Why did dinosaurs have both "bipedal" and "quadrupedal" types? | The untold struggles of dinosaurs that "re-acquired" quadrupedalism

Introduction: Bipedalism and Quadrupedalism

This might be sudden, but what is your favorite dinosaur?

"Tyrannosaurus, Velociraptor, Stegosaurus, Parasaurolophus, Triceratops, Brachiosaurus, Pachycephalosaurus..."

You can probably think of many, but if you look at them again, you will notice that there is a mix of bipedal and quadrupedal types. (The quadrupedal ones are almost all herbivores.)
Isn't it a bit strange, considering that almost all mammals, except for humans, are uniformly quadrupedal?

Why did dinosaurs have both bipedal and quadrupedal types coexisting?
Let's look at the trial-and-error evolution of dinosaurs as they became quadrupedal.

Chapter 1: The Challenge of the Forelimbs

It might be surprising, but the massive group known as dinosaurs started with bipedalism.
The starting point for dinosaurs was the ability to walk upright on two legs, evolving from primitive quadrupedal reptiles.
(See past articles below)

After that, within the dinosaur lineage, some individuals repeatedly chose the path of "returning" to quadrupedalism due to factors like body size increase, defense, and feeding habits.
I think you can see that this was a different route from mammals, which evolved on the premise of quadrupedalism from the start.

However, for dinosaurs that had already perfected bipedalism, returning to quadrupedalism was not easy.
The biggest hurdle was how to place the forelimbs on the ground.

When we humans crawl on all fours, we can naturally place our entire palms flat on the floor. Even a baby can crawl by placing their palms on the ground.
What makes this possible is a mechanism called "pronation," where the radius and ulna of the forearm cross, allowing the palm to rotate downward.

Not limited to humans, mammals that walk on their finger pads (digitigrade), such as cats and dogs, use the same mechanism to face their palms toward the ground, allowing them to lower their forelimbs straight down and walk smartly.

Pronation is an ability possessed only by mammals, and dinosaurs, without exception, had hand structures that could not pronate.
The palms were fixed facing inward (the direction where the palms face each other)at all times, and movements like turning the palm over were impossible.
(See past articles below)

In other words, "turning the palm to face the ground to make contact"—something that seems simple from a human perspective—was impossible for dinosaurs.
Because of this, for bipedal dinosaurs to return to quadrupedalism, they needed the ingenuity of "making contact without pronation".

Chapter 2: Three Different Solutions

Quadrupedal dinosaurs found solutions to this problem using different approaches.
Let's look at representative examples: Thyreophora, Ceratopsia, and Sauropoda.

1. Thyreophora: A makeshift approach

Stegosaurus and Ankylosaurus are representative of Thyreophora, which are thought to have needed to transition to quadrupedalism to handle the weight increase from armor and to maintain stability.
However, with hands that could not pronate (palms fixed facing inward), if they lowered their arms straight to the ground, a problem arose where they could only make contact with the tips of their fingers. (Even if they bent their wrists, the hands would point outward from the body in an unnatural posture.)

In response, Thyreophora arrived at a method that did not involve major structural changes, which feels a bit makeshift: spreading the elbows slightly and thrusting the arms forward to press the finger pads onto the ground.
To use an analogy, it is like the image of a push-up where you place your hands in front of your chest and support yourself by pushing with your arms. It might also be similar to the traditional etiquette of "bowing with three fingers on the floor."

For Thyreophora, falling down or being flipped over by a predator would be the most vulnerable state and could be fatal. Therefore, this seemingly makeshift "bracing" posture might have been the best overall choice for Thyreophora, which prioritized stability.
In fact, this basic policy was passed down to the Thyreophora of the end of the Cretaceous period (such as Ankylosaurids).

2. Ceratopsia: A smarter upright posture

Triceratops and other Ceratopsians seem to have forelimbs that approach an upright posture, with an increased sense of structural perfection. This is thought to be for supporting their massive heads and ensuring the mobility needed to utilize their prized horns.
As a result of modifications to the shoulder joint range of motion and bone shape, it became possible to pull the elbows toward the torso and make contact with the finger pads at a position closer to directly beneath the body.
It is the image of a compact push-up posture with the elbows tucked in.

Although the palms themselves still cannot "pronate," they succeeded in getting closer to a more natural quadrupedal gait, similar to an upright posture, by compensating for this limitation through the bending of the elbows and shoulders.

