I've had this problem, and I think most paleoartists have had this problem - what to illustrate as the ground cover of the Mesozoic? Often times dinosaurs are just thrown onto dusty patches of bare earth with remote patches of cycad/ferns/conifers (the Mesozoic big three of plant types) discrete from the dinosaurs who are conveniently framed by bare earth. Not only is such a look discongruous with how animals actually move through and interact with their veggie costars but is discongruous with how a functioning and healthy ecosystem works and looks. Having such vast expanses of bare, dusty soil would literally bleed minerals, organic matter, all the vital nutrients from the system. You know, the exact stuff needed to grow something like a Dreadnoughtus? Intense solar radiation and heat endemic to a hot house climate would bake such expanses into concrete pans. Without vegetation holding soil together monsoonal rains would wash away all of the valuable topsoil. Dinosaurian megafauna would compress and damage the soil in this compacted, nutrient deficient, scoured out shell of an ecosystem. Any larger trees or shrubs that managed to grow would be very vulnerable to large dinosaurs compacting the soil around their root systems - analogous to how large machinery parked under trees can fatally damage the root systems simply by being there. In short Mesozoic ecosystems with such vast expanses of exposed, barren soil literally paints a picture of ecosystems on the verge of complete collapse, much less capable of robustly supporting the diverse hordes of dinosaurian megafauna we know that such systems hosted.
The go to answer has generally been to just "throw in some ferns and horsetails that'll get her done!". But is that good enough? Too often I feel like the ubiquitous "fern prairie" look is a cop out - grasslands are just replaced with ferns and voila there is your typological replacement!! I'm not saying that such "fern prairies" did not exist, they did and there is some evidence of such mosaic fern/cycad open spaces existing as I discussed in Dino Turf I. I am leery of the ubiquitous "fern prairie" panacea answer usually parroted out because I know from living in a semi-arid habitat (much of the Mesozoic is described as semi-arid) of southern California that ferns generally don't thrive very well in such exposed, water deficient environs. There could have been ubiquitous drought tolerant families of ferns that thrived in such habitats during the Mesozoic , that we have since lost, but we have not found evidence of widespread xerically adapted ferns that would more or less "replace" the role of grass. Just keep in mind that ferns and horsetails - collectively referred to as pteridophytes - are like the amphibians of the plant world - they require a water medium to complete reproduction. Long story short I'm not satisfied and I think others are not as well. Various gymnosperm type plants could have filled the "grass" niche and probably did: cycads, gnetales, bennettitales, ground cover conifers, the mysterious cheirolepediaceae aka "the cheiros". However I can't help but feel we are missing something that holds it all together. A type of plant - or maybe actually a suite of plants & other organisms - that could serve as a buffer to the stresses of megafauna, intense heat, drought, and monsoons emblematic of the Mesozoic? A collection of organisms that actually ameliorate the conditions for and serve as "nursery" beds for the other, larger, and more well known Mesozoic vegetals?
Such a suite of ground cover organisms exists today right under our noses. They store nutrients; anchor soil and inhibit runoff; fix nitrogen; store carbon; serve as nurseries for larger and "higher" plants; thrive in semiarid climes; and generally fulfill all of what is most needed to support healthy and vibrant Meszoic ecosystem. This ground cover is referred to as biological soil crusts or for short bio-crusts, or crypto-biotic crusts.
|cryptobiotic soil crust Natural Bridges Nat'l Monument, Utah
CC3.0 credit Nohonjoe
Cryptobiotic soil crusts sure are neat things to look at and would add a neat textural touch to Mesozoic landscapes. Best of all these crusts are composed of living communities of fungi, lichens, cyanobacteria, bryophytes, and algae - which means that no angiosperms need apply - they are composed of organisms that were all around in the Mesozoic actually before the Mesozoic…
The ancient pedigree of the constituent organisms have caused some researchers to speculate that biological soil crusts are the original land plants. That in fact communities of soil crusts were the first colonizers of land and it is their action via nitrogen and carbon fixation, soil stabilization, and soil aggradation that laid the groundwork for larger, higher plants. Soil crusts don't actually compete with other plants for nutrients but they do ameliorate conditions for other plants, especially by nitrogen and carbon fixation. Over time bio-crusts can actually create soil from impoverished environments.
Is there any evidence of biological soil crusts in the Mesozoic? I am glad you asked because we don't actually have to speculate that these communities were around for at least some of the Mesozoic (arguably all of it) we have proof. The blog Geomdermatophila dedicated to soil crusts hooked me up with this knowledge as they did a piece on a paper documenting the oldest known occurrence of biological soil crusts coming from none other than the Grand Staircase-Escalante Nat'l Monument of New Mexico near the contact of the Wahweap and Kaiparowits formations (Simpson et al., 2010 abstract). Cool!!
|credit Simpson et al., 2010
A couple of observations:
Today biocrusts are emblematic of arid to semi-arid environments. I have found no evidence of widespread aridity in either the Wahweap formation or Kaiparowits. In fact quite the opposite view is most supported.
