Piecing Together Its Appearance from Fragmentary Fossils

Currently, the only Abelisaurus fossil discovered is a skull found in Argentina, leaving its complete picture shrouded in many mysteries.

The discovered skull was remarkably massive, measuring 1 meter long and 85 centimeters high.
Inside its mouth was an array of countless small, serrated teeth, and its snout was rounded.
There were ridges above its nose and eyes, and it is thought to have had keratinous horn-like structures when it was alive.

The shape of the skull was short and robust like that of a Tyrannosaurus, and it possessed extremely powerful jaw muscles.
It is said that once it bit into its prey, it could snap its jaws shut with such speed that it allowed no chance for a counterattack.

Possessed extremely powerful jaw muscles

Many researchers believe it is highly likely that Tyrannosaurus and Abelisaurus represent separate lineages that diverged during the evolutionary process.

Its body length is estimated from the skull to be around 6.5 to 9 meters, with a weight of about 3 tons.

Members of the Abelisauridae Family

Fossils of dinosaurs belonging to the family Abelisauridae have been discovered not only in South America but also in Africa and Asia, and include species such as Carnotaurus and Majungasaurus.
The overall appearance of Abelisaurus is inferred from the fossils of these closely related species.

However, experts remain divided on whether the skull discovered in Argentina truly belongs to a fully grown adult Abelisaurus.
New discoveries in the future will hold the key to revealing the full picture of this mysterious dinosaur.

Abelisaurus was given the scientific name “Abelisaurus comahuensis” in 1985 by Argentine paleontologist José Bonaparte and his colleagues.

Unusual Physical Features and Bony Secrets

Acrocanthosaurus, whose scientific name means “high-spined lizard,” possessed a very unique appearance and remarkable physical abilities.

The Mysterious Back Spines and Their Role

The most distinctive feature of this dinosaur was the elongation of its neural spines, which formed a row of spike-like projections up to 60cm long running from its neck to its back in life.

A row of spike-like projections ran from its neck to its back.

While this structure is reminiscent of the sail of Spinosaurus, it is speculated that it was not a true sail but may have been covered in a fleshy membrane or a camel-like hump of muscle.
The projections are thought to have had roles in thermoregulation or as display features to attract mates, but their exact function is still unknown.

Powerful Forelimbs and Shark-like Teeth

Although Acrocanthosaurus was more slender in build than the power-type Tyrannosaurus, its teeth and forelimbs were very formidable.

Teeth

It had sharp teeth and powerful jaws. Because its teeth were prone to breaking, they were constantly and quickly replaced by new ones, much like a modern shark’s. This meant there was no functional problem in securing prey.

It Had Sharp Teeth and Powerful Jaws.

Forelimbs

Compared to other large carnivorous dinosaurs, its hands and arms were robust, and the claws on its fingers were more than sufficient for catching prey.

Hunting Abilities

Acrocanthosaurus did not rely on the “one-hit kill” power like the later Tyrannosaurus but instead hunted by relying on mobility and agility.

Speed

Fossilized tracks suggest that it could run at speeds of up to 40 km/h.
This is a very high level of athletic ability for a large dinosaur.

It Could Run at Speeds of Up to 40 km/h.

Hunting Style

It is believed to have been a swift hunter that chased down prey using its agility.

Fossil Quality

In addition to four well-preserved skeletal specimens, many footprints have been discovered.
These footprints have allowed research to progress significantly, offering clues about its movements while hunting.

With its unique appearance and high physical ability, Acrocanthosaurus dominated the ecosystem of the North American continent during the Early Cretaceous period.

A Discovery That Pushed Back the Clock and Unique Features

The fossils of Afrovenator were initially thought to be from the Early Cretaceous, but a re-examination of the geological strata corrected the dating to the Middle Jurassic.
This was a critically important discovery that showcased the diversity of dinosaurs on the African continent during that time.

While its skeleton resembles that of the Late Jurassic Allosaurus, it was generally more slender and retained more primitive features.
Its skull was low and elongated, with a narrow, pointed snout.
Additionally, the back half of its tail was stiff, and its overall build was quite slim.

