What makes a mammal?

| 59 Comments

I saw a tweet wondering about what makes an animal a mammal:

@realscientists If a tiger shark’s eggs hatch inside it and are born live (I saw a pic recently) is it a mammal or a fish? :-)
– Fiona Howie (@MummyFiFi) September 29, 2013

So, I thought I’d go through a few of the common ideas about shared physical features of mammals.

What makes a mammal? 
Is it giving live birth? Or having hair/fur? What about feeding their babies milk?

Well, kind of (I’ll tell you at the end what really does). First, let’s go through these three:


Live birth.
Not all mammals give live birth. Monotreme mammals including the echidna:


Short-beaked echidna. Photo by Fir0002/Flagstaffotos

and platypus:

Swiming Platypus. Photo by Peter Scheunis


don’t give birth to live young; They lay eggs.

Okay, so not all mammals give birth to live young.

Hair.
What about having hair/fur. All mammals have hair/fur, right?

Well, I suppose technically baby dolphins have whiskers, but you wouldn’t know it from the adults.

NMMP dolphin with locator
By U.S. Navy photo by Photographer’s Mate 1st Class Brien Aho. [Public domain], via Wikimedia Commons

Pangolins, with their armor-like plates, actually have a little bit of fur on their underside (although it’d be hard to tell):

Tree Pangolin
By Valerius Tygart (Own work) [CC-BY-SA-3.0 or GFDL]

And naked mole rats technically aren’t completely naked, they also have whiskers (but they are mostly naked):

Naked Mole Rat Eating
By Ltshears - Trisha M Shears [Public domain], via Wikimedia Commons

So, we’ll give hair a maybe. Mammals do have hair, but there are several cases where one might mistake a mammal for not having hair.

Milk.
Do all mammals feed their babies milk?

Milk Bar - geograph.org.uk - 474410
By Trish Steel [CC-BY-SA-2.0], via Wikimedia Commons

First, the snarky answer - No. Male mammals generally do not make milk (although they could).

Male-lion-new
By User:Ltshears, edited by User:julielangford [Public domain]


Second, more serious answer - Yes. I don’t know of any species of mammal that nourishes their offspring with anything other than milk. To learn more about mammals, lactation, and milk, check out the blog “Mammals Suck (… Milk)” by Dr. Hinde.

Breast feeding
Breastfeeding. By honey-bee [CC-BY-2.0]


Actually, what makes a mammal is more than just whether they give live birth (because not all do), and have hair, and lactate. And doing each of these things does not necessarily mean the animal should be classified as a mammal. (Note: Although it gives live birth, a tiger shark is not a mammal, it is a shark; sharks are a kind of fish.)

So, what does make a mammal?

Shared evolutionary history
Mammals are a group of species related by their evolutionary history. The picture below is a phylogenetic tree showing the evolutionary relationship between many different species.

All mammals share a common ancestral population.

Phylogeny
Modified from: CT Amemiya et al. Nature 496, 311-316 (2013) doi:10.1038/nature12027


The classification of “Mammals” was made based of shared physical and anatomical characteristics. But, underlying those, is a shared evolutionary history.

We do make sub-divisions within that larger grouping of mammals. For example, the egg-laying mammals, platypus and echidna are called “Proto-theria”, while all other mammals are called “Therians”. There are many other sub-classifications, but they are all still part of the broader group of mammals.

There are also larger groupings. For example, on the picture above you can see all the species highlighted in pink are called Tetrapods. These are all descended from a common ancestral population of tetrapods that are generally four-limbed vertebrates.


Although their physical characteristics may change, all species that descended from the common ancestral mammal population will all be mammals.

So, how do you tell what a mammal is? 
Well, the broad rules of thumbs still apply. If you are a Naturalist, roaming through some uncharted region, and you happen across an animal you’ve never seen, you can start with some of the general physical characteristics (e.g., is it warm-blooded? does it have fur?). But, now you can also take a look at its DNA as another line of evidence.

