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When it comes to the Mesozoic Era, we can always imagine a scene of reptiles thriving, pterosaurs flying in the sky, dinosaurs running on the ground, ichthyosaurs swimming in the sea, plesiosaurs and dragons, crocodiles lurking in rivers and lakes... The beautiful and expansive stage of the earth is packed with members of the reptile family, and it seems that there is no place for us mammals anymore.
Mammals dominated by dinosaurs
However, to our surprise, the Mammals of the Mesozoic Era were not monotonous, and their diversity was beyond our imagination. To understand these mesozoic ancestors, let's start with mammals, mammals.
What is a mammal? As the name suggests, mammals already have the structure of mammals, but they are not real mammals. The most basic anatomical structure of mammals is thought to be that the lower jaw has only one bone, the tooth bone, and the lower jaw and the skull are connected through the tooth bone-scale joint. The reptile jaw is composed of multiple bones, and the lower jaw and the skull are connected by the joint bone- the square bone. You can touch it, our jaw bone is a whole piece, and the reptile chin has several bones, which can be taken apart left and right when swallowed (such as a snake).
Some taxonomic experts believe that true mammals contain only the "crown group", that is, the latest common ancestor of all living mammals, and all descendants of that ancestor. According to this definition, the most primitive mammals are single-porous (platypus). Some fossil groups, although not belonging to the mammalian crown group, already have a single mandible, we call them mammalians. Thus, mammals are transitional taxa in the evolution of mammalian ancestors to true mammals.
Mammalian phylogeny diagram: the red box is mammalian, and the blue box is true mammal
<h1 class="pgc-h-arrow-right" > earliest mammalian class</h1>
The ancestors of mammals were zygomatics. Zygomatics, which used to be a group of reptiles, are called mammal-like reptiles, because traditional evolutionary taxonomy holds that they are more morphologically reptile-like, that is, still in the evolutionary stage of "reptiles".
However , modern systematic taxonomy excludes them from the reptile range , taking into account the fact that homothymes are far from being related to extant reptiles and forming an evolutionary branch with mammals. Zygomatic arches, in fact, are mammalian ancestors with reptile characteristics.
Some of the differences between mammals and reptiles summarized in the early literature, the reptiles shown in the figure are actually homophthalmic
The Hexagons were once the land overlords of the Permian, but the mass extinction at the end of the Permian caused the Hegong Empire to collapse, reptiles took advantage of the opportunity to rise and quickly occupy the Earth, and the remaining canines of the Zygomatics were forced to move underground, and the former royals became the low-level untouchables.
In the late Triassic period, canines evolved the earliest mammals, including the Hidden King, Morganodont, Large Bandodont and Chinese Ceratosaurus. Their breeding method is still laying eggs, although it is already possible to breastfeed their offspring, but this way of breastfeeding is very primitive, the mother beast does not have a developed breast, can only secrete milk from the sweat glands, and the young animals have to lick the milk flowing from the mother's body with their tongues.
The Amniotic Evolutionary Tree: Illustrates the difference between reptiles and zygoids, and how our ancestors gradually changed from the appearance of large lizards to the way they are today
At this time, mammals retreated to underground caves to avoid predators; in order to reduce competition with other animals, they chose to act at night and seek insects for food. Because of this, mammals have stronger night vision and weaker color vision. Even today, with the exception of primates represented by us humans, many mammals are red-green blind or even only have black-and-white vision.
In nocturnal activities, the role of vision is limited, and hearing is particularly important. Thus, mammals strengthen their auditory organs. Like reptiles, zygotes have only one ossicle on each side, while mammals have three. Where did the two extra ossicles on each side come from?
As we mentioned earlier, the most critical feature of mammals is the mandibular joint composed of tooth bone-scale bone, which replaces the joint bone of their ancestors, the joint bone of the arch and the living reptiles, the square bone joint. In the evolution of zygotes into mammals, the square bone and joint bone are useless, and gradually degenerate, and gradually move towards the ear, eventually evolving into part of the ossicles.
This change began with the mammalian class. However, the joint bone of the primitive mammal, the square bone joint, still remains, and later mammals completely disappeared, and the two bones were completely separated from the jaw and transferred to the middle ear.
