True grasses of the family Poaecae are a late product of plant evolution, but not as late as was previously thought. They seem to have existed as early as Late Cretaceous, according to some grass-like phytoliths found in 66-million-year-old coprolite, or dinosaur dung, and pollen record on several continents.
However, the vast grasslands known today have not existed for long. For 25 million years or so after the demise of non-avian dinosaurs, grasses account for less than 1 % of all pollen in the fossil record. Grasslands only appeared after the global rainforests of Eocene Epoch receded, making way to dry-adapted plants like grasses. Evidence of large grass-dominated ecosystems appears by the Early Miocene, some 20 million years ago. (photo: Miroslav Duchacek/Wikimedia Commons)
Despite the lack of actual grasses, there is a number of other, older plants that look very grass-like even though they’re not closely related. Paleoartists often avoid depicting anything that looks remotely like a grass, but the general shape – thin leaves and/or fronds standing more or less upright, with reproductive structures on top – is fairly common and seems to work for both xeric and mesic plants.
Pilularia, a member of the order Salviniales, was briefly mentioned in the first part of this series. It’s an aquatic member of the true ferns, with sister taxa dating all the way back to the Late Jurassic. By the Cretaceous, the distribution of Salviniales was already pretty much global. (photo: Christian Fischer/Wikimedia Commons).
The presence of Pilularia-like filiform ferns in the Mesozoic is, however, doubtful. Their fossil record only goes back few tens of millions of years, though it’s fragmentary nature makes it hard to say if the plants weren’t there of just haven’t been found yet. It could be, however, that Pilularia evolved from it’s clover-like ancestors relatively recently.
In any case, Pilularia is tiny. Mediterranean P. minuta is the smallest known fern with fronds about 2 cm tall. It’s perhaps more fitting to call it grass for ants than grass for dinosaurs.
Psilotum, or whisk fern, looks disturbingly ancient. Lacking both leaves and true roots, and with very primitive-looking sporangia scattered here and there on the fronds, they look similar to the very earliest land plants (P. nudum growing in a rock crack near the Sydney Opera House, Pete the Poet/Flickr).
Looks are often deceptive when it comes to plants, and Psilotum is no exception. It is not closely related to Cooksonia or other Silurian plants, but is a true fern, though belonging to a very old lineage. It has a sister taxon relationship with Tmesipteris, a hanging epiphyte native to Australia, New Zealand and New Caledonia, which does have both leaves and roots.
The lineage leading to Psilotum and Tmesipteris (called Psilotales) is an ancient one, perhaps a sister to all other ferns, though it’s exact placement is still controversial. In any case, the lineage must have been there through all of Mesozoic (photo by me, Psilotum nudum in Kaisaniemi Botanical Garden, Finland).
Unfortunately, there are no fossil psilotales. None. We know they must have been there, but where and how diverse is anyone’s guess. A speculative paleoartist could add them to almost any scene, especially as modern species are apparently equally at home growing terrestrially, on rock cracks or as epiphytes, in forest undergrowth and full sunlight. They are rather weedy and apparently very hard to get rid of.
Equisetum fluviatile from California. Photo by Joel Kabahit/Flickr.
Horsetails (Equisetum) are famously ancient plants, often depicted in paleoart. Species very similar to modern ones range from the Early Mesozoic. It has even been suggested the earlier genus Equisetites is similar enough to be lumped. This would make Equisetum the oldest living genus of vascular plants, dating at least from the Permian, with uncertain early members from the Carboniferous.
Most paleoart shows horsetails with abundant branches. These horsetails look very unique and can’t, without a lot of hopeful thinking, be called grass-like.
However, a significant portion of the 15 or so living species are branchless or sparsely branched and, essentially, look like thick grasses. There’s plenty of variation within species. Equisetum fluviatile, for instance, is branchless on most sites, but in particularly nutrient-rich habitats it may suddenly sprout whorls of branches.
This kind of sparsely branched horsetails are also known from the Mesozoic fossil record, though they are a minority. For example, Equisetum thermale formed thick monitypic stands near (or in) hot springs in Jurassic Patagonia. E. filum from the Middle Jurassic of England was also unbranched. Swamp or lakeshore scenes with dinosaurs grazing on grass-like horsetails are quite plausible all over Mesozoic.
