The Jurassic Period
201 mya - 145 mya
The Jurassic Period was the first stratigraphic unit to be mapped and classified in a formal manner. Its genesis began in the late 1700s when the English engineer William ‘Strata’ Smith noticed different rock layers while excavating waterways across southern and central England. As excavators dug into the ground, he noticed that different layers contained different sets of fossils. He had a eureka moment and realized that one could identify rock strata based upon the fossils the rocks contained. He was the first to promote ‘The Law of Faunal Succession,’ which has remained a cornerstone – no pun intended – of stratigraphy to this day. Building upon his discoveries, between 1797 and 1815 he compiled the first geological maps using different colors to signify different strata. The workmen excavating the waterways knew the varying sequences as the ‘lias’ and the ‘oolite’; the ‘lias’ was a term used by Dorset quarrymen to refer to the ‘layers’ – consisting of alternating beds of limestone and clay – that sprouted across England from the south to the northeast; and ‘oolite’ was the thickly-bedded limestones that were excellent for building projects. The geological system Smith mapped and his quarrymen excavated dated to the Jurassic Period, though it had yet to be known by that name.
The supercontinent Pangaea had marked the Triassic Period, but by the Early Jurassic it was beginning to come apart at the seams. By the end of the Jurassic, the continents would more or less be aligned at their modern latitudes (with minor fluctuations), and three modern seas would be in their infancy: the Atlantic, Pacific, and Indian Oceans. The disintegration of Pangaea was accompanied by constant volcanic activity so that murals and paintings of the Jurassic tend towards showcasing volcanoes erupting in the distance. This isn’t artistic license: at any given point during the Jurassic, volcanic activity would’ve been happening somewhere on the planet. Volcanoes spitting hot lava across the terrain or shooting magma kilometers into the sky would’ve been commonplace, and many creatures would’ve met their end by sudden volcanic eruptions that scoured the landscape with superheated, hurricane-force winds. An example of the cataclysmic nature of these eruptions is seen with the Karoo landscape of South Africa, a popular hunting ground for Triassic fossils: the whole area was inundated by more than 3300 feet of lava due to volcanic eruptions.
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 – but, as an interesting side-note, not all rifts succeeded. Some are considered ‘failed rifts’, in which they failed to tear the continent apart, and some of these form modern major river courses, such as the Amazon and the Mississippi rivers.
At the beginning of the mid-Jurassic, the splitting of Pangaea had created northern and southern blocks. As Pangaea ruptured between north and south, new continents were born: northern Laurasia (comprising modern North America and Eurasia) and southern Gondwana (comprising South America, Africa, and Australia). Southwestern Laurasia (North America) remained attached to northern Gondwana (Africa), but they were slowly zippering apart. By the end of the mid-Jurassic, North America and Africa had completed separated, and Africa was in the process of untangling from South America (though the severance wouldn’t take place until the mid-Cretaceous). Along Gondwana’s northeastern edge, a pair of large continental fragments – which would in time become part of the Himalayan Mountains – were drifting north across the eastern end of the Tethys Sea. Much of western North America was flooded by rising sea levels, though island chains and continental fragments thrust out of the sea far to the west; in time these islands would combine with coastal mountain ranges and become part of the North American west.
Throughout the Middle Jurassic, Laurasia was suffering its own tectonic splitting, and by the beginning of the Late Jurassic – about 150 million years ago – Laurasia was almost completely divided between modern-day North America and Eurasia. The waterway between them was the slender, miniscule genesis of the North Atlantic Ocean. Another rift was forming between North America and what would become Greenland (the two would be completely separated by the end of the Jurassic). In western North America, the Sundance Sea had formed during the mid-Jurassic. This was a shallow continental sea that had spread southwards across the Midwest. Late in the mid-Jurassic it began to dry up and retreat northwards, leaving behind a vast plain of sediment laid down by a shifting network of rivers and streams coursing down from the ancestral Rocky Mountains to the west. This was the formation of the Morrison Formation (see below under the subheading Jurassic Dinosaurs). By the Late Jurassic this valley was semi-arid, but it was so flat and so close to sea level that it was soggy most of the year. Massive lakes swelled and shrank on a seasonal basis; rivers flooded, spreading silt, and then dwindled to mere streams. Trees and permanent flora were confined to the riverbanks, and vast sand dunes stitched the landscape between the accordion-like waterways.
Gondwana, the southern continent, was a little more stable, but rifting was beginning to occur. South America and Africa remained solidly conjoined, but a waterway was forming between the African and Indian plates. By the end of the Late Jurassic and the beginning of the Cretaceous, South America and Africa had broken away from Antarctica and Australia, and India was a large island in the Tethys Sea. Australia and Antarctica would be conjoined until long after the Mesozoic Era; their combined mass helped channel warm water toward Antarctica and kept it from freezing over.
The oldest part of the modern ocean floor dates to 180 million years ago, during the Jurassic Period. New species of phytoplankton appeared in the oceans, and corals formed extensive tropical reefs, especially in the burgeoning Tethys Seaway. The shallow shelf seas bordering the Tethys brimmed with life: bivalves, gastropods, and cephalopods flourished. Cephalopods blossomed into a variety of shapes and sizes: there were coiled ammonites and straight squid-like belemnites; some ammonites were the size of a button while others grew to be as large as the wheels of massive construction trucks; all had great variety in the ornamentation of their ribs, ridges, and spines. Starfish were common, as they had been since Paleozoic times. Crinoids – also known as ‘sea lilies’ – thrived, growing wherever there was hard substratum and even clinging to driftwood. Horseshoe crabs prowled the shorelines, remarkably similar to the modern horseshoe crabs you can pet at most aquariums today. Pterosaurs fished the waters – and often they died there, as their remains show up regularly in marine sediments.
Jurassic sharks prowled the shallow coastal waters; sharks first appeared in the Devonian and have been continuing strong ever since. One of the most successful Mesozoic sharks was Hybodus, which belonged to a family of sharks that appeared in the Lake Permian and continued into the Late Triassic. The six-foot-long Hybodus cruised the shallow Tethys Sea, hunting slippery prey and chowing down on crustaceans and mollusks when it desired (it had two kinds of teeth designed for both catching slippery prey and crushing shelled organisms). It had a bony blade on its dorsal fin that it likely used for defense. Rays also existed during the Jurassic Period; the Jurassic ray Spathobatis had flattened teeth – likely for eating shellfish – and an elongated snout that would’ve been used for probing for food on the muddy seafloor.
The rays would’ve been on the lookout for the seagoing crocodylomorphs – such as Teleosaurus and Metriorhynchus – that hunted in the shallow seas. Though some modern crocodilians, such as the saltwater crocodile, can swim in the ocean, the seagoing thalattosuchians had made the ocean their home, venturing onto land only to lay eggs. Thalattosuchians 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.
