The Early Jurassic

~  The Hettangian Stage  ~

201.3 to 199.3 mya

Early Jurassic

The Hettangian Stage is the first stage of the Jurassic Period; it follows the Rhaetian (the last stage of the Triassic Period) and is followed in turn by the Sinemurian Stage. The base of this stage is the appearance of the ammonite genus Psiloceras; the top of the stage is the first appearances of the ammonite genera Vermiceras and Metophioceras. The Hettangian gets is name from its type district located at the village of Hettange-Grande in France. In northwestern Europe, the Lower Hettangian is referred to as the Planorbis Zone, the Middle Hettangian as the Liasicus Zone, and the Upper Hettangian as the Angulata Zone.

a snapshot of the first days of the Hettangian Stage

During the Triassic Period, the continents were jammed together in a supercontinent called Pangaea. During the Late Triassic, seams began to appear in the continent as tectonic plates shifted apart. These ‘rift basins’ were often filled with waterways, becoming channels or seas. As the rifting continued, volcanic activity became the norm. By the time we get to the Hettangian Period, volcanic activity is happening all over the face of the earth due to Pangaea ripping apart. The Jurassic Period begins in the wake of the Triassic-Jurassic Extinction Event, likely caused by climate change due to the severe increase of volcanic activity. The planet’s temperature warmed at a nauseating pace, and life struggled to keep tempo. The ocean became more acidic, resulting in the collapse of marine food webs; shellfish were particularly hard-hit, and ammonites almost entirely died out (during the Hettangian, however, the ‘Neoammonites’ developed quickly so that by the end of the Hettangian they were widespread). The climate became more variable: periods of intense heat were followed by rapid cooling, megamonsoons became far more severe, and coastal regions became wetter while interior regions became drier. The hotter temperatures led to the death of ninety-five percent of plant life; herbivores suffered from the lack of food, and the carnivores that preyed upon them suffered in turn. Most reptiles, amphibians, and early mammals died out. During the Triassic Period, dinosaurs lived ‘in the shadows’ of more successful creatures, but the Triassic-Jurassic Extinction shuffled the playing board. As the first million years of the Jurassic dawned, the landscape emptied of many creatures that called it home: the supersalamander amphibians, the piglike dicynodonts, the beaked rhynchosaurs, the long-snouted phytosaurs, the tank-like aetosaurs, and the apex-predator rauisuchians were creatures of the past. The crocodylomorphs continued, albeit in a shadowy form that would never assume the dominance enjoyed during the Triassic. Another survivor was the dinosaurs, and with the environment ripe for the plucking, they diversified so that they would become the dominant creatures of the Mesozoic Era.

The shifting tectonic plates in the Early Jurassic created a rift to Pangaea’s east; as the rift blossomed westward, it was inundated with water from the Tethys Ocean. This extended body of water is called the Tethys Seaway. The Tethys Seaway would eventually become the mid-Atlantic Ocean. The Pangaean supercontinent, zigzagged with rifts, was coming apart. The Early Jurassic saw lower temperatures than those at the end of the Triassic, and the Jurassic experienced an increase in rainfall and rising sea levels (there was no water tied up in polar ice caps); the rising sea levels flooded portions of North America, Europe, and North Africa, and the rifts created by the fracturing of Pangaea became waterways called epicontinental or ‘inland’ seas. These factors increased the planet’s humidity so that the Jurassic became a ‘tropical’ world rather than the ‘desert’ one of the Triassic. The high sea levels and inland waterways helped to stabilize temperatures and decrease seasonality so that by the Late Jurassic, the climate was relatively uniform across the globe. As we examine the eleven stages of the Jurassic Period, we are forced to migrate across the continents: because Pangaea was no more, similar organisms took different evolutionary routes in different areas. We will travel an east-west ‘Silk Road’ from southern Laurasia (modern Asia) to northern Laurasia (Europe and North America), and then to northern Gondwana (Africa) to southern Gondwana (South America, Australia, and Antarctica). But first we will examine something more homogeneous: the Hettangian Oceans.

a school of Ichthyosaurus

We look first at the ichthyosaurs, which first appeared during the Triassic Period (though some scientists speculate they may have evolved as early as the Late Permian). The ichthyosaurs dominated the Triassic seas, but following the Triassic-Jurassic Extinction Event, they were outdone by the plesiosaurs, which overwhelmed many of the ecological niches that Triassic ichthyosaurs had dominated. This didn’t mean ichthyosaurs died out; it meant they had to adapt. As a general rule, Jurassic ichthyosaurs didn’t grow as large as their forebears, for the role of ‘large oceanic predator’ would be stolen by Jurassic plesiosaurs; and the specialized suction/ram feeders and durophageous species (who fed on shellfish) all but disappeared. Most Jurassic ichthyosaurs were streamlined and dolphin-like, much like Ichthyosaurus, which appeared in the Rhaetian of the Triassic and which would survive through the Pliensbachian of the Early Jurassic. Ichthyosaurus was one of the few ichthyosaurs to weather the storm of extinction into the Jurassic, and it did remarkably well: hundreds of specimens have been found in Germany, many which contain fossilized embryos (this has led researchers to speculate that the ichthyosaur bonebeds of Germany – marine environments during the Jurassic – were ‘nesting grounds’ for these marine reptiles). Ichthyosaurus grew to eleven feet in length and had huge eyes protected by bony shields called sclerotic rings. Ichthyosaurus coprolites show that it subsisted off fish and squid. It may have lived close to the shoreline.

