November 15, 2022
Dickinson Research Extension Center Updates

Finding Microfossils of Ancestral Grasses with Cretaceous Age, 113 Million Years Ago





Llewellyn L. Manske PhD
Scientist of Rangeland Research
Dickinson Research Extension Center
701-456-1118
The evolutionary development of the grasses and grasslands is poorly understood. There are few grass macrofossils and none older than 56 million years ago (Ma), at the Paleocene-Eocene boundary. Grass pollen is only diagnostic to the grass family level, Poaceae, and graminoid (grass and sedge) dominated ecosystems cannot be identified by pollen analysis. Fortunately, three groups of resourceful scientists have recently been able to collect and identify microscopic fossil ancestral grass phytoliths of Cretaceous age. Phytoliths are nearly indestructible silicon structures that all grasses produce in leaf epidermal short cells to provide structural rigidity to increase light interception. Each grass subfamily and tribe produce unique phytolith morphotypes that are taxonomically diagnostic.
 
The first study was from India and described Cretaceous (67-65 Ma) grass phytoliths extracted from coprolites (fossilized feces) of titanosaur sauropod dinosaurs. The scientists were able to identify ten distinctly different phytolith morphotypes that were ascribed to four subfamilies with three morphotypes ascribed to the subfamily Oryzoideae which were ancestor grasses of rice (Prasad et al. 2005, 2011).
 
The second study was from Myanmar that described a grass spikelet, a leaf fragment with phytoliths, and a grass floret infected by an ergot sclerotia with each specimen embedded in amber from the resin of an araucarian tree (large conifer). The Cretaceous (110-100 Ma) grass parts were from an ancient herbaceous bamboo ascribed to the subfamily Bambusoideae (Poinar 2004, 2011, 2015).
 
The third study was from northwestern China that described two grass epidermal pieces and three bilobate phytoliths in grass fragments extracted from maxillary teeth of a Hadrosaur (duck billed dinosaur). The Cretaceous (113-110 Ma) grass specimens were from the subfamily Anomochlooideae (the oldest known basal subfamily of true grasses) (Wu, You, and Li 2018).
 
These three astonishing studies were able to discover Cretaceous grass microfossil phytoliths that doubled the age of ancestral grasses to 113 Ma. This much longer development timeline will completely change the previous concepts of how ancestral grass evolution occurred. The grass flowers had previously been reduced to three stamens each with an anther and filament, and a single chambered ovary with two stigmas at the top for pollen reception, and a pair of lodicules at the base.
               
The angiosperms had previously underwent rapid genome downsizing which allowed faster rates of growth resulting in a huge competitive advantage. With these improvements, angiosperms were able to achieve rapid diversification and great radiation becoming the dominant plants during the mid Cretaceous (140-100 Ma). This increase in angiosperms greatly increased the quantity and improved the quality of available forage which facilitated a huge increase in new herbivorous dinosaurs. The intensified grazing pressure on grass ancestors from herbivorous dinosaurs became an important driver that influenced the development of grazing defense mechanisms.
 
The grazing pressure from herbivorous dinosaurs required ancestral grasses to produce a low growing point below grazing height, to produce double the herbage biomass greater than that needed for photosynthesis, to develop the structures and hormone systems to improve water use efficiency, to develop a system for compensatory physiological growth to replace leaf and stem structures, to develop a competitive belowground system with symbiotic fungi for uptake of soil water and nutrients, to produce axillary buds for vegetative reproduction of tillers, and to shed the ability to produce antiherbivory toxic substances. 
 
Today, modern perennial grasses posses the same four primary defoliation resistance mechanisms, that permit grasslands to be the primary forage source for the livestock production industry because grazing herbivorous dinosaurs drove ancestral grasses to develop grazing defense mechanisms. The twice-over rotation grazing strategy has been designed to activate these ancient defoliation resistance mechanisms by partial defoliation by grazing livestock that removes 25% to 33% of the aboveground biomass of the vegetative lead tillers between the 3.5 new leaf stage and the flower stage during 1 June to 15 July each year.

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