Hunting Style and Physical Characteristics

The forelimbs of Afrovenator were robust, and its three sharp, hooked claws were powerful weapons used for grasping and tearing prey.
Its teeth were blade-like and thin, perfectly suited for slicing through meat.

However, because its tibia (shinbone) was shorter than its femur (thighbone), it does not seem to have been a runner type built for high-speed, long-distance chases.
Therefore, rather than relentlessly pursuing prey, it is believed to have hunted primarily through ambushes or surprise attacks.

During the Middle Jurassic, the North African region where Afrovenator lived was a rich floodplain with vast rivers flowing through it.
Fossils of the sauropodomorph Jobaria and a theropod similar to Suchomimus have also been found in the same strata, suggesting that Afrovenator may have preyed on these dinosaurs.

Classification and Close Relatives

Afrovenator is considered to be closely related to Dubreuillosaurus and Eustreptospondylus, both discovered in Europe. Some theories suggest that together with these dinosaurs, it forms a subgroup known as the “Afrovenatorinae.”

A Defense Specialist Covered in Armor

The most prominent feature of Agustinia was its heavy armor that covered its entire body.
Its skin was covered with hard pieces of bone called osteoderms, giving it a robust armor similar to an ankylosaur.

Additionally, its back had two rows of thick, rod-like spikes that reached up to 75cm long, which protected its sides.
These spikes are believed to have been used as a weapon for self-defense against large carnivorous dinosaurs.
It is also suggested that similar thick spines may have protected it from its neck to its tail, which would have made it an extremely rare kind of sauropod.

It’s possible it was protected by thick spikes from its neck to its tail

Mysterious Evolution and Fragmentary Fossils

Currently, only a partial skeleton of Agustinia has been discovered, and its complete appearance remains a mystery.

Initially, based on the fragmented fossils found, it was thought to be a type of titanosaur.
However, after features reminiscent of Diplodocus were found in the fossils, it has now been reclassified as a unique, independent species that evolved by inheriting characteristics from both.

If new fossils are discovered in the future, the full picture of Agustinia may be revealed, showing an even more bizarre and unique appearance than is currently imagined.
Research is highly anticipated to find out how this sauropod with a unique defense system lived in South America during the Cretaceous period.

A Scientific Name with a Storied Fate

This dinosaur was once known by the name Ingenia.
It was described in 1981, named after the Mongolian place where it was discovered, but it was later found that the genus name had already been used for a type of nematode (roundworm).
Because the International Code of Zoological Nomenclature dictates that a species must have a unique scientific name, the original genus name could no longer be used.
As a result, it followed the unusual path of being renamed to its current genus, Ajancingenia, in 2013, 32 years after its discovery.

Distinctive Body and Adaptation to the Desert

Ajancingenia was small, about 1.8m in length, and belonged to a group of oviraptorids that had a relatively small or absent crest.

Habitat

The fossil was discovered in the Barun Goyot Formation in Mongolia, suggesting it was adapted to live in a harsh, semi-arid, desert-like environment with dunes and rivers.

Skeleton

Compared to other oviraptorids, it had robust, short forearms. Its skeleton was lightweight for its overall size, suggesting it was capable of agile movements.

Gigantic Thumb and Enigmatic Diet

The ecology of Ajancingenia is believed to have been omnivorous, based on its unique forelimbs and jaw structure.

Gigantic Thumb

The bone at the tip of its first digit (thumb) was very large, approximately twice the length of the second finger.
This sturdy thumb is thought to have been used to dig for subterranean prey or plant roots, or to crack hard nuts and shells.

Jaws and Mouth

Its head was very short, and its jaws were deeply curved.
Instead of teeth, its toothless beak had bony projections on both the upper and lower jaws.
This is a characteristic suggesting it ate and crushed fairly hard materials like the shells of shellfish.

Reproduction Secret

A fossil of its nest has been discovered, revealing its egg-laying strategy.
The eggs were laid in pairs, arranged helically in three layers, totaling about 24 eggs.

The unique jaw and nesting evidence of Ajancingenia show how oviraptorids adapted to diverse environments and survived the harsh desert conditions.