The ichthyosaur Leptonectes, which appeared in tandem with Ichthyosaurus in the Late Triassic, was streamlined with an elongated rostrum that gave it a swordfish-like appearance. It lends its name to a class of 'spear-headed' ichthyosaurs known as the Leptopterygiidae. ‬This group of ichthyosaurs are sometimes envisioned using these snouts to stir up sediment on the sea floor‭ ‬to find buried prey,‭ ‬however because their jaws resemble those of swordfish it is thought that they may have used them like swordfish to kill prey.‭ ‬This would involve using their long rostrum to swipe at fish in front of them and causing injuries to their flanks so that they could not swim away,‭ ‬thus significantly increasing the chance of capture.

a Temnodontosaurus ventures close to shore for a snack

Another Hettangian ichthyosaur, Temnodontosaurus, was one of the last ‘large and in charge’ ichthyosaurs. At this point in the Early Jurassic its niche as apex deep seas predator hadn’t yet been seized by the plesiosaurs. Temnodontosaurus grew between thirty and forty feet in length and prowled the deep Hettangian oceans. It had incredibly large eyes – approximately eight inches in diameter – that are believed to be the largest of any known animal (though, because of the angle at which its eyes were pointed, it had blind spots directly above its head). Temnodontosaurus had a fish-shaped body that was long, robust, and slender. Its tail was either as long as its body or longer. This predator likely fed mainly on vertebrates such as fish and other marine reptiles, such as plesiosaurs and ichthyosaurs. One specimen shows remain of an ichthyosaur in its abdominal cavity. Temnodontosaurus probably used ‘ram feeding’ to catch its prey: it would shoot towards its prey with its mouth open, engulfing the prey along with the water around it.

Atychodracon, a heavy-headed plesiosaur

Though Jurassic oceans would become dominated by plesiosaurs, with ichthyosaurs living in their proverbial shadow, this wasn’t the case during the Hettangian. Two plesiosaur species are known from this early point in the Early Jurassic: Macroplata and Atychodracon. Both were ‘long-necked’ plesiosaurs (in contrast with the later ‘short-necked’ versions known as pliosaurs). Macroplata grew to about fifteen feet in length and likely fed on fish, which it caught using its needle-like teeth. Its large shoulder bones indicate it was a fast swimmer. Its neck was twice the length of its skull. Atychodracon was slightly larger at sixteen feet in length. It had a relatively large skull, which comprised sixteen percent of its total length. It likely hunted fish and marine reptiles such as Ichthyosaurus.

When we move from the oceans to dry land, we find that dinosaurs weren’t the only animals around. Though they would take the spotlight throughout the rest of the Mesozoic, they were by no means the only participants. Crocodylomorphs continued, and their ancestors are with us today in alligators and crocodiles. The Jurassic Period saw the emergence of the first true lizards (snakes would appear in the Cretaceous) and the radiation of modern amphibians (‘modern’ amphibians include frogs, salamanders, and the snake-like caecilians). Though the mammal-like dicynodonts died out during the Triassic-Jurassic Extinction, their compatriots the cynodonts continued – and true mammals assuredly appeared. Though it was assumed for quite some time that mammals lived ‘undercover’ during the Jurassic Period, forced into underground burrows and shady night-time lifestyles by hungry dinosaurs, the last few decades of fossil discoveries have shown that they were wildly successful throughout the Mesozoic. They may have been underfoot the dinosaurs, but they weren’t shy about making their environmental niches their own. The Jurassic saw a proliferation of flying reptiles called pterosaurs; though they, too, suffered during the early days of the Jurassic, they were rebounding in full force by the Middle Jurassic. We’ll examine terrestrial lifeforms by making a long, planet-wide journey across the continents. We begin in southern Laurasia, and our first pit-stop is the Jurassic ecosystem in what is now Yunnan Province, China.

a pair of Yunnanosaurus in Hettangian China

The First Stop: China. Our knowledge of the Hettangian ecosystems in modern China come from the Lufeng Formation in Yunnan Province. This Lower Jurassic sedimentary rock formation has two units: the first is the lower Dull Purplish Beds from the Hettangian stage, and the second is the Dark Red Beds from the Sinemurian age (which followed the Hettangian). This offshore environment consisted of early Jurassic forests of conifers, cycads, and tree-ferns. Fossils of turtles, amphibians, and early mammals have been found in the area. The largest known creature of Hettangian China was Yunnanosaurus, a prosauropod that grew up to twenty-three feet in length. These prosauropods would’ve plodded through the murky forests, feeding on soft leaves, shoots, and buds, all while keeping an eye out for the largest predators of the area: the theropod dinosaurs Sinosaurus and Shuangbaisaurus. Both were ‘dilophosaurids’ in that they had dual head crests; these wouldn’t be used in combat or for defense, as they were rather flimsy, but were likely used for sexual display to attract mates and intimidate rivals or as species recognition. These predators likely hunted Yunnanosaurus and caught early, bipedal, fast-moving ornithischians when they could.

a Sinosaurus enjoys a prosauropod

A smaller theropod of the area was Panguraptor, a turkey-sized coelophysoid that likely fed on lizards, mammals, and insects. Panguraptor was a runt in its ecosystem and did well to avoid not only the larger theropod dinosaurs mentioned above but also the crocodylomorphs. The Lufeng Formation has uncovered Dibothrosuchus and Phyllodontosuchus, both proto-crocodiles. Dibothrosuchus was a long-tailed, long-limbed quadruped with a pointed snout. This four-foot-long crocodylomorph was fully terrestrial, and two rows of armor plates ran along its back.  Phyllodontosuchus was a sphenosuchian, and it’s unusual because some of its teeth were leaf-shaped like those of many herbivorous dinosaurs. This has led scientists to doubt a strict carnivorous diet; perhaps Phyllodontosuchus was an optimistic omnivore who liked to eat plant material from time to time? An odd-ball of the Lufeng Formation is the possible therizinosaur Eshanosaurus; this would be quite the find (it’s currently being hotly debated), since the next therizinosaur in the fossil record doesn’t appear until the Late Cretaceous!

