Lecture 19 - The Evolution and Ecological Rise of Angiosperms

General Context: The rise of flowering plants, or angiosperms, represents the transition to modern forests during the Cretaceous radiation.
Fossil Records and Plant Groups: Modern forests evolved through significant shifts in biodiversity from non-flowering groups to angiospermae. Major groups include: * Pteridophytes: Ferns and lycopods. * Gymnosperms: Non-flowering seed plants such as conifers and cycads. * Angiosperms: Flowering seed plants.
Diversity Statistics (Number of Fossil Species): * Devonian: 23 * Carboniferous: 145 * Permian: 66 * Triassic: 201 * Jurassic: 299 * Cretaceous: 359 * Tertiary (Paleogene/Neogene): 444

The Age of Flowering Plants: While the exact origin remains unknown, molecular and fossil data provide estimated timeframes. * Angiosperm Stem Node: Dated between $310 \times 370 \, Ma$ . * Angiosperm Crown Node: Dated between $140 \times 270 \, Ma$ . * Pollen Records: Angiosperms appear in the pollen record circa $137 \, Ma$ . * Neotropical Rainforests: Established around $58 \, Ma$ , following the Cretaceous-Paleogene (K-Pg) mass extinction.
Extinct Lineages (Stem Relatives): * +Glossopteridales * +Pentoxylales * +Bennettitales * +Caytoniales * +Petriellales

Monocots: * Seeds: Single cotyledon. * Leaves: Long, narrow leaves with parallel veins. * Stems: Scattered vascular bundles. * Flowers: Floral parts in multiples of 3.
Dicots: * Seeds: Two cotyledons. * Leaves: Broad leaves with a network of veins. * Stems: Vascular bundles arranged in a ring. * Flowers: Floral parts in multiples of 4 or 5.

Floral Organs: * Petal: Modified leaves surrounding reproductive parts; often brightly colored to attract pollinators. * Sepal: Modified leaves that protect the flower bud and support petals during bloom. * Tepals: Outer parts of the flower where sepals and petals cannot be easily distinguished (e.g., Magnolia). * Bract: A modified leaf associated with a reproductive structure (e.g., bracts on Rattle, Bougainvillea, or Chloranthaceae).
Stratigraphic Shift: Fossil evidence (Friis et al., 2006) illustrates a shift in protection by the early Late Cretaceous, featuring the first occurrence of flowers with a corolla (organized petals).

The ANA Grade: The first flowering plants to diverge from the ancestral lineage. Characteristics include flowers with multiple parts and bisexual structures (or derived from bisexual ancestors). * Amborella: A single species of shrub native to New Caledonia. * Nymphaeales: Water lilies and related aquatic plants. * Austrobaileyales: Woody aromatic plants, such as star anise.
Fossil Records: * Chloranthaceae: Evidence from the Early Cretaceous of Portugal (Friis et al., 1994) includes female flowers and male inflorescences. * Earliest Monocot Flowers: Discovered in the Upper Cretaceous of New Jersey (Gandolfo et al., 1998). Features include 6 tepals and monosulcate pollen grains.

Key Biological Innovations: * Flowers and Diverse Fruits: Increased reproductive efficiency and specialization to local scaling, facilitating coevolution with pollinators and dispersers. * Dense Vein Networks: Allowed for higher hydraulic efficiency. * Dense Stomata: Increased rates of transpiration and photosynthesis ( $100$ to $1000 \, \text{per mm}^2$ ). * Vessels: Improved water transport, leading to greater productivity and structural complexity.
Quantifiable Global Changes (Benton et al., 2021): * Leaf Vein Density: Increased from $4 \, \text{mm/mm}^2$ to $9 \, \text{mm/mm}^2$ . * Stomatal Density: Increased from $100 \, \text{per mm}^2$ in gymnosperms to $1000 \, \text{per mm}^2$ in angiosperms.

Timeline: Occurred approximately $100$ to $50 \, Ma$ .
The Concept: An expansion of the Cretaceous Terrestrial Revolution (KTR) into the Paleocene, providing an "explosive boost to terrestrial diversity."
Ecological Outcomes: * Appearance of multistratal (layered) angiosperm-dominated forests in the early Paleocene. * Radiation of epiphytes and polypod ferns.
Environmental Context: * Greenhouse climate with a long-term cooling trend. * Occurrences of Ocean Anoxic Events. * Paleocene-Eocene Thermal Maximum (PETM) and Early Eocene Climatic Optimum. * Tectonic continental movements and the K-Pg bolide impact.

Pollen Records (~137 Ma): Early representatives include diverse tricolpate and monosulcate types.
Macrofossil Ecology (~100 Ma): * Early Forms: Fast-growing herbaceous plants like Ternaricarpites floribundus (Middle Albian, Russia), Archaefructus sinensis (Aptian, China), and Potomacapnos apeleutheron (Aptian, Virginia). * Habitat Progression: * Initially: Lakes, ponds, and braided rivers (aquatic pioneers points toward diversification in aquatic settings). * Later: Floodplains, levees, freshwater swamps, and estuaries. * Growth Habits: Increase in platanoids and the appearance of woody monocots.
Size and Structure: Largest known Cretaceous angiosperm is the "Mother Stump" from the McRae Group (south-central New Mexico), with a Diameter at Breast Height (DBH) of $1.8 \, m$ .

Ecological Lag: A delay between the initial increase in species richness and the widespread ecological dominance of angiosperms.
Western Interior of North America (76–72 Ma): A model system showing a gradient in diversity. * Latitudinal Pattern: Flowering plants were more prevalent at lower latitudes within the tropical thermal belt (Mean Annual Temperature, MAT > $20 \, ^\circ\text{C}$ ).
Key Fossil Floras: * Big Cedar Ridge (BCR) (~73 Ma): Diverse flowering plants, but ferns remained the most abundant (palmetto-fern wetlands). * Kaiparowits Formation (K) (~75.7 Ma): Extremely diverse with many platanoids; angiosperms were the most abundant group in channel settings. * Jose Creek Member (JCM) (~74.6 Ma): Extremely diverse; first record of angiosperms being abundant across the entire landscape (terra firma). Features a mixed canopy of flowering plants and conifers.

Spatial Expansion: Angiosperms spread from the equator to the poles and moved from unstable riparian/aquatic settings to stable landscapes.
Vegetation Zones: * Tethys: Hot monsoonal climates and megathermal forests. * High Latitudes: Continued dominance of Pinaceae. * Mid Latitudes: Radiation of Eudicots and Rise of Fagales under cooler climates.
The K-Pg Boundary Trajectory: * Loss of plant diversity (appx. $45\%$ in neotropics) and a decline in conifers. * Low diversity persisted for approximately $6 \, \text{million years}$ . * Recovery: Ushered in modern closed-canopy, multistratal rainforests and an increase in fruit size.