Are we all lizard people? Talking about the evolution from "crawling" to mammals: mammals

Text/Yuyu

When we mention the Mesozoic Era, we can always imagine a scene of thriving reptiles in our minds: pterosaurs flying in the sky, dinosaurs running on the ground, ichthyosaurs, plesiosaurs and mosasaurs swimming in the sea, crocodiles lurking in rivers and lakes... The beautiful and broad stage of the earth is crowded with major members of the reptile family, and there seems to be no room for us mammals.

Mammals under the dominance of dinosaurs

However, to our surprise, the mammals of the Mesozoic Era were not monotonous, and their diversity was beyond our imagination. To understand these old predecessors of the Mesozoic Era, we must start with the beginning of mammals - the mammal type.

What are mammals? As the name suggests, mammals have the structure of mammals, but they are not true mammals. The most basic anatomical structure of mammals is considered to be that the lower jaw has only one bone, the dentary, and the lower jaw is connected to the skull through the dentary-squamosal joint. The lower jaw of reptiles is composed of multiple bones, and the lower jaw is connected to the skull through the articular bone - the quadrate bone. You can feel it and find that our jaw bone is a whole piece, while the jaw of reptiles has several bones, and can be separated left and right when swallowing (such as snakes).

Some taxonomic experts believe that true mammals only include the "crown group", that is, the latest common ancestor of all existing mammals, and all descendants of this ancestor. According to this definition, the most primitive mammals are monotremes (platypus). Some fossil groups, although not belonging to the crown group of mammals, have a single mandible, and we call them mammalomorphs. Therefore, mammalomorphs are transitional groups in the evolution of mammal ancestors to true mammals.

Mammal-type phylogeny: red boxes represent mammals, blue boxes represent true mammals

The earliest mammal-like ancestors were synapsids. Synapsids were once considered a group of reptiles and were called mammal-like reptiles because traditional evolutionary taxonomy believed that they were more like reptiles in morphology and structure, that is, they were still in the evolutionary stage of "reptiles".

However, modern systematic classification schools consider that synapsids are distantly related to existing reptiles and form an evolutionary branch with mammals, so they are excluded from the scope of reptiles. Synapsids are actually mammal ancestors with reptilian characteristics.

Some differences between mammals and reptiles summarized in early literature. The reptiles shown in the figure are actually synapsids

Synapsids were once the dominant land mammals in the Permian period, but the mass extinction event at the end of the Permian period caused the Synapsid Empire to collapse. Reptiles took the opportunity to rise and quickly occupied the Earth, while the remaining cynodonts of the Synapsids were forced to move underground, and the former royal family became the lowest class of untouchables.

In the late Triassic period, cynodonts evolved into the earliest mammal-like animals, including Cryptodon, Morganodon, Megalodon and Sinospinodon. Their reproduction method was still egg-laying. Although they could breastfeed their offspring, this breastfeeding method was very primitive. The mothers did not have developed mammary glands and could only secrete milk from sweat glands. The cubs had to lick the milk from their mothers with their tongues.

The amniote evolutionary tree: diagram showing the difference between reptiles and synapsids, and how our ancestors evolved from lizard-like creatures to what we are today

At this time, mammals lived in underground caves to avoid enemies; in order to reduce competition with other animals, they chose to move at night and look for insects for food. Because of this, mammals' night vision ability became stronger and stronger, while color vision became weaker and weaker. Even today, except for primates represented by us humans, many mammals are red-green color blind or even have only black and white vision.

During nocturnal activities, vision is limited, and hearing becomes particularly important. Therefore, mammals have strengthened their hearing organs. Synapsids, like reptiles, have only one ossicle on each side, while mammals have three. Where do the two extra ossicles on each side come from?

As we have mentioned before, the most critical feature of mammals is the mandibular joint composed of dentary and squamosal bones, which replaced the quadrate joint of their ancestors, the articular bones of synapsids and modern reptiles. In the process of evolution from synapsids to mammals, the quadrate and articular bones were no longer useful, so they gradually degenerated and gradually moved toward the ear, eventually evolving into part of the auditory ossicles.

