I RECENTLY READ a paleontology book that irked me by referring to the basal synapsids as “mammal-like reptiles”. This term pre-dates modern phylogenetics. It violates several rules of phylogenetics, the first being that all clades (groups of related organisms) should contain organisms sharing a single common ancestor in that clade, and the second being that clades should nest within each other, so that each more recent clade fits into its ancestors’ clades as well. In other words, clades should not be polyphyletic, but monophyletic. An example of a polyphyletic group is the pachydermata, an obsolete category containing elephants, hippopotami, rhinoceri, horses, tapirs, and pigs simply on the basis that these organisms have thick skin and hooves. Now these animals are placed into separate clades since they do not share a recent common ancestor. Polyphyletic groups are a great embarrassment to phylogenicists.
Before modern phylogenetics it was often said that amphibians evolved from fish, then the reptiles evolved from amphibians, with the mammal-like reptiles evolving from the reptiles, and the mammals from the mammal-like reptiles. In modern phylogenetics this picture is painfully misleading. In actuality the mammals and the reptiles come from two distinct evolutionary branches, and neither descended from the other–not to mention that also neither group descended from the amphibians. Yet this misrepresentation lives on.
The old model is based on sloppy definitions. A tetrapod lives chiefly in water? It’s an amphibian. A tetrapod lives on land and is not a mammal? It’s a reptile. A tetrapod has some early mammalian characters? It’s a mammal-like reptile. Again, we return to my earlier complaint about the sloppy use of the term “worm”–when you use a term so indiscriminately, it ultimately provides no useful information at all. Since evolutionary taxonomy is about discerning relationships, and sloppy terminology obscures this information, it’s important to have classifications nailed down hard.
The drawback of this is that it requires specific names for extinct organisms that do not fall into the modern groups (Amphibia, Reptilia, Mammalia, etc.) Laymen are familiar with these groups, so it seems easier to stick extinct animals into those groups when talking about evolutionary history–who wants to hear about temnospondyls, synapsids, amniotes, and cynodonts? But by misclassifying extinct groups like this, we only succeed in spreading misinformation and confusion. How is it possible that fur-bearing, milk-producing mammals could evolve from reptiles with their hard scaly skins? It’s not. But it is possible for furry skin and milk glands to evolve from a bare, glandular skin possessed by the synapsids.
So the true picture, briefly given: The early tetrapods evolved from fish and made their way on to the land. A split occurred that produced the amphibians (modern amphibians and a variety of extinct forms) along one branch, while the other led to the amniotes, organisms that produce several characteristic membranes during embryonic development, one of them the amnion. The early amniotes split into two branches, the synapsids (based upon characteristic skull features) and the sauropsids. The sauropsids include all modern reptiles and their descendants and a variety of extinct reptiles (the sauropsids contain Reptilia). The synapsids, on the other route, passed through the early therapsids, through the mammaliaforms, and eventually produced the mammals.
Also note that before the divergence of Amphibia there were multiple amphibious groups that have been labeled “amphibians”. The group Amphibia has also recently been called Lissamphibia to distinguish these organisms from their ancestors. While this works, it has the drawback that most people don’t know what a lissamphibian is. For this reason I think we should refer to the non-lissamphibian amphibians as “amphibious” or “amphibian-like”, meaning they lived an amphibious lifestyle while not being grouped among the amphibians themselves.
Because of the nesting of the phylogenetic tree, you can trace back the human descent through ancestral clades. So we are mammals, mammaliformes, therapsids, synapsids, amniotes, tetrapods. . . Things get a little strange once we reach the fish, since Linnaean taxonomy was invented first and these names carried directly into phylogenetics. It ends up we might technically be sarcopterygians, or lobe-finned fish, and teleosts, or bony fish. But before this we fall neatly into the vertebrates, chordates, bilaterians, and down on the line. We are definitely not mammal-like reptiles!
While most groups currently recognized are monophyletic, Reptilia is a paraphyletic group. Above I said that the reptiles are contained within sauropsids, but there is one group of sauropsids that are not classified as reptiles–the birds. The birds descended from theropod dinosaurs or a close relative of these dinosaurs, so actually birds are technically dinosaurs! But when taxonomists were developing Linnaean taxonomy, the modern reptiles obviously fit into Reptilia together, but birds have so many different features that they were grouped on their own in Aves. Now that we know more about extinct transitional organisms there are paleontologists who have argued about whether some dinosaurs should be placed in Aves!
Keep in mind that these routes of descent are incredibly simplified. Consider that 75% of the types of mammals that have ever lived are now extinct, include multiple major groups of mammals as distinct as the surviving mammals, the eutherians (placental mammals), monotremes, and marsupials. There are many different extinct clades. You can get an idea of some of the diversity of extinct life by scanning through the Tree of Life web project. This website has the ambitious goal of compiling accurate phylogenies for as many living organisms as possible, and will continually be updated as our knowledge advances.
