study notes part 2

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University of Toronto Mississauga
Michaelde Braga

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. 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, 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,
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