Here’s another old friend: Ephedra, also mentioned in the second part of my paleobotany series, as some species produce colourful false-fruit (photo by José María Escolano/Flickr).
Ephedra forms broad, low shrubberies that, along with the upright, leafless branches give it a tussock-like appearance. Fossils classified into this genus date from the Early Cretaceous. While it is mostly known from pollen and seed record, whole plant fossils are known from the Yixian Formation of China. They belong to a species that closely resembles modern E. rhytidosperma, which, incidentally, also grows in China.
Other Ephedra-like members of Gnetales are also known from the Early Cretaceous from all over the world. Some, such as Yixian Siphonospermum above, are so grass-like that they have actually been erroneously described as true grasses (photo: Rydin & Friis 2010/BMC Biology. Scale bar: 1 mm).
True grasses (Poaceae), as mentioned before, were already there in the Late Cretaceous. However, they might not have yet been similarly adapted to dry conditions and frequent grazing as they are now. Poales – the group including grasses, sedges, rushes, cattails and bromeliads – probably originated in wet, nutrient-poor environments, which some basal groups still inhabit. Afterwards, they went through a stage as forest undergrowth plants, only after which (and well in the Cenozoic) the major diversification of dry-adapted grassland species happened. Thus, it might be most plausible to depict Late Cretaceous grasses in either forest or wetland habitats.
But where and when exactly did true grasses originate? Both molecular and fossil record indicates the very latest Cretaceous, about 70 or so million years ago. It has been suggested their original home was in South America, part of the supercontinent Gondwana at the time.
Grasses apparently spread rapidly: grass pollen aged 70 to 60 million years is known from South America, India and North Africa. Interestingly, grass phytoliths (abrasive pieces of silica meant to deter herbivores) were found from late Cretaceous coprolites, or fossilized dinosaur dung. The coprolites, which probably belonged to titanosaur sauropods, represented five different species of grass among with cycad and conifer remains, apparently belonging to multiple lineages. While grasses apparently only formed a small portion of their diets, the sauropods did actually dine on grass!
There’s no evidence, as of yet, that Mesozoic grasses occurred on northern continents at the time, but in late Cretaceous Gondwanan scenes, they are a plausible addition. Unfortunately, as there are no Mesozoic grass macrofossils, we have no idea what they looked like.
Perhaps the best hint available is that one of the phytolith types found in India and assigned to species Matleyites indium is very similar to two closely related genera of modern grasses: the rice (Oryza) and cutgrass (Leersia). Cretaceous rice, anyone? (photo: rice field, Dr. David Gealy, USDA. Public Domain.)
First part: Not all Ferns Look Like Ferns.
Second part: Fruit for the Dinosaurs.
References and further reading:
Puebla et al. 2014: Aquatic ferns from the upper Cretaceous Loncoche Formation, Mendoza, central-western, Argentina. Plant Systematics and Evolution.
Hermsen et al. 2014: New marsileaceous fossils from the Late Cretaceous of South America and a reevaluation of Marsileaceaphyllum. Plant Systematics and Evolution.
Channing et al. 2011: Equisetum thermale sp. nov. (Equisetales) from the Jurassic San Agustín hot springs deposit, Patagonia: Anatomy, paleoecology, and inferred paleoecophysiology. American Journal of Botany.
Harris 1979: Equisetum filum sp. nov. from the Middle Jurassic of Yorkshire. Review of Paleobotany and Palynology.
Cao et al. 1998: Discovery of fossil monocotyledons from Yixian Formation, western Liaoning. Chinese Science Bulletin.
Wang & Zheng 2010: Whole fossil plants of Ephedra and their implications on the morphology, ecology and evolution of Ephedraceae (Gnetales). Chinese Science Bulletin.
Yang et al. 2005: Morphology and affinities of an Early Cretaceous Ephedra (Ephedraceae) from China. American Journal of Botany.
Rydin & Friis 2010: A new Early Cretaceous relative of Gnetales: Siphonospermum simplex gen. et sp. nov. from the Yixian Formation of Northeast China. BMC Biology.
Linder & Rudall 2005: Evolutionary History of Poales. Annual Review of Ecology, Evolution, and Systematics.
Prasad et al. 2005: Dinosaur coprolites and the early evolution of grasses and grazers. Science.