The Origin of Its Peculiar Name: “Lonely (Unmarried)”

Dinosaurs discovered in China are typically named after the locations where they were excavated, but Alectrosaurus unusually bears a name that does not originate from its discovery site.

Derived from the Greek Word for “Lonely”

The genus name “Alectro” is derived from “ἄλεκτρος” (alektros), an ancient Greek word meaning “lonely” or “unmarried.”

Discovered in Isolation

This name stems from the circumstances of its fossil discovery.
When the fossils were discovered in 1923 by an American expedition team in the Iren Dabasu Formation, the remains of one or two individuals were found completely isolated from one another in different, solitary locations.
Due to this unique situation, it was given a scientific name meaning “lonely lizard” in 1933.

A Slender Build and a “Speed-Oriented” Predatory Style

Fossil characteristics have revealed that it is a primitive tyrannosaur relative, but it possessed features distinct from the typical robust and massive Tyrannosaurus.

Unique Hind Limb Proportions

Analysis of the discovered right hind limb bones revealed that “the femur (thigh bone) and the tibia (shin bone) were the same length.”
For a tyrannosauroid, this indicates a relatively short tibia.

Hunting with Speed

Its overall physique was that of a medium-sized yet slender build.
Therefore, rather than overpowering opponents with brute force, it is believed to have lived a “speed-oriented lifestyle,” utilizing its agility to move swiftly, outmaneuver its prey, and devour their flesh.

Excavation History and the 2025 Taxonomic Update

To date, fossils such as the maxilla (upper jawbone), scapula (shoulder blade), coracoid, and parts of the forelimbs have been discovered in addition to the hind limbs.
Although there is a decent number of specimens, their state of preservation is not particularly good; however, compared to highly mysterious dinosaurs known from only a single tooth, it can be said that they provide a sufficient amount of information.

Furthermore, dinosaur research progresses daily, and it is not uncommon for classifications to be revised.
In fact, other specimens found in the “Bayan Shireh Formation,” which is older than the Iren Dabasu Formation, were long considered to belong to Alectrosaurus.
However, following the latest research, the classification was revised, and in 2025, they were reclassified into a new genus called “Khankhuuluu.”

Alectrosaurus, the “lonely lizard,” etched its name in history as Asia’s first tyrannosauroid.
With its slender, speed-oriented ecology and recent taxonomic revisions brought about by modern research, it remains an incredibly fascinating creature for understanding the evolutionary process of primitive theropods.

Resolving the Evolutionary Contradiction: The “Temporal Paradox”

The discovery of Anchiornis solved a major contradiction within the scientific community.
This was the resolution of the “temporal paradox.”

In the past, Archaeopteryx, considered the oldest known bird, lived about 150 million years ago. However, many of the feathered dinosaurs thought to be its ancestors had only been found in newer rock layers from the Cretaceous period.

The Past Contradiction

Why did the ancestors only exist in a newer era than their descendant (Archaeopteryx)?

The Discovery of Anchiornis

It was discovered in Jurassic strata approximately 10 million years older than Archaeopteryx.

This proved that dinosaurs with bird-like features already existed before the appearance of Archaeopteryx, making the theory of evolution from dinosaurs to birds even more solid.

A Historical First! “Dinosaur Colors” Restored with Scientific Evidence

The biggest reason Anchiornis is world-famous is that it became the “first dinosaur in history to have its entire body coloration scientifically restored.”

In 2010, a research team analyzed traces of feathers preserved in the fossils using an electron microscope and discovered microscopic pigment cells called “melanosomes.”
By comparing their shape and density to those of modern birds, the team successfully identified the dinosaur’s body colors.

The Restored Stylish Colors

• Entire body: Primarily black to dark gray
• Wings (front and hind legs): Bold white and black striped patterns
• Head: Gray base with a striking reddish-brown, crest-like crown of feathers
• Cheeks: Red spots

This discovery strongly suggests that dinosaurs utilized advanced coloration for communication with peers and visual appeals (displays) to attract mates.

