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
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.
Producers & Reefs
Extinction at the end of the Triassic disrupted the adaptive
radiation of dinofagellates and coccolithophores. Eighty percent
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).
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
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
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,
fish and bony fish continue to diversify into new forms during
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
for capturing prey, but not tearing the prey. In contrast, the
jaws of neoselachian (modern sharks) open widely and have great
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).
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
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.
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).
(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.
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.
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).
(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.
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
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.
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).
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.
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).
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
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.
(Infraorder Ceratosauria) were closely related to the coelophysoids.
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,
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.
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
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
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
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
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
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.
the Lower Jurassic of South Wales is known mostly from isolated teeth.
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
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
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,
pp. 550 & 556).
The order Dermaptera (earwigs) makes it first undisputed appearance
(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,
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.
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
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,
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).
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,
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 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
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
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
representing the earliest known bird, are the most famous Solnhofen
fossil. Compsognathus is
the only dinosaur found in this deposit. Pterosaurs are represented
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.
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).
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