You can collect and sequence a sample of DNA from hair, or blood, or a toenail, or even from scat (aka poop), then compare the sequences you find with sequences that are already available to learn more about the creature you sampled. You can build a tree (like the one above) based on the similarity between the sequences. The relationship between the sequences for any one region or gene may not reflect the broader species tree, but it will give you an idea of where your species fits. And, the more DNA sequence you analyze, the better your resolution will be come (although it sometimes happens that biology is just messy).

Mammals
Mammals” is the term we use to describe the group of species that generally share a defined set of characteristics (warm blooded, lactating, give live birth, have fur/hair), AND share an evolutionarily recent (300ish million years ago) common ancestral population.

59 Comments

Which shows that an evolutionary context is necessary for a rational taxonomy. If you don’t have that basis, you will have to make many arbitrary decisions and be content with an artificial classification scheme. Just one more reason why evolution is the unifying principle of all modern biology.

DS said:

Which shows that an evolutionary context is necessary for a rational taxonomy. If you don’t have that basis, you will have to make many arbitrary decisions and be content with an artificial classification scheme. Just one more reason why evolution is the unifying principle of all modern biology.

Yes!!

It’s one of the ways in which Stephen Meyer was (deliberately) confused in ‘Darwin’s Doubt’. A current phylogenetic group might have a number of common features, but there’s no reason why they had to evolve simultaneously. Or that all of the features have to evolve in all members. Or that they have to be subsequently retained.

Meyer claims that arthropods and nematodes can’t have a common evolutionary ancestry because a modern arthropod can’t evolve into a nematode. Or vice versa. Because the ‘body plans’ are so radically different.. Ignoring the fact that the body plans in both have developed over hundreds of millions of years. And that the proto-arthropod would have looked almost identical to the proto-nematode.

In my opinion, all female mammals feed their babies milk and if they didn’t do that, why do they cal mammals and why do they have breasts? So they feed their babies with milk.

There are some organisms that have what superficially looks like hair- tarantulas for example. I believe there is also a species of cockroach where the female feeds the nymphs a whitish high protein fluid produced from glands. I heard about this years ago, but I forgot where. I just remember thinking “roach milk”

…all species that descended from the common ancestral mammal population will all be mammals.

Will this always be true, Wilson? I’m thinking of, say, another 100M years in the future: would a hypothetical far-future taxonomist still classify all descendants of mammals as mammals even if some of them have lost all of the classic features of mammals (i.e. no fur, no endothermy, no mammaries, no live birth)? We don’t classify birds as theropods. I’m wondering where the point of divergence is where we decide to give the descendants of one group their very own category?

I love this post and the cheery thread, but it’s not what a phylogeneticist would write :-).

Chris Lawson said:

…all species that descended from the common ancestral mammal population will all be mammals.

Will this always be true, Wilson? I’m thinking of, say, another 100M years in the future: would a hypothetical far-future taxonomist still classify all descendants of mammals as mammals even if some of them have lost all of the classic features of mammals (i.e. no fur, no endothermy, no mammaries, no live birth)? We don’t classify birds as theropods. I’m wondering where the point of divergence is where we decide to give the descendants of one group their very own category?

The future is now:

http://en.wikipedia.org/wiki/Devil_[…]mour_disease

RodW,

In the original post, Wilson does say that he is not aware of any mammals that don’t lactate, but his point is that there are lots of mammals missing one or two of the classic mammalian traits, so it wouldn’t necessarily mean that a hypothetical non-lactating animal would *have* to be kicked out of the mammalian clade.

Imagine a descendant of, say, bears that had fur, endothermy, a mammalian neocortex, 3 middle ear bones, and a 4-chambered heart, but whose females had vestigial mammaries that never produced milk.

Nick,

That’s an really interesting point. I guess the DFTD cancer cells can be considered a separate species that reproduces asexually, is an obligate parasite, and has no fur/hair, endothermy, neo-cortex, or middle ear bones. Should we still call it a mammal? I don’t know what the best answer should be.

I have a few comments. First, thanks for introducing me to pangolins, which I’d never heard of before.

