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Jurassic Introduction

The Jurassic period extends from 199.6 to 145.5 million years ago. The Jurassic period was named for the Jura Mountains by the French chemist Alexandre Brongniart (1770-1847). The Jura Mountain range lies in an area between France, Germany and Switzerland and contains limestone exposures of Jurassic age. Although set back by the Triassic crises, life recovers and diversifies.

Primary Producers & Reefs

Extinction at the end of the Triassic disrupted the adaptive radiation of dinofagellates and coccolithophores. Eighty percent of marine species were eliminated by this crisis. One genus representing coccolithophorids and two the dinoflagellates crossed over into the Jurassic period. During the early Jurassic primary production was dominated by green algal phytoplanktons, which grow well in anoxic conditions (Payne & Schootbrugge, 2007, p. 179). However, dinoflagellates and coccolithophorids not only recover they are joined by a new type of protist, the diatoms. Diatoms (phylum Heterokontophyta) make their first appearance during the Jurassic. Diatoms are unicellular or colonial eukaryotic phytoplankton. Diatoms are the most successful of the phytoplankton in today’s oceans in terms of biomass and net primary production. Diatoms are encased in a two-part, asymmetrical silica cell wall. The two halves of the silica case fit together like the parts of a petri dish. The diatom’s silica shell is called a frustule. With the appearnce of diatoms, the transition to the modern primary producers could now occur. Diatoms, dinoflagellates, and coccolithophores would assume their dominant role as the base of many modern marine ecosystems by Cretaceous times. Reefs were absent during the early Jurassic. Sponges, Tubiphytes, corals, and microbial mounds were the major contributors to reestablishing reefs. Toward the end of the Jurassic scleractinian corals and stromatoporoids would once again be the primary reef builders (Webb, 2001, p. 176).

Marine Invertebrates

Ammonoids, bivalves, and gastropods recover and undergo adaptive radiations. Ammonites evolved rapidly during the Jurassic with entire groups succeeding each other in less than a million years. This makes Ammonites very useful for studying rock strata (Buttler, Cope, & Owens, 2009, p. 234). Belemnoids become common predators of the sea. Some well preserved specimens reveal belemnites had ten arms and an ink sac. Belemnites had an internal chambered shell supported by a cylindrical calcium carbonate structure called the guard. The guards are bullet-shaped and are the most commonly preserved part of a belemnite. Sea urchins representing irregular echinoids with a bilateral symmetry superimposed on a radial symmetry appear in the Jurassic. These echinoids had smaller spines and were adapted for moving forward, burrowing and feeding on detritus (Prothero, 2004, p. 336). The Decapods (crabs, lobsters, crayfish and shrimps) underwent a great adaptive radiation during the Mesozoic. Crabs and some lobsters make their first undisputed appearance in the Jurassic. Predation during the Mesozoic from crabs and lobsters may have forced mollusks to burrow and many brachiopods to go extinct (Prothero, 1998, p. 265).


Cartilaginous fish and bony fish continue to diversify into new forms during the Jurassic. The neoselachian sharks experienced bursts of adaptive radiations during the Jurassic and Cretaceous. Modern shark lineages are a continuation of the initial Jurassic adaptive radiation. Neoselachian sharks lived side-by-side with the hyodonts, which went extinct by the end of the Cretaceous. (Benton, 2005, p. 169). Hyodonts, such as Hybodus, had a mouth that opened at the end of the snout. The teeth of Hybodus had multiple points, which made them good for capturing prey, but not tearing the prey. In contrast, the jaws of neoselachian (modern sharks) open widely and have great mobility. The mouth opens beneath the head and the jaws move forward to capture the prey. The serrated teeth gouge and butcher the prey. Nasal clues indicate neoselachians had an improved sense of smell. The cartilage vertebrae of modern sharks is calcified, which provides more support for swimming (Benton, 2005, p. 166).

Skates, rays, and sawfish are sharks that evolved flattened bodies for life on the seabed. This group of Chondrichthyes makes its appearance in the Jurassic. The rays were the first to evolve. Spathobathis looks very much like a modern Guitar or Banjo fish. This primitive ray had a flattened body with a pair of broad pectoral fins that acted as “wings”. Its eyes and spiracles were positioned on top of the head, while its gill slits were on the underside. Spathobathis had a snout for probing the seabed and teeth designed for eating shellfish (Dixon, 1988, pp. 28-29).

Towards the end of the Paleozoic the primitive ray-finned fish (Paleoniscids) had evolved some characteristics similar to the modern ray-finned fishes and are referred to as neopterygians. Neopterygians continued to diversify during the Jurassic period. Dapedium still possessed the protective scales covered with enamel. It had jaws and teeth designed for eating mollusks. Aspidorhynchus looked like a modern gar, but is related to the modern bowfin.

