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- Pteranodon sternbergi lived in the coastlines of Late Cretaceous North America, just like its pop-cultural relative, Pteranodon longiceps. Even larger than the latter, it also had a more striking look: its toothless beak curved upwards instead of being straight, and its crest was taller, shorter and more developed. Another pterosaur from the same habitat, Nyctosaurus, was also similar to Pteranodon longiceps but smaller, and had an extraordinary, two branched crest (the two "branches" probably substained a flap of skin). This was also the only pterosaur without fingers on its wing, making it the most aerial reptile ever discovered. Pteranodon sternbergi was briefly portrayed in Disney's Dinosaur (perhaps the only correct pterosaur-portrait ever made in fiction); while Nyctosaurus appears in the Walking With spinoff Prehistoric Park.
Return on land: Azdharchids
- Azdharchids were the only pterosaur family still alive at the end of the Cretaceous, it was hit by the C/T extinction event. The largest pterosaurs known to science (Quetzalcoatlus and Hatzegopteryx) belong to this family, but there were also many other smaller-sized members as well (like the Asian namesake Azdharcho), worldwide-distributed. They were among the less aerial pteros, with relatively short wings and very developed neck and hindlimbs; now ptero-scientists think they were a sorta stork-like in habits. Their head was vaguely Pteranodon -like with toothless mouths but very small crests or none.
Winged nutcracker: Dsungaripterus
- Dsungaripterus lived in Early Cretaceous Asia. Smaller than Pteranodon but larger than Rhamphorhynchus, it's easily recognizable thanks to its skull specialized to eat shellfish: bill curved upwards to extract mollusks from their shells, and nutcracker-like jaws with small crushing teeth at the bottom, adapted to grind hard food items. Indeed, most pterosaurs known to science were somewhat related to aquatic environment: we still have very little knowledge about non-water-loving pterosaurs.
Flying duckbill: Istiodactylus
- Istiodactylus was once called Ornithodesmus, although that name turned out to be from a dromaeosaurid. It was a large European pterosaur from the Early Cretaceous, characterized by a spatula-like bill lined with small teeth. It might have been a scavenger, as the teeth are serrated and small, unlike the long, conical teeth of fish eating pterosaurs. Indeed, the several pterosaurian kinds from the Cretaceous were very diversely head-shaped and had very different food habits, just like modern birds.
A whale of pterosaur: Pterodaustro
- Pterodaustro was perhaps the most specialized of all pterosaurs. Its name means "austral pterodactyl": it's the first-discovered among the numerous South American pterosaurs. Often seen as a sort of Cretaceous flamingo (but with upward-curved jaws unlike flamingoes): this because its teeth were very apt to a filter-feeding way of life in lakes or coastal lagoons. It had extremely abundant, baleen-like lower teeth which acted as a filter for minute planktonic crustaceans, while upper teeth were tiny, probably adapted to chew the plankton.
The biggest? Ornithocheirus
- Another Early Cretaceous animal, found in both Europe and South America (its South American remains were formerly known as Tropeognathus), Ornithocheirus has recently become one of the most famous pterosaurs, thanks to its apparition as the main pterosaur within the whole Walking with Dinosaurs series: exagerrately oversized and portrayed as "the biggest pterosaur ever" (the true record-holder among known ptero-species is Quetzalcoatlus or Hatzegopteryx). Interesting though, despite its memorable apparition Ornithocheirus has not become a stock animal since that, unlike the equally oversized marine reptile Liopleurodon. Real Life Ornithocheirus was large, but slighty smaller than the famous Pteranodon. Like many other pterosaurs, it had a specialized head, with sharp teeth for catching fish, and a couple of flattened crests protruding respectively from the upper and the lower jaw, with uncertain purpose - once thought to be a sort of "keel" to better plough the water with the mouth, allowing to catch fish in flight (the synonymous "Tropeognathus" means "keel jaw"). It may have been for display.
South American cocktail 1: Cearadactylus
- Another South American discovery: Cearadactylus is another large pterosaur from the Early Cretaceous. All pterosaurs already seen pertain to the Pterodactyloid subgroup, aka the literal "pterodactyls". These were the most advanced pteros, all with a vestigial tail, and usually with long, slender jaws, longer wings and weaker hindlimbs than the more primitive relatives, Triassic and Jurassic Rhamphorhynchoids (the "rhamphorhynchs"). The appearence of Cearadactylus looks like that of an oversized Pterodactylus, with no easily recognizable traits. It was choosen as "the pterosaur" in the first Jurassic Park novel: useless to say, in the air-born terror role.
South American cocktail 2: Tapejara
- Tapejara was a neighbor of Ornithocheirus. Again a large-sized animal, it was unusual among Early Cretaceous pterodactyloids because of its short and toothless beak; toothless pterosaurs were mainly in the Late Cretaceous, among them two stock animals, Pteranodon and Quetzalcoatlus. Some scientists hypothesize Tapejara was a fruit-eating, toucan-like animal, a rare example of a non-zoophagous (animal-eating) flying reptile. Its most striking feature was once a huge crest, taller than the head itself (the popular pteranodont's is unpretentious in comparison). However, the owner of the crest has been known to be the new pterosaur Tupandactylus since 2007. Tapejara/Tupandactylus appears in the same episode of Walking With featuring Ornithocheirus.
South American cocktail 3: Ludodactylus
- And then, there's Ludodactylus, a recently-discovered pterodactyloid which, strangely, lived in Early Cretaceous South America. It has gained some notoriety, and it's easy to tell why: with its long crest paired with toothed jaws, it's the very first-discovered pterosaur which looks just like the fictional Pteranodons, trasforming mere fiction in some kind of reality. Its prefix Ludo- derives from a Latin root meaning "play" or "toy", as a sort of Real Life Lampshade Hanging. Its species name, sibbicki, honors renowned paleoartist John Sibbick.
- Now we get out of the "True Pterodactyls" world and enter the "Rhamphorhynchs" one, with one of the most ancient pterosaurs, the European Eudimorphodon from Late Triassic. Despite its earliness, it already had all features of a typical pterosaur. But it was still small: all Triassic/Jurassic flying reptiles were small, eagle-sized the most. Giant Flyer -related pteros were only Cretaceous. Eudimorphodon was very similar to the similar-named Dimorphodon, with the typical long, rigid tail of a rhamphorhynchoid, but with a smaller, thinner head. Its contemporary Peteinosaurus (also from Europe) was actually more Dimorphodon-like; it has appeared in Walking with Dinosaurs to represent the start of pterosaur evolution.
Tiny piranhas? Anurognathus
- Anurognathus was one of the tiniest pterosaurs ever, just larger than a sparrow! Lived in Late Jurassic Europe alongside many other pterosaurs, either Rhamphorhynchoids or Pterodactyloids (among them the two namesakes Rhamphorhynchus and Pterodactylus), and it had the possibility to see Archaeopteryx as well. Anurognathus was an exception among rhamphorhynchoids because was short-tailed, but its stub, rounded, frog-like head is typical for a "rhamphorhynch". It was probably insectivorous. Despite having been one of the most harmless Mesozoic creatures in Real Life, not even it has managed to escape the Pop-Cultural fate which hits all its relatives: Primeval has show to us a sort of Zerg Rush flying piranha, while the more benevolent Walking with Dinosaurs has make it a Jurassic oxpecker.
