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The Cretaceous period extends from 145.5 to 65.5 Million years ago. The name Cretaceous is derived from the Latin word "creta", which means chalk. Thick beds of Cretaceous aged chalk are characteristic of Western Europe. The chalk beds were formed by the calcium carbonate shells of marine invertebrates, mostly coccolithophores, during the Upper Cretaceous. The period was defined by a Belgian geologist Jean-Baptista-Julien d'Omalius d'Halloy (1783-1875) using strata he studied in the Paris Basin.
Primary Producers & Reefs
Earth's oceans have experienced two major shifts in the composition of primary producers. Initially, cyanobacteria along with other photosynthetic bacteria were the primary producers during the Proterozoic eon. The first shift occurred during the early Paleozoic era when eukaryotic green algae joined cyanobacteria in being major primary producers. Dinofagellates and coccolithophorids first appear in the Triassic (Payne & Schootbrugge, 2007, p. 166). The second shift would occur during the Mesozoic era when dinoflagellates and coccolithofores would be joined by diatoms in the Jurassic. Diatoms, dinoflagellates, and coccolithophores finally assume their dominant role as the base of many modern marine ecosystems during the Cretaceous. (Knoll, Summons, Waldbauer, and Zumberge, 2007, p. 155).
During
the Cretaceous period seas were elevated, spreading over large
continental areas. The shells of nannoplankton, such as
coccolithofores, accumulated into thick deposits of chalk from
Denmark to France and in the western interior of the United
States. The most famous of these deposits are The White Cliffs
of Dover in England (Stanley, 1987, p. 124).
Early Cretaceous reefs represented a continuation of Late
Jurassic forms. Scleractinian corals and stromatoporoids
continued to be the primary reef builders. During the Early
Cretaceous rudist bivalves started to occupy positions in reefs
along with corals. Rudist bivalves are mollusks with a conical
lower valve (shell) that is covered by a second cap-like valve
(Stanley, 1987, p. 124). Rudist bivalves produced copious amounts
of carbonate sediments and sometimes accumulated into bound,
rigid frameworks. By the Middle Cretaceous rudist bivalves
forced corals into
a
subordinate
reef-building
role
in many
shallow-water shelf reef settings (Webb, 2001, pp. 176-177).
In is interesting to note that mollusks competitively displaced
cnidarians as the major reef builders in many locations during
the Mid to Late Cretaceous. Rudist bivalves would become extinct
at the end of the Cretaceous allowing corals to once again
reclaim their dominance.
Marine
Invertebrates
Although mollusks were hit hard by the Jurassic extinction
few higher taxonomic groups dissappeared and they retained
their prominent position (Stanley, 1987, p. 122). Ammonites
recovered from the Jurassic extinction and underwent a great
adaptive
radiation
reaching
their greatest
diversity
during the Cretaceous. Cretaceous aged sediments layed down
in the Western Interior Seaway of North America preserve amazing
sequential assemblages of fossils representing ammonite evolution
(Johnson & Stucky, 1995, p. 96). Baculites, a straight-shelled
form of ammonite, make their first appearance during the Cretaceous
and spread worldwide. Ammonites were major invertebrate predators
within the marine realm. Crabs and snails were the major canivores
of the seafloor. Crabs and predaceous gastropods (snails) underwent
an adaptive radiation
during
the Cretaceous
most (Stanley, 1987, p. 123). Stalkless, floating crinoids
such as Uintacrinus became common during the Cretaceous.
These crinoids formed floating mats and fed on plankton (Johnson
& Stucky, 1995, p. 92).
Fish
The neoselachian sharks experienced bursts of adaptive radiations
during the Jurassic and Cretaceous. The more modern sharks continued
to live
side-by-side with the hyodonts, which went extinct by the end
of the Cretaceous. Modern
shark lineages are a continuation of Jurassic and Cretaceous
evolutionary
lines,
which are marked by adaptations for improved feeding and swimming
mechanisms. Modern
sharks have greater mobility about their jaw (when the shark
gapes it drops its lower jaw and protrudes its upper jaw),
serrated teeth, larger brains and enhanced sensory areas (especially
olfactory), and calcified vertebrae that enclose the notochord.
Primitive
sharks had a cartilaginous sheath that covered the notochord.
The cartilaginous vertebrae of modern sharks improve swimming
ability (Benton, 2005, pp. 164-169).
The radiation of teleosts (subdivision Teleostei), which began in the Triassic accelerated during the Cretaceous. Teleosts make up 99% of all living fish and account for half of all living vertebrates (Prothero, 1998, p. 352). The flexibility of the jawbone increases with the teleosts allowing them to protrude their mouth in a circular shape, sucking up their food, rather than biting hard. The skull of the teleost is lightweight and flexible. The swim bladder evolved into a more efficient organ in the teleost making them neutrally buoyant. This allowed the pelvic and pectoral fins to become thin and flexible adapted for fine steering control and hovering. In more primitive ray-finned fish heavy fins were designed primarily for thrust. The vertebrae of teleosts become increasingly ossified. Teleosts have a symmetrical, fully homocercal caudal fin with distinctive radiating elements known as uroneurals modified from the spinal column. The bodies of advanced teleosts became covered with thin, flexible, rounded, overlapping scales with no enamel.
Living
teleosts are represented by four clades. Osteoglossomorphs (order
Osteoglossomorpha "bony-tongued") is a primitive, small group
of freshwater ray-finned fish. Representatives of this order
may date back to the Late Jurassic. Aquarium enthusiasts may
be familiar with the elephant fish and the arowana, both of which
belong to this order. Elopomorphs (cohort Elopomorpha), which
include eels, tarpons, and bonefishes are known from the Early
Cretaceous. The new subcohort Otocephala includes clupeomorphs
(marine herring-like fish) and ostariophysians (Ostariophysi,
most freshwater
fish,
such as carp, goldfish, minnows, and catfish). Ostariophysians
are a very successful group and are characterized by a specialized
hearing system known as the Weberian apparatus, which links the
swim bladder to the ear. Otocephala representatives are known
from the Early Cretaceous. Euteleosts (subcohort Euteleostei)
is the largest teleost group and includes salmon, pike, and derived
teleosts.