This change started with mammals. However, the articular bones of the original mammals, the quadrate joints, still remained, but they disappeared completely in later mammals. These two bones were completely separated from the mandible and moved to the middle ear.

Comparison of the ear regions of mammals and reptiles

Let's take Morganodon as an example to analyze the characteristics of early mammals. Morganodon lived in the UK during the Triassic period. It belongs to the same order as the Megalodon and feeds on insects. Compared with the synapsids of the Permian period, Morganodon already has more obvious tooth differentiation, which is a characteristic of mammals. We know that reptiles have no differentiated teeth, and all teeth are the same shape. Morganodon used the canine teeth in the front to catch and kill prey, and then relied on the cheek teeth with three cusps in the back row to crush the food.

The teeth of Morganucodon were capable of biting, but unlike today's mammals, which chew by moving their jaws up and down, Morganucodon chewed by moving its jaws forward and backward.

In addition, Morganucodon had a fully developed secondary palate. The secondary palate is a bony hard palate above the tongue (we also have it, we can lick it with our tongues), which is used to separate the nasal cavity from the oral cavity, allowing mammals to chew and breathe at the same time. This structure is dispensable for reptiles that do not chew, but it is necessary for mammals.

Morganucodon teeth

Morganucodon's canine teeth could still be replaced multiple times, but its cheek teeth were replaced only once in a lifetime (from milk teeth to permanent teeth), just like today's mammals. At the same time, the more primitive Sinosphenodon still replaced its teeth multiple times in its lifetime, just like today's reptiles.

The ventral ribs of Morganucodon have degenerated, providing more space for the lumbar spine to move up and down, allowing them to move flexibly in various gaps. At the same time, they have slender limbs, and the hind limbs have the characteristics of mammals, showing an upright gait, allowing them to run quickly, helping them to chase prey and avoid natural enemies, but the forelimbs still retain the original crawling posture.

Due to its nocturnal and burrowing characteristics, the size of Morganucodon has been evolving in a trend of miniaturization, so it may have limited growth. In addition, according to some anatomical analysis, we can know that this animal has endothermy, so its metabolic rate is high and it is a warm-blooded animal. These are all considered to be characteristics of mammals.

Reconstruction of Morganucodon

Entering the Jurassic At the end of the Triassic, another mass extinction occurred. The crocodile's distant relative, Raoisuchus, declined, and the dinosaurs, which had been struggling at the bottom with mammals, rose up, ushering in the famous dinosaur age in geological history. Among the early mammals, more advanced rhesus, columnar toothed beasts and giant skull beasts appeared.

1. Thief Beast

The molars of the thief beast have two rows of vertically arranged tooth tips, and the upper and lower molars bite each other. They have a preliminary grinding function, which is somewhat similar to the wedge-shaped molars of modern mammals. Scientists infer that the thief beast was the earliest mammal-type vegetarian, feeding on seeds and leaves of plants.

The most famous of this group is the megalodon, which was a squirrel-like creature that lived on the ground and had poisonous stingers on its hind legs for defense. The smaller mythical beasts and fairy beasts moved to trees, had grasping forelimbs, and fed on leaves and ginkgo biloba, playing the ecological niche of lemurs in the Jurassic period.

The Lu family's mythical beast (above) and the Song family's fairy beast (below), Zhao Chuang

In the late Jurassic period, a group of the earliest mammalian aerial explorers appeared: Ahodontia, a tree-dwelling mammal that lived 160 million years ago, with slender limbs, flexible toe bones, and the ability to grasp, which shows that this little guy was very good at climbing.

The huge membrane between the limbs and tail of the tree thief indicates that this species has a strong gliding ability. We can imagine that this is an elf that constantly travels through the trees to catch insects and collect ginkgo. When encountering dangerous predators, it will even jump into the air and glide tens or even hundreds of meters to hide in a distant tree, just like today's flying squirrels and sugar gliders.

Jin's Tree Thief Beast, Zhao Chuang

In addition, the middle ear region of the tree thief has formed a relatively complete structure of three ossicles, and two new ossicles have begun to separate from the lower jaw, which is a more advanced feature of the thief than Morganucodon. However, unlike modern mammals, the tree thief has two bones supporting the eardrum, while modern mammals only have one. Scientists speculate that this may have a great relationship with the formation of new jaw joints and the advancement of chewing.