From: At the Top of the Grand Staircase (Titus & Loewen, Indiana University Press 2013) Chapter 6: The Kaiparowits Formation: A Remarkable Record of Late CretaceousTerrestrial Environments, Ecosytems, and Evolution in Western North America Pp 85:
…the Kairpowits Formation was deposited in a wet alluvial to coastal plain setting with an abundance of large river channels and perennial ponds, lakes, and wetlands…
….the unusually thick (860 m) succesion was deposited in a remarkably short - 2 Myr interval, recording one of the fastest sediment accumulation rates of any richly fossiliferous continental sedimentary sequences in the world (Roberts, 2005).
Not only are these formations not semi-arid, it appears they document one of the wettest fossiliferous environments ever!! The sedimentation rate was just out of control.
One would expect biocrusts to be documented from the Morrison formation or some arid Cretaceous formation from Mongolia… but the Grand Staircase? It is possible that this biocrust occurred in some type of microhabitat, or that there was some sort of anomalous blip of aridity near the contact of the Wahweap formation and the Kaiparowits. Possibly… it's also not impossible to grow biocrusts in wet environs. It's also entirely possible that biocrusts were a lot more widespread, emblematic, and occurring in multiple environments during the Mesozoic. Due to their small, fragile nature, that during sedimentation events (i.e. flooding) their characteristic features get washed out and damaged it is entirely possible that their prevalence has been overlooked due to fossil bias. It is this latter interpretation that I most favor - that biocrusts were emblematic of the Mesozoic in all environments -we've just been missing them.
Biocrust Fossil Bias at Play? Why We Might Be Missing the Most Important Ground Cover Component and Foundational Ecosystem Community of the Mesozoic?
FIrst of all let's state the obvious: paleosols are not the sexiest avenue of research to invest in when it comes to paleontology. Paleosol researchers are certainly not getting the prestige of tyrannosaur researchers. So there is that.
Secondly biocrusts are weird and, despite their vast importance in modern day arid and semi-arid environs, are still greatly under appreciated and a bit of a niche crowd follows them. I'm willing to bet a lot of the people reading this blog have never heard of bio-crusts much less thought of their potential importance during the Mesozoic…
Third of all biocrusts are weird, small, easily dismissed, and simply ignored. Just as biocrusts are trampled on and walked on today without even a cursory question of what is this? it is entirely possible that a great number of paleontologists have trampled over and dismissed preserved traces of bio-crusts simply because they are ignorant of bio-crusts and just dismissed the preservation as some weirdly textured paleosol.
Fourth of all bio-crust detection might be largely escaping detection in the pollen fossil record. Because they are slow growing and low to the ground they are not reproducing and sending up the various spores in huge broadcast spawnings like large trees do with pollen. Instead biocrusts can spread by low rate transmittance. As sort of a background player in the fossil pollen record. But once established on a patch of land they can simply spread vegetatively/colonially/filamentously. Or, if disturbed and moved about, crop up in a new spot. Or possibly pass through the gut of organisms and colonize new areas.
Bio-crusts and the Seasonally Arid/Monsoonal Climate of the Mesozoic: A Perfect Pairing?
Painting in broad strokes here - but Mesozoic environments are often characterized by a highly seasonal wet/dry pattern. A dry period with little to no precipitation for several months, punctuated by a wet monsoonal type deluge of water to various degrees. This regime is actually tailor made for bio-crusts. Let me explain why. Bio-crusts can shrivel up, desiccate, and go into a dormancy period. The various mosses, fungi, bryophytes, lichens etc. etc. look essentially dead. But rehydrate these crusts and voila, the powers of regeneration life coming back from the dead right before your eyes!!
However there is a caveat to mention when illuminating this mode of life, as I will outsource the explanation from this excellent piece: Why mosses can grow in the desert and why their future is uncertain from the blog Geodermatophilia:
In short, multiple small rain events with periods of desiccation can decrease biocrusts. But more singular rain events i.e. a "wet season" or monsoonal type rain pattern followed by one long mainly uninterrupted dry period is great for them.
So should we plop abundant bio-crusts in our Mesozoic paleoart? Do they answer the question of Mesozoic ground cover for us? I say that there is a there… there with regards to bio-crusts in the Mesozoic. That we do have evidence for bio-crust in a very wet environment not typical for bio-crust formation begs the question were bio-crusts more diverse, widespread, and emblematic of the Mesozoic? Are the bio-crusts in today's ecosystems something of a relict from a more widespread diverse condition? That bio-crusts have been winnowed down from a larger and more diverse pedigree?
Such a thought, that bio-crusts were formerly more diverse and widespread, potentially helps answer a question fomenting in your mind: what about dinosaur trampling? After all, that is a major issue with regards to preserving contemporary bio-crusts: human trampling, off road vehicles, cattle grazing, horses etc. etc. How did bio-crusts deal with the large dinosaurian megafauna? Where a single footfall could obliterate decades of growth?