Second, I think you could have made more out of the fact that all female mammals nourish their young with milk. This is the defining characteristic of mammals, and it ties into your main point because it was an evolved trait that every mammal still retains.

Third, in the third to last paragraph you have “it’s” when you mean “its”. “It’s” is a contraction, “its” is the third person neuter singular possessive pronoun.

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Chris Lawson speculated:

RodW,

In the original post, Wilson does say that he is not aware of any mammals that don’t lactate, but his point is that there are lots of mammals missing one or two of the classic mammalian traits, so it wouldn’t necessarily mean that a hypothetical non-lactating animal would *have* to be kicked out of the mammalian clade.

Imagine a descendant of, say, bears that had fur, endothermy, a mammalian neocortex, 3 middle ear bones, and a 4-chambered heart, but whose females had vestigial mammaries that never produced milk.

This dairy-free bear would still be considered a mammal if only because it is (assumed to be) obviously apparent that it is descended from milk-producing mammalian ancestors.

Now, if we were able to find a live “mammal-reptile” like a gorgonopsid, today, would that critter be considered a mammal, or not?

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DS said:

Which shows that an evolutionary context is necessary for a rational taxonomy. If you don’t have that basis, you will have to make many arbitrary decisions and be content with an artificial classification scheme.

As brilliant as Carl Linnaeus and a few other pre-evolutionary scientists were (Linnaeus is often considered to be the father of modern taxonomy), they still only gave us “man-made” classification systems. As DS touched on, such systems are highly arbitrary; thus, even the pioneering classification system that Linnaeus developed was of rather limited practical use.

It was only after biological evolution came along that taxonomy was finally grounded into a naturally occurring physical reality. As a result, taxonomy was no longer strictly from arbitrarily constructed human schemes.

DS said:

Just one more reason why evolution is the unifying principle of all modern biology.

This link explains how evolution finally provided an underlying physical reality to base taxonomy on. Be advised the link has a few minor gaffs (such as saying all mammals have internal gestation, overlooking egg laying mammals like the platypus) but the link still has some nice graphics and such that well drive home the point.

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Nick Matzke said:

Chris Lawson said:

…all species that descended from the common ancestral mammal population will all be mammals.

Will this always be true, Wilson? I’m thinking of, say, another 100M years in the future: would a hypothetical far-future taxonomist still classify all descendants of mammals as mammals even if some of them have lost all of the classic features of mammals (i.e. no fur, no endothermy, no mammaries, no live birth)? We don’t classify birds as theropods. I’m wondering where the point of divergence is where we decide to give the descendants of one group their very own category?

The future is now:

http://en.wikipedia.org/wiki/Devil_[…]mour_disease

Well, mammals aren’t usually regarded as fish, amphibians, or even reptiles, even if they are in those clades.

Nick Matzke said:

I love this post and the cheery thread, but it’s not what a phylogeneticist would write :-).

Not sure I know where you’re going here, Nick. Would be happy to hear more.

balloonguy said:

I have a few comments. First, thanks for introducing me to pangolins, which I’d never heard of before.

Second, I think you could have made more out of the fact that all female mammals nourish their young with milk. This is the defining characteristic of mammals, and it ties into your main point because it was an evolved trait that every mammal still retains.

Third, in the third to last paragraph you have “it’s” when you mean “its”. “It’s” is a contraction, “its” is the third person neuter singular possessive pronoun.

1. You’re welcome. 2. It is, but it could have been lost in some mammalian lineages that haven’t yet been discovered, or in the future, and they would still be part of the larger group of mammals. 3. Thanks - I’ll fix that.

RodW said:

There are some organisms that have what superficially looks like hair- tarantulas for example. I believe there is also a species of cockroach where the female feeds the nymphs a whitish high protein fluid produced from glands. I heard about this years ago, but I forgot where. I just remember thinking “roach milk”

And now I know about roach milk. Aw, they’re so adorable.

apokryltaros said:

Now, if we were able to find a live “mammal-reptile” like a gorgonopsid, today, would that critter be considered a mammal, or not?