Pushing farther out from the shallow shelf seas into the deeper oceans, life on the seafloor became scarce, likely because there was little oxygen on the seabed. The large number of islands prevented bottom currents, which in turn limited depth circulation of nutrients and oxygen. This isn’t to say that the deep ocean was barren, only that its creatures kept near the surface. It was in the deep ocean that many marine reptiles plied their trade, hunting squid and bony fish and one another. The deep oceans were prowled by marine reptiles such as the plesiosaurs and ichthyosaurs. The plesiosaurs first appeared in the Triassic Period, and though they started off small, they bloomed to gigantic sizes by the end of the Cretaceous. Plesiosaurs diversified during the Late Triassic and Early Jurassic and continued to radiate. Most plesiosaurs of the early Jurassic were around sixteen feet in length, but by the Middle Jurassic they were far more numerous, and some species developed extremely long necks. Plesiosaurus – the group’s namesake – lived off the coasts of Early Jurassic England. It had a small head, a long and slender neck, a broad turtle-like body, a short tail, and two pairs of large, elongated paddles. It grew to about eleven feet long. Arminisaurus grew a few feet longer and also lived in the Early Jurassic. Yuzhoupliosaurus grew to twelve feet in length and prowled the freshwater inlets of modern-day China.
In the Middle Jurassic, behemoth pliosaurs developed. Though these were technically still plesiosaurs, they evolved shorter necks with large heads. Gallardosaurus of the Late Jurassic hunted in the shallow seas of the burgeoning Caribbean Seaway. Megalneusaurus prowled the Sundance Sea of the Late Jurassic; it grew to twenty-one feet in length and hunted marine reptiles. The most famous pliosaur of all, Liopleurodon, grew to twenty-one feet in length and dominated Jurassic seaways from the Middle Jurassic to the Late Jurassic. While their long-necked brethren likely hunted smaller, fast-moving prey, the pliosaurs were designed for attacking larger-sized marine reptiles or Jurassic sharks. They were the apex marine predators of the Jurassic.
No doubt many ichthyosaurs fell to pliosaur bandits. Ichthyosaurs emerged during the Triassic Period (though some paleontologists believe they may have first evolved in the Late Permian) and they flourished. Following the Triassic-Jurassic Extinction Event, however, 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 merely meant they had to adapt or die – and adapt they did. Jurassic ichthyosaurs didn’t grow as large as their forebears (for the position for ‘large oceanic predator’ was stolen by the plesiosaurs), and the specialized suction/ram feeders and durophagous species (who subsisted on shellfish) disappeared; but sleeker forms evolved to continue the ichthyosaur line. Most Jurassic ichthyosaurs were streamlined and dolphin-like. Ichthyosaurus appeared in the Late Triassic and survived until the middle of the Early Jurassic; it was a smaller-sized ichthyosaur, growing to eleven feet in length; it’s one of the best known ichthyosaurs after its initial discovery in the late 19th century, and it hunted the Laurasian seas of modern Europe and Asia. Eurhinosaurus of the late Early Jurassic grew to twenty feet in length and hunted the deep seas of Laurasia; it had a distinctive jaw anomaly in that its upper jaw was twice as long as the lower jaw and covered with downwards-pointing teeth. Excalibosaurus of Early Jurassic England grew to twenty-two feet in length, and its upper jaw was longer than the bottom, giving it the look of a swordfish. The Early Jurassic ichthyosaur Temnodontosaurus was widespread in both Laurasia and Gondwana; this ichthyosaur was humongous, clocking in at around forty feet in length, rivaling the earlier Shonisaurus of the Late Triassic. It likely hunted fish, cephalopods, and even other ichthyosaurs and plesiosaurs of the deep oceans. Prior to the emergence of the pliosaurs in the Middle Jurassic, Temnodontosaurus would’ve been the apex predator of the Jurassic seas.
Little is known about ichthyosaurs in the Middle Triassic, but this isn’t surprising, as the Middle Jurassic tends to have a spotty fossil record (in many ways, this period of history is a ‘black hole’ of paleontology). By the time we get to the abundant bone-beds of the Jurassic Period, we find that ichthyosaurs have been diversifying into several different shapes and sizes. Nannopterygius lived from the Middle Jurassic to the Early Cretaceous, and its fossils have been found in Europe where there was a warm, shallow sea; it would’e lived in shallow seas, probably around coral reefs. It was small, growing to just over ten feet, with more than a third of its length being in its tail. It was flexible, agile, and a fast swimmer; it’s theorized that it was an ambush predator that plunged into shoals of fish to snag a bite. Opthalmosaurus lived from the Middle Jurassic to the Late Jurassic, and it grew to nineteen feet in length. It had a long, toothless snout probably used for catching squid. It hunted in the deep seas of both Laurasia and Gondwana. Its chief claim to fame is its monstrous eyes; these eyes were protected by sclerotic rings that would’ve helped maintain the shape of the eyes under enormous water pressure. The size of the eyes and the sclerotic rings suggest that Opthalmosaurus hunted prey in great depths where light was meager; it may also have hunted at night when its prey species were more active. As an interesting aside, ichthyosaurs are abundant in the Holzmaden region of Germany, and some paleontologists believe it’s because that area was a favorite ‘birthing spot’ for these marine reptiles.
Modern fish, the teleosts, came to the fore in the Jurassic Period and diversified throughout the Mesozoic. The teleosts (‘complete bone’) are considered ‘true fish,’ and they comprise 96% of living fish. They evolved from more primitive bony fish, such as the Middle Jurassic Leedischthys, which reached up to seventy-two feet in length and holds the world record for the world’s largest known fish. It belonged to the group Ostelchthyes, or ‘bony fish,’ so-called because their skeletons are made of bone rather than cartilage. Teleosts are distinguished from more primitive bony fish by lighter scales and hinged jaws (teleosts have a movable premaxilla and jaw musculature that enables them to protrude their jaws outwards so that they can grab prey and draw it into the mouth). A common Jurassic teleost (which first appeared in the Triassic) was the sixteen-inch-long, herring-like Pholidophorus. It had large eyes and was a fast-swimming predator that probably ate small crustaceans and smaller fish.
By the end of the Jurassic Period, many hexacoral species (organisms formed of polyps with a six-fold symmetry) were building mind-boggling reefs that make the modern Great Barrier Reef off northeast Australia look like Amateur Hour. A vast sponge reef spread across the northern part of the Tethys Sea, stretching from modern Spain to Romania and Poland, a distance of 1800 miles. The Tethys Sponge Reef grew in deeper waters than modern corals, reaching about five hundred feet deep, and was anchored to the continental shelf. When the reef came close to the surface, however, sections of the reef began to die off. The skeletal remains of the sponge reef were used as anchors for other types of corals that could live closer to the surface. As the reefs broached towards the surface, they created wave-breakers that prevented the circulation of nutrients. As a result, the water isolated by these reefs was inundated with lime and salt, and the water became anoxic, killing any animal or plant that came in contact with it.
Pterosaurs first emerged during the Triassic Period and reached their heyday in the Late Jurassic. The earliest pterosaurs are considered rhamphorhynchids based on their body morphology, in that they had short heads and long tails like the Late Jurassic Rhamphorhynchus. These types of pterosaurs flitted about the Jurassic skies unchallenged until the Late Jurassic when another group of pterosaurs, the pterodactyloids, emerged. These pterosaurs were long-skulled with long necks and short tails. The rhamphorhynchids began to decline in the Late Jurassic as the pterodactyloids overtook their niches. By the end of the Jurassic Period and the beginning of the Cretaceous, the rhamphorhynchids were more or less extinct and the skies belonged to the pterodactyloids. Some scientists speculate that the rhamphorhynchids went into decline due to the emergence in the Late Triassic of another flying predator: the avian dinosaurs (but more on that anon).