Dracoraptor, a small theropod of South Wales

The Second Stop: Europe. Leaving Jurassic China behind, our excursion carries us northwest across Laurasia until we reach Europe. Early Jurassic England was connected with the rest of Europe, and these lands were coated in vast swathes of conifer forests. Roaming these forests was the thirty-foot-long and three-ton prosauropod Camelotia. These forests were also stalked by the medium-sized theropod predator Lophostropheus, which could grow up to ten feet in length and probably weighed 220 to 300 pounds. It may have hunted Camelotia and kin in the fern-covered valleys and muddy floodplains. As we venture farther west to the English seacoast, we look out to sea and see numerous islands rising from the shallow waters teeming with ichthyosaurs preparing to give birth to live young. In modern times this area is South Wales, but during the Hettangian it was a collection of shallow, forest-covered islands. The seven-foot-long theropod Dracoraptor prowled these islands, swimming the channels between them and hunting small vertebrates such as lizards.

a pair of Anchisaurus

As we island hop across the warm, shallow sea, we reach another coastline thick with conifer forests. Unbeknownst to us we just crossed the genesis of the North Sea and have reached North America, considered the northwestern portion of Laurasia. These forests were home to the thirteen-foot-long prosauropod Anchisaurus. It had a wide range: its remains have been found as far south as modern Africa, and it may have made some trackways recently discovered in Nova Scotia. As we head southwest, leaving the burgeoning Atlantic coastline and making our way into the interior of the modern United States, we may come across some Coelophysis, successful survivors from the Triassic Period who would diversify and radiate throughout the Early Jurassic (some even show up in China). Coelophysis would’ve needed to be wary of predators such as the crocodylomorph Protosuchus, which grew to three feet in length and weighed close to ninety pounds. This terrestrial, carnivorous proto-croc had pillar-like legs and its body was covered by osteoderms in a double row along the back and more covering the bottom of its body and running the length of its tail. Its five toes were clawed, and it would’ve been both a good runner and a good swimmer. If we were to disembark our caravan and comb through the topsoil, we might come across Eocaecilia, an amphibian that shared many characteristics with salamanders. It was small, only fifteen centimeters in length, and unlike modern legless caecilians, it possessed small legs and had better-developed eyes.

The North American crocodylomorph Protosuchus

Massospondylus, a Triassic holdout

The Third Stop: Africa. Our journey cuts south from North America, and we plunge into what is now modern Africa. This is the northeastern portion of the southern continent of Gondwana, and we get a fantastic look at the Early Jurassic environment from the Elliot Formation of South Africa. The Elliot Formation gives us a window into the Late Triassic and Early Jurassic. It’s been divided into the Lower Elliot Formation, which dates back to the Norian-Rhaetian stages of the Late Triassic, and the Upper Elliot Formation, which dates primarily to the Hettangian stage of the Early Jurassic but which also dips into the subsequent Simenurian stage. The Upper Elliot Formation – or UEF for short – is famous for preserving a ‘continental record’ of the Triassic-Jurassic boundary. The UEF’s sandstones are also infamous for containing ‘trace fossils’ of animal trackways, especially those belonging to early ornithischians. Due to the reddish color of the rocks, the Elliot Formation is often called the ‘Red Beds.’ Along with dinosaur fossils, the UEF preserves several crocodylomorphs, fishes and crustaceans, and turtles. At this point in time, southern Africa was a hotspot of volcanic activity that threaded much of southern Africa and Antarctica’s landscape with extensive lava flows. It’s speculated that the UEF gives us a snapshot of an arid floodplain rimmed with dense conifer forests close to the ocean. The fossils preserved were likely inundated by periodic flooding, entombed in sediment, and fossilized for our discovery.

Ledumahadi, a giant early sauropod of the Hettangian

The largest creatures of the UEF include the prosauropod Massospondylus¸ which survived the Triassic-Jurassic Extinction Event. This twenty-foot-long prosauropod was one of the kings of the Late Triassic, but it was being outdone by emergent sauropods such as Pulanesaura (twenty-six feet long), Antetonitrus (thirty feet long), and Ledumahadi (up to fifty feet long, the giant of Early Jurassic South Africa!). As the Jurassic progressed, the prosauropods would be squeezed out, and sauropods would become the dominant – and bigger-than-life – sauropodomorphs of the Mesozoic. The genesis of their rise is documented in Africa, and two more early sauropods from other African locations are known: Vulcanodon was uncovered slightly north of South Africa in Zimbabwe, and Tazoudasaurus was fossilized in northern Africa. Thus even in the Hettangian, early sauropods were dominating various niches. The sauropods of the Elliot Formation likely coexisted due to niche partitioning, in which different genera focused on different kinds of vegetation. Some may have been ‘high-browsers,’ stretching their long necks to feast off trees high off the ground, whilst others were ‘low-browsers,’ sweeping their necks back-and-forth over the ground to devour swathes of ferns and cycad shoots.

a Dracovenator disturbs a flock of heterodontosaurs

Theropods of the UEF include a variant of Coelophysis – these small, likely pack-hunting dinosaurs were able to survive the extinction at the end of the Triassic – and a medium-sized carnivore called Dracovenator. Because our knowledge of this dinosaur comes from partial skull fragments, we can’t precisely narrow down its length, though we assume it’s medium-sized because of similarities to the dilophosaurids (such as Sinosaurus and Shuangbaisaurus of China): like them and the later Dilophosaurus, the back end of its lower jaw features a series of lumps and bumps. Thus paleontologist speculate that it was a medium-sized predator that may have had dual crests on its head. Dracovenator may have hunted weak or juvenile sauropods, and it may have chased the early ornithischians that bloomed in Early Jurassic South Africa.

a pair of Heterodontosaurus

Ornithischian origins are murky, but by the Hettangian in South Africa, several species were flocking around in social groups. These early ornithischians generally grew to about five feet in length and could weigh up to twenty pounds. Their bodies were short with flexible tails; their five-fingered forelimbs were long and robust and capable of grasping; their hind limbs were long, slender, and four-toed; their skulls were elongated and narrow and triangular when viewed from the side; and most scientists believe these early ornithischians had long, hollow, feather-like fibers over much of their body. These early ornithischians are known as ‘heterodontosaurs’ because they had ‘heterodont’ (or ‘mismatched’) teeth. While most dinosaurs have a single kind of tooth in their jaws, heterodontosaurs had three: they had small, incisor-like teeth in the upper jaw followed by long, canine-like teeth, and they had chisel-like cheek-teeth behind their fleshy cheeks. Because several different species coexisted – the Hettangian conifer forests and floodplains were home to Heterodontosaurus, Abrictosaurus, Lesothosaurus, and Pegomastax – it’s been assumed that they, like their sauropod neighbors, practiced niche partitioning.

