Megaherbivore Multiplier Effect: Maybe We Should Not Get So Excited About Giant Herbivorous Dinosaurs Coexisting

Ok for this post I want to get back to the terrestrial realm, although I do have a load of posts on aquatic stuff planned, and talk about some ideas in ecology/evolution that may have gone unnoticed in paleo circles with some pertinent thoughts to dinosaur niche partitioning. Why would ecology papers go unnoticed in paleontology? Well because Nanuqasaurus, and umm Cambrian filter feeding invertebrates, and Cosmos was on TV and creationists got all butt-hurt about it... No really it is important for paleontology to stay abreast with ecology/evolutionary theory because if you accept that the present is the key to the past then maybe you should keep your mind open to all avenues of thought and guard against a myopic view of the history of life.

Now dinosaurian niche partitioning has been a rather en vogue avenue of study with regards to dinosaur paleoecology.  It is an elegant and, with morphometrics, quantitative way to potentially explain how such large animals, orders of magnitude larger than current land mammals, coexisted in chunks of land often times significantly smaller than contemporary continents (higher eustatic sea levels in Mesozoic). The skull ecomorphology paper on late Cretaceous Albertan herbivores by Mallon and Anderson (2013) which I talked about here (although I have changed my views a bit as noted in post) might be good review and also the various papers on sauropod skull differences/niche partitioning you can find on the web/blogs etc etc may be useful. What these papers suggest is that through niche partitioning, resources can be in a way divvied up allowing situations like we see in Cretaceous Laramidia or the late Jurassic Morisson formation with multiple multi-tonne taxa coexisting. Through character displacement, species- especially closely related and/or morphologically similar ones -that might compete for food, space, resources will tend to diverge more and more from each other where their ranges over lap. Sauropods might have differing neck lengths to feed at different heights or perhaps have stronger jaws/teeth to handle rougher forage (i.e. Camarasaurus) versus weaker slender jawed taxa (diplodocids). Ornithischians coexisting on Laramidia are construed to be high to mid browsers (hadrosaurs), mid level browsers (ceratopsids), or low level grazers (ankylosaurs). But all in all this partitioning of resources is construed as a necessary corollary of the crucible of evolution- competition - as the guiding light that allowed such a diverse Mesozoic bestiary to coexist without overwhelming the resources.

But how secure are we in positing competition as the engine that drove these adaptations towards niche partitioning in dinosaur communities?

I want to direct your attention to an article Competition May Not Be the Driving Force of Species Diversity After All, which suggests an alternative take on the evolution of ecological diversity. The article, more or less a summary of several papers by Joseph Tobias of Oxford University, calls into question the seeming ubiquity of character displacement driven by competition as the primary impetus towards diversity. Looking at the natural and evolutionary histories of species often interpreted as textbook examples of character displacement driven by competition Tobias found the evidence lacking in a majority of these cases. Instead, Tobias offers, species tend to diverge- with or without competition -given enough time to evolve. Looking at ovenbirds, a new world tropical bird family, Tobias performed a rather exhaustive study of bill shapes. Although he found that the species that lived together had the most divergent bills, as predicted with classic Darwinian competition, when the evolutionary history of these cohabiting birds was taken into account the signal for an evolutionary bump due to competition was lacking. Instead the ovenbirds that lived in the same environment also had the longest evolutionary histories. It was evolution over time and in isolation, allopatric speciation, that best explained the signal of species diversity in ovenbirds. That species which cohabited had the longest evolutionary histories made sense- they had enough time to evolve on their own unique evolutionary trajectories- that ultimately allowed them to effectively partition resources when their ranges once again overlapped after isolation.

To study the role of competition in evolution, Joseph Tobias and collaborators mapped out the evolutionary relationships and variation in beak size among 350 lineages of ovenbirds. Image: Joseph A. Tobias and D. Seddon

I should clarify Tobias' work by pointing out that he maintains character displacement does occur- it has just been overstated by researchers. Allopatric speciation and then later cohabitation with niche partitioning is the more common scenario Tobias suggests. Controversial for sure, but very interesting and pertinent to dinosaur speciation/character displacement.