The “Glider” Dancing Through the Forest on Four Wings

A major feature of Anchiornis is that it was in a “four-winged” state, possessing well-developed flight feathers not only on its forelimbs but also on its hindlimbs.

While it is believed that they could not achieve powered flight by flapping their wings like modern birds, it is speculated that they skillfully used their hind limb wings for the following purposes:

• “Gliding” when leaping from tree to tree
• Acting as “stabilizer wings” when swooping down to target prey

This discovery supports the evolutionary scenario in which dinosaurs went through a “four-winged gliding stage” before their hind limb wings eventually regressed, transitioning into the “two-winged flight” seen in modern birds.

The Discovery of “Pellets” Revealing Realistic Ecology

Among the hundreds of fossils, there were also fossils of “pellets” (regurgitated masses) that reveal what they ate while alive.

A pellet is a mass of indigestible bones and scales that is regurgitated from the mouth after swallowing prey whole.
Bones of lizards and fish have been found in the pellets of Anchiornis, revealing that they preyed on these creatures near the water or up in the trees.

Furthermore, the behavior of regurgitating unnecessary matter is an adaptive trait to lighten the body and make flying or gliding easier. From this perspective as well, we can see that they were incredibly close to birds.

A Fluctuating Classification: Dinosaur or Bird?

Passionate debates continue among paleontologists today regarding the exact classification of Anchiornis.

The Troodontidae Theory

A common theory suggesting they are phylogenetically close.

The Archaeopterygidae Theory

A leading theory arguing that they should be included in this family because they are extremely close to birds.

The Anchiornithidae Theory

A theory that establishes a new family grouping to include closely related species (such as Caihong).

The sheer number of these debates, with even encyclopedias differing in their classifications depending on the publisher, shows that this creature truly sits on the “boundary line between dinosaurs and birds.”

Anchiornis, whose name means “almost bird,” has helped unravel the mystery of bird origins and showed us the “true colors of a dinosaur” for the first time.

With its four wings spread and its reddish-brown crest fluttering as it glided through the Jurassic forests, this creature can truly be called a “miraculous bridge” connecting dinosaurs and birds.

The Discovery of the “Brave One” and Exceptionally Well-Preserved Fossils

The fossils of Angaturama were discovered in the world-renowned Santana Formation (specifically the Romualdo Formation) in Brazil, a geological site that has also yielded numerous pterosaur and fish fossils.

In February 1996, it was officially described and published by paleontologist Dr. Alexander Kellner and his team, based on the excavated front half of a skull (the snout).
It was given a proud name fitting for a predator that dominated the water’s edge with its imposing physique.

A Crocodile-Like Head and Specialization for a “Piscivorous” Diet

Angaturama underwent a unique evolutionary path that set it apart from typical carnivorous dinosaurs like Tyrannosaurus.

A Narrow Snout to Minimize Water Resistance

Its most prominent feature was its long, slender snout (the area from the tip of the nose to the mouth), which was extremely narrow in width, much like that of a crocodile.

“Conical Teeth” Lacking Serrations

While typical carnivorous dinosaurs had serrations (jagged edges) for tearing flesh, the teeth of Angaturama lacked serrations and were “conical” with vertical ridges on the surface.
This structure closely resembles that of modern fish-eating crocodiles and is specialized for firmly “gripping and piercing” thrashing, slippery fish.

Based on these characteristics, it is speculated that rather than chasing prey on land, they lived a “semi-aquatic” lifestyle, using their swimming abilities to prey primarily on fish near the water, much like modern crocodiles.

The Role of the Back Sail and Head Crest

On its head, it had a crest-like structure known as a “sagittal crest.”
Furthermore, based on the skeletons of closely related species, it is believed to have had a “sail” on its back formed by highly developed neural spines.
Two main theories have been proposed regarding its purpose:

The Thermoregulation (Radiator) Theory

The sail was laced with numerous blood vessels, allowing the dinosaur to raise its body temperature by basking in the sun or lower it by catching the wind.

The Display Function Theory

It was used as a flashy visual display to make the dinosaur appear larger and more attractive during territorial disputes or courtship behaviors.