A revolutionary guy: Sordes
- Among ptero-examples, last but not least: Sordes. Very similar to Rhamphorhynchus, this small Late Jurassic "rhamphorhynch" from Central Asia has had an enormous relevance in ptero-science; it was the first pterosaur ever discovered with fur-like covering (20 years before the description of the first feathered non-bird dinosaur), and thus led the start to the "Pterosaur Renaissance" briefly described in the pterosection of Stock Dinosaurs.
Mesozoic dolphins: Stenopterygius
- The small (2.4 m) namesake Ichthyosaurus is the pop-culture member of the ichthyosaurian group because it was the first ever discovered (1821, before the first dinosaur): lived in Early Jurassic seas, and with its dolphin-like shape, it had no particular traits compared to other ichthyosaurs, as well as the contemporary, almost-identical Stenopterygius. The latter is worth of note, however, because is the species from which the famous "mother-with-young-inside" skeleton comes from. That mother died just when its son was going out of its body.
Mesozoic swordfishes: Eurhinosaurus
- Most "fish-lizards" were Early Jurassic just like Ichthyosaurus and Stenopterygius, and some had some specialization. Eurhinosaurus, for example, had a swordfish-like head plus tiny teeth on its "sword"; the same about its relative Excalibosaurus.
Mesozoic orcas: Temnodontosaurus
- There were also bigger guys in the Early Jurassic seas: Temnodontosaurus was much larger than Ichthyosaurus, reaching 8 m in length, as a modern killer-whale. It was once called Leptopterygius and was one of the top-predators of its time, but its shape was that of a generic ichthyosaur.
Mesozoic whales: Shonisaurus
- However, the largest ichthyosaurs known to science were surprisingly the earliest, Triassic ones: Shonisaurus reached 18 m and even more, as large as a sperm-whale. Not only, it had also several specializations: its four flippers were long and plesiosaur-like, its body was stockier than most other ichthyosaurs and its jaws were partially toothless. Just like modern sperm-whale compared with other toothed cetaceans. Shonisaurus is one of the candidates for "the biggest marine reptile ever" title, along with the biggest pliosaurs; and yet, has not received much attention even in documentaries.
Mesozoic eels: Cymbospondylus
- On the other hand, the very unichthyosaur-like Cymbospondylus has received a "better" treatment, showing up as the "biggest ichthyosaur" in the Triassic seas in Sea Monsters. Even though it was large as well, reaching 9 m, was far smaller than Shonisaurus (the series' accompanying book got this right). Unlike the latter, Cymbospondylus was one of the most basal ichthyos known, being similar to a mosasaur, with only a hint of caudal fin and a very elongated body (it may have even been too primitive to be an ichthyo proper). However, its head was already ichthyosaurian, and had no visible neck.
The start...: Mixosaurus
- In dinosaur books, the traditional prototypical Triassic ichthyo has been Mixosaurus. Even smaller than a human and with a still-underdeveloped caudal fin, it had already the classic fish-like form of more advanced ichthyosaurs, showing how the ichthyosaurs' strong adaptations to water were already achieved well before the success of, to say, the land-living dinosaurs and the flying pterosaurs.
...and (almost) the end: Ophthalmosaurus
- Ichthyosaurs reached their prime in the Jurassic. But, surprisingly, in the Late Jurassic they had already become rare. Ophthalmosaurus is the most well-known among these. Unusual for having toothless jaws, its name means "eye lizard" because of its enormous orbits: it's often said to have had "the largest eyes of all vertebrates ever". Even though was actually a specialized ichthyosaur, Ophthalmosaurus shows up in Walking with Dinosaurs' episode dedicated to marine reptiles as the icon of the whole ichthyosaur group - at least averting Stock Dinosaurs by non-showing Ichthyosaurus in this role. Very few ichthyos survived in the Early Cretaceous: among them, the quite unspecialized Platypterigius. Ichthyosaurs went to extinction far before the comet, about 100 million years ago: since they were the best adapted to sea among all Mesozoic reptiles, the reason of this remains still unclear. They may have been outcompeted by sharks and other fish.
Heads or tails? Elasmosaurids
- Plesiosaurs are divided in two main lineages, Plesiosauroids (the "plesiosaurs" sensu stricto), and Pliosauroids (the "pliosaurs"), which descended from true plesiosaurs in the Triassic. Other than the spectacular Elasmosaurus, there were many other animals in the plesiosauroid subgroup. The closest Elasmosaurus relatives are called "Elasmosaurids" from their namesake: some of them were as large as the latter and sometimes even more. They generally lived in the Late Cretaceous, and were among the latest sea-reptiles before the Great Extinction. Let's talk a bit more about the prototypical Elasmosaurus: it was victim of an astonishing paleontological blunder in the middle of the XIX century, when it was first discovered. Before the notorious Bone War began in the USA, Edward Cope (one of the two "warrior" scientists) discovered its first skeleton, but its skull was found separated by the rest. Looking at the unbelievable length of its neck and the comparatively short tail, Cope decided, after infinite second thoughts, to put the skull... on its tail-tip. The other paleontologists obviously laughed at Cope when the mistake was cleared, and the legend says that Othniel Marsh (his future rival) was among them; and this would have caused the hate between the two, and thus, the upcoming Bone-Wars.
Stones for lunch: Cryptoclidus
- Among non-elasmosaurian plesiosauroids, Cryptoclidus and the prototypical Plesiosaurus are the most portrayed. The former was medium-sized and the classic Late Jurassic plesiosauroid, with a typical look but a not-so-oversized neck as the elasmosaurs; some remains show stones in its ribcage, whose purpose is uncertain (see Stock Dinosaurs). Plesiosaurus was even smaller and even shorter-necked, and lived in Early Jurassic along with many ichthyosaurs.
Short-necked longnecks: Dolichorhynchops
- Few media seem to pay attention to the interesting Polycotylids, even though they were the only short-necked plesiosauroid subgroup. They lived in Late Cretaceous, and achieved a shape deceptively similar to pliosauroids (see below), but were far smaller, often man-sized. However, one National Geographic Special featured one polycotylid (Dolichorhynchops) as the main character. On the other hand, museum mounts are rare.
- Unlike "true plesiosaurs", pliosaurs hadn't a great variety in the Mesozoic: most of them had the same size and the same appearance of the two stock members, the gigantic Kronosaurus and Liopleurodon (although the former is about 20 ft smaller than first thought). There were smaller pliosaurs as well, though: the Late Jurassic Peloneustes is a good example. Some giant pliosauroids managed to reach the end of the Cretaceous and lived alongside Elasmosaurus, Tylosaurus and Archelon, but are rarely considered unlike their earlier predecessors, suffering the rivalry of the aforementioned stock sea reptiles. However, the earliest, Early Jurassic pliosauroids were very different-looking than a Liopleurodon: Macroplata, for example, had a long neck and a small head. At the Early Jurassic, plesiosauroids and pliosauroids weren't still differentiated from each other: Plesiosaurus and Macroplata may easily get confused (and people are still arguing about the Late Triassic Thalassiodracon). Late Jurassic forms were well-defined, Liopleurodon and Cryptoclidus were very different-looking.
Other sea reptiles
NOTE: There were many other groups of sea reptiles in the Mesozoic other than the following ones. Among them, giant turtles in the Cretaceous (such as Archelon); "sea crocodiles" in the Jurassic, some of them very fish-like (Metriorhynchus, Geosaurus); while Triassic was represented by two primitive, still partially terrestrial groups: Placodonts and Nothosaurs. All these groups are better-described in the following sections.