On more of a cultural note ( I do have a background in anthro after all) it does make sense for biologists to perhaps foster a bit of bias towards competition as being a chief catalyst towards diversity. Competition is very Darwinian, competition is a very masculine, and competition is very western and often times intimately associated with capitalism (social Darwinism). And let's face it, biology and science is still very masculine and western dominated. Of course scientists imagine that they are above social biases and cultural leanings, but I beg to differ. We are all cultural animals and bring some amount of baggage with us into any endeavor, no matter how rigorous, we pursue.

Ok now with those thoughts in mind I want to direct you to a recent paper on Plos One: Reconstructing Grazer Assemblages for Protected Area Restoration (March 2013). You should go read it but what these researchers were looking at is the most optimal way to recover successful and complete herbivore guilds on a managed, but depauperate, African range. What is critical is that they looked at the question from the lens of size classes, competition, and facilitatory effects. And basically what they found was that competition inhibited a high diversity of species that occupied the same general size cluster- which plays into what is fairly well established in terms of the link between herbivore and digestive efficiency, optimal grazing sites etc etc. But what was most critical in terms of stabilizing the whole system and allowing for maximum diversity, sustainability, and health of all size classes of herbivores was what they referred to as the facilitatory effects of large megaherbivores such as rhinos, hippos, and elephants. Basically the movements and feeding envelopes of these megaherbivores enhanced pasture, broke up woody debris, altered fire regimes, and created pathways that benefitted all the players in a highly disproportionate manner.

Venter et al. 2014. Plos One

"... the lack of large grazers creates an ecosystem devoid of facilitatory effects which in turn leads to an ecosystem which is unable to maintain its herbivore assemblage structure."

Hopefully you can see that the two examples to the far right with abundant megaherbivores, show the most diversity and balance between all the size guilds. It might at first seem counterintuitive but a healthier, more stable system is the one with abundant and diverse megaherbivores.

I like to call this phenomena the megaherbivore multiplier effect and if you start to look at where you see megaherbivores today guess what you see generally- more megaherbivores - as well as more small and medium sized herbivores. African savanna- large herbivores and all sorts of small, medium, and large guys. In India places like Kaziranga park host Indian elephants, Indian one-horned rhinos, wild water buffalo,  gaur, sambar, swamp deer, Indian muntjac, wild boar, hog deer. You do not need to go very far back into the Pleistocene to see that this pattern of abundant and diverse megaherbivores with complete guilds of various sized herbivores was the rule rather than the exception. Now you might be thinking well that is just because those systems are productive enough to support a diverse assemblage... But even in semiarid habitats such as in the Namib and Kalahari deserts there are more abundant and diverse large herbivores- including elephants, rhinos, and giraffes as well as smaller guys -found there than in say a place like the Mojave desert. At the other extreme let's compare rain forests. West African rain forests are heavily modified by forest elephants which are pivotal engineers of open bai habitats which provide food/habitat/mineral resources for forest buffalo, lowland gorilla, giant forest hog, red-river hog, and bongo among others. Contrast that situation with the depauperate large herbivore fauna of south/central American rain forests- what do you have... a couple of tapirs, some peccaries, some rodents and a couple of deer? But chances are you will not see much in the way of large herbivore activity in south American rain forests if you go there. Just loads of leaf-cutter ants.

Dzanga Bai

Ok so now back to the dinosaur dilemma where multiple mega ton species are stacked on top of each other in a seemingly untenable manner.

But the dinosaurs were sooooo much bigger- does this comparison really hold true for berbivores several orders of magnitude larger?

True but bear in mind that dinosaur reproductive strategy was a lot different than large mammalian reproductive strategy. At any one time in a dinosaur population there would be gazillions of newborn/hatchlings running around, one year olds, two years olds, three year olds from previous breeding cycles..... and then a bunch of promiscuous teenage dinos making babies even before their skeletons were ossified and paying no heed at all to responsible family planning. And then maybe just a couple of old stodgy, weather-beaten adults talking about the good ol' days. It was a live fast die young strategy and for the most part seems to have worked for dinos. More importantly, and diverging strongly from mammals, is that the average size of a dinosaur would be a lot smaller than the maximum adult size that could be reached. The mean was brought down quite a bit. Which is why whenever I see a herd of fully mature ceratopsids/sauropods in movies/pictures, with no variety in size classes, I throw up in my mouth just a little bit... Look at the range of sizes in a population of say nile crocodiles, this gives a better view, in the broadest strokes, of how these populations may have varied in size. Again contrast with large mammals, where small broods and intensive natal care via mothers milk brings up baby quickly to adult size.