I imagine there would be a lot of fighting over that, but you could use its DNA to get a clue. If it fell outside of all mammals, including monotremes, then it might deserve its own super-group. If it fell within mammals, I would say it is a mammal.

Chris Lawson said:

…all species that descended from the common ancestral mammal population will all be mammals.

Will this always be true, Wilson? I’m thinking of, say, another 100M years in the future: would a hypothetical far-future taxonomist still classify all descendants of mammals as mammals even if some of them have lost all of the classic features of mammals (i.e. no fur, no endothermy, no mammaries, no live birth)? We don’t classify birds as theropods. I’m wondering where the point of divergence is where we decide to give the descendants of one group their very own category?

I’m not sure I want to get into the taxonomy debates, but I would think that the larger “Mammalia” would still exist, but that new sub-orders would evolve. I suppose a good way to estimate how this might happen would be to learn more about classification of ancient samples from 100 million years ago or more. I don’t know much about this, but sure other people on PT can chime in.

phylogenetically, sharks are much more different from bony fish than an echidna is from you. Bony fish split off from sharks hundreds of millions of years ago.

we really shouldn’t be calling sharks fish any more (or calling hagfish “fish” either for that matter), but how does one go about changing such ancient common classification?

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Uh, guys, out of all of the sites on the entire internet that I’d expect to know that the defining character of the Class Mammalia is just those three inner ear bones derived from bits of the jaw, it’s this one. None of that other stuff (hair, milk, heart shape, etc) fossilizes, making it useless for 99% of all of the mammalian species who have ever lived. We know that Smilodon was a mammal not because today we have furry lions, but because it has that same inner ear structure as all the rest of the clade.

Uh, guys, out of all of the sites on the entire internet that I’d expect to know that the defining character of the Class Mammalia is just those three inner ear bones derived from bits of the jaw, it’s this one. None of that other stuff (hair, milk, heart shape, etc) fossilizes, making it useless for 99% of all of the mammalian species who have ever lived. We know that Smilodon was a mammal not because today we have furry lions, but because it has that same inner ear structure as all the rest of the clade.

I think you are right. There are no fossils of mammary glands and few of hair, but there are other ways to link living species of mammals to extinct ones. The synapsid opening in the skull and 3 middle ear bones are examples.

Andy White said:

In my opinion, all female mammals feed their babies milk and if they didn’t do that, why do they cal mammals and why do they have breasts? So they feed their babies with milk.

Not all mammals have breasts. Monotremes don’t - they secret milk through the skin. Marsupials have nipples but I think they’re in their pouches.

balloonguy said:

Andy White said:

In my opinion, all female mammals feed their babies milk and if they didn’t do that, why do they cal mammals and why do they have breasts? So they feed their babies with milk.

Not all mammals have breasts. Monotremes don’t - they secret milk through the skin. Marsupials have nipples but I think they’re in their pouches.

I didn’t know that. t was interesting. Thanks for your notification.

Chris Lawson said:

In the original post, Wilson does say that he is not aware of any mammals that don’t lactate, but his point is that there are lots of mammals missing one or two of the classic mammalian traits, so it wouldn’t necessarily mean that a hypothetical non-lactating animal would *have* to be kicked out of the mammalian clade.

Imagine a descendant of, say, bears that had fur, endothermy, a mammalian neocortex, 3 middle ear bones, and a 4-chambered heart, but whose females had vestigial mammaries that never produced milk.

Of course. But this thread started with the assumption that we pretty much know what mammals are and the question is what should we exclude. I think its interesting to consider a situation in which we dont really know what constitutes a mammal and so we consider other organisms that have some of the ‘mammal traits’

It seems to me that if some highly derived mammal that was missing most of these traits became the base of a huge radiation, its descendents would not be considered mammals in common parlance 100mya from now, for the same reason birds arent considered reptiles. I think if we discovered an underived therapsid on some tropical island it would be sufficiently different that we wouldnt call it a mammal,

perfusionman said:

Uh, guys, out of all of the sites on the entire internet that I’d expect to know that the defining character of the Class Mammalia is just those three inner ear bones derived from bits of the jaw, it’s this one. None of that other stuff (hair, milk, heart shape, etc) fossilizes, making it useless for 99% of all of the mammalian species who have ever lived. We know that Smilodon was a mammal not because today we have furry lions, but because it has that same inner ear structure as all the rest of the clade.

