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Final

study notes part 2

11 Pages
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Department
Biology
Course Code
BIO356H5
Professor
Michaelde Braga

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Origin of Amniota, the amniote egg
-Thing that you should focus on so far: Diadectomorphs are the sister-taxon to amniotes.
Whether or not they had more of an amniote character trait than were aware of is still
questionable. The one thing that would definitely prove them to be amniotes is the
possession of an amniotic egg or cleidoic egg and thats the one thing we cant prove. It
would be fantastic if we could find eggs associated w/ some remains of Diadectomorphs
but its unlikely b/c its very likely that if they did lay eggs, they would have just left them
there. They didnt have any parental care (most reptiles dont). In crocodiles, dinosaurs,
birds: yes but other examples, reptiles just lay their eggs and thats it. So even if we do
find eggs, we may not know that they belong to Diadectomorphs at all. What would be
really fantastic is if we find an egg w/ an embryo in it so the Cleidoic egg is the key
- Amniote radiation: Amphibians are common up until the Permian-Triassic boundary then
amphibians really die off rapidly. And it seems that a lot of reptile groups do as well.
There was certainly a major extinction event that took place at this point. Amphibians
survived it and continued w/ more or less similar diversity all the way through. Reptiles
becomes more diverse as we go through the Mesozoic and then were hit very hard at the
end of the Mesozoic (birds were believed to have come off it). Mammals or the synapsids
that led to mammals arrived somewhere in the Permian, even earlier than that such as the
carboniferous synapsids, Dimetredon.
W/in this, somewhere here (we probably can trace it all the way back to the Carboniferous), the
amniotic egg must have evolved. Whether that amniotic egg was already present in the sister
taxon to amniotes or if they evolved w/in the group is a big question. One of the things that we
have recognized is that the earliest reptiles (that we know in terms of osteological similarities to
extant reptiles), is that they were small. The Limnoscelis and Diadectes were large, Seymouria
wasnt that small either... so they were all very big. But the earliest reptiles or true reptiles were
quite small. Some issue around size seemed to have permitted the reptile radiation to eventually
take place. But why size is so significant is problematic.
1.)Early Amniotes:
-So we know they were small. The first taxa is Hylonomus.
www.notesolution.com
-The character traits that diagnose the amniotic clade have a lot of uncertainty and many
different interpretations. Have 4 or 5 different cladograms showing the possible
relationships of amniotes and of the taxa nearest amniotes. Some amniote character traits
better than others:
Transverse flange of the pterygoid bearing a single row of teeth: that transverse
flange- that modified region of the palate associated w/ the pterygoid which is a
paired and large element in the palate - seems to be a key to the evolution of the
amniotes. There are no amphibians w/ this transverse flange. Seymoriamorphs
appear to have this flange but they dont really have a prominent row of teeth on
it- they have these tiny little denticles all the way through on their palate – a lot of
amphibians have those. Its likely an aid to holding on to objects they trap in their
mouths but no real dentition to pierce or kill what theyve captured. So when do
the teeth and the transverse flange become a character trait that you can separate
amniotes from non-amniotes is the question.
The transverse flange of pterygoid for some authors (like Benton) doesnt even use
it as a character for amniotes b/c hes so unconvinced that it is. He argues that
Diadectes has a transverse flangeyes but it has a lot of denticles on it.
Limnoscelis has a transverse flange and it appears that Limnoscelis has a row of
prominent teeth (even though it still has denticles). That is supposed to be a reptile
trait so is Limnoscelis an amniote and Diadectes not? Then you have a problem
b/c diadectomorph is no longer a monophyletic group, its now paraphyletic. You
can see the confusion here. Therefore, Limnoscelis is a very important creature.
Frontal contacts the orbit: this is a weak character. The frontal bone is the bone
associated w/ the top part of the skull, usually the orbit (eyes) are bordered by the
pre-frontal and post-frontal and the frontal doesnt make contact. But in amniotes,
the frontal does. The prefrontal and postfrontal have become smaller and pushed
aside and the frontal makes contact. The problem is, that frontal contact is
inconsistent among amniotes. Some amniotes have secondarily reversed back to
the primitive state where the pre-frontal and post-frontal are in contact. So again,
controversy...as prof goes through these, hes throwing out these characters
Tabular, supratemporal and postparietal dont form part of the skull table:
Skull table is the top part of the skull. The occipital region would be the back part
of the skull. What this character trait is saying is that these 3 elements: the tabular,
www.notesolution.com
supratemporal and postparietal are not associated w/ the skull table part- they are
only associated w/ the back. Those elements primitively are on top, reptiles have
them at the back. Limnoscelis seems to have them towards the back but not
Diadectes. Yet again, another problem. So this is not a plausible character.
3 distinct and separate ossifications of the shoulder girdle: yet another
controversy. Our shoulder girdle has a couple elements that ossify separately.
How they ossify is inconsistent throughout any group that may or may not be an
amniote. Some anthracosaur have an unusual ossification. You cant look at these
animals developmentally and how they develop in an embryo b/c they dont exist
today. So the number of ossifications in the shoulder girdle, not very powerful.
Astragalus- ankle bone
No more than 2 centralia (ankle bone) present in foot: no more than 2 is vague.
What does no more mean. What if one is so poorly ossified, that you dont have
evidence for it because it was a cartilaginous element in the carpus. Does that
mean it wasnt there? You want to modify that character to say, no more than 2
ossified elements. You cant say that because in fossils, only ossified elements
fossilize. Again, weak character.
Therefore, all these characters traits are weak and we have absolutely no characters that unite
amniotes. The only character that unite amniotes (the amniotic egg), we cant test and verify its
presence in the fossils. So we really dont know what amniotes are, we only have really good
sense of what they should be. Any one of these characters are flawed b/c can say, what about
Limnoscelis. Limnoscelis is a very important creature. If we can get this thing described, we may
have a better sense of which of these characters are real and which arent. The potential pitfalls is
that Limnoscelis may create a problem w/in Diadectomorpha as a whole-> it may no longer be a
monophyletic taxa.
2.)Hylonomus : early reptile! b/c it looks like it has all of the character traits that prof just
said are inconsistent. And its small. But size is not always a very good character trait
b/c it can be caused as a result of many things (ex. forced to live in small
environments, dwarfism occurs)
www.notesolution.com