The derived euteleosts (division Neoteleostei) include such fish
as lantern fishes, cod, haddock, anglerfishes, clingfishes, flying
fishes, guppies, seahorses, flatfishes, tunas, porcupine fishes,
etc. The neoteleosts are characterized by a specialized muscle
in the upper throat region that helps in manipulating prey. As
a whole, this group is known from the Late Cretaceous (Benton,
2005, pp. 179-184). By the Late Cretaceous teleost had become
the dominant fish in both marine and freshwater habitats. The
teleost adaptive radiation that started in the Jurassic and accelerated
during the Cretaceous continues to this day.
Amphbians
A few primitive Australian amphibians representing
the order Temnospondyli surived into the Cretaceous. Koolasuchus was
a large freshwater carnivore with a somewhat salamander-like appearance.
The decline of temnospondyles is attributed to competition with crocodilians
(Benton, 2005, p. 97). Temnospondyles finally go extinct at the end
of the Cretaceous.
Many modern families of amphibians start to
appear in the Jurassic and Cretaceous. Albanerpeton is a small,
well known salamander-like amphibian from North America. Beelzebufo
from Madagascar is the largest known toad at over 40 cm long and
up to 4 kg. This massive Late Cretaceous anuran is known as the "devil
toad". Beelzebufo was a predatory toad related to present
day South American horned frogs. This relationship is further evidence
that South
America, India, and Madagascar were at one time connected during the
Cretaceous (Guerrero, A.G., Frances, P., & Stradins, I., 2009, p. 308).
Reptiles
As we have already noted, many reptile groups
were hit hard by the Jurassic extinction event including marine crocodiles,
icthyosaurs, stegosaurs, and sauropods. However, reptiles would recover
and maintain their dominance. In fact, dinosaurs would reach the
peak of their abundance and diversity during the Cretaceous period.
Marine turtles diversified and became abundant during the Cretaceous. Although plesiosaurs, marine crocodiles and icthyosaurs survived, a new group of swimming monitor lizards, the mosasaurs would achieve the status of keystone predator in the marine realm.
Mosasaurs
(Order Squamata) became important shallow marine predators in
the Late Cretaceous. Mosasaurs ("Meuse river lizard")
were air-breathing lizards adapted for a marine life that looked
somewhat like a crocodile with flippers. Mosasaurs have an elongate
body, deep tail, paddle-like fins, and large skulls lined with
sharp conical teeth. Mosasaurs ranged in length from 2 to 10
meters and ate fish and other marine animals. Mosasaurs were
so fully adapted to a aquatic life that they gave birth to live
young in their marine habitat. Platecarpus and Plotosaurus are
two mosasaurs found in the Late Cretaceous chalk deposits of
Kansas. Ammonite shells have been found with mosasaur tooth marks.
Mosasaurus hoffmani was one of the largest and most derived mosasaur marine
reptiles. A 1-meter long mosasaur jaw found in 1786 was known as the "Beast
of Maastricht" named for the town in Holland where it was found. Napoleon's
troops occupied Holland and brought the jaw to Paris in 1795, where it was studied
by the great French anatomist George Cuvier (1769-1832). The jaw is still housed
in the Natural History Museum in Paris. The Beast of Maastricht was important
because it encouraged scientists to consider and debate the possibility of extinction,
which was a very controversial idea at the time (Palmer, 1999, pp. 120-121).
Snakes make their first appearance in the Late Cretaceous. Fossils of three marine snakes with hindlimbs (Pachyrachis problematicus, Haasiophis terrasanctus and Eupodphis descouensi) have been used to support the hypothsis that snakes evolved from the marine lizards Mosasauroidea. Najash rionegrina, a more recent find from Patagonia, provides evidence that snakes have a terrestrial origin. Najash rionegrina was a terrestrial snake with well developed hindlimbs. Unlike the previously mentioned marine snakes, the basal snake Najash possessed a sacrum supporting a pelvic girdle with robust legs outside the ribcage (Apesteguia & Hussam, 2006, pp. 1037-1040). Evidence suggests that snakes evolved from the varanid lizards.
Although the Rhamphorhynchs would not survive into the Cretaceous Pterosaurs continued their success in the form of the Pterodactyles. Pterodactyls ("winged finger") are probably the best-known flying reptiles. Pterodactyls had the same general structure as the rhamphorhynchs; however, the tail was shorter, the neck longer and the skull more elongate. Pteranodon ("wing toothless") is one of the largest and best-known pteranosaurs from the Late Cretaceous. Pteranodon had a wing span of up to 8 meters and was probably a glider. A crest on the back of the head doubled the skull length. The crest may have acted as stabilizer during flight, although it was sexually dimorphic. The jaws of Pteranodon were toothless, which is unusual for a pterosaur. Pteranodon may have fed on fish like the modern Pelican. The cervical vertebrae had pneumatic foramen that served to reduce weight and increase respiratory efficiency.
Pterosaur
wings were 1 mm thick and made of several layers including striated
muscles, collagenous fibers, dermis, and epidermis. The membranes
of several species were reinforced with parallel stiff fibers,
termed actinofibrils. The actinofibrils embedded in the wing
helped to ensure a stable aerodynamic shape and proper folding
when not in use. The fact that Pterosaurs could fly and were
covered with insulation (hair) is strong evidence that they were
endothermic or warm blooded (Wellnhofer, 1991, p. 40).
When walking, pterosaurs used all four limbs with legs in the middle and hands
a short distance in front and to the side, wing tips (formed from the elongated
fourth finger) slanted upwards on either side of the head. The largest known
flying vertebrate of all time is Quetzalcoatlus from the upper Cretaceous
of Texas. Quetzalcoatlus is known from a single wing that measures 12
meters (Benton, 2005, pp. 224-229).
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 came to dominate Jurassic and Cretaceous terrestrial faunas. In the Jurassic Carnosaurs, Sauropods, and Stegosaurs became the major carnivores and herbivores. As successful as dinosaurs became during the Jurassic, one could argue that dinosaurs reached the pinnacle of their evolution during the Cretaceous in the form of tryannosauroids, ornithopods, and ceratopsians.
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. The suborder Sauropodomorpha ("lizard feet") includes both the prosauropods and the sauropods. In general, they were herbivorous quadrupeds with a small head, long neck, large body with legs tucked beneath, and a long counterbalancing tail. Sauropods were on the decline during the Cretaceous, while one clade of theropods enjoyed a resergence.
