Among all the dinosaurs discovered to date, “Spinosaurus” is the most unique and has rewritten many of the established rules regarding theropods.
While its features—such as its elongated skull, crocodile-like teeth, and a body adapted to life by the water—are endless, what captures the imagination most is the giant “sail” on its back, reaching up to about 1.8 meters in height.

Why did one of the largest carnivorous dinosaurs in history, which lived in North Africa during the mid-Cretaceous period (about 112 to 93 million years ago) and is said to have reached lengths of 15 to 18 meters, evolve such a flamboyant structure?
In this article, we will explain in detail the true functions (and the functions it was not suited for) of the Spinosaurus sail, which are becoming clear through the latest research involving biomechanical modeling.

Why Was the “Thermoregulation (Radiator)” Theory Rejected?

One of the long-believed hypotheses regarding dinosaur back sails was that they “assisted in thermoregulation, absorbing or releasing heat.”
This was based on an analogy with the synapsid “Dimetrodon” from about 300 million years ago.

However, this hypothesis was overturned by a paper published in the academic journal “Geological Magazine” in 2016.

Few Traces of Blood Vessels

The neural spines that made up the sail were highly ossified and showed almost no traces of blood vessels.

Lack of Blood Flow

Animals that rely on heat exchange at the body surface possess a massive blood flow just beneath the epidermis, but Spinosaurus fossils lacked this feature.

Because a dense network of blood vessels is essential to function as a radiator or heat pump, the theory that the sail was purely a thermoregulatory organ is currently rejected.

The Most Plausible Interpretation: Its Role as a “Display (Ornamentation)”

So far, the most widely supported theory for the sail’s function is that it served as a visual signal to other dinosaurs, namely an “ornamental trait.”

Even today, prominent visual structures are widely seen, such as the flashy plumage of birds, the giant antlers of deer, and the throat fans (dewlaps) of lizards.
These typically evolve for sexual selection or social signaling.

Appealing to the Opposite Sex and Intimidating Rivals

Because it was visible from a distance whether on land or walking in the water, it was useful for conveying attractiveness to potential mates or assessing the strength of rivals.

Avoiding the Risk of Injury

By showing off its dominance, it may have been used in “ritual combat” to settle disputes without the costs of direct physical conflict.

It is argued that structures like sails develop significantly in species with pronounced differences in appearance between males and females, or in species that exhibit complex social behaviors.

A “Rudder” Underwater? The Semi-Aquatic Lifestyle and Hydrodynamics

To solve the mystery of Spinosaurus, one must look at its entire body, not just the sail.
A 2014 paper in “Science” argued that they were not purely terrestrial, but “semi-aquatic” predators living by the water and underwater.

Fossil Traits Indicating a Semi-Aquatic Nature

High-Density Bones

Assisted in buoyancy control, much like modern penguins.

Webbed or Paddle-Like Feet

Specialized for moving on muddy ground or in shallow waters.

A Tall, Flexible Tail

Adapted for side-to-side undulating propulsion.

An Elongated Snout

Ideal for preying on fish.

Given these traits, it is thought that the sail may have had a “hydrodynamic function” in addition to being an ornament.

Posture Control During Lunges (2021 Study)

A 2021 paper (in the academic journal “Life”) compared the Spinosaurus sail to the drop keel of a boat or the “dorsal fin of a sailfish.”
It argued that when lunging toward fish in the shallows of rivers and lakes, the sail helped maintain balance by reducing rolling and pitching.

Drag During Swimming and Walk Stabilization (2022 Study)

On the other hand, a 2022 paper (in the academic journal “eLife”) suggested through biomechanical modeling analysis that “it did not possess high swimming capabilities like whales or marine reptiles.”
This is because if the entire sail was underwater, it would actually create drag and reduce swimming efficiency.
However, even if its hydrodynamic functions were limited, it is highly likely that it helped “ensure stability” when entering or exiting the water, or when making sharp turns while hunting by walking underwater.

Conclusion: A “One-of-a-Kind Adaptation” Born of Evolution

Current researchers generally agree on the view that the reason for the Spinosaurus sail’s existence “cannot be reduced to just a single function.”

• “Ornamentation (visual signaling)” to mates and rivals
• “Stabilization (hydrodynamic function)” to assist hunting and moving in shallow waters

By fulfilling these multiple complementary roles, it was able to become a one-of-a-kind large predator dominating the rivers and lakes during the Cretaceous period.
Modern biomechanics and computer modeling will likely continue to uncover the process by which evolution integrated form and function, transcending categories like “ornamentation” and “aquatic adaptation.”