My point was to go through what is stereotypically thought to represent a mammal (because it is widespread, as the question about sharks shows), and then explain that it doesn’t matter if a species has these features or has lost them. The same is true for the three inner ear bones. If any living mammal were identified today that clearly fell within mammalian diversity, but had lost one of those inner ear bones, it would still be classified as a mammal. I wasn’t discussing all of the features used to classify extinct mammals, although that would be a pretty cool post.

RodW said:

It seems to me that if some highly derived mammal that was missing most of these traits became the base of a huge radiation, its descendents would not be considered mammals in common parlance 100mya from now, for the same reason birds arent considered reptiles. I think if we discovered an underived therapsid on some tropical island it would be sufficiently different that we wouldnt call it a mammal,

I agree. We would likely come up with a new classification for such a species.

RodW said:

There are some organisms that have what superficially looks like hair- tarantulas for example. I believe there is also a species of cockroach where the female feeds the nymphs a whitish high protein fluid produced from glands. I heard about this years ago, but I forgot where. I just remember thinking “roach milk”

Tsetse flies produce a whitish nutritious fluid to feed the larvae before they are laid. Yep, live-born, and final instar if I remember correctly. Very odd flies, both sexes as adults feed exclusively on blood.

^secrete, not secret, in my last post

RodW said:

Chris Lawson said:

In the original post, Wilson does say that he is not aware of any mammals that don’t lactate, but his point is that there are lots of mammals missing one or two of the classic mammalian traits, so it wouldn’t necessarily mean that a hypothetical non-lactating animal would *have* to be kicked out of the mammalian clade.

Imagine a descendant of, say, bears that had fur, endothermy, a mammalian neocortex, 3 middle ear bones, and a 4-chambered heart, but whose females had vestigial mammaries that never produced milk.

Of course. But this thread started with the assumption that we pretty much know what mammals are and the question is what should we exclude. I think its interesting to consider a situation in which we dont really know what constitutes a mammal and so we consider other organisms that have some of the ‘mammal traits’

It seems to me that if some highly derived mammal that was missing most of these traits became the base of a huge radiation, its descendents would not be considered mammals in common parlance 100mya from now, for the same reason birds arent considered reptiles. I think if we discovered an underived therapsid on some tropical island it would be sufficiently different that we wouldnt call it a mammal,

Yes, I agree. We would most likely give them a new name. But phylogenetically that would be incorrect in exactly the same way that it is incorrect not to classify birds and mammals as reptiles.

perfusionman said: We know that Smilodon was a mammal not because today we have furry lions, but because it has that same inner ear structure as all the rest of the clade.

Nope. We know it’s a mammal because it’s phylogenetically nested within Mammalia, and we know that not because of its ear bones, which few people ever look at and which are often unpreserved, but because it shares derived characters with other cats, and we know that cats are mammals. The whole ear bone thing is an arbitrary distinction useful for diagnosing some fossils, but it’s no more essential than 7 cervical vertebrae or deciduous teeth.

apokryltaros said:

Now, if we were able to find a live “mammal-reptile” like a gorgonopsid, today, would that critter be considered a mammal, or not?

It would be if we chose to define “mammal” that way, and wouldn’t if we didn’t. Labels are arbitrarily attached to clades. These days we like to use crown group definitions, and gorgonopsids are not mammals because they’re outside the synapsid crown group. If there were living gorgonopsids, they would be inside the synapsid crown group, but would we call that group “mammals”? Unclear.

M. Wilson Sayres said:

And now I know about roach milk. Aw, they’re so adorable.

Don’t forget pigeon milk. And there are various sorts of fish milk too.