Only a handful of pterosaur genera are known from the Early and Middle Jurassic. The Middle Jurassic has a bad rap, because a lack of fossil bonebeds dating back to that time has made it difficult to construct detailed analyses of the time period. This held true for the pterosaurs, as well, at least until mid-Jurassic bonebeds were discovered in China. These bonebeds have given us a wealth of information of mid-Jurassic pterosaurs. By the Late Jurassic, pterosaurs were radiating like kudzu in the American south. Dozens upon dozens of genera are known, belonging to both the rhamphorhynchids and the pterodactyloids. Jurassic pterosaurs showcased a variety of lifestyles: some, such as Dimorphodon, likely hunted small terrestrial creatures; some, like Dorygnathus, caught fish in ‘spike traps’ created by wildly-arranged teeth; others, such as Dendrohynchoides, subsisted off a diet of insects. Towards the end of the Late Jurassic, filter-feeding pterosaurs evolved, such as Ctenochasma. One of the oddest pterosaurs was a furball named Jeholopterus, which may have been a blood-sucking pterosaur that operated much like modern vampire bats. The largest pterosaur of the Jurassic skies was the eight-foot-wingspan Pterodactylus – not to be confused with the larger Cretaceous Pteranodon. Most Jurassic pterosaurs were mid-sized, with wingspans around three to four feet, though there were many smaller pterosaurs such as Batrachognathus, which likely flitted above prehistoric lakes, gobbling up dragonflies and mayflies. Pterosaurs in general seem to have been coastal critters, hunting out at sea or along the shoreline or among the scattered islands created by Pangaea’s rifting and flooding. Though some pterosaurs ventured farther inland to prowl landlocked lakes and ponds, the predominant rule-of-thumb was a coastal lifestyle.
The Pterosaurs: A Closer Look
Jurassic Pterosaurs: A Closer Look
After the world-breaking Triassic-Jurassic Extinction Event, dinosaurs were free to become the dominant life form on land. It was a dominance they kept for 135 million years. This doesn’t mean, of course, that there was nothing else going on beyond the Jurassic shorelines. Dinosaurs co-existed with a host of other terrestrial creatures: lizards and amphibians, burrowing mammals (our own ancestors!), and crocodylomorphs were just some of the organisms that shared the terrestrial ecosystems with Jurassic dinosaurs.
We begin by looking at the crocodylomorphs, a group of archosaurs that continues today with modern crocodiles and their kin. The crocodylomorphs reached their zenith in the Mesozoic as they diversified and enjoyed far more abundance than they possess today. It was during the Age of the Dinosaurs that they reached titanic sizes that make modern crocodiles look like puny weaklings. The first crocodylomorphs appeared in the Late Triassic, and they looked nothing like crocodiles as we know them today. They were small, lightly-built, agile, and quick-footed creatures whose limbs were built directly beneath their bodies (they didn’t ‘sprawl’ like modern crocs). Mesozoic crocodylomorphs generally fall into three groups: the sphenosuchians were the first to evolve, and these were the ‘reptilian wolves’ of the Triassic. They didn’t last past the Late Jurassic, but there are plenty of them found in the Early Jurassic. The next group of crocodylomorphs were the protosuchians, primitive proto-crocs adapted for life on both land and in the water. None of the protosuchians got very big, and they survived through the Jurassic and into the Early Cretaceous Period. Next came the mesosuchians (or ‘Middle Crocs’), which had more in common with the crocodiles we know today. The mesosuchians themselves are divided into four different groups, each with their particular characteristics: the ocean-dwelling thalattosuchians (which were superbly successful during the Jurassic Period), the heavily-armored notosuchians who decided to go vegan, and the ‘eusuchians’ (or ‘true crocs’), which consist of all living crocodiles and their closest extinct relatives.
The Early Jurassic saw a panoply of the lightly-built, agile sphenosuchians that emerged in the Triassic. The sphenosuchian Dibothrosuchus grew to just over four feet in length, had a keen sense of hearing, and was probably a vocal creature like modern crocodiles. It was slender, long-tailed, and long-limbed. It had a pointed snout and two rows of armor plates running along the midline of its spice. Litargosuchus was also small and gracile, 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. Protosuchians also appear in the Early Jurassic: Orthosuchus had a body proportion similar to lizards and only reached just over a foot and a half in length. It had a flattened skull and massive eyes. Orthosuchus had features that helped it keep breathing when submerging in water, and like modern crocs, its ears were protected by earflaps that prevented water inflow when it was submerged. Its shape and snout is reminiscent of the Indian gharials who prey on small fish; perhaps Orthosuchus, though a poor swimmer because of the design of its legs and tail, slowly prowled towards schools of small fish before lunging in attack. Another protosuchians, Protosuchus, was three and a half feet long, had armored skin and powerful legs, and was likely a terrestrial hunter.
Moving into the Middle Jurassic, we find that some terrestrial crocodylomorphs were growing larger: Hsisosuchus of China lived during the Middle and Late Jurassic and grew up to ten feet in length. However, smaller sizes remained the norm. Junggarsuchus of the Middle Jurassic was a smaller, terrestrial sphenosuchian; Macelognathus, another sphenosuchian from the Late Jurassic, was also small, and it lived in what is now the state of Wyoming. Fruitachampsa of the Late Jurassic lived in modern-day Colorado, and it was a long-limbed terrestrial crocodilian that grew to about three and a half feet long. It had a short face and prominent pair of canine-like teeth in the lower jaw. Its snout was similar to that of modern caimans, though it wasn’t flattened as with modern crocs. The bony armor running in two overlapping rows on its back were thinner than in modern crocodiles. An odd crocodylomorph of the Late Jurassic was Knoetschkesuchus, which was small and short-snouted and reached just under two feet in length. It probably fed on small prey such as invertebrates, amphibians, and early mammals; this specialization was due to the fact that it lived in an island ecosystem of Jurassic Europe that may have resembled modern mangrove swamps.
Rhynchocephalians – lizard-like reptiles that first appeared in the Triassic Period – were popular during the Jurassic and Cretaceous, though only one species – the tuatara of New Zealand – survives in the present day. Many of the niches occupied by modern lizards were held by rhynchocephalians and their relatives in the Mesozoic, though true lizards – which appeared first in the Jurassic – began to overtake these niches in the Cretaceous. Though the modern tuatara is carnivorous, the diverse rhynchocephalians of the Mesozoic enjoyed a variety of lifestyles: some were omnivorous, some were herbivorous, and others were durophagous (eating shellfish and crustaceans). Some even became aquatic, such as the Jurassic Pleurosaurus, which was long, thin, and around five feet in length.
Squamata, the reptilian branch that includes lizards and snakes, got its start during the Jurassic (though snakes wouldn’t appear during the Cretaceous). One of the earliest lizards was Ardeosaurus, one of the first geckos, which grew to about twenty centimeters in length. Eichstaettisaurus of the Late Jurassic Germany (Laurasia) had a flattened head, forward-pointing and somewhat symmetrical feet, and tall claws; it had several adaptations for a climbing lifestyle. Bharatagama, from the Early Jurassic, is the oldest known iguana. It’s known from over one hundred specimens from Germany, indicating it did well for itself. It was part of the group that includes modern chameleons. Turtles first appeared during the Triassic Period, and they continued into the Jurassic. Kayentachelys lived in the early Jurassic of modern-day Arizona, and Eurysternum was a sea turtle of the Late Jurassic.