The modern ray-finned fish (order Teleostei) first appear in the Triassic and continued to diversify during the Jurassic. In the Cretaceous teleosts would establish themselves as the dominant bony fish and modern forms would make their appearance. The dividing line between the neopterygians and the primitive teleosts of the Jurassic is fuzzy. Hypsocormus had primitive enamel-covered, rectangular (rhomboid) scales, although they were smaller and more flexible. Hypsocaormus also had some more advanced features such as a second dorsal fin, symmetrical caudal fin, and an advanced jaw structure. Pholidophorus looked like a small herring, but still possessed the heavy ganoid scales and backbone that was not completely ossified. Leptolepis another herring-like fish, mentioned in the Triassic section, was the first teleost to have a completely ossified spine. It is believed Leptolepis moved in sholes as they are often found on the same slab in great numbers (Dixon, 1988, p. 40).


Lissamphibians (Infraclass Lissamphibia) or the modern amphibians include the extinct albanerpetontids and the living anurans (frogs and toads), the urodeles (newts and salamanders), and the gymnophionans (caecilians). Modern amphibians are represented by roughly 4000 species. Albanerpetontids range from the Jurassic to the Miocene and are very much like salamanders. Frogs and toads (Order Anura) have a skeletal structure modified for jumping. Prosalirus is the earliest known jumping frog from the Early Jurassic of the South-West USA. Vieraella, the oldest known frog with essentially the same anatomy as modern frogs, makes its appearance in the early Jurassic. Salamanders and newts (Order Urodela) first appear in the Mid-Jurassic. They have elongate bodies with four short walking limbs and a flattened tail for swimming. Karaurus, one of the first salamanders, makes its appearance in the late Jurassic. Karaurus has essentially the same anatomy as the modern salamander (Dixon, 1988, p. 57). The caecilians (Order Gymnophiona) look somewhat like earthworms. They have lost their limbs and burrow through leaf litter, soil, and swim in ponds. Eocaecilia is the earliest known caecilian from the Early Jurassic of Arizona, USA. Eocaecilia has the typical features of caecilians, but also has small, reduced legs. Based upon tooth structure, modern amphibians have their origins among the temnospondyls (Benton, 2005, pp. 102-103).


Several orders of turtles make their first appearance during the Jurassic period. Pleurodires (suborder Pleurodira) or side-neck turtles are aquatic and retract their neck with a sideways flexing. Some pleurodires still survive today. Cryptodires (suborder Cryptodira) are the most successful chelonians; most modern forms belong to this group. Cryptodires can retract their head with a vertical motion, moving it directly under the spine. Cryptodires evolved from pleurodires and displaced them by the end of the Jurassic (Dixon, 1988, pp. 68-69).

Plesiosaurs (order Plesiosauria) make their first appearance during the Triassic, but do not become well established until the Jurassic. Plesiosaurs can be divided into two major groups. Plesiosaurs (suborder Plesiosauroidea) have long necks, short heads and paddle-like flippers. Plesiosaurs fed on fish and cephalopods. Pliosaurs (suborder Pliosauroidea) had short necks, long heads, and paddle-like flippers. Pliosaurs fed on fish, cephalopods, sharks, ichthyosaurs, and even plesiosaurs.

Plesiosaurus (early Jurassic) used its flippers to quickly maneuver and its long neck to snatch swimming fish. Muraenosaurus (late Jurassic) had a neck and flippers that were longer than Plesiosuarus. The elongation of the neck and flippers along with an increase in the rigidity of the body was the evolutionary trend in plesiosaurs.

Interestingly, pliosaurs exhibit an almost opposite evolutionary trend, an increase in head size, a shortening of the neck, and lengthening of the paddle-like flippers. Plesiosaurs forelimbs were always longer than their hindlimbs, while pliosuars evolved to have hindlimbs longer than the forelimbs. Pliosuars were built for speed and maneuverability. Plesiosaurs specialized in maneuverability.

Macroplata (early Jurassic) was an early pliosaur that still had a fairly long neck, making it look like a plesiosaur. Liopleurodon (late Jurassic) exhibits the classic pliosaur look, large streamlined body with a heavy head and short neck. It almost has a whale-like appearance (Dixon, 1988, pp 76-77).

Although placodonts and nothosaurs did not survive, ichthyosaurs would evolve into two groups. Members of the family Ichthyosauridae had short, broad-like paddles, while members of the family Stenopterygiidae had longer, narrow paddles. Some very special fossils of Ichthyosaurus have given us a window into the life of these amazing marine reptiles. Fossils showing female Ichthyosaurs in the process of giving birth provide evidence that they gave birth to their young live while at sea. Stomach contents reveal a diet of fish and cephalopods, especially belemnites. The remains of pigment cells suggest a smooth thick skin colored reddish brown (Dixon, 1988, p. 81).

Terrestrial lizards undergo an adaptive radiation at the end of the Jurassic. Primitive forms of geckos, iguanas, and skinks make their first appearance. Geckos were the first of the modern lizards to appear. Ardeosaurus is the ealiest known gecko. Ardeosaurus had a flattened head; large eyes and jaws specialized for eating insects and spiders. It is not known if Ardeosaurus friction pads like modern geckos (Dixon, 1988, p. 88). Semi-aquatic and terrestrial forms of crocodiles evolved during the Jurassic. Modern forms of crocodiles and alligators would appear in the Cretaceous (Dixon, 1988, 100-101).