Paleontology’s Greatest Mystery: The Relationship With “Irritator”

The greatest mystery that cannot be ignored when discussing Angaturama is its relationship with “Irritator,” a very similar dinosaur named in 1996 from the same Romualdo Formation.

Because their era, discovery location, and classification match perfectly, it has long been suggested that “these two might be the exact same dinosaur (the same species).”
However, there is a major dilemma preventing physical proof.

Angaturama

Only the “front half” of the skull has been found.

Irritator

Only the “back half” of the skull has been found.

Because there are no overlapping parts, a direct physical comparison is impossible. Arguing that they would fit together like a puzzle to form a single skull, many suggest treating Angaturama as a “junior synonym” (an invalid alternate name given later) and integrating it into Irritator in encyclopedias and other texts.

On the other hand, a detailed analytical study in 2017 concluded that “these did not come from the same individual,” based on differences in fossil preservation, color, and structure.
In particular, it was pointed out that the snout of Angaturama clearly belonged to a slightly larger individual.

Were Angaturama and Irritator the exact same dinosaur, or were they separate, closely related species coexisting while dividing ecological niches near the water?
A definitive conclusion has yet to be reached.
Precisely because a complete skeleton has not been found, it stirs our imagination, leaving us waiting eagerly for the full picture of this fascinating “brave one” of the wetlands to be revealed.

Discovery and Name Origin: “Jaw Without a Tail”

Well-Preserved Fossils and a Connection to Japan

The fossils of Anurognathus were discovered in the same geological strata as Archaeopteryx.
What is particularly noteworthy is the excellent condition of the discovered fossils.
Despite being an extremely small creature, it was preserved in such pristine condition that there were more than enough features to identify the species.
Interestingly, although the discoverer was a German scholar, he is said to have had deep ties to Japan.

Meaning of the Name

The genus name Anurognathus directly describes its physical characteristics, meaning “jaw without a tail.”
True to its name, its most prominent features were its “short tail” and “unique head,” which were unseen in other pterosaurs of its lineage at the time.

A Taxonomic Paradox: An Advanced Tail and Primitive Wrists

The taxonomic placement of Anurognathus is highly unique, possessing a combination of features that hint at a transitional phase in evolution.

A “Short Tail” Despite Being a Rhamphorhynchoid?

Taxonomically, they are classified under the “rhamphorhynchoids.”
This group, which also includes Dimorphodon—familiar from the Jurassic Park film series—is typically characterized by having a “long tail” with a “diamond-shaped vane” at the end.

However, despite belonging to this group, the tail of Anurognathus is exceptionally short.
The characteristics of its tail resembled the tailbones seen in modern birds or the advanced pterosaurs (pterodactyloids) that appeared much later, such as Pteranodon.

Wrists Remained “Primitive”

At first glance, it looks like an advanced pterosaur, but it had not entirely evolved past its roots.
Their bodies still firmly retained features typical of early pterosaurs like the rhamphorhynchoids, such as short carpal (wrist) bones.
In other words, as a representative pterosaur of the family Anurognathidae, Anurognathus was a peculiar creature where primitive and seemingly advanced features coexisted.

Bat-Like Appearance and Physical Characteristics

Anurognathus was a very small pterosaur.
It is said that its appearance was akin to that of a modern bat.

Size

Its body length was a mere 9 centimeters (3.5 inches), with a wingspan of about 0.5 meters (1.6 feet).

Head

Its skull was tall and short from front to back.
The snout was rounded.

Teeth

Inside its mouth, small, sharply pointed teeth grew straight up, structured to ensure prey could not escape.

Wings

Compared to its body length, its wings were elongated and well-developed, giving it excellent gliding capabilities.

Ecology as an Aerial Hunter

Preying on Insects Like a Nightjar

Given these physical characteristics, Anurognathus is believed to have been insectivorous.
Their hunting style is often compared to that of the modern bird, the “nightjar.”
Utilizing their excellent flight capabilities and agility, they swiftly flew through the air, snatching up flying insects one after another.