Amphibians made leaps and bounds during the Jurassic. This period witnessed the genesis of two of the three modern amphibian groups, frogs and salamanders (the caecilians – limbless, serpentine amphibians that resemble snakes – emerged in the Late Triassic). Prosalirus is considered the earliest ‘true frog’; it lived in the Early Jurassic and three specimens have been found in Arizona. Its skeleton was designed to absorb the forces of hard take-offs and landings, its hind legs were designed for jumping, and it’d lost its tail. It had long hip bones, long hind legs, and long ankle bones like modern frogs. The Jurassic also saw the first salamanders (lizard-like amphibians with slender bodies, blunt snouts, short limbs projecting at right angles to the body, and which had tails both as larvae and into adulthood). Karaurus is one of the earliest-known salamanders; found in modern-day Kazakhstan (Laurasia back then), it had a wide head, short body, and grew to about 7.5 inches. It likely preyed on aquatic invertebrates. As for the caecilians, they are represented by the Early Jurassic Eocaecilia, which was about 15 centimeters long. It differed from modern legless caecilians in that it had small legs and well-developed eyes (modern caecilians have poor eyes, as they spend most of their time underground).
Temnospondyls also lived during the Jurassic, though they were nowhere near as successful as they’d been since their genesis in the Carboniferous Period. Temnospondyls were small- to giant-sized ‘primitive amphibians’ that flourished during the Carboniferous, Permian, and Triassic Periods. Though they survived into the Jurassic, they didn’t fare well in the burgeoning ‘Age of Reptiles.’ Their numbers and lineages drastically dwindled, and only a few species continued into the Early Cretaceous before being wiped out. Despite their eventual extinction, temnospondyls were evolutionary marvels; during their time, they adapted to all sorts of environments – fresh water, ocean water, and terrestrial habits were claimed. Though most were semiaquatic, a few were terrestrial, returning to the water only to breed. Though temnospondyls are considered ‘primitive amphibians,’ they differ from modern amphibians in many ways. The most visible differences include the fact that many temnospondyls had scales, claws, and some even had armor-like bony plating. Debates rage over modern amphibian relations with the temnospondyls. Though some argue that temnospondyls were the ancestors of modern amphibians, others hold that modern amphibians are more closely related to the ancient lepospondyls (another group of ‘primitive amphibians’ which existed from the Carboniferous to the Permian; still others speculate that frogs and salamanders evolved from temnospondyls while caecilians evolved from lepospondyls. An example of an Early Jurassic temnospondyl was Siderops, which grew to about 2.5 feet long and had a large, heavy skull.
Insects became smaller during the Jurassic Period than they’d been in the Triassic. Scientists speculate its due to a variety of factors: first, the atmosphere during the Triassic was richer in oxygen than during the Jurassic, which is important because insects don’t have lungs and get their oxygen via breathing tubes; as oxygen dropped, larger insects couldn’t breathe, and most of the large types died off. Towards the end of the Jurassic, however, oxygen levels increased, but none of the insects grew larger. This brings us to the second factor: larger insects were easier prey for the pterosaurs and burgeoning avian dinosaurs. Pterosaurs were more common in the Jurassic than they were in the Triassic, and many were designed for preying on insects; and many of the small avian dinosaurs likely preyed on insects. Larger, more cumbersome insects would’ve been easier to spot and seize, so smaller insects have an advantage. Thus Jurassic insects were small, and they resembled modern-day dragonflies, beetles, flies, and grasshoppers. Worms, snails, and spiders were also present. When angiosperms began appearing towards the end of the Jurassic and the beginning of the Cretaceous, pollen-gathering insects such as bees, moths, and butterflies began to appear.
The Crocodylomorphs: A Closer Look
The Temnospondyls: A Closer Look [Coming Soon]
Mammals evolved from a therapsid offshoot called the cynodonts, and though they may have emerged as early as the Triassic, it’s known that they were definitely present during the Jurassic. For decades it was believed that Mesozoic mammals lived ‘undercover’, hiding by day and scurrying by night to avoid being stamped out by the dinosaurs. Over the last few years, however, that perspective has changed, largely thanks to amazing fossil discoveries in China. China has become a paleontological hotspot, for several of its bone-beds have provided a wealth of detailed information about the past. During the Jurassic Period, China was inundated with volcanic activity, and animals were frequently buried under ash. Most Mesozoic fossils are comprised of scattered bones, usually disarticulated, but the ash-entombed fossils from China often include entire skeletons, and their unique fossilization process has even preserved such fragile remains as fur, feathers, skin, and internal organs. When the new Chinese discoveries are coupled with other discoveries around the world, we begin to see that Mesozoic mammals were ecologically diverse and experimenting with a wide range of lifestyles: there were burrowers and climbers, swimmers, and even gliders! They came in all sorts of sizes: some were as small as voles, others grew to rat- and badger-sized, and others even grew as large as modern beavers (a far cry from the traditional view of ‘mammals undercover’ during this period of history).
Cynodonts were alive and well during the Triassic Period, but even they suffered during the Triassic-Jurassic Extinction Event. One type of cynodont, the tiny, shrew-like Morganucodonta, survived the extinction and thrived throughout the Jurassic Period. Many paleontologists believe that Morganucodonta was the evolutionary precursor to modern mammals, including human beings (it’s worth noting, of course, that mammal origins are shaky; many different creatures have been labeled as ‘the first true mammal’ or our ‘evolutionary ancestor’; this is due both to the nature of evolution and common ancestry and to the fact that paleontology is a puzzle more than anything else!). Though there were relatively few mammals in the Jurassic when compared to the large numbers of reptiles and dinosaurs, they nevertheless underwent a rapid variation during the Early and Middle Jurassic. This diversification slowed during the Late Jurassic. Though mammal evolution experienced other ‘radiation hotspots’ during the Mesozoic, none compared to that of the Early and Middle Jurassic; in fact, the rapidity of mammal diversification early in the Jurassic wouldn’t be outdone until the Cenozoic. Paleontologists believe this Jurassic Radiation can be explained by the opening of new ecological niches due to the breakup of Pangaea. The radiation seen at this period in time defined the main mammalian morphologies that would dominate the rest of the Mesozoic underworld.
Most of the Jurassic mammals were small and shrew-like burrowers, often nocturnal or diurnal. Megazostrodon survived the Late Triassic and lived into the Early Jurassic. Often considered one of the first true mammals, this shrew-like creature was less than five inches long. It’s believed to have been nocturnal due to a larger brain than its cynodont relatives; the enlarged parts of the brain are those areas that, in mammals, process sounds and smells. The theory goes that it evolved this way to avoid competing with reptiles or becoming prey to dinosaurs. While most mammals were of the shrew-like burrowing variety – such as Docofossor, which had paws and claws designed for digging and which looked like a modern mole – there were outliers. Agilodocodon could climb trees and likely gnawed into bark to access sap. Several mammals were gliders like our modern flying squirrels: Maiopatagium, Vilevolodon, and Volaticotherium all had long limbs, long hand and foot fingers, and wing-like membranes for tree-to-tree gliding. Many Jurassic mammals show trademark mammalian characteristics: Microdocodon had a form of the hyoid bone that enables mammalian infants to suckle and is the earliest known mammal known to have suckled milk; and the mouse-like, paper-clip-sized Hadrocodium may be the earliest animal to have a nearly full mammalian middle ear.