Theropods would once again rise to prominance during the Cretaceous in the form of coelurosaurs (division Coelurosauria). Coelurosaurs are a diverse clade of theropods that are more closely related to birds than to the carnosaurs, such as Allosaurus and Megalosaurus of the Jurassic period. Coelurosaurs include the tyrannosaursids (formerly grouped with carnosaurs) of the Late Cretaceous, ornithomimids, and maniraptorans.
Tyrannosaurids of the Late Cretaceous, like Tryrannosaurus ("tyrant lizard") are among the largest known terrestrial carnivores. Tyrannosaurus measured up to 12 meters long and weighed up to 6 tonnes. Tyrannosaurus had a large skull, over 1.35 meters in length. It's jaw was lined with serrated teeth up to 16 cm long and 2.5 cm wide. It is estimated that Tyrannosaurus had a bite force of up to 13,400 Newtons. Tyrannosaur coprolites contain bones of Triceratops and pachycephalosaurids (Benton, 2005, p. 193). Tyrannosaurs had small, but powerful forelimbs equipped with two clawed fingers. The large powerful hind limbs possessed three large claws. It is estimated that T. rex could achieve speeds of up to 40 km/h.
Ornithomimids of the Early Cretaceous were slender theropods with ostrich-like bodies, small heads, relatively long necks, limbs and fingers. Ornithomimids would reach their greatest diversity during the Late Cretaceous period. Struthiomimus ("Ostrich mimic") from the Late Cretaceous possessed a toothless jaw covered with a keratinous beak. Struthiomimus's anatomy suggests that it was a fast organism, reaching speeds of up to 60 km/h. Their diet consisted of small lizards and mammals.
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, such as Sinosauropteryx, Beipiaosaurus, Protarchaeopteryx, Microraptor, and Caudipteryx, 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).
Sauropods were hit hard by the Jurassic extinction event and their diversity decreased drastically during the Cretaceous. Sauropods were no longer the dominant herbivores of North America. However the story was different in the southern landmasses, where sauropods continued to be the dominant herbivores. One group of sauropods, the titanosaurids (family Titanosauridae) flourished during the Late Cretaceous in South America. Representatives have also been found in Australia, Europe, and China. The skin of many titanosaurids possessed armore-like scales. Saltasaurus had osteoderms, amore-like bony plates embedded along its back that would remind one of ankylosaur armore. Some titanosaurs, such as Argentinosaurus, at over 100 tonnes, may have reached the theoretical maximum size for any terrestrial land animal (Benton, 2005, p. 204).
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).
Ornithopods
("bird feet") were the most diverse and successful
group of ornithischians and included the heterodontosaurids,
hypsilophodontids, iguanodontids, and hadrosaurids. Heterodontosaurids
were the most primitive Ornithopods and range from the Early
Jurassic to the Early Cretaceous. Representatives of the remaining
ornithopod groups became the dominant herbivores of the Cretaceous
in North America. The evolution of a sophisticated chewing mechanism
facilitated their success.
Ornithopods evolved complex chewing mechanisms making them unique among reptiles.
Two different solutions to chewing can be seen in the jaw structures of Ornithopods.
Basal ornithopods, the Heterodontosaurids possessed a ball and socket joint
that
allowed
the
lower
jaw to rotate, creating
a shearing action between the cheek teeth. All later ornithopods
possessed pleurokinetic hinges in the upper jaw, which allowed the sides of the
upper jaw to flap in and out, creating a lateral shearing action between the
cheek teeth. Ornithopods were the most successful herbivores during the Cretaceous
because of their ability to chew (Benton, 2005, p. 207).
Iguanodon ("lizard tooth") was the second dinosaur ever described.
Dr. Gideon Mantell, an English amateur geologist, described Iguanodon from
some teeth in 1822 and credited their discover to his wife Mary Ann Mantell. Iguanodon's hand
is unusual, digit 1 is reduced to a thumb spike, and digits 2 and 3 have small
hooves. Iguanodon could walk both bipedally and on all fours. The thumb
spike of Iguanodon was first believed to be a horn positioned on the
snout.
Iguanodon, Megalosaurus, and Hylaeosaurus were the first
dinosaurs to be represented as three-dimensional restorations. They were part
of London's Sydenham Park built to showcase the glass and iron structure named
Crystal Palace, which had been featured at the 1851 Great Exhibition held in
London. Twelve guests dined inside the incomplete mould of Iguanodon at
a New Year's dinner party on December 31, 1853. Richard Owen supervised the restorations,
which suffered from misinterpretations and incomplete information (Sarjeant,
1997, pp, 161-164).
Hadrosaurs ("sturdy lizard") or duck-billed dinosaurs were the most
successful ornithopod clade. Hadrosaurs had long rows of grinding cheek teeth
arranged in closely packed batteries. Plant material was ground with a sideways
shearing movement as the pleurokinetic hinge pushed the cheeks in and out with
each bite. The jaws also moved forward and backward providing additional grinding
action (Benton, 2005, p. 209).
Hadrosaurs all have similar skeletons and skulls; however, many possessed various
shaped crests. Parasaurolophus ("by lizard crest") was a highly
derived hadrosaur of the Late Cretaceous. Parasaurolophus could walk
on all fours as well as on two legs. The 9 meter long, two tonne hadrosaur had
a head equipped with a curved horn-like crest up to 1.8 meters long. The crest
had two hollow passages that ran from the nostrils back to the tip of the crest
and curved back down to the throat region. It is believed that hadrosaur crests
may have acted as resonating chambers. It is common to find several species of
hadrosaurs in the same formation, so they probably roamed in mixed groups. Hadrosaurs
were the dominant herbivores towards the end of the Mesozoic and one can imagine
the reverberating sounds of dinosaurs with different shaped crests filling the
air in ancient North American and Mongolian forests of the Late Cretaceous.
The hadrosaur Maiasaura peeblesorum is the state fossil for Montana. Maiasaura nests
on Egg Moutain provide evidence that this dinosaur was nest bound as a hatchling
and required parental care. The ends of the hatchling leg bones are not fully
formed and the egg shells are found in pieces. Jack Horner and Robert
Makela, American paleontologists, found Maiasaura. Horner named the
dinosaur Maiasaura ("good mother lizard") because of the evidence
that it took care of its hatchlings. Hadrosaurus foulkii is the state
fossil for New Jersey.
