LIFE OF THE MESOZOIC

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I. MEZOZOIC REEFS

Early in the Triassic, the final major group of corals evolved, the scleractinian corals. These corals differ from rugose corals in their pattern of skeletal growth, septa are inserted by the mesenteries in multiples of six. Scleractinians are both hermatypic and ahermatypic, with ot without symbiotic zooanthellae. Modern reefs are comprised of hermatypic scleractinians.

No reefs existed in the Early Triassic.

A. Middle Triassic

• low mounds (<10' high)
• some in deep water (ahermatypic corals)
• few species

B. Latest Triassic

• larger reefs
• 20 or more species
• hermatypic scleractinians
1. < 50m water depth
2. 20^o C water temp.
3. normal salinity

C. Jurassic

• proliferation of colonial scleractinians

D. Cretaceous

• increasing displacement of corals by rudistid clams
• by late Cretaceous all reefs were dominated by rudistid clams
• rudistid clams
1. assymetrical bivalves
2. one valve in shape of a cone, the other a small hinged lid.

E. Rifting of Pangaea and Mesozoic Reef Development

• Important class discussion

II. TRIASSIC

A. Predators and invertebrate assemblage changes

The Mesozoic brought a major increase in predators to the marine environment. Among the vast array of forces facing the defenseless were:

1. marine reptiles with shell crushing "tooth pavements"
2. fish
3. rays (Jurassic and Cretaceous)
4. crabs and lobsters (Late Triassic and Jurassic)
5. sea stars (Jurassic)
6. molluscs (with drilling devices)
• boring gastropods (e.g. naticids and muricids)
7. ammonites

Responses to the attack

1. brachiopods recover slowly from the Permian extinction and did not adapt well in response to increased predation. Minor radiation of terebratulid brachiopods and final extinction of spirifers.
2. pelecypods proliferated after the mid-Triassic, many occupied a deep burrowing habitat. Pelecypod expansion almost totally replaced brachiopods.
3. echinoids develop armor and others burrow
• sea urchins first dominant group
• heart-shaped urchins (first in Early Jurassic)

B. Mesozoic changes in trophic structure

• During most of the Paleozoic, multitiered suspension feeders dominated invertebrate assemblages.
• The Triassic and Jurassic witnessed a permanent change in the feeding strategy of invertebrates that has persisted to this day. This change involes an expansion and radiation of infaunal suspension feeders and infaunal deposit feeders. Previously, Paleozoic infanal feeders had included bivalves, worms, and crustaceans.
• The Mesozoic expansion of infauna was prompted by the evolution of siphonate heterodont bivalves.
1. siphon- an extension of the mantle that acted as a snorkel, supplying them with oxygen and food from the above the sediment/water interface.
2. the bivalve radiation included both suspension- and deposit-feeders
• Multi-tiered epifaunal suspension feeding communities
1. highest tiers
• crinoids (50-100cm)
2. mid-tier (20-50 cm)
• sponges
• soft corals
• crinoids
3. low tier (5-20 cm)
• sponges
• corals
• bryozoans
• bivalves
• crinoids
• deposit feeders
1. annelid worms
2. bivalves
3. crustaceans
4. holothurians
5. echinoids

C. Plants

• The large sphenopsids and lycopods of the Paleozoic persisted but were much less common.
• Lycopods extinct in Triassic
• Sphenopsids survived into Jurassic
• Four types of plants dominated the Triassic and Jurassic landscape
1. understory plants were ferns
2. middle story plants:
• tree ferns
• seed ferns
• cycads
1. gymnosperms
2. still living
3. large palm-like leaves growing in a cluster at the tip of the stem
• cycadeoids
3. upper story plants:
• conifers
1. primitive conifers called araucarians
2. Norfolk Island Pine a living relative
3. Upper Triassic trees of the Petrified Forest (AZ.) Chinle Formation are araucarians
• ginkgos
1. single sex trees
2. fan-shaped leaves, parallel veins, split margin
3. one living species, Ginkgo biloba

D. Index Fossils

• Ammonites
• Pelecypods

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III. JURASSIC

A. Expansion and proliferation of plankton

Phyotplankton did not begin to recover from the Permian extinction unitl the Jurassic. The Jurassic marks the beginning of the rise of modern planktonic single cellular groups that have dominated the oceans to today.

1. dinoflagellates underwent extensive diversification beginning in the Middle Jurassic
2. diatoms first appear in the Early Jurassic
3. coccoliths firat appear in the Early Jurassic
4. the planktic foraminifera globigerinids first appeared in the Middle Jurassic and proliferated during the Jurassic
5. ostracods began new radiation in Early Jurassic

B. Invertebrate trends

1. most surviving brachipod groups disappeared
2. pelecypods expanded owing to their burrowing or hiding strategy
• swimming pelecypods (scallops) radiated
• first occurrence of attached pelecypod oyster group
3. urchins
1. regular urchins
• surface browsers and grazers
2. irregular urchins
• burrowers and filter feeders
• as diversified as regular urchins by end of period
• shift of anus from top to side
• regulars give rise to five orders of irregulars early in the period
4. ammonites
1. spectacular radiation

C. Index fossils

• Ammonites
• Beleminites
• Pelecypods
• Radiolaria
• Coccoliths
• Gastropods
• Echinoids
• Scleractinian corals

CRETACEOUS

A. Faunal habitats

• Since the Cretaceous infaunal deposit feeders and infaunal suspension feeders have dominated in shallow water settings away from reefs
• deposit feeders more abundant in mud settings
• suspension feeders more dominant in sandy settings
• example of infaunal suspension feeding community
1. depth down to a meter
2. bivalves
3. worms
4. decapod crustaceans
5. gastropods
• example of an infaunal deposit feeding community
1. worms
2. bivalves
3. crustaceans
4. holothurians
5. echinoids

B. Invertebrate trends and changes

• sea urchins and sckeractinian corals: diversified but no major adaptive change
• benthic foraminifera: a large percentage of the "modern" fauna appeared
• encrusting bryozoans (cheilostome) first appeared in the Jurassic but expanded ito >100 genera in the Late Cretaceous
• "new snails" or neogastropods diversified, carniverous with long siphon
• crabs: more modern types appeared
• Inoceramid bivalves- shaped like dinner plates up to 2 m across
• stalked crinoids became restricted to depths >100m

C. Plants

• evolution of 'sea grass", important for pelecypods (scallops)
• During the Cretaceous Period, angiosperms began overcoming non-flowering plants as the dominant land plants. Since this time they have become the dominant land plants.
1. Very dominant group today - 96% of all vascular plant species
2. Success the result of development of flower and enclosed seed
1. specialized flower reproductive structure: - both seed and pollen producing structures
2. colored leaves to attract pollinators
3. Double fertilization: one pollen grain fertilizes ovary, and a second triggers vigorous growth of the nutritious fruit, nut, or kernel.
4. seed covering: soft, fleshy, edible, aid in seed dispersal
5. other dispersal mechanisms
6. exhibit great diversity
7. geologic range: few in Triassic and Jurassic; dominant since Cretaceous
8. fossil record of Angiosperms

D. Index Fossils

• ammonites
• molluscs
• planktic foraminifera
• radiolaria
• calcareous nannofossils
• scleractinian corals

E. Extinction

• see separate notes

F. Ammonites

• see lecture notes