The winner-takes-all of mammalian diversity during the Jurassic Period is undoubtedly the carnivorous Gobiconodon. This mammal appeared in the Middle Jurassic and was so successful that it lasted until the Late Cretaceous. While it was long believed all Mesozoic mammals were herbivorous, omnivorous, or insectivorous, Gobiconodon has shown that many were likely predators in their own right. This odd-looking mammal weighed 10-12 pounds, had a beaver-like tail, otter-like paddling limb with (likely) padded feet, and seal-like teeth. Its jaws included shearing molar teeth and large canine-like incisors. It had a powerful jaw and strong limbs, indicating that it hunted vertebrate prey. It’s interesting to note that another carnivorous mammal, the badger-sized Repenomamus of the Early Cretaceous, was found with the bones of a Psittacosaurus in its stomach!
After the world-breaking Triassic-Jurassic Extinction Event, dinosaurs were free to become the dominant terrestrial landforms across the world, and they went at this goal with gusto. It was during the Late Triassic that the earliest dinosaurs - the theropods and prosauropods - flittered among the margins of earth's ecosystems, but during the Jurassic they radiated in all directions; it was during the Jurassic that you saw some of the most famous dinosaurs come to the fore. The sauropods, whose genesis began in the Late Jurassic, rose to spectacular new heights; such heavyweights as Diplodocus, Apatosaurus, and Brachiosaurus roamed the savannah-like western United States in the Late Jurassic. Giraffatitan, pictured in the image to the left, lived in northern Africa. The theropods, whose lineage stretches back to the Jurassic, diversified and grew larger: the Jurassic saw the emergence of the dilophosaurids - such as Dracovenator and Dilophosaurus - and megalosaurs - such as Magnosaurus and Megalosaurus - and even the first tyrannosaur, Proceratosaurus. The infamous predators Allosaurus and Ceratosaurus vied neck-and-neck with the sauropods of western North America.
The ornithischian (or 'bird-hipped') dinosaurs saw the most variation. The cat-sized heterodontosaurs - such as Heterodontosaurus and Pegomastax - scurried among the early and middle Jurassic underbrush. Stegosaurs, who would be one of the dominant low-browsing herbivores of this period, first emerged in Europe and North America. Scelidosaurus, though technically not a stegosaur, came onto the scene in the Early Jurassic and was a cousin to the true stegosaurs, the first of which were Huayangosaurus and Lexivosaurus, who appeared in Asia and Europe respectively in the Middle Jurassic. By the end of the Jurassic Period, true stegosaurs would be found on nearly every continent throughout the world. The most famous of these were Stegosaurus of North America and Kentrosaurus (pictured here) of northern Africa. Sadly, stegosaurs didn't last far into the Cretaceous and were soon eclipsed by new low-browsing ornithischians: the hadrosaurs and ceratopsians. The latter were overshadowed by the stegosaurs during the Jurassic, and early ceratopsians looked nothing like the horned tanks of the Cretaceous: the earliest ceratopsians were cat- and dog-sized likely-bipedal beaked herbivores scurring about in the underbrush. These early ceratopsians emerged in China and would radiate throughout the globe by the end of the Mesozoic. Ankylosaurs, which would be common low-browsers in the Cretaceous, first appeared in the Jurassic: Gargoyleosaurus and Mymoorapelta took the stage during the Late Jurassic in North America, and they were the forerunners of more popular Cretaceous cousins such as Ankylosaurus and Euplocephalus. These early ankylosaurs are considered 'nodosaurids' since they had yet to develop the bony tail club often associated with ankylosaurs; these bone clubs wouldn't evolve until the Cretaceous. A final class of Jurassic ornithischians are the ornithopods, such as Camptosaurus and Dryosaurus: these were the forerunners of the iguanodonts and hadrosaurs and are often viewed as the 'livestock' of the Jurassic, roaming in herds much like modern-day cows and having few means to defend themselves.
It was during the Jurassic Period that ‘flying dinosaurs’ (or ‘birds’) first evolved. Theropod dinosaurs and modern birds share hundreds of characteristics, not least of all the presence of feathers. The first ‘proto-feathers’ likely evolved for thermoregulation (to help keep dinosaurs warm) or, more likely, for display purposes (as with modern peacocks). As certain theropod lineages evolved, some of these ‘dino-birds’ were able to take flight. Feathers, originally for a purpose other than flight, became essential to flight in what has been called a ‘happy accident.’
How did flying dinosaurs come about? Scientists have devised a proposed ‘route of emergence’ for many features that serve as ‘spring-pads’ for the emergence of avian birds. The basic ‘bird blueprint’ – long, straight legs and feet with three skinny toes – first appeared in the Late Triassic some 230 million years ago as dinosaurs evolved into upright, bipedal, fast-running creatures. These hind-limb features are staples of both dinosaurs and birds. Some time later, some of these dinosaurs of the theropod lineage fused their left and right collarbones into a new structure, called a wishbone; this stabilized the shoulder girdle and enabled them to absorb the extreme shock forces of grabbing prey. This wishbone, designed to protect the hunter from the physical effects of hunting methods, would later serve as a spring to store energy for flying dinosaurs when in flight. Farther down the line, some theropods called maniraptorans evolved a curved neck (perhaps for scouting prey), and some of these species shrank in size. Smaller sizes gave them access to new ecological niches, such as trees and brush and even underground caves or borrows that were inaccessible to larger dinosaurs. Even later, some of these shrinking dinosaurs began to fold their arms against the body (probably to protect the delicate quill-pen feathers that were evolving around the same time). These were the paravians – a subgroup of the maniraptorans – and would be the immediate ancestors of birds. These paravians were smaller, nimbler, and smarter than their larger-sized cousins. The paravians include the dromaeosaurs (such as Deinonychus and Velociraptor) and the troodontids (such as Troodon). They had feathers, many of them had wings, and most likely looked and acted like birds. Somewhere among these paravians lies the blurred line between ‘bird’ and ‘non-bird’ (at least in modern parlance).