the Hettangian crocodylomorph Litargosuchus

The UEF of the Hettangian has revealed two crocodylomorphs and a cynodont (the ancestral heritage of modern mammals). Sphenosuchus could grow up to five feet in length, making it one of the largest sphenosuchian proto-crocodiles. It was terrestrial and a great runner, and it likely hunted heterodonts or even unwary Coelophysis. Another sphenosuchian was Litargosuchus; this proto-croc was smaller than Sphenosuchus, and it had unusually elongated limbs with its hind limbs slightly longer than its front. Paleontologists believe that Litargosuchus hunted its prey much like a wolf, though these creatures were probably solitary rather than pack hunters. The Hettangian cynodont Megazostradon – widely viewed as one of the first mammals – was a small, shrew-like creature that grew between four and four and a half inches long. It was likely nocturnal and fed on insects and small lizards.

the Hettangian cynodont Megazostradon, considered by many to be one of the earliest mammals

Vieraella, one of the earliest true frogs 

The Fourth Stop: South America. Heading west across Africa, we cross into what is now South America. Though Africa and South America would begin separating via a north-south split in the back-end of the Late Cretaceous, at this point there’s no Atlantic Ocean – not even a seaway – separating the two. If it weren’t for GPS, one wouldn’t know they crossed from one tectonic plate to another. In modern-day Argentina, the small prosauropod Leonerasaurus prowled conifer forests and fern-swept valleys. Argentina also showcases Vieraella, the earliest-known true frog. Far to the south of the continent, the five-feet-long theropod Tachiraptor preyed upon the bipedal, fast-running ornithischian Laquintasaurus.

a Tachiraptor plunges into a flock of Laquintasaurus in Hettangian Venezuela

~  The Sinemurian Stage  ~

199.3 to 190.8 mya

Early Jurassic

The Sinemurian Stage is the second stage of the Jurassic Period; it follows the Hettangian Stage and is followed in turn by the Pliensbachian Stage. The base of this stage is the appearance of the ammonite genera Vermiceras and Metophioceras; the top of the Sinemurian is the emergence of the ammonite species Bifericeras donovani and the ammonite genus Apoderoceras. The Sinemurian takes its name from the French town of Semur-en-Auxois, near Dijon. The chalky, limey soil formed from the Jurassic limestone in the region is partly responsible for the character of Sancerre wines. In the Sinemurian oceans, plesiosaurs radiated, and the infamous Plesiosaurus appears in the fossil record, and a spear-wielding ichthyosaur roamed the seas. One of the earliest new Jurassic pterosaurs, Dimorphodon, steps onto the scene, and dinosaurs continue to radiate: more sauropods evolve, theropods get larger, and we have the first inklings of the armored thyreophorans that will one day bloom into stegosaurs and ankylosaurs. We look first to the oceans, then to the skies, and then we take a journey across splitting Pangaea to see how terrestrial life is evolving.

a Plesiosaurus on the prowl

The sixteen-foot-long, fish-eating plesiosaur Attenborosaurus prowled the shallow seas of prehistoric England. Though this creature ‘walked and talked’ like a proper plesiosaur, scientists classify it as a pliosaur; though it didn’t have the classic ‘pliosaur’ morphology of a short neck and thick head, anatomical features indicate it was more closely related to the later Pliosaurus than to earlier, more basal plesiosaurs. Another long-necked pliosaur was Macroplata, which was slightly smaller than its aforementioned contemporary at fifteen feet. It, too, was a fish-eater, using its sharp needle-like teeth to catch prey. Its large shoulder bones imply a powerful forward stroke for fast swimming. Its neck was twice the length of its skull, an anatomical feature more in line with the long-necked ‘proper’ plesiosaurs than with the short-necked pliosaurs. The infamous Plesiosaurus hunted alongside Attenborosaurus and Macroplata during the Sinemurian Period; it would be a staple of northern Laurasian seas up through the Toarcian stage of the Early Jurassic. Plesiosaurus lends its name to the plesiosaur group, and it’s become associated with the Loch Ness Monster of Scotland. It was much shorter than its contemporaries at only eleven feet (thus ‘Nessy’ would be a behemoth!). The prehistoric Plesiosaurus had a small, narrow head; a long and slender neck; a broad, turtle-like body; a short tail; and two pairs of large, elongated paddles. Plesiosaurus likely fed on belemnites (squid-like Mesozoic cephalopods) and fish. Its U-shaped jaw and sharp teeth would’ve acted like a fish trap as it propelled itself by the paddles. It may have used its neck as a rudder when navigating. We know that Plesiosaurus gave birth to live young in the water like sea snakes, and the young may have lived in estuaries or bays before moving out into the open ocean. Because its long neck may have hindered its pursuit of prey – any slight movement would increase water turbulence – scientists believe it hunted with its neck held directly along the plane of its path. Another theory is that Plesiosaurus was an ambush hunter, lying in wait and using its elongated neck and big eyes to scan for prey; when prey came close, perhaps its neck darted out, snatching the prey out of the water. Perhaps it was brightly-colored to blend in with prehistoric reefs, nestled among corals as it waited for large fish or cephalopods to wander by. Ichthyosaurus, Temnodontosaurus, and Leptonectes continued into the Sinemurian, but two new 'spear-headed' ichthyosaurs emerged: Eurhinosaurus and Excalibosaurus.