- ... "antediluvian" sea reptiles were already familiar guys to scientists. Mosasaurids, in particular, had a crucial role in vertebrate paleontology: their namesake, Mosasaurus has been the
very firstsecond "antediluvian reptile" ever discovered, at the end of the XVIII century in Netherlands (the record-holder is Pterodactylus). The "Mosasaur" was described by the Father-of-Paleontology, French naturalist Georges Cuvier, and its first-found remains were object of an awesome tangle which almost deserves to be mentioned apart. One detail is astounding: the Mosasaur was ultimately discovered.... thanks to some bottles of wine. All mosasaurs lived in the Late Cretaceous, and thus were short-lived compared to other sea reptiles. The gigantic Tylosaurus and Mosasaurus are the two stock genera, often confused each other in paleo-art. Among other mosasaurs, we can mention: the rather ichthyosaur-like Plotosaurus, a specialized mosasaur which was as large as Tylosaurus; the much smaller, more traditional-looking Platecarpus and Clidastes, both very common in the famous inland sea that covered central North-America at the time (which was also home to Tylosaurus), the unusual shellfish-eater Globidens, with a typical mosasaur shape but uniquely blunt teeth, and the first mosasaur: the amphibious Dallasaurus, which had functioning legs. While some scientists speculated that Mosasaurs evolved from the same ancestor as snakes, the discovery of legless snakes predating Dallasaurus debunked that, and it is now known that they are varanoids, making them the Badass marine cousins of monitor lizards such as the Komodo dragon and Megalania.
Successful underdogs: Champsosaurus and the Choristoderans
- While technically not exclusively marine, Choristoderans (better-known as Champsosaurs) are thought to have been fully aquatic, even in freshwater ecosystems - to the point that in some species only the females had limbs that were strong enough to move on land. Originated in the Triassic period, Champsosaurs were often similar to miniaturized crocodiles (but not related at all with them), and are of particular interest as they managed to survive beyond the age of the dinosaurs and into the age of mammals where most of its brethren went extinct at the K/T extinction event. They are the only now-extinct group of reptiles that lasted after the end of the Mesozoic era. The Late Cretaceous Champsosaurus is the namesake of the group: croc-shaped, it was only 5 ft long, a perfect underdog when compared with its neighbour, the 45 ft long "true crocodilian" Deinosuchus.
Still-living reptile groups
Contrary to what many shows make to believe, Dinosaurs, Pterosaurs, Dimetrodonts and Sea-Reptiles were not the ancestors of any modern reptile; instead, some of them were at the origin of bird and mammal groups. However, there were true relatives of modern reptilian species in the past as well, and they have existed since the beginning of the Age of Dinosaurs or even before that time (except snakes, which are a rather young group evolutionarily speaking). Some of these animals were rather similar to their modern-days relatives, while other were quite different (remember that every animal group does evolve during the time). In general, most media and even documentaries will go with the larger species of each group. The smaller relatives are almost never mentioned because they are not spectacular enough, even though they were more abundant that their gigantic versions, just like what happens to modern animals in general.
An enduring success: Prehistoric crocodylomorphs
- Crocodilians, the only surviving members of the clade Pseudosuchia, are the only still-living reptiles that it wouldn't be too wrong confounding them with dinosaurs. OK, they aren't dinos in a strict sense, but they were their closest non-avian relatives, and shared with dinosaurs much more traits it may seem at first glance. Both dinosaurs and crocs have/had alveolate teeth with a bit of heterodonty: to make things clear, their teeth were more similar in their structure to the mammalian ones than, to say, those of lizards. Both dinos and crocs show/showed complex parental care, again just like mammals and unlike lizards/turtles. And both dinos and crocs did descend from bipedal ancestors. Quite so. The first common ancestors of both dinos and crocs, the Triassic Archosaurs (see further), were a sorta mixup of dinosaurian and crocodilian features, and some ancient croc relatives were deceptively dinosaur-like - the most striking case is the Struthiomimus-like Effigia. But at the Jurassic, their evolution diverged more, and since then, separating crocs from dinos becomes an easier task. However, don't think ancient crocs were boring things: it's anything but. Within their enduring success, they were almost as diversified as dinosaurs, and their size and body plan was very variable. Some were as small as a chameleon, others larger than T. rex. Some were powerful predators of large land animals; other became fish-lovers or insect-hunters; and some were even aquatic filter-feeders. As a group, they roamed all the three main Earth environments: land, oceans and freshwater - even though the latter was their favourite, because here they didn't suffer any competition, unlike dinosaur-ruled inlands and Sea Reptile-ruled seas. Some examples of ancient crocodilians are following.
- Let’s start with giant freshwater crocodilians, for obvious reasons. Deinosuchus (“terrible croc”, also called Phobosuchus “fearsome croc”) belonged to the Eusuchians aka the “true crocodilians”, which appeared only in the Cretaceous but with the same anatomy we can still see today. More precisely, it was closer to alligators and cayman than to true crocodiles, hence the nickname “giant alligator”. Like gators, the deinosuchus’ skull had wide strong jaws and relatively blunt teeth. Its head was as long as a fully-grown man, but the length of its body is unknown because the skull is the only left remain. Comparing with modern alligator, Deinosuchus could have reached 15 m in length and weighed more than a Tyrannosaurus. Its home were freshwater basins in Late Cretaceous North America, but could also have frequented the inland sea that divided the continent at the time. Since its fossil is from 75 mya, Deinosuchus could not have lived long enough to meet T. rex in Real Life, but only the latter’s smaller relatives. Sarcosuchus (“meat-eating croc”) was not an Eusuchian but only a “crocodilomorph” distantly related with true crocodilians. Its shape was that of a gigantic gharial, with long thin jaws and numerous needle-like teeth. First found in Cretaceous Northern Africa in the same habitat of Spinosaurus, it was recently found also in South America, where Giganotosaurus roamed. 15 m long, Sarcosuchus was basically the same bulk of these giant theropods. Recently CGI documentaries have popularized these animals with the nickname “supercrocs”. Given their size they could have eaten giant dinosaurs if they’d the chance, and some portraits show them defeating even the biggest theropods. In Real Life, it’s more likely such powerful predators tried to avoid each other. Few of us know, however, that two enormous crocs lived just few million years ago, in full Mammal Age, when the first hominids just started their evolutive journey: the gharial-like Rhamphosuchus from India and the cayman-like Purussaurus from South America. And they were at least as big as (if not bigger than) the two dinosaur-eating docu-stars. Finally, don’t forget that most prehistoric freshwater crocodiles were not bigger than ours: the Early Cretaceous Goniopholis is one of the best-known examples, and was not bigger than a modern alligator or cayman. Another, Kemkemia (initially considered a theropod dinosaur) lived alongside the fish-eating dinosaur Spinosaurus and probably competed with it for fish & other water-based prey.
- And now, let's discover the opposite end: the smallest crocodilomorphs ever lived were land-loving, long-legged, graceful things with a bit of dinosaur inside. Early Jurassic Protosuchus has been perhaps the most portrayed. Land crocodilians were not related each other however: several croc lines reached this body-plan independently. It's simple to undestand why they remained small: competition from dinosaurs was too strong in dry land, and they could survive only occuping the niche of small, fast-reproducing hunters, just like proto-mammals which shared the same niche. Some scientists hypotize that the nocturnal adaptments mammals underwent during the Mesozoic (herited by all modern mammals, even the daylight-living ones like humans) were not determined by dinosaurs' predation, but rather by the competiton of the (arguably) diurnal land-crocs. Some of the latter were even partially bipedal: for example Jurassic Sphenosuchians (the most primitive crocodilomorphs known so far), which were smaller than Compsognathus and could become their prey. Some land-crocs became larger however, expecially in Cretaceous South America and Australia, and were powerful predators in competition with theropod dinosaurs. The most extreme example known of a running croc appeared just after the mass-extinction (that wiped out many crocodilomorphs as well): the large, hoofed Pristichampsus. The most astonishing thing is, however, that some small-sized land crocodiles managed to survive almost until the start of human history: the Australasian Mekosuchians.