Ok but where are all the small and medium dinosaur herbivore species? Many locations show really small species and then really big species- but few in betweeners?

Two points here:

1) The notion of dinosaurs acting as various ecological species- ontogenetic species -throughout their growth trajectories has gained considerable traction. Additionally if we look at this idea through the lens of what I discussed earlier where megaherbivores enhance systems for small and medium species- then it could be possible that the activities of adults megaherbivorous dinos augmented habitat for their own young!!

2) We may in fact be missing quite a bit of the diversity of small and medium sized herbivorous dinosaurs. Fossil bias exists and if they tended to stay in more upland/drier habits they may have stood less chance of fossilization.

And one final word on niche partitioning/competition in dinosaurs. I think it is a bit overstated. Did it exist? Sure. Various sized mouths, tooth structure, jaw muscle leverage, neck length all point to various herbivores being better equipped to handle such and such resource better than others. But if we are talking about the big guys here, the megaherbivores, I think these suggestions of partitioning are more or less moot. They just ate everything. I know, I know what about tooth wear studies? Well I will offer how much do tooth studies really tell us? If dinos were shedding their teeth constantly maybe the wear patterns reflect what they were eating during that particular season? There are lots of examples of modern herbivores partitioning resources but I see a lot of examples of herbivores overlapping considerably and there just being enough of that green stuff that it doesn't really matter. Maybe there was room for 'em all with some partitioning but often times high degrees of dietary overlap. And in the Mesozoic with high eustatic seas, year round balmy temperatures, dino-dung fertilizer, monsoonal climate regimes, high CO2, and often times rich volcanic soils maybe these were just systems that could foster multiple taxa of berbivores with highly overlapping diets and that was just ok.

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Laramidian Dino Morpho-Eco-Head Space Trippin'

* New data has come to light on ceratopsid biting mechanics. I no longer subscribe to the view I espouse here. Update 11/5/13

Available for free here on PLOSone is a new paper analyzing the ecomorphology of some herbivorous dinosaurs from Dinosaur Park, Alberta Canada dating to the Upper Campanian of the Cretaceous. The authors developed 5-6 distinct ecomorphs from the 12 herbivorous dinosaurs examined. They analyzed the skulls based on size, width, ventral deflection of the snout, distal extension of the tooth row, and depth of the mandible. The paper addresses the ongoing debate on how such large and diverse megaberbivores coexisted in such exceptional diversity on the relatively small island continent of Laramidia.

I have talked about Laramidia before here on Laramidia: The Great Dinosaur Species Pump. I discussed a paper suggesting transverse mountain building events- orogonies- as providing a geographic barrier promoting dino vicariance speciation events and ultimately great diversity. I also theorized in that post that nest site fidelity may have worked in coupling with mountain building to further promote dinosaur speciation.

Now keep in mind that the question of dino diversity on Laramidia, like many complex questions, is not necessarily an either/or one. That is, it is not necessarily a simple one answer question. Just because niche partitioning was occurring does not mean mountain building/isolation did not play a role and vice/versa. Never-the-less the paper does add another layer to the debate and I suggest you read it before you move forward reading this post as I do not want to write a simple summary but address some of the more pertinent points in the paper as they relate to my own opinions about dino faunas of Laramidia.

(c) Julius Csostonyi

First of all, here is where I think the paper left me wanting more in several spots.

Where are all the small guys? Now I know people love big stuff and dinos were big stuff but the more we look at these faunas the more diversity of smallish dinosaurs we uncover. Dinosaur Provincial Park was no exception with lots of small to medium sized ornithomimids, pachycephalosaurids, oviraptors, various ornithopods, and probably some other stuff I am forgetting. How important were these guys ecologically? I don't know. Fossil bias likely skews the preserved remains in favor of large dinos like ceratopsids, ankylosaurids, and duck-bills- which happen to be what the study focuses on. Furthermore our friends the ankylosaurs are given the ecomorphological treatment like the ceratopsids and duck-bills but as the authors mention themselves they could not conduct much statistical work on ankylosaurs as the sample size was rare. There it is again, rare but ubiquitous ankylosaurs. Maybe ornithomimids were more important herbivores in DPP than ankylosaurids? Predation pressure and fossil bias likely account for larger species being preferentially preserved. Additionally, due to their R-strategy reproduction, the populations  was likely skewed towards smaller class sizes- but the paper does not address how these subadult/teenage/hatchling dinos acted.