an artistic rendition of the ichthyosaur Excalibosaurus

a flock of Dimorphodon on a wooded island of modern England

Though we know pterosaurs survived the Triassic-Jurassic Extinction – they proliferated like kudzu during the Jurassic, after all – our evidence of Early Jurassic pterosaurs is scant. The first well-known pterosaur of the Jurassic is Dimorphodon, a long-tailed, blunt-headed rhamphorhynchid. It shows up during the Simenurian and survived through the Pliensbachian. This puffin-like pterosaur had a five-foot wingspan and dominated the flurry of wooded islands that was prehistoric England. It stretched 3’3” head-to-tail and its head measured eight inches. The head was large and heavy with a deep, narrow snout. The beak may have been brightly colored in life and used in visual displays; like modern puffins and hornbills, it may have used vibrant colors to ‘stake out’ its territory, to attract mates, or to intimidate rivals. Studies of its hips, legs, and feet indicate that it may have stood upright and balanced on its toes while walking, just like modern birds. This stance makes Dimorphodon unusual, as most pterosaurs had weak legs that splayed out to the sides. Dimorphodon’s tail was long and stiff, and its three-fingered hands were strong and curved with sharp claws designed for climbing rocks or trees. The nature of this pterosaur’s eating habits have been hotly debated: was it a fish-eater? an insectivore? or did it hunt terrestrial prey, such as small lizards and early mammals? The current consensus (if we can call it that) is that it was a specialized carnivore, being too large for an insectivorous diet and therefore subsisting off small lizards and mammals (though it may have fished from time-to-time). Given its ecosystem of wooded islands, such a lifestyle wouldn’t be surprising, as islands are often host to smaller organisms than are found on the mainland.

A herd of Barapasaurus in Sinemurian China

The First Stop: China. As we begin our journey across the Sinemurian landmasses, our first stop is the offshore wooded forests of prehistoric China in what was southern Laurasia. This brooding forest of conifers was interspersed with cycads and tree-ferns, and fossils of turtles, amphibians, and early mammals are abundant. Our knowledge of Sinemurian China comes from the Red Beds of the Lufeng Formation. It was during the Sinemurian that China witnessed its first sauropods, such as Barapasaurus – which reached up to sixty-six feet in length and, judging by the abundance of fossils, was wildly successful and traveled in herds – and the thirty-foot-long Kotasaurus, which also traveled in herds. The prosauropods Yunnanosaurus and Lufengosaurus, though dwarfed by the emergent sauropods, continued into the Sinemurian. Bienosaurus, an early but not-well-known thyreophoran – ancestral to the later armored ankylosaurs and stegosaurs – also appeared. The predator theropods didn't experience much evolution, indeed it appears they went through a setback: though the small Panguraptor and the medium-sized dilophosaurid Sinosaurus survived into the Sinemurian, the Hettangian Shuangbaisaurus was nowhere to be seen. The Hettangian crocodylomorphs Dibothrosuchus and Phyllodontosuchus are present in addition to several early mammals and kin. Yunnanodon, whose skulls only reached about an inch and a half in length, were one of the few non-mammalian therapsids to survive the Triassic-Jurassic Extinction Event. Early Chinese mammals of the Sinemurian include Sinocodon and Hadrocodium. The latter is impressive, because it has a relatively large brain cavity – like modern mammals – and is one of the first early mammals to have a nearly fully-mammalian middle ear.

the early mammal Hadrocodium of Sinemurian China

Saltriovenator prowls an Italian beach

The Second Stop: Europe. Moving north across the supercontinent Laurasia, and nearing Europe, we reach modern Italy to meet the mega-predator of the Sinemurian. Fossils from Italy are rare, and while we don’t know what Saltriovenator hunted (though we can assume the environment was home to early ornithischians, prosauropods, and emergent sauropods), we know it was more than capable of taking down anything that it came across. Saltriovenator was the giant of its day, stretching twenty-five feet in length and dwarfing the other predators of its time (theropods its size wouldn’t show up for another twenty-five million years!). Heading further north, we eventually reach western Europe. The ancient waterways cut along Germany and France, and England was a mere collection of low-lying wooded islands. Among these islands was the lightly-built, eleven-foot-long theropod Sarcosaurus. It’s believed it may have swam between the hundreds of scattered islands in the emergent seaway. We also find the early thyreophoran Scelidosaurus. This thirteen-foot-long herbivore had a small head with a horny, beak-like front to its mouth. It was quadrupedal, with its rear legs longer and stronger than its forelegs, so that its back sloped up towards the hips (a feature that would be exaggerated with the later stegosaurs). The most notable facet of Scelidosaurus was the ‘light armor’ it sported on its neck, body, and tail. This armor was comprised of small, pebble-like scales and large bony plates called scutes. Scelidosaurus’ place in the ‘dinosaur family tree’ is debated: was it ancestral to the later thyreophorans (the armored ankylosaurs and stegosaurs), was it ancestral to one of those two families, or was it an early thyreophoran family all on its own? What is known is that Scelidosaurus migrated to northern Laurasia (modern North America), and we follow its journey across the emergent Atlantic Seaway where we run into one of the most famous theropods of all time: Dilophosaurus.

a family of Scelidosaurus are watched by predators in modern Connecticut

two Dilophosaurus fight over a Scutellosaurus

The Third Stop: North America. As we track into northern Laurasia, we reach what is now modern Connecticut. We see a family of Scelidosaurus walking along the beach as it’s eyed by a pair of small Coelophysis and a curious juvenile Dilophosaurus (yes, I basically copied-and-pasted from the image above!). Though Dilophosaurus is mainly known from Arizona, trackways have been discovered in Connecticut. As we move southwest towards the interior of North America, we may see scattered Anchisaurus. These prosauropods grew up to thirteen feet in length and were capable of rearing back on their hind legs to reach higher foliage. Though it used to be thought that they were some of the earliest prosauropods, it’s now known they were rather late bloomers. Despite their morphologically basal sauropodomorph anatomy, they developed in tune with the sauropods of southern Laurasia and Gondwana.