- Initially, crocodiles were land animals. Then, many of them became amphibian and fresh-water living, as they still are today. But some of them went even further, trying to colonize open seas. Here, they have always had trouble, because of the strong competition with the classic Sea-Reptiles (ichthyosaurs, plesiosaurs, mosasaurs). But a group of them did manage to coexist with the latter: the mainly Jurassic Thalattosuchians (literally "sea crocodiles"). The most archaic ones were still gharial-like, the most known being Teleosaurus; the most evolved didn't even resemble crocs. Rather, they looked like slender ichthyosaurs, because they developed the same caudal fins of the latter, lost their armor altogether, and transformed their limbs in paddles. They were probably the only fully-marine archosaurs ever, maybe they didn't even lay eggs and gave to birth alive newborn, just like ichthyosaurs, mosasaurs and (perhaps) plesiosaurs. Metriorhynchus and the ironically-named Geosaurus ("land lizard") are the two most-portrayed examples. Dakosaurus andiniensis, a species of Metriorhynchid crocodile, was an especially bizarre example, as it possessed a theropod-like skull and teeth, earning it the nickname "Godzilla".
- Turtle Power is Truth in Television. Turtles have literally been among the longest-lived reptiles ever, since appeared 230 million years ago and are still-living today. But their origin is really mysterious. The very first turtles ever discovered, among them Proganochelys from Triassic, had already the classic turtle-shape, shell and toothless beak included; since then, they have not changed their body-plan at all for 250 million years. Mesozoic turtles were very similar to ours. They have had a great success, colonizing all three main habitat just like crocs: terrestrial, marine, freshwater. And just like crocs, freshwater has been the favourite one, while terrestrial species have always been a minority. Marine turtles reached gigantic sizes in the Cretaceous (the aforementioned Archelon was 20 ft long and weighed several tons), and were the only group of Mesozoic sea reptiles which managed to survive the mass-extinction: modern marine turtles do descend from some ancestors already present before the cataclysm happened (though not from Archelon: it went eventually extinct without leaving descendants). Fossil record of Chelonians is extremely abundant (like that of Crocodilians) since freshwater aid the fossilization, and hard-boned shells / bony armors do preserve very well. Most non-marine turtles were small, just like today: but there were two large land-living species just 1 million years before modern history: the "Atlas Tortoise" from India was very Galapagos Tortoise-like but as large as a small car; the Australian "Horned Tortoise" was smaller but with a cooler look: it had small bovine-like hornlets on its head. Extra-note: recent research seems to show turtles were not the most ancient still-living reptiles as traditionally said. Lizards and Tuataras were perhaps more basal, and turtles might even be Archosaur relatives: that is, closer to birds than to lizards, just like crocodiles. But this is an age-old discussion among paleontologists. Everything Is Long-Living With Turtles, literally.
- Contrary to turtles and croc, lizard's fossil record is extremely poor: their gracile skeletons do not usually fossilize. Ironically, the best-preserved lizard-rests known so far were discovered... into other creatures' ribcages. It's particularly famous the case of Bavarisaurus, a small Jurassic lizard found into the first discovered Compsognathus skeleton. We don't know exactly which kind of modern lizards lived already in the Age of Dinosaurs: we're sure there were at least geckos, monitors, and proto-iguanas; while chameleons seem to be a recent evolution, after the non-avian dinosaur extinction, derived from iguana-like ancestors. Lizards occupied the same niche ruled by mammals and the apparently similar land-crocs, as small insectivores or omnivores. Many modern-days lizards are still compared with dinosaurs, or even passed off as "mini-dinosaurs", in documentaries and pop-books; ironically, just because they were used in the past as a model for the early dinosaur paintings and models. This spread the popular notion that all prehistoric reptiles were nothing but "giant lizards": a notion then adopted by films, comics and whatnot, which has given to us the Slurpasaur trope. But lizards actually pertain to a very different group of reptiles than dinosaurs and even crocodiles (both Archosaurs); this group is called the Squamates (literally "the scaly ones"). Together with the "sphenodonts" (see below), Squamates form in turn the "Lepidosaurs". One may even hear the largest modern-day lizards literally passed off as dinosaurs in documentaries or other non-fictional works; the predestined victim is, obviously, the large-sized monitor-lizard nicknamed Komodo dragon. The astonishing thing is, our Indonesian "dragon" did have in the recent past a close Australian relative much, much larger than itself: Megalania was 15 ft long, twice as long as its Komodan kin; and lived just 1 million years ago. It was, arguably, one of the most powerful predators of its habitat (but don't forget the contemporary "marsupial lion": though not larger than a lion, some scientists think it was the most efficient mammalian predator ever, maybe even capable to kill a fully-grown Megalania!). Megalania is by far the largest lizard that ever lived. But wait... have we forgotten something? Yeah, the Mosasaurs. It's so easy to forget this, but they were true lizards, and evolved from monitors to boot; with more than 30 ft in total length, the ultimate size-record belongs definitively to them. Along with Megalania and the Komodo dragon, Mosasaurs are the only "giant lizards" which are Truth in Television. But wait… we’ve still forgotten something: yeah, anacondas and reticulated pythons. See below.
- Snakes are the great exception among still-living reptiles: they are a very recent thing, appeared only in the Late Cretaceous, just before the mass-extinction. Their success was obtained only since the beginning of the Mammal-age 65 million years ago, and venomous species appeared even later, 30-20 million years ago. The most ancient still-living snakes are probably boas and pythons, or at least their closest relatives: the most common kind of snakes, the Colubrids (the Garden-Snake and relatives), appeared in fully Cenozoic settings. As bird are nothing but winged dinosaurs, snakes are nothing but legless lizards. They descend from a still unknown kind of Cretaceous lizard which did elongate its body loosing the limbs at the same time. Curiously, the lizard-group closer to snakes in phylogeny is not to be searched among the small slithering ones, like slow-worms or amphisbaenians. One hypothesis suggests snakes evolved from burrowing varanid-like lizards, or shared an ancestor with the gigantic Mosasaurs instead, although recent genetic analysis and the discovery of Najash, a genus of two-legged snakes, is calling that relationship into question. The latter's reputation as the Cretaceous "sea-serpents" seems thus justified. Prehistoric snakes are not much portrayed in books or paleo-art: this is probably due to the fact that their remains are very, very scant, even more than those of their lizardy ancestors: all that we often have are few isolated vertebrae, which don't allow to understand even how long they were. Hence, speculation and exaggerations tend to be common. Gigantophis (which lived just after the dinosaur extinction) is a prime example: only known for fragmentary remains, it may get described as twice the length of an anaconda despite it more probably was only a bit longer than the latter, if it was. If dealing with Cretaceous species, it'll probably be a Dinilysia.