Pachycephalosaurs ("thick head lizard") are the dome-headed dinosaurs. These bipedal herbivores range in size from 1 to 5 meters long. Fossils of thickheaded dinosaurs are restricted to the Cretaceous. In one specimen of Pachycephalosaurus wyomingensis the skull was 22 cm thick. The thickened skull bones of Pachycephalosaurs suggest to many that they engaged in a head-butting behavior not unlike moder day bighorn sheep (Sues, 1997, p. 512).
Ceratopsians
("horned faces") are a diverse group of ornithischians
from the Late Cretaceous. Ceratopsians have a triangular shaped
skull when viewed from above and a beak-like rostral bone on
the upper jaw that meets with the predentary bone on the lower
jaw. Ceratopsians evolved neck frills and horns. Later forms
also had skeletons adapted for galloping.
Triceratops ("three horn face") is the best-known horned dinosaur. Triceratops was
8 meters long and weighed in at 4.5 tonnes. The brow horns of Triceratops reached
lengths of 1 meter and the neck frill up to 2.5 meters wide. The teeth of Triceratops were
elongated blades designed for shearing. This herbivore did not chew, it may have
browsed on fibrous plant material like cycad or palm fronds. Triceratops is
the state dinosaur for Wyoming and the state fossil for South Dakota.
Stegosaurs reached their zenith during the Late Jurassic. Stegosaurs were hit hard by the Jurssic crises and declined in numbers during the Cretaceous. Sometime in the Early Cretaceous the stegosaurs would go extinct.
Ankylosaurs arose in the Mid-Jurassic and diversified during the Early Cretaceous. Ankylosaurus ("curved lizard") is the largest known ankylosaur and survived to the end of the Cretacous. Ankylosaurus was up to 10 meters long and weighed in at 3.6 tonnes. The body was broad (up to 5 meters wide) and squat supported by powerful legs. Ankylosaurs had a massive bony club at the end of their tail, which would have made a formidable weapon.
Mammals
In general, Cretaceous mammals remained small nocturnal insectovores and carnivores. However, Cretaceous mammals continued to evolve traits critical to the success of their modern descendents. Among mammals basal groups continued to be the most successful. However, the first monotremes, marsupials, and placental mammals appear in the Cretaceous.
Multituberculates (order Multituberculata) are an extinct group of rodent-like organisms that have the longest evolutionary history of any mammalian lineage.Multituberculates first appear in the Mid-Jurassic and go extinct in the Oligocene. During the Cretaceous they were the most successful mammal group. Multituberculates get their name from their large grinding molars that have rows of cusps or tubercles. Multituberculates first appear in the Mid 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.
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). Jeholodens is a Cretaceous-aged triconodont mammal known from the Liaoning Province of China. A single complete skeleton represents Jeholodens. Skeletal evidence indicates that this small primitive mammal, like many Mesozoic mammals, was a nocturnal insectivore. Repenomamus is the largest mammal known from the Cretaceous of China. Repenomamus was a carnivore up to 1 meter long. One specimen of Repenomamus had the partial skeletal remains of a juvinelle Psittocosaurus preserved within the stomach region (Rose, 2006, p. 62). Psittocosaurus was a ceratopsian dinosaur. This incredible fossil is the first evidence of a mammal preying on a dinosaur.
Several closely related groups of Mesozoic mammals exhibit molar teeth with a triangular cusp pattern. 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. Zhangheotherium is one of the few symmetrodonts known from almost a complete skeleton. Zhangheotherium lived in China during the Early Cretaceous and possessed skeletal characteristics intermediate between monotremes and therians.
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.
Mammal
groups examined thus far tend to have cheek teeth with cusps
oriented either in a linear fashion or a primitive triangular
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.
Sometime in the early Cretaceous the advanced triangular cusp pattern that
defines modern mammals, the tribosphenic tooth, appears. The linear and primitive
triangular tricuspid patterns represent cheek teeth that are good for cutting
and tearing, but not crushing. The more advanced tribosphenic tooth has a triangular
cusp pattern that creates occlusion surfaces good for crushing or grinding,
like a pestle and mortar. The tribosphenic tooth is defined by the presence
of a large cusp on the upper molars called the protocone. The protocone of
the upper molar works against a basined area named the talonid on the corresponding
lower molar. The protocone thus acts as a pestle, while the talonid acts as
the mortar. The tribosphenic molar provided a basic form that would later be
modified into the wide variety of dentitions exhibited by therian mammals (marsupials
and placentals). This dental structure allowed mammals to expand into a wide
variety of specialized dietary niches (Rose, 2006, p. 67). Fossil representatives
of monotremes, marsupials and placental mammals are known from the Cretaceous.
Extant (living) mammals are traditionally divided into two subclasses based upon reproductive strategies. The subclass Prototheria includes the egg-laying mammals, while the subclass Theria includes marsupials and placentals, which bear young live. The subclass Prototheria unites monotremes with many ancient Mesozoic mammal groups, but is now no longer in use. Monotremes were thought to be related to basal mammals with a linear arrangement of cusps such as morgonocodontids, triconodonts, and multituberculates.
Determining relationships among mammals requires teeth and monotremes do not have teeth as adults. Finally, in 1985 a fossil monotreme was found in which teeth were retained into adulthood. The Cretaceous aged Steropodon was found in the famous opal mine of Lightning Ridge, New South Wales (Kemp, 2005, p. 176). Steropodon was the first Mesozoic mammal fossil found in Australia and is an ancestor to the platypus. However, unlike the living platypus the adult Steropodon had cheek teeth that exhibit a primitive triangular cusp arrangement (tribosphenic) similar to young monotremes and an extinct southern hemisphere mammal family Ausktribosphenidae. Monotremes are now grouped with these extinct organisms into the superdivision Australosphenida (Benton, 2005, p. 399).
