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 6: Giant Insect Size Through Time

Ok so its been a little bit since I put up a paleo-myth since my last one here where I covered the issue of dinosaur competitive exclusion of mammals during the Mesozoic ( Paleo Myth 10, 9, and 8 btw). In it I argued that it was a trifecta of immature dinosaurs, terrestrial crocodiles, and sphenodonts putting ecological pressure on mammals. Like that post I will again be dealing with size constraint on a clade of organisms except this time it will be insects.

In the above picture we see a gargantuan mosquito filling up on Will Ferrel's blood in what I thought was a genuinely funny movie Land Before Time. While the giant mosquito was the least of the films scientific oversights the idea of giant bugs coexisting with dinosaurs is still intertwined in many people's minds. But this is a bit of an error because the hey-day of giant exists was the Carboniferous (300mya) period so dinosaurs, who arrived mid Triassic (230mya), missed guys like Meganeura by about 70 million years. In other words, dinosaurs were separated by giant insects for a span of time longer than what we are now separated from non-avian dinosaurs (65mya). So depiction like the one above, or the nightmarish arthropods of the King Kong remake below are erroneous.

So why such big bugs back in the coal era of the Carboniferous? Well the prevailing view is that higher oxygen concentration in the atmosphere (35% vs 21%) allowed insects, as well as other arthropods like arachnids and millipedes, to grow larger than today owing to all these groups having relatively inefficient repiratory schemes (spiracles, book lungs).  An insect essentially lets air "soak" into it from holes in the sides of its body. As an organisms surface area increases the volume increases at a far greater rate. So eventually an insect reaches a size where it is limited by the amount of surface area it needs to respirate its internal volume. Oxygen becomes limiting to insect size. Other arthropods, spiders for instance, are less constrained by oxygen but still run into problems from their exoskeleton breaking down in strength with increasing size. If insects or other arthropods found a way to actively respire they could at least get lobster size on land before the exoskeleton became limiting, regardless of oxygen concentration. Overall an abiotic factor, oxygen concentration, appears to be the limiting factor preventing hawk sized dragonflies from buzzing the Kardashians. Or is it?

Meganeura, crow sized dragonfly of Carboniferous

In a recent study from UCSC (go Slugs) researchers tracked the size of insect wings with oxygen concentrations throughout earth history. What they found was that insect size correlated nicely with oxygen concentration until about 150mya. They found that insect size dropped during this time period, despite a worldwide increase in oxygen levels. Why were the bugs not as big as they could be? The authors argue it was the worldwide emergence of sophisticated aerial theropods (i.e. "birds") that put predatory pressure on flying insects to select maneuverability over size (all flying organisms lose agility with increasing size).

Large flighted insects would both be easier to catch and also offer a greater caloric food reward than small ones for birds- they were optimal prey. The researches also found another dip in insect size later in the Cretaceous, possibly due to increasing sophistication of bird flight and the emergence of bats. They found no correlation between pterosaur emergence and insect size. perhaps bolstering the argument that pterosaurs were predominantly terrestrial foragers and not as well designed for aerial "hawking" as birds are.

Interestingly modern flying insects are not as large as they could be- and we can thank birds for that.

Pertinencia

Mechanisms of Maneuverability

Harison, Jon. Atmospheric oxygen level and the evolution of insect size. Proceedings of the Royal Society Biological Sciences. March 10, 2010.
http://rspb.royalsocietypublishing.org/content/early/2010/03/04/rspb.2010.0001.full

Reign of the Giant Insects Ended With the Birds

Abstract

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Eyelids of Mourning

Olympic Mens b-ball champion Russel Westbrook bringing it

So the annual "shark week" put on by the Discovery Channel is winding down this week here in the states and I have a couple of thoughts...

First of all they should just name the week "Great White Shark Week" because every show seems to be about the "man in a grey swimsuit" (surfers will understand reference). Don't get me wrong- I love the great white- but to exclude all the other species of sharks really does a dis-service to shark biology, awareness, and conservation efforts. I mean it gets a little disingenuous when Discovery keeps on giving you reels of shark attack clips and leaping macropredatory events off seal island - and then reminds you at the end that sharks are "gravely imperiled in the world's oceans". And for much of the general public the great white- because of shark week, the Jaws franchise, and its penchant for nibbling on us- has become emblematic of all sharks. But seen in light of the diversity of shark species worldwide I think it is fair to point out that the great white- Carcharodon carcharias - is actually very aberrant, specialized, and downright different from most sharks.

First of all white sharks, along with other lamnids such as makoes, are endothermic poikilotherms, that is they can maintain an elevated temperature from their surroundings but it is not constant like in mammals. Secondly they are just unusually massive and girthy - which is related to their ability to maintain heat in cold environments, perform long distance migrations, and fusiform shape. A great white the same length as a tiger shark could easily weigh twice as much as the tiger. Third of all is their behavior- white sharks do some pretty un-sharklike things like spy-hopping- generally characteristic of marine mammals but not fish.

For a person on a surfboard it would be very unnerving to see such mammalian type behavior on the part of a fish but great whites do appear to engage in this behavior to satisfy some curiosity on their part and thus display some innate awareness of a world view larger than their immediate watery realm. 

And finally those eyes, "black eyes, like a doll's eyes."- Jaws, Quint. Well it actually is a bit of a myth that the great white has total jet black eyes as the photos below attest.

He is watching you...

People who have been this close to see the jet blue iris of the great white will report it tracking them

Although sharks were once considered to have bad vision it turns out that they have quite good visual adaptations. And great whites, which can contract their pupils (no teleost fish can do this), have rods and cones, color vision, tapetum lucidum, and something approximating "sunglasses" to diminish glare, have some of the most advanced visual adaptations among sharks- again setting the great white apart from its peers. Check out this article.

"When he comes at ya, he doesn't even seem to be living...until he bites ya, and those black eys roll over white..."- Jaws, Quint.

Well it turns out great whites are a little different here too. You see while great whites do indeed roll their eyes back into their skull while biting the largest family of large predatory sharks, the carcharinoids, whom Quint was most certainly referring to, have a nictitating membrane that protects the eye.

Caribbean Reef Shark nictitating eyelid

A quick perusement of the internets reveals quite a few animals including some reptiles, birds, and even some mammals have nictitating eyelids.

Nictitating eyelid masked lapwing

Like sharks, birds of prey and herons will close their nictitating eyelids while struggling with prey.

Hmmmm... birds are derived theropods and many theropods, like sharks, had a "head first"predatory attack mode. Both sharks and birds are very visual critters that use their nictitating eyelids to protect their eyesight. Theropods were very visual critters and likewise would have needed to protect their eyes during struggles with prey and such...it begs the question- did theropods have nictitating membranes?

Well, I can't say for certain- but the fact that their living descendants have nictitating eyelids and many "headfirst predators" like sharks have them I find it extremely likely that theropods had nictitating eyelids as well.

So why don't I ever see restorations of allosaurus with a spooky eyelid covering its eye when it chomps onto its prey? I don't know- maybe this potential aspect of their behavioral anatomy has not yet been explored...whatever the case I don't think that is beyond the limits of conjecture to depict theropods with nictitating eyelids.

I don't even think it would be out of bounds to depict a T-rex rolling its eyes back into its skull like the great white does.

Cheers!!!

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