As we reach what is now modern Arizona, we come across a litany of rivers and prehistoric Lake Dixie, a massive body of water that stretched from St. George, Utah east to Colorado City, Arizona; and from Cedar City, Utah south to the middle of Arizona. By the Middle Jurassic, this area was being encroached upon from the north by a sandy dune field, but in the Sinemurian it was a lush paradise of vibrant forests, seasonal rainfall, and terrain cut by streams, ponds, rivers, and lakes. It’s no surprise that it was resplendent with a host of creatures calling it home. Here we encounter another early, albeit smaller, thyreophoran, Scutellosaurus. These five-foot-long, herbivorous early thyreophorans were lightly armored and likely ran around in packs, and they may have been hunted by the larger, twenty-three-foot-long Dilophosaurus. These predators are famous for their twin swooping head-crests, though there’s no reason to believe they were capable of spitting venom. They may have been pack hunters, and it’s very likely they were actually primarily fish-eaters (they had a lot of adaptations for catching fish, and their frail head-crests would be damaged in fierce combat when taking down larger prey). Dilophosaurus lived alongside large prosauropods such as Sarahsaurus, early ornithischian heterodontosaurs, and the lightly-armored Scutellosaurus and Scelidosaurus. Smaller theropods such as Coelophysis hunted in the undergrowth. The environment of Lake Dixie and its environs consisted of freshwater bivalves and snails and various crustaceans. Fossils of hybodont sharks, bony fush, lungfish, salamanders, caecilians, and the early frog Prosalirus have been discovered, along with the Early Jurassic turtle Kayentachelys. The remains of numerous lizards and crocodylomorphs, as well as a panoply of early mammals, have been uncovered. The remains of a relatively-unknown pterosaur, Rhamphinion, have also been studied. It’s safe to say, then, that Lake Dixie was a happening place!

a Dilophosaurus relaxes on the beach at Lake Dixie

the prosauropod Aardonyx of Sinemurian South Africa

The Fourth Stop: Africa. As we head south into Gondwana, we come to the volcanically-active region of the Elliot Formation of modern South Africa. During the Early Jurassic, southern Africa was a hotspot of volcanic activity that threaded much of southern Africa and Antarctica with extensive lava flows. It wouldn’t be uncommon to see plumes of acrid volcanic smoke rising distantly above the conifer treetops. The Elliot Formation gives us a snapshot of an arid floodplain rimmed with dense conifer forests close to the ocean. South Africa’s Jurassic menagerie didn’t change much from the Hettangian into the Sinemurian, but there was at least one newcomer, the twenty-foot-long prosauropod Aardonyx, which had features belonging to both primitive sauropodomorphs and more advanced sauropods. Aardonyx is considered by many paleontologists to be a ‘snapshot’ of the sort of creature that was transitional between prosauropods and sauropods. Aardonyx lived alongside the prosauropod Massospondylus and sauropods such as Antetonitrus, Ledumahadi, and Pulanesaurus. The Hettangian and Sinemurian African sauropods may have diversified and done so well due to a lack of larger theropod predators. The heterodontosaurs of the Hettangian scurried across the forest floor, perhaps hunted by the small theropod Coelophysis. Crocodylomorphs such as Litargosuchus and Orthosuchus may have hunted the heterodontosaurs, as well.

the Antarctic dilophosaurid Cryolophosaurus

The Fifth Stop: Antarctica. Moving south from Africa, we reach Antarctica. Though nowadays Antarctica is blistery and cold, in the Jurassic Period it was much warmer with a temperate climate. The Hanson Formation has given us glimpses into Antarctica’s Jurassic past: there were late synapsids, herbivorous mammal-like reptiles, and crow-sized pterosaurs that resembled Dimorphodon. Prosauropod remains, and even fossils belonging to a large sauropod, have been discovered. The most interesting find of all, of course, is that of Cryolophosaurus, the ‘Elvis Presley’ of the theropods due to backward-sweeping head-crests that look remarkably like that musician’s classic hairdo. Cryolophosaurus grew up to twenty feet long and may have hunted Antarctic prosauropods. Some scientists believe it was a scavenger or fish-catcher, as its head-crests were fragile and would easily be broken in combat. However, fossilized remains of what appear to be an herbivorous dinosaur’s rib bone was found wedged inside the bones of a Cryolophosaurus’ throat. Some postulate that this specimen choked on its dinner, though others believe the rib belongs to Cryolophosaurus and was wedged into its throat postmortem. The purpose of the head-crests were likely for sexual displays or species identification.

The Jurassic Antarctica is not the Antarctica you know!

~  The Pliensbachian Stage  ~

190.8 to 182.7 mya

Early Jurassic

The Pliensbachian stage of the Early Jurassic follows the Sinemurian and is followed in turn by the Toarcian. Its name comes from the hamlet town of Pliensbach in the Swabian Alps. The base of the Pliensbachian is the first appearances of the ammonite species Bifericeras donovani and the genera Gleviceras. The top of the Pliensbachian is the appearance of the ammonite genus Eodactylites. At the end of the Pliensbachian there was a minor extinction event known as ‘The Toarcian Turnover.’ Global temperatures had cooled down after the Triassic-Jurassic Extinction Event, but they began to warm up around 190 million years ago at the beginning of the Pliensbachian; about ten million years later, around the beginning of the Toarcian, temperatures began to cool off once more. Cooler temperatures – at least relative to the skyrocketing temperatures of the desert-like Triassic – would prevail throughout the rest of the Jurassic Period. The Pliensbachian witnesses a growing diversity in the oceans, and the pterosaur Dimorphodon continued to dominate European skies. Our terrestrial knowledge of the Pliensbachian ecosystems, when compared to the wealth of information we have for the Hettangian and Sinemurian, is rather lacking due to few bone-beds and fossilized remains.