Chris Lawson said:

Will this always be true, Wilson? I’m thinking of, say, another 100M years in the future: would a hypothetical far-future taxonomist still classify all descendants of mammals as mammals even if some of them have lost all of the classic features of mammals (i.e. no fur, no endothermy, no mammaries, no live birth)? We don’t classify birds as theropods.

Yes we do classify birds as theropods. You’re a bit behind the times. The rule of cladistics is “once a duck, always a duck”. Descendants of a clade never leave that clade, no matter how much they change. Snakes are still tetrapods, even though they lack the defining tetrapod feature of legs. Mammals are still amniotes, even though most of them no longer lay eggs (OK, so they still have amnions; no shells, though). The only feature that never gets lost is ancestry, and that’s what we go by.

The three middle-ear-bones definition isn’t perfect either, as there are a variety of early mammal-ish fossils that almost or barely have this character, and it might have evolved independently a few times amongst these closely related almost-mammals. This is why there was a scientific debate, sometimes dug up by creationists, about whether or not “mammals” evolved twice.

And, anyway, picking that character as the dividing line is almost as arbitrary as picking any other. It may or may not correlate with other characters, such as mammary glands and hair, which might have evolved at other times.

This sort of thing is why node-based definitions of taxa are becoming the most popular amongst taxonomists. Nodes define the groups, and characters (as many as possible, in a phylogenetic analysis) are used to diagnose whether or not a species of interest falls into a group of interest.

Melissa wasn’t trying to dig into all of this, but for people who want a starter, here’s an intro:

http://en.wikipedia.org/wiki/Phylog[…]nomenclature

John,

Thanks for clearing that up. From a phylogenetic point of view, it makes sense to classify everything from the root downwards. I had never heard of birds being called theropods, but I’m not in the field. Without meaning to get into all the no doubt innumerable complicated arguments in taxonomy, I imagine this system of classification still has controversies over what to do about endosymbiosis. (I guess this makes humans eukaryotes *and* prokaryotes *and* proteobacteria.)

In the small and recent scale, things seem reasonably easy to work out:

primates > hominoids > hominids > hominines > hominins > humans

On the larger scale I notice most phylogenetic trees for humans on the web only go back as far as eukaryotes, which is I guess where things get hard. Conversely, on the very small recent scale, say the last 5M years, there also seems to be a lot of controversy about where to put various Australopitheci and Homos on the branch.

perfusionman said:

Uh, guys, out of all of the sites on the entire internet that I’d expect to know that the defining character of the Class Mammalia is just those three inner ear bones derived from bits of the jaw, it’s this one. None of that other stuff (hair, milk, heart shape, etc) fossilizes, making it useless for 99% of all of the mammalian species who have ever lived. We know that Smilodon was a mammal not because today we have furry lions, but because it has that same inner ear structure as all the rest of the clade.

There are a few mammal fossils that do show hair, like the Mesozoic Eomaia, Volaticotherium, Casturocauda, and Late Paleocene coprolites containing hair of Lambdopsalis.

Tom said:

phylogenetically, sharks are much more different from bony fish than an echidna is from you. Bony fish split off from sharks hundreds of millions of years ago.

With the discovery and study of the Silurian placoderm, Entelognathus, which has jaws anatomically similar to primitive bony fish, bony fish, including the ancestors of lungfish and tetrapods, branched off from the arthrodire+ptyctodontid branch of Placodermi during the early Silurian. Whole body fossils of the early Devonian shark Ptomacanthus, which was originally described as an acanthodian, or “spiny shark,” suggest that true sharks/cartilaginous fish diverged from the acanthodians during the start of the Devonian, well after bony fish diverged from the placoderms.

we really shouldn’t be calling sharks fish any more (or calling hagfish “fish” either for that matter), but how does one go about changing such ancient common classification?

The term “fish” is an imprecise term that technically encompasses hagfish and all non-tetrapod craniate vertebrates (i.e., every aquatic vertebrate that breaths through gills from lampreys to sharks), and even a few amphibians and some invertebrates (i.e., “congo eels,” and “silverfish”).