[Edited to correct some errors pointed out by David Marjanović. Thank you!]
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February 26, 2008 at 7:49 pm
A Decimated Lineage « Nimravid’s Weblog
[…] Evolution, extinction, Jurassic, lactation, mammal, mammaliaform, science I MENTIONED in a recent post that 75% of all of the types of mammals that have ever lived are now extinct. We have a tendency to […]
February 27, 2008 at 7:51 pm
Christopher Taylor
Originally, the term “worm” was any animal with an elongated body shape and no limbs. Snakes are worms. This is why dragons, which in European tradition were generally imagined as giant snakes before the wings and the fire got added, are often referred to as worms (or wyrms, which is just a variant spelling). It’s also why there is a legless lizard known as a slow-worm.
February 27, 2008 at 7:53 pm
Christopher Taylor
Oops, don’t know how that happened – that comment was supposed to go on the post about the term “worm”.
February 27, 2008 at 9:46 pm
Carl Buell
I feel your pain! It’s hard enough to get most people past “evolution is just a theory”; good luck with the whole anapsid/synapsid/diapsid thingie! I’ve tried using the proper terms for years and what happens is… “huh?” … followed by long explanations. You’re perfectly correct, but “mammal-like reptile” is ingrained in our culture and at least acknowledges evolution. As you note, the diversity of life on our planet both present and (especially) past is beyond easy description. Evolution is NOT a soundbyte.
That said… Keep trying… I’ll help what little I can.
February 27, 2008 at 10:22 pm
Nimravid
“Oops, don’t know how that happened – that comment was supposed to go on the post about the term “worm”.”
That seems to happen here, I’m not sure why.
You’re right about the historic usage of the word. Maybe we could require everyone to insert the phrase “in a taxonomically meaningless way” whenever they call something a worm?
February 27, 2008 at 10:24 pm
Nimravid
“I’ve tried using the proper terms for years and what happens is… “huh?” … followed by long explanations.”
This must be surmountable! Dinosaurs are terribly popular and little kids spout off multisyllabic dinosaur names, so I’m sure we can manage at least introducing “synapsid”. That would go a long way. It just requires the proper publicity campaign. Beany-baby gorgonopsids, perhaps?
February 28, 2008 at 9:49 pm
mythusmage
For my part I’ve gone with the following;
Amphibian-like Amphibians -> Amphibian
Amphibian-like Reptile -> Reptile-like Reptile -> Reptile
Amphibian-like Mammal -> Reptile-like Mammal -> Mammal-like Mammal -> Mammal
Yes, it is, in part, snark. :)
Note though that Seymouria, once considered the first reptile and now counted among the earliest amniotes, is also a synapsid, and thus an Amphibian-like Mammal -> Reptile-like Mammal transitional form.
February 28, 2008 at 11:31 pm
Christopher Taylor
Erm, no, Seymouria is not a synapsid. It’s not even an amniote. Most authors, however, have regarded Seymouria as more closely related to amniotes than to modern amphibians, which would make it a stem-amniote.
March 4, 2008 at 6:30 pm
David Marjanović
“Clade” is not newspeak for “taxon”. If it’s polyphyletic, it’s not a clade — by definition. “Clade” and “monophylum” are synonyms; the definition of “clade” is “an ancestor and all its descendants”.
No, Lissamphibia is the crown-group, while the name Amphibia is usually used for the total group ( = everything more closely related to Lissamphibia than to Amniota; this includes Lissamphibia itself, obviously). Amphibious lifestyle doesn’t enter the question at all.
What do you mean by “types”?
Here you want to say “clades”. A classification is either a hierarchical list of taxa, and classification is the act of making such a list.
…and as the professionals who are supposed to contribute find time! That’s the limiting factor.
—————–
Seymouria probably isn’t even a crown-group tetrapod ( = amniotes and lissamphibians are more closely related to each other than to Seymouriamorpha).
March 5, 2008 at 9:28 am
Nimravid
Thanks for the advice! I will neaten up my terminology in future.
May 9, 2008 at 11:15 am
Who are you calling ‘primitive’? « Nimravid’s Weblog
[…] If we say marsupials have reptilian traits, then they must have descended from reptiles. As I have mentioned before, mammals are vehemently not descended from reptiles. Reptiles (and birds, who often get left out) […]
November 12, 2013 at 10:50 am
EVOLVER
It has LOOOOOOOOOOOOOOOOOOOOOOOOOOOONG been known that mammals are NOT descended from reptiles. As far back as 1911. Four-color vision, someone? But the new age lunatic fringe and their pop psych gurus would be very skeptical and suspect you of the worst conspiratorial tendencies if you said that there is no such thing as a “reptilian brain”, cause, eh, guess what! we’re not descended from reptiles, Einstein.