I'm almost a lizard: Prehistoric sphenodonts
- The Tuatara is the modern reptile more often cited for being a "living fossil", and with reason. It is the most ancient and primitive still-living Amniote (amniotes = reptiles + birds + mammals), a survivor which has miracolously managed to be alive today, while all its relatives went extinct before the end of the Mesozoic. The tuatara group is called the Sphenodonts, a sibling group of the Squamates. Once called "Rhynchocephalians", the Sphenodonts' natural history is completely distinct to lizards and so on. Sphenodonts are Lepidosaurs just like lizards, but have retained more primitive traits still present in our tuataras; they appeared in the Triassic, like almost all the main reptilian lineages. Dinosaurs (both Ornithischian and Saurischians), Pterosaurs, Ichthyosaurs, Plesiosaurs, Crocodilians, Turtles, Lizards, even Mammals: all these appeared in the Triassic Period. And tuataras as well. Like turtles, they didn't change much since then; fossils show that prehistoric tuataras were almost identical to their modern relative; and lived around the world, while they are limited only to New Zealand today. Like komodo dragons, tuataras are often cites as "living dinosaurs" in pop-books. Once the "Rhynchosaurs" (see "Triassic non-archosaurs") were considered early tuatara-relatives (they are now considered closer to archosaurs): this explains why tuataras used to be called Rhynchocephalians in the past. The latter means "beaked head", and yet nobody'll ever see a tuatara with a beak! This term was actually referred to the parrot-billed rhynchosaurs, which were once considered Rhynchocephalians as well. And this explains why the term rhynchocephalian has fallen in disuse for indicating the tuatara lineage. While "sphenodont" has always been referred only to tuataras, never to rhynchosaurs: thus, scientists now use only this more correct term when referring to our spiky New Zealander.
These were once named thecodonts, but is actually an artificial assemblage of basal archosaurs: among them, the ancestors of crocs, dinos and pteros. Actually some of them (Teratosaurus, Ornithosuchus, and others) were once believed true dinosaurs, precisely the very first large carnivorous dinosaurs, but most belong to the clade Pseudosuchia, today represented only by crocodilians. Basal archosaurs, often being somewhere between Crocodilians and Dinosaurs, tend to get token appearances more in art and documentaries than anywhere else, and are rarely named.
- Rauisuchians are the most commonly represented Triassic archosaurs, as they were the top predator of Late Triassic and the competition for the early dinosaurs. Rauisuchians were more related to crocodilians than to dinosaurs, but their look was more that of a small, four-legged tyrannosaur rather than a crocodile, and their skull was very theropod-like. In fact, one of them, Teratosaurus (mentioned in the Theropod list for convenience) was long considered a dinosaur (even though its remains were mixed with those of a prosauropod). Today the 18 ft long Postosuchus has become the new prototype of the group, expecially after its memorable apparition in Walking with Dinosaurs as the competitor of Coelophysis. Other rauisuchians, like the small European Ticinosuchus and the South American Saurosuchus (the biggest ever discovered, 25 ft long) make one rauisuchian subgroup, the Prestosuchians; another, very specialized subgroup is the Poposaurs (see later).
Plant-eating crocodiles: Desmatosuchus and the Aetosaurs
- The only "thecodont" group that was entirely herbivorous, with small jaws and teeth, Aetosaurs looked like Ankylosaurs crossed with Crocodiles: they had a heavy armor covering not only their back but also their underbelly, making them the most armored archosaurs that ever lived. Like Rauisuchians, Aetosaurs reached large sizes (5 m the most) and had pillar-like limbs. Despite this, these animals were not dinosaurs, and their erect limbs were obtained by other means than those of dinosaurs. Desmatosuchus is the most portrayed, because has the coolest-look among them with its long shoulder-spikes, and lived alongside Coelophysis in Triassic North America.
- Despite many of the creatures above may superficially look dinosaurs, the only true basal archosaur related to dinosaurs (and pterosaurs) was the tiny Lagosuchus and its relatives, called Dinosauriformes. Together, dinosauriformes, pterosaurs and dinosaurs make one of the two main archosaur subgroups, the Ornithodirans - literally "bird-necked", because many had flexible bird-like necks, countered against the almost-always short-necked Crurotarsans (the other great archosaur subgroup). Only one foot long, Lagosuchus could stay on an adult man's hand without problems - hence its fanciful name, "croc-rabbit"! It was a long, thin animal, a bit like a miniaturized Coelophysis but also capable to walk on all fours - complete bipedality was reached only by its descendents, dinosaurs, thanks to the shape of their hips. However, Lagosuchus has been classified as a dinosaur in the past just because its dinosaurian appearence, but its skeleton is too primitive to make it a proper dinosaur. Lived in Triassic South America alongside the gigantic rauisuchian Saurosuchus and the very first true dinosaurs (what an "incidence"!), such as Eoraptor and Pisanosaurus. It's worth noting however, some partially bipedal croc-relatives, such as Ornithosuchus and Saltoposuchus (both European and living alongside Plateosaurus), were believed the real ancestors of dinosaurs because they too had a dinosaur-like appearence. Science Marches On has been a very common thing when coping with dinosaur ancestry.
When crocodiles were like dinos: Effigia and the Poposaurs
- Poposaurians are the least known among triassic archosaurs, but perhaps the more interesting. They were the most dinosaur-looking among all Crurotarsans, (that is, the archosaurs more closely related to crocs than to dinosaurs). Many of them had sailbacks resembling Dimetrodon, and the most specialized were toothless and beaked. The most surprising one is Effigia: if you take a look on it, you'll surely say it's an ostrich-mimic dinosaur!
Plant-eating crocodiles 2: Rutiodon and other Phytosaurs
- Phytosaurs have a very weird name: "plant lizards". A much more apt name for this group is Parasuchians, "near crocodiles". "Phytosaurs" indeed were the most crocodile-like among all Triassic Archosaurs, and occupied the freshwater predator niche outcompeting giant amphibians. But then, true crocodilians took their place in turn, after the Triassic/Jurassic extinction event. Parasuchians too could reach large size (5 m the most), but had very short limbs and long, thin jaws often similar to a modern gharial. However it's easy separate them from crocs, by one feature: their nostrils were just in front of their eyes. Though once thought to be croc-line archosaurs (pseudosuchians), new analyses have shown that phytosaurs are likely outside of Archosauria altogether. Rutiodon is one of the most portrayed, because lived alongside Coelophysis and was a potential predator of the latter.
The first runner: Euparkeria
- There were several archosaur offshoots as well in the Triassic. Some of them are closely related and very similar to the archosaurs above, and lived in the Early Triassic (while true archosaurs were mainly Middle- and Late- Triassic): among them, the most iconic is surely Euparkeria. Discovered in South Africa in the Early Triassic, it was contemporary to the two stock cynodonts Cynognathus and Thrinaxodon (see "Origin of Mammals" below). This merely 3 ft long insect-eater resembled a bit a theropod dinosaur in shape, but is thought to be only partially bipedal, and with slightly sprawling legs unlike dinosaurs and Lagosuchus. Euparkeria has been perhaps the most commonly-portrayed "thecodont" in classic sources, sometimes mis-interpreted as "the first ancestor of dinosaurs" (like in Walking With Monsters). Once, it was also considered "the first animal that could walk on two legs"; but now it seems having lost this record.
The first crocodinos: Proterosuchus
- Proterosuchus and its relatives were the first archosauromorphs which reached the mixed croc-dino shape of triassic archosaurs. Indeed proterosuchians were once considered the most basal "thecodonts". Also called chasmatosaurs, they were medium-sized reptiles with long heads and croc-like shape, but had also a strange overbite which, along with the lack of armor and longer legs, made them distinctive. We don't know if they were semi-aquatic as usually stated: being so basal, they could still have been terrestrial. Another croc-shaped group of basal archosauriforms is very rarely portrayed in books: the gharial-like Proterochampsids.
