An Ultimate Guide to Segnosaurus: The Slow Lizard

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Name Meaning“Slow lizard”Height2-2.5 meters (6.5-8.2 feet)
PronunciationSeg-noh-sore-usLength6-7 meters (20-23 feet long)
EraMesozoicLate CretaceousWeight1.3 metric tons (1.4 short tons)
ClassificationDinosauria,‭ Saurischia & TheropodaLocationMongolia, Asia

Segnosaurus Pictures

A modern rendition of Segnosaurus with feathers
A modern rendition of Segnosaurus with feathers | PaleoNeolitic via Wikipedia CC BY-SA 4.0

The Segnosaurus

Gage Beasley Prehistoric's Segnosaurus Concept
Gage Beasley Prehistoric’s Segnosaurus Concept

Having lived in what we now call Mongolia during the Late Cretaceous, approximately 102-86 million years ago, the Segnosaurus is now known mainly as the slow lizard.

The only species in the genus, Segnosaurus galbinensis, was a therizinosaurid in the Theropoda clade.

Most fossils attributed to this prehistoric creature were discovered in the 1970s.

Although no complete skeleton was found, the fossils discovered in Mongolia’s Gobi Desert were enough for scientists to name the species and the genus, as well as outline the appearance, behavior, diet, and evolution of this bipedal dinosaur.

A colored illustration of Segnosaurus
A colored illustration of Segnosaurus | FunkMonk via Wikipedia CC BY 3.0

However, it’s essential to note that many of these aspects have been deduced from details already known about other therizinosaurid species.

Scientists have debated the Segnosaurus classification for years. This is because even the Therizinosaurus, which contains the type species of the Therizinosauridae family, was thought, at first, to have been a turtle-like creature.

As such, it’s not surprising that scientists weren’t sure where the Segnosaurus belonged.

In this article, we aim to describe the species by listing its most interesting details!

Moreover, we’ll share information on how the fossils were discovered and some facts about the animal’s evolution and history.

Keep reading to discover more!

Gage Beasley's Prehistoric Shirt Collection
Gage Beasley’s Prehistoric Shirt Collection
Gage Beasley's Prehistoric Plush Collection
Gage Beasley’s Prehistoric Plush Collection

Physical Characteristics

The Segnosaurus was a large therizinosaur. It measured roughly 6-7 meters (20-23 feet) long and 2-2.5 meters (6.5-8.2) tall.

It had an average weight of 1.3 metric tons (1.4 short tons).

However, if we compare it to the type species of the Therizinosauridae family, Therizinosaurus cheloniformis, we could say it wasn’t that large, as the latter measured around 9-10 meters (30-33 feet) long and roughly 3.96-4.87 meters (13-16 feet) tall!

Gage Beasley Prehistoric's Segnosaurus Size Comparison Chart
Gage Beasley Prehistoric’s Segnosaurus Size Comparison Chart

Besides the average size of the species, fossils, as well as general therizinosaurid characteristics, helped scientists discover the following details about the Segnosaurus:

  • It was likely bipedal and had a robust build.
  • The trunk of the body was likely tilted upward.
  • The head was small and featured a horny beak known as a rhamphotheca.
  • The neck was long and slender.
  • The forelimbs were short and featured three somewhat short fingers with large claws. A study shows that the forelimbs of the Segnosaurus represent 69% of Deinocheirus limbs and 73% of Therizinosaurus limbs.
  • The finger claws were curved, compressed from side to side, and pointed.
  • The belly was large.
  • The hindlimbs were long and robust and featured four toes, of which the first never touched the ground. One unique thing about the hindlimbs of a Segnosaurus is that the metatarsus (the part between the ankle and toes) comprises only 30% of the tibia length, thus being the shortest metatarsus known among theropods.
  • The toes had robust, sharply curved claws.
  • The dentary’s front-most part was deflected downward at an angle of 30 degrees, which hasn’t been observed in other therizinosaurid dinosaurs.
  • The dentary’s hindmost part was toothless.
  • The teeth were leaf-shaped, and the crowns were tall and sideways compressed; the teeth of other therizinosaurids, like Erlikosaurus, for example, were simpler and more symmetrical.

Naturally, scientists wrote numerous research papers that discussed various features of the Segnosaurus mandible and postcranial skeleton.

However, many of the details they discovered belong to what we’d like to call a scientific paleontological handbook and might not be useful to dinosaur enthusiasts.

That’s why we’ve selected only the most important characteristics to help you imagine what the Segnosaurus looked like!

Habitat and Distribution

Segnosaurus fossils were discovered in today’s southeastern Mongolia.

More precisely, they belong to the Amtgay locality of the Gobi Desert.