Flying reptiles continued to diversify during the Jurassic. Dimorphodon and Rhamphorhynchus, two well known rhamphorhynchs make their appearance during the Jurassic. Rhamphorhynchs become extinct at the end of the Jurassic. Pterodactyls (suborder Pterodactyloidea) make their first appearance in the Jurassic and diversify into many forms. Pterodactyls had shorter tails and longer skulls than rhamphorhynchs. Species of Pterodactylus are the best-known pterodactyls from the Jurassic (Dixon, 1988, pp. 104-105). Pterodactyls, like rhamphorhynchs specialized in eating fish.

A recently discovered pterosaur, Darwinopterus modularis, from the Middle Jurassic of China represents a transitional fossil linking primitive long-tailed pterosaurs and more advanced short-tailed pterodactyls (Lu, Unwin, Jen, Liu, & Ji, 2009, p. 1). The genus Darwinopterus (“Darwin’s Wing”) is for Charles Darwin, honoring the 2009 anniversaries of his birth (200 years) and the publication of On the Origin of Species (150 years). The species name modularis is Latin for “composed of interchangeable units”.

The species name focuses on a key aspect of this transitional fossil. Darwinopterus does not exhibit characteristics intermediate between basal pterosaurs and derived pterodactyloids; rather, it possesses the body and limbs of a primitive pterosaur and the head and neck of a pterodactyloid. This mosaic of primitive and derived characteristics provides evidence for modular evolution. Natural selection may have acted on modules instead of individual characteristics. In the first phase of pterosaur evolution the head elongated, teeth were reduced, the braincase enlarged, and the neck became more flexible. In the second phase, body and limbs modifications improved locomotion on the ground (Lu et al., 2009, p. 6).

Sir Richard Owen (1804-1892), a British comparative anatomist and paleontologist, created the taxon Dinosauria to describe large terrestrial reptiles that walked upright, clearly different from other fossil or living reptiles. He based Dinosauria on the grouping of three taxa including Megalosaurus, Iguanodon, and Hylaeosaurus. Dinosaurs (Superorder Dinosauria "terrible or fearfully great lizards") range from the Triassic to the Cretaceous (to the present if you include birds).

In 1887 Harry Seeley (1839-1904), a British paleontologist, proposed that Dinosauria could be divided into two groups based on their hip structure, braincase, and vertebrae (Padian, 1997, p. 494). Seeley's scheme has persisted to this day. The order Saurischia includes dinosaurs with a lizard-like hip structure. The order Ornithischia includes dinosaurs with hip structures reminiscent of birds. Representatives from both groups appear in the Triassic period, although Ornithischians remained relatively rare until the Jurassic. The oldest dinosaurs, like Coelophysis, were small bipedal, carnivorous organisms. Dinosaurs diversified into many forms and came to dominate terrestrial faunas during the Jurassic and Cretaceous.

Jurassic aged dinosaurs are found on all seven continents (Dodson, 1997, p. 388). Dinosaurs diversified into many forms and became the dominant terrestrial herbivores and carnivores. Prosauropods were widespread during the Early Jurassic, but would be displaced by Sauropods. Sauropods evolved into gigantic forms and were the most abundant dinosaurs in their communities. Stegosaurs were also abundant. A variety of small to medium sized theropods became the dominant carnivores. Ornithopods evolved into small to medium sized forms. Ankylosaurs appear in the Late Jurassic, but remained relatively rare.

Saurischian dinosaurs have a "primitive" pelvic girdle with the pubis pointing forwards and the ischium back. Saurischians also share an elongate, S-shaped neck, and asymmetrical hands with a distinct thumb (Prothero, 1998, p. 372). Saurischian dinosaurs can be placed into two major groups, the theropods (Suborder Theropoda) and the Sauropodomorphs (Suborder Sauropodomorpha).

The suborder Theropoda ("beast feet") includes the bipedal, carnivrous dinosaurs, which range from chicken size to the 6-tonne giants of the Cretaceous. Theropods have hollow, thin-walled bones. In general, the forth and fifth digits on the hand and foot are reduced. Only three toes on each foot are weight-bearing. Most theropods have sharp recurved teeth and claws at the end of each finger and toe (Wagonner, 1995, Theropod Page). Let's take a look at three major theropod groups.

Coelophysoids (Infraorder Coelophysoidea) were the first theropods. Coelophysoids were small, slender, bipedal, dinosaurs with a long tail and long narrow-snouted skull. These small carnivorous dinosaurs range from the Triassic to the Jurassic and were widespread geographically. Coelophysis is well known from Triassic beds, but is also found in Early Jurassic strata.

Ceratosaurs (Infraorder Ceratosauria) were closely related to the coelophysoids. Many ceratosaurs had crests on their skulls (Benton, 2005, p. 191). Dilophosaurus ("two crests"), from the Early Jurassic, had two flat-sided crests oriented vertically on either side of its skull. The crests may have been used in sexual displays. Dilophosaurus was up to 6 meters long and is the creature that kills Dennis Nedry in the film Jurassic Park. There is no scientific evidence to support the poisonous nature of these dinosaurs as depicted in the film. Eubrontes giganteus are believed to be the tracks of Dilophosaurus and are the state fossil for Connecticut. Dinosaur tracks in general are the state fossil for Massachusetts.