Comparison of ear areas in mammals and reptiles
Let's take morgan-toothed beasts as an example to dissect the characteristics of early mammals. Morgan toothed beasts lived in areas of England during the Triassic period. Belonging to the same order as the great banded toothed beast, which feeds on insects, morganodonts already have a more pronounced tooth differentiation than the Permian zygomatics, which is characteristic of mammals. We know that the teeth of reptiles are not differentiated, and all teeth are one shape. Morgan toothed beasts use their front canine teeth to catch and kill their prey, and then crush food by relying on the cheek teeth in the back row with three pointed teeth.
Morganodonts' teeth have been able to bite, but unlike mammals today, which use the up and down movement of the jaw to chew, Morganodonts chew by moving their jaws back and forth.
In addition, Morgan-toothed beasts already have fully developed secondary palates. The secondary palate is the bony hard palate above the tongue (we also have it, which can be licked by sticking out the tongue) and acts to isolate the nasal cavity from the mouth so that mammals can breathe while chewing. This structure is dispensable for non-chewing reptiles and necessary for mammals.
The teeth of morgan toothed beasts
Morgan toothed teeth can still be replaced many times, and the posterior buccal teeth have been replaced only once in a lifetime (deciduous teeth for permanent teeth) as in today's mammals. At the same time, the more primitive Chinese sharp-toothed beasts still changed their teeth many times in their lives, just like today's reptiles.
Morgan toothed rods have degenerated, giving the lumbar spine more room to move up and down, allowing them the flexibility to move freely through various crevices. At the same time, they have slender limbs, and the hind limbs already have the characteristics of mammals, showing an upright gait, allowing them to run fast, helping them chase prey and avoid predators, but the forelimbs still retain the original crawling posture.
Due to its nocturnal and burrowing characteristics, morganodonts have been tending to miniaturize and therefore may have limited growth. In addition, according to some anatomical analysis, we can know that this animal has internal heat, so the metabolic rate is high and belongs to the thermostatic animals. These are considered characteristic of mammals.
Morgan tooth beast restoration
< h1 class= "pgc-h-arrow-right" > into the Jurassic</h1>
At the end of the Triassic period, another mass extinction occurred, the crocodile's distant relative, the Laurent crocodile, decayed, and the rise of dinosaurs who struggled with mammals at the bottom of the ladder ushered in the famous era of dinosaurs in geological history. Early mammals, on the other hand, saw the emergence of more advanced thiefs, columnar-toothed beasts, and megacraniums.
1. Thief Beast
The molars of the thief have 2 rows of pointed columns arranged longitudinally, and the upper and lower molars bite each other. It has a preliminary grinding function and is somewhat similar to the grinding wedge molars of living mammals. Scientists deduce that the thief was the earliest vegetarian of the mammalian type, feeding on the seeds and leaves of plants.
This taxon is best known for its megalodons. Megalodon resembles a squirrel, living on the ground, with stingers on its hind legs for defense. The relatively smaller mythical beasts and fairy beasts have moved to live in trees, with forelimbs having a grip function, feeding on leaves and ginkgo biloba, playing the ecological niche of lemurs in the Jurassic period.
Lu Clan Divine Beast (top) and Song Clan Immortal Beast (bottom), Zhao Chuang
In the late Jurassic period, a group of the earliest aerial explorers of the mammalic type emerged: the Aho's tree thief. This is an arboreal mammal that lived 160 million years ago, with slender limbs, flexible toe bones, and the ability to grasp, which shows that this is a little guy who is very good at climbing.
The tree thief has a huge skin membrane between its limbs and tail, indicating that this species has a strong ability to glide. We can imagine that this is an elf who constantly travels among the trees to catch insects and collect ginkgo biloba, and when encountering dangerous predators, it will even jump into the air, glide tens of meters or even hundreds of meters, and hide from the distant trees, like today's shrews and honey bag shrews.
