To Ms. Lerna, to anyone who might not be in the know, could you please explain (in a chart) what exactly are Paleo Tyrants and other Retrosaurs and what separates them from the other fauna of their time frame?
Yes, that would be an excellent mental exercise to help me calm down and think more clearly. Let’s do that.
Retrosaurs were (are? I suppose they are now) a diverse group of archosaurs that were closely related to, and possibly even descended from, crocodilians. The name comes from the word retrograde, which means backwards moving, and the Latin word for lizard, “saur,” - “Backwards Lizards,” a cheeky reference to the (unscientific) early belief that these animals were freaks of nature that had since been corrected - a notion that the plethora of fossils we’ve discovered since then disproves. In fact,they were anything BUT a mistake. Retrosaurs dominated the earth during the Mesozoic era, filling almost every niche until their sudden (in geologic time) extinction 65 million years ago. The most famous retrosaurs were megafauna - massive animals that put our modern giants like elephants to shame.
None… none were nearly as massive as the beast I am currently riding, mind you. Not even close.
As I said, retrosaurs were diverse in addition to being successful, filling many niches. At the moment we separate them into ten distinct families, though as with all things in taxonomy, that could be subject to change.
The Primitive Retrosaurs are, as the name suggests, the oldest clade, and the ones that most resemble their modern crocodilian cousins. All members of this clade were quadrupedal, a trait some of their descendants would leave behind. Most also have sprawling gaits, which implies they may be closer to modern day crocodilians than they were to the other archosaurs. However, were set apart from crocodilians by their three toed feet and their fully digitigrade hind legs.
While primitive retrosaurs survived up until the end of the Mesozoic, they were most successful in the Triassic before their more successful descendants would rise up and take the world for their own. The retrosaurs that would rule the Jurassic held their legs straight under their bodies like mammals and birds do today, rather than sprawling to the sides like other reptiles. Retrosaurs were clumsier than the primitive birds that developed alongside them, as their heels were less stable, but they compensated by using their tails for balance. It seemed to work well enough, as they won the arms race against prehistoric avians and went on to rule the rest of the Mesozoic era.
After gaining the ability to stand erect, the family split into two main groups: the “carnivorous” retrosaurs, which include five distinct clades, and the “herbivorous” retrosaurs, which include the remaining four. I put those terms in air quotes because they are technically misnomers, as there are retrosaur species on each side of the divide that are omnivorous.
We’ll start on the carnivorous side of the family because, aheh, it just happens to be on my mind for some reason right now. Most famous and prolific of the carnivorous retrosaurs are the Paleo Tyrants. These stood completely upright on their hindlegs, much like modern day kangaroos. Their muscular and flexible tails provided a counter balance, allowing these reptilian killers to move with great speed while also being able to root themselves to the ground when they needed to. Their forelimbs were used for grasping, and seemed to shrink in size and importance as the clade continued to refine its weaponry throughout the Mesozoic. Their vicious predatory prowess, while highly sensationalized in pop culture, was still formidable - these were powerful hunters that, before their extinction, hunted some of the largest and most heavily armored land animals ever to exist.
The closest relatives to the Paleo Tyrants were the Flying Retrosaurs, who were exactly what they sound like. They had leathery, bat-like wings made from their forelimbs, and short, grasping talons for their hind limbs. Some fossils indicate they may have been partially covered in fur, feathers, or some other downy sort of covering, and some species have beaks; this is yet another case of retrosaurs coincidentally resembling birds and mammals of today.
The most controversial clade of retrosaurs are the Transitional Tyrants, many of whom have been accidentally classified as Primitive Retrosaurs in the past. At some point, some Paleo Tyrants began to adapt to an underwater lifestyle. They slowly became quadrupedal again, their legs became squat, and they took on a more lizardlike appearance. Their resemblance to their primitive ancestors was only skin deep, however. Their legs were still designed to be held directly under their body (though some could alternate and sprawl), and their spines moved with a vertical motion (up and down) instead of a horizontal (side to side). Which retrosaurs belong in this clade, and whether the clade even actually exists, is a subject of passionate debate.
Nevertheless, two notable aquatic clades of retrosaurs eventually evolved from this line: the short-necked and fierce Sea Tyrants, and the elegant, long-necked Paleo Leviathans. Of the two, Sea Tyrants were more prolific and diverse, with some taking on elongated, almost serpentine shapes while other became very squat and very, very large. Paleo Leviathans, on the other hand, had a basic shape that they rarely strayed from: a long neck, a large, barrel-chested body, and either a short or very long tail. They tended to be more peaceful creatures, feeding on schools of fish rather than other megafauna.
Then we get to the other side of the family: the herbivorous retrosaurs. Of these, the earliest clade to evolve were the Long-Necked Behemoths. With short legs, massive torso, and long, serpentine necks and tails, they were majestic in their enormity, and produced the largest animals ever to walk the land (or rather, we thought they did; the beast I’m riding now is at least three times as large as any Long-Necked Behemoth fossil we’ve ever discovered, but it belongs to the Paleo Tyrant clade). While some were amphibious, spending as much time in swamps as they did on land, none were fully aquatic like their Paleo Leviathan cousins on the other side of the family.
Duck Billed Retrosaurs are a hard to place clade, and one of the reasons that paleontologists aren’t quite sure when the herbivorous and carnivorous parts of the family split. Their bipedal locomotion made us believe they might have split from the Paleo Tyrant clade, but the shape of their hips and jaws have more in common with Long Neck-Behemoths, which suggests they developed bipedal locomotion through parallel evolution. Their bills only resembled a duck’s on the surface, as underneath them were many rough teeth they used to grind plant matter into a digestible pulp. Some members of this clade had armored skulls and necks, which gave them a well-needed defensive weapon against their Paleo Tyrant adversaries.
While the Long-Necked Behemoths survived till the K-T extinction, they were slowly losing niches to their own descendants as the Jurassic turned to the Cretaceous. Of these descendants, the Horned Behemoths were the most successful. Standing low to the ground and sporting a variety of long cranial horns and shield-like frills, this diverse group proved the perfect challenge for the increasingly deadly Paleo Tyrants, as they were armed with just as many weapons as their predatory foes. The Armored Behemoths weren’t quite as successful, but they still carved out a niche for themselves, since very few predators could get through their thick scales and boney armor. These two clades were caught in an ever-escalating evolutionary arms race with the Paleo Tyrants all throughout the Cretaceous, with each generation growing thicker hides and crueler weapons up until their untimely extinction.
Retrosaurs were utterly bizarre, unfathomably violent, and astounding beyond belief. While they are romanticized to mythic proportions nowadays, studying them is a fascinating look back at a world that no longer is - one that is familiar and yet utterly alien. I have been fascinated with this subject all my life in part because of that distance - of the fact that these horrifying yet marvelous creatures once ruled in mankind’s stead. So perhaps you can imagine just how many passionate and conflicting feelings I have now that these creatures - one of them, at least - are colliding with our time now.
It’s terrifying, yes, but thrilling. Completely thrilling.
