Old Earth Ministries Online Earth History Curriculum

Presented by Old Earth Ministries (We Believe in an Old Earth...and God!)

This curriculum is presented free of charge for use by homeschooling families.

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Chapter 7 - The Carboniferous Period

Lesson 39: Synapsids


    Synapsids ('fused arch') are a group of animals that includes mammals and everything more closely related to mammals than to other living amniotes. Primitive synapsids are usually called pelycosaurs; more advanced mammal-like ones, therapsids. The non-mammalian members are described as mammal-like reptiles in classical systematics, but are referred to as "stem-mammals" or "proto-mammals" under cladistic terminology. Synapsids evolved from basal amniotes and are one of the two major groups of the later amniotes, the other major group being the sauropsids (reptiles and birds). They are distinguished from other amniotes by having a single opening (temporal fenestra) in their skull behind each eye, which developed in the ancestral synapsid about 324 million years ago (mya) during the late Carboniferous Period.


Chapter 7 - The Carboniferous Period


Lesson 36: Overview

Lesson 37:  Coal

Lesson 38:  Oil

Lessson 39: Species In-Depth - Bivalves


 Synapsid Skull


The synapsids are distinguished by a single hole behind each eye (Picture Source)

     Synapsids were the dominant terrestrial animals in the middle to late Permian period. As with almost all groups then extant, their numbers and variety were severely reduced by the Permian extinction. Some species survived into the Triassic period, but archosaurs quickly became the dominant animals and few of the non-mammalian synapsids outlasted the Triassic, although survivors persisted into the Cretaceous. However, they included the prehistoric ancestors of mammals and in this sense, synapsids are still very much a living class of vertebrates. Synapsids, most recently and notably humans, again became the dominant land animals after they outcompeted birds following the K-T extinction event.




The pelycosaurs (from Greek πέλυξ - pelyx "axe" or perhaps "bowl"/"pelvis" and σαῦρος sauros "lizard") were primitive Late Paleozoic synapsid amniotes. Some species were quite large and could grow up to 3 meters or more, although most species were much smaller.
 Life restoration of Archaeothyris (Picture Source)

Archaeothyris and Clepsydrops are the earliest known synapsids. They lived in the Pennsylvanian subperiod of the Carboniferous Period and belonged to the series of primitive synapsids which are conventionally grouped as pelycosaurs. The pelycosaurs were the first successful group of amniotes, spreading and diversifying until they became the dominant large terrestrial animals in the latest Carboniferous and Early Permian Periods. They were sprawling, bulky, cold-blooded and had small brains. They were the largest land animals of their time, ranging up to 3 m (10 ft) in length.  The pelycosaurs appeared during the Late Carboniferous and reached their acme in the early part of the Permian Period, remaining the dominant land animals for some 40 million years. A few continued into the late Permian. They were succeeded by the therapsids.

     At least two pelycosaur clades independently evolved a tall sail, consisting of elongated vertebral spines: the edaphosaurids and the sphenacodontids. In life, this would have been covered by skin, and possibly functioned as a thermoregulatory device and/or for mating display. Pelycosaur fossils have been found mainly in Europe and North America, although some small, late-surviving forms are known from Russia and South Africa.

Unlike lepidosaurian reptiles, pelycosaurs lacked epidermal scales. Fossil evidence from some ophiacodonts shows that parts of the skin was naked, but that the belly was covered in dermal scutes, looking like the scutes present in other reptile groups, which are of a different type of structure.

     Dermal scutes are also found in extant mammals, in the tails of rats, beavers and Opossum, and as regular dermal armour with underlying bone in the armadillo.


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