③ Sauropods: Columnar limbs through power play

Brachiosaurus and Titanosaurus and other sauropods needed their forelimbs to have a mechanically strong structure to support their massive bodies. As I mentioned earlier regarding thyreophorans, if you were to lower a non-pronating hand straight down, the tips of the fingers would touch the ground; however, sauropods broke through this by evolving a structure that "stands with the tips of the hands planted on the ground." During the course of evolution, the phalanges (finger bones) degenerated, and it was mainly the metacarpals (the bones in the back of the hand connected to the phalanges) that supported the load. When viewed from above, they were arranged in an arch like an "∩" and firmly connected, and by placing a round, pad-like tissue underneath, they distributed their body weight. To use an analogy, although it is impossible for a human with fingers to replicate, it is like doing push-ups using only the knuckles (the heads of the metacarpals) when you clench your fist.
“stands with the tips of the hands planted on the ground” evolved into a structure like
, mainly the load was supported by the metacarpals (bones of the back of the hand connected to the phalanges). When viewed from above, they were arranged in an arch like an "∩" and firmly connected, and by placing a round, pad-like tissue underneath, they distributed their body weight. It is like doing push-ups using only the knuckles (the heads of the metacarpals) when you clench your fist.

Phalanges and claws degenerated as time went on, and in major sauropods of the Jurassic period, the phalanges had almost completely degenerated, leaving only the parts near the base of the fingers, and the claws had also degenerated, leaving only one claw-like structure on the first digit (thumb). By the time of the Cretaceous titanosaurs, not only the claw of the first digit but all the phalanges had disappeared, and it was common for them to have a smooth, completely columnar shape. Note that while illustrations of sauropods often depict claws on the forelimbs like those of elephants or rhinoceroses, as mentioned above, except for very early sauropods, there were no such claws on the forelimbs, so such depictions are inaccurate. My own icon image of a Brachiosaurus also has claws clearly drawn on its forelimbs. This is a bad example. (A reminder to myself)

Chapter 3: Even when quadrupedal, the basis is bipedal

Dinosaurs, which began as bipeds, were fundamentally based on propulsion centered on the hind limbs, and that basic philosophy was inherited by those that became quadrupedal. Except for some examples like Brachiosaurus, in many cases, the hind limbs were more developed, and the center of gravity was also shifted toward the rear. This tendency was particularly pronounced in giant sauropods, where the forelimbs were dedicated to supporting body weight and maintaining balance, while propulsion was mainly handled by the hind limbs. As evidence of this, the hind limbs continued to have spike-like claws, while the claws on the forelimbs degenerated over time. It is believed that they walked in a way where the forelimbs themselves generated almost no propulsion, but were pushed forward like a billiard ball by the force from the hind limbs.
the hind limbs were more developed in many cases, and the center of gravity was also shifted toward the rear. Especially in giant sauropods, that tendency was pronounced, and the forelimbs were dedicated to supporting body weight and maintaining balance, while propulsion was mainly handled by the hind limbs. As evidence of this, the hind limbs continued to have spike-like claws, while the claws on the forelimbs degenerated over time.
The forelimbs themselves generated almost no propulsion, and they walked in a way where they were pushed forward like a billiard ball by the force from the hind limbs

.

Summary: Diversity born from constraints

For some dinosaurs that started as bipeds and chose the path back to being quadrupeds, the fact that they "could not pronate" was a major hurdle. Even so, they each found unique answers, and in the case of sauropods, they evolved into the largest land animals in Earth's history. The evolution of quadrupedal dinosaurs makes me realize once again the underlying strength that the dinosaur race possessed. There is no such thing as a perfect creature in this world, and there are always inconveniences and compensations due to constraints inherited from ancestors somewhere. Life is desperately trying to survive while overcoming or skillfully avoiding these. If you think that dinosaurs, which lived in the distant past tens of millions of years ago, were no exception, perhaps you can feel them as more familiar beings?
the largest land animals in Earth's history to. The evolution of quadrupedal dinosaurs makes me realize once again
the underlying strength that the dinosaur race possessed. There is no such thing as a perfect creature in this world, and there are always inconveniences and compensations due to constraints inherited from ancestors

somewhere. Life is desperately trying to survive while overcoming or skillfully avoiding these. If you think that dinosaurs, which lived in the distant past tens of millions of years ago, were no exception, perhaps you can feel them as more familiar beings?

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■ List of image credits used

・Article header image:
AI-generated image

・Leopard walking: Leopard (Panthera pardus) (51984659585).jpg, by
Bernard DUPONT, Wikimedia Commons, CC BY- SA 2.5

・Stegosaurus skeletal specimen: Journal.pone.0138352.g001A.jpg by Susannah Maidment et al. & Natural History Museum, London,
Wikimedia Commons, CC BY-SA 4.0

・Triceratops skeletal specimen: LA-Triceratops mount-2.jpg by
User:MathKnight, Wikimedia Commons, CC BY-SA 2.0, Source:File:LA-Triceratops mount-1.jpg (by Allie_Caulfield )

・Argentinosaurus skeletal specimen: Argentinosaurus skeleton, PLoS ONE.png by William Irvin Sellers, Lee Margetts, Rodolfo Aníbal Coria, Phillip Lars Manning,
Wikimedia Commons, CC BY-SA 2.5