Ceratosaurus ("horned lizard"), from the Late Jurassic, was the same size as Dilophosaurus. Ceratosaurus had a pair of horns, possibly used for mating displays, on the nasal bones. Ceratosaurus had small bony plates running down its neck, back, and tail, giving it a serrated crest. Footprints thought to be that of Ceratosaurus found in the Morrison Formation of the western USA suggest these dinosaurs moved in groups.

The remaining theropods and birds belong to the infraorder Tetanurae. Tetanurans first appear in the Jurassic and can be grouped into two major divisions the Carnosauria and the Coelurosauria (Benton, 2005, p. 397).

Carnosaurs are the first well-known tetanurans and were large predators. Megalosaurus ("great lizard") was a 9 meter long carnosaur that ranges from Early to Late Jurassic. Megalosaurus was the first dinosaur to be scientifically named and described in the 1820's. It was one of the creatures that prompted Sir Richard Owen to coin the term Dinosauria in 1841.

At 12 meters long, 4.6 meters tall, and up to 2 tonnes, Allosaurus ("different lizard") was the largest carnosaur of the Late Jurassic. Joints in the upper and lower jaws allowed Allosaurus to wolf down large chunks of flesh. Allosaurus was equipped with more than seventy teeth up to 7 cm long and three sharp claws up to 25 cm long on each forelimb. Horn-like structures were located above and in front of each eye (Lessem & Glut, 1993, pp. 19-20). Allosaurus is the state fossil for Utah. Saurophaganax maximus is an Allosaur-like theropod that is the state fossil for Oklahoma.

Coelurosaurs are a diverse clade of theropods that are more closely related to birds than to the carnosaurs. Coelurosaurs include the tyrannosaursids (formerly grouped with carnosaurs) from the Late Cretaceous, ornithomimids, and maniraptorans.

The first coelurosaurs appear in the Jurassic and include such dinosaurs as Coelurus (“hollow tail") and Compsognathus ("elegant jaw"). Ornithomimids of the Early Cretaceous were slender theropods with ostrich-like bodies, small heads, relatively long necks, limbs and fingers. The Jurassic aged Elaphrosaurus may represent one of the first known ornithomimids; however, the first undisputed ornithomimids are found in the Lower Cretaceous (Osmolska, 1997, p. 501). Ornithomimids would reach their greatest diversity during the Late Cretaceous period.

Maniraptorans are the most derived theropods and include such familiar organisms as troodontids, dromaeosaurids, and birds. Eshanosaurus from the Early Jurassic of China may represent the first known maniraptoran. Maniraptoran theropods from the Early Cretaceous of China provide evidence that feathers evolved in the earliest coelurosaurs and functioned as insulation and possibly for display. Maniraptoran fossils exhibit an evolutionary progression through different types of feathers from simple bristles to advanced contour feathers. Although contour feathers do appear on some maniraptorans they may not have played a role in flight until the first known bird Archaeopteryx (Benton, 2005, pp 199-201).

The suborder Sauropodomorpha includes both the prosauropods and the sauropods. In general, they are herbivorous quadrupeds with a small head, long neck, large body with legs tucked beneath, and a long counterbalancing tail. The small head contained jaws with lanceolate or spatulate teeth used for stripping vegetation from branches. Food was ground in the gizzard by gastroliths. Their pillar-like legs supported a deep body to accommodate an enormous digestive system. Sauropodomorphs had five-toed, spreading feet. They possessed a large thumb claw that was probably used for digging and no claws on fingers 4 and 5. Sauropodomorphs had large nostrils set higher on the skull than most vertebrates.
Although basal sauropodomorphs were bipedal omnivores the evolutionary trend in this group was towards quadrupeds of increasing size. To accommodate ever larger sizes the vertebrae evolved with increasing bone pneumaticity (hollow spaces). Bone was developed only along the lines of stress. The massive pelvic girdle became firmly fused to the backbone by 4 and later 5 sacral vertebrae. The spaces in these bones may have been filled with air sacs connected to the lungs as in modern birds. These hollowed out bones with air sacs would have reduced weight and enhanced respiratory efficiency (Benton, 2005, p. 202).

Several Late Jurassic sauropods are familiar to many people. Camarasaurus ("chamber lizard") is the best- known sauropod from North America. This 18 meter long compact browser had a relatively short neck and tail. The forelimbs were only slightly shorter than the hindlimbs. Camarasaurus had a box-like skull with large nasal openings. Camarasaurus had heavy, spoon-shaped teeth that could handle tough plant material. Adults and juveniles have been found in the Morrison formation indicating they traveled in herds. Within these deposits are found isolated piles of polished stones, which may represent regurgitated gizzard stones. The eggs of Camarasurus may have been laid in lines instead of nests.

Apatosaurus ("deceptive lizard") was once known as Brontosaurus ("thunder lizard"). Othniel Charles Marsh (1831-1899), an American paleontologist, described two specimens as separate animals; Apatosaurus in 1877 and Brontosaurus in 1879 (McIntosh, 1990, p. 349). Later, it was realized that the two specimens represented separate species of the same genus. Apatosaurus was the first genus given, so by taxanomic convention it is the name used. Apatasaurus was 21 meters long and up to 30 tonnes. For almost one hundred years the skull of Camarasaurus was used as a model for Apatosaurus. The relatively small head of Apatosaurus was found in 1975. The teeth of Apatosaurus were all at the front of the mouth and served to rake foliage off branches. Like most sauropods Apatosaurus had five toes on each foot. The front foot had a large claw on the thumb, while each back foot had 3 claws. Each ankle joint had thick wedges of weight-bearing cartilage. The long whip-like tail in many sauropods like, Apatosaurus, may have been used for defense as well as a counter balance to the long neck.

