Mesozoic Vegetation Before Flowering Plants: exhaustive Study Guide

Presentation Overview and Mesozoic Global Context - Presenter: Jaemin Lee

• Date: March 12th, 2026

• Chronological focus: Late Permian, Late Triassic, Middle Jurassic, Late Jurassic, Early Cretaceous, and the Late Cretaceous periods.

• Geographical Features and Timeframes:   - Late Triassic ($220\,\text{Ma}$): Global maps depict the supercontinent Pangea surrounded by the Panthalassa ocean, with the Tethys sea to the east (Blakey 2014).   - Middle Jurassic ($170\,\text{Ma}$): Ongoing tectonic shifts (Blakey 2014) and the emergence of the Central Atlantic.   - Late Jurassic: Presence of the Sundance Seaway (Price 2009).   - Early Cretaceous ($105\,\text{Ma}$): Emergence of the Western Interior Seaway (Blakey 2014; Price 2009).

Evolutionary Transitions in Mesozoic Flora

• The Mesozoic era spans from $251\,\text{mya}$ to $66\,\text{mya}$.

• Land plant diversity transitioned from the Paleophytic flora to the Mesophytic flora (Niklas 1986, modified by I. Duijnstee and Lee).

• Paleophytic Flora Components:   - Lycopsids (lycophytes).   - Sphenopsids.   - Marattialean ferns.   - Early pteridosperms (e.g., medullosans).   - Cordaites.   - Walchian and voltzian conifers.

• Mesophytic Flora Components:   - Leptosporangiate ferns.   - Acrogymnosperms: Including cycads, ginkgos, gnetaleans, and modern conifers.   - Later pteridosperms: Including peltasperms, corystosperms, bennettitaleans, and caytonialeans.   - Angiosperms: Eventually emerging during the later Mesozoic.

The Carnian Pluvial Episode (CPE)

• Occurred approximately $234-232\,\text{Ma}$, lasting between $1.2-1.7\,\text{million years}$.

• Drivers of Change:   - Wrangellia Large Igneous Province (LIP) activity.   - Massive input of greenhouse gases leading to increased global temperatures.   - Feedback loop: Increased temperature led to increased evaporation, which in turn increased precipitation.

• Geological and Biological Impacts (Dal Corso et al. 2020):   - Siliciclastic input and crisis of microbial carbonate platforms.   - Near-shore anoxia.   - Development of hygrophytic microflora and tropical humid paleosols.   - Volcanic $SO_2$-linked acid rain.   - Increased water stress and more frequent wildfires.   - Significant biological event: The first global appearance of amber.   - Notable fossil: Ampezzoa triassica, a phytophagous mite from the Superfamily Triasacaroidea (Seyfullah et al. 2018).

• Analogy: The January 2022 eruption of Hunga Tonga—Hunga Ha’apai is used to contextualize massive atmospheric impacts.

Ferns in the Mesozoic

• Ferns occupied diverse ecological niches (Shen et al. 2018):   - Wet Environments: Peat-forming swamps, freshwater marshes, riverbanks, and understory vegetation.   - Drier Conditions: Coastal areas, mountain slopes, and prairies.

• Taxonomical Diversity:   - Orders/Families: Gleicheniales (Gleicheniaceae, Dipteridaceae, Matoniaceae), Marattiales (Marattiaceae), Cyatheales (Cyatheaceae, Dicksoniaceae), Osmundales (Osmundaceae), and Schizaceae.   - Fern Prairies: Modern analogues include Dicranopteris (Gleicheniaceae) thickets in French Polynesia and Sierra Negro, Galapagos Islands.

• Mangrove-like Flora: Weichselia reticulata was abundant from the Middle Jurassic to the Early Cretaceous. It is characterized by unresolved phylogenetic affinity (potentially Matoniaceae or Marattiaceae) and a mangrove-like ecology.

Seed Plants and Cycads

• Seed plant groups include Acrogymnospermae (extant gymnosperms: Cycadales, Ginkgoales, Cupressales, Pinales, Gnetales) and extinct groups.

• Cycads Attributes:   - Usually trunk-forming with pinnately compound leaves.   - Dioecious: Separate male and female plants (Zhang et al. 2015; Coiro et al. 2023).   - Symbiosis: Multiple evolutions of symbiotic relationships with nitrogen-fixing cyanobacteria via coralloid roots (Kipp et al. 2023).

Ginkgophytes

• Fossil History:   - First known from the Mesozoic with high diversity (e.g., Ginkgo huttoni in the Jurassic; Ginkgo adiantoides in the Paleocene).   - Ginkgo biloba was "rediscovered" in the 1700s; one wild population survives in China, but it is now globally common in landscaping.

