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University of Toronto Mississauga
Michaelde Braga

Anthracosauria and Diadectomorpha (closest sister taxa to amniotes) Anthracosaurs- as name implies, they are cold reptiles; intitially thought of as being reptilian-like based on general appearance but they are amphibians . They are a fairly diverse assemblage of amphibians. - Believed to be related to amniotes - Maybe they are paraphyletic because some members of anthracosaurs are more closely related to amniotes than other anthracosaurs. - The group is not a natural assemblage so from a phylogenetic point of view, the word anthracosaurs doesn’t mean anything. It’s just a generic term that has remained in usage but as far as what a real anthracosaur is, we don’t have one. There are no synapomorphies to diagnose the group specifically. There are synapomorphies that diagnose members w/in the group; but we just continue to group them as a general assemblage of amphibians that are reptile-like, but may not necessarily share a lot in common other than being reptile-like. - Greatest similarity (to amniotes) is associated w/ the skull and has a lot to do w/ potential development of amphibian’s matching ear. B/c of that otic notch at the side of the skull, they would have had these matching ear. But we now know that the stapes of these animals are too large to have functioned properly-> it wouldn’t have been useful as a system that amplified sound- probably heard in the same way that many primitive groups have -> simply feeling vibrations. Certainly, the stapes in its contact b/w the cheek region, in its association w/ the jaw and its direct contact w/ the braincase is in a perfect place for the transmission of sound into the braincase. Therefore, it was pre-adapted to become an impedance-matching system and eventually, it does. - But in every group that it appears as an impedance-matching system (ie. An opportunity to amplify sound waves so that you can actually hear), it has evolved independently. It evolved independently in amphibian groups and birds but all related to those bones and the stapes is one of them. 2.) Anthracosaur synapomorphies (depending on whether you believe anthracosaurs to be monophyletic or not) • tabular contacts the parietal (tabular is a bone associated w/ the back end of the skull)- primitively, that contact is absent • otic capsule is firmly attached to the tabular - Proterogyrinus: typical anthracosaur 3.) Anthracosaur Symplesiomorphies (features considered to be primitive) • Intertemporal is present- (mentioned in previous lecture: the intertemporal is lost in many taxa- it’s not particularly useful as a synapomorphy is many taxa b/c it’s regularly lost; common trend is a reduction of elements in the skull) • Closed palate (no interpterygoid vacuity)- recall temnospondyls had huge holes in their palate; anthrocosaurs don’t have that - they have solid palate • Skull table is not firmly sutured • Presence of 5 fingers (are they primitive or derived; Ichyostegids and Ancanthostega had more than 5 fingers. Generally, higher # of fingers seems to be primitive but is 5 fingers a derived feature of this group or is it a primitive feature that is present in this group, that is retained later on in amniotes. 4.) Anthracosaur unpolarized characters (means no one really knows what these characters mean): • Recall: Otic notch was initially described to perhaps supporting a tympanum (which would have been the eardrum) but clearly there is no indication fossil wise or in terms of function that was the case. Massive stapes, very unlikely there was a tympanum present 5.) Proterogyrinids: best known groups of anthracosaurs; differ from many anthracosaur in having crescentic pleorocentra and intercentra; pleorocentra forms almost a complete circle and the intercentra are a little bit smaller. 6.) Vertebral morphology: • As you progress away from fish, to early tetrapod, to more derived members of the tetrapod lineage, the large intercentra disappears. In amniota, it’s virtually absent or very small if present. The reverse happens in amphibians – the intercentra often becomes the larger element In fish and Ichthyostegids (Ichthyostega and Acanthyostega): Pleurocentra is very small, virtually non-existent (intercentrum is the largest element). The only difference is the articulating zygopophesis (fish don’t have them). If you only look at the intercentrum and pleurocentrum, may not have enough information to determine if that’s a fish or a tetrapod. - Microsaurs and other amphibians (the Eryops-one of the temnospondyl-like amphibians looked at before), intercentra is still fairly large. In anthracosauroids, the intercentra and pleurocentra are both large. - Go further up to Seymoria (which is a basal didectomorph)- the intercentrum is small - The pleurocentra is the most significant element of the vertebral column that evolves w/in w/in the group leading toward the amniotes. We’re fairly confident of that and if we find a fossil taxa that’s unknown, and you examine the vertebral column of these taxa, you can determine if there is a pleurocentrum present (it is likely going to be in line w/ the amniotes, certainly an anthracosauroid amphibian if you have nothing else to go on).And if you are missing this pleurocentrum or if you have a much larger intercentrum, then you know you are perhaps looking at something more amphibian-like. So that’s a fairly good character. The vertebral morphology seems to be consistent all the way through. 7.) Anthracosauroids: 2 large intercentra in the front; Seymoria (heading toward the amniotes): intercentra becoming smaller. If this has been a taxon other than Anthracosauroids leading towards the amphibian groups (like temnospondyls), we would see the intercentrum much larger and the pleurocentrum virtually absent. 8.) c. Rachitomous type: similar to fish & Ichthyostegids: large intercentra, Small pleurocentra d. Stereospondylous type: pleurocentra is completely absent and all you have is a intercentrum (it’s almost a completely circular spool,just like spool of thread) And have a big, prominent hole going through the center, at least in the very young organisms, suggesting there is some form of notochord still persistent) e. Embolomerous type (seen in anthracosaurs): have 2 fairly large components to the vertebral centrum. A slightly smaller intercentrum w/ a larger pleurocentrum but still 2 elements clearly present. f. Amniote condition: a very large pleurocentrum (that eventually becomes fused to the neural arch) and intercentrum is extremely small and most cases, lost. Zygopophesis present in all of these taxa (except only fish) 9.) Not going to spend too much time; Embolomeres (group of anthracosaurs) – fairly common; by the end of the Carboniferous and beginnings of the Lower Permian, that’s when they reach their peak and after that, they disappear; one basic character that seems to unite the embolomeres is the structure of the pleurocentra and intercentra 10.) Discosauriscids - another type of anthracosaur; these differ from the other groups in being aquatic in their general appearance. They were probably neotenic (ie. They retained a larval lifestyle and matures as larval forms. Imagine the tadpole that never metamorphose into a frog. That’s the case for the Discosauriscids). • not ver
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