Brachiosaurus ("arm lizard") was one of the tallest and largest sauropods, designed for reaching high in the trees. Brachiosaurus was up to 25 meters long, 16 meters tall, and weighed in at 80 tonnes. Unlike other sauropod families Brachiosaursids had front legs that were longer than the hind legs. The neck of Brachiosaurus accounted for half of its height. The humerous was enormous accounting for over 2 meters of the animal's height and giving it the name "arm lizard". Brachiosaurus had a relatively short tail and the whole body sloped down from its highest point at the shoulders, not unlike the modern giraffe. The vertebrae were marvals of engineering with large hollowed out spaces on the sides making a lightweight framework that provided maximum strength along the lines of stress with a minimum amount of bone (Dixon, 1998, p. 129). Pleurocoelus is a brachiosaurid and the state dinosaur for Texas. The Cretaceous-aged sauropod, related to Brachiosaurus, Astrodon johnstoni is the state dinosaur for Maryland.

Ornithischian dinosaurs have a pelvic girdle in which the pubis runs back parallel to the ischium. There is also a prepubic process pointing forwards. Ornithischians were all herbivorous dinosaurs and possessed a predentary bone, which is a beak-like bone in front of the lower jaw. The predentary bone is matched with the premaxilla or the rostral (in ceratopsians) in the upper jaw. These bones helped Ornithischians clip vegetation. Ornithischians possessed cheek teeth that are inset into the jaw, suggesting they had fleshy cheeks for holding food (Prothero, 1998, p. 372).

Ornithischian dinosaurs can be divided into two major groups. The suborder Cerapoda includes ornithopods (Infraorder Ornithopoda), pachycephalosaurs (Infraorder Pachycephalosauria), and ceratopsians (Infraorder Ceratopsia). The suborder Thyreophora includes the ankylosaurs (Infraorder Ankylosauria) and stegosaurs (Infraorder Stegosauria). Stegosaurs were the most abundant Ornithischian dinosaurs during the Jurassic.

Ornithopods ("bird feet") were the most diverse and successful group of ornithischians and included the heterodontosaurids, hypsilophodontids, iguanodontids, and hadrosaurids. Ornithopods reached their greatest diversity during the Cretaceous, but some representatives appear in the Jurassic.

Othnielosaurus (Othnielia) is a primitive hypsilophodont from the Late Jurassic of Colorado, Wyoming, and Utah. Othnielosaurus is named for the famous 19th century American fossil hunter professor Othniel Charles Marsh of Yale University. Heterodontosaurus (“different toothed lizard”) from the Early Jurassic of South Africa is named for its differentiated teeth. Heterodontosaurus had stabing-like teeth in the front of its mouth, canine-like teeth, and molar-like teeth. Othnielosaurus and Heterodontosaurus were small lightly-built bipedal ornithopods. Both were herbivorous and had five fingers on their forelimbs.

Stegosaurs are known mainly from the Late Jurassic. Stegosaurs have small heads, massive bodies, and rows of alternating bony plates that extend down either side of the backbone. Their heavy tail is equipped with pairs of long, sharp spikes. Stegosaurs hindlimbs are much longer than their forelimbs, which hint at a bipedal ancestry.

Stegosaurus (roof lizard) is the state fossil for Colorado and is the largest and most well known of the stegosaurs. Stegosaurus was up to 9 meters long and weighed in at over 2 tonnes. Stegosaurus was not equipped to chew food, so it probably utilized a gizzard to help grind food, as did many other herbivorous dinosaurs. The Stegosaurus brain was the size of a walnut. A cavity in the hip vertebrae of Stegosaurus housed a nerve ganglion to help control the hindquarter movements. In some big-hipped dinosaurs this nerve ganglia or sacral "brain" was larger than the brain (Dixon, 1988, p. 156). The back plates of Stegosaurus show evidence of extensive vascularization and probably functioned to thermoregulate body temperature. They may have also been used for sexual and deterrent displays (Benton, 2005, p. 217).


In general, Jurassic mammals remained small nocturnal insectovores and carnivores. However, Jurassic mammals continued to evolve traits critical to the success of their modern descendents. Traits possessed by you and me.

Kuehneotherium from the Lower Jurassic of South Wales is known mostly from isolated teeth. Kuehneotheriids possess teeth with a triangular cusp pattern that may represent a precursor to the tribosphenic molars that help to define modern mammals. Kuehneotherium also retained some primitive traits such as a double jaw articulation (Rose, 2006, p. 54).

Hadrocodium is known from a single shrew-sized skull from the Lower Jurassic of China (195Ma). Hadrocodium is one of the smallest known mammals and is estimated to have weighed only 2 g (Rose, 2006, p. 55). Hadrocodium possesses primitive teeth similar to morganucodonts. However, it has a single jaw articulation between the dentary and squamosal bones, which suggests that the quadrate and articular bones had become ear ossicles (Kemp, 2005, p. 149).

