Paleo-Myth Number 5: Cenozoic: The Age of Mammals?

Speak of the Cenozoic (New Life) and you implicitly speak of what is commonly referred to as "the Age of Mammals". Although mammals were around during the Mesozoic and were very diverse in small size guilds- its only in the Mesozoic do we see them expand into large and dominant carnivores, herbivores, and omnivores. We also see expansion into the marine realm and a certain aberrant, bipedal ape became especially dominant and widespread during the Holocene. Certainly when one looks across the breadth of mammalian diversity one is astounded by the myriad forms: from shrew to bat to giraffe to beaked whale they are all mammals.

And this post is not discounting the obvious success mammals have sustained during this time period. But what it is addressing is how this notion- how the Cenozoic became synonymous with "The Age of Mammals"- has distorted our view of the relative ecological importance of other vertebrates during this time period- namely birds.

Now some of you reading this might be well aware that the Cenozoic always represented a significant expansion of Aves and especially Neognathene passerines- this post and the Paleo-Myth series is not necessarily aimed at you but at those perhaps less conscious of the importance of birds in the Cenozoic.

Chances are wherever you are at on the planet right now as you read this, if you step outside and just start counting species you will see more of and greater diversity of birds than mammals.

Consider this:

Most estimates place the number of extant bird species at about 10,000. Mammals? roughly half the number of species as birds at about 5,500 species. And of the mammal species alive the vast majority are rodents, bats, and shrews (red, blue, and yellow in the graph below).

So basically about 75% of all mammals can be characterized as being small, primarily nocturnal, and insectivorous/omnivorous- they basically have not changed a bunch from the Mesozoic. And if you look at the largest order Rodentia it comprises a little less than 2,500 species- or roughly half the amount of species that compromise Passerines at roughly 5,000. This comparison is especially telling as rodents and passerines do share a lot in terms of ecological overlap. And this diversity among birds is not slowing down, if anything it is picking up!

Looking at species diversity offers us one perspective on the bird vs mammal question. It should be noted that the peculiar adaptations and specializations of many birds might have a way of inflating bird species counts compared to mammals. Also because birds can be quite mobile they are able to fly off to different areas and speciate- unlike rodents. But the sheer number of bird species, compared to mammals, can not be ignored.

But lets look at biomass, as this may offer another perspective on the issue.

Tsavo. (c) Antero Topp. The most abundant bird, red-billed Quelea, vs largest land mammal, African Elephant

Now biomass estimates are tenuous at best and it is hard to estimate the population of even people- but lets look at the most common bird in the world- Africas Red-Billed Quelea (Quelea quelea). Estimates for the population range from 3 billion to up to 10 billion- pretty respectable numbers. But here the mammals probably have the birds beat. Most experts believe the brown rat (Rattus norvegicus) to be the only land based vertebrate to come close to humans in terms of biomass. Some estimate New York City alone to have a population of rats at up to 100 million! Ok so mammals hold the #1 and #2 spot for terrestrial vertebrate biomass- who come in at #3? a bird, the common chicken (Gallus gallus domesticus).

Another point to be made with the rodent question is that birds of prey- owls, falcons, hawks- and also birds like corvids, herons, gulls, and shrikes are the most important predatory control on exponential rodent growth in many habitats. And, unlike mammal predators, can converge from long distance to exploit areas with rodent plagues. They thus serve as a last line of defense for rodent scourges. Another point for the birds.

Another avenue to explore in regards to the bird/mammal debate is extent of ecological niches. Both extant and extinct forms of birds show that they have made significant ecological inroads into niches of large terrestrial herbivore and carnivore traditionally occupied by mammals. Examples include modern flightless birds (ostriches, rheas etc) and in the past phorusrhachids, moas, and elephant birds. But mammals have not yet exploited many niches that are the exclusive domain of birds. There are no soaring mammals that scavenge carcasses (vultures, condors etc). There are no mammals that exploit shores/intertidal zones to the extent the myriad waders, shorebirds etc do. There are no filter feeding freshwater or brackish water mammals like ducks, flamingoes, anseriformes etc. Birds show a much greater diversity in their insectivorous forms than mammals. Although bats are important pollinators, seed dispersal agents in some areas they are less successful/diverse than the myriad birds that perform these same functions. And there are no bats that exploit marine resources, unlike the myriad diversity of marine birds.

In short birds can often do the same tasks that mammals do when given a chance- but mammals do not seem to succeed in the areas where birds do exceptionally well.

The Cenozoic: The Age of Mammals and Birds

(c) Flip Nicklin

Pertinencia

Along with Humans, Who Else is in the 7 Billion Club?

Global Diversity of Birds in Space and Time

Influence of Predation on Rodent Population

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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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