I don't know maybe it was the dataset that they were forced to work with but any more complete approximations of this ecosystem include how smaller dinos/immatures fit into it.

Scant attention is given to the vegetative community. The authors briefly mention the paleo-flora but argue that much of the flora alive at DPP is also found today and their morphometric study is not invalidated by this fact. No argument from me on this one but I believe it is imperative that we perhaps acknowledge that the environment of DPP, a high-latitude subtropical biome, is one we have no modern proxy for and how herbivores exploited this environment will likely invoke animal ecologies we likewise have no modern proxy for. Did plant growth slow or just stop mid-winter with only 7-8 hours of day? Conversely during the summer there must have been a riot of growth...

Again, these are not so much criticisms of the paper- you can only do so much in one paper- but just some ways I think we can see and understand DPP in more resolution. I do applaud the work put in and it appears very solid in terms of showing how these various subfamilies partitioned the resources. But here I do see a little bit of problem in that the language and tone of the paper is couched in terms of "competition" i.e. how did the herbivores coexist? They minimized competition by eating differently on different plant types the paper suggests. But is their another way to look at how these herbivores coexisted?

Consider a beaver pond in modern day Alberta, Canada. You have a beaver which creates the habitat of the beaver pond and directly enhances it for other creatures. One such creature that directly benefits from the beaver pond is moose, attracted to the luxuriant growth and aquatic vegetation. Now the moose and beaver have significant dietary overlap, leaves/branches of cottonwood, poplar, and willow for example. There might be some truth in saying that they compete for this resource. But at the same time the moose disproportionately benefits from the actions of the beaver by the beavers' creation of ideal habitat providing a great example of commensalism. Where Beaver Lead, Moose Follow.

Imagine, if beaver and moose were extinct before paleontologists came about, the problems in elucidating this relationship between moose and beaver for a putative paleontologist. You keep finding this seeming cohabitation beween moose and beaver in the fossil deposits. One a short, stubby, squat rodent equipped with massive incisors and browsing dentition. The other a spindly legged quadrupedal browser. You might wonder how this little critter made its living...how could it get away from predators?....why those teeth?....it certainly could not reach very high into the vegetation. What about the moose? An upland browser maybe washed into wetland settings? Would you ever arrive at a strong seasonal aquatic habit for it? Did the two compete? partition resources? What problems might you encounter in getting to the relationship between moose and beaver? Would you ever?

Now let's get back to the question of Laramidian dinosaurs with the example of moose/beaver commensalism in mind. Currently much of the debate concerning Laramidian megaherbivores centers on niche partitioning i.e. how they minimized competitiion. But let's reframe the debate and allow for various levels of competition, niche partitioning, and commensalism all possibly taking place- maybe even concurrently.

In the paper the authors note that the morphometric breakdown suggests discrete herbivorous adaptations among the families and subfamilies. One of the more interesting aspects of the species breakdown is that the pattern of herbivore types represented at DPP tends to repeat over time remaining stable. That is these "chronofaunas" keep stable even as the individual species change. For example a fauna will likely contain maybe one nodosaur, one ankylosaur, a chasmosaurine, a centrosaurine, several  lambeosaurine, and maybe several hadrosaurine types. But you never see the fauna dominated by a single morpho-type; for example you never see five or six lambeosaurines but no chasmosaurine. That these distinct morpho-types keep repeating in a pattern is indeed, as the paper suggests, consistent with niche-partitioning; but let's examine this trend of "chronofaunas" not from a perspective dominated by competition, but one in which commensal relationships dominated.