Docodonts (order Docodonta) are known only from Jurassic deposits. Some of the first docodonts were found in the Morrison formation of Wyoming and Colorado along with the bones of large sauropod dinosaurs (Rose, 2006, p. 55). Docodonts have broad, rectangular cheek teeth that show precise occlusion that may indicate an omnivorous diet (Kemp, 2005, p. 147). Haldanodon is known from a complete skeleton found in the Late Jurassic Guimarota lignites of Portugal. The skeletal features of this organism suggest that it may have been adapted for burrowing. The fact that it is found in lignites (swamp deposits), indicate a semi-aquatic lifestyle. Castorocauda, from the Mid Jurassic of China, is the largest docodont at half a meter in length. Castorocauda had a skeleton adapted for burrowing and swimming. Castorocauda is also important because it is the first mammal fossil preserving evidence of fur (Rose, 2006, p. 56).

Multituberculates (order Multituberculata) are an extinct group of rodent-like organisms that have the longest evolutionary history of any mammalian lineage. Multituberculates get their name from their large grinding molars that have rows of cusps or tubercles. Multituberculates first appear in the Middle Jurassic and evolved into many forms, which ranged from mice to beaver sized organisms. Many of these organisms had blade-like teeth that may have been used to eat hard seeds. Multituberculate hip structure suggests that they gave birth to undeveloped young like marsupials. Multituberculates had a single dentary/squamosal jaw joint and true inner ear ossicles. Kermackodon and Hahnotherium from the Mid Jurassic of England are the oldest known multituberculates (Rose, 2006, p. 56). Multituberculates possess many rodent-like features, which represent convergent evolution as they predate the first rodents by 180 million years. Multituberculates were abundant in many Mesozoic and Cenozoic communities in the northern continents, but went extinct in the Oligocene, perhaps even displaced by true rodents that had evolved and diversified by this time.

The order Eutriconodonta is a taxon that represents a diverse group of extinct mammals that span from the Mid Jurassic to Late Cretaceous. Triconodonts were rat to cat-sized mammals that lie at the core of this group. Triconodonts had the dentary/squamosal jaw joint and the three inner ear ossicles. Eutriconodonts are named for their teeth, which have three linear cusps on their molars. The lower molars were interlocked by a unique tongue-in-groove articulation. Eutriconodonts had the derived mammalian pectoral girdle (limbs tucked underneath the body), but retained the ancestoral pelvic girdle (sprawling hind limbs).

Several closely related groups of Mesozoic mammals exhibit molar teeth with a primitive triangular cusp pattern. Except for Kuehneotherium, all of the Mesozoic mammals described up to this point possess molar teeth with cusps arranged in a linear fashion. When linear, the cusps on upper molars fit between the cusps on lower molars. When triangular, the cusps on upper molars fit into V-shaped valleys between the tricuspid patterns on the lower molars. The evolution of the tricuspid pattern is important because it represents an innovation in processing food. The primitive triangular cusp pattern would evolve into the tribosphenic molar, a characteristic of higher mammals, sometime in the Cretaceous.

The symmetrodonts and eupantotheres (Dryolestoidea and Peramura) represent mammals that are closely related to the therians (marsupials and placentals). We will briefly discuss two of these groups, the dryolestids (Order Dryoletida) and symmetrodonts (Order Symmetrodonta).

Symmetrodonts were shrew to mouse sized and are known from the Early Jurassic to Late Cretaceous. Symmetrodonts are believed to be at the base of the therian radiation because of the triangular cusp pattern on their molars. Dryolestids, the most diverse eupantotheres, range from the Late Jurassic to the Late Cretaceous. Dryolestids have a more advanced triangular cusp pattern on their molar teeth than the symmetrodonts and possessed three inner ear bones. It is believed by many that the ancestors to modern therians can be found among the dryolestids.

Mammals played a subordinate role to the reptiles within terrestrial ecosystems. Mesozoic mammals may seem insignificant, but nothing could be further from the truth. These small, highly active, relatively large brained creatures of the night evolved adaptations that would allow their descendents to secure dominant roles in most ecosystems during the Cenozoic.


Birds make their first appearance during the Jurassic. Cladistic analyses favor that birds are derived theropod dinosaurs, most closely related to dromaeosaurids or deinonychosaurs (Benton, 2005, p. 261).

In 1861, two years after the publication of Charles Darwin’s On the Origin of Species, a fossil filling the role of “missing link” between reptiles and birds was discovered in the Upper Jurassic limestones of Solnhofen, Bavaria. In fact, what had been found was the first known bird Archaeopteryx lithographica. The fine grained limstone was excavated to produce limestone slabs that could be etched with acid to make lithographic plates used to print illustrations, thus the species name lithographica (Prothero, 2007, p. 257). The Solnhofen limestone is a fossil lagerstatten that represents a Jurassic-aged shallow, saline lagoon community situated in a subtropical climate (Selden & Nudds, 2004, p. 107). Archaeopteryx is perhaps the most famous fossil organism from this deposit and represents an excellent transition fossil in time and structure between reptiles and birds.

