ZOO 2090 Lecture Notes - Lecture 12: Sauropsida, Squamata, Skull
1 May 2018
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ZOO2090 – Sauropsid Amniotes
*See Tree Diagram
• Diapsida (two holes on each side of skull) – large majority within sauropsids
o Archasauromorphs
▪ Archosaurs = dinosaurs and birds
o Lepidosauromorphs
▪ Lepidosaurs
• Squamates = lizards and snakes
• Divergence of turtles is unknown and highly debatable
o In sauropsids?
o In lepidosaurs?
o In squamates? *before archosaurs – most recent
• Birds are derived from dinosaurs
• Ichthyosaurs – flying; not actually in the dinosaurs group (but birds are)?? *in
lepidosauromorphs
• K-T boundary – where dinosaurs and other reptiles become extinct *major boundary
• In Mesozoic (Jurassic) dinosaurs were very successful and dominant
Testudines (Turtles)
• Already present in late Triassic
• There are ~341 species today with a wide range of habitats and diets
• Hard shell and solid toothless skull have been stable
• Dorsal carapace and ventral pastron, both made up of fused bony elements covered by
keratin scutes, are attached on the sides
• Recent molecular evidence: turtles derived from diapsids (not as ancient as previously
thought)
• *see tree C – most closely related to birds and crocodiles
• Origin of turtle body plan:
o Shell: (vertebrae+ribs+osteoderms) + scutes (keratin) *bone plates develop in
dermis
o Position of scapula: inside rib cage**
1. The classic Transformationist view
2. The emergentist view
rapid evolution via changes in developmental regulation = gradual transformation
o Started with small plates (osteoderms) which come together to form one big
shell that eventually encloses rib cage = forms boundary
o No osteoderm in tail or neck (disappears)
o Fossil species represent intermediate stage
o Movement of spatula = changes muscle connection
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*mesosaurs were first sauropsids to adopt specialized aquatic lifestyle paddle shaped feet,
swimming tail, long neck/snout
Early Anapsids - not important
First Diapsids
o Origins in the Carboniferous
o Petrolacosaurus is similar in form to earliest amniotes smaller head and longer
limbs/neck
o Skull has larger orbits and two temporal fenestrae
o Diapsids did not radiate until late Permian, when gliding weigeltisaurids lived in Europe
and Madagascar
o Horizontal wings were made of elongated ribs that could be folded back when the
animal was not in the air
o Terrestrial and aquatic younginiformes also appeared at this time
Lapidosauromorphs: Lepidosauria
o Extinct forms leading to Sphenodontians (tuatara) and squamates (snakes and lizards)
o Oldest known lizards from mid-Jurassic, while snakes appeared in early Cretaceous
o Snakes diverge from legless lizards
o Relationship of snakes, dibamids, and amphisbaenids (all legless) to lizards is still
uncertain
o Snake Differences: snakes have short tail and long body; lizards have long tail
▪ Snakes have a forked tongue (so do large lizards)
o Skull Adaptation in Squamates:
o Lower temporal bar is absent in lizards and both upper and lower temporal bars
are absent in snakes improved jaw mobility and ability to swallow
o Cranial kinesis (mobility) in lizards modifies jaw mechanics *extra joints
(mesokinetic and streptostylic)
o Snout tips up when jaws open (top) but it tips down when they close allowing
equal perpendicular forces by both jaws on food item (eliminates outward force
for better handling of food)
o Snake:
▪ Contains 8 moveable links very mobile skull; can swallow food larger
than head
Crawling Locomotion: Life without Limbs
• Several thousand amniote species are limbless and this situation has evolved several
times successful adaptation
• Lateral Undulation is most common in land and water *similar to fish
o Moving waves push sideways against contact points (at least 3 are required)
generating a reaction force with forward component (lateral components cancel)
• Concertina movement:
o (costly) is used in narrow spaces like tunnels
o involves stationary coils wedging animal into place while free body parts move
forward
find more resources at oneclass.com
find more resources at oneclass.com