The formation the dinosaur lived in is called the Bayan Shireh Formation, and it belonged to the Albanian-Santonian ages of the Cretaceous period.

The ecosystem that Segnosaurus inhabited is believed to have had a semi-arid climate and many water sources.

Studies show that the territory had a large meandering fluvial system with rivers connecting directly to the sea.

The formation primarily consists of claystone and sandstone, but gray, red, and brown mudstone and conglomerates are also present.

The remains of the Segnosaurus were discovered in gray sands consisting of gravel, gray claystone, and intraformational conglomerates.

Behavior and Diet

Segnosaurus was initially thought to be a quadruped but was later proved to be bipdal
Segnosaurus was initially thought to be a quadruped but was later proved to be bipdal | Hammerbro123 via Dinopedia

As its common name suggests, the Segnosaurus was thought to have been a slow-moving dinosaur.

This detail is backed up by the unusually large toes and the short, massive metatarsus.

While some researchers proposed a quadrupedal lifestyle for the Segnosaurus, others quickly disapproved of this theory, stating that vertebral characteristics served as evidence of a bipedal lifestyle.

There’s another interesting thing you’ll probably want to know about therizinosaurids.

A study shows that the Segnosaurus might have led a lifestyle similar to that of large terrestrial extinct sloths.

More precisely, they likely used their claws to defend themselves against predators.

Moreover, young Therizinosaurus specimens are thought to have led an arboreal lifestyle.

While there’s no confirmation that the same is valid for the Segnosaurus, we can assume that at least part of this was true for the genus!

A. Perle, in a study about the Erlikosaurus, another therizinosaurid dinosaur, mentions that it might have had a semiaquatic lifestyle, which is why other researchers attribute this amphibious behavior to the Segnosaurus as well.

Segnosaurus near a body of water
A Segnosaurus near a body of water | Scanova via Additional Creatures Wiki

Another study on the endocranial anatomy of Therizinosauria shows that the members of this clade had well-developed senses of smell and hearing, which might’ve been useful while foraging, escaping predators, and communicating with other animals.

As for what the Segnosaurus preferred to eat, this is quite a debated topic.

Some specialists assumed the Segnosaurus was a piscivorous species, but this doesn’t explain the horny beak.

Another assumption is that therizinosaurid dinosaurs, including the Segnosaurus, fed on social insects. In the end, though, most factors indicated that the species had been predominantly herbivorous.

One unique thing about the Segnosaurus is that it probably relied on highly specialized feeding strategies.

This information is carried by the distinctive characteristics of the species’ teeth, which are indicators that the slow lizard possessed a higher degree of oral food processing.

Furthermore, since many scientists compare the Segnosaurus to the Erlikosaurus, its contemporary, they concluded that the former preferred tougher vegetation and used its dentition to feed.

At the same time, the latter exhibited a more flexible feeding strategy and relied on its beak for cropping.

Life Cycle

Fossilized eggs of Segnosaurus
Fossilized eggs of Segnosaurus | Ninjatacoshell via Wikimedia CC BY-SA 3.0

All dinosaurs reproduce by laying eggs. Compared to modern birds, which have only one functional oviduct, thus being able to lay only one egg at a time, female dinosaurs had two oviducts and laid eggs in pairs.

This is valid for the Segnosaurus as well.

Paleontological expeditions discovered colonies of egg nests in the Javkhlant Formation of the Mongolian Gobi Desert.

Researchers found at least 17 egg clutches within the study area, each containing eight spherical eggs.

They are thought to have belonged to therizinosaurid dinosaurs, suggesting that these creatures nested in colonies.

This can be an indication that the Segnosaurus exhibited the same reproductive characteristics.

Another paleontological expedition to China’s Nanchao Formation ended with the discovery of dinosaur eggs with embryos attributed to therizinosaurid dinosaurs.

Paleontologists couldn’t associate any adult dinosaurs with the nests they discovered, thus assuming that, like most dinosaurs, baby therizinosaurs were precocial, meaning they didn’t require adult help to grow.

Segnosaurus lived in groups for protection
Segnosaurus lived in groups for protection | CoreyFord via iStock

As such, we can safely assume that Segnosaurus babies were also precocial.

Some sources suggest that theropods lived slightly less than sauropods, which usually reached 50-100 years.

We can include the Segnosaurus in the same category, assuming it lived for around 40-80 years.

Evolution and History

We’d like to make a long story short regarding this genus’ evolution, history, and classification, but it would be impossible.

There were so many debates about what kind of dinosaur Segnosaurus was and what it evolved from that it took scientists decades to be somewhat sure of the species lineage.