Jin's Tree Thief Beast, Zhao Chuang
In addition, the middle ear area of the tree thief has formed a relatively complete structure of three auditory ossicles, and the two new ossicles have begun to appear separate from the lower jaw, which is a feature of the thief beast's more advanced attitude than the Morgan toothed beast. But unlike living mammals, there are two pieces of bone that support the eardrum, while living mammals have only one, which scientists speculate may have a huge relationship with advances in the formation and chewing of new jaw joints.
2. Column-toothed beasts
Then there are the more advanced cynodonts. The body structure of the columnar tooth is closer to that of mammals, and the tips on the molars are no longer arranged in a straight line like the previous mammals, but are instead arranged in an isosceles triangle by a small tip attached to each of the front and back of a tooth tip. This kind of tooth is the prototype of the later grinding wedge molars, and its working principle is that the tip of the upper molar and the groove of the lower molar fit with each other, thus playing the role of cutting off and grinding food.
The teeth of the columnar toothed beast
It is this complex tooth that allows mammals to easily handle a variety of different foods, thus opening up the possibility of diverse evolution of mammals. However, the molars of the columnar tooth are still relatively primitive, and the tip fit is not as good as that of living mammals.
The Portuguese Haldan is a ground-dwelling columnar-toothed beast with short hind limbs and developed forearms, good at digging and punching holes, and has a curved cochlear tube in the ear, indicating that the mammalian ear is being further developed and perfected.
The dexterous columnar tooth has slender, curved toes, and its ankle joint is very flexible and good at turning, which is a characteristic of being good at climbing. With shovel-like front teeth in its mouth, much like some of today's sap-feeding primates, scientists have deduced that it is an animal that relies on its claws to cut through the bark and suck the sap.
Dexterous Columnodont (Top) and Digger Columnodont (Bottom)
The toes of the excavator columnar tooth beast show a shortening trend, and even degenerate some toe joints, the soles of the feet are wide, flat, slender, the forelimbs are strong, and the head is relatively narrow and long, which is very different from the dexterous columnar tooth beast, which may be a good digging animal, living underground.
The most peculiar species is the otter-shaped tanuki, an otter-like animal with a beaver-like flat tail, webbed feet, defensive stingers at the ankles, and sharp teeth that curve inwards and line up in a straight line. This freak, which resembles a combination of platypus, otters, and beavers, suggests that it is an animal that is good at swimming and digging, and loves to eat fish. These fossils suggest that mammals of the Mesozoic Era were also diversifying.
Restoration of the otter-shaped tanuki
The tiny columnodont has another important feature of mammalian evolution, compared with the rod-like hyoid bone of the toxoplasmic arch and living reptiles, the tiny columnodont has a saddle-shaped hyoid bone at the throat, which can connect more muscles around it, helping to improve the efficiency of chewing, transmission and swallowing. Combined with a wide molar with a tip and ridge, the tiny columnodon can handle delicious insects efficiently.
In addition, there are more joints on the saddle hyoid bone, which allows the tongue to move more flexibly. Therefore, scientists deduced that the tiny columnar toothed beast had bid farewell to the early mammalian way of life of licking breast milk with their tongues and could suck the delicious milk of the mother.
3. Megacranium
The giant skull beast on the evolutionary tree, only 3 centimeters long, has a huge head that is not proportional to the body, so it is a relatively intelligent animal that can handle more complex information. At the same time, the ossicles are separated from the lower jaw, the middle ear area begins to form, and the cochlea structure is more advanced than that of the columnodonts. A series of anatomical features suggest that megalopods are somewhere between columnar-toothed mammals and true mammals, and mammals are only one step closer to mammals.
Giant skull beast, weighing only 2 grams
It is these small, insignificant mammalian ancestors, exposed to frost and thorns, struggling to find a way out in the dark crypts and steep canopy, dodging the repeated hunting of predators, and struggling to pass on the incense of the bow to this day. When we bask in the brilliant sun, drink the sweet spring water, breathe fresh oxygen, smell the fragrant flowers, and taste the delicious fruits, it looks like the overlord of the earth, have you ever thought that our ancestors were afraid in the dark time?
Don't be surprised if you dream of Godzilla one night when everything is silent, because your genes now recall the fear of being dominated by dinosaurs and the humiliation of being imprisoned in a hole in a tree underground.