Archaeopteryx is now known from eight skeletons and one feather impression (Benton, 2005, p. 257). As an intermediate, Archaeopteryx exhibits both reptilian and bird features. The reptilian features include: thecodont teeth, a long bony tail, a dinosaurian hand with three clawed fingers, and a dinosaurian foot. The bird-like characteristics include a furcula (collarbones fused into a wishbone) and the presence of asymmetric feathers (Prothero, 1998, pp. 374-375). Archaeopteryx was a pigeon-sized bird that lived in a subtropical environment. These birds had long legs for running and wings formed from forelimbs. Although Archaeopteryx did not have a keeled sternum like modern birds reconstructions of the flight muscles suggest that it possessed the power for takeoff and sustained flapping flight (Benton, 2005, p. 264). Many think that Archaeopteryx hunted for insects and could glide and climb among the trees.


Insects continued to diversify with many families making their first appearance during the Jurassic period. Although extinct families continued to dominate Jurassic communities many existing families representing the orders Diptera, Hymenoptera, and Coleoptera make their first appearance. The order Glosselytrodea goes extinct during the Jurassic. These small insects are thought to be relatives of Orthoptera (Grimaldi & Engel, 2006, p. 333). The earliest fossils of Trichoptera (caddisflies) and Lepidoptera (moths and butterflys) appear in the Jurassic (Grimaldi & Engel, 2005, pp. 550 & 556). The order Dermaptera (earwigs) makes it first undisputed appearance in the Jurassic (Carpenter & Burnham, 1985, p. 310). The oldest larval caddisfly cases (Trichoptera) are found in the Jurassic (Grimaldi & Engel, 2005, p. 51).


Mesophytic flora continued to diversify during the Jurassic period with seed plants dominating communities. Cycads, ginkgos, bennettitaleans, and conifers underwent adaptive radiations. Modern conifer families including Pinaceae, Taxodiaceae, Cupressaceae, and Cephalotaxaceae make their first appearance (Willis & McElwain, 2002, p. 148). Gymnosperms were the dominant trees including a variety of conifers and ginkgos. Bennettitaleans dominated the role of small trees and bushes. Ferns continued to flourish as the dominant herbaceous plants (Dodsen, 1997, p. 388). Representatives of the order Ginkgoales date back to the Permian, but the genus Ginkgo makes its first appearance in the Jurassic.

Holzmaden Shale

The supercontinent Pangaea started to break up during the Triassic. As the continents rifted apart epicontinental seas formed supporting coral reefs and providing an environment in which reptiles could diversify and flourish. The Holzmaden shale, located near Stuttgart in Germany, is a conservation lagerstatten that provides a window into one of these Jurassic-aged epicontinental seas. The Holzmaden shale and its fossils represent a community living in a subtropical epicontinental marine basin.

Echinoderms (such as crinoids, echinoids, and ophiuroids) and mollusks (gastropods and bivalves) were the primary consumers. Bony fish and cephalopods (ammonites, squid, and belemnoids) preyed on these primary consumers. Ichthyosaurs, plesiosaurs, sharks, and crocodiles were the top predators. Small sauropod dinosaurs, horsetails, ginkgos, cycads, and conifers provide clues to the vegetation supported by nearby landmasses.

The Holzmaden lagerstatten is most famous for the important insights it gives paleontologists into the anatomy and life of ichthyosaurs. On some specimens a black organic film provides an outline of the body around the skeletal structure. The black outline revealed that the ichthyosaur Stenopterygius had a fleshy dorsal fin and upper lobe on the tail. Female ichthyosaurs fossilized with embryos in their bodies indicate they gave live birth at sea. Stomach contents reveal a diet of fish and cephalopods (Selden & Nudds, 2004, pp. 79-87).

Morrison Formation

The Morrison Formation of North America represents Jurassic-aged terrestrial environments. The age of the Morrison formation has been determined to be Late Jurassic (155 to 148 million years ago) independently using microfossil analysis and radiometric dating. The Morrison Formation of North America is known from twelve different states. Museums around the world display Morrison Formation fossils.

The Morrison Formation covers an area greater than 1.5 million square kilometers in the western United States. The Morrison Formation represents a variety of terrestrial environments. In the Southwest eolian sandstones mark the existence of past hot, arid deserts. Towards the north, sandstones and conglomerates mark the paths of ancient meandering rivers. Next to the ancient rivers mudstone deposits tell the story of sediments spilling out over the riverbanks into the floodplain. Thin limestone deposits mark the location of lakes and ponds. In Montana, Morrision coal deposits indicate a wet, swampy environment (Carpenter, 1997, p. 451).

