Lecture 5 : Emerging Terrestrial Ecosystems: The Rhynie Chert and Early Paleozoic Life

Introduction to Emerging Terrestrial Ecosystems and Geologic Context

Timeline of Terrestrialization: Terrestrial ecosystems before $440$ ,million years ago were largely barren or dominated by microbial crusts.
- By $530$ \,million years ago, early complex terrestrial ecosystems began to emerge
- By $430$ \,million years ago (Silurian), more complex structures appeared
- By $370$ \,million years ago (Devonian), ecosystems became significantly more complex.
Geologic Time Scale Reference:
- Paleozoic Era: Includes the Cambrian ( $542$ - $488.3$ \,Ma), Ordovician ( $488.3$ - $443$ \,Ma), Silurian ( $443$ - $416$ \,Ma), Devonian ( $416$ - $359.2$ \,Ma), Carboniferous (Mississippian and Pennsylvanian, $359.2$ - $299$ \,Ma), and Permian ( $299$ - $252.2$ \,Ma).
- Mesozoic Era: Triassic, Jurassic, Cretaceous
- Cenozoic Era: Paleogene, Neogene, and Quaternary (including the Holocene)
- Major Extinctions: A "Gigantic Extinction" occurred at the end of the Permian ( $252.2$ \,Ma), and a "Big, Big Extinction" occurred at the end of the Cretaceous ( $65.5$ \,Ma).

The Rhynie Chert: A Windows into Early Ecosystems

Definition of Fossil Lagersttte: A German term meaning "resting/storage place."

Refers to a sedimentary deposit extraordinarily rich in the diversity or quality of preservation.

Concentration Lagersttten: Large numbers of fossils, often fragmented, representing significant time spans (e.g., dinosaur bone beds).
Conservation Lagersttten: Exceptional preservation, often of entire in situ (in place) ecosystems (e.g., Burgess Shale, La Brea Tar Pits, Rhynie Chert).
Discovery and History:
- Discovered in $1912$ by Dr. William Mackie, a medical practitioner, in Aberdeenshire, Scotland.
- Found unusual rocks in a dry-stone wall; thin sections revealed plant tissues with cellular detail.
- Kidston and Lang ( $1917$ - $1921$ ) provided the first publications describing plants, fungi, and algae/bacteria
- Arthropods were described shortly after by Hirst ( $1923$ ) and others.
Formation Process (Silicate Permineralization):
- The environment was a modern equivalent to Yellowstone: a volcanic area with geysers and hot springs.
- Hot, mineral-rich (silica-rich) fluids from depth coated and trapped organisms on land or in shallow ponds.
- Organic structures were mineralized as sinter deposits turned into chert (a fine-grained silica-rich sedimentary rock).
- Paleogeography: During the Early Devonian ( $408$ \,Ma), the Rhynie area was part of Laurussia, situated in the subtropics approximately $28^{\circ}$ south of the equator.

Biological Components of the Rhynie Chert Ecosystem

Rhynie Plants:
- Seven taxa total; five are well-known: Horneophyton, Aglaophyton, Asteroxylon, Nothia, and Rhynia
- Aglaophyton is notable as the only non-vascular plant in the group.
- No bryophytes are known from this site
- All plants were "knee-high" or lower, measuring less than $40$ \,cm tall.
Bacteria, Algae, and Protists:
- Palaeonitella cranii: A charophyte (green alga/algal relative).
- Winfrenatia reticulata: An early fossil lichen representing a symbiosis between fungus and cyanobacteria
- Palaeoleptochlamys hassii: A testate amoeba
- Presence of cyanobacteria and simple green algae.
Rhynie Fungi:
- Saprotrophs: Decomposers of dead plant material; often associated with degraded plant axes
- Parasites: Targeted plants, algae, and other fungal spores (e.g., Chytrids growing on glomeromycotan spores).
- Mycorrhizae: Evidence of arbuscular mycorrhizae (AM) in Aglaophyton suggests this partnership was essential for early land plant adaptation. Endomycorrhizae occupy intracellular spaces; "arbuscular" means tree-like mycelia used for nutrient exchange.
Prototaxites: The Devonian Giant:
- Organisms up to $1$ \,m wide and $8$ \,m tall, far exceeding the height of contemporary plants.
- Francis Hueber ( $2001$ ) hypothesized it was a giant fungus based on tube-like internal tissues.
- Recent study by Loron et al. ( $2026$ ) suggests Prototaxites (specifically P. taini) was chemically and structurally distinct from true fungi, possibly representing a completely extinct group of multicellular eukaryotes.