Feathered dinosaurs are those believed to have possessed the same quality of plumage as modern birds, belonging primarily to the Theropod group.
Their discovery firmly established the theory that “theropods are the ancestors of birds,” finally resolving the long-standing debate over the origin of birds.
These feathered dinosaurs are sometimes affectionately referred to as “fuzzy dinosaurs,” and their popularity is growing.

The Origin of Feathers Lies in “Insulation”

Feathered theropods are considered a specific group within the Saurischian lineage—one of the three major dinosaur groups—that evolved into birds.

The Role of Early Feathers Was Insulation

The reason why non-flying theropods had feathers is speculated to be due to the high insulating capacity of the plumage.
This provided a significant advantage over other cold-blooded animals by helping them maintain a constant body temperature, enabling them to be active regardless of the season or time of day.

Early feathers were simple, hair-like structures that evolved from the scales of lizards.
Fossils of some Oviraptorids have been found in an incubating posture, indicating that some species had the habit of warming their eggs.

Evolution to Flight

It is believed that these insulating feathers gradually evolved into more complex shapes and ultimately into the form seen in modern birds, where they are used for flight.
Maniraptorans, which chose to stay airborne for extended periods, evolved their feathers into flight feathers (remiges).
They also fused their collarbones to create a structure called the furcula (wishbone), which would eventually be repurposed as a spring to anchor the pectoral muscles necessary for flight.

The Shock of Consecutive Discoveries That Supported the “All Dinosaurs Were Feathered” Theory

Initially, feathered dinosaurs were thought to be confined to a portion of the theropods, but successive discoveries around the world—especially in China and Siberia—dramatically challenged this assumption, providing strong evidence for the possibility that “all dinosaurs had feathers.”

Definitive Evidence in Theropods

The “dinosaur origin of birds” theory was proposed in 1870 but was initially rejected due to the lack of feathered dinosaur fossils.
However, in 1996, a dinosaur fossil with feather traces was discovered for the first time in the layers of Liaoning Province, China, astonishing the world.

Sinosauropteryx

Sinosauropteryx

The first dinosaur discovered with traces of feathers, which clearly showed a filamentous, downy structure.

Color Analysis

In 2010, melanosomes—intracellular organelles containing melanin pigment—were found preserved in the feathers of Sinosauropteryx, revealing that it had reddish-orange feathers along its back and tail.
This suggests that, similar to modern birds, dinosaur feathers may have served a role in display for attracting mates or for intimidation.

Details on Sinosauropteryx here

Shocking Discovery in Ornithischians (Bird-Hipped Dinosaurs)

In addition to the theropod discoveries, the finding of feather traces in the Ornithischian group—a lineage separate from the one leading to birds—became the strongest evidence for the “All Dinosaurs Were Feathered” theory.

Discovery of Kulindadromeus (2014)

Kulindadromeus

This herbivorous dinosaur (Ornithischian), discovered in Siberia, was covered entirely in feathers.
This finding demonstrated that soft plumage was present in groups other than just theropods.

Details on Kulindadromeus here

Bristles of Psittacosaurus (2001)

Psittacosaurus

Hollow, quill-like tubular bristles—different from avian feathers—were found on the tail of Psittacosaurus.

Details on Psittacosaurus here

Regression to a Common Ancestor

These discoveries strongly support the idea that if the acquisition of feather tissue occurred only once in biological history, then dinosaurs must have acquired feathers before the divergence into the Saurischian and Ornithischian lineages.
Therefore, the theory that “the common ancestor of dinosaurs had feathers, and all dinosaurs possessed some form of plumage” is now considered the most likely hypothesis.

The Origin of Wings and the Connection to the Modern Day

Research on feathered dinosaurs has also led to the unraveling of the mystery of how birds acquired the ability to fly.

Ornithomimus Solved the Mystery of Wing Origin

Ornithomimus

Fossils of Ornithomimus discovered in Alberta, Canada, were the first feathered dinosaurs found in North America and greatly contributed to clarifying the origin of wings.

Details on Ornithomimus here

Changes Depending on Growth Stage

Studies of specimens ranging from juveniles to adults suggested that feathers initially covered the entire body, and as the animal matured, the feathers on the arms of the largest adults developed, potentially becoming the origin of the flight wing.

