Cretaceous Period: 145.5-65.5 Million Years Ago

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.

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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.

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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.

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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.

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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).

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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.

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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.

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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.

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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).

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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.

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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.

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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).

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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.

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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.

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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.

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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.

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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).

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