Triassic Tyrannosaurs: Erythrosuchus
- The most spectacular basal archosauriformes were the large, bulky, carnivorous Erythrosuchus and its relatives, the Erythrosuchians, "red crocodiles". Nicknamed "crimson crocs", they occupied the top predator niche in the Early Triassic, substituting the mammal-like Gorgonopsids (see further). However, they were stocky animals arguably slower-moving than the more evolved Rauisuchians, and finally get replaced by the latter in Late Triassic.
Triassic Triceratopses: Hyperodapedon and the Rhynchosaurs
- Rhynchosaurs were four-legged, plant-eating reptiles that were only distant archosaur relatives; once they were thought to be in the same group of the modern Tuatara (see above). They were the most successful groups of herbivores in the Triassic, thanks to their slit, parrot-like beak (their name means "beaked lizards") and powerful grinding jaws which allowed them to chew even the toughest vegetation. They were rather small compared to other triassic reptiles, lived worldwide, and shared their habitat with the very first dinosaurs such as Plateosaurus and Coelophysis. Rhynchosaurs went to extincion only at the end of the Triassic, like rauisuchians and the other basal archosaurs above. Hyperodapedon (also known as Scaphonyx) is the most portrayed.
Triassic Brontosaurs: Tanystropheus and the Prolacertiforms
- Tanystropheus was one of the most enigmatic among all prehistoric reptiles. Lived in Triassic Europe. 15 ft long, its body was lizard-like but its neck was the longest respect-to-the-body that any creature known, to the point it almost challenges physical laws. We haven't any precise idea how Tanystropheus lived: but it was almost certainly a partially aquatic creature. Its neck was particularly stiff, having only few very elongated vertebrate: since its neck was flexible only at the base, some hypothized Tanystropheus used it as a fishing-rod for catching fish from ashore. Other relatives were more lizard-like, but with very long hindlimbs and also quite elongated necks. All these reptiles belonged to the group called Prolacertiformes.
- Even though are often nicknamed "gliding lizards", these reptiles were not lizards. They were small and lizard-shaped nonetheless, except for one thing: they were able to glide, just like a modern lizard species called "flying dragon". They weren't related each other, and adopted several different gliding structures and mechanisms: North American Icarosaurus and European Kuehneosaurus had elongated ribs which substained a skin membrane acting as a parachute, just like that of the "flying dragon"; Asian Sharovipteryx had membranes extending from limbs to the body, in a way rather similar to Pterosaurs. While the most enigmatic of all, Longisquama (its name means "long scale") had two rows of long scales protruding from each side of its body. However, nobody knows what these things exactly were (Real scales? Proto-feathers? Or a simple fossilization artifact as it seems according to recent research?)
- The small tree-specialists Avicephalians lived in Early Triassic, and were among the most basal "Diapsids" - that is, the group containing all reptiles sensu-stricto (except Anapsids and maybe turtles). Some avicephalians were gliding forms similar to those already mentioned above: ex. the deceptively dinosaur-sounding Coelurosauravus (literally "coelurosaur ancestor") had elongated ribs like those seen in Icarosaurus, but was not related with the latter. Another subgroup was more similar to chamaeleons but with a neck of a bird, ex. Megalancosaurus; even though are very rarely portrayed, they were among the most specialized and weird-looking reptiles ever.
- Placodonts and Nothosaurs (already mentioned in the "sea reptile" section) were the two main groups of Triassic sea reptiles, both relatively small compared to the most famous Jurassic/Cretaceous marine reptiles, and both still partially terrestrial. Placodonts were bulky animals with strong jaws and crushing teeth specialized to eat shellfish; the most evolved of them (ex. Henodus) had an armor and were very turtle-like, with a beak and short tails, and swum with their limbs like turtles; while the most basal ones (ex. the namesake Placodus) were almost armor-less, with "incisor"-like teeth and a long, robust tail for swimming, resembling a bit the modern "Galapagos islands' marine iguana". Placement of placodonts in the reptilian phylogenetic tree is still uncertain, but are traditionally regarded as distant plesiosaur-relatives, thus not related with turtles despite their resemblance.
- Nothosaurs were very different-looking than placodonts: slender fish-eaters with streamlined bodies, flat tails, long necks and long, thin jaws with pointed teeth. Some of their features were plesiosaur-like: this because nothosaurs were close plesiosaur relatives, and some of them might have even been their ancestors. However, nothosaurs still swum using their tails like modern crocodilians, while their possible descendents the plesiosaurs had rigid body and used their flippers to propel themselves through the water. Nothosaurus is considered the prototype of the nothosaur group and was 4 m long, but the most basal nothosaurs were much smaller, like Neusticosaurus. The most evolved one were pratically plesiosaurs: ex. Pistosaurus. Another group of Triassic aquatic reptiles, Thalattosaurs, resembled miniaturized Nothosaurs, but weren't related with them. Still another, the Hupehsuchians, looked like a cross between an Ichthyosaur and a Placodont, and were perhaps the ancestor of ichthyosaurs.
The Origins of Mammals
This sections talks about both mammal-like reptiles (not actually reptiles in a cladistic sense), and their descendents the Mesozoic mammals. However, if you're searching only for Dimetrodon and nothing else, see Stock Dinosaurs Non Dinosaurs.
- We traditionally call "pelycosaurs" the most basal Synapsids (the correct name for the mammal-like "reptiles"). They were the dominant group of land animal in the Early Permian (for the record, Permian Period was just before the Mesozoic Era), until they were replaced in the Middle Permian by their descendents, the Therapsids. "Pelycosaurs" were still lizard-like in general body shape but showed already mammalian traits: their head was laterally flattened and high-settled above the ground, and their teeth started to show some resemblance with ours. By far the most popular are Dimetrodon and, to a lesser degree, Edaphosaurus (see below), because both shared a similar crest (the so-called "sail") on their back substained by elongated neural spines, still for uncertain purpose: a solar panel/radiator as traditionally said? A courtship device? Or both things? Most other "pelycosaurs" didn't have such a sail: these seem not to receive any attention, even in books. However, from little animals like the Carboniferous Archaeothyris, several big, cool-looking animals originated, not only the sailbacks. The plant-eating Cotylorhynchus, for example, was not only the biggest pelycosaur known so far, but also the oddest-looking, with its disproportionately small head compared to the bulky body; the more normal-looking Ophiacodon was one of the very first Amniotes to reach large size (it was already living at the end of the Carboniferous); while Sphenacodon was a sort of sail-less Dimetrodon. If you see old paleo-art, expect to see scaly-skinned mammal-ancestors. This is due to a long-standing Taxonomic Term Confusion: since they are classically called reptiles in Linnean systematics, most old-fashioned artists use to draw them using actual modern reptiles as model. But horny scales are a exclusive thing of the Diapsids aka True Reptiles (and possibly Anapsid aka Near-Reptiles). It's very unlikely that Dimetrodon and its kin developed horny scales only to loose them altogether after becoming mammals. On the other hand, modern birds still retain the old reptilian scales on their hindlimbs... indeed, cladistically speaking, birds are True-Reptiles.
The dimetrodon's vegan twin: Edaphosaurus
- After Dimetrodon, Edaphosaurus is the only "pelycosaur" which has some possibilites to appear in non-documentary media. A bit larger than the dimetrodont, the edaphosaur was very similar to the latter, with a sail on its back, long tail and splayed legs. Its sail was bigger and more complex however: it had a more rounded shape and its spines had regularly-placed tubercles for uncertain purpose. Edaphosaurus head was much smaller than Dimetrodon and with round teeth all with the same shape. With this dentition, it was arguably herbivorous, but could also have eaten shellfish according to some. Living alongside Dimetrodon in Late Permian North America, Edaphosaurus is sometimes shown in paleo-art as one of its possible preys. This could be realistic, even though Dimetrodon almost-certainly hunted young Edaphosauruses more often than the powerful adults.