The tribosphenic therian mammalian lineage (Subclass Theria) split into marsupials (Infraclass Metatheria) and placentals (Infraclass Eutheria) by the late Early Cretaceous. Marsupials are often referred to as the pouched mammals because females possess a marsupium or specialized pouch in which newborn young are carried, protected and nourished during development. The marsupial embryo develops for only a few weeks after which it is born underdeveloped. After birth the marsupial embryo crawls to the pouch where it attaches to a nipple and completes development.North America serves as the primary source of fossils revealing early marsupial evolution. Marsupials first appear in the Mid-Cretaceous. Kokopellia, from Utah, USA may be the oldest marsupial at 100 Ma (Kemp, 2005, p. 196). Alphadon represents the first undisputed marsupial. Alphadon from the Upper Cretaceous of North America is known mostly from teeth and jaws. Alphadon is a member of the family Didelphidae and is often considered a model archetype marsupial. Alphadon had the marsupial dental formula of three premolars and four molars (placental mammals have three molars). Like living marsupials these early forms exhibited a tooth replacement related to their nursing habits. Only the last premolar is replaced, anterior dentition is not because of extended nursing (Benton, 2005, p 309).
In the late Cretaceous marsupials underwent an adaptive radiation in North America. These early marsupials were small and adapted to various niches. Teeth indicate that various marsupials were specialized as insectivores, carnivores, and omnivores. Although more common than placentals, they were not as diverse or numerous as the multituberculate mammals.
Placental mammals (infraclass Eutheria) first appear in the Early Cretaceous and differ from marsupials in having a reproductive system that allows the fetus to develop within the female for a longer period of time. Embryos of placental mammals are connected to the mother’s uterus wall by the placenta organ. The placenta supplies the developing embryo with maternal nutrients and allows embryo waste to be disposed by the maternal kidneys. The placenta and embryo form from the same group of cells; this allows the placenta to act as a barrier against the mother’s immune system. Marsupials do not enjoy such protection, which explains why offspring are born underdeveloped. Eomaia ("dawn mother") from the Early Cretaceous (dated at 125 MA) of China is currently the oldest known placental mammal. Eomaia was a shrew-sized animal that possessed finger and toe bones adapted for climbing. The exceptional preservation of this specimen reveals that Eomaia was covered in fur (Benton, 2005, p. 311).
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
The first bird, Archaeopteryx lithographica appears in the Upper Jurassic. Birds underwent a great adaptive radiation during the Cretaceous that resulted in many now extinct primitive forms as well as a sister group to modern birds. Cladistic analyses favor that birds are derived theropod dinosaurs, most closely related to dromaeosaurids or deinonychosaurs (Benton, 2005, p. 261). The difficulty encountered in determining the proper taxonomic position of possible basal Cretaceous birds seems to only reinforce the theropod/bird connection. Rahonavis was a raven-sized dinosaur/bird from the Upper Cretaceous of Madagascar that had a reversed hallux (backwards pointing first digit) and papillae on the ulna for the insertion of wing feathers. It still retained a long tail like Archaeopteryx and possessed an enlarged sickle claw on the second toe like the dromaeosaurid Velociraptor. Jeholornis from the Lower Cretaceous of China is a turkey-sized bird that possessed a tail with feathers arranged in a fan. It had broad wings with asymmetrical wing feathers and the structure of the hand was more advanced than Archaeopteryx. The type specimen of Jeholornis has seeds of the conifer Carpolithus preserved in its crop. Rahonavis and Jeholornis represent the most basal Cretaceous birds. Confuciusornis is a primitive crow-sized bird from the Early Cretaceous of China. The genus was named for the Chinese philosopher Confusius. Confuciusornithids may be the first birds to have a toothless beak. Confuciusornis had a slight keel and a more flexible wrist than Archaeopteryx. The tail was modified with the caudal vertebrae fused forming a pygostyle. The wing retained the three long fingers with claws like those of Archaeopteryx.
The order Enantiornithes represent the most diverse bird clade of the Cretaceous. These primitive birds were distributed worldwide and ranged form sparrow size to birds with wingspans of 1.5 meters. Enantiornithines were more advanced than Archaeopteryx and Confuciusornis but more primitive than modern birds. Most birds in this clade had teeth and retained the three-clawed fingers on the hand. Sinornis of China was a sparrow-sized bird with a larger ossified sternum and a pygostyle tailbone. Sinornis was capable of sustained flight as it hunted for insects. Sinornis had a toothed beak and retained the three-clawed fingers on its wing. Sinornis possessed a wrist joint that allowed it to fold the wings against its body and an opposite first toe for perching. Enantiornthines went extinct at the end of the Cretaceous.
A second major clad of Mesozoic birds was the Ornithurines. Ornithurines are a sister taxa to the radiation that gave rise to the modern birds. Members of the order Hesperornithiformes were strong swimming predatory birds. These birds were flightless and propelled themselves through the water by kicking their feet. Hesperornithiformes had teeth lining their jaws, which helped secure the fish they captured. Coprolites of these organisms show their diet consisted of sea fish. Hesperornis and Baptornis are found in the Upper Cretaceous Niobrara Chalk Formation of Kansas, USA. Members of the order Ichthyornithiformes were strong fliers that also fed on fish. Ichthyornis of the Niobrara Chalk Formation of Kansas was a gull-sized bird. Like modern birds Ichthyornis had a deeply keeled ossified sternum, unlike modern birds it had jaws lined with teeth. It is thought that Ichthyornis caught fishes in the Great Interior Seaway by diving into the water from the wing (Benton, 2005, pp. 267-274).
Insects
The Cretaceous period is of great significance to the evolution of insects. Insects represening the orders Isoptera (termites), Siphonaptera (fleas), Strepsiptera (twisted-winged parasites), Embioptera (webspinners), Mantodea (mantises) and Zygentoma (silverfish) make their first appearance. In fact, most Cretaceous insects can be assinged to modern families (Carpenter & Burnham, 1985, p. 310). The origin and adaptive radiation of the Angiosperms (flowering plants) occurred during the Cretaceous. Many insects are intimately associated with flowering plants as pollinators and consumers; evidence of coevolution. The three main insect groups with advanced sociality, ants, termites, and vespid wasps make their first appearance during the Cretaceous (Grimaldi & Engel, 2005, p. 76). Celliforma is a fossil bee nest (in the form of subterranean excavations) that is first found in Late Cretaceous deposits. Celliforma is found from the Cretaceous to the Pliocene (Grimaldi & Engel, 2005, p. 51). Termite borings appear in the Cretaceous and represent the oldest undisputed fossil nest for social insects (Grimaldi & Engel, 2005, p. 54).
