Evolution What the Fossils Say and Why it Matters

Parietal foramen

Dentary

Large clavicles,

interclavicles,

and coracoids Long phalanges

Increased number

of sacral vertebrae

Limb placed under body

Expanded

iliac blade

Short tail

Reduced pubis

and ischium

Calcaneal heel

Zygomatic arch

Reduced clavicles,

interclavicles,

and coracoids

Dentary-squamosal

jaw joint

Rod-
shaped
ilium

Loss/reduction of

cervical ribs

Loss of

lumbar ribs

Postorbital bar

Secondary palate

Differentiated teeth

Coronoid process

of dentary

Short phalanges

Temporal fossa

Large pubis and ischium

Large processes on

caudal vertebrae

Long tail

Pelycosaur (Haptodus)

Noncynodont therapsid (Lycaenops)

Cynodont therapsid (Thrinaxodon)

Early mammal (Megazostrodon)

1 cm

1 cm

1 cm

1 cm

FIGURE 13.4. The transformation of

the synapsid skeleton from primitive

pelycosaurs like Dimetrodon through

cynodonts to true mammals. (Drawing

by Carl Buell).

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