Modern Prosperity

Feathered theropods passed on their characteristics to modern birds.
Birds inherited not only bipedalism but also the feature of having hollow bones, which contributes to a lightweight skeleton.
With approximately 10,000 species, modern birds are more diverse and globally distributed than mammals, suggesting that the prosperity of the dinosaur lineage continues to this day.

The story of feathered dinosaurs teaches us about the resilience of life and the magnificent journey of evolution.

Unlike modern ecosystems, which are filled with diverse omnivores, the Earth during the dinosaur age is believed to have had a clear division between carnivorous and herbivorous diets.

However, among these, a small number of omnivorous dinosaurs did exist.
Even within the theropod group—the archetype of carnivorous dinosaurs, which includes Tyrannosaurus—there were omnivorous species.

Representative Omnivorous Dinosaurs

Struthiomimus

Struthiomimus

This dinosaur had an ostrich-like appearance, and evidence from its stomach contents shows it ate both meat and plants.

Details on Struthiomimus here

Ajancingenia

Ajancingenia

A small dinosaur believed to have eaten hard-shelled seeds and insects.

Details on Ajancingenia here

In addition, some ancient creatures closely related to dinosaurs were also omnivorous.

Confuciusornis

Confuciusornis

An early form of bird believed to have eaten plant seeds and insects.

Gastornis

Gastornis

A giant, flightless bird that reached a height of about 3 meters, thought to have eaten both meat and plants based on the shape of its jaw.

Why Were Omnivorous Dinosaurs So Few?

Why were omnivorous dinosaurs a minor presence on ancient Earth?
The reason is still not fully understood.
However, as research progresses, there is a possibility that dinosaurs previously classified as carnivores or herbivores may be reclassified as omnivorous.
We look forward to future new discoveries.

What kind of herbivorous dinosaurs come to mind?

In this article, we will delve into the appeal of representative herbivorous dinosaurs, ranging from the super-gigantic species—the largest animals in Earth’s history—to those with uniquely shaped bodies.

The Largest Land Animals: Colossal Sauropodomorphs

The epitome of herbivorous dinosaurs is the quadrupedal Sauropodomorpha.

This group includes Seismosaurus (over 40m long) and Argentinosaurus, among others.

Argentinosaurus

These dinosaurs are among the largest land animals in Earth’s history, and their colossal size is truly overwhelming.

Sauropodomorphs are also characterized by their long necks and long tails.
The length of their necks is evidence of the incredibly tall and large plants that grew in ancient times, telling the story of how much food they needed to sustain their gigantic bodies.

Appealing Diversity! Other Representative Herbivorous Dinosaurs

In addition to Sauropodomorphs, there were many diverse groups of herbivorous dinosaurs.

Horned and Dome-Headed Dinosaurs (Marginocephalians)

This group includes the bipedal Pachycephalosaurus and Triceratops, with its armor-like body and two large horns.
Triceratops is particularly popular, known for its distinctive head frill.

Triceratops

Stegosaurs (Plated Dinosaurs)

The most famous is the quadrupedal Stegosaurus, known for the two rows of large bony plates on its back.
Its unique appearance makes it widely recognized by children and adults alike.

Stegosaurus

Duck-Billed Dinosaurs (Hadrosaurs)

This group includes Parasaurolophus, which is characterized by a large crest extending from the back of its head.

Parasaurolophus

Unique Herbivores Covered in Armor

Furthermore, some herbivores had truly unusual appearances.

For example, Euoplocephalus had countless spikes growing on a back that resembled a rigid turtle shell.

Euoplocephalus

This appearance, reminiscent of a heavily armored medieval knight, may have been an evolutionary necessity for self-defense against the predators of the time.

Summary

So, what did you think? Herbivorous dinosaurs were not just big; they were fascinating creatures that underwent diverse evolution.
The next time you look at a dinosaur encyclopedia, be sure to pay attention to their unique characteristics!

When you hear “carnivorous dinosaur,” giant predators like Tyrannosaurus and Allosaurus probably come to mind.

Tyrannosaurus, a representative carnivorous dinosaur

Called “theropods,” they reigned at the top of the terrestrial ecosystem during the Mesozoic Era.

However, the rulers of the ancient Earth were not just the kings of the land.
Surprisingly diverse carnivorous animals existed, including pterosaurs soaring in the sky and marine reptiles lurking in the sea.
In this article, we explain the rulers of the ancient Earth field by field, from the unknown ecology of theropods—the protagonists of the dinosaur era—to the hunters that dominated the sky and sea.