Let's address the issue of megaherbivore diversity from the perspective of "keystone" species- species that have dramatic influence over the whole ecosystem. And the keystone species, I purport, in DPP park were the ceratopsids. A few posts ago in The Devil's Chewtoy I spelled out my view of ceratopsid paleo-ecology. I envision ceratopsids not as archosaurian rhinos/buffalo but as giant, terrestrial parrot/beavers. These guys, I suggest, were using those over-sized heads, parrot beaks, and stout builds to chew up and topple trees and then have their pickings
of the best browse. Seen in this light ceratopsids were not ungainly rhino-wannabes in an awkward foreleg half push up. Instead that unique stance, rostral beak, and robust build bespeak an exquisitely adapted tree toppler. Indeed the authors in the paper characterize ceratopsids as acting as browsers from their analysis but are perplexed that the scope of browse is limited to the first meter of vegetation. But if ceratopsids brought the browse down to their level the problem is solved.

One of the more intriguing issues in dinosaur biogeography is the scarcity of ceratopsids in Asia despite it being connected to North America (with abundant ceratopsids). Could the prevailing arid conditions of Asia not have provided enough trees for ceratopsids? We don't see guys like Sinoceratops in China until things get wetter. Additionally although hadrosaurids seem to have penetrated into South America, ceratopsids were either absent or extremely scarce in colonizing South America. Could an equatorial arid belt prohibited their migration into the south? To add another layer to my argument remember there are no clear high browsers in DPP. No sauropods, not even any especially tall therizinosaurs or ornithomimids. Is it not a little weird that some of the best paleoart of DPP shows all these low slung herbivores walking underneath towering, unexploited trees? But tree toppling ceratopsids adequately fulfill that seeming ecological vacancy.

Seen in this light ceratopsids were the engineers of these ecosystems. They would have moved into forests and toppled trees. The effect would be to lower the browse level not only for ceratopsids but for any other dinos that wanted to partake; maybe bands of lambeosaurines followed along the ceratopsids waiting for a leafy dinner. Abundant root sprouting/coppicing would commence for redwoods and angiosperms providing abundant browse in reach of herbivores. Additionally this tree toppling would have created open areas with sun penetration that benefitted lower browsers and grazers like big wide mouthed ankylosaurs and hadrosaurids.  Remember those wood chomping hadrosaurs I talked about in Rot n' Roll in the Mesozoic Muck? Yeah tree toppling ceratopsids would have benefited them as well... Maybe ceratopsid tree cutting set a whole ecological cascade in motion of forest succession ultimately creating and enhancing habitat for a whole myriad of dinos. plants, and other critters? The point is you can imagine all kinds of unique ecological corollaries based off of ceratopsid tree toppling.

And if ceratopsids were the lumberjacks/forest clearers of Laramidia, ankylosaurs were the janitors.

As I talked about in Ankylosaurs Are Still Weird I believe these guys were up to eating all sorts of weird stuff. Not saying they did not eat stuff like ferns, fruits and stuff as preserved in the oolite of Minmi, but I believe the Mesozoic offered up ecological opportunities just not found today which these guys capitalized on. You have to imagine that these subtropical high latitude environments with abundant water were interesting places without exact analogue today. During those long, hot summer days plant growth was likely off the charts. And then during those winter months you have all those deciduous plants, leaf litter, dino dung, moisture- it must have just been rife with fungal growth. Ankylosaurs may have relished those fungal treats. Additionally stuff like dung, rotten dead stuff, insect hives, eggs,  detritus, tree sap, roots, fruits and bones were possibly delectable tid-bits for ankylosaurs. They were, in my view, like walking compost bins with their adept smell leading them to their next toxic meal eagerly slurped up with their robust tongues.  Ankylosaurs kept the flow of nutrients going hard and fast in these super-charged Laramidian ecosystems in my view.

And the hadrosaurids in my view were the middlemen ecologically. Relatively narrow snouted lambeosaurines perhaps benefited in the forest regeneration stage where browse was nutritious and within reach. Wide beaked hadrosaurids, along with wide mouthed ankylosaurs, likely thrived in recently cleared areas. Periodic fires would have cleard excessive debris and stimulated new growth. Ornithomimids, pachys, and small ornithopods likewise exploited these putative ceratopsid modified habitats. Tyrannosaurids, raptor type guys, and azhdarchid pterosaurs maintained a strong top down influence preventing the herds from overbrowsing, keeping the ceratopsids moving, preventing clear-cutting, and allowing forest regrowth. Egg eating lizards, mammals, and ankylosaurids would have also checked explosive herbivore populaiton growth.

