Origin of Mammals Flashcards
Which group of reptiles did mammals descend from?
Synapsids
How do anapsids, synapsids, diapsids, and and euryapsids differ?
The number of temporal openings in the skull behind the orbit. Anapsids had none, synapsids had one, diapsids had two. Euryapsids also have one, but they are an offshoot of the diapsid lineage where one of the openings became fused shut again
Which group of synapsids were the ancestors of mammals during the Permian period?
The pelycosaurs like Dimetrodon. They were predominantly vegetarian, sail-backed synapsids. They lived in the northern hemisphere. Didn’t survive the Permian period, went extinct in the mass extinction
Which group did the Pelycosaurs evolve into that did survive the Permian extinction?
The therapsids
What are some characteristics of therapsids?
They were the first group to show different types of teeth, and the first appearance of canines in the amniotes. They were predatory, with large maxilla bones and long faces
Why are the types of jaw bones and their connections important when considering the origin of mammals?
The jaw bones are what links mammals to the synapsids
How did jaws in early vertebrates come around?
There were bony elements associated with the pharyngeal gill slits, and the front two moved forward and became associated with the skull. They supported the mouth cavity and one became the upper jaw and the other the lower jaw. This allowed these fish to bite and become predators
What characteristic jaw feature defined the first true mammal?
Squamosal-dentary jaw articulation
What two bones form the jaws of fish and most non-mammalian tetrapods?
The articular and the quadrate
How was the squamosal bone changing in the therapsids?
It was extending down towards the mouth cavity
What are the characteristics of the cynodonts? Did it have the dentary-squamosal jaw articulation?
They were a specialized group of therapsids that survived the Permian extinction, but went extinct in the late triassic. The quadrate and the articular still formed the jaw articulation, but they were getting smaller
What was the extension of the squamosal bone selected for?
Provided better jaw support and made those individuals better predators
What were the jaw characteristics of Probainognathus, an advanced cynodont?
The dentary became larger and connected with the squamosal and formed a jaw connection. However, the quadrate and articular were getting smaller, but they were still there and still formed the main jaw articulation. The upper jaw was nearly all squamosal and the lower jaw was nearly all dentary. They also had double jaw articulation, one with the quadrate and articular, the other with the squamosal and dentary
What were the characteristics of Morganucodon, a late cynodont that was nearly a mammal?
The quadrate bone was gone, but the articular was still there. It was a small, shrew-like insectivore. Finding nests suggest that they had social behaviour and maybe even mammary glands
When did the first true mammals appear?
Late jurassic