Paleontologist Steve Brusatte writes in The Rise and Fall of the Dinosaurs, “The totality of the evidence—that wings first evolved in dinosaurs too large and ungainly to fly, that these wings were originally colored, and that modern birds use their wings for display—has led to a radical new hypothesis. Wings originally evolved as display structures—as advertising billboards projecting from the arms, and in some cases, like Microraptor, the legs, and even the tail. Then these fashionably winged dinosaurs would have found themselves with big broad surfaces that by the unbreakable laws of physics could produce lift and drag and thrust. The earliest winged dinosaurs… probably would have considered the lift and drag produced by their billboards to be little more than an annoyance. In any case, whatever lift was generated wasn’t nearly enough to get such large animals into the air. But in more advanced paravians, which had the magic combination of bigger wings and smaller body size, the billboards would have been able to take on an aerodynamic function. These dinosaurs could now move around in the air, even if awkwardly at first. Flight had evolved—and it had happened totally by accident, the billboards now repurposed as airfoils.” (pp. 297-298)
We turn now to the critical question: “How did paravians become flyers?” We know that flight has evolved in the animal kingdom (excepting insects) at least three times: first in pterosaurs during the Triassic Period, then in birds sometime during the Mesozoic (likely in the Jurassic or Cretaceous Period), and finally in bats during the Cenozoic. All three of these animal groups evolved flight independently in episodes of convergent evolution. There are three main anatomical requirements for flight, and these are shared by pterosaurs, birds, and bats: (1) a lightweight body, generally accomplished via lightweight bones and sometimes fewer bones in the back; (2) airfoil to produce lift, in which the wing is an airfoil that, when combined with forward motion, produces lift, which enables the body to become airborne; and (3) energy to take off (most modern birds are strong enough to become airborne from a standing start, but heavy pterosaurs would’ve needed a good run to take off, and how the earliest dino-birds took flight is debated). Studying the fossil record, we can determine that flying dinosaurs didn’t evolve from just one branch of the paravian tree; rather, it looks like flight evolved multiple times. Brusatte notes, “Flight probably evolved multiple times in parallel, as different species of these dinosaurs-with their different airfoil and feather arrangements—found themselves generating lift from their wings as they leaped from the ground, scurried up trees, or jumped between branches.” (298, Brusatte) Some of these early birds were gliders; incapable of powered flight, they could nonetheless soar on air currents. The four-winged maniraptor Microraptor could glide, as proven by experiments in which scientists built an anatomically correct, life-size model and put it in a wind tunnel. Brusatte continues, “Most theropods were evolving at ho-hum rates, but then, once an air-worthy bird had emerged, the rates went into overdrive. The first birds were evolving much faster than their dinosaur ancestors and cousins, and they maintained accelerated rates for many millions of years… [Other] studies have shown that there was a sudden decrease in body size and a spike in rates of limb evolution right around this same point on the genealogy, as these first birds were quickly getting smaller and growing larger arms and bigger wings so that they could fly better.” (302, Brusatte)
It’s interesting to note that Archaeopteryx, one of the oldest birds, was capable of powered flight; this means that the origin of ‘small, winged, flapping, bona fide birds’ likely stretched back to the Middle Jurassic, around 170 to 180 million years ago. This means there was around 100 million years in which flying dinosaurs coexisted with their ground-loving counterparts. By the end of the Cretaceous Period, there were at least fourteen branches of the modern bird family tree that had taken root; the Cretaceous-Tertiary Extinction wiped out the non-avian dinosaurs and many of these bird lineages so that only eight branches went on to flourish in the Tertiary Period.
The Avian Genesis: A Closer Look
The supercontinent Pangaea had marked the Triassic Period, bu
Notes on The Toarcian Turnover
Notes on the Jurassic Ice Age
The term ‘Jurassic’ comes from the Jura Mountains in northern Switzerland, and in 1795 – just before Smith began compiling his geological map – Alexander von Humboldt coined the term ‘Jura Kalkstein’ to describe the limestone outcroppings in northern Switzerland. In 1829, just shy of fifteen years following Smith’s work, the French geologist Alexandre de Brogniart used the term ‘Terraines Jurassiques’ for outcrops of oolite in mainland Europe. It wasn’t long before ‘the Jurassic’ came to refer to a certain assemblage of rock layers with their unique fossils. These rock layers fell into three main parts: the Lias, equivalent to the Lias of the Dorset quarrymen (which is also known as the Early or Lower Jurassic); the Dogger, equivalent to the lower part of the Oolite (the Middle Jurassic); and the Malm, equivalent to the middle and upper Oolite (the Late or Upper Jurassic). Between 1842 and 1849, geologists identified eleven stages within these three parts; all stages are named after the places where they were first identified, and these ‘stages’ have continued as part of our understanding of the Jurassic Period.
~ Geography and Climate ~
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 – but, as an interesting side-note, not all rifts succeeded. Some are considered ‘failed rifts’, in which they failed to tear the continent apart, and some of these form modern major river courses, such as the Amazon and the Mississippi rivers.
At the beginning of the mid-Jurassic, the splitting of Pangaea had created northern and southern blocks. As Pangaea ruptured between north and south, new continents were born: northern Laurasia (comprising modern North America and Eurasia) and southern Gondwana (comprising South America, Africa, and Australia). Southwestern Laurasia (North America) remained attached to northern Gondwana (Africa), but they were slowly zippering apart. By the end of the mid-Jurassic, North America and Africa had completed separated, and Africa was in the process of untangling from South America (though the severance wouldn’t take place until the mid-Cretaceous). Along Gondwana’s northeastern edge, a pair of large continental fragments – which would in time become part of the Himalayan Mountains – were drifting north across the eastern end of the Tethys Sea. Much of western North America was flooded by rising sea levels, though island chains and continental fragments thrust out of the sea far to the west; in time these islands would combine with coastal mountain ranges and become part of the North American west.
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| The Sundance Sea of western North America |
Gondwana, the southern continent, was a little more stable, but rifting was beginning to occur. South America and Africa remained solidly conjoined, but a waterway was forming between the African and Indian plates. By the end of the Late Jurassic and the beginning of the Cretaceous, South America and Africa had broken away from Antarctica and Australia, and India was a large island in the Tethys Sea. Australia and Antarctica would be conjoined until long after the Mesozoic Era; their combined mass helped channel warm water toward Antarctica and kept it from freezing over.
The Early Jurassic saw lower temperatures than those at the end of the Triassic. Temperatures warmed up about ten million years into the Jurassic, but about ten million years after that, temperatures cooled off again. The end result was that the Jurassic Period was somewhat cooler than the desert-like conditions of the Triassic, but this isn’t to say the Jurassic was cold: the mean temperature for the Period was sixty degrees, and it never got cold enough to trigger a lengthy Ice Age (though there was a miniature Ice Age near the start of the Late Jurassic).
The amorphous breakup of Pangaea changed ecosystems and weather systems: the Jurassic saw an increase in rainfall (due to the splitting of the continents, more regions were nearer the sea where moist winds could bring rain) 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.
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| a rainforest reminiscent of the coastal Jurassic |
The Jurassic’s warm and wet climate promoted the growth of rainforests, especially in the tropical areas, and these rainforests increased carbon dioxide levels to forge a ‘greenhouse’ climate that fostered global warming. The groundswell of Jurassic forestation provided the Jurassic coalfields used today. The position of the continents during the Jurassic allowed warm water to circulate even in polar regions, and the result was a globally uniform climate that was warmer and wetter than it is today. The lands bordering the Tethys Sea and its off-shooting waterways were low and thickly vegetated. The arid continental interiors, which had been nearly lifeless wastelands during the Triassic, began to grow green as the climate enabled them to root and flourish where they had previously dared to tread. Studies of Jurassic plants indicate that warm conditions reached as far north and south as sixty degrees latitude, up to our modern world’s polar zones. The northern and southern poles weren’t tropical like most of the Jurassic world, but they remained temperate with only seasonal snows in the higher latitudes. There were no polar ice caps.
