended from a group of mammals that split off from the mammalian family tree more than 200 million years ago and have since evolved separately from the rest of the world’s mammals. To better understand the evolutionary history of these puzzling animals, scientists are currently striving to sequence the platypus genome.

Up Next:

The 10 Oldest Living Mammals

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Oldest Living Animals on Earth Today

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