For paleontologists hunting dinosaurs it is the river and lake deposits that have been the most productive. Bones of large dinosaurs and other Mesozoic animals deposited by flash floods form Concentration Lagerstatte in Colorado, Utah, and Wyoming, These deposits provide important insights into some Jurassic terrestrial ecosystems. The Morrison Formation was deposited in a semi-arid basin with meandering rivers and lakes following the retreat of the Sundance Sea. Evidence suggests this environment was influenced by cycles of drought and flood. During times of drought dinosaur herds concentrated around disappearing water sources. Drought created mass death assemblages. Carcasses of the dead decomposed and dried. Periodic floods deposited the disarticulated bones in river channels. Allosaurus, Diplodocus, Apatosaurus, Camarasaurus, and stegosaurus are among the most famous Morrison Formation dinosaurs. The Morrison biota also includes: lizards, crocodiles, turtles, pterosaurs, many primitive mammals, fish, invertebrates, bryophytes, ferns, cycads, ginkgos, and conifers (Selden & Nudds, 2004, pp. 88-98).

The Morrison Formation became famous due to an intense competition between two American palaeontologists, which started in 1877. Professor Othniel Charles Marsh (1831-1899) of Yale’s Peabody Museum and Edward Drinker Cope (1840-1897) were fierce rivals that strove to scientifically outdo one another by discovering and describing new fossil organisms. The feud between these two noted paleontologists is known as the “bone wars”. The bone wars continued until Cope’s death in 1897. Marsh had described 75 new dinosaur species of which 19 are valid today. Cope had described 55 new species of dinosaurs of which 9 are valid today (Selden & Nudds, 2004, p. 90).

The bone wars had a tremendous impact on paleontology. Spectacular discoveries of complete dinosaur skeletons improved our understanding of dinosaurs and the evolution of life. The discoveries made in the Morrison Formation helped to fuel explorations worldwide (Breithaupt, 1997, pp. 347-350).

Solnhofen Limestone

The Solnhofen Limestone of Bavaria in Southern Germany is an important Conservation Lagerstatten that preserves both terrestrial and marine life of the Late Jurassic (150 MA). The deposit consists of finely laminated, micritic limestone known as lithographic limestone. The fine grained limestone was excavated to produce limestone slabs that could be etched with acid to make lithographic plates used to print illustrations. The fine grained micritic limestone helped to preserve the intricate details of feathers, insect wings, and squid tentacles as impressions. Examples of organic material preservation include cephalopod ink sacs and feathers. Soft tissues, such as the muscles of fish and cephalopods are sometimes replaced by francolite (calcium phosphate).

Solnhofen Limestone represents a subtropical, saline lagoon community with a semi-arid monsoonal climate. The bottom and lower layers of the lagoon were inhospitable to life. Over 600 species make up the Solnhofen Limestone biota. The majority of fossils represent organisms swept into the lagoon during storms. These organisms lived in adjacent land and reef communities as well as the open sea.

Archaeopteryx specimens, representing the earliest known bird, are the most famous Solnhofen fossil. Compsognathus is the only dinosaur found in this deposit. Pterosaurs are represented by Rhamphorhynchus, Scaphognathus, and Pterodactylus. Crocodiles, turtles, lizards, and the teeth of ichthyosaurs and plesiosaurs are known. Ray-finned fish, lobe-finned fish, and cartilaginous fish have been described. Shrimps, lobsters, crabs, and horseshoe crabs represent crustaceans. Perhaps the most interesting fossils are those of Mesolimulus, horseshoe crabs preserved at the end of their spiraling death trails. It is believed they quickly died after being swept into the toxic lagoon bottom. Insects are represented by mayflies, dragonflies, cockroaches, termites, water skaters, locusts, crickets, water scorpions, cicadas, lacewings, beetles, caddis flies, true flies, and wasps. Many marine invertebrate groups are represented such as sponges, cnidarians (jellyfish and corals), annelids, bryozoans, brachiopods, mollusks (gastropods, bivalves, and cephalopods), and echinoderms (crinoids, starfish, brittle stars, sea urchins, and sea cucumbers). Plant life preserved in Solnhofen Limestone includes seed ferns, bennettitales, ginkgos, and conifers (Selden & Nudds, 2004, pp. 99-108).


A minor extinction event near the end of the Jurassic period affected both marine and terrestrial life. The primary victims included marine invertebrates and dinosaurs. Among marine organisms bivalves and ammonoids suffered the most. Many marine crocodiles and ichthyosaurs would not survive into the Cretacous. Among terrestrial organisms the stegosaurs and most sauropod groups went extinct. It was once thought that drops in sea levels contributed to the Jurassic loses, but current evidences suggests this is not the case (Stanley, 1987, pp. 121-122).


Back to Jurassic Period


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Prothero, D.R. (1998). Bringing Fossils to Life: An Introduction to Paleobiology. New York: McGraw-Hill.

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Rose, K.D. (2006). The Beginning of the Age of Mammals. Baltimore: The Johns Hopkins University Press.

Selden P. & Nudds, J. (2004). Evolution of Fossil Ecosystems. Chicago: The University of Chicago Press.

Stanley, S.M., (1987). Extinction. New York: Scientific American Books.

USGS Publication: Major Division of Geologic Time see:

Waggoner, B. (1995). Theropod Dinosaurs:

Webb, G.E. (2001). Biologically Induced Carbonate Precipitation in Reefs through Time. In Stanley, G.D. Jr. [Ed] The History and Sedimentology of Ancient Reef Systems (159-203). New York: Kluwer Academic/Plenum Publishers.

Willis, K.J. & McElwain, J.C. (2002). The Evolution of Plants. New York: Oxford Univeristy Press.

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