A size comparison of Segnosaurus with other Therizinosaurids
A size comparison of Segnosaurus with other Therizinosaurids | Qilong via Wikipedia CC BY 3.0

The history of the Therizinosauridae family starts with the discovery of the Therizinosaurus fossils in the Nemegt Formation of the Mongolian Gobi Desert.

At first, they were thought to belong to a giant marine turtle, although this was later disproved.

After discovering the first Segnosaurus fossils, Altangerel Perle concluded that the species differed from therizinosaurids and created a new family for it – Segnosauridae.

Nevertheless, that same year, a pelvis analysis showed that the Segnosaurid was, in fact, very different from traditional theropods, so paleontologists proposed to create a completely separate grouping.

However, this hypothesis was later dropped, as scientists concluded that segnosaurids were closer to theropods than sauropods or ornithischians.

Another assumption proposed by Gregory S. Paul was that segnosaurids were, in fact, a mix between prosauropods and ornithischians.

They resembled the former in mandible, snout, and hindfoot morphology and the latter in cheek, pubis, palate, and ankle morphology.

Segnosaurus teeth is unique among theropods
Segnosaurus teeth is unique among theropods | Lindsay E. Zanno​ et al. via Wikipedia CC BY 4.0

The discovery and description of the Alxasaurus genus shed some light on this group of dinosaurs and confirmed that segnosaurids were theropods.

Apart from the numerous issues and debates concerning the evolution and classification of Segnosaurus, there were multiple problems with the fossils belonging to the genus.

Some were damaged or wrongly identified, while others completely disappeared.

For example, scientists who redescribed the holotype mandible mentioned that most tooth crowns lacked tips and had been severely damaged over the years.

Interactions with other Species

It is well known that the Segnosaurus was a contemporary species to Erlikosaurus and Enigmosaurus, two other therizinosaurids whose fossils were discovered in Mongolia’s Bayan Shireh.

Since the three were considered herbivorous, they were likely competing species.

Segnosaurus was often prey for Alectrosaurus during the late Cretaceous within Mongolia
Segnosaurus was often prey for Alectrosaurus during the late Cretaceous within Mongolia | Jaguar123 via Cool Dino Facts Wiki

However, it is thought they learned to “use” their environment in different ways by developing different behaviors and feeding strategies.

This way, they could coexist in the same ecosystem.

Other prehistoric creatures thought to have inhabited the Bayan Shireh Formation are the following:

  • Tyrannosaurs like the Alectrosaurus
  • Ornithomimids like Garudimimus
  • Ankylosaurs like Talarurus
  • Hadrosaurs like Gobihadros
  • Ceratopsians like Graciliceratops
  • Sauropods like Erketu
  • Some crocodyliforms
  • Freshwater molluscs
  • Ostracods
  • Turtles

It is unknown whether the Segnosaurus had any predators.

Cultural Significance

As you’ve probably noticed, the discovery of Segnosaurus fossils represented a major contribution to outlining the evolution and history of therizinosaurs and the clade they belong to, Theropoda.

Holotype mandible of Segnosaurus
Holotype mandible of Segnosaurus | Lindsay E. Zanno et al. via Wikipedia CC BY 4.0

Naturally, scientists are still researching the species, hoping to discover more distinctive details.

Since there’s no complete skeleton and much of the information about Segnosaurus is derived from what paleontologists know about other therizinosaurid species, it can only be hoped that future discoveries in the field will enable further description of the species from various points of view.

Scientists aren’t the only ones concerned with the Segnosaurus, as the species is popular in the entertainment world, especially in video games.

The slow lizard appears in Jurassic Park III: Park Builder and Jurassic World: The Game.

Moreover, it is a toy in the Jurassic Park collection, being one of the therizinosaurs.

Additionally, the Segnosaurus can be seen in Additional Creatures 2: Wild ARK.


The Segnosaurus roamed the Earth about 102-86 million years ago during the Late Cretaceous.

During that time, it “traveled” through Mongolia, looking for suitable plants to eat.

Other therizinosaurs, like Enigmosaurus and Erlikosaurus, probably accompanied it.

The slow lizard was bipedal, slightly feathered, and had a distinctive horny beak and long claws on its forelimbs.

It is believed to have had well-developed senses of smell and hearing and various adaptations that permitted it to coexist with other species.

The discovered fossils were of tremendous significance to understanding our world’s evolutionary history, which is why the Segnosaurus is undoubtedly worthy of any enthusiast’s attention!


Segnosaurus vs T-Rex: who would win?

If a Segnosaurus were to fight a T-Rex, the latter would probably be the winner.

T-Rex was an apex predator and likely had well-developed hunting and attacking techniques that could put down any Segnosaurus.


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