Anyways, those are my thoughts on how some aspects of the DPP ecosystem functioned. I know it is not a typical color by the numbers approach and, as always, highly speculative but I do believe what I have portrayed is in the realm of possibilities and may offer insight into how to approach these ecosystems.

Doug Henderson sketch for Tony McVey project (c) 2008

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Paleo-Myth Number 7: Dinosaur Suppression Was the Primary Stifling Agent of Mammals During the Mesozoic

Great competitors need, and are often times defined by, a great rival who will make them rise to their greatest potential. Magic needed Bird. The Yankees need the Red Sox. John McEnroe needed Bjorn Borg. And the US Woman's soccer team needs the Japanese Woman's soccer team.

It was scarcely a year ago that a Japanese squad captured the world's attention by beating a heavily favored American squad in the World Cup. Emboldened team play of the Japanese, inspired by the plight of their tsunami stricken homeland, provided some of the most captivating sports drama of the new millennium. In the 2012 Olympics the two teams squared off yet again, this time the US girls prevailed.

And while history will no doubt record the Japanese Woman's team as a special unit who seized a special "zeitgeist" moment in time, the US through their continued dominance of the sport, will no doubt be recognized as the greatest female soccer team of all time.

The Mammals Are Winning

And since, oh, about 65 million years ago it is the mammals who have been winning in many of the earths' ecosystems. Although not always the most numerous in their environment, one is hard pressed to find examples of places in the world where the largest critter is not a mammal. The great morphological diversity of mammals should also be appreciated. Just consider the difference between a fruit bat and a beaked whale or a gibbon and a moose. Definitely more diverse than both extinct and extant dinosaurs, squamates, amphibia or arguably even fish in terms of body styles.

Clearly mammals are doing something right....

But it wasn't always golden days for class mammalia.

The suppression continues....

That herons and egrets have an inordinate fondness for all things cute and fluffy may be a shock to some- but it is a common behavior, especially among great blue herons. That these birds are doing something that their ancient progenitors did is not much of a stretch- tasty mammal flesh is high up on the preferred dinner of a lot of carnivores, past and present. Those tasty bundles of soft, scaleless cholesterol rich flesh have been sought after morsels for quite some time I would imagine.

Tokay gecko

One of the stranger behaviors I observed with my Tokay Gecko was that it took a particular interest in a batch of kittens that I had brought home. What it really keyed in one was the distinctive plaintive call of the kittens....Meeewww. Now some people feed their Tokay "pinkies" or even small mice- but I had never done that. The way the Tokay came out of hiding was almost like a predatory response to the mammalian yelps. Could this response to helpless mammal baby cries have been hardwired into the Tokay or perhaps other reptilian predators in the past? ....which inspired the pic below where I have drawn a sphenodont reptile taking off with a baby mammal. Mom has arrived, a little too late.

Illustration by author. Raiding the Cradle.

Indeed it is the persistent predatory pressure of predatory dinosaurs as well as dinosaurs monopolizing all the medium/large bodied terrestrial niches during the Mesozoic that is most often interpreted as the primary factors for mammalian suppression during that time. But how true is this notion?

While no doubt terrestrial stalking small theropods, as well as the young of large species, placed a heavy predatory toll on mammals- they were probably mostly generalist predators, like the heron, and ate whatever they could catch. Probably an equal threat to mammals during the Mesozoic were pterosaurs. Recently liberated from their ecological stereotype of "demented seagulls" the vast majority of pterosaurs are now interpreted as terrestrial predators- gleening a living from scavenging, dino hatchlings, and whatever other small fry they could overwhelm and put down their gullet. In addition to theropods and pterosaurs, various terrestrial crocs as well as evolving squamates- snakes and lizards- were all putting predatory pressure on the mammals.

So it appears that predatory dinos were only one component of a suite of predators that enjoyed mammal flesh during the Mesozoic. But what about the dinos dominance in terrestrial ecosystems? Did that thwart mammal growth?

The idea of dinosaurs being "big" is not without merit. The average dinosaur weighed in the order of several tonnes while, for the whole of the "Age of Mammals"- the Cenozoic, the average weight of a mammal is estimated at about 2-5 kg (5-10 lbs). So dinosaurs did not even particularly dominate in the size range that mammals appear to cluster at. With a few exceptions, why do we not see Mesozoic mammals thriving at the size range of say a small cat? Dinosaurs were not particularly specialized to exist at that size so why do we not see more mammals at this size that seems so optimal for them?