Plants
Flowering plants or angiosperms (Magnoliophyta) make their first unmistakable appearance during the Early Cretaceous (140 Ma) (Kenrick & Davis 2004, p. 195). Angiosperms became the dominant flora across the globe by the Paleogene a mere 70 million years after their first appearance. Flowering plants continue to dominate the world’s flora today; extant pteridophytes species number 10,000, gymnosperms 750, and angiosperms up to 300,000 species. Angiosperms appear 300 million years after the first vascular plants and 220 million years after the first seed plants (Willis & McEwain, 2002, p 156). Angiosperms underwent a rapid adaptive radiation soon after their first appearance. These new seed plants possessed a number of important characteristics that separate them from other seed plants.
Flowering plants evolved distinctive characteristics that help to define this plant division. Angiosperms possess flowers, develop fruits, contain specialized conducting cells in their vascular tissues, develop a double-layered seed coat, exhibit a distinctive column-like structure in their pollen grain walls, and undergo double fertilization during their life cycle.
New
reproductive strategies helped angiosperms become a great success
and diversify into the forms we know today. Male and female structures
develop within flowers. When pollen comes into contact with a
flower's stigma the growth of a pollen tube is activated. Each
pollen grain carries two sperm. One sperm fertilizes the egg
in the ovule; the other sperm unites with two haploid cells in
the same ovule. This process is known as double fertilization
and is an important adaptation found in angiosperms. The fertilized
egg will undergo cell division to become a zygote and then an
embryo. The second fertilization results not in offspring, but
rather the development of endosperm, which acts as a nutrient
for the embryo. Cells in the endosperm have three sets of chromosomes.
Endosperm not only serves as an important food source for the
embryos of flowering plants it also is important to other animals.
Humans depend upon the endosperm of rice, wheat, and corn. Recent
research indicates the endosperm may also act as a fertilization
sensor helping to abort embryos of incompatible crosses (Juniper & Mabberley
2006, p.27). A seed is formed when the endosperm and the embryo
become enveloped in a part of the ovule that hardens into the
seed coat. The ovary or other parts of the flower in angiosperms
develop into a fleshy fruit surrounding the seeds. Many organisms
such as birds, bats, and insects have coevolved to help pollinate
angiosperms. The fleshy fruits of angiosperms are an adaptation
for seed dispersal. Many animals use the fruit as a food source,
which results in the dispersal of seeds encapsulated within a
natural fertilizer!
Traditionally angiosperms
are divided into the monocotyledons and dicotyledons. Today
angiosperms are divided into the monocots, eudicots, and
magnoliids. Monocots and eudicots are monophyletic groups.
Eudicots contain most of the dicots. It is useful to known
the major differences between monocots and dicots (eudicots & magnoliids)
when studying both extinct and extant plants.
Monocots
have one cotyledon (seed leaf) at germination. Monocots usually
have flower parts in threes, one aperture or furrow on their
pollen, parallel leaf venation, a scattered arrangement of
vascular bundles, and usually no secondary woody growth. Grasses
and palms are well known examples of monocots. Petrified
plam wood or Palmoxylon is the state stone for Texas
and the state fossil for Louisiana. The state stone for Mississippi
is petrified wood and much of the fossil wood found in the
state is Palmoxylon.
Dicots have two cotyledons when they germinate. Today there are six times as
many dicots as monocots. Dicots usually have flower parts in fours or fives,
possess three apertures on their pollen (except the magnoliids, which have
one), netlike leaf venation, vascular bundles arranged in rings, and commonly
have secondary woody growth (Willis & McElwain, 2002, pp. 156-157). Woody
dicots possess eustele stems; a central pith surrounded by secondary wood and
bark. Woody deciduous trees such as oak, elm, and maple are good examples of
dicots. When looking at permineralized wood in cross-section one can quickly
distinguish between gymnosperms and angiosperms with a 10x loupe.
Most angiosperms have two cell types that are distinctly different in size. The large, water conducting cells, are called vessels; the smaller diameter, more abundant cells are fibers. Gymnosperm wood is made of small diameter tracheids. Tracheids are more easily seen with a 20x loupe. Angiosperms also have tracheids for water conduction. Among the angiosperms we can also distinguish between dicots and monocots. Dicots have their vessels and fibers arranged in rings while monocots have their vascular bundles scattered throughout the stem giving a speckled appearance even to the naked eye (Kenrick & Davis, 2004, p. 74).
The first angiosperms had small seeds, which may indicate they were small herbaceous weedy generalists (Willis & McElwain, 2002, p162). The lack of angiosperm wood in the early Cretaceous would also support the idea that the first flowering plants were small herbaceous plants. Fossil evidence from flowers, leaves and pollen suggests that dicots evolved before monocots. Cladistic analysis indicates a close relationship between Bennettitales, Gnetales and angiosperms (Willis & McElwain, 2002, p. 184).
By the late Cretaceous the adaptive radiation of angiosperms produced shrubs and trees that make up a significant part of today's flora. Representatives of the following dicot families make their first appearance duirng the Cretaceous: Magnoliaceae (Magnolia), Platanaceae (Sycamore), Ulmaceae (elm), Betulaceae (birch), Juglandaceae (walnut), Fagaceae (beech) and Gunneraceae (Willis & McElwain, 2002, p. 187). The following monocot famalies make their first appearance during the Cretaceous: Pandanaceae, Arecaceae or Palmae (palms), Potamogetonaceae (pondweeds) and Araceae (aroids) (Taylor, Taylor & Krings, 2009, p. 917). Fossil pollen indicates that the first grasses (Poaceae) probably evolved during the Cretaceous; although, the earliest unequivocal macrofossil evidence is from the Eocene (Willis & McElwain, 2002, p. 207).