• Morphology:   - Dioecious plants.   - Paired ovules or pollen organs in G. biloba.   - Evolutionary trend: Reduction of ovule numbers.   - Seed feature: The outer layer of the ovule becomes fleshy, known as the sarcotesta.

• Related Fossil Genera: Ginkgo, Baeira, Sphenobaeira, and Umaltolepis (Vajda et al. 2021).

Conifers (Pinophytes)

• General Traits: Tracheids with large bordered pits, resin canals, simple leaves with single or parallel veins, and pollen/ovulate cones.

• Temporal Shift (Condamine et al. 2020):   - Triassic: Dominance of Paleozoic conifers like the Voltziales (e.g., Voltzia).   - Jurassic–Cretaceous: Rise of modern families and extinct families like Cheirolepidiaceae.

• Pinaceae:   - Includes $220-260$ species, representing $35-40\,\%$ of all conifer taxa.   - Predominant in high latitudes and altitudes in the Northern Hemisphere.   - Features: Cone flexing and cone shedding.

• Araucariaceae and Podocarpaceae:   - Currently southern hemisphere taxa; includes tropical understory plants.   - Araucariaceae: Known for cone shedding.   - Podocarpaceae: Known for fleshy cones (e.g., Nageia, Afrocarpus).   - Historical Example: Petrified Forest National Park (AZ), Late Triassic ($225\,\text{Mya}$), features abundant Araucarioxylon arizonicum.

• Cupressaceae:   - Cupressaceae s.s.: Junipers, Monterey cypress.   - Cupressaceae s.l.: Includes Sequoias and bald cypresses (formerly Taxodiaceae).   - Metasequoia: Wide global distribution seen at $70\,\text{Ma}$, $60\,\text{Ma}$, and $50\,\text{Ma}$, shrinking significantly by the present day (Zhang & Wang 2015).

• †Cheirolepidiaceae (Extinct: $252-66\,\text{Ma}$):   - Pollen type: Classopollis, an indicator of aridity.   - Habitats: Ranged from arid environments (with thick stomatal protection) to tropical halophytes (saline environments).   - Example: Brachyphyllum sp. and Pseudofrenelopsis capillata.

• Podozamites:   - A morphotaxon of conifer shoots with broad, multi-veined leaves (Pole et al. 2016).   - Includes deciduous forms that drop entire shoots.   - Represented in the Krassilovia Mongolica clade (Voltziales) and associated with Araucariaceae and Podocarpaceae (Herrera et al. 2021).

Gnetophytes and Bennettitales

• Gnetophytes: Includes Gnetum, Welwitschia (e.g., W. austroamericana), and Ephedra (e.g., archaeorhytidosperma).

• Bennettitales (Cycadeoids):   - Cycadeoidaceae: Characterized by stout trunks and bisporangiate cones (e.g., Cycadeoidea, Cycadella, Monanthesia).   - Williamsoniaceae: Slender, branched trunks with bi- or monosporangiate cones (e.g., Williamsonia, Wielandella, Ischnophyton).   - Historical Site: Fossil Cycad National Monument ($1922-1957$).

• Caytoniales: Middle Triassic to Late Cretaceous shrubs/trees; likely seasonally deciduous and considered close relatives of angiosperms.

Case Study: Early Cretaceous Jinju Formation

• Location: Jinju area, Milyang Subbasin, Gyeongsang Basin, South Korea (Gyeongsang Supergroup).

• Stratigraphy: Part of the Sindong Group (Nakdong, Hasandong, and Jinju Formations), followed by the Hayang and Yucheon Groups.

• Age Determination (Lee et al. 2010; Chae et al. 2020):   - Palynology: Late Neocomian (Berriasian to Hauterivian).   - U-Pb Zircon Dating: Range of $112.4 \pm 1.3$ to $110.4 \pm 2.0\,\text{Ma}$.   - Youngest detrital zircon: $106.0 \pm 1.9\,\text{Ma}$.   - Uppermost formation age: $106.5\,\text{Ma}$.

• Fossil Record:   - Identified as a Vertebrate ichnofossil Konservat-Lagersttte.   - Jinju entomofauna: Includes high-resolution insect fossils.   - Flora: Ferns and conifers; includes Krassilovia (voltzian conifer), Doylea (corystosperm), and Umaltolepis (ginkgophyte).

• Paleoenvironment Reconstructions (Lee et al., accepted):   - Temperate swamp inhabitants (Mongolian analogues).   - Intermontane savanna environment.   - Palynological climate indicators:     - Abundant Classopollis (Cheirolepidiaceae) and Ephedroid pollen (Gnetales) indicate an arid climate.     - Inaperturate and bisaccate pollen (Cupressaceae s.l., Pinaceae) indicate temperate conditions.