Having said that, it’s probably easier to catch the moon in a bucket than trying to convince laypeople, or even other scientists to stop calling sharks “fish.” Hell, it’s a painfully uphill battle to even convince people to stop using the terms “jellyfish” and “starfish.” People are very resistant to changing their vocabulary over what they perceive as a minor quibble that requires a lot of hard to understand explanation.

apokryltaros said:

Tom said:

phylogenetically, sharks are much more different from bony fish than an echidna is from you. Bony fish split off from sharks hundreds of millions of years ago.

With the discovery and study of the Silurian placoderm, Entelognathus, which has jaws anatomically similar to primitive bony fish, bony fish, including the ancestors of lungfish and tetrapods, branched off from the arthrodire+ptyctodontid branch of Placodermi during the early Silurian. Whole body fossils of the early Devonian shark Ptomacanthus, which was originally described as an acanthodian, or “spiny shark,” suggest that true sharks/cartilaginous fish diverged from the acanthodians during the start of the Devonian, well after bony fish diverged from the placoderms.

we really shouldn’t be calling sharks fish any more (or calling hagfish “fish” either for that matter), but how does one go about changing such ancient common classification?

The term “fish” is an imprecise term that technically encompasses hagfish and all non-tetrapod craniate vertebrates (i.e., every aquatic vertebrate that breaths through gills from lampreys to sharks), and even a few amphibians and some invertebrates (i.e., “congo eels,” and “silverfish”).

Having said that, it’s probably easier to catch the moon in a bucket than trying to convince laypeople, or even other scientists to stop calling sharks “fish.” Hell, it’s a painfully uphill battle to even convince people to stop using the terms “jellyfish” and “starfish.” People are very resistant to changing their vocabulary over what they perceive as a minor quibble that requires a lot of hard to understand explanation.

Agreed. But we shouldn’t just let them call whales and dolphins “fish”. You gotta draw the line somewhere.

There are a few mammal fossils that do show hair, like the Mesozoic Eomaia, Volaticotherium, Casturocauda, and Late Paleocene coprolites containing hair of Lambdopsalis.

You can see mammoth hair at the American Museum of Natural History

Karen S. said:

There are a few mammal fossils that do show hair, like the Mesozoic Eomaia, Volaticotherium, Casturocauda, and Late Paleocene coprolites containing hair of Lambdopsalis.

You can see mammoth hair at the American Museum of Natural History

Come on, is it really that big?

DS said:

Agreed. But we shouldn’t just let them call whales and dolphins “fish”. You gotta draw the line somewhere.

In this case, the reason why people don’t think of whales and dolphins as “fish” is because they don’t have gills/don’t breath water. That Aristotle determined them to be mammals helped a lot, too.

Chris Lawson said:

John,

Thanks for clearing that up. From a phylogenetic point of view, it makes sense to classify everything from the root downwards. I had never heard of birds being called theropods, but I’m not in the field. Without meaning to get into all the no doubt innumerable complicated arguments in taxonomy, I imagine this system of classification still has controversies over what to do about endosymbiosis. (I guess this makes humans eukaryotes *and* prokaryotes *and* proteobacteria.)

“Prokaryote” isn’t currently a taxonomic term, as it doesn’t describe a clade. But yeah, cladistic classification – any classification, really – has a bit of a problem with reticulation, as in endosymbiosis and hybrid speciation. I solve the problem by trying not to think about it, which works fine.

I suspect that most people use the words “fish”, “reptile”, etc., to mean something having some set of traits (or some subset of that set), rather than the clade that includes all descendants of the first member species of that group.

Henry

Not sure about the Echidna but I have read (god knows where), that the milk of the Platypus actually oozes through large pores in the skin. This is possibly a modified sweat? Concentrate the screting glands, expand and localise the pores (nipple), hey presto, breast!

Tom said:

phylogenetically, sharks are much more different from bony fish than an echidna is from you. Bony fish split off from sharks hundreds of millions of years ago.

we really shouldn’t be calling sharks fish any more (or calling hagfish “fish” either for that matter), but how does one go about changing such ancient common classification?

