List of Transitional Fossils - Synapsid ("mammal-like Reptiles") To Mammals

Synapsid ("mammal-like Reptiles") To Mammals

The Synapsids → Mammals Evolutionary Series
Appearance Taxa Relationships Status Description Image
??? Ma

Genus:

  • Protoclepsydrops
 Known from very fragmentary finds, Protoclepsydrops may be the earlies Synapsid (mammal-like reptile) A low-slung, lizard-like animal of moderate size.
306 Ma

Genus:

  • Archaeothyris
 The oldest undisputed Synapsid (mammal-like reptile) Primitive traits
  • A relatively flat, reptile-like skull
  • Typically reptilian sprawling gait
  • Generally lizard-like proportions with a dorso-ventrally flattened body

Derived traits

  • Temporal opening low on the side of the skull roof, between the zygomatic bone and the elements above.
  • Tendency to enlarged forward teeth on the maxilla
297 Ma Genus:
  • Haptodus
 A primitive member of the Sphenacodontidae, or possibly just outside the group. A Pelycosaur-grade synapsid Derived traits
  • Two or three moderatly large canine-like teeth about a third down the maxilla.
  • Dentary bone the largest element of the lower jaw
  • The skull deeper than in Archaeothyris
265 Ma

Genus:

  • Dimetrodon
 An advanced member of the family Sphenacodontidae, from which the therapsids (advanced synapsids) evolved A Pelycosaur-grade synapsid. At up to 4 meters, Dimetrodon was one of the largest animals of its time. The distinct sail of the back makes it the most recognized synapsid known

Primitive traits

  • Cold blooded metabolism dependent of external heat source (hence the "sail")
  • Sprawling gait
  • No secondary palate
  • No enlarged side teeth in the lower jaw

Derived traits

  • Distinctly elongated 2nd and 3rd tooth on the maxilla, corresponding to the canine in mammals. The first canine generally longer than the second.
  • Skull deep and narrow
  • Body overall deeper than in earlier forms
267 Ma

Genus:

  • Biarmosuchus
 A primitive therapsid. About the size of a large dog, Biarmosuchus was a lightly built and likely fairly agile animal for its size.

Primitive traits

  • No repiratory turbinates indicate limited overall oxygen consumption and hence bradymetaboliic metabolism
  • Sprawling legs, but the legs longer and more slender than in pelycosaurs
  • Long pelycosaur-like tail

Derived traits

  • A single canine as the first tooth on the maxilla, all other maxillary teeth small
  • Tendency for an enlarged caninelike tooth on the dentary
  • Internal nostrils covered by a partial fleshy palate
  • Enlarged temporal opening giving more powerful bite
248-245 Ma

Genus:

  • Thrinaxodon
205 Ma

Genus:

  • Morganucodon
125 Ma

Genus:

  • Yanoconodon
 An early crown group mammal.

Read more about this topic:  List Of Transitional Fossils