The Tricky Question of Biocrusts and Dinosaur Trampling…
The issue of trampling on biocrusts is not as one dimensional as it may seem. Indeed it appears that bio-crust composition can shift in response to varying degrees of disturbance and there are situations where a certain modicum amount of disturbance may in fact benefit bio-crusts. Too much trampling would be a bad thing, too little disturbance might also have negative consequences… a certain sweet spot might be optimal.
I point you to this abstract:
Take home message: the trampling effect of ungulate hoofs (up to about 25% coverage) actually bolstered moss growth by providing ideal microhabitat for moss growth!! Pretty cool.
This observation leads to some interesting patterns that might occur with where and how bio-crusts accumulated in dinosaur mediated habitats. They probably did not get a good foothold in "dino-turbation" prone areas - near water ways or on game trails that dinosaurs probably made just as large game animals do today. The lack of bio-crust accumulating near watering hole areas or on game trails would lend towards more preservation bias against bio-crusts. Game trails can act as small drainages transporting material to areas of deposition and fluvial habitats are of course prime areas for fossilization.
Also take a closer look at the authorship of that last abstract, recognize one of the names? An interesting synchronicity that paleoart own's Julius Csotonyi does a bit of biocrust research on the side...
Another issue with regards to crusts is how different dinosaur feet are from hoof dominated mammalian herbivorers. I discussed this issue here: Dino Turf II. Could it be that soft footed dinosaurs enacted a differing regime of disturbance than sharp hoofed mammals? A more compressive type force than shearing and cutting?
Biocrust Grazing Dinosaurs?
You know I had to go there right? If biocrusts were widespread during the Mesozoic, possibly more diverse and important than in today's ecosystems, they possibly comprised a large percentage of the biomass in certain areas. This possibility lends itself to questions of dinosaur exploitation of biocrusts for food… it is not so strange when we consider that modern caribou subsist primarily on lichens and mosses in the Arctic, not the classic grass dominated diet of other ungulates.
Should we envision fern prairies in areas such as the Mesozoic equatorial arid belt - that almost assuredly lacked them based on the evidence on hand. From the Complete Dinosaur Second Edition:
As I discussed on this post Brazilian Death Valley in the Cretaceous fern spores generally diminish towards arid areas in the Mesozoic while Classopolis pollen - indicative of cheirolepidiaceaen conifers - denotes arid environs. A lack of fern pollen makes sense for arid areas - and especially the arid equatorial belt - that seems to have been a semi-permanent fixture for the Mesozoic. Despite the paucity of evidence for abundant ferns at the equatorial arid belt, fern grazing is the go to answer for a group of low latitude herbivorous dinosaurs: rebbachisauridae, especially Nigersaurus, Tataouinea and similar kin.
Could such animals have subsisted primarily on biocrusts? Their jaws are weak, teeth show evidence for a gritty, low and dirty diet… if it fits the bill?
If I had to choose between a more likely dominant ground cover type to thrive in the Mesozoic equatorial arid belt that could have easily been scraped up by the weak jaws of rebbachisaurids - remember ferns decline towards the equator and cycads, gnetales, and horsetails are not especially soft, pliable food stuffs - through process of elimination I'm going with biocrusts.
Like solitary desert schooners, the slow motion march of Tataouinea through the vast North African desert was vast and primarily lonely. A complex system of air sacs and efficient S.I.G.I.L. (sub dermal interstitial gridded insulating layer) provided the animal with an internal air conditioning system in this infernal habitat. The S.I.G.I.L. "bubble-wrap" insulating layer of keratinized skin allowed the animal to shield itself from the hottest temperatures and keep warm during the coolest nights or monsoonal storm. It also allowed the animal an efficient metabolic transfer of food energy to growth, as little metabolic energy was needed to keep the metabolism elevated. Apart from the occasional flush of green growth following monsoonal rains and sporadic mast fruiting events of cycads the mainstay of the dinosaurs' diet was the omnipresent biocrusts that flourished everywhere in this landscape of extremes. Biocrusts sprang up on dunes, stabilizing the sand for higher plants to further colonize. Biocrusts covered every spot of bare soil not covered with clumps of cycad and cherioleopiaeacean groves. Biocrusts were present in all areas except for the most highly trampled locales and game trails - which were few and far between in this desolate realm. Tataouinea could survive on this slow growing resource simply because of the vast expanse of land these crusts covered. The relationship was not one sided however. The micro topography of the dinosaurs footprints served as adequate colonizing grounds for new biocrusts. Spores passed through the dinosaurs digestive tract survived and, deposited within a pile of dung, allowed for new colonization opportunities. The dinosaurs travels within the realm of the massive tidal rivers of this low lying land was infrequent. The higher incidence of large predators discouraged a long stay but the locale served as a reconnaissance point for mating opportunities and egg laying.
|credit Emiliano Troco. form here used w/permission
It should also not go unnoticed that biocrusts may have not just been important in the Mesozoic but also increasingly evident that biocrusts are critical in todays arid lands, especially with regards to changing climates.
"A Long habit of not thinking a thing wrong, gives it a superficial appearance of being right, and raises at first a formidable outcry in defense of custom". Thomas Paine