[Land] The Pinnacle of Diversity! The Bipedal Kings “Theropods”

Theropods are a group of reptiles belonging to the order Saurischia.
Their most distinctive feature is “bipedal locomotion” using developed legs.
They underwent unique evolution in both the Northern Hemisphere (Laurasia) and the Southern Hemisphere (Gondwana), thriving all over the world.

Diversification of Size and Diet

Their evolution was not limited to just being “carnivorous.”

Size Diversity

They ranged from massive terrestrial monsters like Tyrannosaurus to agile small dinosaurs like Velociraptor and Stenonychosaurus.

Tyrannosaurus

Velociraptor

Stenonychosaurus

Dietary Diversity

While basically carnivorous, there were species like Spinosaurus that primarily ate fish, and others that changed to omnivorous or herbivorous diets during their evolution.

Spinosaurus

Feathered Dinosaurs and Evolution into Birds

The biggest topic in recent years is the “feathered dinosaurs” discovered in China and Mongolia.
Early feathers are thought to have been used for insulation and display (appealing to the opposite sex), not for flying.
Modern birds evolved from “avialans,” a subgroup of theropods, meaning that birds today continue to live as part of the theropod group.

[Sky] From Giant Pterosaurs to Archaeopteryx! Rulers of the Sky

While theropods fought for supremacy on the ground, the sky was also a hunting ground for carnivores.

Prosperity of Pterosaurs

Dominating the sky were “pterosaurs,” reptiles of a lineage different from dinosaurs.

Pterodactyloids

Large species such as Quetzalcoatlus and Pteranodon.
With huge wings, they flew freely through the sky, hunting prey.

Quetzalcoatlus

Pteranodon

Rhamphorhynchoids

Smaller species that utilized superior maneuverability to dance through the air.

Arrival of Archaeopteryx

In this era, “ancient birds,” considered ancestors of modern birds, also appeared.
A representative example, Archaeopteryx, was a carnivore eating fish and insects, and is a significant existence showing the process of evolution from dinosaur to bird.

Archaeopteryx

[Sea] Shark-Like Figures? Ferocious “Marine Reptiles”

Turning our gaze to the sea, there lived “marine reptiles” that had evolved to resemble modern sharks.

Underwater Hunters

They possessed streamlined bodies like fish, with large mouths and sharp teeth like crocodiles.

Mosasaurus, a representative marine reptile

Feeding mainly on aquatic animals like fish, they dominated the ancient seas as ferocious hunters that let no prey escape.

Summary: A World of Evolution Beyond Imagination

The ancient Earth was a dynamic world where super-sized carnivores, unimaginable today, flourished.

• Land: Theropods that conquered with bipedalism
• Sky: Pterosaurs and ancient birds that spread their wings and soared
• Sea: Marine reptiles that swam freely

By learning about the diverse carnivores of the land, sea, and sky—not just star dinosaurs like Tyrannosaurus and Tarbosaurus—the image of the ancient Earth will come back to life more vividly.

Dinosaurs are broadly divided into two groups, Saurischians and Ornithischians, based on a decisive characteristic: the shape of their pelvis (hip bone).
This classification is extremely important for understanding the diverse ecology and evolutionary paths of dinosaurs.
While the “saurus” included in dinosaur scientific names means “lizard,” academically, it refers to a specific group of reptiles that evolved a unique upright walking gait.

The Two Pillars of Dinosaur Classification: “Saurischians”

Saurischians are characterized by resembling a lizard’s pelvis, with the pubis pointing diagonally forward and down.
This group includes professional carnivores and the largest giants in history.

1. Theropods: Carnivorous Professionals and Ancestors of Birds

Representative theropod dinosaur ‘Tyrannosaurus’

Characteristics

Popular carnivorous dinosaurs such as Tyrannosaurus, Spinosaurus, and Velociraptor belong to this group, and all species are fundamentally bipedal.

The Secret of Evolution

Theropods evolved into birds in later eras.
In that lineage, the pubis tilted diagonally backward, changing into a shape resembling a bird’s pelvis.
Archaeopteryx is also included in this group.

Dietary Diversity

Most are carnivorous, but in recent years, it has been discovered that there were also herbivorous dinosaurs like Jianchangosaurus, indicating a diversity in diet.

Intelligence

Some species, such as Velociraptor, are known to have possessed high intelligence and exhibited social behaviors, such as hunting in packs.