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Description
Origin of Amniota, the amniote egg - Thing that you should focus on so far: Diadectomorphs are the sister-taxon to amniotes. Whether or not they had more of an amniote character trait than were aware of is still questionable. The one thing that would definitely prove them to be amniotes is the possession of an amniotic egg or cleidoic egg and thats the one thing we cant prove. It would be fantastic if we could find eggs associated w some remains of Diadectomorphs but its unlikely bc its very likely that if they did lay eggs, they would have just left them there. They didnt have any parental care (most reptiles dont). In crocodiles, dinosaurs, birds: yes but other examples, reptiles just lay their eggs and thats it. So even if we do find eggs, we may not know that they belong to Diadectomorphs at all. What would be really fantastic is if we find an egg w an embryo in it so the Cleidoic egg is the key - Amniote radiation: Amphibians are common up until the Permian-Triassic boundary then amphibians really die off rapidly. And it seems that a lot of reptile groups do as well. There was certainly a major extinction event that took place at this point. Amphibians survived it and continued w more or less similar diversity all the way through. Reptiles becomes more diverse as we go through the Mesozoic and then were hit very hard at the end of the Mesozoic (birds were believed to have come off it). Mammals or the synapsids that led to mammals arrived somewhere in the Permian, even earlier than that such as the carboniferous synapsids, Dimetredon. Win this, somewhere here (we probably can trace it all the way back to the Carboniferous), the amniotic egg must have evolved. Whether that amniotic egg was already present in the sister taxon to amniotes or if they evolved win the group is a big question. One of the things that we have recognized is that the earliest reptiles (that we know in terms of osteological similarities to extant reptiles), is that they were small. The Limnoscelis and Diadectes were large, Seymouria wasnt that small either... so they were all very big. But the earliest reptiles or true reptiles were quite small. Some issue around size seemed to have permitted the reptile radiation to eventually take place. But why size is so significant is problematic. 1.) Early Amniotes: - So we know they were small. The first taxa is Hylonomus. www.notesolution.com- The character traits that diagnose the amniotic clade have a lot of uncertainty and many different interpretations. Have 4 or 5 different cladograms showing the possible relationships of amniotes and of the taxa nearest amniotes. Some amniote character traits better than others: Transverse flange of the pterygoid bearing a single row of teeth: that transverse flange- that modified region of the palate associated w the pterygoid which is a paired and large element in the palate - seems to be a key to the evolution of the amniotes. There are no amphibians w this transverse flange. Seymoriamorphs appear to have this flange but they dont really have a prominent row of teeth on it- they have these tiny little denticles all the way through on their palate a lot of amphibians have those. Its likely an aid to holding on to objects they trap in their mouths but no real dentition to pierce or kill what theyve captured. So when do the teeth and the transverse flange become a character trait that you can separate amniotes from non-amniotes is the question. The transverse flange of pterygoid for some authors (like Benton) doesnt even use it as a character for amniotes bc hes so unconvinced that it is. He argues that Diadectes has a transverse flange yes but it has a lot of denticles on it. Limnoscelis has a transverse flange and it appears that Limnoscelis has a row of prominent teeth (even though it still has denticles). That is supposed to be a reptile trait so is Limnoscelis an amniote and Diadectes not? Then you have a problem bc diadectomorph is no longer a monophyletic group, its now paraphyletic. You can see the confusion here. Therefore, Limnoscelis is a very important creature. Frontal contacts the orbit: this is a weak character. The frontal bone is the bone associated w the top part of the skull, usually the orbit (eyes) are bordered by the pre-frontal and post-frontal and the frontal doesnt make contact. But in amniotes, the frontal does. The prefrontal and postfrontal have become smaller and pushed aside and the frontal makes contact. The problem is, that frontal contact is inconsistent among amniotes. Some amniotes have secondarily reversed back to the primitive state where the pre-frontal and post-frontal are in contact. So again, controversy...as prof goes through these, hes throwing out these characters Tabular, supratemporal and postparietal dont form part of the skull table: Skull table is the top part of the skull. The occipital region would be the back part of the skull. What this character trait is saying is that these 3 elements: the tabular, www.notesolution.com
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