Protoclepsydrops was an early amniote

Amniote

The amniotes are a group of tetrapods that have a terrestrially adapted egg. They include synapsids and sauropsids , as well as their fossil ancestors. Amniote embryos, whether laid as eggs or carried by the female, are protected and aided by several extensive membranes...

, and its skeletal remains indicate that it may have been more closely related to synapsids than to sauropsids, making it a possible synapsid member. If so, it is the oldest synapsid

Synapsid

Synapsids are a group of animals that includes mammals and everything more closely related to mammals than to other living amniotes. They are easily separated from other amniotes by having an opening low in the skull roof behind each eye, leaving a bony arch beneath each, accounting for their name...

 known, though its status is unconfirmed because its remains were fragmentary. Protoclepsydrops lived slightly earlier than Archaeothyris

Archaeothyris

Archaeothyris was a very early mammal-like reptile, which lived in the late Carboniferous period. Dated to 306 million years ago, it is the oldest undisputed synapsid known....

. Like Archaeothyris, Protoclepsydrops resembled modern lizard

Lizard

Lizards are a widespread group of squamate reptiles, with nearly 3800 species, ranging across all continents except Antarctica as well as most oceanic island chains...

s. However, Protoclepsydrops still had primitive vertebrae with tiny neural processes typical of the first (stem-) amniotes.

See also

• List of synapsids

• Evolution of mammals
  Evolution of mammals
  __FORCETOC__The evolution of mammals within the synapsid lineage was a gradual process that took approximately 70 million years, beginning in the mid-Permian. By the mid-Triassic, there were many species that looked like mammals, and the first true mammals appeared in the early Jurassic...
  
  
• List of transitional fossils
• Carboniferous tetrapods
  Carboniferous tetrapods
  Carboniferous Tetrapods include amphibians and reptiles that lived during the Carboniferous Period.During this time, amphibians were the predominant tetrapods, and included the Temnospondyli, Lepospondyli, and Reptiliomorpha/Batrachosauria...
  
  
• Clepsydrops
  Clepsydrops
  Clepsydrops was a primitive amniote from the early Late Carboniferous that was related to Archaeothyris and the synapsids—the ancestors of mammals. Like many other terrestrial early amniotes, it had the diet of insects and smaller animals. It also laid eggs on land rather than in the water, as...