With no expertise whatsoever on the subject, I have accepted that “sharks are fish” but I’ve never been comfortable with that classification, there’s something about them that I find rather un-fishy.

apokryltaros said: Having said that, it’s probably easier to catch the moon in a bucket than trying to convince laypeople, or even other scientists to stop calling sharks “fish.”

I recognize that the old Class Pisces has been abandoned for good reason. But this is the first that I have heard of the suggestion that one might exclude sharks from being fishes. I presume that this means that “fish” is to be identified with Osteichthyes, the “bony fishes”? I have heard it said that if there is to be a genuine clade of fishes, one which includes both sharks and tunas, for example, then it would also include giraffes and ostriches (as well as whales and penguins).

I’m not arguing with the idea that “sharks are not fishes”, it’s just a new way of dealing with the taxonomy that I hadn’t heard of before.

TomS said: I presume that this means that “fish” is to be identified with Osteichthyes, the “bony fishes”? I have heard it said that if there is to be a genuine clade of fishes, one which includes both sharks and tunas, for example, then it would also include giraffes and ostriches (as well as whales and penguins).

You do understand that Osteichthyes already does include giraffes and ostriches, right? It isn’t sharks that make this necessary. It’s lungfish and coelacanths (and a host of extinct groups). Sharks join in the group Gnathostomata.

Just yesterday, Discover magazine came out wih an article on another trait all mammals evidently share. I don’t vouch for the research’s accuracy, but I will tentatively vouch for its amusement value…

John Harshman said:

Chris Lawson said:

John,

Thanks for clearing that up. From a phylogenetic point of view, it makes sense to classify everything from the root downwards. I had never heard of birds being called theropods, but I’m not in the field. Without meaning to get into all the no doubt innumerable complicated arguments in taxonomy, I imagine this system of classification still has controversies over what to do about endosymbiosis. (I guess this makes humans eukaryotes *and* prokaryotes *and* proteobacteria.)

“Prokaryote” isn’t currently a taxonomic term, as it doesn’t describe a clade. But yeah, cladistic classification – any classification, really – has a bit of a problem with reticulation, as in endosymbiosis and hybrid speciation. I solve the problem by trying not to think about it, which works fine.

Originally, “prokaryote” used to be synonymous for “bacterium,” and described how bacteria differed from eukaryotes in that they had yet to evolve a nuclear envelope for their genomic DNA (or evolved membranes for any organelles in the first place). However, “prokaryote” eventually evolved into an imprecise term like the word “fish” when scientists discovered archaeans (aka “archaeobacteria”), and, appreciated how different archaeans are from (eu)bacteria. {/pedantry}

What makes a mammal?

A female mammal, a male mammal, and some romantic music.*

*(Or whatever it is that members of the particular species happen to regard as romantic.)

A female mammal, a male mammal, and some romantic music.*

*(Or whatever it is that members of the particular species happen to regard as romantic.)

Most likely that would be the right smell. Not to mention beating the crap out of the competition.

Karen S. said:

There are a few mammal fossils that do show hair, like the Mesozoic Eomaia, Volaticotherium, Casturocauda, and Late Paleocene coprolites containing hair of Lambdopsalis.

You can see mammoth hair at the American Museum of Natural History

You can buy it from web vendors at times. I saw a small sample for sale for around $75. I just looked it up–at least one sample is for sale on Ebay now (good luck knowing if it’s real, though).

Glen Davidson

Thnaks You Very İnteresting

Another defining characteristic of mammals is a single lower dentary bone. All mammals possess this.

I consider monotremes to be mammalian heretics.

Or evolutionary deviants, having deviated from the proper path lo these many millions of years.*

Same thing, really.

Glen Davidson

*Well, they really are the long-diverged “exceptions” that indicate how taxonomic categories are fairly arbitrary for evolving organisms, apart from extinctions.

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This page contains a single entry by M. Wilson Sayres published on October 16, 2013 1:37 PM.

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