The diversification of flowering plants during the Cretaceous helps to mark a significant change in the world's flora. Paleozoic flora was dominated by ferns and clubmosses (Paleophytic flora). The Paleophytic flora gave way to a Mesophytic flora during the Triassic period. Woody seed-bearing plants and their relatives dominated Mesophytic flora. Thus, the change from Paleophytic to Mesophytic represented a change in reproductive strategy; from spore producers to seed producers. Conifers, cycads, and ginkgoes diversified during this time and dominated the landscape. Flowering plants first emerge during the Early Cretaceous and undergo a great adaptive radiation during the Middle Cretaceous. Flowering plants quickly became a major constituent of species diversity and the world entered the third great age of plant life known as the Cenophytic by the Late Cretaceous (Kenrick & Davis, 2004, p. 143).
The transition from Mesophytic to Cenophytic represents a change in reproductive strategies. Gymnosperms and their relatives relied mostly on wind pollination and bore naked seeds clustered in cones or on the end of stocks. Flowering plants coevolved with animal pollinators, underwent double fertilization, and encased seeds in a fleshy ovary that encouraged seed dispersal. Our modern plant world is a continuation of the Cenophytic age of plants.
Crato
The Crato Formation is a conservation lagerstatten famous for its excellent preservation of Cretaceous insects. The Crato and Santana Formations are two Cretaceous aged fossil-lagerstatten in Ceara, Brazil that make up part of the Araripe Basin stratigraphy. The formations are believed to be around 112 Ma. Formation of the Araripe Sedimentary Basin was associated with the rifting of South America and Africa during the Early Cretaceous. German naturalists Johann Baptist von Spix and Carl Friedrich Philipp von Martius from the Academy of Sciences in Munich collected fish nodules in 1817 and 1820. Their findings were illustrated and published between 1823 and 1831.
The Crato Formation represents a freshwater lake that was increasing in salinity due to an arid environment. High salinity and or oxygen deficient waters prevented benthic organisms from inhabiting this lake. Fossils formed from episodes of mass death and also from carcasses floating or blowing into the lake. Organisms were entombed in a micritic limestone (Plattenkalk), not unlike Slonhofen. Insects and plants have been pyritized and oxidized to goethite, no original carbon remains. Microstructure and even color patterns are preserved.
The Crato Formation is key to our understanding of Cretaceous insects. The insect assemblage includes aquatic and terrestrial forms, most of which can be assigned to modern families. The insects found in the Crato Formation are diverse, examples include: mayflies, damselflies, dragonflies, cockroaches, termites, locusts, crickets, grasshoppers, earwigs, leafhoppers, true bugs, water bugs, lacewings, snakeflies, beetles, weevils, caddis flies, true flies, wasps, and bees. Other arthropods, like scropions, spiders, centipedes, and crustaceans are also found. Gymnosperm shoots and Angiosperm leaves, roots, flowers, fruits, and seeds are preserved. Fish, pterosaurs, frogs, lizards, turtles, feathers, and birds have also been reported. Many of these specimens await description. Many of the insects are found by workers who quarry the stone near the town of Nova Olinda for use as ornamental paving stone (Selden & Nudds, 2004, pp. 109-120).
Santana
The Santana Formation may represent a shallow embayment near a coastal region that periodically experienced marine incursions. Organisms are preserved within calcium carbonate concretions.
Preservation is so good that even delicate soft tissues, such as gills, muscles, stomachs, and eggs are fossilized. The organisms themselves are preserved in calcium phosphate (francolite). Francolite precipitates in acidic environments low in oxygen, which would occur during decomposition by bacteria. The exquisite preservation of soft-tissue indicates that the decomposition by bacteria could not have lasted long. The phosphatized fish then became nucleation sites for the precipitation of calcium carbonate (limestone). The precipitation of these limestone concretions occur under the same conditions, except that a rise in pH in the microenvironment is needed, possibly facilitated by the presense of cyanobacterial mats.
The Santana Formation is best known for its fossil fish. Most of the fish are collected by farmers and sold to local commercial fossil dealers. The majority of fish taxa represent the ray-finned fish (subclass Actinopterygii). The pike-like Vinctifer, with its distinctive extended rostrum, is often found with its back arched, a sign of dehydration after death. Some fish, such as Tharrhias and Rhacolepis are often found grouped together within a single concretion. Lobe-finned fish (subclass Sarcopterygii) are represented by two coelacanths, Mawsonia and Axelrodichthys. The class Chondrichthyes is represented by the hybodont shark Tribodus and the ray Rhinobatos. Over 20 different taxa of fish are known from the Santana Formation.
Reptiles found in this formation include: pterosaurs, theropod dinosaurs, crocodiles, and the oldest known examples of side-necked turtles (pleurodires). One of the crocodiles, Araripesuchus, is a terrestrial form that is also known from West Africa. This find indicates a link between Africa and South America after the origin of this lineage. Invertebrates include some small shrimp, gastropods, and bivalves. Among invertebrates, only ostracods are common (Selden & Nudds, 2004, pp. 109-120).
Hell
Creek
The
Hell Creek formation is a concentration lagerstatten that
preserves dinosaurs of the Late Cretaceous.
Some of the bone beds contain the disarticulated remains of thousands
of individuals. However, some bone beds produce catilaginous
structures and skin impressions. A specimen of Anatotitan,
a hadrosaur, was recently found with over 50% of its skin preserved.
Hell Creek beds outcrop in Montana, North Dakota and in South Dakota. Equivalent strata in Wyoming are known as the Lance formation. Barnum Brown (1873-1963) first described the Hell Creek formation in 1907. Brown discovered the first Tyrannosaurus rex in Wyoming in 1900. He discovered two more specimens in the Hell Creek formation of Montana in 1902 and 1908.
The Hell Creek formation is bounded by the Fox Hills Formation below and the Fort Union Formation above. The Fox Hills Formation represents near-shore beach deposits layed down as the Western Interior Seaway retreated. In general, the Hell Creek formation represents a fluvial deposit made by meandering rivers flowing east out of the Rocky Mountains across a floodplain into the Western Interior Seaway. Fossils are found in both channel and floodplain deposits.
The fossils and geology of the Hell Creek formation paint a picture of a semi-tropical environment with abundant rivers and open forests. The forests were dominated by small to medium sized flowering plants including laurels, sycamores, magnolias, cericidiphyllum, and palms. Barberry, buttercups, nettles, elm, mallow, rose, coffeeberry, and dogwood were less common. Rare but present were bryophytes, ferns, cycads, ginkgos, and conifers.