- Scientific names are often misleading. Tetraceratops, for example, means "four-horned face" but is not a middle-way between a Triceratops and a Pentaceratops: it was a synapsid, thought by some a "missing link" between "pelycosaurs" and true Therapsids, but with four small horns above of its head. Tetraceratops could actually belong to a the most primitive group of therapsids, the obscure Biarmosuchians, whose Biarmosuchus is the prototype.
- “Therapsids”: is the classic name for the most advanced mammal-like "reptiles"; cladistically speaking, however, it contains mammals as well, so we humans are Therapsids in this sense. We'll use this term with the traditional meaning. It's worth noting that most Therapsids have been discovered in two precise places in the world: South Africa and Russia. Dinocephalians included the largest therapsids. They were bulky-bodied and large-headed, and lived in the Middle/Late Permian, but were wiped out by the Permian/Triassic extinction event. Some of these were herbivorous, like Moschops (see below); and others were more likely carnivorous (Anteosaurus) or omnivorous (Titanosuchus). The meat-eating ones were the top-predators of the Middle Permian, but were later substituted by another group of therapsid, the Gorgonopsids. Despite their diversity, all dinocephalians shared common traits in their dentition.
Hulky beast: Moschops
- Moschops is the most famous dinocephalian, and also one of the biggest. Characterized by front-legs longer than hind-legs and a rather Hulky frame, it has a thickened skull-roof possibly for headbutting. Some old sources said it had even a third eye on the middle of the skull; actually this "eye" was just a tiny bunch of light-sensitive cells shared by many other primitive vertebrates (such as the modern Tuatara). Even though is less-portrayed than the moschops, the most awesome-looking among dinocephalians is Estemmenosuchus, with its bony protrusions on its head whose purpose is uncertain.
- Made famous by Walking With Monsters, Gorgonopsids were the top-predators of the Late Permian, but they too were deleted by the aforementioned mass-extinction. More slender and usually smaller than Dinocephalians, they are nicknamed "sabertooth" just like their mammalian namesakes; however their upper canines, though longer than most therapsids, were far less developed than those of a sabretooth-cat! The wolf-sized Lycaenops and the larger Inostrancevia are perhaps the most portrayed. Gorgonopsids, Cynognathus and other carnivorous therapsids are often described as dog-looking; indeed, in modern depictions, this resemblance is even more evident than in the older, more reptilian-like portraits.
The first known mammal-ancestor: Dicynodon
- The first mammal-like “reptile” ever described was Dicynodon in the middle of the XIX century, the time in which Darwin popularized his revolutionary concept of “evolution”. His pupil Thomas Huxley (nicknamed “Darwin’s Mastiff”) proposed a surprisingly modern hypothesis, that land vertebrates should be divided in only two branches instead of the Linnean tripartition Mammals/Birds/Reptiles. These lineages were: Theropsids (“beast-looking”, not to confound with Therapsids) and Sauropsids (“lizard-looking”). The former were basically the mammals; the latter were the reptiles (including dinosaurs and birds). Since Dicynodon was initially not thought a mammal-ancestor, in Huxley’s classification was put in the “sauropsid” branch. This was the start of the tradition to classify these animals as reptiles, and to depict them with reptilian traits. Even though the relationship therapsid-mammal was cleared at the start of the XX century, the “Mammal-like reptiles” thing has endured until the XXI century.
- Dicynodon has given his name to the Dicynodonts, the most successful group of herbivorous therapsids. They appeared in Late Permian as small diggers such as the tiny Diictodon (portrayed in Monsters); flourished in the Early Triassic with Lystrosaurus (see below); and then become bulky and vaguely elephant-like at the end of the Triassic, conviving with the first dinosaurs: Kannemeyeria and North-American Placerias are two main examples. Traditionally thought to have disappeared at the Triassic/Jurassic border, a recent discover seems indicating some Australian dicynodonts managed to make their way even in the Cretaceous. One thing does unify dicynodonts: their jaws. They had only two teeth at all, the upper canines vaguely Dracula-like, while the tip of their mouth was a sort of tortoise-like beak for cutting vegetation. Some species however lacked even these teeth, but still they made their way wery well.
Dracula was a hog: Lystrosaurus
- One note more about the most iconic dicynodont, Lystrosaurus. One of the first animals to have recuperated after the Permian/Triassic extinction event, Lystrosaurus was one of the most successful animals of every time. Its remains have been discovered everywhere in southern continents, even Antarctica - don't forget that this continent began to freeze only few million years ago, in full Mammal-Age. The lystrosaur has the typical dicynodontian shape: bulky, short-tailed, with strong semierect limbs, and the typical dentition made only by two upper “canines”. Formerly, it used to be depicted as a freshwater-dweller like a hippo, but now is mostly believed a grazing land-animal. It is often shown as the favourite prey of Cynognathus (see below).
Doggie jaws: Cynognathus
- Cynodonts ("dog teeth") were the most advanced, mammal-like of all Therapsids. They had a very mammal look, had certainly at least some hair and a quasi-mammalian dentition. They were also the smallest therapsids, being mostly cat-sized: the largest known, Cynognathus, was no bigger than a German Shepherd. Cynognathus ("dog jaws") has traditionally been considered not only the prototypical cynodont, but the prototypical mammal-like reptile as well (after Dimetrodon of course...); and yet, the cynognath has not received much attention in films or other pop-media, maybe being not so impressive-looking compared with the dimetrodonts or naturally dinosaurs. However, it should be an excellent predator, perhaps capable to kill therapsids bigger than itself like the dicynodonts (please don't confound them each other!). Even though was almost certainly hairy, its hair should have been less-dense than modern mammals. We don’t know if cynodonts had auricles or mammary glands (two distinctive mammalian traits). Similarly, we have no idea about how therapsids were colored. The usually-bland coloration typical of mammals is thought an adaptation for darkness –according to scientists, every modern mammal (even the diurnal ones like us humans) descend from night-dwellers. Nocturnal habits, however, were achieved within the synapsid lineage only in the Triassic, to avoid competiton with dinosaurs (or at least, that’s what scientists usually say). If true, this would mean non-mammalian synapsids like Cynognathus and Dimetrodon could have been very colorful guys, like modern reptiles and birds.
Fur and whiskers: Thrinaxodon
- After Cynognathus, the most represented cynodont is the cat-sized Thrinaxodon: for example, it appeared in New Blood episode of Walking with Dinosaurs (identified simply as "cynodont", oversized and misplaced). The thrinaxodont has sometimes been cites as "the most mammal-like among mammal-like reptiles": ironically, recent research indicates it was actually one of the most basal cynodonts, even more basal than Cynognathus. With its small size, it was arguably a hunter of small animals, a bit like a modern badger. Careful analysis of its skull show it was certainly covered with fur, and has also sensitive whiskers just like modern mammals.
- The closest-to-mammals cynodonts were very diverse in habits, and often not predatory at all: Traversodontids, for example, were omnivores or even plant-eaters; Tritylodontids even developed rodent-like teeth for gnawing. However, both achieved their traits convergently with modern herbivores, and were not direct ancestors of mammals. However, the last common mammal-ancestor has surely to be searched among cynodonts. In the Late Triassic, cynodonts were the only successful therapsid group: dicynodonts were still surviving but were rare at that point. Non-mammalian cynodonts survived until the Early Jurassic (Oligokyphus) and perhaps even in the Early Cretaceous (Xenocretosuchus), but they made a minor part of the synapsid fauna after Triassic: their true-mammalian descendent were dominant at this point.
