Segnosaurus

The Mystery of the “Unclassifiable” Skeleton That Shook the Academic World

When Segnosaurus was discovered, paleontologists were baffled.
This was because its skeleton contained a mix of “contradictions” unthinkable by the common knowledge of the time.

An “Ornithischian” Pelvis Despite Being a Saurischian?

Dinosaurs are broadly divided into “Saurischia” (like Tyrannosaurus) and “Ornithischia” (like Triceratops) based on the shape of their pelvis.
Segnosaurus possessed characteristics of theropods (Saurischia), but both the “pubis” and “ischium” of its pelvis pointed backward.
This is a characteristic originally seen in “Ornithischia.”

For this reason, they were called “Ornithischians pretending to be Saurischians” or suspected of being “descendants of an independent ‘third dinosaur group’.”

Resembling “Prosauropods” That Should Have Been Extinct?

What caused further confusion were the features of the head and teeth.
The small, weak teeth lining its long skull bore a striking resemblance to “prosauropods” (such as Plateosaurus) that were supposed to have gone extinct long ago (Triassic to Early Jurassic).
Because of this, the theory that they were “descendants of prosauropods that miraculously survived until the Late Cretaceous” was seriously debated.

*Currently, these theories have been refuted, and it has been determined that they are “theropods that secondarily evolved to become herbivores (Therizinosaurus family).”
The unique pelvis is considered to be a result of evolution to support the massive internal organs needed to digest plants.

A Hybrid of Carnivore and Herbivore? A Strange, Patchwork Body

The Therizinosauridae, including Segnosaurus, have a unique appearance that looks as if parts from different dinosaurs were patched together.

Sharp Claws and Weak Teeth

Their bodies contained a mix of carnivorous and herbivorous dinosaur parts.

Forelimbs

They possessed three sharp, long claws, a remnant of carnivorous dinosaurs.

Mouth

Lined with 48 small, weak, pencil-like teeth, with a beak-shaped tip for plucking plants.

In the past, there was a “piscivore theory” suggesting they used these claws to catch fish, but currently, based on the shape of their teeth and body structure, they are considered to have been completely herbivorous.

Rare “Four-Toed” Feet

The structure of their feet is also unique.
While many theropods stood on three toes, Segnosaurus walked with all of its wide “four toes” touching the ground.
The hind limbs were broad and sturdy but short; this was a feature not for running, but for supporting their heavy weight.

Name Means “Laid-back”: Position Within the Therizinosaurs

Meaning of Scientific Name and Ecology

The scientific name Segnosaurus means “slow lizard” in Latin.
The pelvis was wide, and the torso was thick and swollen like a barrel.
True to their name, they likely trod the earth with their heavy bodies, moving in slow motion while eating plants.

A Medium-Sized Dinosaur Representing the Group

The total length was approximately 6.5m.
Although small compared to Therizinosaurus which reached 11m, it is a relatively large size within the group and is one of the representative species.
Before the name Therizinosauridae became established, this group was sometimes called “Segnosaurs.”

A complete skeleton has not yet been found, and it still holds many mysteries.
If a definitive fossil is discovered in the future, they may once again become the “eye of the storm” in the paleontological world.