Early Terrestrial Animals: The Arthropod Invasion

Myriapods:
- Millipedes (Diplopoda): Oldest known terrestrial arthropods (late Silurian, UK). Examples: Kampecaris. Primarily detritivores with two leg pairs per segment.
- Centipedes (Chilopoda): Predatory and venomous; appeared shortly after millipedes. Possess one leg pair per segment.
Euthycarcinoids:
- Mysterious group of aquatic or possibly semi-terrestrial arthropods.
- Heterocrania rhyniensis provides evidence for their relationship to myriapods.
Arachnids:
- Trigonotarbids: Extinct spider-like predators (Silurian to Permian). Palaeocharinus rhyniensis is a key Rhynie taxon; specimens show evidence of "coldest lungs."
- Harvestmen (Opiliones): Non-venomous predators known as daddy-long-legs
- Mites: Small arachnids like Palaeotydeus devonicus, which likely ate Aglaophyton spores (sporivory) or acted as detritivores
- Scorpions: Present in the Silurian (e.g., Ontario) but not the Rhynie Chert; terrestrial or semi-terrestrial predators.
Hexapods: Defined by having six legs
- Rhyniella praecursor: A springtail (collembolan), not a true insect but a close relative. Small microbivores/fungivores.
- Rhyniognatha hirsti: Once thought to be the oldest winged insect ( $400$ \,Ma), but its status is debated; it may belong to a centipede.
Crustaceans (Freshwater):
- Lepidocaris rhyniensis: Most common animal remains; similar to modern fairy shrimp; likely biofilm scrapers.
- Castracollis wilsonae: Similar to modern tadpole shrimp (Triops); omnivores.
Nematodes: Found in Aglaophyton stems; could be parasites or feeding on dead tissue.

Trophic Ecology and Food Web Reconstruction

Evidence for Trophic Interactions:
- Coprolites (Fossil Feces): Different types classified via ichnotaxonomy (e.g., Lancifaex simplex, Rotundafaex aggregata). Contents include plant spores, fungal hyphae, and amorphous organic matter.
- Direct Evidence: Rare, such as Palaeocharinus found inside plant stems
- Morphological Inference: Comparing mouthparts (chelicerae) to living relatives.
Food Web Framework:
- Nodes: Represent populations, species, or higher taxa.
- Edges/Links: Represent feeding interactions
- Metabolic Modeling: Grouping organisms into guilds (e.g., terrestrial primary producers, aquatic detritus) to generate multiple Species Level Networks (SLNs) to account for taxonomic uncertainty.
Phytophagy/Herbivory Delay:
- The Gap: There is a $90$ -million-year gap between the appearance of land plants ( $460$ \,Ma) and widespread arthropod herbivory ( $360$ \,Ma). A $50$ -million-year gap exists for vertebrate herbivory.
- Hypotheses for Delay:
  - 1. Digestive Constraints: Plants are tough, high in structural fibers (cellulose/lignin), and low in nutrients ( $P$ , $N$ ) compared to unit of $C$ .
  - 2. Co-evolution: Time needed to develop gut microbiota for fermentation.
  - 3. Taphonomic Bias: Small, soft-bodied herbivores or light damage might not preserve well in the fossil record.
  - 4. Top-Down Control: Suppression of early herbivores by predators subsidized by aquatic food sources (trophic subsidies).
Energy Flow Models:
- Detrital Pathway: Early ecosystems were primarily detritus-based (decomposers and detritivores)
- Herbivory as "Short Circuit": Herbivory allows energy to skip the detrital stage, flowing directly from producers to higher consumers, potentially increasing ecosystem efficiency.

Comparison: Early Devonian vs. Modern Communities

Modern Ecosystems: Feature seed plants, flowering plants, winged insects, amniotes (mammals, birds), web-spinning spiders, and land mollusks.
Rhynie (Early Devonian) Ecosystems: Dominance of seedless plants (lycopods, ferns), Prototaxites, trigonotarbids, and wingless hexapods. Shared groups include arachnids, centipedes, and mycorrhizal fungi.