2. Stigmata

Then came the more advanced columnar toothed animals. The body structure of columnar toothed animals was closer to that of mammals, and the cusps on the molars were no longer arranged in a straight line like the previous mammal-type animals, but were formed into an isosceles triangle with a small cusp attached to each cusp. This type of tooth was the prototype of the later grinding wedge-shaped molars, and its working principle was that the cusps of the upper molars fit into the grooves of the lower molars, thereby cutting and grinding food.

Teeth of acanthotherium

It is this complex tooth that allows mammals to easily handle a variety of different foods, thus providing the possibility for the diverse evolution of mammals. However, the molars of the column tooth beast are still relatively primitive, and the degree of fit of the tooth tip is not as good as that of modern mammals.

Haldanetheus of Portugal was a terrestrial column-toothed animal with short hind limbs and well-developed forearms. It was good at digging and burrowing. Its ears were equipped with curved cochlear ducts, indicating that the mammalian ears were further developing and improving.

The dexterous columnar tooth has slender, curved toes and very flexible ankle joints that are good at turning, which are characteristics of its climbing ability. It has shovel-shaped incisors in its mouth, which are very similar to the characteristics of some primates that feed on sap today. Therefore, scientists infer that this is an animal that relies on claws to scratch the bark and suck sap.

Dexterous columnar tooth (top) and digging columnar tooth (bottom)

The toes of the digging columnar tooth tend to shorten, and some joints have even degenerated. The soles of the feet are broad, flat, and slender, the forelimbs are strong, and the head is relatively narrow and long. It is completely different from the dexterous columnar tooth. It may be an animal that is good at digging and lives underground.

The most unique species is Platypus, which looks like an otter but has a flat tail like a beaver, webbed feet, and defensive stingers on the ankles. Its sharp teeth are curved inward and arranged in a straight line. This freak that looks like a combination of a platypus, an otter, and a beaver indicates that it was an animal that was good at swimming and digging and loved to eat fish. These fossils show that mammals in the Mesozoic Era were actually developing towards diversification.

Restoration of the otter-shaped raccoon-tailed beast

The Microcodont has another important feature of mammalian evolution. Compared with the rod-shaped hyoid bones of synapsids and modern reptiles, the Microcodont has a saddle-shaped hyoid bone in the throat, which can connect more surrounding muscles and help improve the efficiency of chewing, transmission and swallowing. With the wide molars with tooth tips and ridges, the Microcodont can efficiently handle delicious insects.

In addition, the saddle-shaped hyoid bone also has more joints, which allows the tongue to move more flexibly. Therefore, scientists infer that the tiny columnar tooth beast has bid farewell to the early mammalian lifestyle of licking breast milk with its tongue and can suck the delicious milk from the mother.

3. Gigantosaurus

The further-developed Hadrocodon on the evolutionary tree was only 3 centimeters long, but had a huge head that was disproportionate to its body, so it was a relatively intelligent animal that could process more complex information. At the same time, the auditory ossicles separated from the mandible, the middle ear region began to form, and the cochlear structure was more advanced than that of the columnar-toothed beast. A series of anatomical features show that Hadrocodon was between the columnar-toothed beast and true mammals, and it was only one step away from mammals to mammals.

The giant skull weighs only 2 grams

It is these weak and insignificant mammal ancestors who, exposed to frost and dew, cut through thorns, struggled to find a way out in dark caves and steep tree canopies, and avoided being hunted by natural enemies, and passed on the incense of synapsids to this day. When we bask in the bright sunshine, drink sweet spring water, breathe fresh oxygen, smell fragrant flowers, and taste delicious fruits, we seem to be the overlords of the earth. Have you ever thought that our ancestors were in constant fear in the dark times?

If one day when everything is quiet and deep in the night, you dream of Godzilla, please don't be surprised, because your genes are now recalling the fear of being dominated by dinosaurs and the humiliation of being imprisoned in a tree hole underground.