Representative Theropod Dinosaurs

• Tyrannosaurus
• Velociraptor
• Spinosaurus
• Allosaurus
• Giganotosaurus

2. Sauropodomorphs: The Group with the Largest Bodies in History

Representative sauropodomorph dinosaur ‘Brachiosaurus’

Characteristics

This includes herbivorous dinosaurs that walked on four legs with massive bodies, represented by Brachiosaurus and Argentinosaurus.
It is the group containing the largest land animals in history (exceeding 30 meters in length).

Secrets of the Skeleton

It is thought that because they possessed pneumatic bones (bones with hollows) during the process of becoming gigantic, the burden on their legs was reduced.
The long tail functioned to maintain body balance and also served as a weapon for defense.

Process of Evolution

Ancestral species such as Eoraptor were bipedal, but as they increased in size, quadrupedalism later became the mainstream.

Representative Sauropodomorph Dinosaurs

• Brachiosaurus
• Apatosaurus
• Diplodocus
• Argentinosaurus
• Amargasaurus

The Other Pillar of Dinosaur Classification: “Ornithischians”

Ornithischians are characterized by resembling a bird’s pelvis, with the pubis extending diagonally backward parallel to the ischium.
All dinosaurs belonging to this group are herbivorous and possess extremely diverse appearances.

1. Ornithopods: The Duck-Billed Group

Representative ornithopod dinosaur ‘Iguanodon’

Characteristics

Iguanodon and the Hadrosaurus family (duck-billed dinosaurs: Parasaurolophus, Edmontosaurus, etc.) are representative.
They are characterized by long, flat mouths (beaks) like ducks, and they ground down plants to eat using cheek teeth called dental batteries.

Representative Ornithopod Dinosaurs

• Iguanodon
• Parasaurolophus
• Maiasaura
• Edmontosaurus
• Heterodontosaurus

2. Marginocephalians: The Group Characterized by Head Ornaments

Representative marginocephalian dinosaur ‘Triceratops’

Ceratopsians

Represented by Triceratops, they are characterized by large horns and frills (neck shields).
It is believed that the frills also played a role in protecting the neck from attacks by enemies.

Pachycephalosaurs

Groups such as Pachycephalosaurus possess thick, dome-shaped skulls.

Representative Marginocephalian Dinosaurs

• Triceratops
• Protoceratops
• Pachycephalosaurus
• Styracosaurus
• Stygimoloch

3. Thyreophorans: Defense Specialists with Armor and Spikes

Representative thyreophoran dinosaur ‘Stegosaurus’

Ankylosaurs

Represented by Ankylosaurus, they specialize in defense, possessing features such as a club at the end of the tail.

Stegosaurs

Represented by Stegosaurus, they are characterized by having plate-like bones (plates) on their backs.
It is thought that this armor and plates were used for defense and counter-behavior.

Representative Thyreophoran Dinosaurs

• Stegosaurus
• Ankylosaurus
• Gargoyleosaurus
• Kentrosaurus

Definition of Dinosaurs and Standards for Scientific Names and Classification

While the word “dinosaur” is often thought to refer to giant ancient reptiles in general, academically it is very strict.

1. Academic Definition and Upright Walking

Academically, dinosaurs are defined as “the most recent common ancestor of birds and Triceratops, and all its descendants.”
Put more simply, dinosaurs can be thought of as “reptiles that walk upright.”
For example, crocodiles walk with their joints bent, but dinosaurs possess the decisive characteristic separating them from other reptiles: they do not bend their joints outward but extend their legs straight down.

2. Creatures Easily Mistaken for Dinosaurs

Creatures that do not fit the definition of upright walking are distinguished as “non-dinosaur reptiles.”
Pterosaurs such as Pteranodon are a sister group to dinosaurs, and marine reptiles such as Mosasaurus and Plesiosaurus are not included in dinosaurs.

3. Secrets and Changes in Dinosaur Names

The “saurus” attached to part of dinosaur names is the Greek word for “lizard (reptile).”
Also, dinosaur research is constantly advancing, and previous common knowledge may be overturned.
For example, if fossils thought to be different species are found to be the same, they may be unified under the name given first, and the other name may be revoked.
Additionally, since dinosaur body lengths are estimated values based on fossils, the figures may change with repeated research.

The Truth That “Birds” Are Dinosaur Survivors

Modern birds are direct descendants of dinosaurs that evolved from small theropods.
Current taxonomy states that “birds are dinosaurs (theropods) themselves,” and they are included in the academic definition.
There are approximately 10,000 species of living birds, widely distributed around the world—more diverse than us mammals (just under 5,000 species).
Seeing the prosperity of these birds, it can be said that the history of dinosaurs is still continuing.