“Agility” Over Speed

While their flight capabilities may not have allowed for high maximum speeds, they seem to have been specialized for “agility.”
In addition to the gliding ability provided by their elongated wings, their short-tailed bodies allowed for tight turns in the air.
This ability would have been immensely helpful when hunting evasive insects like butterflies mid-air.

In the skies of Jurassic Germany, Anurognathus—with its adorable tiny body and peculiar “tailless” appearance—played a vital role in the ecosystem.

Filling the “Gap” of North American Sauropods

Sauropods, famous for dinosaurs like Brachiosaurus, first appeared in the Triassic and reached their peak in the Late Jurassic.
During that time, a wide variety of sauropods roamed the Earth, but the situation changed drastically as the era transitioned into the Cretaceous.

The sauropods that had once spread across the globe began to drastically decline, with the exception of species in South America.
In North America specifically, there is a “gap” (a fossil hiatus) where these long-necked dinosaurs seemed to have been relegated to the past.

However, Alamosaurus is the dinosaur that, defying this adversity, took root in North America—the ancient homeland of its ancestors.

Potential for Record-Breaking Size and a Body Shrouded in Mystery

Alamosaurus is classified in the family Saltasauridae within the titanosaurs. It had relatively long forelimbs and a thick neck, giving it a body shape similar to Brachiosaurus.

It had a body shape similar to Brachiosaurus.

For a long time, it was thought to be about the same size as an Apatosaurus, with a total length of 21 meters (69 feet) and a weight of 30 tons.
However, recent fossil discoveries suggest it reached lengths of 30 meters (98 feet) and weights of 50 tons, indicating it may have been one of the largest dinosaurs in history.

Unfortunately, very few Alamosaurus fossils have been found, and no complete skeleton—let alone a skull—has ever been discovered.
As a result, the reality is that its detailed appearance and exact size remain largely unclear even today.

Ecology and Habitat: An Immigrant From South America?

The scientific name Alamosaurus sanjuanensis is derived from the Ojo Alamo Formation in San Juan County, New Mexico, where its fossils were first discovered.

A Unique Habitat and the “Alamosaurus Fauna”

In Late Cretaceous North America, herbivorous dinosaurs like Triceratops and hadrosaurs were the dominant species in the northwest (such as Montana).
As if to avoid competition with them, Alamosaurus was distributed in the more arid, inland plains of the southwest (such as Utah and Texas), where those other dinosaurs were scarce.

It was distributed in the arid inland plains of the American Southwest.

This unique ecosystem is known as the “Alamosaurus fauna,” and fossils of the giant pterosaur Quetzalcoatlus have also been found sharing its skies.
(Despite being “arid,” it is believed that the vegetation was rich and forests were abundant.)

The South American Immigrant Theory

Given its close relatives, its habitat, and the long “gap” (hiatus) where no sauropod fossils are found in North America prior to Alamosaurus, it is widely theorized that its ancestors likely migrated from South America across a temporarily connected land bridge.

An “Invincible” Being With No Natural Enemies

Because Alamosaurus possessed such an extraordinarily massive body, it is believed that fully grown adults had virtually no natural enemies.
As a single individual, it was likely the absolute largest and strongest dinosaur—and animal—on the North American continent at the end of the Cretaceous, presenting a target that even an adult Tyrannosaurus could not easily take down.

Of course, only a tiny fraction of individuals survived to reach that size, and juveniles, young adults, or weakened individuals would certainly have been preyed upon by carnivores like Tyrannosaurus.

A Survivor of the K-Pg Boundary (Mass Extinction)?

One of the greatest mysteries surrounding Alamosaurus is the theory that they “might have survived the great mass extinction.”

In 2011, researchers from the University of Alberta in Canada radiometrically dated an Alamosaurus femur excavated in New Mexico. The results estimated it to be from 64.8 million years ago—700,000 years after the K-Pg boundary—suggesting the possibility that they survived the mass extinction event.

However, many researchers heavily criticize this theory and there are numerous counterarguments, so it will likely be a long time before a clear consensus is reached.

With its massive body and its mysterious ecology as a dinosaur that (perhaps) lived on past the end of the age of dinosaurs, Alamosaurus continues to fascinate researchers today.