Earth Sciences 2266F/G Lecture Notes - Lecture 15: Draco Volans, Diapsid, Protorosaurus

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Lec 15
We’ve talked about the Anapsida; turtles (testudines) will eventually survive; they’re related to
anapsids
Diapsids eventually led to modern reptiles, and another group evolved into Archosauria:
crocodiles, dinosaurs, and eventually birds
Diapsids are easy to identify: on the back side there are 2 holes (fenestrae) and other side are
another 2 so total 4 holes
Hole is in between parietal and squamosal and postorbital bones
From this group on, they always have these 2 holes including dinosaurs
Diapsids evolution: in their early evolution, they are geared towards agility – light body, light-
boned for fast movement; thus their bodies are very agile including dinosaurs, and that’s why
birds can fly since they’re so light-bodied
In their adaptation onto land, they started off as insectivores; chasing insects require very quick
movement
Light-weight skull: huge eye for judging distance and positioning, and holes at the back make
their skull very light (not much weight in skull)
So if they have a long neck, there’s quick movement with a light skull
Eating insects and dragonflies, which are big at this age (almost a metre long)
So if you catch just one, it’s already a big meal – can serve a couple of meals for several diapsids
Co-evolution of forests and insects, and then vertebrates followed
This is a very small (40cm) diapsid, running around trying to catch insects
But dragonflies and other arthropods can grow to a huge size as well, so some larger insects may
be preying on these diapsids as well
Petrolacosaurus has tough scaly skin to trap moisture inside; can venture away from water but
thus encountering new predators
Evolved a complex heart (template for our own), pumping lots of O2 around so it can run at high
speeds
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Some of the early diaspids like the ones that can fly that evolved into Pterosaurs, have extended
rib-like bones
There’s a debate whether they’re extended ribs or separate bones
Now people tend to think they’re separate bones and not ribs
They can stretch these out, and skin can thus stretch out too
They don’t have muscles though, so can’t fly; but they can glide to escape danger or find more
prey
Their skull is very lightweight; big eye socket and big holes; skull has been reduced
They look almost the same as modern dragon lizard (Draco volans, a diapsid glider); normally,
their flaps are hidden but they can spread them out to glide
There are separate bones in these flaps
Eventually they grow bigger and they become more specialized predators
One of the groups in diapsids, right before the dinosaurs, is the Protorosaurus
Half a metre long, and their legs got longer  so they can run very quickly
Longer neck as well, which is a trait of dinosaurs
Insects are flying, so longer neck is easier to catch prey instead of using their whole body
Pelvic girdle becomes much bigger (with 3 bones: ileum, ischium, and pelvis); later, dinosaurs
have even bigger bones in this area
Bone is also curved (femur), for easier jumping and for absorbing impact better
A straight femur won’t be good for absorbing impact
Protorosaurus and flying reptiles pave the way for dinosaurs (Archosauria)
Now we will turn to the branch that led to mammalia
In yellow are the Paleozoic forms (synapsids)
- So we belong to the synapsid group, not diapsid
In our evolution, if you look at our ancestors, they weren’t built for agility and
lightweightedness; they were built for heavy body, clumsiness, and brutal power
The hole at the back of the skull was very small, and heavy bones were at the front of the skull
Large, powerful big teeth
They started off eating insects, but then started eating bigger prey: fish and other lizards
Sometimes even the same body size as themselves
Archaeothyris has stretched legs sideways and a heavy body, living in tree trunk (as tree trunks
are hollow at this time)
Looking at the skeleton, the head is clearly very heavy
Deeper skull for more larger teeth, and also stronger muscles (for biting other prey larger than
insects)
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