Arminisaurus of the Pliensbachian Stage

In the oceans, the plesiosaurs Attenborosaurus and Plesiosaurus continued to hunt, mate, and die; but new plesiosaurs came onto the scene. Meyerasaurus prowled the prehistoric seas of prehistoric Germany and grew up to eleven feet in length; Arminisaurus, also of German origins, grew up to thirteen feet; and Westphaliasaurus, discovered in France, grew up to fifteen feet. Ichthyosaurs diversified, as well, but there were holdouts from earlier stages: the titanic Temnodontosaurus continued as the apex predator of the deep oceans, and the much smaller Ichthyosaur continued to rove the shallower seas like modern dolphins. Ichthyosaur newcomers included the thirteen-foot-long Suevoleviathan, which may have competed with Ichthyosaurus in overlapping ecosystems. In the air, Dimorphodon continued as the unchallenged pterosaur of the Early Jurassic, though its throne would be upset in the Toarcian.

the amphibian-pretender Siderops

On land, at least two new non-dinosaurian creatures make big entrances. The first was the temnospondyl (or ‘amphibian wannabe’) Siderops, which grew to about two and a half feet long and had a large, heavy skull. It likely hunted among the rivers, streams, and lake beds of Early Jurassic Australia in then-southern Gondwana. The second was the terrestrial crocodylomorph Kayentasuchus from Arizona. This crocodylomorph lived and breathed and had its being among the waterways and lake shores of prehistoric Lake Dixie. It had a tall, narrow snout and a flattened ‘skull table’ at the back of its skull. Its lower jaw was slender at the tip and curved upwards. The conical teeth had distinct unserrated edges. Its thigh bones were slender, and it had bony scutes on its back and belly. The scutes on its back ran in the typical parallel double rows seen in most crocodylomorphs.

a head-shot of the terrestrial crocodylomorph Kayentasuchus

the bird-like Segisaurus

When it comes to dinosaurs, the Pliensbachian fossil record is rather scant, and those fossils we do have show a general continuation of the Sinemurian. Such continuations aren’t surprising, as Mesozoic ‘stages’ have little to do with terrestrial comings and goings but with ammonite biodiversity. Though ammonite biozones may undergo revolutions, life may continue largely ‘as is’ above surface level. This doesn’t mean, of course, that there are no new arrivals. In China, two new sauropods – Yimenosaurus and Gongxianosaurus – emerge. In North America, Dilophosaurus remains the ‘tyrant king’ of Lake Dixie, but several smaller theropods eek it out underfoot. These include a small tetanuran theropod named Kayentavenator and two new coelophysoids: the swift-running Podokesaurus and the ‘bird-like’ Segisaurus. This latter theropod was just under three and a half feet long and would’ve stood just over one and a half feet tall at the hips. It was nimble and likely preyed upon insects, lizards, and small mammals (and there’s no reason to think it wouldn’t scavenge when the opportunity presented itself). Segisaurus had a flexible, elongated neck and stout body; its powerful legs were crowned with three-toed feet; and it had a long tail and long forearms. Segisaurus is remarkable for the fact that it seems to have had clavicles, which are unknown in other theropods from that era. This find demonstrates that the clavicle was primitively present in early theropods and reinforces the theory that modern birds are descended from theropod dinosaurs. This little dinosaur coexisted with the Sinemurian prosauropod holdover Sarahsaurus as well as a new prosauropod named Seitaad. In Antarctica, the prosauropod Glacialisaurus shows up, upon which Cryolophosaurus may have fed.

a juvenile Glacialisaurus flees a pair of adult Cryolophosaurus

At the end of the Pliensbachian, around 182 million years ago, the world underwent a minor extinction (though it was the seventh-largest extinction in earth’s history). This extinction was oceanic, in that it mainly hit marine creatures. Evidence of the so-called ‘Toarcian Turnover’ has been found in Japanese waters, the Andean basin, and the floor of the former Tethys Sea. Mollusks – such as ammonites, belemnites (soft-shelled cephalopods), gastropods, and bivalves – suffered the worst. Because these critters comprised some of the bedrock of the marine food chain, the suffering affected other marine organisms such as crinoids, crustaceans, fish, and even marine reptiles. The plesiosaurs Arminisaurus, Attenborosaurus, and Westphaliasaurus all went extinct; and both Ichthyosaurus and Leptonectes, which had dominated the ichthyosaur niches since the Late Triassic, went extinct. The few marine reptile survivors would be crowded out by newcomers in the burgeoning Toarcian. The cause of the Toarcian Turnover seems to be due the spread of ocean anoxia and ocean acidification, likely due to a ‘double-punch’ of global warming and severe volcanic activity from the rifting of the African and Australian plates at the beginning of Gondwana’s slow dismemberment.

~  The Toarcian Stage  ~

182.7 to 174.1 mya

Early Jurassic

The Toarcian stage is the last stage of the Early Jurassic. It follows the Pliensbachian and is in turn followed by the Aalenian, the first stage of the Middle Jurassic. The stage’s name comes from the city of Thouars, just south of Saumur in the Loire Valley of France. The base of the Toarcian is the emergence of the ammonite genus Eodactylites, and it coincides with the Toarcian Turnover, the seventh-largest extinction event in earth’s history and which primarily affected marine ecosystems. The top of the Toarcian is the first appearance of the ammonite genus Leioceras. During the Toarcian, marine ecosystems underwent a rapid shuffling in the wake of the Toarcian Turnover: many of the staple plesiosaurs and ichthyosaurs of the earlier Jurassic went extinct, and new genera showed up in their place; the Toarcian is also where we witness the emergence of the ocean-going crocodiles known as thalattosuchians.

the plesiosaur Rhomaleosaurus

The plesiosaurs of the Pliensbachian were hit hard: Attenborosaurus, Arminisaurus, and Westphaliasaurus went extinct. The plesiosaurs Meyerasaurus and Plesiosaurus weathered the storm, and they witnessed the emergence of several new plesiosaurs, such as the dolphin-sized Microcleidus and the sixteen-foot-long Hauffiosaurus. A tubby plesiosaur named Bishanopliosaurus is unusual in that it seems to have favored freshwater environments (depending upon its fossil locale), and the tyrant plesiosaur of the Toarcian was the twenty-three-foot-long pliosaurid Rhomaleosaurus. This fierce plesiosaur may have been able to pick up scents while submerged by forcing water through skull passages containing sensory organs; if this is true, then it may have hunted its prey in a manner similar to some modern shark species. Ichthyosaurs, too, suffered in the Toarcian Turnover: though the ‘large and in charge’ Temnodontosaurus remained the apex predator of the deep oceans, the shallow seas witnessed the disappearance of both Ichthyosaurus and Leptonectes – ending a dominance that had begun in the Late Triassic – as well as the extinction of Eurhinosaurus and Suevoleviathan. New dolphin-sized ichthyosaurs emerged to fill the niches left vacant: the fourteen-foot-long Dearcmhara (Gaelic for ‘marine lizard’) swam the warm, shallow seas in what is now northwest Scotland, and its contemporary Steopterygius was hugely successful, with over one hundred specimens discovered in Germany (they may have perished while congregating for mating or birthing). Ichthyosaurs weren’t the only marine reptiles to fill the tumbled niches left in the wake of the Toarcian Turnover; it’s in the Toarcian that we see the emergence of the ocean-going crocodiles known as thallatosuchians.