Typical Mesozoic Mammal 4-6" long Cronopio dentiacutus

Well for one dinosaurs were occupying that size range, just not adult dinosaurs. Hatchling and infant dinosaurs would have easily been in the 5-10 lb range and would have acted as separate ecological species from the adults at this stage. Every season a new crop of hatchlings/yearlings would arise competing for food, burrows, hiding spots etc, etc. putting a lot of stress on that particular morpho-ecospace.

Although predatory pressure and competition from hatchling dinos doubtless suppressed mammal size during the Mesozoic the major obstacle to the mammals was neither. You see, just like the US woman's soccer team, mammals faced some direct competition that had to be bested in order for the furry ones to truly prosper.

A growing picture is emerging suggesting that sphenodonts (represened today by the Tuatara) and several radiations of terrestrial crocodiles- most notably the spenosuchians and the notosuchians- were the primary competitors to mammals during the Mesozoic.

I have already talked a bit about the unexpected diversity of Mesozoic crocodiles a bit here.
What I did not dwell on is the increasing realization that many of these crocs, especially the notosuchids, were experimenting with herbivorous/omnivorous diets and  evolving heterodont dentition.

Pakasuchus kapilimai a notosuchid crocodile- NOT a mammal

What is interesting is that although several of these crocs weighed several hundred lbs, the majority were rather smallish- in the crucial 5-10 lb size range that mammals seem to covet so much. And mammals, before they could become water buffalo size would have first needed to diversify into that size range. These peculiar land crocs, some of which were the most common vertebrate in their habitat, were an impassable barrier to mammalian size diversification in many areas, especially Gondwana, during the Mesozoic.

Armadillosuchus arrudai

You may have heard of the "living fossil" the Tuatara (Sphenodon) an island endemic of New Zealand. You may even have heard it is the last survivor of a formerly widespread family of lizard like lepidosaurs- the Spenodontia. But for most of us that is where our knowledge of sphenodonts ends- an isolated island endemic that can be active in 40 degree weather, which may not in fact be typical of Mesozoic sphenodonts and an "ancient, lizard like family". Well for many of us who don't live in the tropics our idea of a lizard might be a small, fly chasing reptile that runs away when you approach it- not this:

Tuatara munching shearwater chick

Tuatara are strong biters, eating large bugs and as in the pic above, small verts- chicks and eggs. It has a peculiar acrodont dentition where the teeth are literally projections of the bone and can chew with a forward/backward motion of the jaws. Oh yeah, it also has some type of third eye, can remain active in the cold, and grow to about 2 feet and 3 lbs.

When we look at the fossil record for this family we see some interesting stuff. Priosphenodon avaiasi was the largest sphenodont at up to a meter long and about 33 lbs.

Copyright Jorge Gonzalez, Priospenodon avaiasi

Pleurosaurus was a marine sphenodont of the early Jurassic.

Pleurosaurus goldfussi

In fact the diversity of sphenodont skull shape and feeding ecology is suggestive of a wide variety of feeding niches- herbivore, omnivore, insectivore...

Hmmm, sound familar? Just like the crocs I talked about earlier, sphenodonts, were a diverse group of critters that competed directly with mammals and actively suppressed them by monopolizing niches in the pivotal 2 lb and up range.

Mammals showed incredible diversity and filled multiple niches in the Mesozoic, they were simply not large. In the famous Morrison formation mammals exceed all other vertebrae groups in species diversity. Yet they do not exceed squirrel size. Interestingly the Morisson contained a cat sized terrestrial croc, Fruitachampsa, and several sphenodonts in the 2 lb range. Where we do finally see badger sized mammals, both sphenodonts and land crocs are rare or absent- such as in Maastrichtian North America or the Jehol Biota of China.

Repenomamus

Sphenodonts had their heyday in the Triassic and Jurassic, becoming locally extinct in many areas during the Cretaceous. Land crocs held sway in the much of Gondwana until the KT event. Only then do we see mammals, having outlasted their stiff competition throughout the Mesozoic, seize their own zeitgeist resulting in the astounding diversity of forms with us today.

And some more Hope Solo just because...

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