Recent research has indicated that 174 million years ago, at the beginning of the Aalenian stage of the Middle Jurassic, the planet underwent an abrupt, mid-latitude ‘ice age’ in which seawater temperatures cooled. This miniature ‘ice age’ has been attributed to a massive volcanic event, known as the North Sea Dome, which lasted millions of years. This volcanic event altered the flow of seawater and the heat that it carried into the equator to areas in the North Pole. A counter-effect was that the planet underwent a significant, rapid cooling in temperature. Scientists wrote in the journal Nature Communications, “Our [research] results show an especially abrupt earliest Middle Jurassic mid-latitude cooling of seawater by as much as ten degrees Celsius in the north-south Laurasian Seaway, a marine passage that connected the equatorial Tethys Ocean to the Boreal Sea [and known as the Viking Corridor].”
Recent research has indicated that 174 million years ago, at the beginning of the Aalenian stage of the Middle Jurassic, the planet underwent an abrupt, mid-latitude ‘ice age’ in which seawater temperatures cooled. This miniature ‘ice age’ has been attributed to a massive volcanic event, known as the North Sea Dome, which lasted millions of years. This volcanic event altered the flow of seawater and the heat that it carried into the equator to areas in the North Pole. A counter-effect was that the planet underwent a significant, rapid cooling in temperature. Scientists wrote in the journal Nature Communications, “Our [research] results show an especially abrupt earliest Middle Jurassic mid-latitude cooling of seawater by as much as ten degrees Celsius in the north-south Laurasian Seaway, a marine passage that connected the equatorial Tethys Ocean to the Boreal Sea [and known as the Viking Corridor].”
~ Jurassic Marine Life ~
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| soft-bodied cephalopods made for tasty pterosaur treats |
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| a Hybodus (upper right) swims among several ichthyosaurs |
The rays would’ve been on the lookout for the seagoing crocodylomorphs – such as Teleosaurus and Metriorhynchus – that hunted in the shallow seas. Though some modern crocodilians, such as the saltwater crocodile, can swim in the ocean, the seagoing thalattosuchians had made the ocean their home, venturing onto land only to lay eggs. Thalattosuchians 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.
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| a pair of Metriorhynchus crocodylomorphs attacking a doomed bony fish |
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| the Early Jurassic Plesiosaurus |
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| a Liopleurodon hunts smaller plesiosaurs |
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| a school of Ichthyosaurus |
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| the Late Jurassic Opthalmosaurus |
Modern fish, the teleosts, came to the fore in the Jurassic Period and diversified throughout the Mesozoic. The teleosts (‘complete bone’) are considered ‘true fish,’ and they comprise 96% of living fish. They evolved from more primitive bony fish, such as the Middle Jurassic Leedischthys, which reached up to seventy-two feet in length and holds the world record for the world’s largest known fish. It belonged to the group Ostelchthyes, or ‘bony fish,’ so-called because their skeletons are made of bone rather than cartilage. Teleosts are distinguished from more primitive bony fish by lighter scales and hinged jaws (teleosts have a movable premaxilla and jaw musculature that enables them to protrude their jaws outwards so that they can grab prey and draw it into the mouth). A common Jurassic teleost (which first appeared in the Triassic) was the sixteen-inch-long, herring-like Pholidophorus. It had large eyes and was a fast-swimming predator that probably ate small crustaceans and smaller fish.
By the end of the Jurassic Period, many hexacoral species (organisms formed of polyps with a six-fold symmetry) were building mind-boggling reefs that make the modern Great Barrier Reef off northeast Australia look like Amateur Hour. A vast sponge reef spread across the northern part of the Tethys Sea, stretching from modern Spain to Romania and Poland, a distance of 1800 miles. The Tethys Sponge Reef grew in deeper waters than modern corals, reaching about five hundred feet deep, and was anchored to the continental shelf. When the reef came close to the surface, however, sections of the reef began to die off. The skeletal remains of the sponge reef were used as anchors for other types of corals that could live closer to the surface. As the reefs broached towards the surface, they created wave-breakers that prevented the circulation of nutrients. As a result, the water isolated by these reefs was inundated with lime and salt, and the water became anoxic, killing any animal or plant that came in contact with it.
~ Jurassic Life in the Skies ~
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| Darwinopterus of mid-Jurassic China |
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| Sordes, a small pterosaur, examines a snail |
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| Rhamphorhynchus of the Late Jurassic |
Jurassic Pterosaurs: A Closer Look
~ Underfoot the Dinosaurs ~
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| the Early Jurassic sphenosuchian Litargosuchus |
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| Protosuchus of the Early Jurassic |
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| Fruitachampsa of the Late Jurassic |
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| the aquatic Pleurosaurus, a lizard-like rhynchocephalian |
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| an artist's rendition of Eichstaettisaurus |
Amphibians made leaps and bounds during the Jurassic. This period witnessed the genesis of two of the three modern amphibian groups, frogs and salamanders (the caecilians – limbless, serpentine amphibians that resemble snakes – emerged in the Late Triassic). Prosalirus is considered the earliest ‘true frog’; it lived in the Early Jurassic and three specimens have been found in Arizona. Its skeleton was designed to absorb the forces of hard take-offs and landings, its hind legs were designed for jumping, and it’d lost its tail. It had long hip bones, long hind legs, and long ankle bones like modern frogs. The Jurassic also saw the first salamanders (lizard-like amphibians with slender bodies, blunt snouts, short limbs projecting at right angles to the body, and which had tails both as larvae and into adulthood). Karaurus is one of the earliest-known salamanders; found in modern-day Kazakhstan (Laurasia back then), it had a wide head, short body, and grew to about 7.5 inches. It likely preyed on aquatic invertebrates. As for the caecilians, they are represented by the Early Jurassic Eocaecilia, which was about 15 centimeters long. It differed from modern legless caecilians in that it had small legs and well-developed eyes (modern caecilians have poor eyes, as they spend most of their time underground).
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| a pair of Karaurus, the first known salamander |
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| Siderops, a temnospondyl holdout of the Early Jurassic (Toarcian Period) |
Insects became smaller during the Jurassic Period than they’d been in the Triassic. Scientists speculate its due to a variety of factors: first, the atmosphere during the Triassic was richer in oxygen than during the Jurassic, which is important because insects don’t have lungs and get their oxygen via breathing tubes; as oxygen dropped, larger insects couldn’t breathe, and most of the large types died off. Towards the end of the Jurassic, however, oxygen levels increased, but none of the insects grew larger. This brings us to the second factor: larger insects were easier prey for the pterosaurs and burgeoning avian dinosaurs. Pterosaurs were more common in the Jurassic than they were in the Triassic, and many were designed for preying on insects; and many of the small avian dinosaurs likely preyed on insects. Larger, more cumbersome insects would’ve been easier to spot and seize, so smaller insects have an advantage. Thus Jurassic insects were small, and they resembled modern-day dragonflies, beetles, flies, and grasshoppers. Worms, snails, and spiders were also present. When angiosperms began appearing towards the end of the Jurassic and the beginning of the Cretaceous, pollen-gathering insects such as bees, moths, and butterflies began to appear.