one of the earliest thalottuschians, Pelagosaurus

The thalattosuchians were crocodylomorphs that embraced a fully aquatic lifestyle. Though the saltwater crocodile is known to transverse wide bodies of oceanic water, it does so in order to reach land; this wasn’t the case with these thalattosuchians. They were, without doubt, fully marine, and they would become staples of the Jurassic oceanic ecosystems and thrive into the Middle Cretaceous. The thalattosuchians are divided into two groups: Teleosauridea and Metrirhynchoidea (the teleosaurids would become the largest-bodied and most successful of the two). Teleosaurids are known mostly from lagoons and coastal marine environments, but some remains have been found from prehistoric estuaries and freshwater ecosystems. They were highly successful and diversified quickly. In most ecosystems there were three types of thalattosuchians: long-snouted forms with lots of small, pointed teeth designed for grasping small fish; long-snouted forms that had a shorter snout and fewer teeth than the first form, though these teeth were larger and capable of taking down larger prey; and short-snouted forms that had even fewer teeth, but in which the teeth were robust with numerous ridges on the enamel. All three types occurred side-by-side in many ecosystems, indicating that they maintained their diversity by occupying different yet overlapping ecological niches. By the end of the Middle Jurassic, a fourth ‘type’ would appear: these ‘Machimosaurini’ were the giant teleosaurids, reaching between sixteen and twenty-four feet in length to become the largest crocodylomorphs of the Jurassic. Though they started off similar to the third type given above, with short snouts and robust teeth, they underwent evolutionary development in the Late Jurassic and Early Cretaceous to have short snouts, huge jaw-closing muscles, and blunt teeth with serrated edges – perfect for feasting on sea turtles. The Metrirhynchoids – the second group of thalattosuchians – is split into two subgroups: ‘true’ metriorhynchids that became dolphin-like with a tail fin, flippers, and loss of bony armor; and ‘basal’ metriorhynchoids that are immediate between the teleosaurids and the dolphin-like metriorhynchids. Many thalattosuchians had porous bones like modern animals – such as whales and seals – that dive deep into the ocean.

a profile shot of the Chinese Peipehsuchus

Thalattosuchians emerged in the Toarcian and began radiating almost immediately. Peipehsuchus is known from southern Laurasia, in modern China; and two genera – Platysuchus and Pelagosaurus – are known from northern Laurasia in the vicinity of Europe. Pelagosaurus grew ten feet in length and weighed about one thousand pounds. It was well-designed for aquatic life: it had a long, streamlined snout; a tail with fin-like attributes; and paddle-like limbs for swimming the warm, shallow seas of the prehistoric North Sea. Its jaws had thirty teeth capable of hunting and grasping fish, crustaceans, and insects while swimming (one fossil specimen was found with an early fish in its stomach). Its forward-facing eyes and streamlined body imply that it was a pursuit predator rather than a scavenger or ambush hunter. It was remarkably similar to modern crocodiles and probably swam in the same way, by whipping its tail from side-to-side (though it would’ve been more agile than modern crocs). It would’ve had to emerge from the water to lay eggs or rest on the banks, but it would’ve spent the vast majority of its time in the water.

a pair of Dorygnathus on a fishing expedition

Pterosaurs began to diversify in the Toarcian skies. The first of these new pterosaurs was Campylognathoides of Laurasia. This was a gigantic pterosaur for its time, with a wingspan of five feet and eight inches. It had a slight upward hook to the front of its jaw, a pointed snout, and large eyes indicating excellent eyesight (leading some to believe it was nocturnal or diurnal). Its conical, recurved teeth would’ve been capable of delivering a piercing bite, and so it’s theorized it hunted small terrestrial animals. Its robust forelimbs and proportionally large wing fingers would’ve made it a fast flyer, and it may hunted in the same fashion as modern falcons. Two other pterosaurs, Dorygnathus and Parapsicephalus, looked a lot alike, except the latter’s skull was longer than the former’s. Dorygnathus was smaller-sized, with a wingspan of three feet three inches, and it was wildly successful, for its remains are abundant throughout Europe. It had long, curved, sharp teeth near the front of both the upper and lower jaws; these teeth pointed forward and interlocked when the mouth was closed to form a ‘spiky grab.’ This arrangement, seen also in Parapsicephalus, was designed for impaling and trapping slippery fish.

a couple of Ohmdenosaurus in prehistoric Germany

Dinosaur remains are scant for this age of Early Jurassic history, but nonetheless paleontologists have made some interesting discoveries. In China of then-southern Laurasia, the sauropod Lingwulong represents the earliest-known member of ‘neosauropoda,’ the  ‘New Sauropods’ to which most sauropods of the Jurassic and Cretaceous belong. Heading north into modern Europe, we come across Emausaurus – the only known Toarcian thyreophoran, which resembled the earlier Scelidosaurus – and the sauropod Ohmdenosaurus. Ohmdenosaurus is the first sauropod known from Europe. Most sauropods of the Early Jurassic stayed near the Tethys Sea; for some reason or another, they didn’t have a tendency of migrating far north. The absence of sauropod remains in Europe – though there are a few other than Ohmdenosaurus – may clue us into how the wooded, island-esque ecosystems were generally unsuitable for sauropods. Nevertheless, Ohmdenosaurus made its home on one of these large islands connected to the mainland by a narrow tract of land. Moving westward into Gondwana and heading into Africa, it’s worth noting the medium-sized theropod Berberosaurus, which some believe to be the first true ceratosaur.

an Emausaurus gets ready to stand its ground against an unnamed Toarcian theropod

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