- Probably the least portrayed among the main therapsid groups, Therocephalians were similar in size and shape to the cynodonts, but less mammal-like, and only distantly related with mammals. Therocephalians began in the Permian and survived until the Early Triassic, but then were replaced by their more evolved relatives, Cynodonts indeed. Just like the latter, they were initially meat-eaters, but then some of the last forms became vegetarian. One unidentified predatory therocephalian appears in Walking With Monsters, portrayed with a totally speculative venomous bite.
Thank you dinosaur! MESOZOIC MAMMALS
The boundary between mammals and non-mammals has always been a hard issue for paleontologists. Since typical mammalian features such as hair, milk glands etc. do not fossilize most the time, the key to separate the two ensembles lays in their skull. True mammals have a mandible made by a single couple of bones, and three ossicles in the mid-ear. Non-mammalian synapsid have several pairs of bones in the lower jaw and a single ossicle in the ear. It's also worth noting that mammalian features probably didn't appear all in the same istant: perhaps some therapsids already produced milk, though they didn't have erect limbs yet, unlike modern mammals.
Some quasi-mammals (more correctly called Mammaliaforms) began in Late Triassic and were tiny and very shrew-like, like Megazostrodon. Then, the first true mammals appeared, and remained shrew-like for all the Mesozoic... at least this is what scientists used to think. Now we know mammals were already very diversified at the Age of Dinosaurs. Some were mole-like diggers, some were beaver-like swimmers, and some were even gliders. The largest Mesozoic mammals were badger-sized, and one of these has been discovered with baby dinosaur remains in its stomach.
A whole group, the Multituberculates, were rodent-like and herbivorous: they were the most abundant ones at the end of the Cretaceous, and managed to survive after the mass-extinction. At the beginning of the Mammal-Age they became even more successful, until true rodents replaced them in the middle Cenozoic. Interestingly, Multitubercolates were the longest-living mammalian group ever before gone extinct.
The direct ancestors of modern mammalian groups (Placentals, Marsupials and Monotremes) appeared in Early Cretaceous but widespread only in the Late Cretaceous. We can mention: the platypus-like Steropodon, an early monotreme; the early possum-like marsupial Didelphodon (portrayed in Walking with Dinosaurs as a scavenger); and the oddly-named placental Purgatorius, which is often considered the first known ancestor of primates, or at least, a close relative.
A classic commonplace it to say Mesozoic mammals were underdogs in a dinosaur-dominated world. Actually, thanks to their dense populations, they affected their ecosystem the same way dinosaurs did; and remember that small animals are always key species in their natural environments. Let's debunk another myth: we must thank if dinosaurs went extinct, otherwise humans couldn't have appeared on Earth. Maybe we could have appeared just the same, maybe a bit later... It's more probable that dinos actually guided mammal evolution in an indirect way. Being competitors of and preying upon our ancestors, they selected actively the most adapted, most evolved traits us mammals are proud of: among them, intelligence and parental care. If you are here to read this now, you have to thank dinosaurs. Everything has always been better with dinosaurs!
The animal listed here were once called “Cotylosaurs”, but this word is now fallen in disuse. This was a catch-all term for all most primitive “reptiles” which couldn’t be placed in another well-defined groups. However, some of them still make a natural grouping, Anapsids. They were a successful group of animals which make a group on their own, intermediate between true reptiles (the Diapsids) and the mammal-like Synapsids: hence their alternate name, Parareptiles ("near reptiles"). Most anapsids were even more ancient than diapsids: they lived mainly in the Permian period alongside Dimetrodon and the other mammal ancestors. Like therapsids, they were mainly discovered in Russia and South Africa. Whether turtles are just surviving anapsids or not is still matter of discussion (see “Still-living reptile groups” above).
Near-reptilian tortoises: Scutosaurus and the Pareiasaurs
- Pareiasaurids were the only anapsids which reached great size, almost like a small rhinoceros at the extreme (but there were also smaller species though). They resembled a bit some plant-eating therapsids in shape (Moschops, Placerias…), with their bulky frame, short tails, strong semierect limbs, and an armored skull. Indeed, pareiasaurs occupied their niche during the Late Permian, substituting Dinocephalian therapsids (which were dominant in the Middle Permian); but were wiped out by the mass-extinction and substituted in turn by other therapsids (the dicynodonts) and the non-therapsid Rhynchosaurs, in the Triassic. An early theory said pareiasaurs could have been the ancestors of turtles and tortoises. Now this seems disproved, as turtle’s anatomy is very specialized and very different to that of a pareiasaurid. Scutosaurus was one of the largest pareiasaurs and the most armored, with a “horned” skull and bony plates on its back (a sort of archaic version of an ankylosaur). Other examples are the namesake Pareiasaurus (as large as the former but armor-less), and the small spiky Elginia.
Near-reptilian crocodiles: Mesosaurus
- Mesosaurs (not confound them with Mosasaurs!) were among the most basal anapsids, but perhaps the most specialized. Semi-aquatic, they were a bit like non-armored gharials, with a croc-like shape and long jaws filled with strongly-crammed teeth. As these teeth look weak and needle-like, the Mesosaurs’ diet is uncertain. They could have been either fishers or filter-feeders. Mesosaurs were small-sized (3 ft long) and arguably weren’t strong swimmers: they are known to be freshwater dwellers. However, their remains have been found in several Southern continents. Since they couldn’t be capable to cross open seas, they were used as a demonstration of the Pangea Hypothesis. In their epoch (Permian) continents were still reuned together, and this allowed mesosaurs to go across landmasses with ease without leaving freshwater. The oldest currently known fossilized amniotic embryos are mesosaur embryos, and it seems that mesosaurs were the oldest group of amniotes known to evolve viviparity.
- Most other anapsids had a very generic look, classically described as “lizard-like”. The most successful ones were the Procolophonids. They were the only anapsid group which managed to survive the awful mass extinction at the end of the Permian, and survived long enough to see the first dinosaurs. Only the small mass-extinction at the Triassic end deleted them definitively. Procolophon is the namesake of the group, resembled an iguana in shape, and could have been omnivorous. But don't forget the tiny Eudibamus. From the Permian period, this is the guy which has recently taken over the title to the Triassic Euparkeria for being the first animal ever capable to run on two legs.
- ...the very first Amniotes appeared. Traditionally we quote Carboniferous reptiles as "lizard-like" because were similar to 1 ft long lizards in shape, but some weren't even reptiles sensu stricto. The most cited is perhaps Hylonomus, because it was the most ancient of all; some hylonomes have been found inside fossilized logs so abundant in the Carboniferous. Another is Petrolacosaurus, which lived slightly after Hylonomus and was a full diapsid - contrary to the latter which was only a relative of diapsids. Walking With Monsters chose to show Petrolacosaurus as "the very first reptile": only.... for some weird reason, it magically reveals to be the ancestor of mammals in this show. This is even more unbelievable if you think that the true first known mammal ancestor was available for a Carboniferous setting: Archaeothyris. Put together, these three animals make the most archaic common ancestors of all land vertebrates that lived from Permian up to the Recent Period (amphibian excluded, which originated before these and remain only partially terrestrial even today). Among their descendents there are indirectly birds and mammals as well, and thus mankind itself. Recently, another animal has been found, which could be the real first amniote: Casineria.