The Crocodylomorphs: A Closer Look
The Temnospondyls: A Closer Look [Coming Soon]
~ Mammals Undercover? ~
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| Morganucodon of the Late Triassic and Early Jurassic |
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| Agilodocodon (upper left) and Docofossor |
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| a family of Maiopatagium |
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| Gobiconodon prowling a Late Jurassic waterway |
~ Dinosaurs of the Jurassic ~
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| a Ceratosaurus in Late Jurassic North America |
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| a Stegosaurus shares a drink at a river with a herd of Dryosaurus |
It was during the Jurassic Period that little theropod dinosaurs first took flight. They emerged in southern Gondwana (modern China). The earliest, Serikornis, hails from the Middle Jurassic. By the Late Jurassic, a flurry of paravians - such as Anchiornis, Caihong, and Xiaotingia - were flittering about the trees of China. These paravian theropods appeared in Europe at the tail-end of the Jurassic Period, the most notable of which was the infamous Archaeopteryx. The 'avian genesis' is described in further detail in the next section.
~ The Avian Genesis ~
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| the infamous Late Jurassic Archaeopteryx |
How did flying dinosaurs come about? Scientists have devised a proposed ‘route of emergence’ for many features that serve as ‘spring-pads’ for the emergence of avian birds. The basic ‘bird blueprint’ – long, straight legs and feet with three skinny toes – first appeared in the Late Triassic some 230 million years ago as dinosaurs evolved into upright, bipedal, fast-running creatures. These hind-limb features are staples of both dinosaurs and birds. Some time later, some of these dinosaurs of the theropod lineage fused their left and right collarbones into a new structure, called a wishbone; this stabilized the shoulder girdle and enabled them to absorb the extreme shock forces of grabbing prey. This wishbone, designed to protect the hunter from the physical effects of hunting methods, would later serve as a spring to store energy for flying dinosaurs when in flight. Farther down the line, some theropods called maniraptorans evolved a curved neck (perhaps for scouting prey), and some of these species shrank in size. Smaller sizes gave them access to new ecological niches, such as trees and brush and even underground caves or borrows that were inaccessible to larger dinosaurs. Even later, some of these shrinking dinosaurs began to fold their arms against the body (probably to protect the delicate quill-pen feathers that were evolving around the same time). These were the paravians – a subgroup of the maniraptorans – and would be the immediate ancestors of birds. These paravians were smaller, nimbler, and smarter than their larger-sized cousins. The paravians include the dromaeosaurs (such as Deinonychus and Velociraptor) and the troodontids (such as Troodon). They had feathers, many of them had wings, and most likely looked and acted like birds. Somewhere among these paravians lies the blurred line between ‘bird’ and ‘non-bird’ (at least in modern parlance).
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| the Middle Jurassic Serikornis |
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| the Early Cretaceous Confuciusornis |
It’s interesting to note that Archaeopteryx, one of the oldest birds, was capable of powered flight; this means that the origin of ‘small, winged, flapping, bona fide birds’ likely stretched back to the Middle Jurassic, around 170 to 180 million years ago. This means there was around 100 million years in which flying dinosaurs coexisted with their ground-loving counterparts. By the end of the Cretaceous Period, there were at least fourteen branches of the modern bird family tree that had taken root; the Cretaceous-Tertiary Extinction wiped out the non-avian dinosaurs and many of these bird lineages so that only eight branches went on to flourish in the Tertiary Period.
The Avian Genesis: A Closer Look
~ Jurassic Plant Life ~
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| An Assortment of Jurassic Plants |
The main trees of the Jurassic were conifers, pines, and monkey-puzzle trees (often called 'Chilean pines'). Cycads and tall tree-ferns were also abundant. The forest floors were carpeted with liverworts and mosses, clubmosses, ferns, and horsetails. At the beginning of the Jurassic, ferns and gingkoes that reproduced via spores were the most common flora on the planet. As the world’s climate stabilized towards sub-tropical generalities, gymnosperms began to take center stage. Gymnosperms are cone-bearing plants, such as conifers and pine trees (the first true pines emerged during this period), and they quickly spread over much of the Jurassic world. The four major groups of gymnosperms alive today are cycads, ginkgos, conifers, and angiosperms; of these four, only one – the ‘flowering plants’, or angiosperms – were absent in the Jurassic. Gymnosperms were the staple trees of the lush Jurassic forests, as they had been during the Triassic. The Jurassic Period didn’t see the emergence of major new plant-groups, though some of the classic plants from the Triassic – such as the seed-fern Glossopteris – went extinct. The plant life seen in the Jurassic represents the diversification of plant life first seen in the Triassic.
Spore-bearing ferns diversified, but the seed-bearing ferns that had dominated southern Pangaea during the Triassic were run to extinction by cycads and conifers. Thus if you were to travel back in time to the Jurassic and take a quiet nature hike – hopefully avoiding the predatory lurkers of these Jurassic forests – you would walk abed carpets of ground-spreading spore-bearing ferns (grass had yet to evolve) interspersed with upward-sprouting tree ferns, gigantic cycads, and towering conifers. As the forest breaks against a river, the bank would likely be coated with horsetails – yes, the very same you often see today in flooded roadside ditches. Horsetails first stepped onto the scene during the Devonian, and they’re considered ‘living fossils’ because they’ve undergone very little change since their genesis. Many of the conifers in the forest would be similar to us today, as many modern conifer families – such as the Taxodiaceae, Taxaceae, Pinaceae, Cephalotaxaceae, and Araucariceae – flourished in the Jurassic. The Podocarpaceae – evergreen trees and shrubs – dominated the southern hemisphere of the Jurassic, especially in modern Antarctica, and they’re found today in many parts of the world. An extinct form of conifer, the Cheirolepidiaceae family, were common in the equatorial regions along with the shrub-like Bennettitales (they resembled cycads but different in the arrangement of their stoma).
The most striking plant life you’d see, though, would be the Jurassic cycads. Cycads were wildly successful during the Jurassic, so much so that paleobotanists call the Jurassic ‘The Age of Cycads.’ Cycads were seed plants that emerged during the Carboniferous Period, and though they’re around today, they’re rare compared to their prevalence in the Mesozoic. Most cycads have a stout and woody trunk with a crown of large, hard, stiff, evergreen leaves. Their leaves are usually pinnate (arranged in a feather-like pattern) and grow in a rosette form from the top of the trunk. Cycads have a cylindrical trunk; as the trunk grows, the leaves – which are generally the same size or larger than the trunk – sprout only from the top, so that cycads are marked by a crown of leaves at their top. Some cycads’ trunks are buried underground, giving the cycad an appearance like a forest-hugging fern. Cycads reproduce by a cone sprouting from the top of the trunk in the middle of the leaves. Cycads are slow growers and can live up to a thousand years. They are only distantly related to palms and ferns, despite their obvious similarities. During the Triassic and Jurassic Periods, cycads comprised twenty percent of the earth’s plant life. Cycads formed the ground cover of the Mesozoic: these primitive cycads were stumpy, with round or barrel-like stems, or they had tall, slender trunks topped by a tuft of palm-like leaves.
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| East Africa in the Late Jurassic |
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| a stegosaur wanders a Jurassic forest of China |
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| an artist's depiction of Antarctica in the Jurassic Period |
~ The Last Days of the Jurassic Period ~
Notes on The Toarcian Turnover
Notes on the Jurassic Ice Age
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