Herds of cerotopsids, composed of Triceratops and Torosaurus, roamed the plains and are the most common fossil of the Hell Creek Formation. Groups of Hadrosaurs, like Edmontosaurus, also fed on the vegetation and are the second most common fossil found in this formation. Ankylosaurs, such as Ankylosaurus and Edmontonia along with Pachycephalosaurs, like Pachycephalosaurus, were present but less common. Ornithomimids, like Ornithomimus and Struthiomimus, were the most common carnivores feeding on insects and small animals. Tyrannosaurus rex, the top predator was the second most common carnivore. Dromeosaurs, such as Dromeosaurus and Saurornitholestes as well as the Trootids, like Troodon are the third most common carnivores and probably hunted in packs.
The Hell Creek Formation is best known for its dinosaurs, but many other organisms can be found. Frogs, salamanders, turtles, crocodiles, and alligators inhabited the waterways. Hesperornithiforms, strong swimming, flighless predatory birds explored bodies of water preying on fish. The bowfin Cyclurus is the most common fish found in the The Hell Creek Formation. Gars, sawfish, paddlefish, and sturgeons also cruised the rivers. Freshwater mollusks lived in and around the bodies of water. Freshwater sharks and rays preyed on the mollusks.
Along side the rivers and in the forested areas were lizards, snakes, and a variety of mammals. Pterosaurs still inhabited the skies. The earliest known boa snake and the last known pterosaurs are found in the Hell Creek Formation. Multiple mammalian representatives coexisted with the dinosaurs. Rodent-like multituberculates lived along side a primitive placental hedgehog. Marsupials, such as the badger-sized Didelphodon and the oppossum-like Alphadon shared the landscape.
Marine mollusks, such as ammonites, are also found in the Hell Creek Formation. Marine fossils indicate a close proximity to the remnants of the Western Interior Seaway. Some marine fossils are also associated with the Breien Member of the Fox Hills Formation, which represents a brief return to marine conditions The Hell Creek Formation, dated at 65 Ma is important because it gives us a window into the last days of the dinosaurs (Nudds & Selden, pp. 168-185).
Mass Extinction
At the end of the Cretaceous, 65 million years ago, 85% of all species would go extinct, making this event second only to the Permian mass extinction (Hooper Museum, 1996). Sixteen percent of marine families went extinct. Ammonoids, belemnoids, rudist bivalves, inoceramid bivalves and many brachiopod groups went extinct. Most of the large marine reptiles (ichthyosaurs, plesiosaurs, and mosasaurs) were lost. Some families of sharks and teleost fishes went extinct. Eighteen percent of terrestrial vertebrate families would go extinct (Siegel, 2000). Dinosaurs, pterosaurs, many lineages of early birds, and some mammals went extinct. In fact most terrestrial animals more than 1 meter in length would go extinct (Nudds & Selden, 2008 p. 169). One third of higher level plant taxa went extinct and for a short time ferns became dominant over the angiosperms and conifers in North America (Stanley, 1987, p. 157). Some of these organisms mentioned went extinct before the KT (Cretaceous-Tertiary) boundary, while others were on the decline. Some groups disappeared catastrophically right at the KT boundary. Some interesting ecological patterns can be observed.
The hardest hit marine organisms were free-swimming or surface forms (plankton, ammonites and belemintes). On the sea floor filter feeders (corals, bryozoans, and crinoids) were hit hard while organisms that fed on detritus were little affected. Open water fish fared well. Mollusks with wide georaphic ranges had a higher survival rate than those with a small geographic distribution. Tropical species were effected more than those who were cold tolerant. In the terrestrial realm, as we have already mentioned, being large was a disadvantage. The only large land animals to survive were crocodilians (Benton, 2005, pp. 248-251). Amphbians seem to have not been affected by the extinction event. At the family level 70 to 75% of taxa surived the event (Benton, 2005, p. 255). What contributed to this mass extinction?
Scientists at the University of California at Berkeley including Luis and Walter Alvarez, Frank Asaro, and Helen Michel discovered an iridium anomaly in a fine-grained clay layer in several K-T (Cretaceous/Tertiary) boundary sites around the world (now the Cretacous/Paleogene boundary or K-Pg). These K-T boundaries are found in both marine and terrestrial deposits and show the same succession, an ejecta layer followed by the clay enriched iridium layer (Benton, 2005, p. 250). The group recognized that iridium is abundant in stony meteorites and proposed that the fallout from a meteorite on the order of 10 kilometers could explain the anomaly and possibly the extinction event. Subsequently, a crater was found beneath the Gulf of Mexico off the Yucatan Peninsula during exploration for oil. The Chicxulub crater is of the right size and age. Volcanic activity may also act as a source of iridium. The Deccan Traps in India represent a large terrestrial flood basalt. Ironically, the Deccan Traps would have been positioned on the opposite side of the Earth at the time of the Chicxulub impact.
There is also evidence for climatic changes as well as floral and fauna changes leading up to these events. Many organisms were already on the decline during the Late Cretaceous. Planktonic foraminiferans experienced major losses before the end of the Cretaceous. Calcareous nonoplankton were also on the decline. Ammonoids, inoceramid bivalves, and the reef building rudists experienced attrition. Multiple lines of evidence including preferential survival of cold water tolerant organisms and isotopic ratios suggest the climate was cooling. There is also evidence to support a decline in abundance and diversity of dinosaurs (Stanley, 1987, pp. 133-171).
However, the iridium anomaly, which in some areas is also associated with shocked quartz grains (quartz grains that bear criss-crossing lines produced by meteorite impacts), glassy spherules close to the impact site (produced from melted material under the crater and then ejected into the air), carbon particles associated with massive fires, the spike in ferns (associated with ash falls), and the Chicxulub crater support that a meteor impact may have caused a final pulse of extinction that occurred on a global scale. Whether this mass extinction was the result of multiple factors or primarily one, its effects on the evolution of life would have great consequences.
The largest mass extinction at the end of the Permian period provided reptiles with the opportunity to become the dominant vertebrate life forms on Earth. Roughly, one hundred and eighty-six million years later the second largest mass extinction would take away Mesozoic reptilian dominance and usher in the Cenozoic, an age for mammals.
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